Jarray Mohamed

Henry Yann

Fatu Aurelian

Souchet Dominique

Sealing Technology

FEATURE

Contactless seals are mainly used in order to avoid possible damage caused by friction. This work focuses on the study of one type of non-contact seal -the viscoseal, which is also known as the helical groove seal.

).

Advantages

The viscoseal has multiple advantageous characteristics, such as zero leakage, radial compactness and, most importantly, an inherent long service life because there is no solid contact between the stator and the rotor, which provides low friction and a low wear rate. However, the viscoseal requires a high degree of radial and axial precision and its initial cost is relatively high.

Although the viscoseal has several advantageous characteristics compared with other types of sealing devices, only a few studies exist, which focus on the experimental validation of numerical models that aim to predict its performance.

Historically, Asanuma was the first to consider the problem -both theoretically and experimentally. In 1951 Asanuma [1] started a series of experimental and theoretical studies of the viscoseal that aimed to determine the optimum channel configurations to achieve effective sealing.

For instance, he found that, for maximum sealing performance, the seal helix angle should be 11°; the groove density should be 0.5 and the thickness ratio, the ratio of the clearance to the clearance and groove depth sum [c/(c + h 0 )], should be 0.2. He also observed that the performance of the grooved shaft within a plain sleeve was the same as a plain shaft within a grooved sleeve.

In 1965, Ludwig et al. [2] analysed the effect of the helical profile of the seal on the pressure distribution, and found that each groove-land pair repeats the same pressure pattern around the seal in a saw-tooth profile.

At about the same time, McGrew and McHugh [3] designed a test facility that included a vertically mounted viscoseal with fluid above the grooved rotor. They predicted that eccentric operation of the viscoseal could cause as much as a 50% increase in the sealing coefficient obtained under concentric conditions. However, one year later, contrary results were found by Bowman. [4] In this case experimental evidence indicated that the effect of eccentricity on the laminar regime is negligible.

Stair [5] reported in his test series a phenomenon termed "air ingestion", which was observed. Air ingestion is a condition in which air is ingested -by the action of the grooves exposed to air -into the seal. Stair reported that air ingestion was not observed during laminar operation. This article, which is published in two parts, describes research that focuses on the analysis -through an experimental study -of the sealing behaviour of a non-contact dynamic sealing device known as a viscoseal (or sometimes called a helical groove seal). Despite its reliability and its advantageous characteristics, compared with other types of sealing devices, the viscoseal is still under-researched because of its complex characteristics. The experimental apparatus used in this work is equipped with two different test cells. This first, made from acrylic glass, is transparent and is used for the visualisation of the liquid-air interface within the viscoseal. The second cell, used for pressure measurement, is composed of bronze. The experimental results are compared with numerical results obtained using the Reynolds model and also with computational fluid dynamics code. The first instalment, which appears here, includes a general introduction and covers the experimental set-up and relevant references. Part II will be published in the September issue.

Experimental and numerical investigation of a viscoseal: part I

Sealing Technology

August 2019

FEATURE

However, as the rotational speed and turbulence increased, air bubbles began to appear in the pressure tap lines. Air ingestion tended to decrease the pressure gradient near the atmospheric boundary of the seal which, in turn, increased the wetted length in the seal.

In other research, [6] Stair conducted an experimental study to investigate the influence of a viscoseal's groove geometry on the sealing coefficient for a Reynolds number varying from 20 to 6000. Stair particularly stated that the viscoseal performance in the turbulent regime is relatively insensitive to variations in the ratio of the axial groove width to the groove depth.

More recently, Kanki and Kawakami [7] studied the leakage characteristics of a viscoseal, and found that the pumping effect of the groove becomes dominant at a high rotational speed, leading to a decrease in the leakage flow-rate.

Exclusively numerical

Most recent research covering the viscoseal is exclusively numerical.

For instance, Meng Zhang et al. [8] studied the friction on the viscoseal for different groove angles, using computational fluid dynamics (CFD) code. A simulation was performed in steady state, employing water as the sealing liquid. The results showed that the friction in the seal has an upward trend with a reduction in the groove angle.

More recently, Targaoui et al. [9] investigated the optimal geometrical characteristics of the viscoseal by applying the finite element method, using a model based on the Reynolds equation. The authors based their analysis on the sealing length, that is, the wetted length in the seal.

The work done by Targaoui et al. showed that in order to provide highly satisfactory sealing performance, the groove angle should be between 20° and 40° at a high speed of rotation, the thickness ratio should exceed 0.35, and for a groove angle of 30°, the groove density should not exceed 0.75, without falling below 0.25.

Jarray et al. [10] investigated the suitability of an interface-tracking approach to predict viscoseal performance. Good concordance between previous experimental work and numerical results were found. Souchet et al. [11] studied viscoseal performance in the laminar and turbulent regime and proposed a numerical model that is able to predict viscoseal performance with acceptable accuracy in the turbulent regime.

The main purpose of the work that is described here is to build an experimental apparatus that is capable of testing viscoseal performance by simulating industrial operating conditions.

Experimental set-up

In order to study the sealing performance of the viscoseal, we designed the test system shown in Figure 2. The test area includes the following main parts:

• a hydraulic power station, containing a cooling system with an air-oil exchanger, to supply 10 l/min of oil at a maximum pressure of 1 MPa (and also enable the pressure to be regulated from 0 MPa to1 MPa); • two electrical cabinets -one for the power supply and the another containing the data acquisition system; • test-bench; and • control station.

Mechanical characteristics

The pressure regulation in the cell is controlled by a hydraulic power unit, where an air-oil exchanger keeps the oil supply at a temperature set by the user. The unit provides the test sleeve with sealant oil that can reach a flow rate of 10 l/min at a controlled pressure. Oil returning from the cell is collected in a sump tank. By using this arrangement, the oil is given more time to settle in the tank in order to prevent the formation of air bubbles. The sealant tank contains a relief valve and pressure gauges to monitor the sealant supply pressure, and to guarantee a safety backup for the pressure regulators.

An electric motor with an operating range of between 500 rpm and 4000 rpm is used to rotate the shaft in the system. The speed of the motor is adjusted by a regulator and monitored by data acquisition software. Figure 3 shows a schematic drawing of the experimental apparatus containing the viscoseal.

Two viscoseal housings were tested -one is made from transparent acrylic glass poly(methyl methacrylate) (PMMA), with an interior diameter of 60.738 mm (Figure 3), whilst the second one, which is made from bronze (Figure 4), has an interior diameter of 60.598 mm.

The transparent housing enabled the rate of oil flow and the sealing length to be observed visually, whilst the bronze housing enabled the pressure along the viscoseal to be measured, via equally distant holes along the cell.

The stator is mounted on two rolling bearings in order to allow an axial degree of freedom. The labyrinth seal is used to ensure that leaked lubricant is collected. Since the labyrinth seal is a contactless seal, its effect on torque measurement is negligible.

Considering the small radial clearance between the seal and the housing, seal concentricity is important and should be maintained within certain limits of precision. Hence, during the mounting of the housing, we conducted a careful and lengthy eccentricity manoeuvre to guarantee that the concentricity with the shaft was lower than 15 µm. Centring is ensured by fine-tuning micrometre screws attached to the housing.

The geometrical characteristics of the viscoseal employed in this test are shown in Table 1. In the table, n is the number of grooves; D the diameter; β is the helix angle; h 0 ,the groove depth; L is the seal length; and Χ is the groove density. Χ = I g /(I g + I r ), where I g is the groove width normal to the groove and I r is the ridge width normal to the groove (see Figure 1).

Cell instrumentation

The purpose of the test-rig is to operate the mechanical seal and record its performance data in real time.

The bronze cell is equipped with nine thermocouples and eight pressure sensors. The distribution of hydrodynamic pressure along the axial direction is an important data output, because besides the pressure distribution, the pressure gradient can give an indication of the effective seal length, which can indicate the increment of pressure per increment of seal length. Therefore, it is important that the limit of precision on the pressure indication system is as small as possible. Hence, in order to obtain the pressure distribution of the sealing elements located inside the housing, 12 pressure sensorsequally spaced (10 mm) apart from one another -are mounted on the housing. The sensors used are the Keyence GP-M10 model. They have a measuring range of -0.1 MPa to1 MPa (with 1% of measurement precision). Data acquired from the sensors are transferred to the 16-channel data acquisition device (manufactured by National Instruments).

Pressure-measuring holes of 0.5 mm were made in such a way that they had a negligible effect on the results.

Figure 5 presents details of the numerical simulation that was carried out using ANSYS

FEATURE

Sealing Technology

August 2019

FEATURE

Fluent software to illustrate the influence of the sensor holes on the pressure distribution in the lubricant channel along which they were expected to be made. A velocity of 2.5 m/s was applied to the inlet (on the left side). Data are extracted along the x axis for y = 0.25 mm, and y = 0.1 mm. As can be seen from the figure the disturbance induced by the holes, through which the measuring instruments pass, is of the order of 2-4 Pa, which is extremely low, and hence, negligible. Therefore, it is assumed that the measuring holes do not affect the measurements.

Metrology

One important consideration that should be taken into account, before mounting the experimental apparatus, is the circularity and vertical straightness of the housing and viscoseal.

Considering the small clearance space, an error in the circularity and vertical straightness may lead to measurements being incorrectly interpreted. Therefore, measuring these features after manufacturing is of paramount importance. Circularity and vertical straightness tests were conducted using an ultra-precision roundness tester (Talyrond 365 instrument). Several tests were conducted to ensure that the measurements were represented correctly in relation to the sample.

