Nataliya Denchik
email: nataliya.denchik@gm.univ-montp2.fr

Stéphanie Gautier

Margaux Dupuy

Christelle Batiot-Guilhe

Michel Lopez

Véronique Léonardi

Muriel Geeraert

Gilles Henry

Denis Neyens

Paul Coudray

Philippe A Pezard

In-situ geophysical and hydro-geochemical monitoring to infer landslide dynamics (Pégairolles-de-l'Escalette landslide, France)

Keywords: Landslide, rainfall, borehole monitoring, electrical resistivity, water geochemistry

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Introduction

A correlation between heavy rainfalls, groundwater circulations and slope movements has been demonstrated at several sites (e.g. [START_REF] Iverson | Landslide triggering by rain infiltration[END_REF][START_REF] Bogaard | Landslide hydrology: from hydrology to pore pressure[END_REF]. In contrast, the internal physicochemical processes related to the groundwater flows and their contributions to slope failures or later reactivations remain poorly understood. Better constraining these internal processes is a real challenge for both landslide understanding and prediction.

Much effort has been deployed at different sites combining geological, geophysical and hydrogeological approaches in order to characterize the 3D geometry of several landslides (e.g. [START_REF] Jongmans | Geophysical investigation of landslides : a review[END_REF][START_REF] Jomard | Electrical imaging of sliding geometry and fluids associated with a deep seated landslide (La Clapière, France)[END_REF][START_REF] Hibert | Characterizing landslides through geophysical data fusion: Example of the La Valette landslide (France)[END_REF], the groundwater flows within the slope (e.g. [START_REF] Guglielmi | Hydrogeochemistry: an investigation tool to evaluate infiltration into large moving rock masses (case study of La Clapière and Séchilienne alpine landslides)[END_REF][START_REF] De Montety | Identifying the origin of groundwater and flow processes in complex landslides affecting black marls: insights from a hydrochemical survey[END_REF][START_REF] Deiana | Chemical and isotopic investigations (δ18O, δ2H, 3H, 87Sr/86Sr) to define groundwater processes occurring in a deep-seated landslide in flysch[END_REF] and the resulting deformation at the surface (e.g. [START_REF] Delacourt | Remote-sensing techniques for analysing landslide kinematics: a review[END_REF][START_REF] Jaboyedoff | Use of LIDAR in landslide investigations: a review[END_REF][START_REF] Stumpf | Correlation of satellite image time-series for the detection and monitoring of slow-moving landslides[END_REF]. For a few years, time-lapse electrical resistivity tomography is used to monitor the spatial and temporal distribution of water content through the landslides (e.g. [START_REF] Wilkinson | Tracking the Movements of Electrodes on an Active Landslide over Time Using Only Time-lapse Resistivity Data[END_REF][START_REF] Gance | Permanent electrical resistivity measurements for monitoring water circulation in clayey landslides[END_REF][START_REF] Palis | Multiyear time-lapse ERT to study short-and long-term landslide hydrological dynamics[END_REF]. This method provides images of both rainfall supplies and groundwater circulations at depth and thus contributes to a better description of the slope hydrology. Nevertheless, the relationships between slope hydrology and water-rock interactions have been so far little studied for landslide investigations, although the inferred coupled hydro-mechanical effect may take part of the slope destabilization (e.g. [START_REF] Cappa | Hydromechanical modeling of a large moving rock slope inferred from slope levelling coupled to spring long-term hydrochemical monitoring: example of the La Clapière landslide (Southern Alps, France)[END_REF][START_REF] Lignon | Hydro-mechanical modelling of landslides with a material instability criterion[END_REF][START_REF] Lehmann | Hydromechanical triggering of landslides: From progressive local failures to mass release[END_REF].

Because the water-rock interactions occur at small scales through the landslide, near-field observations by boreholes are essential to better describe the internal landslide processes. Investigations by boreholes contribute to gather lithological (core samples, logging), geotechnical (tiltmeters, extensometers) and hydrogeological data (piezometers) (e.g. [START_REF] Crawford | Geologic, Geotechnical, and Geophysical Investigation of a Shallow Landslide[END_REF]. In particular, borehole data allow characterizing the internal structure and fissure patterns [START_REF] Lofi | Geological discontinuities, main flow path and chemical alteration in a marly hill prone to slope instability: Assessment from petrophysical measurements and borehole image analysis[END_REF] and locating preferential water flows within the system [START_REF] Tsao | A case study of the pore water pressure fluctuation on the slip surface using horizontal borehole works on drainage well[END_REF]. However, these conventional borehole measurements are not well adapted to monitor the landslide dynamics due to the low frequency of the time series and the lack of crucial data as the water chemistry [START_REF] Bogaard | Hydrogeochemistry in landslide research: a review[END_REF].

The aim of this work is to investigate at small scales the relationships between rainfalls, groundwater circulations and the hydro-physicochemical processes within the landslide. For this purpose, we investigate a slow deep-seated landslide located in the central part of Languedoc (South of France). This area is affected by well-known heavy rainfall events with short durations that occur over the Cevennes ridge [START_REF] Delrieu | The catastrophic flash-flood event of 8-9 September 2002 in the Gard Region, France: a first case study for[END_REF][START_REF] Fresnay | Heavy precipitation events in the Mediterranean: sensitivity to cloud physics parameterisation uncertainties[END_REF]. In 2012, we developed a new monitoring strategy based on coupled surface (rainfall) (off-site between 2012 and 2017, on-site since 2017) and borehole (hydrogeology, hydrochemistry, deformation, electrical resistivity) observatories.

The downhole in situ permanent instrumentation produces original high-frequency measurements at depth, in particular in the vicinity of the sliding zones. For the first time, we evaluate the ability of downhole observatories to monitor the impact of rainfalls on slope processes.

In this paper, we present the downhole observatories and the data recorded during the five years of monitoring. We aim to show that this database allows characterizing both the slip zones and the behavior of the different hydrogeological units in the landslide. We also attempt to identify peculiar geophysical and geochemical signatures related to large rainfall events that might provide some insight into groundwater transfer modalities inside the landslide body.

General setting

The Pégairolles-de-l'Escalette landslide (PEL) is located in the central part of Languedoc, South of the French "Massif Central", 60 km northeast of Montpellier city, on the left bank of the Lergue River, close to the village of Pégairolles-de-l'Escalette (Fig. 1). In this area, the landslide activity may affect important social and economic infrastructures, as the A75 motorway. The PEL destroyed a nineteenth-century bridge and damaged another one in the 1990s. A persistent scatterer analysis (e.g. [START_REF] Hilley | Dynamics of Slow-Moving Landslides from Permanent Scatterer Analysis[END_REF][START_REF] Del Ventisette | Multitemporal landslides inventory map updating using spaceborne SAR analysis[END_REF]

Geological and geomorphological setting

The fringing anthropic rock piles (Fig. 1d). On this area, a several hundred meters large, very irregular and chaotic surface, with possible counter slope, separates discontinuously the scree slope mantle and the remaining ruled slope indicating successive large nested slides with clear differentiate slide scars. The general fabric of the slide units shows a systematic tilting toward the footwall of the main fault that is compatible with a listric geometry. Furthermore, the mean dip of the tilted series increases from the inner to the outer slide unit suggesting an overall retrogressive rotational process rooted at the top of the evaporitic pile during the river incision. The Pégairolles-de-l'Escalette landslide (PEL) is the most typical of the valley (Fig. 1 and2). His freshness, position in the upstream part of the enlarged valley and location of the shear sole close to the present day thalweg is compatible with a Late Pleistocene to Holocene history. This hypothesis is reinforced by the extensive travertine flat seated in the village area and developed at the same level that the PEL slide front, two meters above the present-day river thalweg, that suggests a same stage of incision and a close age of emplacement i.e post-Wurmian [START_REF] Alabouvette | Notice explicative de la feuille Le Caylar à 1/50000[END_REF]. The lithological succession of the Carnian and Norian are here dominated on about 60 m thickness by alternating white gypsum/anhydrite and blue claystone layers including massive beds, chicken-wire fabric and isolated nodules and dispersed blebs (Fig. 3). In this tectonostratigraphical context the outcropping of the Norian at the base of the valley may be responsible of the complete change in the incision process, passing progressively from an overall sheet erosion at dynamic equilibrium to mass movement processes including: (1) ductile creep, evaporite solution and removal by river incision, (2) fracture corridor formation and opening by lower valley flank deconfinement, (3) increase of vertical seepage of meteoric waters, (4) rotational sliding and finally

(5) tilted block brecciation and passive rafting. The lithological succession of the Carnian and Norian is particularly compatible with this model. In this case, the faults that were initiated on a pre-existing fractured corridor on the Hettangian carbonates flattened downward along a main evaporite layer that plays the role of shear sole.

Hydrogeological setting

This region is characterized by a low rainfall level on average, but suffers from heavy rainfall events (300 to 500 mm in a few days), called "Cévenol events" which mainly affect the Mediterranean region in autumn, in particular the Cevennes ridge [START_REF] Blanchet | Co-occurrence of extreme daily rainfall in the French Mediterranean region[END_REF]. These heavy precipitations lead to a rapid recharge of the different reservoirs as suggested by some authors [START_REF] Marc | Investigation of the hydrological processes using chemical and isotopic tracers in a small Mediterranean forested catchment during autumn recharge[END_REF][START_REF] Aquilina | Water storage and transfer in the epikarst of karstic systems during high flow periods[END_REF]. Both the short duration and the abundant volume of meteoritic water related to those specific events may provide a clear input signal for landslide processes that may originate from seepages, lateral groundwater flow or vertical leakages.

The major hydrogeological unit implied in the landslide dynamic is a semi-confined aquifer (Fig. 4, R1 label), localized inside the Norian level and that becomes artesian in the foot of the landslide. High hydrostatic pressures in this specific level may participate to the slope instabilities. This groundwater flow system may also result in the dissolution of evaporites [START_REF] Binet | Variability of the groundwater sulfate concentration in fractured rock slopes: a tool to identify active unstable areas[END_REF]. R1

reservoir is overlaid by an unconfined Rhaetian carbonate reservoir (Fig. 4, R2 label). Hettangian karstic reservoir (Fig. 2b) may supply the deeper hydrogeological unit by vertical or lateral rainwater seepages from the surrounding slope through the gravitational listric faults system and may contribute to the recharge of the sliding zone.

Experimental site

Two nearby boreholes (one cored) of 65 m depth were drilled in the middle part of the landslide in order to investigate different potential sliding surfaces, rainfall infiltrations and also groundwater circulations (Fig 1c and2b). Both the description of the borehole core and geophysical logging allowed us to produce a well-resolved lithological and geophysical characterization of both slope-forming materials and discontinuities at depth (Fig. 4). Because scree slope deposits were encountered between 10 to 32 m depths, no core (only cuttings) and no data (casing) were collected in the superficial part of the borehole.

