Laurent Barbiéro

Abdallahi Ould Mohamedou

Lucien Roger

Soˆnia Furian

Alain Aventurier

Jean Claude Rémy

Serge Marlet

The origin of Vertisols and their relationship to Acid Sulfate Soils in the Senegal valley

Keywords: Acid Sulfate Soil, Vertisol, Soil genesis, Senegal valley, Senegal delta

In the Senegal valley, it is commonly considered that the Acid Sulfate Soils of the delta are fossil soils overlain by more recent sediments, and that the Vertisols, which abruptly overlie a thick sandy horizon, result from a change in the sedimentation mode of the river. However, we show that both soils belong to the same pedological system extending from the delta to the limit of the last marine transgression. This conclusion is based on (1) the study of soil profiles intermediate between Acid Sulfate Soils and Vertisols along a 100-m sequence in the delta, (2) the mineralogy of the clay fraction ( < 2 Am) in a 200-km transect along the river, and (3) the similarity of the sand size distribution across the textural discontinuity between the horizons. The following processes are involved in the pedological transformations: (1) development of acidity by oxidation of pyrite, (2) neutralization of acidity initially by the carbonate in shell beds, and later by the hydrolysis of easily weatherable silicate clays. The slightly alkaline river water precipitates kaolinite and later smectite at the contact between the strongly acidic and slightly alkaline environments. This results in the formation of a superficial vertic clay horizon surmounting a sandy horizon. Therefore, we emphasize that the soil morphology results from development and control of acidity and not from changes in past climates.

Introduction

During the Quaternary, the sea transgressed several times into the Senegal River valley [START_REF] Faure | Geoidal changes and shore-level tilts along Holocene estuaries: Sénégal river area, West Africa[END_REF]. In the middle Holocene, soon after 7000 BP, the Nouakchottian trangression penetrated Pleistocene sand dunes and formed extensive shallow bays and lagoons. Between 4000 and 1800 BP, the shoreline retreated westwards and the Senegal river mouth was forced southwards during the Taffolian regression [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie. Etude géomorphologique[END_REF]. About 2000 BP, a final transgression formed a large gulf as far inland as Boghé, which is now 380 km from the coast, and a mangrove vegetation developed in the fluviomarine environment [START_REF] Lezine | Evolution of the West African mangrove during the Late Quaternary: A review[END_REF]. The sea regressed from Boghé to the present Senegal delta in the medieval period accompanied by the mangrove vegetation, and the remains of this vegetation still occur along creeks in the delta. The sequence of transgression -regression cycles is thought to have had a great influence on soil formation in the Senegal valley.

Vertisols are common in the Senegal valley and are widely used for irrigated agriculture, particularly rice cultivation. About 75% of the total cultivated area are Vertisols occurring in depressions [START_REF] Fao-Sedagri | Etude hydro-agricole du bassin du fleuve Sénégal[END_REF][START_REF] Favre | Water movement and soil swelling in a dry, cracked Vertisol[END_REF]. In the Vertisols, the superficial clay horizon abruptly overlies a sandy horizon, which is coloured by iron oxides around former roots of mangrove vegetation. The clay and sandy horizons are both 0.5 m thick in the Senegal delta but together reach 6 m in the middle valley around Podor. The clay and sandy horizons have been attributed to changes in sedimentation during and after the Nouakchottian transgression. [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie. Etude géomorphologique[END_REF] attributed the sandy horizon to redeposition of Pleistocene (Ogolian) sand dunes during the Nouakchottian transgression, and [START_REF] Deckers | Major soils and their formation in African Sahel[END_REF] attributed the clay to post-Nouakchottian sedimentation during the Taffolian regression. Potential acidity accumulated as pyrite in the mangrove roots. The pyrite was later oxidized as the water table level lowered, causing high actual soil acidity. [START_REF] Deckers | The fate of Salic and Thionic Fluvisols under irrigated rice in the Senegal river delta[END_REF] suggested that in many instances, the resulting Acid Sulfate Soils (thionic Fluvisols) are buried under more recent sediments of fluviatile origin, and can therefore be considered as fossil soils. The transgression was also responsible for incorporation of salt into the sediments. A complex redistribution of the salt occurred subsequently in the landscape, involving several processes: (1) concentration by evaporation, (2) aeolian deflation and clay dune formation, (3) transformation of shell banks into gypseous layers, and (4) runoff and deposition of salt in the depressions around the clay dunes (Barbiéro et al., in press). After the last regression, the soils in the valley developed from the sediments. The most recent soils should occur in the delta, particularly in the present remnants of mangrove areas, whereas the most developed ones should occur upstream, at the limit of the transgression. We have shown that this upstream sequence influences the distribution of salinity and its chemical composition along the valley (Barbiéro et al., in press), and have suggested that the various processes formerly responsible for soil development and features in the middle valley could still be active downriver in the lower middle valley or delta. The aim of this paper is to show that the sequence from the delta to the middle valley is relevant to understanding development of the soil profiles. We focus on the area of transition between the two main soil types (Acid Sulfate Soil and Vertisol), and particularly on the origin of the abrupt textural discontinuity at the base of the Vertisol profile.

Materials and methods 2.1. Site

Fourteen sites were studied along the Senegal valley, on both the Mauritanian and Senegalese sides of the river, and from the delta to the Podor region (Fig. 1). The semiarid climate is characterized by a wet season (approximately 200 mm of rainfall) from July to September, a cold and dry season from October to February, and a hot dry season from March to June. The average temperature is 24 jC in January and 39 jC in May. The dry and warm winds increase evaporation, so that average Class A pan evaporation exceeds average rainfall for each month and by >2000 mm annually [START_REF] Verheye | Impact of climate and soil conditions on conception and implementation of irrigation schemes in the Senegal River basin[END_REF]. In the floodplain, the water table fluctuates from the topsoil to 1 m depth in the delta, and from 4to 8-m depth in the Podor region [START_REF] Voinovitch | Apport de la méthodeélectrique pour la reconnaissance hydrogéologique et l'étude des variations superficielles en zone sédimentaire subsaharienne[END_REF][START_REF] Hammecker | Suivi etévaluation de la qualité des eaux et de l'impact environnemental des aménagements du secteur N'Gallenka[END_REF].

Soils

Thionic Fluvisols containing pyrite (FeS2) and jarosite [KFe3(SO4)2(OH)6] occur near the remaining mangroves [START_REF] Brusq | Nouvelles paragenèses à sulfates d'aluminium, de fer, et de magnésium, et leur distribution dans les sols sulfatés acides du Sénégal[END_REF][START_REF] Deckers | Major soils and their formation in African Sahel[END_REF]. Beyond these areas, the most representative soil types are Gypsic Fluvisols, Calcic Gypsic Fluvisols and Vertisols [START_REF] Fao | World Reference Base for Soil Resources[END_REF]. Upriver in the middle valley, Vertisols predominate in the depressions (locally Gypsic or Calcic), and Fluvisols occur on the river banks.

Fieldwork

The first step of the fieldwork was in the Diawling National Park, which is a narrow triangle of land between the coast and the Senegal River in southern Mauritania. The flooded area of the park consists of the permanently flooded Gambar basin, the dam reserve of the Organisation for the Development of the Senegal Valley, and the periodically flooded Bell Basin, which includes the estuarine area, whose elevation is greater than that of the Gambar. The fieldwork was concentrated in the Bell Basin between the Birette and Ziré dunes (16j20VN, 17j25VW) close to the Khurumbam Creek, which is a tributary of the Bell Creek (Fig. 1). A 100-m soil sequence was studied perpendicular to the Khurumbam creek bed, and intersecting the present edge of the mangrove area. The soil morphology was based on 11 hand boreholes and 3 pits (P1 to P3), excavated at the lateral transitions of the soil. The horizons are described according to the classification for Acid Sulfate Soils of the International Institute for Land Reclamation and Improvement [START_REF] Dent | Acid sulphate soils : a baseline for research and development[END_REF].

