Influence of glenoid component design and humeral component retroversion on internal and external rotation in reverse shoulder arthroplasty: A cadaver study

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ORIGINAL

ARTICLE

Inﬂuence

of

glenoid

component

design

and

humeral

component

retroversion

on

internal

and

external

rotation

in

reverse

shoulder

arthroplasty:

A

cadaver

study

J. Berhouet

∗

,

P. Garaud ,

L. Favard

UniversitéFranc¸ois-RabelaisdeTours,FacultédeMédecinedeTours,CHRUTrousseau,Service d’orthopédietraumatologie2A,1C,avenuedelaRépublique,37170Chambray-les-Tours,France

Accepted:21August2013 KEYWORDS Reverseshoulder prosthesis; Glenoidcomponent combinations; Humeralcomponent retroversion; Rotations Summary

Background: Acommondisadvantageofreverseshoulderarthroplastyislimitationoftherange ofarmrotation.Severalchangestotheprosthesisdesignandimplantationtechniquehavebeen suggestedtoimproverotationrangeofmotion(ROM).

Hypothesis: Glenoidcomponentdesignanddegreeofhumeralcomponentretroversion inﬂu-encerotationROMafterreverseshoulderarthroplasty.

Materialandmethods: TheAequalisReversedTM _shoulder_prosthesis_(Tornier_Inc.,_Edina,_MN,

USA) wasimplantedinto 40cadavershoulders.Eightglenoid componentcombinationswere tested,ﬁvewiththe36-mmsphere(centredseating,eccentricseating,inferiortilt,centred witha5-mmthicklateralised spacer,andcentredwitha7-mmthicklateralisedspacer)and threewiththe42-mmsphere(centredwithnospacerorwitha7-mmor10-mmspacer).Humeral component positionwas evaluatedwith 0◦,10◦,20◦,30◦,and40◦ ofretroversion.External andinternalrotationROMstoposteriorandanteriorimpingementonthescapularneckwere measuredwiththearmin20◦ofabduction.

Results: Thelargeglenosphere(42mm)wasassociatedwithsigniﬁcantly(P<0.05)greater rota-tionROMs,particularlywhencombinedwithalateralisedspacer(46◦internaland66◦external rotation).RotationROMsweresmallestwiththe36-mmsphere.Greaterhumeralcomponent retroversion was associated with a decrease ininternal rotation anda signiﬁcant increase (P<0.05) inexternalrotation.The bestbalance between rotationROMs was obtainedwith thenativeretroversion,whichwasestimatedat17.5◦onaverageinthisstudy.

∗_{Corresponding}_author._Tel.:₊₃₃₀₂₄₇₄₇₄₇_47;_fax:₊₃₃₀₂₄₇₄₇₅₉_12.

E-mailaddress:julien.berhouet@gmail.com(J.Berhouet).

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Discussion: Ouranatomicstudyinalargenumberofcadaversinvolvedadetailedand repro-ducible experimental protocol. However, we did not evaluate the variability in scapular anatomy.Earlierstudiesoftheinﬂuenceoftechnicalparametersdidnottakehumeral compo-nentretroversionintoaccount.Inaddition,nopreviousstudiesassessedrotationROMs.

Conclusion:RotationROMshouldbeimprovedbytheuseofalarge-diameterglenospherewith aspacertolateralisethecentreofrotationofthegleno-humeraljoint,aswellasbypositioning thehumeralcomponentatthepatient’snativeretroversionvalue.

Introduction

Reverseshoulder arthroplasty (RSA)isnowa ﬁrmly estab-lishedtreatment methodfor olderpatientswitheccentric gleno-humeralosteoarthritis[1,2].TheeffectivenessofRSA is ascribable to two major changes in shoulder morphol-ogy:alarge-diametermetallichemispherewithnoneckis implantedintotheglenoidcavity,andapolyethylenesocket coveringlessthanhalfoftheglenosphereandhavinga non-anatomicneck-shaftangle is usedtoreplacethe humeral

head[3,4].Thebiomechanicaleffectofthisuniquedesign

consistsof medialisationanddistalisationofthehumerus, whichimprovesdeltoidperformance.Thefunctionaleffects includeimprovedjointstabilityanddecreasedshearforces ontheglenoidcomponent.Potentialdisadvantagesofthese morphologicalchangesareadecreaseinrotationrangeof motion(ROM)withtheelbowbytheside[4—6]andscapular notching[7].

Loss of arm rotation with the elbow by the side was evaluatedbyBoileau etal.[2].Afterameanfollow-upof 53months,internalrotationimprovedbyonlytwovertebral levelsandexternalrotationbyonly4◦.Factorsresponsible for loss of rotation may include the small lateral gleno-sphere offset and the medialisation of the humerus [4], whichresultinimpingementofthehumerusontheanterior or posterior part of the glenoid cavity, withthe develop-mentofscapularnotching.Thus,thechangessuggestedto preventscapular notchingalsoimprovetherangeof rota-tionwiththeelbowbytheside.Decreasedmedialisationof thecentreofrotationwasassociatedwitha36◦increasein externalrotationwiththeelbowbythesideafter33months

[8].Intwostudies,lateralisationofthecentreofrotation by8.5mmimprovedexternalrotationby15◦ afteramean follow-up of 36months [9,10]. Lateralisation achieved by bone graft implantation between the glenoidsurface and baseplate(BonyIncreasedOffset-ReverseShoulder Arthro-plasty, BIO-RSA)[4,11] improved external rotation by 10◦ andinternal rotation by1.4 points onthe Constantscore

[12]afterafollow-upof28months.Grammontinhisseminal workthenWalchetal.[2]reportedthatlesshumeral com-ponentretroversionwasassociatedwithbetter outcomes. However,tothebestofourknowledgenootherstudiesof theimpactof humeral retroversion onrotationROM have beenreported.