We identify the massive Norian evaporitic-claystone unit (Fig.

Data and methods

Two in-situ observatories were installed in the two holes (Fig. 1c), since April 2012, in a permanent way, for both geophysical (deformation and electrical resistivity) and hydro-geochemical (hydrogeology and hydrogeochemistry) monitoring. This instrumentation provides direct access to the slip zones and allows near-field investigations at depth. Therefore, this approach aims to evaluate groundwater transfers related to heavy rainfall events and to constrain both the geophysical and hydrochemical signatures of the related internal landslide processes. This downhole monitoring strategy is the main originalities of this study since the coupling of borehole observatories for geophysics, deformation, hydrogeology, and hydrochemistry has never been achieved in the past to study the landslide processes.

Rainfall

The daily rainfall is recorded by Météo-France at the station "Les Plans", located ~ 6.4 km away from the experimental site. Thanks to a new station deployed on the site since 2017, we verified that the local conditions are similar. From 2012 to 2017, the average annual rainfall was 1359 mm with a minimum value of 1058 mm (year 2017) and a maximum one of 1727 mm (year 2014) (Fig. 5). The mean intra-annual rainfall distribution was: 45% in autumn, 26 % in spring, 17 % in winter, and 12%

in summer. This partition underlines the major contribution of the autumnal heavy rainfall events.

Downhole hydro-geochemical observatory

A multilevel groundwater device (WestBay System®) was deployed in the destructive borehole (Fig. 1c, WB). This device allows combining piezometric monitoring and hydrochemistry surveys in order to better understand the effects of rainfall events on groundwater transfer and water-rock interaction within the slope [START_REF] Marc | Groundwater-Surface waters interactions at slope and catchment scales: Implications for landsliding in clay-rich slopes[END_REF][START_REF] Deiana | Chemical and isotopic investigations (δ18O, δ2H, 3H, 87Sr/86Sr) to define groundwater processes occurring in a deep-seated landslide in flysch[END_REF]. This system is based on a multipacker completion designed to collect pore fluid samples and to monitor both the pore fluid pressure and the temperature from chosen horizons along the borehole [START_REF] Black | Multiple-level groundwater monitoring with the MP System[END_REF].

Here, the borehole has been equipped with four ports, located respectively at 36, 45, 48 and 56 m depths (Fig. 4e). These levels were chosen accordingly to both the core description and the logging data in order to investigate the two evaporate rich zones (U1 and EL) and the two reservoirs (R1 and R2). Physico-chemical parameters and fluid samples are collected on a monthly basis. Electrical conductivity and pH are measured in the field for each sample. During the monitoring period, 44 pore fluid samples were collected at 45 m depth, 44 samples at 48 m, and 43 samples at 56 m depth. Only sparse data were collected at 36 m depth because of an insufficient amount of water in the upper part of the slope, which coincides with the unsaturated zone. Geochemical analyses of the collected fluid samples were performed in the laboratory. The following parameters were measured: major and minor cations (Ca 2+ , Mg 2+ , Na + , K + , Sr 2+ ), major anions (Cl -, SO4 2-, NO3 -, HCO3) and total organic carbon (TOC). The total dissolved solid parameter (TDS) is deduced from those analyses. The precision error was typically <±5%. For each sample, the result is the mean of at least three satisfactory injections in terms of standard deviation (< 0.1) and variation coefficient (< 2%). The analytical uncertainty in the concentration of TOC is less than à 0,1 mg/l.

The hydro-chemical parameters are first used to discuss the origin and the pathways of the waters inside the landslide [START_REF] Guglielmi | Hydrogeochemistry: an investigation tool to evaluate infiltration into large moving rock masses (case study of La Clapière and Séchilienne alpine landslides)[END_REF][START_REF] De Montety | Identifying the origin of groundwater and flow processes in complex landslides affecting black marls: insights from a hydrochemical survey[END_REF]. In particular, the TOC parameter may give indications on water infiltration in fractured or heterogeneous media [START_REF] Emblanch | Dissolved organic carbon of infiltration within the autogenic karst hydrosystem[END_REF][START_REF] Batiot | Carbone organique total (COT) et magnésium (Mg2+) : deux traceurs complémentaires du temps de séjour dans l'aquifère Karstique[END_REF]. The water chemistry (Ca 2+ , Na + , SO4 2-) can also contain information on the water-rock interactions highlighted by dissolutions or precipitations of the few minerals [START_REF] De Montety | Identifying the origin of groundwater and flow processes in complex landslides affecting black marls: insights from a hydrochemical survey[END_REF]. A joint interpretation of hydraulic head and water chemistry variations may therefore help to identify pressure (piston effect) and mass (mixing effect) transfer mechanisms within the landslide [START_REF] Deiana | Chemical and isotopic investigations (δ18O, δ2H, 3H, 87Sr/86Sr) to define groundwater processes occurring in a deep-seated landslide in flysch[END_REF].

Downhole geophysical observatory

The downhole geophysical observatory (DEO, Fig. 1c) is based on an automatic multi-electrode system (ImaGeau®) for electrical formation soundings and is coupled with fiber optic sensors for strain measurements. This monitoring system was deployed in the cored hole since 2012 in order to study the temporal variation of the subsurface electrical properties and the related deformation at depth. Assuming that electrical resistivity variations can be interpreted in terms of hydro-geochemical quantities (water content, composition) over the monitoring period, the recorded data may provide information on the coupling between groundwater flows and deformation at depth. Because the dissolution of the evaporites influences the groundwater composition within the landslide, this instrumentation may also help to understand internal fluid-rock processes.

With an inter-electrode spacing of one meter and a daily sounding, the multi-electrode system is designed to provide near-field and high-resolution measurements of the electrical resistivity for depth ranging from 31 to 53 m depth (Fig. 4e). A cement-bentonite grout was injected inside the borehole in order to ensure a good electrical signal and to avoid the inter-connection between the different aquifers. The data are recorded using quadripoles in dipole-dipole configuration in order to obtain a 1D profile sensitive to resistivity contrasts along the borehole [START_REF] Dahlin | A numerical comparison of 2D resistivity imaging with 10 electrode arrays[END_REF]. The accuracy of the apparent resistivity measurements is about 2 %. The multi-electrode system is coupled with 8 optical fiber strain sensors (Kloé®), working on the Fiber Bragg Gratings principle, and distributed in-line between 37 m to 58 m depth every each 2 m (Fig. 2e). The strain sensors provide an estimation of the structural deformation at depth by measuring elongations with a sensitivity of 1.5 pm/μm. Due to the weakness of the acquisition system, we were able to collect the data only 3 times per year from March 2012 to September 2014.

Results

In-situ hydro-geochemical monitoring

All the hydro-chemical parameters measured monthly during the last five years, in the field or in the laboratory, were integrated into the public database (https://data.oreme.org/observation/omiv, Table 1).

Periodic measurements of both pore-water pressures and temperatures reveal clear seasonal variations (Fig. 5) coinciding with groundwater discharge-recharge dynamics. We observe a reduction of about 1 bar of the pore-water pressures during the summer dry season and the low-water period occurs at the beginning of autumn. Autumnal and winter rainfalls induce an increase of the pore-water pressures that reflect the recharge of the different hydrogeological units. The same general trend is observed for the four monitoring levels displaying a similar temporal behavior of the two hydrogeological units. We also notice low-temperature periods marked by a water temperature reduction of less than 1°C during autumn and winter periods.

Geochemical analyses performed on the pore-water fluid samples point out that the transition between the two reservoirs R1 and R2 is located between 45 and 48 m depth ( and Mg 2+ (~ 200 mg/l) and SO4 2-(~ 1000 mg/l). This composition argues for an evaporitic origin of the waters (Fig. 6), probably related to dissolution processes that may take place within the main evaporate-claystone unit (U1). R2 shows chemically intermediate waters, between a carbonate and an evaporite pole (Fig. 6), which demonstrates a major contribution of less mineralized waters that might come from rainwater directly infiltrated in the landslide (Fig 2b).

The chemical monitoring shows only slight seasonal variations of the water chemistry in the landslide (Fig. 5). We notice that the mean Total Organic Carbon (TOC) value is ~1.6 mg/l in average for all the levels, except some singular higher values observed mainly at 56 m depths (Fig. 5). The main anomaly (12/2015), characterized by an increase of 3 mg/l of the TOC values, is consistent in time with a decrease of calcium (~350 mg/l), magnesium (~190 mg/l) and sulfate (~1600 mg/l) concentrations. They also coincide with a reduction of the electrical conductivity (~ 1.0 mS/cm) and an increase of HCO3 concentration (~100 mg/l) at the same depth. These chemistry variations suggest that heavy rainfall events induced occasional dilution effects related to rainwater infiltrations in R1 [START_REF] Batiot | Carbone organique total (COT) et magnésium (Mg2+) : deux traceurs complémentaires du temps de séjour dans l'aquifère Karstique[END_REF].

In-situ geophysical monitoring

During the first year of the strain monitoring, we observe an increasing strain (~ 400 pm) for all the sensors along the borehole (Fig. 7a). This first pattern may be explained by the setting of the cement-bentonite grout, which is responsible for a continuous deformation over time for the whole borehole. From March 2013, the strain soundings show only slight changes at depth, except at around 45 m depth. This zone is marked by a strong negative strain anomaly (~ 1100 pm) that becomes more important over time (-700 pm in 18 months). At this depth, the temporal evolution of the strain values underlines the presence of an active slip horizon in agreement with the core description that pointed out the presence of clayey-evaporitic gouge materials probably associated to shear mechanisms.

Moreover, the presence of preferential water flow at the same depth (Fig. 4d) may explain the localization of the deformation because of a coupled high-water pressure and low-frictional strength effect [START_REF] Tsao | A case study of the pore water pressure fluctuation on the slip surface using horizontal borehole works on drainage well[END_REF][START_REF] Bogaard | Landslide hydrology: from hydrology to pore pressure[END_REF].

We also consider apparent resistivity changes relatively to a reference baseline [START_REF] Samouëlian | Electrical resistivity survey in soil science[END_REF] (Fig. 7b). We aim to remove the lithological signature, which is assumed to be constant over the monitoring sequence, and focus on the water contribution, both water content, and concentration.

Because of low temperature variations (Fig. 5), we assume that thermal effects are negligible comparing to the other factors mentioned previously. The reference baseline (Ro) we used is an average of apparent resistivity measurements made in June-July 2015 which corresponds to a representative dry season (only 37 mm of precipitations in two months). The resistivity profile recorded at time t (Rt) was then subtracted from the background geological model (Ro) to compute a 2D (depth and time) interpolated distribution of the apparent resistivity differences (Rt-Ro).