The second step of the field work was based on 13 sites along the Senegal valley. Since no notable lateral variations were noticed, the soil morphology was described in isolated profiles from boreholes or excavated pits (P4 -P16). Samples were taken from the main horizons and additional samples from 5 cm above and below the discontinuity between the clay and sandy horizons.

Laboratory work

Soil pH was measured in the field (pHfield) and in the laboratory after hydrogen peroxide (30% H2O2) treatment (pHH2O2) in order to oxidize FeS2 [START_REF] Van Beers | Acid Sulphate Soils[END_REF]. The soil water content (Wc) was measured by weighing soil samples before and after oven drying at 105 jC. Detailed particle size distribution (clay, 2 silt fractions and 17 sand fractions) was determined by sieving and sedimentation on 100 g samples. Total carbon content in the samples was determined by dry combustion in an elemental analyser (Thermoquest CN 2100). Organic matter was estimated as 1.724 total carbon. The clay ( < 2 Am) fraction of 30 samples was separated by sedimentation after destruction of the organic matter with dilute H2O2. Its mineralogy was investigated by X-ray diffraction (XRD) using a Rigaku CN4012A2 diffractometer (cobalt anticathode) on orientated specimens studied under 6 different conditions: K saturation (K), Mg saturation (Mg), glycerol treatment (Gly), and heating at 110, 450 and 550 jC (110, 450 and 550). The samples were scanned from 0.35j to 18j at 0.5j per minute. Al and Fe between the layers were removed from four samples by treatment with Na-citrate buffered at pH 7.3, followed by K-or Mg-saturation and finally heating at 110 jC overnight (Cit-K-110 or Cit-Mg-110 treatment) [START_REF] Tamura | Identification of clay minerals from acid soils[END_REF][START_REF] Robert | Méthode de préparation des argiles des sols pour desétudes minéralogiques[END_REF].

Cation exchange capacities (CEC) of the soil samples were measured by the cobalt hexamine trichloride non pH-buffered method (AFNOR NF X31-130, of AFNOR, 1994). The pH of the collected samples was adjusted to within the range over which the cobalt hexamine complex is stable [START_REF] Ciesielski | A comparison between three methods for the determination of cation exchange capacity and exchangeable cations in soils[END_REF]. Cobalt ion concentration was measured by ICP Varian (Vista) in the exchange solution. The CEC of the organic matter was estimated as 250 meq 100 g_ 1 organic matter. The remainder of the CEC was attributed to the clay fraction.

For six samples collected along the valley in the clay horizon of profiles P4, P5, P8, P10, P14 and P15, the < 2-Am fraction was dissolved with hydrofluoric acid, and the extracted potassium, aluminium and silica were measured by atomic absorption spectroscopy. After digesting the clay from the same samples in HF + HClO4 in teflon bombs, iron was determined by flame atomic absorption spectroscopy using the 373.7-nm wavelength [START_REF] Voinovitch | Apport de la méthodeélectrique pour la reconnaissance hydrogéologique et l'étude des variations superficielles en zone sédimentaire subsaharienne[END_REF].

The relative abundance of clay mineral groups (kaolinite, smectite and illite) was calculated by three methods:(i) (Method M1). The percentage composition of the clay groups was determined semi-quantitatively from the area under selected XRD peaks, following standard procedures of [START_REF] Biscaye | Mineralogy and sedimentation of a recent deep-sea clay in the Atlantic Ocean, adjacent seas and oceans[END_REF]. As the XRD method is semi-quantitative with an accuracy of about 10% of the relative clay mineral abundance, we quote values to the nearest 1% [START_REF] Kahle | Review of XRD-based quantitative analyses of clay minerals in soil: the suitability of mineral intensity factors[END_REF].(ii) (Method M2). The potassium measured after HF +HClO4 digestion was attributed entirely to illite. The percentage of illite in the samples was calculated as follows:

illite sample% = 100. (K sample % / K illite ) (1)
where illitesample% is the percentage of illite in the sample, Ksample% is the percentage of K in the solution after HF + HClO4 digestion and Killite is the weight percentage of K in the illite. An illite with iron in the tetrahedral position was considered for this calculation: illite = [Si 6.7 (Al 0.65 , Fe 3+ 0.65 )] O 20 [(Al 1.95 , Fe 3+ 1.95 ) (Fe 2+ 0.05 , Mg 0.05 )] (OH) 4 K 1.4

(2)

which gives Killite = 6.4%.The contribution of illite to the CEC of the clay fraction was calculated assuming that illite has an exchange capacity of 25 meq 100 g_ 1. The relative proportions of kaolinite and smectite were calculated from the CEC of the clay fraction based on their assumed exchange capacities (kaolinite: 10 meq 100_ 1; smectite: 100 meq 100 g_ 1).(iii) (Method M3). The potassium measured after HF + HClO4 digestion was attributed to illite. The proportion of Fe (Fea) attributed to illite was calculated as follows:

Fe a = (Fe illite . illite sample% )/100

(3

)
where Feillite is the weight percentage of Fe in the illite calculated from the structural formula in Eq. ( 2) (Feillite = 2%).

In a recent study of clay minerals in the Podor region, [START_REF] Favre | Iron reduction and changes in cation exchange capacity in intermittently waterlogged soil[END_REF] calculated that structural Fe (tetrahedral and octahedral) content is about 120 cmol kg_ 1 in the smectite. Therefore, after subtracting this value (Fea), the remaining Fe (Feb) was attributed entirely to a smectite of ironbeidellite type: beidellite = [Si 7 (Al 0.5 , Fe 3+ 0

.5 )] O 20 [(Al 2.2 , Fe 3+ 2 ) (OH) 4 Ca 0.1 Na 0.2 (4)
The percentage of beidellite was calculated as follows:

beidellite sample% = 100 (Fe b / Fe beidellite )

where Febeidellite is calculated from the structural formula of the beidellite in Eq. (4) (17.3%). The proportions of Al attributed to illite (Ala) and beidellite following:

Al a = (Al illite . illite sample% )/100 (6) Al b = (Al beidellite . beidellite sample% )/100 (7)
where Alillite and Albeidellite are the respective weight percentage of Al in the illite and beidellite (1% and 9%). After removing Ala and Alb, the remaining Al (Alc) was attributed to kaolinite with no substitution of Fe for Al:

kaolinite = Si 4 O 10 Al 4 (OH) 8 (8)
The percentage of kaolinite in the samples was calculated as follows: kaolinite sample% = 100 (Al c / Al kaolinite ) (9

)
where Alkaolinite is the weight percentage of Al in the kaolinite calculated from the structural formula of kaolinite (21%). Finally, the remaining silica was attributed to quartz, which was detected in small quantities in the < 2-Am fraction by XRD.