Since milling PMMA is difficult, we focus primarily on the transparent test-cell. Figure 6 presents the vertical straightness measured along fours vertical lines 90° apart, around the transparent housing. It can be seen that the maximum deviation occurs at the cell extremity, inlet side, and does not exceed 20 µm.

Geometrical measurements of circularity were also carried out. Different circularities were measured at subsequent locations along the cylindrical axis. The plot obtained, shown in Figure 7, shows the maximum, minimum and average "out of roundness" taken across 13 planes, equally spaced along the housing. It is perhaps worth noting that the out of roundness is the maximum deviation inside and outside a perfect reference circle.

The reference circle is defined by the radial separation of two concentric circles positioned to enclose the measured profile such that their radial departure is minimum. The black plot in Figure 7 represents the average circularity. It was found that the circularity error of the transparent housings was less than 20 µm.

Figure 8 shows circularity measurements across the bronze cell. These were obtained using the Talyrond 365 instrument. The bronze test-cell has a higher manufacturing accuracy, with an average interior radius of 60.598 mm.

Figure 9 shows the viscoseal and the vertical straightness obtained along two vertical lines, 90° apart, around the seal. An imperfection in the bottom of the groove is noticed and should be kept in mind when analysing the experimental results. It is noteworthy that numerical tests were conducted to assess the effect of this imperfection. It was found that it has a negligible effect on the pressure distribution and sealing length.

In parallel, measurements using a threedimensional measuring machine (Mc 1004 E) made it possible to accurately determine the diameter of the grooved shaft. The average radius measured 60.016 mm at room temperature (25°C). The radial clearances of the transparent test-cell and bronze cell are 0.361 mm and 0.291 mm, respectively.

Control station

A control station located (see Figure 2) near the experimental apparatus was used to control of the systems used in this experiment. The control software was written in the LabView development environment (National Instruments).

During the entire signal-acquisition process, the LabVIEW program monitored, controlled, analysed and displayed the experimental results, with the data acquisition being managed by subroutines that enabled recording at an imposed frequency and sampling size.

Security conditions were also incorporated into the software to stop the test-bench in the event of an anomaly. The conditions for initiating the security process are based on a critical threshold of the friction torque, pressure, supply flow-rate or average oil temperature. The userinterface for controlling the test-bench is shown in Figure 10.

Start/stop buttons are located in the upper left, whilst indicators in the bottom left give instantaneous temperature measurements. In the centre the rotational speed is displayed beside the two torque measurement dials, and immediately

Next month

Part II, which will be published in the September 2019 issue of Sealing Technology, will present the results by comparing the experimental findings with the predictions of numerical models, and provide a conclusion. This feature article is based on a paper entitled 'Experimental and numerical investigation of viscoseal' that was presented at the 17th EDF-Pprime workshop, which was held in EDF Lab Paris-Saclay, France, on 4 October 2018.

For further information on this conference, contact:

Figure 1 .

 1 Figure 1. Illustrative view of the viscoseal.

Figure 3 .

 3 Figure 3. Experimental apparatus -viscoseal housing.

Figure 2 .

 2 Figure 2. Experimental test system.

Figure

  Figure 4. Experimental apparatus -bronze housing. n D (mm) β (°) h o (mm) L (mm) Χ 4 60 20 0.6 150 0.5

Figure 5 .

 5 Figure 5. The pressure distribution in the seal channel along which sensor holes are expected to be made.

Figure 6 .

 6 Figure 6. Vertical straightness of the transparent cell, measured along four lines equally spaced by 90°.

  Figure 7. Circularity measurements along the transparent cell.

  Figure 8. Circularity measurements along the bronze cell.

References 1 .

 1 Figure 9. Tested viscoseal (a); close-up view of the groove (b); vertical straightness in the grooved surface (c); and planar reconstruction of the seal based on the groove depth (d).

  17th EDF-Pprime workshop 'Green sealing: how to combine both low leakage and low friction?', Noël Brunetière, InstitutPprime -UPR 3346, Dépt. Génie Mécanique et Systèmes Complexes, CNRS -Université de Poitiers -ENSMA, SP2MI -Téléport 2, Bd Marie et Pierre Curie -BP 30179, F86962 Futuroscope Chasseneuil Cedex, France. Tel: +33 5 4949 6531, Fax: +33 5 4949 6504, Email: noel.brunetiere@univ-poitiers.fr, Web: http://edf-pprime-2018.sciencesconf.org.

Figure 10 .

 10 Figure 10. User-interface for bench control.

Table 1 . Viscoseal characteristics.

 1


François Bonne

Christine Hoa
email: christine.hoa@cea.fr

Quentin Le Coz

Louis Zani

Benoît Lacroix

Jean-Marc Poncet

Optimization of the cooling capacity of the cryo-magnetic system for EU DEMO at the pre-conceptual design phase

Keywords: DEMOnstration fusion reactor (DEMO), fusion, simulation, refrigeration, optimization

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In the framework of EU DEMO pre-conceptual design phase, different Toroidal Field (TF) coil conductor designs are under study [START_REF] Corato | Progress in the design of the superconducting magnets for the EU DEMO[END_REF]. In the thermal hydraulic analyses [START_REF] Vallcorba | Thermohydraulic analyses on CEA concept on TF and CS coils for EU DEMO[END_REF], the inlet temperature and pressure of the supercritical helium flow are commonly chosen at 4.5 K and 5 bar to simulate the cooling circuits of the conductors, with a maximum pressure drop of 1 bar. These boundary conditions and design parameters rely on previous magnet designs (JT-60SA, ITER) and an arbitrary fixed interface with the cryogenic system. This study aims at investigating these common values and propose an optimization method to calculate the total refrigeration power on a complete model of the supercritical helium loop and a liquid helium bath (cold source) connected to the cold end of the refrigerator (Fig. 1). CEA has developed Simcryogenics [START_REF] Bonne | A simulink library of cryogenic components to automatically generate control schemes for large cryorefrigerators[END_REF] to model and perform optimization process calculations.

1D thermal hydraulic model for the conductor

Simcryogenics model

The Simcryogenics model (Fig. 2) is composed of a liquid helium bath (PS1) supplied by a Joule-Thomson valve (CV1). The bath pressure is regulated by the cold compressor C1. Two heat exchangers are immersed in the liquid helium bath and extract the thermal loads of the supercritical helium loop driven by the cold circulator C2. The 111.56 kA TF conductor is a CEA design inspired from ITER magnets, with pancakewound coils, using Nb3Sn cable in conduit conductor (CICC) [START_REF] Corato | Progress in the design of the superconducting magnets for the EU DEMO[END_REF][START_REF] Vallcorba | Thermohydraulic analyses on CEA concept on TF and CS coils for EU DEMO[END_REF] (Table 1). The CICC is modelled with two 1-D pipes in parallel for simulating the repartition of the flows in the bundle and the central channel. The friction factor and heat transfer coefficient correlations are described in [START_REF] Vallcorba | Thermohydraulic analyses on CEA concept on TF and CS coils for EU DEMO[END_REF]. The heat transfer between the central channel and the bundle has been modeled and the mass transfer has been assumed negligible.

Boundary conditions

The thermal loads on the TF conductor (mostly nuclear heating and a small contribution of conductor losses) is non-uniformly distributed along one pancake length (Fig. 3). The total amount of loads is about 10 Watts. This thermal hydraulic analysis includes the thermal coupling of the inter-turn along the conductor and also the thermal coupling between two adjacent pancakes. One TF coil is composed of eight double pancakes and each pancake has 8 turns featured on Fig. 4 although the two last turns are less visible. The current sharing temperature is derived from the magnetic field distribution along the TF conductor (Fig 4). The temperature margin is critical in the first 50 meters of the conductor, where the magnetic field is the highest.

The total refrigeration loads are calculated at the interface of the refrigerator with an enthalpy balance calculation. At the interface with the cold end of the refrigerator, the high pressure flow is fixed at 16 bar and 5 K upstream the Joule Thomson valve, whereas the outlet of the compressor is fixed at 1.3 bar. These boundary conditions are common values in the helium refrigeration process of tokamaks, for example in JT-60SA [START_REF] Hoa | Performance of the JT-60SA cryogenic system under pulsed heat loads during acceptance tests[END_REF].

The total refrigeration load covers the thermal load along the TF conductor, the static heat losses along the cryogenic lines but includes also the pumping powers of the cold circulator and the cold compressor, which represent a cryo-distribution load depending on the process parameters ∆ and .

Optimization problem

The optimization problem aims at minimizing the refrigeration power by playing on the pressure drop (or the mass flow) along the supercritical helium loop and the cold source Tbath under the constraint that the temperature margin along the conductor, with respect of the current sharing temperature, is higher than ∆ = 1.5 !.

" #$% (∆", & '()* ) = + ,-.: " #$% 0. ). : & 1-11 (2) ≤ & 10 (2) -5& ,
The THEA+Cast3M model is run again with the optimized boundary conditions (∆ , ℎ ) 6 to evaluate an update of the heat load profile. Several iterations are performed to converge towards an optimal set of thermal-hydraulic parameters (∆ , ℎ ) 6 .

Optimization results

Reference conditions

The reference thermal hydraulic boundary conditions based on previous magnet design for JT-60SA and ITER are commonly: ∆P=1 bar (corresponding to a mass flow of 12.5 g/s per conductor) and = 4.45 K, which corresponds to 4.5 K at the inlet of the TF coil. The total required refrigeration power is 23.6 watts (Fig. 6). A large contribution is due to pumping power of the cold circulator C2: 13 W (55%).

Optimization

The optimization algorithm finds a minimum refrigeration power of 13 W after three iterations with the following thermal hydraulic parameters: ∆ 6 = 0.23 bar, corresponding to a mass flow of 5.35 g/s and = 4.16 !.