The monitoring data (ftp://omivelecftp.oreme.org, please ask the authors for access) indicate, that the electrical resistivity is not constant over time at different depth (Fig. 7b). From 43 m to 47 m depth, the data show important resistivity variations that match with the active deformation horizon identified in the time-lapse strain soundings. This level is marked by a major transition from A first order analysis of the geophysical monitoring shows no systematic correlation between resistivity anomalies and rainfall events (Fig. 7c). In contrast, a comparison with the water-pore pressure monitoring points out that the temporal resistivity variations at 47 m depth match the seasonal discharge-recharge dynamic of the landslide. The same trend is observed at 50 m depth. In fact, the resistive anomalies are correlated with the low-water periods and the more conductive ones to the recharge periods with rainwater infiltrations at depth that induces internal high-pressure values as well as an increase of evaporite leaching.

Example of "Cevenol event"

September 2015 was marked by a heavy rainfall event (~ 300 mm in 24 h), followed by two important rainfalls occurring at the end of October (~100 mm) and at the beginning of November ( 110mm) (Fig. 8a). Some smaller pre-event rainfalls were also recorded in July and August (~ 50-70 mm).

The hydro-geochemical observatory shows a two-step response of the system to the large rainfall event. First, the hydrological monitoring points out that the intense precipitations lead to an increase of 1 to 1.5 bar of the water-pore pressure in less than one month at the four levels in WB (Fig. 8a). This reveals an immediate hydrodynamic response of the system to the meteorological pulse and involves a pressure transfer at the landslide scale. In contrast, a reduction of both the pore-fluid conductivity (from 3.3 mS/cm in average to 2.0 mS/cm) and the sulphate concentration (from 2436 mg/l in average to 817 mg/l) correlated with an increase of the TOC value up to 4.3 mg/l are observed 3 months after the heavy rainfall event and were visible only at 56 m depth. These changes in the water chemistry of R1 display a dilution of the sulfate-saturated waters by rainfall supplies. The mixing of the deeper groundwater with less mineralized waters is confirmed by the chemistry of the related fluid sample (16.12.2015) close to the carbonate pole in the binary diagram (Fig. 6). This underlines that the mass transfer to the deeper part of the slope does not match the pressure transfer.

Comparing the hydrodynamic and the geophysical monitoring, we notice that the abrupt porepressure variation entailed by the heavy rainfall event is consistent over time with slight reductions of the resistivity difference values (decrease of ~10 Ω.m) at around 47 m depth (Fig. 8b-c) 10 days after the heavy rainfall event. Resistivity changes (from ~20 Ω.m to -10 Ω.m) coinciding with this meteorological event are also observed at around 50-52 m depth 5 days after the event. Nothing is detected at 34-37 m depths. Unfortunately, the lack of resistivity data in December 2015 does not allow a comparison between changes observed in the pore-water chemical parameters (EC, sulphate concentration and TOC) with the resistivity data. Nevertheless, the five years of resistivity monitoring does not show any changes related to pore-water chemical variations for the other large rain events (Fig. 5). Assuming the porosity and the ground structure constant with time, this indicates that the electrical resistivity is more sensitive to the water saturation than to the water ionic concentration.

Discussion

The geophysical and hydro-geochemical monitoring data have demonstrated the presence of two major active sliding surfaces in the landslide body. We identified a narrow failure zone at around 45 m depth corresponding to both a thin evaporite rich level and a preferential water pathway, which gives rise to shear deformation localized around this horizon (Fig. 4 and7). A second massive evaporite-clayey level (U1) is detected from 52 m depth down to the maximum depth of the borehole.

The strain monitoring does not show any deformation related to this deep active zone whereas resistivity variations over time indicate the occurrence of some internal landslide processes at this depth (Fig. 7). Based on these observations, we propose that the upper evaporite level (EL) corresponds to the currently active sliding surface in the landslide that may accommodate most of the slow moving recorded at the surface.

Both the geophysical and the hydro-geochemical observatories point out the seasonal behavior of the unstable slope with clear hydro-geophysical signatures related to discharge-recharge periods (Fig. 5 and7). Furthermore, the monitoring data suggest a rapid transfer of pressure at the landslide scale after a large rainfall event and a slow mass transfer down to the deeper part of the system (Fig. 8). Two different mechanisms or a combination of both can be evocated to explain this two-time response of the hydrological system to rainfall solicitations. First, the delay between the pressure response and the incoming of less mineralized waters at depth may be related to a piston flow mechanism, which requires the contribution of post-event precipitations [START_REF] De Montety | Identifying the origin of groundwater and flow processes in complex landslides affecting black marls: insights from a hydrochemical survey[END_REF][START_REF] Brönnimann | Effect of groundwater on landslide triggering[END_REF]. The large rainfall event may push out the stored groundwater, while the following rainfalls may supply meteoritic waters in the landslide body down to the deeper reservoir (Fig 2b). The other possible explanation for this two-step response may originate from the upper evaporite level (EL), which constitutes a discontinuity at the slope scale. In the vicinity of this slip zone, the presence of open discontinuities, brecciated zones, and clayey gouges (Fig. 4) demonstrates that both the mechanical and hydraulic properties are modified. In particular, this failure zone seems to act as a major drain for groundwater flow (Fig. 2b) that probably implies reduced direct vertical rainfall infiltrations in R1. In this case, less mineralized waters at depth may be mostly controlled by slower lateral supplies from the surrounding slope (Fig. 2b).

Even if the results of the coupled geophysical and hydro-geochemical monitoring are promising, a number of limitations are outlined and should be taken into account for the next stage of instrumentation. While the downhole resistivity observatory provides automatic and higher frequency sampling rate over time, the monthly sampling of the hydro-geochemical observatory is not sufficient to determine precisely the hydrological timescale response of the system. In particular, we are not able to discriminate accurately the contribution of the post-event or pre-event rainfalls. Maybe short-term responses to the meteorological solicitations were missed. Moreover, the sampling step does also not provide a sufficient basis for taking into consideration a cumulative effect of rainfalls in the slope dynamic and for investigating threshold effects. In particular, we need to increase the time step of the fluid samplings in order to better document both short and long-term chemical responses of the different hydrogeological units to large rainfall events. Considering the objectives of the study, the temporal resolution of the resistivity monitoring is large enough to report the general evolution of the water content. A comparison with the hydro-chemical monitoring shows that the time-lapse resistivity sounding shows poor sensitivity to water chemistry variations linked to internal processes. A reduction of the electrode spacing to improve the vertical resolution, in particular in the vicinity of the two active zones, can allow verifying if this lack of sensitivity is associated to the array configuration or not.

Further work on the baseline determination may improve the monitoring of the landslide processes [START_REF] Pezard | On baseline determination and gas saturation derivation from downhole electrical monitoring of shallow biogenic gas production[END_REF]. A more detailed statistical analysis of the electrical resistivity data could also contribute to detect changes in apparent resistivity, in particular high frequency variations (day, week), and their possible links to rainfall and groundwater processes [START_REF] Lebourg | Landslides triggered factors analysed by time lapse electrical survey and multidimensional statistical approach[END_REF][START_REF] Gance | Permanent electrical resistivity measurements for monitoring water circulation in clayey landslides[END_REF]. Improvement in more frequent strain acquisition is also needed to investigate the effect over time of both rainwater infiltration and groundwater flows on deformation. Finally, the downhole monitoring produces accurate near-field vertical data but this instrumentation does not enlighten about lateral variations within the slope. In order to investigate the spatial complexity of both the landslide structure and dynamics, additional data are required. The lateral variations along the slope may be assessed by drilling additional boreholes along the slope [START_REF] De Montety | Identifying the origin of groundwater and flow processes in complex landslides affecting black marls: insights from a hydrochemical survey[END_REF] or deploying electrical resistivity tomography monitoring system at the surface [START_REF] Gance | Permanent electrical resistivity measurements for monitoring water circulation in clayey landslides[END_REF][START_REF] Palis | Multiyear time-lapse ERT to study short-and long-term landslide hydrological dynamics[END_REF]. In particular, borehole resistivity data may be used to constrain the inversion of ERT models [START_REF] Fortier | Internal structure and conditions of permafrostmounds at Umiujaq in Nunavik, Canada, inferred from field investigation and electrical resistivity tomography[END_REF]. A joint interpretation of surface and borehole data may improve our knowledge regarding both the landslide structure and processes (Linde and Doetch, 2016). In our case, the spatial and temporal monitoring may provide arguments upon the distribution of preferential flows, in particular at the upper slip horizon, and slow matrix flows down to the deeper unit.

Conclusion

In this study, we proposed to test a new integrated monitoring strategy in order to characterize the landslide hydrology and the related slope movement of the Pégairolles-de-l'Escalette (PEL) site in response to heavy rainfall events. The new instrumentation consists of two permanent downhole observatories for both geophysical and hydro-geochemical investigation of the slope. This instrumentation provides original near-field data with direct access to the failure zones in the underground that gives an opportunity to investigate the relationships between rainfall events, groundwater processes and deformation at depth. The monitoring data of the PEL, well constrained by the core description and conventional borehole logs, help to characterize both the structure and the dynamic of this unstable slope. Two main active zones were identified in the landslide body. Our data point out that the upper failure zone plays a key role in both the hydrology and the dynamic of the landslide. In particular, our results suggest that this horizon is important in regulating the water infiltration towards the deeper part of the slope. Nevertheless, the relationships between major rainfall events, meteoric water supplies and groundwater flows are complex. In particular, a better knowledge of the timing in response to meteorological solicitations requires an improvement of the sampling step of both the hydro-geochemical and strain monitoring. This knowledge is a key point for performing suitable short and long-term landslide modelling.

  of the 30 descending Envisat SAR archives acquired from 2003 to 2011 indicated a current slow surface displacement of 3-4 mm/year that is mainly concentrated on the upper part of the landslide (Fig 1c).