Results

Soil morphology

Fig. 2 shows the soil distribution pattern in the delta. In the creek bed at the extreme west of the transect, unripe silty clay rich in pyrite occurs at 0.4-m depth (Gr horizon, pHfield = 5.6, pHH2O2 = 2.7, Wc = 60%), and is overlain by a greyish brown, half ripe and strongly acidic silty clay with yellow mottles of jarosite (Gj and Bj horizons, pHfield = 4.5, pHH2O2 = 2.6, Wc = 42%). The superficial greyish brown horizon has a coarse prismatic structure mottled with reddish brown iron oxide at the base (Bg horizon, pHfield = 4.7, pHH2O2 = 4.3). The surface soil has a soft powdery structure. Near 12 m to the east, a bleached sandy horizon occurs at 0.2-m depth and is discontinuous between 12 and 24 m (Fig. 3). Its lower boundary is abrupt, wavy and emphasized by reddish hematite mottles, whereas its upper boundary is gradual and irregular. The thickness of this horizon increases progressively from 0.2 to 0.95 m between 24 and 100 m. It is locally bleached, but around the former mangrove roots, it is coloured by iron oxide (Fig. 4). Various undulating clay laminae 1 -5 cm thick occur in the sandy horizon, and are sometimes anastomosing. In the unripe mud located deep below, the pHfield reaches 3.9 (pHH O2 = 2.9) and Wc is about 33%. Near 66 m east of the creek bed, the surface structure changes abruptly from soft powder to centimetric platelets, where there is an abrupt increase in topsoil clay content from 19 to 29%. The thickness of this clay topsoil horizon increases irregularly from 66 m to the end (100 m) of the surveyed transect (Fig. 5), where a decimetric vertic structure appears and the clay content reaches 55%(pHfield = 6.1, pHH O2 = 5.9 and Wc = 17%). The lower boundary of the vertic clay horizon is abrupt and many of the laminae located in the sandy horizon merge into its base. Two layers rich in shells and shell fragments occur in the western part of the sequence. The first layer at 0.6-m depth is composed of Anadara senilis fragments and the second (0.9-m depth) of Thympanotonus fuscatus shells. In both layers, the shells are strong only to distances of 33 and 62 m, respectively. Further east, they are replaced by shells weathered by dissolution, and yet further east centimetric gypseous nodules replace the two shell layers. Up the Senegal valley, the Profiles P4 to P16 have the same horizon sequence as that described in the final part of the sequence (Profile P3, Fig. 5). Three main horizons are distinguished from top to bottom: a brown -grey clay with decimetric vertic structure (pHfield ranging from 5.9 to 6.7 along the valley), underlain by a sandy horizon with an abrupt, slightly undulating boundary. The sandy horizon is usually bleached but sometimes coloured by iron oxides, and contains several undulating clay laminae. Finally, an irregular merging boundary leads to a black mud. Profiles P3 -P16 differ only in the thickness of the horizons (Fig. 6).

Sand fraction granulometry

In the delta (P1 to P3), there is a bimodal distribution of the sand fraction, with modes at 0.13 and 1.25 mm in all the horizons (Fig. 7). Along the valley (P4 to P16), the grainsize distribution varies significantly between profiles. A coarse unimodal distribution occurs in P6 and P7, a fine unimodal distribution in P4 and P14, a bimodal distribution in P15, etc. (Fig. 8). However, the sand grain-size distribution is always similar on either side of the abrupt discontinuity between the vertic clay horizon and the sandy horizon.

Clay mineralogy

X-ray diffraction of the < 2-Am fractions indicated the presence of kaolinite, illite, smectite, interstratified illite -smectite and quartz. Kaolinite, illite and smectite are dominant, whereas quartz and interstratified minerals are present in only small quantities, as reported by [START_REF] Haefele | Long-term fertility experiments for irrigated rice in the West African Sahel: agronomic results[END_REF] at Ndiaye in the delta. Examples of typical XRD diffractograms are given in Figs. 9 and 10. Kaolinite gives a sharp main peak ranging from 7.12 to 7.19 Aº in all the samples. However, in the samples from the unripe mud, the peak disappears after heating at 550 jC, whereas in the clay laminae and the vertic clay horizons throughout the valley, the kaolinite peak decreases after heating to 550 jC but does not disappear completely. Smectite occurs in the unripe mud, the vertic clay horizon and the clay laminae in Profiles P3 to P16, whereas in Profiles P1 and P2, it is present only in the unripe mud. The main peak is located around 15 -17 Aº (Fig. 9), shifts to 19 Aº after Mg-Gly treatment, and decreases to 10 Aº after heating at 400 jC. After this treatment, some peaks with interlayer distances >10 Aº persisted, though heating at 550 jC removed them. After sodium citrate treatment with K-saturation and heating at 110 jC (Cit-K110), a 12.5-Aº peak appeared. Using the same treatment but with Mg-saturation (CitMg-110), the 12.5 Aº peak was absent or less pronounced. In most of the samples, after Cit-K-110 processing, the amplitude of the 10-Aº peak increases compared to Cit-Mg110 processing. In contrast, smectite is hardly detectable in the clay lamina of Profile P2, where it forms a plateau between 14 and 24 Aº. In these samples, some peaks correspond to an interlayer spacing >22 Aº. A regular change in the smectite occurs along the 100-m sequence in the delta from the clay lamina in P2, where smectite is hardly identified, to the vertic clay in P3, where the peak is clearly defined (Fig. 10). A mica, whose main peak is located between 9.83 and 10.1 Aº, occurs in the unripe mud and the clay horizon from Profiles P2 to P13. Interstratified minerals exhibiting a main peak >20 Aº occur in Profiles P4 to P8. Lastly, a slightly weathered illite with interlayer distance ranging from 10.5 to 12 Aº may occur in Profiles P8 to P13, and also in two samples of the unripe mud in P1 and P2.

Semi-quantitative composition of the clay fractions

Table 1 shows the relative proportions of kaolinite, smectite and illite groups in the samples from the vertic horizon in Profiles P4 -P16. The mineral ratios calculated from methods M1, M2 and M3 are presented in Fig. 11. Upriver, the ratio smectite/kaolinite slowly increases, illite/kaolinite decreases and smectite/illite increases.

Discussion

Soil morphology

The soil observed on the western part of the transect (Fig. 2) has the typical morphology of an Acid Sulfate Soil. Gr, Gj, Bj, Bg and A horizons (criteria used in the ILRI classification; [START_REF] Dent | Acid sulphate soils : a baseline for research and development[END_REF] occur from the bottom to the top of the profile. The horizon sequence is determined by two main pedological processes: ripening of the mud involving principally an irreversible loss of water, and the oxidation of iron minerals. The pyrite in the Gr horizon was produced by the activity of the sulfate-reducing bacteria in waterlogged sediments rich in organic matter. The pale yellow colour in the Gj and Bj horizons results from jarosite, which is generally observed when pH is < 3.7 and Eh >400 mV [START_REF] Dent | Acid sulphate soils : a baseline for research and development[END_REF]. At higher pH values, jarosite is metastable with respect to goethite and ultimately, it is hydrolysed to iron oxide, predominantly geothite and sometimes hematite ( [START_REF] Van Breemen | Genesis and solution chemistry of acid sulfate soils in Thailand[END_REF], as observed at the top of the Bj horizon. At the top of the profile and towards the eastern end of the sequence, there is evidence of chemical reactions induced by the acidity, following oxidation of pyrite, and also evidence of acid neutralization by the soil alkalinity. The alkalinity is mainly provided by carbonates, exchangeable bases and easily weatherable silicates. The first control of acidity is the progressive dissolution of the shells, leading to disappearance of the calcareous layers. The calcium released in the saline and sulfate-rich environment precipitates as centimetric gypsum nodules, indicating the former presence of the shell beds in the soil [START_REF] Dent | Acid sulphate soils : morphology and prediction[END_REF][START_REF] Dent | Acid sulphate soils : a baseline for research and development[END_REF]. According to Van Breemen (1973a,b, 1976), acid neutralization by protonation of clay minerals occurs after dissolution of the carbonates. The resulting protonated clays are not stable [START_REF] Laudelou | La stabilité chimique des suspensions d'argile saturées par l'ion hydrogène[END_REF] and their crystalline structure is attacked by H+, liberating silica and metal ions, principally aluminium, iron, magnesium and potassium into the soil solution [START_REF] Fitzpatrick | Acid Sulfate Soil Assessment: Coastal, Inland and Mine spoil conditions[END_REF]. Van Breemen (1973a,b, 1976) showed that there is significant dissolution of silicate clays at pH < 4. The acid hydrolysis of aluminosilicate clays can contribute 10% of the buffering of the acid produced by oxidation of pyrite [START_REF] Dent | Quantitative model to predict the rate and severity of acid sulfate development : a case study, the Gambia[END_REF]. We therefore interpret the sandy bleached horizon as resulting from acid hydrolysis of aluminosilicate clays, leading to relative accumulation of quartz, and not as a result of sedimentation. This is based on the following observations: 1. The sandy material appears first at about 20 -m depth, where presently the oxidation of pyrite and jarosite creates the most acid field conditions. 2. In the unripe mud, the horizontal shell beds indicate stratification, which is intersected by the lower boundary of the sandy horizon. 3. The bimodal sand size distribution (Fig. 6) persists throughout the sequence in spite of the textural differences in the three horizons, namely, mud, sand and clay. This would not happen if the soil profile were a result of different sedimentation episodes, as the different sedimentation regimes required for the deposition of these three materials would not preserve the same sand size distributions. The textural discontinuity between the mud and the sandy material is therefore interpreted as a pedological transformation front, resulting from acidification, dissolution of aluminosilicate clays and relative accumulation of quartz skeleton. The origin of the textural discontinuity between the sandy horizon and the clay vertic horizon will be discussed below using other data collected along the valley.