The total refrigeration power has been reduced from 23.6 to 14.5 W (-39%). The algorithm has optimized the cryo-distribution loads with a trade-off between the cold circulator and the cold compressor pumping powers. The energy saving is mainly due to the decrease of the pumping power of the cold circulator C2 from 13 W to 1.2 W. The cold compressor C1 for pumping the liquid helium bath down to the saturated pressure corresponding to 4.16 K, requires 2.9 W instead of 0.8 W (4.45 K). Its contribution has increased from 3% to 12% of the total refrigeration load. The additional cost required to reach 4.16 K in the bath is largely counterbalanced by the decrease of the mass flow and pressure drop of the SHe loop (5.35 g/s and 0.23 bar).

Exergetic comparison

From an exergetic point of view, the rigorous comparison is performed by calculating the equivalent refrigeration powers at 300 K. With an ideal Carnot factor, the refrigeration powers at 300 K would be 2.9 kW for cooling one conductor pancake with the reference conditions. The optimized case would require at 300 K 1.7 kW (-44%). The real process of large helium refrigerator is known to have efficiency around 25% with respect to Carnot factor, so the expected electrical powers are much higher. The global efficiency depends on the other cooling processes related to the cryogenic users, not only the TF magnets but also the central solenoid and poloidal coils. The thermal shields cooled at about 80 K and the high temperature superconducting current leads cooled at about 50 K also contribute to the total refrigeration power, with specific efficiency related to different temperature levels.

Conductor temperature profiles

The resulting spatial distribution (after optimization) of the conductor temperature 9699 (:) is described in Fig. 7 and is compared with the distribution with the reference conditions (before optimization). The temperature profile has not much changed, except for the first 50 meters, where the temperature of the conductor is close to the acceptable margin of 1.5 K with respect to the current sharing temperature. For this double pancake design, the driving parameters for the cooling is related to the first 50 meters.

Conclusion and perspectives

A Simcryogenics model and an algorithm have been developed to optimize the thermal-hydraulic parameters of the cryo-distribution of one conductor pancake. An optimal configuration has been found which reduces significantly the equivalent refrigeration power at 300 K (-44%). Hence the common values for the boundary conditions (∆P=1 bar, 6; < = =4.5 K) of the TF coil can be revised for an optimization of the refrigeration power.

In the case of the CEA design with pancake wound conductor, a lower pressure drop (0.23 bar) and a lower inlet temperature (4.16+0.05K) could benefit from a reduced refrigeration, with an acceptable temperature margin> 1.5 K along the conductor length.

The model and the algorithm will be relevant for comparing different conductor designs (graded layer designs) [START_REF] Corato | Progress in the design of the superconducting magnets for the EU DEMO[END_REF]. This optimization method is well adapted to the pre-conceptual studies on TF conductors and can be included in the iterative conductor design calculations in complement with the thermal hydraulic analyses.

The Simcryogenics model would be also improved to include the inter-turn transverse coupling along one pancake which has been accounted for in the THEA/CAST3M model and which also includes the cooling channels of the structures in the present study. Hence the optimization process could be simplified with a single calculation with Simcryogenics.

Another potential study of the Simcryogenics model is the evaluation of the impact of the temperature margin on the required refrigeration power.

Fig. 1 .

 1 Fig. 1. Optimization model of the refrigeration power of a supercritical helium loop

Fig. 3

 3 Fig. 3 Heat load profile distribution along the TF conductor

Fig. 5 6 .Fig. 6 .

 566 Fig. 5 Iterative process for optimal thermal hydraulic parameters (∆ ,

Table 1 . CEA Toroidal Field coil conductor characteristics

 1

	Conductor current (kA)	111.56
	Material		Nb3Sn -Cu
	(T)		12.3
	Central spiral (mm) o.d*i.d	10*8
	Bundle	(m)	0.48*10 -3
	Bundle Wetted perimeter (m)	5.39
	Void fraction (%)	29
	Friction factor Bundle	Darcy-Forchheimer
			correlation
	Friction factor Central spiral	Smooth tube
			correlation
	Heat transfer coefficient	Dittus Boelter
			correlation
	Hydraulic Length (m)	363.86
	Number of double pancakes	8
	Number of turns per pancake	8

Acknowledgments

This work has been carried out within the framework of the EUROfusion Consortium for the EU-DEMO magnet work package WPMAG. The views and opinions expressed herein do not necessarily reflect those of the European Commission.


Dr Harald Köpping

The Moon Village and Space 4.0: the 'open concept' as a new way of doing space?

During the first decades of the 21 st century, humanity's approach to space has undergone significant changes. The European Space Agency has referred to these changes as Space 4.0 (European Space Agency, 2016). Whereas the first space era dealt with astronomical observation, during the second space age nation-states sent objects and people into outer space. The third space age was marked by cooperation between national space agencies and culminated in the construction of the International Space Station. Space 4.0 is characterised by opening space to private and academic organisations and individuals. In terms of space exploration, recent discourse has been dominated by a debate on whether the focus on future human space travel should lie on the Moon or on Mars. Since 2005 there has been a tendency in favour of the Moon, although plans for future human Moon and Mars exploration have been pursued simultaneously. Nevertheless, Bush's Constellation programme and Obama's subsequent SLS/Orion programme have put NASA on course for returning astronauts to lunar orbit in the 2020s. Most recently, the Trump administration has expressed its intention to land American astronauts on the Moon by 2024. China's Chang'e programme aims at a sample return mission before the end of the decade, and Russia has signed a cooperation agreement with the US to build a space station in cislunar orbit. At the same time, private enterprises such as SpaceX and Blue Origin are attempting to make lunar exploration economically selfsustainable, with Elon Musk aiming to launch tourists to lunar orbit in the near future. As Stoica confirms, "all prominent space agencies are focusing their near term efforts on the Moon" (2018,1).

ESA too has contributed to the debate between the Moon and Mars having made a decisive turn towards the Moon. ESA chief Jan Wörner's idea of the 'Moon Village' makes the Moon the next goal of international collaboration in space exploration. However, being firmly rooted in the Space 4.0 paradigm, the Moon Village is conceptually very different from the SLS or Chang'e 5. During an interview which forms much of the empirical basis of this article, Wörner described the Moon Village as an open concept, which is merely meant to facilitate cooperation. Participation in the Moon Village is not limited to nations and space agencies, but to private companies who may not even necessarily hold an expertise in space matters.

So far, academics have paid very little attention to the Moon Village concept. Wörner's idea has been tremendously misunderstood, and even space policy experts continue to refer to the Moon Village as an attempt to colonise the Moon. This article therefore has three targets. Firstly, it places the Moon Village concept within the increasing international momentum for returning to the Moon. This will be accomplished using a discussion of the advantages of going to the Moon rather than Mars, whereupon the most important contemporary projects for returning to the Moon will be briefly summarised. Secondly, being based on an interview with Jan Wörner, the article intends to provide an overview of what the Moon Village actually is and how it came about. Rational choice and institutionalist arguments will be presented to explain the Wörner's motivations behind the Moon Village. The world café metaphor is then used to illustrate that the Moon Village is a process rather than a project. In the future, it will serve as an umbrella for different projects rather than as a managing organisation. Thirdly, it evaluates the feasibility of the Moon Village using a SWOT/PEST analysis. This provides insight into the strength and weaknesses of Wörner's proposal.

The new race to the Moon

The Moon has become popular again. All major space agencies as well as several commercial actors with credible space ambitions have set their sights firmly set on the Moon. After briefly summarising the most important arguments used in the Moon/Mars debate, this section will outline some contemporary lunar projects, highlighting that interest in the Moon is indeed experiencing a renaissance.

Moon or Mars?

In her 2017 book, The Politics and Perils of Space Exploration, Linda Dawson highlights how the debate between the Moon and Mars has shaped recent discourse on the future of human space exploration. Indeed, whichever target is chosen as a destination for human exploration, the agency carrying out this project will necessarily become path dependent (Bruggemann, 2002). The infrastructures required for landing and sustaining human beings on the Moon and on Mars are quite different. While both NASA's Space Launch System (SLS) and SpaceX's Falcon Heavy would theoretically be capable of landing humans on the Moon [START_REF] Bergin | Dual SLS launch campaign required for NASA's lunar return[END_REF], neither rocket could be used for human missions to Mars. Permitting the latter would require building an altogether new rocket, which including the design phase may easily take up to 15 years (e.g. Space Shuttle, see Bizony, 2011, 13). As the result, space agencies' discourse has been shaped by the idea that the Moon may serve as a "launching platform to deep space," as the lower lunar gravity makes it easier to transport heavy, crewed payloads deep into the solar system (Dawson, 2017, 72). Nevertheless, this approach is arguably naïve, as it ignores the technical difficulties required in getting to the Moon in the first place, apart from the current technical infeasibility of building a lunar spaceport. As a result, it is meaningful to briefly summarise the main arguments used by both sides of the Moon vs Mars argument, as this helps illustrate why the Moon has recently been favoured as the prime target destination.