  Pégairolles de l'Escalette valley is incised by the Lergue River through the tabular Middle to Lower Jurassic carbonate series of the southern border of the Larzac plateau forming a typical Steephead karstic valley (Fig.2a). Northward of the village, the valley shows an overall V-shaped morphology with a ruled slope of about 40° along the thick suboutcropping Hettangian crypalgal dolomudstones. The border of the plateau is dominated by two major structural rock cliffs, respectively at the top of the Plienbachian massive sandy grainstones and the Aalenian to Bathonian massive mudstones and dolomites. These cliffs originate in the rheological contrast with the Toarcian black marls sandwiched between the two massive carbonate slabs that suffer a differential erosion and rock fall retreat. Southward the village, the valley enlarges abruptly with a wide U-shape morphology in the lower part and a remaining ruled slope which progressively fades downstream. This major change of the landform is clearly associated to the intersection point of the main river thalweg with the Norian claystones and evaporites as evidence during geological mapping (Fig.2a-b). Downstream of this point the lower part of the valley is crowded by a thick scree mantle, partly onlapping fringing rock scarp established on the wavy mudstones of the Parlatges facies (Base of Hettangian) and the underlying Rhaetian dolomitized grainstones and mixed siliciclastic-carbonate deposits. The scree slope mantle is partly cemented at the base, indicating a major drainage pathway for the seepage of meteoric waters above the clayey Lower Rhaetian-Norian permeability barrier. It shows multiple flat terraces and solution collapse depressions compatible with the vicinity of evaporite layers. This area is punctuated by large isolated blocks (> 1 m 3 ) and slide slabs (>10 m²) dominated by Rhaetian and Parlatges facies. The flat to low inclined surfaces are still used for vineyard growing with typical

  4, U1 label) at around 53 m depth down to the bottom of the borehole. It is characterized by low values of total gamma ray well correlated with high resistivity values. The observed high-frequency negatively correlated variations in GR and resistivity logs show the alternating evaporite-claystone interlayers. The unit U1 is overlaid by a Rhaetien dolomudstone-marls level (Fig. 4, U2 label), at around 42-52 m depths, marked by intermediate values of gamma ray and low resistivity values. In this level, the core description points out a narrow evaporite rich level (EL), at around 45 m depth, which coincides with a high concentration of open discontinuities, brecciated zones and clayey gouge that underline the presence of both water circulations and deformations. This EL is less visible in the geophysical logs, but correlates with clear changes in fluid conductivity and temperature (Fig. 4d), confirming the existence of a major drainage pathway at this depth. Above 42 m depth, high gamma ray values combined with intermediate resistivity ones underline a slight different mudstone-marls unit U3.

  December 2012 to March 2013 between high (~40 Ω.m) and low (~-10 Ω.m) apparent resistivity differences that may be explained by the cement setting and the related thermal effects. After March 2013, the resistivity monitoring points out transient resistivity increases ranging from -10 Ω.m to 20 Ω.m in average. These changes are observed in October-November 2013, mid-July-September 2014 and end-August-September 2015. Few resistive anomalies (~25 Ω.m) are also observed at around 50 m depth, which coincides with the top of the massive evaporite-claystone unit (U1). These anomalies occurring at the same periods, the monitoring emphasizes similar temporal resistivity behavior for these two levels (44-47 m and 50 m). Some slight resistivity variations that may be related to water flows through open discontinuities in U3 (Fig 4) are also observed at 35-37 m depth.

Figure captures Figure 1 .

 captures1 Figure capturesFigure 1. (a) Photo of one bridge damaged by the Lodeve landslide since 1996; (b) Location of the Lodeve site (GeoMappApp); (c) Aerial photography (Google Earth) of the experimental site with landslide limits (red line), mean LOS velocity (PS-InSAR) displacement from 30 Envisat images in the descending pass and the field set-up: the downhole hydrodynamic observatory based on a multipacker completion from the WestBay system (WB, white star) and the imaGeau downhole electrical resistivity observatory (DEO, blue star). White rectangles show a damaged (A) bridge and a destroyed (B) one. The distance between WB and DEO observatories is about 4 m. (d) View of the Lodeve landslide.

Figure 2 .

 2 Figure 2. Geological and geomorphological setting of the Pégairolles de l'Escalette landslide. Detailed geological and geomorphological map (A) and cross-section (B) (Personal communication LOPEZ M.). Main lithostratigraphy: (a) Ladinian sandstones; (b) Carnian claystones and evaporites; an1: Main anhydrite key bed; (c) Norian lower claystones and evaporites an2: Secondary anhydrite key bed; (d) Norian upper claystones and evaporites; (e) Rhaetian marls and dolomudstone interbeddings; (f) Rhaetian mixed siliciclastic-carbonate; (g) Rhaetian dolomitized grainstone; (h) Basal Hettangian wavy mudstone (Parlatges key bed facies); (i) Hettangian cryptalgal dolomudstones; (j) Sinemurian-Plienbachian sandy grainstones and wackestones. White arrows show possible water pathways related to rainwater infiltrations in the landslide.

Figure 3 .

 3 Figure 3. Core sample of the Norian upper claystone and evaporite interval (Fig. 2b) showing the nodular to chicken-wire fabric of partly rehydrated anhydrite into gypsum (G-An) and gypsyferous wavy layers (G) alternating with blue claystones (C). This core sample was recovered at 60 m depth of an initial exploration hole close to the WB observatory.

Figure 4 .

 4 Figure 4. Downhole structure and physical properties in DEO hole (Fig. 1c). At left: Core photos with lithostratigraphic log (modified from Giovannageli, 2014). At right: (a) Unit labels from the core description; (b) Total Gamma Ray log; (c) Deep and shallow resistivity logs recorded in the cored borehole; (d) Pore fluid conductivity and temperature logs recorded in the same hole; (e) Positions of the four monitoring levels in the WB hydrodynamic observatory (blue ovals), positions of the electrodes (ER, in green circles) and locations of the optical fiber sensors (OF, in violet circles) coupled to the DEO observatory. R1 and R2 labels indicate the assumed location of two reservoirs. The water table was about 34 m depth during logging.

Figure 5 .

 5 Figure 5. In situ monitoring (April 2012 -March 2017) of hydrochemical parameters recorded in the downhole hydrodynamic observatory (WB in Fig.1c, Westbay system) at 36, 45, 48 and 56 m depths in comparison with the record of rainfalls (Les Plans station). Vertical grey bars show summer seasons (from June 1 to August 31 of each year). Dashed lines indicate the noticeable changes of hydrochemical parameters after the "Cevenol event". R1 and R2 labels indicate two reservoirs.

Figure 6 .

 6 Figure 6. Scatter diagram of some geochemical parameters for water samples collected during the observation period at depth of 36, 45, 48 and 56 m compared with the evaporate, carbonate facies, and rainwater pole composition estimated from Ladouche et al. (1999).

Figure 7 .

 7 Figure 7. Time-lapse geophysical monitoring (April 2012 -March 2017) in DEO hole (Fig. 1c): (a) Strain survey from March 2012 to September 2014; the different curves represent the different soundings; (b) Resistivity differences with respect to the baseline (average value from June-July 2015 data). The results are given in apparent resistivity. The blue colour indicates a decrease in the resistivity value whereas an increase of the resistivity is represented with red colours. Green dots show electrode position in DEO hole. Velvet dots show strain measurement points over time. (c) Porepressure monitoring from the WB observatory at 45 and 48 m depths in correlation with rainfall (Les Plans station) and apparent resistivity at 47 and 50 m depth. The vertical grey bars in (b) and (c) images show summer seasons (from June 1 to August 31 of each year).

Figure 8 .

 8 Figure 8. Example of the "Cevenol event" recorded in September 2015 (~ 300 mm of rain in 24 hours): (a) The hydrochemical parameters from the water samples collected at 4 depths in the WB observatory are given; (b) The apparent resistivity difference recorded from the DEO observatory (Rt-Rt0) is represented; (c) Pore-pressure monitoring from the WB observatory at 45 and 48 m depths in correlation with rainfall (Les Plans station) and apparent resistivity at 47 and 50 m depth.

Table 1 ,

 1 Fig.5). The physical and chemical parameters measured for the different samples (see 3.1.2 and Table1) point out that the upper reservoir R2 (36 and 45 m) is characterized by both lower mean electrical conductivity values (~1.8 mS/cm) and lower mean total dissolved solids concentrations (~1800 mg/l) than the deeper one R1 (48 and 56 m) described by much more conductive and mineralized waters (EC ~ 3.2 mS/cm, TDS ~3430 mg/l). Moreover, the relative abundance of ions in R1 in comparison with R2 reveals that the deeper reservoir corresponds to sulphate-rich waters with higher proportions of Ca 2+

Table 1 .

 1 Physico-chemical parameters from the fluid samples collected monthly from 2012 to 2017 in the WB observatory at the four monitoring levels. Number of samples collected per level: 7 samples at 36 m depth, 44 samples at 45 m, 44 samples at 48 m, and 43 samples at 56 m depth.

	Tables
	Depth		Cond.	pH	HCO3 -	Cl -NO3 -SO4 --	Ca ++	Mg ++	Na +	K +	TOC	TDS
	m		mS/cm	-		Majeur cations and ions concentrations (mg/l)		mg/l	mg/l
		Min	1.6	6.7	204	10.0 1.7	1077	329	89	5.5	4.1	0.8	1748
		Max	1.9	7.8	226	14.3 3.0	1224	392	100	7.5	6.3	2.6	1942
	36 m	Mean	1.8	7.2	214	11.6 2.1	1119	346	94	6.2	5.5	1.7	1806
		Median	1.7	7.2	213	11.2 1.9	1095	335	93	5.9	5.7	1.9	1761
		Standard deviation	0.1	0.4	8.4	1.5	0.6	57	24	3.6	0.8	0.8	0.7	83
		Min	1.6	6.8	165	4.1	1.5	1021	288	78	3.2	2.1	0.6	1665
		Max	2.4	8.0	239	20.3 6.0	1563	483	145	20.4 16.3	2.9	2353
	45 m	Mean	1.9	7.5	188	9.7	2.7	1161	350	114	6.8	5.8	1.5	1813
		Median	1.9	7.4	184	9.6	2.4	1119	332	110	6.4	5.0	1.4	1784
		Standard deviation	0.2	0.3	18.5	2.3	1.1	139	52	13	2.3	2.5	0.7	329
		Min	2.8	6.9	136	4.8	-	1769	437	196	5.8	1.9	0.5	2674
		Max	3.4	8.0	202	17.2	-	2509	580	329	18.0 12.9	3.9	3583
	48 m	Mean	3.1	7.5	160	10.7	-	2314	512	293	12.0 5.5	1.6	3128
		Median	3.1	7.5	153	10.3	-	2330	512	297	11.8 4.9	1.3	3309
		Standard deviation	0.2	0.3	19	2.1	-	125	31	25	1.6	2.1	0.8	748
		Min	2.1	6.4	163	5.3	-	817	188	109	6.5	4.4	0.4	1414
		Max	3.6	8.0	264	22.3	-	2730	579	368	27.0 20.9	4.3	3853
	56 m	Mean	3.3	7.4	193	12.1	-	2436	518	322	19.8 12.1	1.7	3500
		Median	3.4	7.3	183	12.0	-	2470	523	329	20.0 12.0	1.5	3551
		Standard deviation	0.3	0.4	27	2.6	-	274	59	44	3.7	2.7	0.9	382

Acknowledgments

This work has been supported by the OREME Observatory, (Observatoire de recherche méditerranéen de l'environnement), the OMIV observatory (Observatoire des Mouvements et Instabilités de Versants), INSU ALEA projects (2014, 2015) and ANR HYDROLSIDE (2016HYDROLSIDE ( -2019)).