Clay fraction of the unripe mud

In the unripe mud (Profiles P1 and P2), the < 2-Am fraction is composed of kaolinite, smectite and micas. The presence of a peak at about 12.5 Aº after Cit-K-110 processing may indicate neoformed smectite [START_REF] Robert | Principes de détermination qualitative des minéraux argileux à l'aide des Rayons X[END_REF]. In the same samples, the increase of the peak at 10 Aº, indicating collapse of the smectite layer after K saturation, should characterize a smectite formed from mica. However, the presence of well-crystallized mica and the absence of interstratified minerals in spite of some slightly weathered illite suggest that the transformation of the mica into smectite did not occur in the mud. The mineralogy of the clay fraction in the mud of Profiles P1 and P2 therefore seems to be that of the original river sediment, which is in agreement with the composition of sediment at the river mouth [START_REF] Gac | Le fleuve Sénégal : I. Bilan hydrologique et flux continentaux de matières particulaires à l'embouchure[END_REF]. To summarize, the mineralogical profile of the < 2-Am fraction in the unripe mud is kaolinite, illite and two types of smectite, one of which is formed by transformation of micas and the other by chemical precipitation.

Clay fraction of the clay laminae and vertic horizon

The kaolinite in the samples taken from the clay laminae is slightly heat-resistant and therefore differs from that of the mud. This suggests that it was precipitated in the laminae. In acid sulfate waters, high Al contents occur as a result of aluminosilicate dissolution, but the geochemistry of Al is significantly modified by sulfate, so that gibbsite and kaolinite are not the most stable phases [START_REF] Bigham | Sulfate minerals. Crystallography, geochemistry, and environmental significance[END_REF]. In some parts of the Senegal delta where the acid soil solution concentrates, Le [START_REF] Brusq | Nouvelles paragenèses à sulfates d'aluminium, de fer, et de magnésium, et leur distribution dans les sols sulfatés acides du Sénégal[END_REF] observed soluble aluminium sulfate minerals. However, during overflow of the Senegal River, a neutral or slightly alkaline water with lower ionic concentrations easily infiltrates the sandy horizon of the acid soil. After dilution and partial control of the acidity by the Senegal freshwater, kaolinite can form. The clay laminae undulate in the soil profiles and are linked to the vertic superficial horizon, and may have originated at the moisture front. The geochemical contrast at the contact between the acid water table and the Senegal water should cause precipitation of aluminosilicate, which first forms a gel, as observed in a similar geochemical environment in Spain [START_REF] Gimeno | Geochemical characterization of the acidic waters in the Arroyo del Val system Zaragoza, Spain[END_REF], and is then quickly reorganized into kaolinite.

The smectite in the superficial vertic horizon at P3 is better organized than that in the deeper clay lamina in Profile P2 (Fig. 10). The large plateau on the X-ray trace (P2 lamina 1) corresponds to an interlayer spacing >22 Aº and therefore suggests weakly organized smectite layers. It cannot be attributed to interstratified minerals because peaks at 14 and 24 Aº are not present simultaneously. Hence, we conclude that smectite was formed later than kaolinite in the clay laminae and vertic horizon, probably when acid conditions were sufficiently buffered in the soil solution. The mineralogy of the < 2-Am fraction in the mud differs from that in the clay laminae. Therefore, contrary to our previous claims [START_REF] Barbiéro | Influence de la maturation des sols de mangrove sur la déflationéolienne et la formation de dunes argileuses dans le delta du fleuve Sénégal[END_REF], the clay laminae are not the remains of the mud, nor are they formed by redistribution of the clay fraction in the soil profile. They can only be formed by dissolution of the clay fraction of the mud and precipitation in the clay laminae.

Changes along the Senegal valley

The sand size distribution is similar on both sides of the textural discontinuity between the sandy horizon and vertic clay horizon, but varies along the river from one profile to another, suggesting that the discontinuity is pedogenic in origin. Indeed, it is unlikely that the sandy and clay horizons arise from different sedimentation processes, because the clay would not retain exactly the same sand size distribution as the sand below. At the contact between the sandy horizon and the vertic clay horizon, the strong acidity is buffered and there is no evidence of clay dissolution. Therefore, we interpret the contact as a pedological transformation front, where clay is precipitated, so that the clay horizon is expanding at the expense of the sandy material. In the vertic clay horizon of Profiles P3 -P16, the kaolinite remains similarly heatresistant to that in the clay laminae, and was probably precipitated in the same conditions, i.e. in a very contrasting chemical environment and in the initial stage of acid buffering. The smectite is better crystallized in the vertic clay horizon than in the clay lamina of P2. The peak at 12.5 Aº observed after the Cit-K-110 treatment disappears after the Cit-Mg 110 treatment, indicating formation by chemical precipitation, which probably occurred after the buffering of acidity. The increase in smectite contents upstream indicates an increasing period of precipitation of smectite. The crystallinity of the illite is good in the downstream Profiles P4, P5 and P6, but upriver the intensity of the peak decreases, the width increases and the peak almost disappears at P12, which agrees with the relative proportions calculated from CEC and chemical data after HF + HClO4 digestion. The parallel increases in smectite contents and the 10-Aº peak after the Cit-K-110 treatment indicate the transformation of illite into smectite. Our results agree with the observation of [START_REF] Haefele | Long-term fertility experiments for irrigated rice in the West African Sahel: agronomic results[END_REF], who also observed the presence of kaolinite and smectite without illite in the clay fraction at Fanaye close to P12. To summarize, taking the kaolinite content as a reference in the clay fraction, the ratio smectite/kaolinite increases upriver because of chemical precipitation of smectite and also because of the progressive transformation of illite into smectite. The maximum proportion of smectite in the clay fraction in our samples was 46%, but upriver beyond our sequence up to 70% of smectite and 30% of kaolinite were reported in more superficial samples of the vertic horizon [START_REF] Favre | Iron reduction and changes in cation exchange capacity in intermittently waterlogged soil[END_REF].