Jan Wörner, Director General of the European Space Agency, puts forward several arguments for focusing on the Moon rather than Mars. Firstly, he claims that the Moon is "not as well researched as Mars" (personal communication, 20 November 2017). Arguably the historical political interest in the Moon was mostly based on geostrategic rather than scientific considerations. Secondly, the Moon could be used for scientific research, for example by building a radio telescope on the lunar far side. Such a device would be able conduct radio astronomy without interference by human-made objects, which may provide unprecedented insight into origins of the universe and the search for extra-terrestrial intelligence [START_REF] Burns | Observatories on the Moon[END_REF][START_REF] Fiebag | Mars: Planet des Lebens -Die Jahrtausendentdeckung der NASA[END_REF]. [START_REF] Galan | Telescope array for extrasolar planet detection from the far side of the Moon[END_REF] are proposing the construction of optical telescope on the far side of the Moon for detecting exoplanets in our interstellar neighbourhood. Thirdly, Wörner views the Moon as a testing ground for technologies needed for deep space exploration. Specifically, he refers to in situ resource utilisation (ISRU), where local resources are used to produce basic materials needed to sustain a Moon base (cf. [START_REF] Spudis | The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources[END_REF]. Another argument for the Moon as opposed to Mars is that a Moon programme may "spur investment from the private sector" (ibid.), which is less likely in the case of a Mars mission, given the higher cost.

In his book The Case for Mars Robert Zubrin, founder of the Mars Society and creator of the Mars Direct plan, provides several arguments against going back to the Moon. Firstly, he puts forward that "we have, after all, been to the Moon six times," implying that yet another Moon landing would not warrant the investment (2011, xv). Secondly, the examination of lunar geology is "trivial [...] in comparison with the questions of the origins and fundamental nature of life that would be addressed by the human exploration of Mars" (ibid.). The third major reason he puts forward against returning to the Moon addresses the question of American pride: "one wonders what it says about America if the highest aspiration of our space program is to repeat a mission it accomplished a half-century before" (ibid.). Apart from that, as was mentioned previously, the idea of the Moon as a testing ground for the technologies needed to get to Mars may be based on false assumptions. As Davis argues, the technologies required for resource extraction from lunar and Martian rocks are very different [START_REF] Davis | Let's talk about this whole Moon vs. Mars thing for human spaceflight[END_REF]. Moreover, the landing equipment required to get to Mars differs considerably from lunar space vehicles, as the former would have to work in the Martian atmosphere.

Zubrin's arguments for a human mission to Mars rest primarily on the planet's potential for colonisation. He puts forward that "in contrast to the comparative desert of the Earth's moon, Mars possesses veritable oceans of water frozen in its permafrost" (2011, xxv). He highlights further that Mars is abundant in all the resources needed for setting up a permanent settlement such as carbon, nitrogen, hydrogen and oxygen. Moreover, Mars possesses a "twenty-four-hour day-night cycle and an atmosphere thick enough to shield its surface against solar flares" (ibid., xxvi). Apart from the points Zubrin mentions, Mars is arguably more interesting as a scientific target, since it is more likely to harbour life than the Moon (see [START_REF] Kite | Persistent or repeated surface habitability on Mars during the late Hesperian-Amazonian[END_REF].

Jan Wörner argues the case against sending astronauts to Mars primarily on ethical grounds. Wörner argues that it is "ethically problematic to send someone away for two years without any opportunity to receive medical treatment" (personal communication, 20 November 2017). Nevertheless, there may be other, even more practical reasons against sending humans to Mars. Citing a NASA scientist, Davis emphasises that there are a number of technologies that still need to be developed before humanity travels to Mars, and that their development "may require several miracles" [START_REF] Davis | Let's talk about this whole Moon vs. Mars thing for human spaceflight[END_REF]. Apart from that, the greater distance between Earth and Mars implies that there would be a communications delay lasting between six and forty minutes. Apart from the fact that Mars astronauts would inevitably have to take many decisions without consulting Earth-based experts, this communications delay greatly increases the risk involved in Mars mission, as emergency interference from Earth would often be impossible. A final argument against sending humans to Mars relates to economics: it is simply cheaper to send people to the Moon rather than to Mars. Given the budgetary constraints the space agencies of Europe and the United States find themselves in, it may be politically impossible to acquire the funds necessary for a Mars mission, whereas a trip to the Moon is more feasible.

[Insert Figure 1] While there are good arguments for both the Moon and Mars, what ultimately matters most is how well these arguments resonate with policymakers. Recent years have been marked by a discourse that involved the idea that the Moon will function as a stepping stone to Mars (e.g. [START_REF] Rothery | How a New Orbital Moon Station Could Take Us to Mars and Beyond[END_REF]. Nevertheless, as was pointed out earlier, it is at best questionable whether the technologies tested on the Moon can simply be copied on Mars. Moreover, according to a Planetary Society Report, in order to "fund any executable human exploration program, NASA must end its large financial commitment to operating the International Space Station" (Hubbard et al., 2015, 10). Should the ISS eventually be replaced by another international programme, a trip to Mars is almost certainly off the table under the current financing framework. A choice between the Moon and Mars ought to be made, and given the higher cost and the higher risk of a Mars expedition, the Moon may "ultimately represent a more compelling political destination" [START_REF] Davis | Let's talk about this whole Moon vs. Mars thing for human spaceflight[END_REF].

Traditional space actors' lunar plans

Since the inauguration of Donald Trump the US has firmly shifted its focus in human space exploration to the Moon. In December 2017 Trump signed a new American Space Policy Directive which states that the "United States will lead the return of humans to the Moon for long-term exploration and utilization" [START_REF] Trump | Presidential Memorandum on Reinvigorating America's Human Space Exploration Program[END_REF]. In March 2019, US vice president Pence tasked NASA with landing humans on the Moon by 2024 "by any means necessary" [START_REF] Chang | The Trump Administration Wants Astronauts on Moon by 2024. But What's the Plan?[END_REF]. The current plan involves the construction of the Space Launch System (SLS), which is to become the most powerful rocket ever built. It will enable the United States and Europe to send astronauts beyond low-earth orbit for the first time since the Apollo era. The current schedule would send an uncrewed Orion capsule into orbit around the Moon in late 2020, although this date has changed many times in the past. A series of SLS launches would then be used to build the Gateway, a habitable space station in orbit around the Moon (NASA, 2018). Apart from an opportunity to test the life support systems required to keep humans alive beyond low Earth orbit, the Gateway will eventually serve as the mothership for lunar landings, using a yet to be designed lander.

The 5-year plan to put human beings back on the Moon mars a significant change from the indecisiveness of previous NASA human exploration programmes. It has the potential to generate American path dependency by necessitating investment in infrastructure that can only be used for lunar missions. Nevertheless, it ought to be stated that the feasibility of the plan is questionable. It appears unlikely that a human-rated lunar lander could be built within five years, considering that it took approximately 10 years to build the first Space Shuttle, and more than 15 years to build the SLS (which is still a work in progress). As Dreier writes, "there is no functional heavy-lift rocket, no functional spacecraft for deep space, and no lunar lander even under formulation" (2019). Nevertheless, Trump's space directive represents a clear reorientation of American space ambitions, and it is arguably the first time in recent US space policy that a choice was made between the Moon and Mars.

Like the United States, China is also pursuing a space policy that is aimed at eventually landing humans on the Moon. In 2019, China spectacularly became the first country to land a probe on the far side of the Moon. The Chang'e 4 probe was part of the Chinese Lunar Exploration Programme, which consists of a series of robotic launches to the Moon. The Chang'e 5 mission is scheduled to carry out the first lunar sample return since 1976 [START_REF] Aliberti | When China Goes to the Moon… Berlin[END_REF]. A sample return mission demonstrates the technical ability to land on and launch a vehicle from an object in space, therefore laying an important foundation for future crewed missions. Nevertheless, according to a January 2019 press conference China's next steps involve the construction of a robotic research station at the lunar South Pole. Space journalist Andrew Jones describes the idea as "likely similar conceptually to ESA's 'Moon Village' concept" (Jones, 2019). This is because the station is intended to be open to international partners, although China has so far been hesitant to have partners involved at critical stages of its exploration efforts. In the past China has also talked about potential lunar missions, including the construction of a human moon base [START_REF] Jones | Living on the Moon: A Chinese conceptual lunar base[END_REF]. US vice-president Pence has described the US as being in a space race with Russia and China [START_REF] Wall | US Is in a New Space Race with China and Russia, VP Pence Says[END_REF]. Nevertheless, while China's robotic exploration programme is very concrete and hitherto highly successful, China's crewed mission proposals remain relatively vague. However, what it clear is that China has its sights firmly set on the Moon.

Finally, Russia also maintains plans to send cosmonauts to the Moon. In the near future, Russia's space agency Roscosmos will cooperate with ESA to relaunch the Luna programme, which had enabled the Soviet Union to orbit around, land on, and collect soil samples from the Moon between 1959 and 1976. ESA and Roscosmos will cooperate on the Luna 27 mission, which will send a lander to the far side of the Moon. The new Luna-Glob programme has the goal of building a robotic Moon base by 2030. This will be enabled by the Angara-A5V rocket, which is currently on the drawing boards [START_REF] Zolfagharifard | Russia's plan to conquer the moon: Nation will send 12 cosmonauts to lunar surface ahead of creating a permanent base by 2030[END_REF]. So far, Russia has launched two Angara rockets in 2014, the A1 and the A5. All rockets of this series will rely on the same engines and first stages, but as opposed to the A5, the A5V requires a hydrogen-based upper stage, whereas the A5 relied on kerosene (Russia Today, 2016). Nevertheless, the 2014 launches show that Roscosmos is serious about the rocket, and that the basic hardware for the heavy-lift vehicles required for transport to the Moon is becoming available. Transport of humans to the Moon is thought to take place using the Ryvok capsule, which is a fully reusable shuttle between the International Space Station and the lunar surface. In the 2030s, Russia is planning to upgrade its robotic base to a human base, with up to ten cosmonauts operating on the Moon simultaneously. In 2017, Russia announced its participation with the NASA-led Gateway, which may undermine the Ryvok programme, as the ISS is no longer needed as a launch platform [START_REF] Zak | Russia will team up with NASA to build a lunar space station[END_REF]. Possibly Ryvok will use the Gateway as a launch platform instead.