The authors would like to thank A. Busiau-Remes, J.-L. Seidel and S. Van Exter for their help in geochemical analyses, and M. Peyret for the PS-InSAR analyses. The authors acknowledge the partners: ImaGeau, Kloé and Schlumberger Services. The authors also thank J. Paris and L. Baqué (ImaGeau) for substantial help in the field with data acquisition and J. Fabre for the database. We also thank the editor, J. Wasowski, the managing guest editor, M. Jaboyedoff for advice, and the two anonymous reviewers who improved the manuscript through constructive comments.


Séverine Ledoux
email: severine.ledoux@lmr.aphp.fr

Ouidad Sami

Daniela Calabrese

Maude Le Gall

Martin Flamant

Muriel Coupaye

Gastric bypass specifically impairs liver parameters as compared to sleeve gastrectomy, independently of evolution of metabolic disorders

Keywords: obesity, bariatric surgery liver, NAFLD

Background: Numerous studies have shown that Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG) differently affect metabolic disorders associated with obesity. While bariatric surgery has been shown to improve non-alcoholic fatty liver disease, very few studies have compared liver parameters after both procedures.

Objectives: To compare the evolution of liver parameters after SG and RYGB and their relationships with improvement of metabolic disorders.

Methods: Metabolic parameters and abdominal ultra-sonography (US) were recorded before and 1 year after bariatric surgery, in all patients who underwent SG or RYGB between 2004 and 2016 in our institution.

Setting:

Introduction

Bariatric surgery is currently the more efficient technique to treat severe obesity. The two surgical procedures most commonly performed in the world are sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB), because they induce remission or amelioration of obesityrelated comorbidities in a majority of cases with an acceptable rate of complications. Notably, numerous studies have shown that these procedures improve non-alcoholic fatty liver diseases (NAFLD) whose prevalence is particularly high in obese patients and thus in candidates for bariatric surgery, reaching 86% on per-operative liver biopsies [START_REF] Caiazzo | Roux-en-Y gastric bypass versus adjustable gastric banding to reduce nonalcoholic fatty liver disease: a 5-year controlled longitudinal study[END_REF].

It is generally admitted that RYGB is more efficient than SG on improving metabolic disorders associated with obesity, because of specific mechanisms beside weight loss related to proximal gut exclusion [START_REF] Schauer | Bariatric Surgery versus Intensive Medical Therapy for Diabetes -5-Year Outcomes[END_REF] . This was notably shown for glucose disturbances, but a specific effect of RYGB on cholesterol level or hypertension was also found in several studies [START_REF] Salminen | Effect of Laparoscopic Sleeve Gastrectomy vs Laparoscopic Roux-en-Y Gastric Bypass on Weight Loss at 5 Years Among Patients With Morbid Obesity: The SLEEVEPASS Randomized Clinical Trial[END_REF] .

Thus, a better effect on NAFLD is expected after RYGB than after SG. However, some cases of severe liver alterations have been reported after RYGB associated with malnutrition [START_REF] Mahawar | Liver Dysfunction with Both Roux-en-Y and One-Anastomosis Gastric Bypass Is Almost Exclusively Seen with Longer Than Standard Limb Lengths[END_REF] .

Very few studies have compared the effects of RYGB and SG on liver parameters (using blood tests, imaging or liver biopsies), some showing a better improvement after SG [START_REF] Billeter | Combined Non-alcoholic Fatty Liver Disease and Type 2 Diabetes Mellitus: Sleeve Gastrectomy or Gastric Bypass?-a Controlled Matched Pair Study of 34 Patients[END_REF][START_REF] Spivak | Omega-loop gastric bypass is more effective for weight loss but negatively impacts liver enzymes: a registry-based comprehensive first-year analysis. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF][START_REF] Kalinowski | Liver Function in Patients With Nonalcoholic Fatty Liver Disease Randomized to Roux-en-Y Gastric Bypass Versus Sleeve Gastrectomy: A Secondary Analysis of a Randomized Clinical Trial[END_REF] , one showing a better improvement after RYGB [START_REF] Nickel | Bariatric Surgery as an Efficient Treatment for Non-Alcoholic Fatty Liver Disease in a Prospective Study with 1-Year Follow-up : BariScan Study[END_REF] , while others [START_REF] Raj | The effect of surgically induced weight loss on nonalcoholic fatty liver disease in morbidly obese Indians: "NASHOST" prospective observational trial. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF][START_REF] Froylich | Effect of Roux-en-Y gastric bypass and sleeve gastrectomy on nonalcoholic fatty liver disease: a comparative study. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF][START_REF] Luo | How bariatric surgery affects liver volume and fat density in NAFLD patients[END_REF] being not conclusive on the superiority of one of these procedures on NAFLD evolution after surgery. Moreover, all were short term studies from 6 to 18 months after surgery. Finally, it was not tested whether the evolution of liver tests after both procedures was only related to the evolution of metabolic parameters or whether other mechanisms of liver alterations could be implicated, including alterations of nutritional parameters.

The aim of our study was thus to compare the evolution of liver parameters after SG and RYGB and their relationships with improvement of metabolic disorders.

Subjects and methods

Patients and surgical procedures

This study is a retrospective analysis of our prospective database of patients who underwent bariatric surgery since 2004. All subjects who underwent either SG or RYGB between 2004 and 2016 with available metabolic explorations both before and 1 year (± 3 months) after surgery were included. The data available more than 3 years after surgery were also recorded in a subgroup of patients. The exclusion criteria for this study were pregnancy at the time of the visits, alcohol abuse or know liver disease of origin other than NAFLD. Bariatric surgery was performed in accordance with the recommendations of international committees and consensus conferences [START_REF] Fried | Interdisciplinary European Guidelines on Metabolic and Bariatric Surgery[END_REF] . All procedures were performed laparoscopically, as previously described [START_REF] Coupaye | Comparison of Nutritional Status During the First Year After Sleeve Gastrectomy and Roux-en-Y Gastric Bypass[END_REF] with a 150 cm alimentary limb and a 60 cm biliopancreatic limb for RYGB.

Intraoperative liver biopsies were systematically planned from 2013 and scored according to the classification of Bedossa [START_REF] Bedossa | Current histological classification of NAFLD: strength and limitations[END_REF] . Pre-and postoperative multidisciplinary management in our institution were previously described elsewhere [START_REF] Ledoux | Relevance of Self-reported Behavioral Changes Before Bariatric Surgery to Predict Success After Surgery[END_REF] . All investigations were performed with a medical care goal. Informed consent was obtained in all patients before surgery and the data collection was approved by our institution and the local ethic committee.

Clinical and biological assessments

All patients underwent a routine examination, an abdominal ultra-sonography (US) and systematic fasting biological analyses in a day-hospitalization, before and one year after surgery and then every 3 years in average after surgery. Clinical, biological parameters and US liver abnormalities were prospectively assessed as previously described [START_REF] Coupaye | Comparison of the incidence of cholelithiasis after sleeve gastrectomy and Roux-en-Y gastric bypass in obese patients: a prospective study. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF] .

Statistical analysis

Statistical analyses were performed with SigmaStat 3.5 software. Quantitative parameters were compared in univariate analysis using unpaired Student's t-test or non-parametric tests when the distribution was not normal. Qualitative parameters were compared using the Pearson's Chi square test or the fisher exact test, when appropriate. Patients with missing data were excluded only from the analyses of the missing parameter. Correlations between the deltas of pre-and postoperative parameters (value after surgery minus value before surgery) were analyzed by Spearman correlation. Decreased alanine aminotransferase (ALT) was arbitrary defined by a delta > 5IU/L whereas increased ALT was defined by a delta > 5IU/L, in order to exclude unspecific changes, taking into account the intra-assay and inter-assay variations that are respectively below 2 and 5 IU/L based on the indications of the manufacturer (Dimension® Siemens Healthineers). ALT was considered abnormal if > 34 IU/L for women and >45 IU/L for men [START_REF] Spivak | Omega-loop gastric bypass is more effective for weight loss but negatively impacts liver enzymes: a registry-based comprehensive first-year analysis. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF] . Results are expressed as mean ± SD or percent when indicated.

Results

Clinical characteristics

Among the 554 subjects evaluated before and one year after surgery, 21 were excluded because of pregnancy at the moment of explorations. None had known liver disease other than NAFLD and none had alcohol abuse. Only 6 subjects reported to drink alcohol on a regular but mild basis before surgery and only 9 after (2 after SG and 7 after RYGB) and their transaminases were not higher in average than in the whole cohort. Thus, 533 subjects were analyzed, including 326 RYGB and 207 SG. Abdominal US was missing in 79 subjects before surgery (15%) and 49 subjects after surgery (8%). Intraoperative liver biopsies were available in 150 RYGB and 80 SG and the percentage of subjects with histologic NASH did not differ between groups (27 vs 24%, respectively). The characteristics of the subjects are indicated in Table 1. Baseline characteristics were similar in RYGB and SG groups. One year after surgery, percent weight loss was significantly higher after RYGB than after SG,with a trend towards lower absolute postoperative weight that did not reach significance. The decreases in caloric and protein intakes were similar, but the subjects ate less lipids after RYGB than after SG. The number of subjects with treatment for obstructive sleep apnea syndrome or diabetes decreased in the same proportion after both procedures. In contrast, the decrease in the number of subjects treated for hypertension or lipids disorders was only significant after RYGB.

Evolution of biological and US parameters

Metabolic parameters were similar before surgery and improved in both groups after surgery (Table 2). However, the decrease in fasting insulin and cholesterol was greater after RYGB than after SG, as for the inflammation marker C-reactive protein. Despite improvement of metabolic parameters, liver tests including alkaline phosphatases and transaminases, decreased less after RYGB than after SG. In contrast, Gamma-GT and ferritin, whose increase is usually associated with metabolic liver disease, were similar or even lower after RYGB (Table 2). In the same line, the number of subjects with persistent US steatosis did not differ (Figure 1A). The liver test alterations after RYGB were not explained by gallbladder disease, the number of subjects with cholelithiasis or who underwent cholecystectomy being similar after both procedures (figure 1A). It cannot be excluded that the lower ferritin level after RYGB was explained by iron malabsorption. However, serum iron concentration increased in the same manner in both groups (Table 1). Again, higher alkaline phosphatases levels after RYGB could be explained by a difference in bone resorption, but parathyroid hormone did not differ one year after both procedures (43.5 ± 21.0 vs. 42.2 ± 25.6 pg/ml after RYGB and SG respectively).