Conclusions

In a previous study, we showed that eolian deposition is a major soil forming factor in the Senegal valley [START_REF] Mohamedou | Geochemistry of clay dunes and associated pan in the Senegal delta (Mauritania)[END_REF]Barbiéro et al., in press). The present paper suggests that the development and buffering of soil acidity is also a major factor in the formation of the soil profiles. Acid Sulfate Soils (thionic Fluvisols) occur in the delta under the remains of mangrove vegetation, and their mineralogical composition suggests they are inherited from sediments of the Senegal River. The oxidation of pyrite causes strong acidity in the soil. This is buffered first by dissolution of the shell deposits, which results in the formation of gypseous layers. Subsequently, it is buffered by protonation and acid hydrolysis of aluminosilicate clays, which result in the formation of a sandy horizon by residual accumulation of the quartz skeleton. Two main pedological fronts occur between the Acid Sulfate Soils and Vertisols. The first pedological front is the undulating textural discontinuity between the deep mud with high pyrite content and the overlying sandy horizon. During the rainy season, the Senegal river overflows and neutral or slightly alkaline freshwater infiltrates into the soil profile. At the contact between the fresh Senegal water and the acidic water rich in Al, Fe and Si, kaolinite precipitates forming clay laminae in the sandy horizon. Subsequently, smectite precipitates when the acid conditions are sufficiently buffered. Clay precipitation results the formation of a superficial clay horizon, which acquires a vertic structure, and produces the second pedological front between the sandy horizon and the vertic clay horizon. The similar morphology of the Vertisols in the eastern part of the sequence described in the delta and those observed along the valley suggests that they belong to the same soil system. Upriver a thickening of the soil cover and progressive mineralogical changes occur in the vertic horizon. The proportion of smectite in the clay fraction increases by smectite neoformation and transformation of illite into smectite. The pedological processes indicated by this study disagree with the previous interpretation of the of soil profiles in the valley. We conclude that Acid Sulfate Soils in the delta are not fossil soils buried under more recent deposits [START_REF] Deckers | The fate of Salic and Thionic Fluvisols under irrigated rice in the Senegal river delta[END_REF], but are undergoing transformations related to the development and buffering of acid conditions. Also, we do not interpret the current Vertisols as resulting from three successive deposits, pre-Nouakchottian acid sulfate mud, Nouakchottian sand and post-Nouakchottian clay deposits [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie. Etude géomorphologique[END_REF]. Instead we suggest that they have formed from clay precipitation in the sand after buffering of the acid conditions. The standard succession of the Vertisol profile over a sandy horizon over unripe mud is the result of pedological processes. This succession is thin in the delta but reaches a thickness of about 6 -7 m in the Podor region 200 km inland. Therefore, it is not necessary to resort to drastic changes in environmental conditions to explain the discontinuities observed in the soil profiles along the valley. Indeed there is no independent evidence of changes in palaeoenvironmental conditions. The soils of the Senegal valley can be considered as a chronosequence, the most recent soils being observed downriver in the delta, and the most developed ones upriver at the border of the last transgression. This chronosequence may prove to be useful in various studies, particularly in understanding the buffering of alkalinity brought during each irrigation. The Senegal water used for irrigation is suspected to cause eventual soil degradation by alkalization [START_REF] Boivin | Soil degradation in irrigation schemes in the Senegal river middle valley: mechanisms, characterization methods and actual situation[END_REF]. In order to calculate the alkalinity balance, the current clay precipitation identified in this study should be taken into account. Irrigated soils of the Podor region should eventually develop similar chemical features to those located upstream between Podor and Boghé, which have been naturally flooded by water of the same quality for a longer time.

Figure captions

Fig. 1 :

 1 Figure captionsFig. 1: Locations of the studied sites: + are points sampled along the Senegal valley (P4 to P16), and of the soil sequence studied in the Diawling National Park, Senegal delta (P1-3). Fig. 2: Soil morphology along the sequence in the delta. Fig. 3: Profile of Acid Sulfate Soil (P1), with the appearance of the sandy horizon at about 20 cm depth. Jarosite appears as yellow mottles and oxidizes to goethite/hematite at the base of the sandy horizon. Fig.4: Profile of Acid Sulfate soil (P2). The sandy horizon is well developed with clay laminae. Clay content increases abruptly from 19 to 29 % in the topsoil on the right side of the photo. Fig. 5: Profile of Vertisol (P3). From top to bottom: topsoil vertic clay horizon, sandy horizon with clay laminae, and unripe mud at 90 cm depth. Fig. 6: Distribution of the main horizons from the delta to the middle valley. Fig. 7: Distribution of sand size fractions along the soil sequence in the delta. Fig. 8: Distribution of sand size fractions on either side of the textural discontinuity between vertic clay horizon (Clay) and bleached sandy horizon (Sand) along the Senegal valley. Fig. 9: Examples of X-ray diffraction traces of the clay fraction from the vertic horizon of P3. Fig. 10: X-ray diffraction traces showing the gradual evolution of smectite from clay lamina at P2 and P3 to vertic clay horizon at P3. 11: Ratios of smectite/illite, illite/kaolinite and smectite/kaolinite along the Senegal valley, using methods M1, M2 and M3 (see text). Straight lines denote linear trends of the ratios.

Table 1 :

 1 Fig. 3 Fig. 4 Fig. 5

Acknowledgements

This work was partially supported by a research grant from AUPELF-UREF (Agence Francophone pour l'Enseignement Supérieur et la Recherche), awarded to Abdallahi Ould Mohamedou. We also thank Vasanthi Dass for the editorial advice.


Cécile Batailler
email: cecile-batailler@hotmail.fr

Didier Hannouche

Francesco Benazzo

Sébastien Parratte

Concepts and techniques of a new robotically assisted technique for total knee arthroplasty: the ROSA knee system

Keywords: ROSA knee system, Robotically assisted system, Total knee arthroplasty, Collaborative robotics, Personalized robotic TKA

Concepts and techniques of a new robotically assisted

technique for total knee arthroplasty: the ROSA knee system.

Introduction

Improving implants positioning and gap balancing are important goals in total knee arthroplasty (TKA). The use of conventional instrumentation in TKA showed limited accuracy with up to 40% of outliers [START_REF] Hetaimish | Meta-analysis of navigation vs conventional total knee arthroplasty[END_REF]. In the past few years, new targets in TKA have been defined with more patient-specific targets both for alignment and gap balancing. Restoring the physiological knee kinematics with these techniques can provide excellent functional results when well executed [START_REF] Courtney | Early outcomes of kinematic alignment in primary total knee arthroplasty: a meta-analysis of the literature[END_REF], but may also have detrimental effects when alignment is outside the classically accepted 0° ± 3°range [START_REF] Lee | Femoral component varus malposition is associated with tibial aseptic loosening after TKA[END_REF]. Different types of new technologies, such as computer-assisted surgery, patient-specific instrumentation, and more recently robotic-assisted TKA, have been developed to improve surgical accuracy and reproducibility, allowing surgeons to reach personalized targets, which may improve patient outcomes and satisfaction after TKA [START_REF] Batailler | New technologies in knee arthroplasty: current concepts[END_REF]. The development of robotic-assisted surgery has been possible thanks to the technological advancements made during the last 20 years starting with the rise of conventional computerassisted surgery in the early 2000s. Different types of roboticassisted systems have been developed with different features and characteristics, depending on robot autonomy, the requirement or not of 3-D pre-operative images, the use of a cutting guide or not, the use of haptic or assistive technologies, and the use of a burr or a saw.

A new robotically assisted system for TKA (ROSA knee System; Zimmer Biomet; Warsaw, IN) has been recently introduced. The previous systems available in the market were either active: the surgeon sets the robotic arm close to the patient and then the robot autonomously performs the surgery [START_REF] Liow | Early experiences with robot-assisted total knee arthroplasty using the DigiMatch ROBODOC(R) surgical system[END_REF][6][START_REF] Song | Simultaneous bilateral total knee arthroplasty with robotic and conventional techniques: a prospective, randomized study[END_REF][START_REF] Song | Robotic-assisted TKA reduces postoperative alignment outliers and improves gap balance compared to conventional TKA[END_REF], or haptic: the surgeon pushes the "Go Button" but the robot keeps the instrument within pre-determined boundaries [START_REF] Hampp | Robotic-arm assisted total knee arthroplasty demonstrated greater accuracy and precision to plan compared with manual techniques[END_REF][START_REF] Marchand | Patient satisfaction outcomes after robotic arm-assisted total knee arthroplasty: a short-term evaluation[END_REF][START_REF] Smith | Improved patient satisfaction following robotic-assisted total knee arthroplasty[END_REF][START_REF] Batailler | MAKO CT-based robotic arm-assisted system is a reliable procedure for total knee arthroplasty: a systematic review[END_REF][START_REF] Collins | Initial experience with the NAVIO robotic-assisted total knee replacementcoronal alignment accuracy and the learning curve[END_REF][START_REF] Bollars | Preliminary experience with an image-free handheld robot for total knee arthroplasty: 77 cases compared with a matched control group[END_REF]. During the last decade, these systems have shown their accuracy and safety, but they also have some limits in terms of usability, as the movements of the surgeon are constrained on a predefined surface. Thus, the surgeon may have the feeling to lose the control of the operation or being framed into a certain pattern. The new system presented here had the intention to keep the surgeon active, performing the cut while the robot is positioning and holding the cutting jig based on the preoperative plan made by the surgeon. This can be considered as collaborative robotics, where the surgeon remains in charge of the procedure and collaborates with a smart robotic tool. After a preclinical evaluation in Australia in 2018, this device has been officially launched in March 2019 in the Middle East (Abu Dhabi), and thereafter in the US and in Europe. The ROSA knee system has been developed based on the accuracy of the ROSA brain [START_REF] Miller | Applications of a robotic stereotactic arm for pediatric epilepsy and neurooncology surgery[END_REF][START_REF] Benedictis | Robot-assisted procedures in pediatric neurosurgery[END_REF]. ROSA stands for Robotic Surgical Assistant and is able to ideally position the instruments allowing the surgeon to perform the surgery with a high accuracy and reproducibility. The ROSA system offers both image-less and image-based options using 2D X-rays which are then transformed into a 3D model of the patient's knee. The integration of real-time intraoperative ligament balancing allows the surgeon to perform personalized Robotic Knee arthroplasties.