Private actors

A turn towards the Moon can be witnessed not only among the world's most important government space agencies, but also among the increasing number of private space actors, most notably SpaceX. Elon Musk's well-known rocket company has been pursuing ambitious plans to colonise Mars. However, since US president Trump's decision to prioritise lunar missions Musk has put forward that SpaceX rockets may as well as used to build a base on the Moon [START_REF] Malik | Elon Musk Teases Images of SpaceX 'Moon Base Alpha' and 'Mars City[END_REF]. This shift in Musk's attitude is probably due to the fact that SpaceX relies on government contracts as its primary source of income. To maintain the US government's support, it was necessary to somewhat align SpaceX's exploration and colonisation strategy to that of NASA. Nevertheless, even before Trump's announcement, SpaceX was preparing to use its recently tested Falcon Heavy rocket to propel a crewed Dragon II capsule on a free-return trajectory around the Moon. This mission mimics the previously mentioned maiden flight of the SLS/Orion system. Until February 2018, SpaceX had intended to use Falcon Heavy for manned missions to cislunar orbit (SpaceX, 2017). This plan has since been cancelled, with more attention being given to the development of BFR (Big Falcon Rocket), which is meant to be used for landing on the Moon and Mars [START_REF] Foust | SpaceX no longer planning crewed missions on Falcon Heavy[END_REF].

Apart from SpaceX, Jeff Bezos' space company Blue Origin pursues very ambitious plans to permit American astronauts to return to the lunar surface. While NASA currently has no concrete plans to build a lunar lander, Blue Origin's Blue Moon lander would enable an SLS-based human mission to the Moon. Bezos has plans for such a mission for the mid to late 2020s. If SLS is unavailable, Blue Origin could also use its New Glenn launch vehicle, which is scheduled for its maiden flight in 2020. The dimensions and payload capabilities of the New Glenn rocket are comparable to the Falcon Heavy. Blue Origin's ultimate goal is to build a human colony on the Moon [START_REF] Davenport | An exclusive look at Jeff Bezos's plan to set up Amazon-like delivery for 'future human settlement' of the moon[END_REF].

Europe's lunar plans: the Moon Village

The previous section outlined two facts. Firstly, momentum is gathering to bring human beings back to the Moon's orbit and surface. Not only have the space agencies of Russia, China and the United States decided for the Moon and against Mars, but several actors are simultaneously developing, building and testing Moon rockets. SLS, Falcon Heavy and New Glenn could all launch humans into lunar orbit, and given the development of a lander, they could all enable humans to land on the Moon. Indeed, the path dependency these developments have brought about are very difficult to reverse by any US government. Since the Apollo era the chances for a human return to the Moon have never been better. Secondly, while the programmes outlined above are carried out by very different actors, they all share one essential characteristic: they all involve plans, mission architectures, concrete, pre-set goals and large-scale financial investment. The Moon Village is radically different.

In terms of space exploration, ESA is probably the second-most important space actor after NASA. Naturally, ESA has thus not remained quiet on the Moon/Mars question. Indeed, Jan Wörner's Moon Village proposal represents ESA's contribution to this debate. The Moon Village is characterised by its neat fit with the contemporary momentum in human space flight policy as well as by being utterly different from all other proposals hitherto discussed. As will be shown in this section, the Moon Village is not a traditional space programme, but rather an open concept, which will be explored using the analogy of the world café conference. Arguably it is precisely this feature, that may determine its chances for success.

Understanding Jan Wörner's lunar dreams

The Moon Village concept is undoubtedly the brain child of ESA director general Jan Wörner. However, since Wörner's idea began making headlines in 2015, the Moon Village has arguably remained the most misunderstood currently discussed concept for lunar exploration. The Moon Village is thus often understood to as a European attempt to build a moon base or a lunar colony. Stenzel et al. describe the Moon Village as a project intending to "enable a group of humans to live and work for a certain period of time on the surface on the moon" (2018,8). In article on additive manufacturing Labeaga-Martínez et al. state that the "aim of [the Moon Village] project is to establish a Moon colony" (2017, 796). As will be shown, these descriptions arguably miss the central point of Wörner's vision.

To better understand the Moon Village, it is meaningful to explore what motivated Wörner to pursue his idea. Indeed, based on an interview with Wörner that was carried out in November 2017, it will be argued that he was motivated by scientific, political and organisational considerations. As such, the emergence of the Moon Village concept differs significantly from the emergence of past visions for space exploration. The most widely known interpretation of the American decision to launch the Apollo programme for instance, is that Apollo was a means of competition with the Soviet Union at the height of the Cold War [START_REF] Logsdon | The Decision to Go to the Moon: The Space Program and the National Interest[END_REF]. While other explanations have been offered [START_REF] Launius | Kennedy's Space Policy Reconsidered: A Post-Cold War Perspective[END_REF], none of them cite the institutional culture of NASA as the underlying motivation behind Kennedy's decision. In the case of the Moon Village however, it may be argued that the concept's architecture is not only the result of 'rational choice', but that it is an outgrowth of ESA's cooperative approach to space exploration.

Wörner's has publically declared his scientific interest in the Moon on many occasions. In a blog post on the ESA website, Wörner describes the Moon as "an archive of Earth's early history" (2016). He views the far side of the Moon as a potential site for a "radio telescope […] to stare deep into the Universe without any interference from human made signals" (ibid.). Apart from that, as was mentioned previously, Wörner argues that "if you want to fly deeper into space, there are many technological challenges that you can try out on the Moon" (personal communication, 2017). He specifically refers to ISRU, which can be tested "perfectly" on the lunar surface (ibid.). Wörner further highlights that the Moon is less explored than Mars and more reachable than the Red Planet. This explanation for Wörner's interest in the Moon Village is closest to the rational choice approach, which has been used to account for Kennedy's decision to go to the Moon.

Politically, Wörner wanted to come up for a project for fostering international cooperation. As such, he views the Moon Village as a social experiment and a "hope for society" (personal communication, 2017). In the interview, he explicitly stated that it would be unwise to export Earth's political divisions and structures to the Moon: "I am also interested in the geopolitical aspect, in working internationally, beyond borders and boundaries of thought, without re-erecting material and psychological fences" (ibid.).

Wörner expressed his strong disapproval of "suggestions" to build "fences and walls" on the lunar surface to protect "national interests" (ibid.). From his point of view, "replicating national interests on the Moon […] would be the death of this development [of space exploration]" (ibid.). Wörner sees the Moon Village as an "opportunity for space science and a chance for society to try out something different" beyond "the capitalist or the communist solution" (ibid.).

Finally, Wörner was motivated by his pursuit to experiment with new ways of project management in space. In fact, he would reject the term 'project' with reference to the Moon Village, and he would argue that the Moon Village requires no 'manager'. Up until the present day, ESA has carried out projects and programmes. ESA projects have budgets, aims and schedules. All involved actors have clearly defined responsibilities. These projects are either government funded or the result of public-private partnerships. Wörner hopes that the Moon Village will be the first example of a new type of endeavour he calls the 'open concept', which he defines as follows:

"It is not a single project. It is not something, where schedules are clearly defined. It is not something, where I define every step. It is not something, where partners are clearly defined. It is not something, where all interfaces are regulated. But it is an open platform, an open idea, that different actors can participate in" (personal communication, 2017).

This can be interpreted in different ways. According to Wörner, the Moon Village is thus an example of Space 4.0. While the American term NewSpace refers merely to the privatisation and commercialisation of space travel that began during the 1980s, Space 4.0 is a wider term that encompasses further developments in the space sector. As was discussed in the introduction, Space 4.0 finally refers to "a time when space is evolving from being the preserve of the governments of a few spacefaring nations to a situation in which there is the increased number of diverse space actors around the world, including the emergence of private companies, participation with academia, industry and citizens, digitalisation and global interaction" (European Space Agency, 2016). This poses the challenge of integrating the space sector with the economy as a whole, which involves providing opportunities for actors of all kinds to become involved with space-based projects. Wörner thus intends for the Moon Village to become a means to multiply and diversify the actors involved with space exploration.

Despite Wörner arguing that the Moon Village is different from past ESA 'projects', his statement that the Moon Village "is an open platform, an open idea, that different actors can participate in" (personal communication, 2017), is remarkably reminiscent of ESA culture in general. ESA is an international organisation that is based on the cooperation of different national governments. It is not an exclusive organisation and exists side-by-side with European national space agencies. ESA is open to participating in other space agencies' projects as a junior partner. The organisation itself is thus firmly geared towards cooperation. Moreover, ESA funding is made up of the member states' mandatory contribution as well as of optional programmes, which member states may or may not sign up to. This is similar to the Moon Village's principle that participating partners do not have to be involved with all aspects of the Moon Village, but only with those elements they consider useful. This arguably underlines that the Moon Village is a translation of ESA culture to the international level. As opposed to the rational choice approach that is typically used to explain the emergence of the Apollo programme, an institutionalist point of view may be better suited to explain the emergence of the Moon Village concept.