Long-term data

At 3 years or more after surgery (Supplemental table 1), the subjects that underwent SG were significantly heavier than those who underwent RYGB and all metabolic parameters were better improved after RYGB, except for transaminases and alkaline phosphatases that were still higher after RYGB. Again, ferritin and gamma GT were lower and the number of patients with US liver abnormalities was not significantly different after RYGB and SG (Supplemental table 1).

Characteristics of the subjects according to the evolution of transaminases after RYGB

The percentage of subjects with abnormal ALT was significantly higher one year after RYGB than after SG (Figure 1B). Furthermore, not far from a third of subjects increased their ALT one year after RYGB as compared to less than 5 % after SG (Figure 1B). However, even after exclusion of subjects with increased ALT, transaminases were still higher after RYGB (19.8 ± 5.3 vs 17.1 ± 5.6 IU/l for AST and 26.2 ± 8.3 vs 22.1 ± 7.5 IU/l for ALT, p< 0.001). The subjects with increased ALT after RYGB were older but did not differ from those with decreased ALT in term of preoperative BMI, weight loss, and caloric intake. However, they had better liver tests before surgery, less US steatosis, less histologic abnormalities on liver biopsies and less metabolic disorders (Supplemental Table 2). In the whole cohort of RYGB, the decrease in ALT after surgery was positively correlated to weight loss and to improvement of other markers of liver metabolic disease and of insulin resistance (Table 3), whereas in those with increase in ALT, the delta of ALT was inversely correlated to the delta of fasting insulin, total cholesterol, transferrin and albumin.

Discussion

The impact of bariatric surgery on NAFLD has been extensively studied, and in our study, NAFLD assessed by blood liver tests and liver US improved after both procedures in parallel with the improvement of metabolic disorders, as expected [START_REF] Cazzo | Nonalcoholic fatty liver disease and bariatric surgery: a comprehensive review[END_REF]18) . Beside weight loss and decrease in insulin resistance, some mechanisms for NAFLD improvement have been proposed, including modifications in incretins release, adipokines secretion, bile acid metabolism and microbiota, notably for surgical procedures with intestinal derivation [START_REF] Cazzo | Nonalcoholic fatty liver disease and bariatric surgery: a comprehensive review[END_REF] . Thus, it can be expected that RYGB should be more efficient on NAFLD remission than restrictive procedures. In this line, a study with liver biopsies performed in a large cohort of 1236 obese patients, has shown that NAFLD improves better 5 years after RYGB than after gastric banding [START_REF] Caiazzo | Roux-en-Y gastric bypass versus adjustable gastric banding to reduce nonalcoholic fatty liver disease: a 5-year controlled longitudinal study[END_REF] . Unfortunately, no study with such a level of proof is available for comparison with SG. The 2 studies that have compared the improvement of NAFLD assessed by liver biopsies after RYGB and SG, were inconclusive on the superiority of one of these procedures, but they included no more than 30 subjects and the follow-up duration was 6 months [START_REF] Raj | The effect of surgically induced weight loss on nonalcoholic fatty liver disease in morbidly obese Indians: "NASHOST" prospective observational trial. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF][START_REF] Froylich | Effect of Roux-en-Y gastric bypass and sleeve gastrectomy on nonalcoholic fatty liver disease: a comparative study. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF] . One study [START_REF] Nickel | Bariatric Surgery as an Efficient Treatment for Non-Alcoholic Fatty Liver Disease in a Prospective Study with 1-Year Follow-up : BariScan Study[END_REF] reported a better improvement of liver stiffness assessed by elastography one year after RYGB but another study using MRI [START_REF] Luo | How bariatric surgery affects liver volume and fat density in NAFLD patients[END_REF] did not find any difference between RYGB and SG, 6 months after surgery. These results are in accordance with our study showing that remission of US steatosis did not differ between both procedures. However, two previous studies based on blood liver enzymes, one in a small cohort of 34 diabetic subjects [START_REF] Billeter | Combined Non-alcoholic Fatty Liver Disease and Type 2 Diabetes Mellitus: Sleeve Gastrectomy or Gastric Bypass?-a Controlled Matched Pair Study of 34 Patients[END_REF] and one in a large registry cohort [START_REF] Spivak | Omega-loop gastric bypass is more effective for weight loss but negatively impacts liver enzymes: a registry-based comprehensive first-year analysis. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF] , showed a better improvement of liver enzymes, including transaminases and alkaline phosphatases but not gamma-GT, one year after SG as compared to RYGB, as in our study. These results are unlikely to be explained by a more severe NAFLD before surgery in candidates for RYGB, because the 2 groups were comparable for metabolic disorders, BMI, liver blood tests, US steatosis and histologic finding at baseline in our study. Furthermore, the difference in liver enzymes persists at 3 years or more, despite weight regain and more marked metabolic alterations after SG. Finally, in the RYGB group, some subjects increased their ALT, as reported by Spivak et al [START_REF] Spivak | Omega-loop gastric bypass is more effective for weight loss but negatively impacts liver enzymes: a registry-based comprehensive first-year analysis. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF] and this increase was not explained by greater insulin-resistance and was unrelated to other markers of NAFLD, including US steatosis, gamma-GT and ferritin. Altogether, these results do not support the hypothesis previously suggested [START_REF] Billeter | Combined Non-alcoholic Fatty Liver Disease and Type 2 Diabetes Mellitus: Sleeve Gastrectomy or Gastric Bypass?-a Controlled Matched Pair Study of 34 Patients[END_REF] that RYGB induces a poorer improvement of NAFLD than SG, but instead argue for mechanisms independent of NAFLD retrospective nature of the analysis of our prospective database; 3) the absence of histological data for some subjects before surgery which did not allow us to match the patients based on histologic degree of liver involvement at baseline, but ALT levels were significantly lower after SG than after RYGB both in subjects with or without NASH (data not shown) and above all 4) the lack of liver biopsies after surgery, the morphological characteristics being evaluated only by ultrasound after surgery.

Conclusion

RYGB has less beneficial effects on liver enzymes as compared to SG, independently of improvement of metabolic disorders and of NAFLD, both in the short and long terms. These alterations could be linked to subtle malnutrition but other mechanisms need to be explored.

Randomization trials should be conducted to confirm the differential effects of SG and RYGB on liver parameters and should be continued in the long-term to determine the consequences of increased liver enzymes after RYGB. If these results are confirmed by other studies, this could have an impact on the choice of surgical procedure, suggesting that SG is the preferred operation in subjects at risk of developing liver failure.

Acknowledgments: the authors thank Pierre Bedossa for the histological analyses

Figure 1 .

 1 Figure 1. Evolution of liver parameters 1 year after sleeve gastrectomy (SG) and Roux-en-Y

underlying the increase in transaminases after RYGB. This effect was unmasked in subjects with baseline normal transaminases but was also present in subjects with NAFLD since after exclusion of subjects with increased ALT, transaminases were still less decreased after RYGB than after SG.

Malnutrition could be an underlying mechanism. Indeed, some cases of liver failure have been described in a context of malnutrition after RYGB but these extreme cases are rare [START_REF] Mahawar | Liver Dysfunction with Both Roux-en-Y and One-Anastomosis Gastric Bypass Is Almost Exclusively Seen with Longer Than Standard Limb Lengths[END_REF]19) . However, more subtle malnutrition could be involved and, in this line, it was shown that omega-loop gastric bypass, with a longer bypassed biliopancreatic limb and therefore greater malabsorption, induces a larger increase in liver enzymes than RYGB [START_REF] Spivak | Omega-loop gastric bypass is more effective for weight loss but negatively impacts liver enzymes: a registry-based comprehensive first-year analysis. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery[END_REF]20) Unfortunately, the authors did not study the link between transaminases and markers of malnutrition. In our study, we observed that the increase in ALT after RYGB was inversely correlated to cholesterol, albumin and transferrin, could argue for this hypothesis. However, no correlations were found with minerals and vitamins usually assessed after surgery [START_REF] Coupaye | Comparison of Nutritional Status During the First Year After Sleeve Gastrectomy and Roux-en-Y Gastric Bypass[END_REF] (data not shown).

On the other hand, the lower concentrations of cholesterol, albumin and transferrin, all synthetized by the liver, could also reflect alterations of liver functions independently of nutritional status in subjects with the higher transaminases after RYGB. In this line, a previous study has shown that patients with non-alcoholic steatohepatitis undergoing RYGB are more susceptible than those undergoing SG to early transient deterioration of liver functions with increase in International Normalized Ratio (INR) and decrease in albumin concentration [START_REF] Kalinowski | Liver Function in Patients With Nonalcoholic Fatty Liver Disease Randomized to Roux-en-Y Gastric Bypass Versus Sleeve Gastrectomy: A Secondary Analysis of a Randomized Clinical Trial[END_REF] . Thus, we cannot exclude that other mechanisms could influence liver functions after RYGB, for example related to changes in bile acids metabolism or perturbations in gut permeability and gut-liver axis induced by gut derivation.

The main limitations of our study are: 1) the non-randomized design that could induce bias, but the baseline characteristics of candidates for RYGB and SG were very similar and the collection of liver parameters was performed by medical staff unaware of the study; 2) the


Louis Dagneaux
email: louisdagneaux@gmail.com

Stanislas Marouby

Cedric Maillot

François Canovas

Charles Riviere

Dual Mobility Device Reduces The Risk Of Prosthetic Hip Instability For Patients With Degenerated Spine: A Case-Control Study La dégénérescence du complexe lombo-pelvien influence le risqué de luxation de prothèse totale de hanche: étude cas-témoin Running title: Spine Degeneration & THA Dislocation

Keywords: total hip replacement, instability, spine-hip relation, spine degeneration, pelvic incidence, dual mobility cup

Introduction:

The pelvic incidence is an anatomical and biomechanical pelvic parameter determining spine sagittal morphology and kinematics. Stiffening of the lumbo-pelvic complex, a result of degeneration, affects the functional cup positioning, putting prosthetic hip patients at risk of instability. The anti-dislocation dual mobility (DM) device may be clinically advantageous by reducing the risk of prosthetic instability for older patients with spine ageing. Our study aims to answer the following questions: 1) is there a relationship between prosthetic hip instability and the standing cup position, 2) is there a relationships between prosthetic hip instability and the pelvic incidence, 3) is there a relationships between prosthetic hip instability and the severity of the spine degeneration? 4) Is the DM cup device an effective option for reducing the risk of prosthetic instability related to spine degeneration? Hypothesis: There is a relationship between prosthetic hip instability and the standing cup position and pelvic parameters.