The aim of this paper was: (1) to describe the concept and the surgical technique of the ROSA knee system for TKA;

(2) to describe its advantages and potential limits; and (3) to report the early experience with this system.

Concept of ROSA knee

Most of the surgeons involved in the development of the ROSA knee platform had previous experience with the pre-existing platforms. They have outlined the constraints and limits of active and haptic systems, and have listed the necessary and desirable features and characteristics of the device. The goal during the development phase of ROSA was to keep the surgeon active and at the center of the operation: here, the surgeon handles the saw and performs the cuts while the robotic arm, which is equipped with a cutting jig, places and holds the guide at the right place, with a high accuracy and reproducibility. After the planning step, the robot places the jig according to the surgical plan for the tibial and the distal femoral cuts, and determines the position of the 4-in-1 resection guide. The robotic arm has 3 modes of action: automatic, collaborative and static. In the automatic mode, when the tip of the robotic arm is far from the knee, the robot moves in the space on its own. When approaching the knee and the surgical field, the robot switches to a so-called collaborative mode. In this mode, the surgeon can collaborate with the robotic arm by applying a gentle force on the guide to move it to the bone within the cutting plane. At this step, the movement of the robotic arm is restricted to the planned cutting surface, but follows the knee and constantly adapts to any joint movement. The position of the jig is verified by looking at live cut values on the screen, then fixed, before switching to a static mode which allows the surgeon to perform the different cuts. Perfectly flat cuts are obtained very naturally using a conventional saw and very reliably due to the rigid construct achieved by the robotic arm. After performing the cut, the jig is freed from the bone and the robot switches back to a collaborative mode to perform the next cut. The aim is to keep a smooth surgical flow and increase the efficiency, the accuracy, and the reliability of the conventional surgical steps. The ROSA knee platform has been developed to assist the surgeon for the distal femoral cut, the sizing and positioning of the femoral component (including the determination of the rotation), the tibial cut and the ligament balance.

Surgical technique

Principles of image-based or image-less robotic system

This robotic system has two options for case creation and planning: image-based with 3D virtual model derived from 2D full-length preoperative plain radiographs; or image-less, exclusively based on intraoperative landmarks acquisition.

For the image-based procedure, standard 2D X-rays are converted into a patient-specific 3D model of the knee thanks to a dedicated algorithm. Using a conventional radiographic system, standing long-leg AP and lateral radiographs are taken with two calibrated trackers disks positioned on the thigh and the calf with a Velcro strap. The 2D radiographic data are then uploaded into a secure image segmentation platform. Using validated algorithms, engineers are able to create a patient-specific 3D virtual model of the patient's knee. Similarly, to what was done for PSI TKA, a 3-D planning of the surgery is then created on these virtual 3-D models and uploaded on the surgeon's platform for edition (Fig. 1). With this technique, implant sizes and positioning can be anticipated at this step. The advantages of an image-based robotic system on the clinical outcomes are not yet demonstrated in knee arthroplasty [START_REF] Jacofsky | Robotics in arthroplasty: a comprehensive review[END_REF][START_REF] Ponzio | Preoperative mapping in unicompartmental knee arthroplasty using computed tomography scans is associated with radiation exposure and carries high cost[END_REF].

For the image-less option, the standard preoperative radiographs are performed according to the surgeon's practice. The planning during the surgery is based on the bony landmarks and on the ligament balancing collected at the beginning of the surgery. The image-less option appears to be also very accurate [START_REF] Parratte | Accuracy of a new robotically assisted technique for total knee arthroplasty: a cadaveric study[END_REF][START_REF] Seidenstein | Better accuracy and reproducibility of a new robotically-assisted system for total knee arthroplasty compared to conventional instrumentation: a cadaveric study[END_REF].

Set-up

The ROSA knee platform comprises two main components which are positioned on opposite sides of the operating table. A robotic unit which consists of a robotic arm and a touch screen, and an optical unit including an infra-red camera mounted on a dedicated arm and a touchscreen. The ROSA knee TKA universal cutting guide is mounted at the tip of the robotic arm. This jig can be used with any of the Zimmer Biomet TKA implants (Persona ® , NexGen ® , and Vanguard ® ). The robotic and optical units, the instruments, and patient's bones are linked by infra-red optical reference frames.

The surgeon and the robot are positioned on the same side of the patient, and the optical system is positioned on the other side of the table (Fig. 2). The robot can be draped and aligned to the patient's knee at the beginning of the procedure in a so-called set-up position. An automatic calibration is performed to link the robotic arm and the camera.

Registration and planning

The robotic procedure requires the installation of two rigid bodies, one in the femur and one in the tibia, as for all the current systems of robotic surgery. The trackers can be installed inside or outside the surgical incision depending on the surgeon's preference. They should be far enough of the knee to avoid any conflict with the instruments during surgery and placed distal enough on the tibial side to not interfere with the keel preparation of the tibial component.

Once the trackers are set in the bone, femoral and tibial bony landmarks are acquired. First the localization of the femoral head center is established by capturing 14 distinct Fig. 1 Radiographs in standing position (AP view and lateral view) with the calibrated markers to obtain a 3D virtual model, used for the per-operative planning Fig. 2 Robotic system positioning in OR positions of the hip during circumduction. The mechanical axis of the femur is determined with the femoral head center and the distal femoral canal entry point. Further landmark registration points of the distal femur include the medial and lateral distal condyles, the medial and lateral epicondyles. The posterior condyles are used to determine the posterior condylar axis, and the anterior and posterior trochlear groove is used to determine the Whiteside's line. The anterior cortex is used for femoral sizing and A/P translational positioning and to determine if notching will occur. The mechanical axis of the tibia is determined by the medial and lateral malleoli distally and the tibial canal entry point. The tibia rotation is determined by the medial third of the tibial tubercle and the PCL insertion. The medial and lateral plateau resection references are also collected. Importantly, when landmarking articular surfaces, care should be taken not to pierce the cartilage with the ROSA knee registration Pointer.

The next step assesses the knee frontal laxity using varus and valgus stresses at different angles of the knee flexion, the most important being the extension and 90° of knee flexion. But the laxity can also be tested and recorded at 30°, 45°, 60° and 120° of knee flexion. The values obtained can then be used to guide implant frontal, sagittal and rotational positioning, implant sizing and consecutively soft tissue balancing. This evaluation of the laxity can be performed before the planning but after the approach and osteophytes removal; during the bone preparation if needed with a spacer; at the end of the procedure with the trials or the definitive implants. This evaluation can be done at any time to adapt the surgical planning if needed. The surgeon can also decide not to use the ligamentous evaluation and perform a pure measured resection technique without considering the ligament at all, as when using a PSI technique.