The Moon Café

As opposed to the American, Russian and Chinese lunar plans, the Moon Village cannot be understood as a project. Indeed, Wörner intended to convey the basic meaning of his idea by calling it a Moon 'village' instead of a Moon base or even a Moon colony. According to Wörner, "a village is something that is not decided upon by a government. When I think of a village on earth, a village emerges when different actors say, 'this is a nice place'. Here we could set up a common settlement. Here we can found a community. Here we can live together and support one another" (personal communication, 2017). From Wörner's point of view, a village is a conglomerate of interests rather than a structured project. Nevertheless, it is debatable whether his definition of a village corresponds to contemporary reality in the Western world. New villages and settlements are usually the result of government planning. Different parts of new villages are zoned for particular purposes and the infrastructure is laid out by planning agencies. Wörner's notion of the village seems to stem from a pre-bureaucratic era when human settlements emerged organically without central planning. Indeed, his idea resembles patterns of colonisation in North America or Siberia. Despite this, Wörner emphasises that the Moon Village is not a human colony, stressing that "this is not colonisation, and I will tell you why. Earth is too beautiful to leave it for long periods of time. I want to see the person, who truly, from the bottom of their heart, decides that they don't want to live on Earth anymore. Anyhow the Moon is no good for an alternative. Neither is Mars. Life in cans, in space suits, everyday […], I simply can't imagine that there are sensible people who truly aspire to that" (ibid.).

Wörner is clearly sceptical about humans colonising the Moon and space in general.

Wörner refers to the Moon Village as an open concept. The idea of the Moon Village is intended to gather together the diverse groups and actors that have an interest in lunar operations. These operations may take place on the surface and in orbit around the Moon. They may involve robots and humans, and they may be carried out for commercial and scientific purposes. Therefore there are no ESA plans regarding the specific architecture of the Moon Village or the types of activities that will be carried out on the Moon. The first step towards the realisation of the Moon Village was Wörner's effort to popularise the idea among potential stakeholders. Thereupon these stakeholders were invited to sign a declaration to announce their interest in being part of the Moon Village. The next step involves a survey of all actors that have signed the declaration. In this survey, the actors are asked about the elements of the Moon Village they can provide, and about the infrastructures they will need:

"For example, someone might say, 'I want to break down water into hydrogen and oxygen on the Moon, I have the competences for that, but I need someone to get me there.' Or someone else says, 'I want to try out a scientific instrument, but I need a rover that will carry it around to different places on the Moon.' That's why we're doing a survey: what do you want to provide, what do you want to do, what do you offer and what do you need" (personal communication, 2017) According to Wörner, one company has already offered to provide transport to the Moon for a set price.

In this sense, seeing the Moon Village concept as a particular type of world café may be a useful metaphor to prevent further misunderstandings (cf. [START_REF] Brown | The World Café: Shaping Out Futures through Conversations that Matter[END_REF]. A world café is a method of hosting conferences with an open outcome. It usually involves a large room with different tables hosting different conversations. The conversation topic at each table is determined by a brainstorming process at the beginning of the conference, where ideas for conversation topics are put forward by the conference participants. The most popular suggestions are assigned to particular tables. The conference participants may then move from table to table, engaging in the conversations they find the most interesting. Only the table hosts remain at fixed tables to moderate and document the conversations. At the end of the world café, the table hosts present the conclusions of the discussions that were held at each table. The world café is very useful for fostering creativity and gathering ideas.

There are several similarities between world cafés and the Moon Village. Firstly, while a world café may be organised by persons or institutions with particular interests, they do not exhibit hierarchies, as there are no audiences and no presenters. This approach is shared by the Moon Village. Wörner foresees no particular role for himself or even the European Space Agency in 'building' the Moon Village, merely viewing the Moon Village as "Europe-inspired" (Wörner & Foing, 2016;Messina et al., 2016). Indeed, in the interview Wörner and I engaged in a semantic discussion on how to describe his role in the realisation of the Moon Village. Wörner sees himself as a broker or facilitator, but not as a project manager or coordinator. The word 'facilitator' is often used by world café organisers with reference to the table hosts. When I pointed out that any village on Earth has a mayor, Wörner stated: "It is my hope that we will grow beyond that, that we won't need it (personal communication, 2017). Beyond accepting a set of standards and regulations to allow for the mutual compatibility of technical equipment, Wörner sees no need for coordination. Furthermore, he does not view ESA as the leading actor in setting up the Moon Village. He displayed great enthusiasm for the American, Russian and Chinese plans to return to the Moon, affirming that "if it allows the international community to work together, I gladly remain in the shadows. […] A village requires no leading power" (ibid.). Another similarity between the Moon Village and world cafés is that both represent an invitation to an audience that may have an interest in a particular goal, in this case, carrying out projects on the Moon. Wörner spoke of a wide range of actors that are interested in lunar operations, including both government and private-sector stakeholders. Thirdly, like the current Moon Village survey, the outcome of a world café is undetermined. Like the Moon Village, a world café is a collective brain storming exercise rather than a project management meeting. It is a process, rather than a project. Given those similarities, it may be meaningful to refer to the current state of discussions on the Moon Village as a Moon Café.

Feasibility of the Moon Village

To test the feasibility of the Moon Village this section will subject the idea to a SWOT/PEST analysis. A SWOT analysis is an organisational management tool designed to test the strengths and weaknesses of a commercial endeavour. A SWOT analysis is essentially carried out by listing the strengths and weaknesses of a project alongside opportunities and threats within unpredictably exogenous parameters [START_REF] Piercy | Making SWOT Analysis Work[END_REF]. To structure the analysis, the SWOT model is combined with a PEST analysis. In a PEST analysis, the political, economic, social and technological factors determining the success or failure of a project are evaluated. For the purposes of analysing the feasibility of the Moon Village, the social dimension is left out, as this variable was intended to predict the success of business strategies.

Several strengths are apparent from the Moon Village's internal properties. Politically, the Moon Village is potentially successful because it does not require immediate funding. This was confirmed by Wörner, who told me that when he presented the Moon Village concept at the ESA Council, the member states did not discuss it: "That's because when they meet, they have two days to focus on defining the budget for the different proposals I have provided. They argue and they talk about how much money for what project. And because I said right away that this project doesn't cost any money, it wasn't discussed. But that's not a problem. That's not a problem because it's just not a project. It is an open concept" (personal communication, 2017).

An ESA concept that requires no funding is by default popular among the member states. Economically, it is one of the strengths of the Moon Village that it builds on government funding as well as private investment. While I showed previously that the Moon Village has no immediate costs, components of the Moon Village (such as possibly the ESA Orion Service Module) do of course require public funds. However, these component projects are not explicitly part of the Moon Village and their funding is secured through international commitments and internal budgetary agreements. Moreover, the success of the Moon Village does not depend on a single project, as several actors are simultaneously producing the technologies required for the realisation of Wörner's idea. Indeed, as Foing points out, current lunar lander missions "will constitute a Robotic Village on the Moon" (Foing, 2018). Technologically, the strength of the Moon Village stems from its ability to pool the technological resources of the world's space actors, including government agencies and private enterprises.

Despite the evident strengths of the Moon Village, it also exhibits several clear organisational weaknesses. Politically, despite the humble word 'village', the Moon Village does at first glance appear to be an attempt to colonise the Moon. European space policymakers traditionally lack the ambition of their American, Russian and even Chinese counterparts. While the Moon Village is not limited to grandiose plans to build a Moon base, being open to robotic exploration, the meaning inevitably conveyed by the concept's title may act as a political deterrent. Moreover, while Wörner stresses that the idea's inherent lack of hierarchy is meant to overcome Earth-based structures, any successful project does require a certain amount of leadership and coordination. This also relates to the Moon Village's economic weaknesses. A project that lacks concrete budgetary requirements is arguably incompatible with the structured funding acquisition process of ESA and other space agencies. However, this problem may be mitigated if one takes into account that the Moon Village is not a grand scheme, but rather an amalgamation of different projects which may follow standard budget allocation procedures. Technologically, it is one of the weaknesses of the Moon Village that ESA lacks its own rockets and capsules for facilitating human space flight. However, this argument is weakened by the fact that there are many potential ways to transport humans to the Moon, including NASA's Space Launch System (SLS) and SpaceX's Falcon Heavy rockets. SLS is a project that ESA supports while the privately built Falcon Heavy may be used for ESA missions.

In terms of exogenous factors, there are many variables that may contribute to the success of the Moon Village. Politically, the Moon Village clearly rides on a wave of increasing international momentum to return human beings to the surface of the Moon. As discussed in the first section of this chapter, a consensus is emerging that a mission to Mars should be preceded by a mission to the Moon. Moreover, the ISS will eventually require replacement and a Moon base seems like the next logical project requiring international cooperation. Moreover, the Moon Village is not regarded to stand in competition with other Moon projects, as it potentially pools the numerous plans for lunar exploration. It is one of the economic opportunities that the Moon Village will consist of several mutually independent projects. Wörner has stressed that "there may be a few things you have to have an agreement on, but no more" (personal communication, 2017). This implies that actors may proceed relatively independently with their own projects, merely placing them within the overall scheme of the Moon Village. This reduces the vulnerability of the Moon Village to economic downturns in particular countries. Technologically, the Moon Village's feasibility benefits from the several projects simultaneously producing the technologies required to get to the Moon. New Glenn, Falcon Heavy and the SLS could all transport humans to the Moon. Similarly, Orion and Dragon 2 could both bring astronauts into orbit around the Moon. If one programme fails, the overall Moon Village concept may nevertheless continue.

The primary political threat to the Moon Village is the geopolitical environment. Firstly, Wörner has to convince the member states of the merits of the Moon Village. While it may currently require no funding, the projects it entails will be very expensive. The construction and upkeep of infrastructure in lunar orbit or on the surface of the Moon is very expensive, requiring regular funding comparable only to the International Space Station. The Moon Village therefore depends on the mercy of its benefactors. Secondly, while I have stated that the SLS/Orion programme as well as Falcon Heavy/Dragon II programme may facilitate the Moon Village, neither programme currently places itself under the Moon Village umbrella. While Wörner stresses that he is willing to remain out of the spotlight if this brings about international cooperation, the Moon Village is still a European idea. If it were successful, and if the name 'Moon Village' was maintained, ESA and Wörner himself would gain a significant amount of prestige. It is unlikely that the United States is willing to participate in such a prestigious programme without being the leader. If the US, Russia and China play a role in the Moon Village, its name is therefore unlikely to remain once the 'world café' phase of the concept is completed. The success of the Moon Village is therefore critically dependent on Wörner's ability to convince ESA's international partners that they should collaborate. Economically, like any major space programme, the Moon Village may fail, if there is a major global economic downturn. Finally, in terms of technology the Moon Village concept rests on several untested technological innovations, such as lunar ISRU. There are no long-term studies on the effects of lunar dust on the human body and it is difficult to shield the human body from the radiation experienced on the Moon.