Methods: Case-control study on prospectively collected data since 2009. From 1672 conventional total hip replacements (THR -5.4% dislocation rate) and 1056 DM-THRs (1.1% dislocation rate) performed at our institute since 2009, we created three groups: 33 patients with unstable THR (group 1-case), 41 patients with stable THR (group 2-control), and 42 patients with stable DM-THR (group 3-control). The cup orientation was measured on standing pelvic radiographs and the spinopelvic parameters were measured on standing EOS TM biplanar images or lateral full spine radiographs.

Results: By comparing patients from group 1 with those of group 2 we found they had similar cup position (57% versus 51% fitting the safe zone, p=0.58), higher pelvic incidence (58° versus 51°, p=0.01), and more severe spine degeneration (smaller anterior pelvic plane Tilt (2° versus 7° (p=0.002)), a larger pelvic incidence -lumbar lordosis mismatch (17° versus 8° (p=0.005)), and a higher proportion of spino-sacral angle < 127° (70% versus 43%, (p=0.02)). Patients from group 3 had similar cup position, pelvic incidence, and spine degeneration compared to patients from group 1.

Discussion

/Conclusion: Patients with spine-hip relation type 2C/D (high pelvic incidence and severe spine degeneration) have an increased risk of instability that is partly compensated for by the use of a DM device. Preoperative screening of patients with abnormal spine-hip relation would improve THR planning and reduce the risk of prosthetic hip instability. The use of a DM device on spinedegenerated elderly patients is probably sound.

Level of evidence: III,case-control study.

1) Introduction

Instability of modern total hip replacement (THR) represents the first cause of early revision [START_REF] Brooks | Dislocation following total hip replacement: causes and cures[END_REF][START_REF] Bozic | Risk of Complication and Revision Total Hip Arthroplasty Among Medicare Patients with Different Bearing Surfaces[END_REF][START_REF] Bayliss | The effect of patient age at intervention on risk of implant revision after total replacement of the hip or knee: a population-based cohort study[END_REF], but its pathophysiology is yet to be fully understood. To illustrate, several authors found that a large proportion of unstable patients had their acetabular component oriented in the radiographic zone recommended by Lewinnek [START_REF] Abdel | What Safe Zone? The Vast Majority of Dislocated THAs Are Within the Lewinnek Safe Zone for Acetabular Component Position[END_REF][START_REF] Esposito | Cup Position Alone Does Not Predict Risk of Dislocation After Hip Arthroplasty[END_REF][START_REF] Reina | Can a target zone safer than Lewinnek's safe zone be defined to prevent instability of total hip arthroplasties? Casecontrol study of 56 dislocated THA and 93 matched controls[END_REF], thus suggesting the static component orientation alone is of low value in predicting the risk of instability. This is probably related to the fact that the acetabular orientation is functional, with values for orientation dependent on the highly inter-individually variable lumbo-pelvic kinematics [START_REF] Huec | Normative values for the spine shape parameters using 3D standing analysis from a database of 268 asymptomatic Caucasian and Japanese subjects[END_REF][START_REF] Philippot | Pelvic balance in sagittal and Lewinnek reference planes in the standing, supine and sitting positions[END_REF] and spine-hip relation [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Rivière | Spine-hip relations in patients with hip osteoarthritis[END_REF]. Following on, the pelvic incidence (PI) is a highly inter-individually variable anatomical and biomechanical pelvic parameter which determines the spine sagittal morphology and kinematics [START_REF] Chen | Pelvic incidence variation among individuals: functional influence versus genetic determinism[END_REF]; its relationship with prosthetic hip instability remains unclear [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Delsole | Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision?[END_REF]. Spine degeneration, a frequent cause of abnormal spine-hip relations, has also been linked to the risk of prosthetic hip instability [START_REF] Delsole | Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision?[END_REF][START_REF] Nam | The Impact of Total Hip Arthroplasty on Pelvic Motion and Functional Component Position is Highly Variable[END_REF][START_REF] Fessy | What are the risk factors for dislocation in primary total hip arthroplasty? A multicenter case-control study of 128 unstable and 438 stable hips[END_REF]. By tending to stiffen the spine with a pelvis that progressively becomes excessively retroverted in the standing position, the process of spine ageing results in a compensatory increased range of hip motion during daily activities of life [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Esposito | Does Degenerative Lumbar Spine Disease Influence Femoroacetabular Flexion in Patients Undergoing Total Hip Arthroplasty?[END_REF] and a poor standing cup position [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Delsole | Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision?[END_REF][START_REF] Suzuki | Postural and Chronological Change in Pelvic Tilt Five Years After Total Hip Arthroplasty in Patients With Developmental Dysplasia of the Hip: A Three-Dimensional Analysis[END_REF], which both contribute to an increased risk of prosthetic hip instability.

The dual mobility (DM) cup, a recently promoted device, aims to reduce the occurrence of prosthetic hip instability. Compared to traditional cup designs, DM cups have an overall higher tolerance to articular impingement (high mobile liner-neck ratio), a higher jumping distance, and are more constrained [START_REF] Ohmori | Increase in Safe Zone Area of the Acetabular Cup Using Dual Mobility Cups in THA[END_REF][START_REF] Heffernan | Does Dual-mobility Cup Geometry Affect Posterior Horizontal Dislocation Distance?[END_REF]. Its benefit for spine-degenerated patients remains to be proven.

Our study aims to answer the following questions: 1) is there a relationships between prosthetic hip instability and the standing cup position, 2) is there a relationships between prosthetic hip instability and the pelvic incidence, 3) is there a relationships between prosthetic hip instability and the severity of the spine degeneration? 4) Is the DM cup device an effective option for reducing the risk of prosthetic instability related to spine degeneration? We hypothesized here is a relationship between prosthetic hip instability and the standing cup position and pelvic parameters

Methods

Patients

The study flow chart is illustrated in figure 1. We performed a retrospective case-control study on prospectively collected data since January 2009. We consulted our database in April 2017 in order to identify all patients who suffered from prosthetic instability following a primary hip replacement.

Amongst the 1672 conventional THRs and 1056 DM-THRs performed during this time period, 90 (5.4%) and 12 (1.1%) dislocations were recorded, respectively. All patients had EOS TM (Biospace, Paris, France) standing imaging or lateral full spine view preoperatively; THRs were performed through a posterior approach with posterior capsule and external rotator repairs by two senior surgeons (LD, FC); a DM device was used if the patient had a recognised risk factor for instability and/or was greater than 70 years of age. Amongst the 90 unstable THRs, 66 (73%) occurred during the first year, and the dislocation was recurrent in for 62 (69%) of patients. After exclusion of patients with a well-recognized cause for instability (lumbar instrumentation, dementia, traumatic dislocation, abductor mechanism deficiency) and/or incomplete data, we were left with 32 patients who had unexplained instability (group 1 -unstable THR). Two control groups were then generated; one group of 41 patients with a stable THR (group 2) and one group of 41 patients with a stable DM-THR (group 3). Patients from groups 2 and 3 were randomly selected from the cohorts of 1582 stable THRs and 1044 stable DM-THRs, respectively. Demographic data and initial diagnosis for each group are shown in Table 1. This study has been fully approved by our local Ethics Committee: CLER 2017_CLER-MTP_10-10.

Methods

The cup position (inclination and anteversion) was measured 6 weeks postoperatively on standing pelvic radiographs using Centricity software (GE Healthcare, Ill, USA) (figures 2 and 3). The spinopelvic parameters (lumbar lordosis (LL), spino-sacral angle (SSA), sagittal vertical axis (SVA) distance, pelvic incidence (PI), pelvic version (PV), sacral slope (SS), anterior pelvic plane tilt (APP Tilt -positive value meaning anterior tilt)) were measured on 6 months postoperative lateral standing EOS TM views as illustrated in the figure 2. The individual spine degenerative status was defined according to published data, with good recognised reliability: sagittal vertical axis distance >50 mm, spino-sacral angle angle <127°, Pelvic version >0.24*Pelvic incidence, and Pelvic incidence-Lumbar lordosis >15° [START_REF] Savage | Fixed sagittal plane imbalance[END_REF][START_REF] Schwab | Sagittal Plane Considerations and the Pelvis in the Adult Patient[END_REF][START_REF] Schwab | Scoliosis Research Society-Schwab adult spinal deformity classification: a validation study[END_REF].

Statistical analysis

Continuous variables were expressed using means and standard deviation, and were compared using Student t-test or Wilcoxon rank-sum test. Categorical variables were reported as frequencies and proportions, and were compared using Chi-square or Fisher exact tests. The level of evidence (p value) was set at <0.05. Statistical analysis was performed using SPSS for Windows version 20.0 (SPSS, Chicago, IL).

Results

Groups 1 and 2 had similar cup anteversion (14°±7 versus 13°±6, p=0.53), inclination (47°±5 versus 49°±6, p=0.13), and proportion of cup orientation within the recommended safe zone (57% versus 51%, p=0.58). Cup orientations for groups 1 and 2 are shown in figure 3 andtable 2.

Group 1 had a larger Pelvic incidence (58°(±13) versus 51° (±11), p=0.01) and Pelvic version (18°±12 versus 13°±9, p=0.03) compared with group 2. Pelvic parameters for groups 1 and 2 are illustrated in table 3.

Group 1 had a smaller anterior pelvic plane tilt (2° versus 7°, p=0.002), a larger Pelvic incidence-Lumbar lordosis mismatch (17° versus 8°, p=0.005), and a higher proportion of spino-sacral angle < 127° (70% versus 43%, p=0.02). Spino-pelvic parameters for groups 1 and 2 are illustrated in table [START_REF] Abdel | What Safe Zone? The Vast Majority of Dislocated THAs Are Within the Lewinnek Safe Zone for Acetabular Component Position[END_REF].

Cup anteversion (14°±7 versus 13°±5, p=0.37), cup inclination (47°±5 versus 46°±5, p=0.32), proportion of cup orientation within the recommended safe zone (57% versus 54%, p=0.8), pelvic incidence (58°(±13) versus 56° (±12), p=0.49), pelvic version (18°±12 versus 19°±9, p=0.95), and all spino-pelvic parameters were similar between groups 1 and 3. Cup orientation, pelvic and spinopelvic parameters for groups 1 and 3 are illustrated in tables 2, 3, and 4, respectively.