After the landmarks collection and the assessment of the frontal laxity, the surgeon can perform the final planning according to her/his surgical preferences. During this planning, several parameters are determined: the femoral and tibial component sizes, the orientations of the bone cuts (femoral distal, posterior and anterior, tibial) and their thickness based on the bony and ligaments references. Predictive values of the final gaps and alignment are provided. (Fig. 3).

Bony preparation

The sequence of bone cuts, either tibia or femur first, can be individualized based on the surgeon's preference. After finalizing the planning, the surgeon can switch to the "Resection panel" on the touch screen. Pressing the button femoral or tibial resection will send ROSA to the knee in automatic mode. Once the tip of the ROSA arm reaches the surgical field, a switch to collaborative mode occurs and the surgeon is allowed to bring the jig exactly where it should be, while ROSA remains coplanar and maintains the accuracy of the cut in terms of frontal and sagittal alignment, and thickness of resection (Fig. 4). Live cut values are shown on the screen, with the agreement between the planed values and the actual ones (compensating for any knee motion). Once the jig is aligned with the planned femoral or tibial cut, two pins are installed to fix the jig and the cut can be performed using a conventional saw. After the bone cut, the validation tool is placed on the bony resection to confirm the adequation between the planned and the actual cut. Each cut can be readjusted anytime if needed.

Different surgical strategies can be performed with the ROSA knee, according to the surgeon's preferences. All the classic techniques can be used with ROSA: measured resection, gap balancing, functional alignment, kinematic alignment. Some techniques recently developed are more ROSA-specific: Robotic personalized TKA and ROSA-FuZion technique. The software has been created to navigate between the different surgical steps according to the surgeon's preference for the alignment, the implants positioning, but also the ligament balancing in flexion and extension.

Classic surgical techniques

The conventional technique aims at achieving a neutral limb alignment (with a 0° ± 3°range), by implanting the femoral and tibial components perpendicular to the mechanical axis [START_REF] Insall | The total condylar knee prosthesis. A report of two hundred and twenty cases[END_REF]. The cuts are either independent of the other (starting usually by the distal femur) or dependent, meaning that the distal and posterior femoral cuts are performed parallel to the tibial cut. Ligament balancing is obtained by a standardized sequence of soft tissue release to equalize the medial and lateral compartments, as well as flexion and extension gaps. Knee joint stability relies on the conformity of the prosthetic components and tensioning of the ligaments in flexion and extension.

The case of a ROSA TKA implanted for a varus deformity will be taken as an example. Once the trackers have been installed, the knee is exposed; the menisci are completely excised together with the cruciate ligaments depending on the type of prosthesis; the medial osteophytes are removed which opens the gap medially and is usually sufficient to balance the knee. Femoral and tibial landmarking are performed at this step. The knee is then assessed in terms of motion and laxity, and the femoral cuts are set by the surgeon on the planning panel overview. Typically, with the mechanical alignment technique, the amount of bone to be resected will be greater on the lateral side on the tibia, on the medial side on the distal femur, and on the medial side on the posterior condyle. After the distal femoral cut and the tibial cut are performed, the extension is checked with a spacer block. The amount of residual varus and flexor contracture is visualized on the screen. Further release of the medial compartment is performed by elevating the medial collateral ligament, and in Fig. 3 The orientation of bone resections, the implant positioning and their sizing allow to adjust the gap balancing in extension (a) and in flexion (b), and the limb alignment (c). The manipulation of the implants will give "live" feedback on implants alignment and gaps in flexion and extension severe constitutional varus knees, by the detachment of the semi-membranous tendon. The knee is flexed, and the ROSA cut guide is moved to the bone to drill the 4-in-1 holes. Rotational alignment is adjusted to the surgical epicondylar axis, which is 3° externally rotated relative to the posterior condylar line. The cut guide is then disengaged, and the 4-in-1 cuts are performed according to the planning. The trial implants inserted, and final knee evaluation is performed to check for full range of motion, mechanical axis, and joint stability both in extension and flexion.

ROSA-specific techniques

Personalized robotic TKA The concept of personalized robotic TKA is to recreate the patient anatomy, avoid ligament release, restore the joint line orientation and the limb alignment. The first step is to verify the absence of major extra-articular deformity on the pre-operative X-rays as this technique is not intended to correct a more than 10° extraarticular deformity inside the joint. The second step is to look at the patellar tracking and make sure that there is not patellar tilt of more than 30°. Intra-operatively, the ROSA set-up and landmarks registration are performed as previously described. The ligament laxity is recorded at 0° and 90°. The femoral planning is then done to restore the anatomy of the femur in a kinematic manner (compensating for the wear), the implant being intended to recreate the anatomy as it was before the OA. Persona TKA is ideal for that (anatomic design with small increment of sizes). The femur is then positioned on average between 0° and 3° of valgus and aligned to the posterior condylar axis in the absence of preoperative patellar mal-tracking. The tibial cut is planned based on the residual laxity to obtain at the end a perfectly symmetrical gap in extension (aiming for 1 mm of laxity both for the medial and lateral side of the knee). The second parameter is the space in flexion intended to be trapezoidal with a perfectly stable medial compartment all along the arc of knee flexion and some opening (from 2 to 6 mm depending of the initial laxity of the knee at the beginning of the procedure) in the lateral compartment. ROSA is used to perform all the cuts based on this planning, femur first and then the tibia to optimize exposure and the surgical flow.

Fuzion technique An optional feature of ROSA knee is the Femoral Rotation Tool, which guides the anterior and posterior rotational cuts of the femur based on a balanced flexion gap. This method is extrapolated from the FuZion method of the conventional technique. The femoral distal cut and the tibial cut are performed first. Then, the ROSA knee system will provide quantitative information about ligament laxity and tension in extension after the surgeon performs a distraction test. During this step, the Fuzion instrument has a 9 mm shim in place. ROSA Knee will accurately measure and record the values of the extension gap and indicate the frontal alignment. The knee is then positioned at 90°of flexion and the Fuzion instrument is positioned again without the 9 mm shim. FuZion ® instrument is tensioned to equally tension the medial and lateral compartments in flexion. ROSA knee will record the flexion gap values and assess the femoral component rotation to match the extension gap. ROSA will then ideally position the 4-in-1 femoral resection bloc to obtain the ideal femoral rotation.

Trials and definitive implants

The trial implants are positioned, and the postoperative knee state evaluation is performed to assess the knee balance and range of motion (Fig. 5). If the results are approved, the final components can be implanted.

Advantages and limits

The characteristics of this system are simplicity, preservation of the surgical flow, minimization of extra-time related to robotic surgery, while achieving high levels of accuracy for the orientation of the bone cuts and resection thickness.

The advantages and limits of the ROSA system as compared to other available systems can be summarized as follows:

Radiographs in standing position

Compared to CT scans, the use of plain radiographs is less costly, requires less radiation exposure, and is less Fig. 4 When the jig is fixed to the bone and the robot is locked in place, an extremely solid construct is provided, allowing the bone cut by the surgeon inconvenient to the patient [START_REF] Ponzio | Preoperative mapping in unicompartmental knee arthroplasty using computed tomography scans is associated with radiation exposure and carries high cost[END_REF]. The image-based system consists of a 3D virtual model derived from plain fulllength radiographs, whose the accuracy and the reliability have been recently demonstrated [START_REF] Masse | Using standard X-ray images to create 3D digital bone models and patient-matched guides for aiding implant positioning and sizing in total knee arthroplasty[END_REF]. The radiographs are performed in a standing position. Contrary to a CT scan performed in supine position, the radiographs of the ROSA knee show a knee in a functional position, that means a full weight-bearing knee. These radiographs allow a more functional analysis of the osteoarthritic knee. If needed, ROSA Knee can be also used without imaging, and still remains very accurate which could be also considerate as an advantage in terms of organization.