[Insert Figure 2]

Conclusion

In this paper it was shown that there is an increasing international momentum to return astronauts to the Moon. The Moon is closer to Earth than Mars, and getting to the Moon does not depend on the development of hitherto non-existent technologies. Most importantly, a lunar programme is much cheaper than a human mission to Mars. Within this context, the feasibility of Jan Wörner's Moon Village proposal is increased. Given the need to replace the ISS in the medium-term future, and given that the Moon Village is merely a platform for all sorts of different projects, it is likely that Wörner's campaign will at least meet some success.

But is the open concept of the Moon Village a 'new way of doing space'? This article's answer would be 'yes', as the evidence lends support to the institutionalist hypothesis that the Moon Village represents the potential translation of the ESA system to the global level. For ESA projects, it is not required that every single member state participates, and according to the geographical return principle, those member states that do participate are certain to have their investment returned into their own national economies. Similarly, the realisation of the Moon Village does not require the participation of all actors in all of its elements. Moreover, resources are not pooled, but operations are merely carried out without a loose set of parameters and under the umbrella term 'Moon Village'. As such, the Moon Village cannot be compared to the International Space Station which requires far more coordination. The ESA system has proven to be very resilient, and the existence of the geographical return principle is a powerful reason against the inclusion of ESA into the EU family of institutions (see Hoerber, 2009 on the EU/ESA debate). The institutionalisation of cooperation in space cooperation on a global level may be an important contribution towards the maintenance of balance on the international system.

1

  The Moon Village and Space 4.0: the 'open concept' as a new way of doing space?

Acknowledgements

I would like to thank Thomas Hörber, Sarah Liebermann and especially Jan Wörner.


Yves Catonné
email: yves.catonne@gmail.comnepasutiliser

Elhadi Sariali

Frédéric Khiami

Jean-Louis Rouvillain

Antony Wajsfisz

Hugues Pascal-Moussellard

associée à une déformation extra-articulaire. Première partie : ostéotomie tibiale. À propos de 26 cas

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Résumé

La cause de la déviation osseuse tibiale était constitutionnelle dans 14 cas, secondaire à un cal vicieux dans 12 cas, que celui-ci soit post traumatique (5 cas) ou après ostéotomie (7 cas).

Parmi des 14 déformations constitutionnelles, 10 étaient en varus dont une séquelle de tibia vara infantile (maladie de Blount) et une dysplasie poly-épiphysaire, et 4 en valgus.

Techniquement, les interventions ont toutes été faites selon le même schéma : temps fémoral prothétique dans un premier temps, puis temps tibial avec ostéotomie première, fixation provisoire par agrafe, et mise en place de l'implant tibial à tige longue après alésage par alésoir rigide permettant de réaliser une coupe tibiale à 90° de l'axe tibial.

Le type d'ostéotomie a varié en fonction du type de déformation : ouverture médiale dans les varus tibiaux (13 cas), fermeture médiale dans les valgus (10cas) (Figures 1 et 2), fermeture antérieure après section de la tubérosité tibiale antérieure dans un cas de flessum intra-osseux (obs. n°1), ostéotomie de varisation et flexion dans le cas de déformation associée en valgus et recurvatum (Figure 3), ostéotomie de translation latérale après relèvement de la tubérosité tibiale antérieure dans le cal vicieux avec translation (Figure 4).

Dans 21 cas une ostéosynthèse complémentaire tibiale a été réalisée. Dans tous les cas sauf un (obs n°1), une tige longue a été utilisée (10 cm le plus souvent, 15 cm dans 6 cas), parfois droite (14 cas), parfois décalée (11 cas).

Le degré de contrainte a été variable : dans 23 cas des implants postéro-stabilisés à plateau fixe ont été utilisés, dans 3 cas il s'agissait d'implants de type Condylar Knee (postérostabilisés à came longue), la décision ayant été prise en per-opératoire. Aucune prothèse à charnière n'a été utilisée.

Les suites post opératoires ont consisté dans tous les cas en une reprise d'appui précoce à J1 avec attelle en extension pendant 45 jours uniquement pour la marche, associée à une mobilisation précoce de l'articulation.

Méthodes

Ont été évalués en pré et post opératoires :

-Cliniquement l'indice de masse corporelle (BMI), la mobilité du genou en flexion-extension, une éventuelle laxité en varus-valgus, le score IKS (genou score et fonction).

-Radiologiquement, des clichés de face et profil en appui monopodal, un cliché de face en schuss, une incidence fémoro-patellaire à 30° et un pangonogramme en charge remplacé à partir de 2010 par une imagerie EOS.

En cas de troubles de rotation et de cal vicieux post traumatique, un scanner pré opératoire était pratiqué. Les autres complications post-opératoires comportaient 2 thrombo-phlébites, deux hématomes (dont un a nécessité un drainage), un déficit transitoire de l'extenseur du gros orteil (régressif en un an), et un déficit d'extension active de 15° persistant au dernier examen (obs. n°5).

Aucun déplacement secondaire, descellement ou pseudarthrose n'a été observé. Une ablation d'agrafes a été faite en raison de la saillie de celle-ci sous la peau.

Discussion

1 Résultats

La mobilité en flexion après PTG et ostéotomie associée (107° en moyenne) était plus limitée que dans séries pour arthrose habituelle mais comparable à celle retrouvée dans l'arthrose sur importante déformation et l'arthrose post traumatique [START_REF] Parratte | Prothèse totale du genou sur cal vicieux intra-articulaire[END_REF]. Il faut noter que la mobilité pré opératoire (98° en moyenne) était nettement moins importante que dans les séries de PTG standard en particulier en ce qui concerne les cas post-traumatiques (77° en moyenne). Dans ces derniers cas (5 patients) le gain moyen de mobilité en flexion est de 28° contre 9° sur l'ensemble de la série.

Les

Autres solutions proposées dans les déformations extra articulaires

La réalisation de coupes orthogonales en association à un relâchement de la concavité est la solution adoptée par la plupart des auteurs [START_REF] Teeny | Primary total knee arthroplasty in patients with severe varus deformity[END_REF], [START_REF] Deschamps | Total Knee Arthroplasty for osteoarthritis secondary to extra articular malunions[END_REF].

Certains pensent qu'elle est toujours réalisable ( 14) : en fait, il existe des cas où il est impossible de se passer d'une ostéotomie, que celle-ci soit réalisée dans un premier temps où dans le même temps que la PTG (Figure 3 et Figure 4).

Le relâchement de la concavité, possible dans de nombreux cas, a des limites et dépend du siège et de l'importance de la déformation [START_REF] Wolff | The effect of extraarticular varus and valgus deformity on total knee arthroplasty[END_REF]. Plutôt que la déformation globale du membre inférieur (angle HKA), il faut prendre en compte pour les déformations tibiales l'angle tibial mécanique et la majorité des auteurs considère qu'un axe tibial mécanique (ATM) inférieur à 80° ou supérieur à 100° constitue la limite de cette option.

Résultats comparatifs

Il est difficile de comparer nos résultats avec ceux des prothèses à charnière ou des PTG avec relâchement majeur car les séries avec résultats à long terme des PTG sur DEA majeure sont rares et il s'agit de séries à court terme et peu documentées sur les laxités post-opératoires [START_REF] Wang | Total Knee Arthroplasty for arthritis of the knee with extra-articular deformity[END_REF]. De même, s'il existe de nombreuses séries étudiant les résultats des PTG après OT, aucune ne concerne uniquement les cas de déformation extra-articulaire majeure avec PTG programmée dans un deuxième temps. Il parait cependant logique de privilégier l'ostéotomie première chez le sujet de moins de 65 ans, car elle peut retarder de façon importante la prothèse. Lorsque la PTG est inéluctable (gêne fonctionnelle majeure, arthrose tricompartimentale), les différentes séquences du traitement (ostéotomie puis éventuelle ablation de matériel puis PTG après consolidation) allongent de façon importante la durée du traitement et sont souvent mal acceptées par le patient (6).

Complications

Une complication majeure aboutissant à une arthrodèse est survenue dans cette série et permet de souligner qu'une PTG associée à une ostéotomie n'est pas une opération anodine.

Nous n'avons repris (avec changement des implants) aucun autre patient de cette série au recul moyen de 9 ans : en particulier nous n'avons à déplorer aucun échec pour laxité résiduelle.