Discussion

Instability represents the first cause of early revision after primary THR. Assessing the influence of the static cup orientation and spino-pelvic parameters on the risk of instability is of importance in gaining a better understanding of the pathophysiology and thus reducing the occurrence. Our main findings are that patients with a spine-hip relation type 2C/D (high Pelvic incidence and severe spine degeneration -Bordeaux classification [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Rivière | Spine-hip relations in patients with hip osteoarthritis[END_REF]) have an increased risk of instability, which the use of a DM device partially compensates for. We were unable to detect a relationship between standing cup position and risk of prosthetic instability. Our finding is in line with those from Abdel et al. [START_REF] Abdel | What Safe Zone? The Vast Majority of Dislocated THAs Are Within the Lewinnek Safe Zone for Acetabular Component Position[END_REF], Esposito et al. [START_REF] Esposito | Cup Position Alone Does Not Predict Risk of Dislocation After Hip Arthroplasty[END_REF], and Reina et al. [START_REF] Schwab | Scoliosis Research Society-Schwab adult spinal deformity classification: a validation study[END_REF]. This is probably because other parameters, such as other acetabular orientations (the functional, the static sitting, etc.) [START_REF] Nam | The Impact of Total Hip Arthroplasty on Pelvic Motion and Functional Component Position is Highly Variable[END_REF][START_REF] Esposito | Does Degenerative Lumbar Spine Disease Influence Femoroacetabular Flexion in Patients Undergoing Total Hip Arthroplasty?[END_REF], combined anteversion [START_REF] Dorr | Combined Anteversion Technique for Total Hip Arthroplasty[END_REF], and the individual spine-hip relation (spine or hips user) [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Rivière | Spine-hip relations in patients with hip osteoarthritis[END_REF], may be more relevant parameters influencing the prosthetic stability.

Similar to Delesole et al. [START_REF] Delsole | Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision?[END_REF], we found unstable THR patients to have a higher Pelvic incidence. This may be because the Pelvic incidence is an anatomical and biomechanical pelvic parameter which determines the sagittal spine morphology and kinematics [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF], thus affecting the risk of prosthetic instability [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Delsole | Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision?[END_REF]. Therefore, the severity of the spine-hip syndrome (determining the risk of prosthetic instability) may be proportional to the value of the Pelvic incidence, as patients switch from being a constitutional "spine user" to an unhabitual new "hip user" [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Rivière | Spine-hip relations in patients with hip osteoarthritis[END_REF]. To illustrate, when flexing the thighs over the trunk (sitting task), patients with normal Pelvic incidence are likely to share this motion between their spine and their hips, defined as being a "spine user". In cases where a stiff degenerated spine has developed, the same task for the same patient would result in a compensatory increased hip flexion (unhabitual "hip user" condition), acting around an abnormally functionally oriented acetabulum [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF]. If the patient has a low Pelvic incidence, they would tend to constitutionally be a "hip user", thus the development of spine degeneration would slightly accentuate their "hip user" condition, moving the patient not far from their constitutional state.

We found that unstable THR patients had more severe spine degeneration compared to stable THR patients. Our findings are in line with those from Delsole et al. [START_REF] Delsole | Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision?[END_REF], Nam et al. [START_REF] Nam | The Impact of Total Hip Arthroplasty on Pelvic Motion and Functional Component Position is Highly Variable[END_REF], and Fessy et al. [START_REF] Fessy | What are the risk factors for dislocation in primary total hip arthroplasty? A multicenter case-control study of 128 unstable and 438 stable hips[END_REF]. This is probably because a degenerated spine is stiffer, with a pelvis excessively posteriorly tilted in the standing position. This is responsible for a poor standing acetabular orientation (abnormally high acetabular anteversion and inclination), potentially compromising the prosthetic stability [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Rivière | Spine-hip relations in patients with hip osteoarthritis[END_REF]. The risk of prosthetic instability is further accentuated by the compensatory increased range of hip motion used for daily activities of life, which results from spine stiffening [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Esposito | Does Degenerative Lumbar Spine Disease Influence Femoroacetabular Flexion in Patients Undergoing Total Hip Arthroplasty?[END_REF]. Figure 4 illustrates an unstable THR patient affected by severe spine degeneration. Interestingly, we found high PI and severe spine degeneration to be associated with the risk of instability; this combination defines patients with spine-hip relation type 2C/D (Bordeaux classification [START_REF] Rivière | The influence of spine-hip relations on total hip replacement: A systematic review[END_REF][START_REF] Rivière | Spine-hip relations in patients with hip osteoarthritis[END_REF]).

We found that stable DM-THR patients (group 3) had similar standing cup position, pelvic incidence, and severity of spine degeneration, but were older. In addition, a sample group of patients had a 5 times lower dislocation rate (1.1% versus 5.4%) compared to unstable THR patients (group 1). This suggests the use of a DM device is probably beneficial in reducing the risk of instability related to spine degeneration. The DM device therefore confirms its status as an anti-dislocation device, which is due to its higher tolerance for articular impingement (high mobile liner-neck ratio), higher jumping distance, and higher constraint, compared to traditional cup designs [START_REF] Ohmori | Increase in Safe Zone Area of the Acetabular Cup Using Dual Mobility Cups in THA[END_REF][START_REF] Heffernan | Does Dual-mobility Cup Geometry Affect Posterior Horizontal Dislocation Distance?[END_REF]. Therefore when faced with older patients, the use of a DM device is probably a sound option to increase the THR stability and make the surgery safer. Before interpreting our results, it is important to acknowledge some limitations in our methods. Firstly, the fact that our control groups were not paired may have generated confounding bias. However, as demographics between groups 1 and 2 were similar, and our results for questions 1, 2, and 3 (comparison groups 1 and 2) are in line with published literature, it is likely that this bias did not significantly affect the quality of our results for these questions. Similarly, the fact that patients from group 3 were older than those from groups 1 and 2, a recognised risk factor for prosthetic hip instability, cannot act as a confounding bias to explain the lower rate of instability of patients with DM-THR compared to those with conventional THR (1.1% versus 5.4%, respectively).

Secondly, the 57 excluded unstable THR patients (identified cause for instability and/or incomplete data) may have generated a selection bias, hindering the quality of all our results. However, by only selecting patients with unexplained instability, we have reduced the risk of confounding bias with regards to causes of instability, which has probably contributed to the identification of a relationship between some spino-pelvic parameters and prosthetic instability. Finally, we found similar a radiographic cup orientation between groups 1 and 2, which may be the result of a lack of statistical power. However, as we found unstable THR patients to have, while not significant, a better cup orientation than stable THR patients, it is very likely that the standing radiographic cup orientation is of poor value in predicting the risk of instability. This finding is in line with the conclusion from previously published reports [START_REF] Abdel | What Safe Zone? The Vast Majority of Dislocated THAs Are Within the Lewinnek Safe Zone for Acetabular Component Position[END_REF][START_REF] Esposito | Cup Position Alone Does Not Predict Risk of Dislocation After Hip Arthroplasty[END_REF].

Conclusion

Patients with spine-hip relation type 2C/D (high Pelvic incidence and severe spine degeneration) have an increased risk of instability that is partly compensated for by the use of a DM device. Preoperative screening of patients with abnormal spine-hip relation would improve THR planning and reduce the risk of prosthetic hip instability. The use of a DM device on spinedegenerated elderly patients is probably sound.

Conflicts of

Figure 1 :Figure 2 :

 12 Figures legends:

Figure 3 :

 3 Figure 3: A scatter plot showing a similar dispersion of the standing cup inclination and anteversion angles between the 33 unstable THR patients (group 1), the 41 stable THR patients (group 2), and the 41 stable DM-THR patients (group 3).

Figure 4 :

 4 Figure 4: Standing EOS TM lateral view of a group 1 patient with significant spine degeneration with a compensated sagittal imbalance. Note the excessive pelvic retroversion (Pelvic Version (PV) = 50°, Sacral Slope (SS) =32°), reduced standing Lumbar Lordosis (LL) (56°), Pelvic Incidence-Lumbar Lordosis mismatch (26°), and excessive anterior projection of the C7 plumb line (Sacral Vertical Axis (SVA) = 97mm) in standing position.

Table 1 :

 1 Comparison of demographic data and origin of hip osteoarthritis between groups. : Significant difference. AVN: Avascular Necrosis of the femoral head, OA: Osteoarthritis. NA: Not

		Group 1	Group 2	Group 3	p (G1 vs	p (G1 vs
					G2)	G3)
	Age	67±11	63±8	77±7	0.10	0.01*
	Female (%)	63%	53%	66%	0.38	0.78
	BMI	27±6	26±3	26±6	0.64	0.37
	ASA Score	2.1±0.7	1.8±0.7	2±0.5	0.07	0.56
	Prosthetic	33±2.4	33±2.4	NA	0.73	NA
	head diameter
	Non-instrumented	18%	7%	7%	0.15	0.15
	spinal surgery
	reasons for THR
	• AVN	9%	2%	5%	0.17	0.38
	• Dysplastic OA	3%	7%	2%	0.41	0.86
	• traumatic OA	6%	10%	2%	0.56	0.4
	• Primary OA	72%	78%	85%	0.59	0.16
	Available

*

Table 2 .

 2 Comparison of the standing cup orientation between groups.

		Group 1	Group 2	Group 3	p (G1 vs	p (G1 vs
					G2)	G3)
	Cup anteversion (°)	14±7	13±6	13±5	0.53	0.37
	Cup inclination (°)	47±5	49±6	46±5	0.13	0.32
	% within safe zone:
	• Cup anteversion	81%	87%	90%	0.47	0.27
	• Cup inclination	75%	58%	64%	0.11	0.28
	• Cup A+I	57%	51%	54%	0,58	0,8

*: Significant difference.

Table 3 .

 3 Comparison of pelvic parameters between groups.

	Group 1	Group 2	Group 3	p (G1 vs G2)	p (G1 vs G3)
	Pelvic Incidence	58±13	51±11	56±12	0.01*	0.49
	Sacral Slope	40±8	37±8	37±7	0.29	0.2
	Pelvic Version	18±12	13±9	19±9	0.03*	0.95
	*: Significant difference.

Table 4 .

 4 Comparison of spino-pelvic parameters between groups.

		Group 1	Group 2	Group 3	p (G1 vs G2)	p (G1 vs G3)
	Anterior Pelvic Plane	2±8	7±6	3±6	0.002*	0.36
	Tilt
	Lumbar Lordosis	41±11	43°±12	41±14	0.47	0.93
	Sacral Vertical Axis	68±64	36±46	70±52	0.02*	0.87
	(mm)
	Sacral Vertical Axis	53%	34%	59%	0.10	0.6
	>50mm
	Sacro-spinal angle	122±11	126±12	121±10	0.16	0.61
	Sacro-spinal angle <	70%	43%	68%	0.02*	0.76
	127°
	Pelvic Incidence-	17±15	8±12	15±14	0.005*	0.61
	Lumbar Lordosis
	Pelvic Incidence-	60%	29%	55%	0.006*	0.61
	Lumbar Lordosis>15°
	Pelvic Version	66%	51%	78%	0.18	0.24
	>0.24*Pelvic
	Incidence

*: Significant difference.

, which has been approved by our institutional ethics committee (CLER 2017_CLER-MTP_10-10).