Collaborative robotic system

The ROSA knee system is considered as a collaborative robotic system, where the robot is holding and placing the cutting jig, while the surgeon keeps the tactile "feel" and remains in full control of sawing through the jig. The concept of the system is to complete the surgeon skills and not to serve as a substitute for the tasks that are easily and well performed by the surgeon. The sequence of bone cuts, the implant positioning, the limb alignment, and the targets of ligament balancing, can be individualized based on the surgeon's preferences.

This system must remain easy to manipulate and should not require much time for the set-up as compared to a conventional technique. That is why the landmark's acquisition is fast without bone morphing or preoperative planning. The planning during the surgery is also simplified to visualize all data on a few screens. To perform the bone resections, only one cutting jig is necessary, easily manipulated with the three modes of action. The system has been conceived not to add steps during the surgery, but just to help the surgeon to improve his accuracy for the bone resections and the ligament balancing.

Implants

This system is associated with femoral and tibial implants with satisfactory outcomes and with good positioning in international registries and the literature [START_REF] Bizzozero | Morphometric tibial implant decreases posterior overhang rate and improves clinical outcomes: results of a prospective, matched controlled study[END_REF][START_REF] Stulberg | Which tibial tray design achieves maximum coverage and ideal rotation: anatomic, symmetric, or asymmetric? An MRI-based study[END_REF][START_REF] Dai | Anatomic tibial component design can increase tibial coverage and rotational alignment accuracy: a comparison of six contemporary designs[END_REF][START_REF] Kim | Cementless and cemented total knee arthroplasty in patients younger than fifty five years. Which is better?[END_REF]. The tibial Fig. 5 Evaluation with the trials or definitive implants to assess the range of motion, the gap balancing in flexion and extension component is anatomical, allowing an optimal bone coverage [START_REF] Stulberg | Which tibial tray design achieves maximum coverage and ideal rotation: anatomic, symmetric, or asymmetric? An MRI-based study[END_REF][START_REF] Dai | Anatomic tibial component design can increase tibial coverage and rotational alignment accuracy: a comparison of six contemporary designs[END_REF]. This "off-the-shelf" knee arthroplasty is appropriate for the majority of the patients, with a large increment of femoral sizes. The aim of these "personalized" implants associated to the accuracy of the robotic system is to restore more easily a native knee, without needing a customized implant.

Predictive robotics

With the rise of robotic surgery and new technologies, the surgical procedure becomes easier, more accurate, more reliable, which may represent an important step towards the improvement of patient outcomes and satisfaction after TKA. One of the missing features currently is mainly the target: "where should we go?" Understanding how other groups of variables (other than the surgical quality), such as patientspecific characteristics, knee deformities, per-operative settings influence clinical outcomes, becomes increasingly important [START_REF] Bini | Machine learning algorithms can use wearable sensor data to accurately predict six-week patient-reported outcome scores following joint replacement in a prospective trial[END_REF][START_REF] Kunze | Development of machine learning algorithms to predict patient dissatisfaction after primary total knee arthroplasty[END_REF]. Conceivably, using relevant data points incorporated into an algorithm (preoperative clinical data, surgical data), the surgeon could improve his practice with this predictive model and incorporate this machine learning in the robotic system. That is why the OrthoIntel Orthopedic Intelligence Platform connects the pre-, intra-and postoperative data gathered through the mymobility application and allows analyzing surgical decisions with quantifiable data provided by the robotic system. The concept is to "close the loop": learn from every surgery to make the next one better. The analysis of surgical data combined with clinical outcomes could improve progressively the TKA procedure.

Limits

Several limits exist also with this robotic system. First, all robotic-assisted TKAs have a significant cost and cannot be available for every surgeon. Second, the significant improvement of the functional outcomes of patients with the use of robotic system is not yet demonstrated. Third, there is also a learning curve with this robotic system, mainly for the surgical planning. Fourth, like every robotic or navigation system, the surgical planning on the screens can be difficult and needs some experience to be relevant and efficient. The data reported on the planning screens are numerous and can sometimes be confusing for the surgeon. And if the surgical planning is not appropriate, there is no feedback of the robotic system to improve the planning currently. Finally, the specific complications of the robotic system, such as breakage of pins or fracture on the pins holes, remain uncommon and can be avoided with better pin placement techniques [START_REF] Beldame | Pin track induced fractures around computer-assisted TKA[END_REF].

Pre-clinical validation

This robotic system is recent and currently no clinical series has enough follow-up to report clinical outcomes. Nevertheless, two cadaveric studies have reported the accuracy of this device. Parratte et al. [START_REF] Parratte | Accuracy of a new robotically assisted technique for total knee arthroplasty: a cadaveric study[END_REF] have demonstrated the accuracy and the reproducibility of this robotic system with an imageless mode in a series of 30 cadaveric knees. The authors have compared three different measurements: the per-operative planning of the bone cuts performed with the ROSA knee system, the real bone cuts measured with a validated computer-assisted navigation system (ORTHOsoft; Zimmer Biomet) [START_REF] Scholes | Patient-specific instrumentation for total knee arthroplasty does not match the pre-operative plan as assessed by intraoperative computer-assisted navigation[END_REF][START_REF] Lustig | Unsatisfactory accuracy with VISIONAIRE patient-specific cutting jigs for total knee arthroplasty[END_REF], and the resection thickness for each cut measured with a caliper. To standardize the procedure, the HKA target was 180°, with 90° for both the tibia and femoral coronal angles. They reported that the cuts performed using ROSA knee were very accurate. Regarding the resection angles, there were no significant differences between the planned and the measured values, except for femoral flexion that had a mean difference of -0.95°. For the resection thickness values, no differences were observed except for the distal medial femoral cut and for the medial proximal tibial cut. The HKA mean difference was computed at -0.03° ± 0.87°. In another cadaveric study, Seidenstein et al. [START_REF] Seidenstein | Better accuracy and reproducibility of a new robotically-assisted system for total knee arthroplasty compared to conventional instrumentation: a cadaveric study[END_REF] have compared the accuracy of the ROSA knee system with a conventional TKA technique. There were two TKA groups: a conventional group of 20 knees operated by conventional technique and a robotic group of 14 knees operated with the ROSA knee system. Each bone resection was controlled by the handheld validation tool of the robotic system and measured with a caliper. The accuracy of bone resection angles was significantly improved for all values in the robotic group compared to the conventional group (p < 0.05). For the robotically assisted knees, the accuracy of all bone resection angles was below 0.6°, except for the sagittal femoral cut. The accuracy of all bone resection levels was below 0.7 mm. All values of the robotic group had 100% of cases within 2 mm of target, except for the distal femoral resection (93%). This robotic system provided accurate bone resections with fewer outliers compared than conventional technique. The distal femoral cut is a little less accurate than the others (sagittal plan and thickness) but remains more accurate than with the conventional ancillary. These results are similar to other TKA robotic systems [START_REF] Hampp | Robotic-arm assisted total knee arthroplasty demonstrated greater accuracy and precision to plan compared with manual techniques[END_REF][START_REF] Bollars | Preliminary experience with an image-free handheld robot for total knee arthroplasty: 77 cases compared with a matched control group[END_REF][START_REF] Casper | Accuracy assessment of a novel image-free handheld robot for total knee arthroplasty in a cadaveric study[END_REF][START_REF] Jeon | Robot-assisted total knee arthroplasty does not improve long-term clinical and radiologic outcomes[END_REF][START_REF] Koulalis | Sequential versus automated cutting guides in computer-assisted total knee arthroplasty[END_REF]. The clinical studies are in process to assess the outcomes for the patients.

Conclusion

The ROSA knee system is a robotically assisted semiautonomous surgical system with some specific characteristics compared to the previous robotic system. The aim of this collaborative robotic system is to improve the accuracy and the reliability of the bone resections and the ligament balancing, without replace the steps well performed by the surgeon. The preliminary results of this system reported a good reproducibility and accuracy in the TKA procedures.

Funding There is no funding source.

Declarations