Conclusion

  Introduction Lorsqu'une gonarthrose évoluée s'accompagne d'une déformation extra-articulaire, la réalisation d'une prothèse totale de genou (PTG) peut poser des problèmes d'équilibrage ligamentaire et certains cas nécessitent soit de corriger la déformation osseuse dans un premier temps soit de réaliser une PTG et une ostéotomie tibiale (OT) simultanées. Nous rapportons les résultats d'une étude prospective de 26 PTG et OT en un temps réalisés entre 1995 et 2014. Hypothèse La réalisation d'une PTG et OT en un temps n'entraine pas une morbidité plus importante que les autres techniques et donne des résultats anatomiques et cliniques satisfaisants. Matériel et méthodes 25 patients ont été inclus, correspondant à 26 PTG et OT associées, avec un âge moyen de 64 ans (29 à 80) et un BMI moyen de 29,6 (24 à 49). Il s'agissait de 16 hommes et 9 femmes présentant une déformation osseuse constitutionnelle (14 cas), post traumatique (5 cas) ou iatrogène après ostéotomie tibiale (7 cas). Aucune PTG à charnière n'a été utilisée. Tous les patients ont été évalués en préopératoire et au dernier recul, cliniquement (score IKS), et radiologiquement (angles HKA, ATM et AFM). Résultats préopératoire, à 170 au dernier recul. La mobilité moyenne en flexion passait de 98° à 107°. Une complication majeure à type de nécrose cutanée massive suivie d'une infection était observée chez un patient de 40 ans présentant les séquelles d'une maladie de Blount multiopérée avec un BMI à 49. Les 25 autres cas ont donné des résultats cliniques et anatomiques satisfaisants. Il s'agit de la seule reprise avec ablation des implants de l'ensemble de la série. Discussion Nos résultats sont concordants avec ceux de la littérature, qui montre, le plus souvent sur des séries courtes, des résultats satisfaisants. Les alternatives à cette technique sont étudiées. Les indications semblent être les arthroses avec déformation intra-osseuse supérieure à 10°. Niveau de preuve :IV,Etude de cohorte Mots clés :Gonarthrose, Déformations extra articulaires du genou, Prothèse totale de genou associée à une ostéotomie tibiale proximale 1. Introduction Une gonarthrose évoluée peut s'accompagner d'une déformation extra articulaire importante pouvant siéger dans le fémur ou dans le tibia. Les déformations extra-articulaires sont souvent de faible angulation et elles peuvent alors être corrigées lors de l'arthroplastie par prothèse totale de genou (PTG) dans la coupe osseuse, en réalisant un équilibrage ligamentaire avec libération de la concavité. Mais en cas de déformation osseuse importante, la réalisation des coupes habituelles oblige à une résection étendue du côté de la convexité et peut entraîner une laxité ou aggraver une laxité préexistante. Dans ces cas se pose le problème de la correction de la déformation au niveau de son siège, c'est à dire en extra-articulaire par une ostéotomie tibiale (OT). En cas d'arthrose globale et chez le sujet plus âgé la correction de la déviation osseuse peut être associée à la prothèse totale de genou (PTG) en un ou deux temps. En 1991, Wolff et Hungerford (1) ont décrit le premier cas d'ostéotomie et prothèse en un temps et ont bien montré les limites de la correction en intra articulaire d'une déformation extra articulaire. Par la suite des cas isolés ou des séries courtes ont été rapportés (2-5

3 . 4 .

 34 Fémoro-Tibial Mécanique (HKA) tracé entre l'axe mécanique du fémur et du tibia. -L'Angle Fémoral Mécanique (AFM) entre l'axe fémoral mécanique et la ligne bicondylienne (mesuré en dedans) pour évaluer la déformation intra-osseuse. Un angle compris entre 90° et 94° était considéré comme normal. -L'Angle Tibial Mécanique (ATM) entre l'axe tibial mécanique et la tangente au plateau sain pour mesurer la déformation intra-osseuse tibiale. Un angle compris entre 86 et 90° était considéré comme normal. -Les complications éventuelles, per-opératoires ou secondaires (immédiates ou tardives) et les éventuelles reprises. Les patients ont été revus à 45 jours, 6 mois, un an, deux ans, 5 ans, puis tous les 5 ans. L'angle fémoral mécanique était en moyenne de 90.5° en pré-opératoire (82°-93°) : on notait un seul cas de varus fémoral franc (82°) concernant le seul cas de dysplasie polyépiphysaire et expliquant le varus post opératoire (HKA 175°). Les 13 genu varum avaient un angle tibial mécanique moyen de 74° (67° à 79°) en pré et de 89° (87° à 92°) en postopératoire. Pour les genu valgum (11 cas), l'ATM était en moyenne de 105° (100 à 118°) en pré et de 89,7° (87 à 94°) en post opératoire. Complications et reprises Les complications per-opératoires ont consisté en un trait de refend tibial chez 4 patients sans conséquences sur le résultat final Une complication majeure post opératoire est survenue (obs. n°24). Il s'agissait d'un patient âgé de 40 ans, présentant une obésité morbide (BMI 49), ayant été opéré à 3 reprises dans l'enfance pour une maladie de Blount. L'impotence fonctionnelle était majeure, le genou très raide (flexion 60°, flessum de 15°). Il existait une déformation extra articulaire majeure (ATM 69°) Une PTG combinée à une OTV d'addition a été réalisée et s'est compliquée d'une nécrose cutanée nécessitant un lambeau de gastrocnémien médial. Secondairement une infection est survenue et a conduit à une ablation des implants puis à une arthrodèse du genou. Cette complication majeure a malgré tout abouti à une nette amélioration clinique (score IKS 50 en pré et 126 en post-opératoire) au détriment d'une ankylose définitive du genou.

FK

  La réalisation d'une PTG associée à une ostéotomie tibiale est une solution satisfaisante dans la gonarthrose évoluée chez les patients présentant une déformation extra-articulaire majeure (déformation globale supérieure le plus souvent à 20° avec déformation intra-osseuse de 10° et plus). Elle permet d'éviter la laxité de la convexité générée par une coupe tibiale orthogonale sur déformation intra-osseuse L'utilisation d'une PTG semi-contrainte à tige longue permet à la fois un bon alignement tibial, une consolidation dans des délais raisonnables et des résultats cliniques satisfaisants. Elle reste une indication rare, la solution la plus souvent adoptée demeurant la correction intra articulaire par libération de la concavité. La morbidité post opératoire parait acceptable en regard des résultats cliniques et radiologiques. Le traitement des déformations extra articulaires par une ostéotomie précoce reste cependant le traitement de choix permettant d'éviter ou de retarder la survenue de l'arthrose. Conflits d'intérêt YC Royalties société Biomet (concernant une tige fémorale de PTH) ESA Royalties Société Symbios (concernant PTH) de patients, Rédaction ESA Etude statistique, suivi de patients FK Tableaux. Suivi de patients JLR Suivi de patients

Figure 1 aFigure 1 b

 11 Figure 1 a Obs n° 13 et 14. Homme de 60 ans : gonarthrose sur valgus tibial constitutionnel bilatéral Figure 1 b Pangonogramme pré-opératoire Figure 1 c PTG et Ostéotomie tibiale de varisation bilatérale par soustraction médiale.

Figure 1 dFigure 2 aFigure 2 bFigure 3 cFigure 4 a

 12234 Figure 1 d Pangonogramme 1 an après le 2 ème côté. Très bon résultat clinique

  Lerat et al.[START_REF] Lerat | Prothèse totale de genou associée à une désostéotomie : technique de l'ostéotomie après la mise en place de la prothèse[END_REF] ont décrit une technique dans laquelle une prothèse de genou à tige courte ou à simples plots d'ancrage était mise dans un premier temps. Une ostéotomie tibiale était pratiquée au cours de la même intervention, sous l'implant tibial.Sur une série de 8 désostéotomies tibiales, les résultats cliniques ont été bons, avec cependant une pseudarthrose ayant nécessité une greffe, et un délai moyen de consolidation de 5 mois.Vielpeau et al[START_REF] Vielpeau | Ostéotomie tibiale et prothèse totale de genou simultanées[END_REF] pratiquaient une ostéotomie tibiale par voie latérale après éversion de la tubérosité tibiale antérieure. L'ostéosynthèse était confiée à la tige de fixation. Sur une série de 14 cas, les auteurs ont rapporté un démontage précoce, avec de bons résultats cliniques. Ils réservaient cette technique aux déformations extra-articulaires supérieures à 8°.

	L'hypothèse est confirmée
	4.2	Comparaison avec la littérature
	4.2.1 Technique opératoire
	L'utilisation de tige d'extension longue non cimentée du côté de l'ostéotomie est constante
	dans cette série (en dehors du cas n°1). Dans tous les cas, l'ostéotomie tibiale a été pratiquée

bons résultats globaux sur l'axe (HKA et ATM) nous semblent liés à la tige longue et à l'alignement « automatique » sur le guide d'alésage puis sur la tige après l'ostéotomie. avant la mise en place de l'implant tibial. Certains auteurs pratiquent des techniques différentes :

La distribution des variables a été testée pour la normalité à l'aide des tests de Ryan-Joiner et de Shapiro-Wilk. Pour les variables normalement distribuées, lorsque deux groupes avaient les mêmes variances, les différences entre eux ont été analysées à l'aide d'un test t de Student.Pour les variables anormalement distribuées ou les variables normalement distribuées avec des variances différentes, nous avons utilisé le test de Mann et Whittney. Une valeur p inférieure à 0,05 était considérée comme significative.3. RésultatsEn dehors d'un décès à un an post opératoire pour insuffisance cardiaque, le recul variait de 3 à 18 ans avec une moyenne de 9 ans (Tableau 2).3.1 Résultats cliniquesLe score IKS moyen était en pré opératoire de 70 (29-104) et en post opératoire de 170 (126-190) avec une augmentation significative de 98.9 ± 17.6 (p<0.0001). Le score IKS Genou passait de 28,5 avant (10-59) à 84 (56-99) après.La mobilité en flexion passait de 98° avant l'intervention (5°-120°) à 107° après (0-130°) avec un gain moyen significatif de 16.3° ± 8.3.3.2 Résultats radiologiquesEn post-opératoire, seuls 2 cas étaient en dehors de la fourchette 180 +_3 avec des angles respectivement à 175° et 184°.