Given these persistent uncertainties, we assessed the inﬂuence of various glenoidcomponent combinations and degrees of humeral retroversion on rotation ROM after RSA.

Materials

and

methods:

shoulders

and

implants

We conducted a study in 40 cadaver shoulders, 20 from leftand20fromrightupperlimbs.Meanageatdeathwas 79.1years(range,61—95years).Therewere21menand19 women. Each specimen included the shoulder girdlewith theclavicleandtheentireupperlimbincludingtheforearm andhand.

WeusedtheAequalisReversedTM_prosthesis_(Tornier_Inc.,

Edina, MN, USA). The speciﬁc instruments were used as recommended by the manufacturer. For all 40 shoulders, we usedthe same humeral componentdesign, witha 36-mmmetaphysisscrewed intoadiaphysealstemmeasuring 6.5mmindiameterand150mminlengthandscrewedinto an intra-medullarynailmeasuring3.5mmindiameterand 250mminlength.Thedistalendofthenailexitedthrough the distalhumeral metaphysis and olecranon andsecured thehumerus tothemeasurementdevice. Weimplanted a removablehumeralinsertmeasuring36mmindiameterand having 6mm of lateral offset. Insert concavity (36mm or 42mm)wasselectedaccordingtothediameterofthe gleno-sphere.

A29-mmglenoidbaseplatewasimplanted,withthe fol-lowingglenospheresandlateralisedspacers:

• 36-mmcentredglenosphere;

• 36-mmglenospherepositionedoffcentreby2mm; • 36-mmglenospherewith10◦tilt;

• 42-mmcentredglenosphere;

• 36-mmcentredglenospherewith5mmlateralisation; • 36-mmcentredglenospherewith7mmlateralisation; • 42-mmcentredglenospherewith7mmlateralisation; • 42-mmcentredglenospherewith10mmlateralisation.

Measurementdevice

Amodularmetallicholderwasdevised(Sawbones,Malmö, Sweden).Avisewasclampedontothescapulaandan arti-culatedarmheldtheremainderoftheupperlimb,towhich it was secured by the intra-medullary nail screwed onto thehumeralprostheticcomponent.Thedeviceallowedthe reproductionofshouldermovementsinallplanesandwas equipped with protractors to record motion ranges. The goal of the assembly was to allow variations in humeral elevation while enabling the measurement of rotation ROMs.

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Experimentalmethod

Theanatomicspecimenwasremovedfromthecadaverand thescapulawasclampedintotheviseofthemeasurement device.Thedeltoidmusclewasdetachedfromtheclavicle andacromiondowntothedeltoid‘V’toexposetheplane of the rotatorcuff. The supra-spinatus and infra-spinatus muscles were removed. To enable a reliable assessment ofanteriorimpingements,thesub-scapulariswasdetached fromthelessertuberosity.Theteresminorwaspreserved. Averticalincisionofthegleno-humeraljointwasperformed andthecapsulewasresectedtogetherwiththelongheadof thebicepstendon.Theglenoidcavityrimandupperportion ofthescapularpillarwerecarefullyexposed,ifneededby detachingthelongheadofthetriceps.Fullexposureofthe anterior,inferior,andposteriorrimoftheglenoidcavitywas achievedtoallowanassessmentofimpingementsduringthe measurements.

For implantationof the glenoid baseplate, the glenoid cavity waspreparedby successivelyremoving the glenoid labrum;insertingathreadeddrillguideperpendiculartothe glenoidsurfacetoensurecentring,incompliancewiththe 12-mmrule[13];poweredreamingoftheglenoidto29mm indiameter;andmanualreamingto36or42mmin diam-eter.The29-mmglenoidbaseplatewasthenimpactedinto theglenoid.Anadditionalscrewwasinsertedifneededto achieveﬁrmbaseplateﬁxation.

Preparation of the humerus started with resection of the humeral head using a cutting guide, after identiﬁca-tionofthereamerentrysitealongtheextendeddiaphyseal axis. The cutting guide allowed humeral head resection accordingtothe nativeretroversion measured with refer-encetotheforearmaxis.Reamingofthediaphysisfollowed by preparationof the metaphysis witha 36-mm burr and ﬁnallybyreamingofthemetaphysisandepiphysiswerethen

performed manually. A drill bit was inserted into the medullarycavityandmadetoexittheolecranon withthe elbowin90◦ ofﬂexion.Thislaststeppreparedthe implan-tation of the humeral component secured to a threaded intra-medullarynail,whosetipwasscrewedontoa protrac-tortoenablemeasurementofrotationROMs.Thehumeral componentimplantedwiththenativeretroversionwasthen securedtothearticulatedarmofthemetallicholder.Atthe levelofthemetaphysis,asmallanti-rotationpinservedto blocktheretroversionofthehumeralcomponent.A later-alised36-mmhumeralinsertwasimplanted(Fig.1).

Baseplateadjustment

Baseplate adjustment was required before the measure-mentscouldbestarted.Aguidesystemcomprisingaspirit levelandapointerwascentredontheimplantedbaseplate. Thissystemserved toadjustthepositionof thebaseplate relative to the metallic holder equipped with the mea-surementinstruments.Adjustmentsweremadeinallthree planestoensureco-axialityofthecentresofrotationofthe gleno-humeraljointandarticulatedholder.Theglenosphere wasscrewedintothebaseplatewhentheglenoidwas prop-erlyaligned(Fig.2).Thisadjustmentstepwasrepeatedat eachchangeinbaseplateorglenosphere.Humeral prosthe-sisretroversionwasdeterminedbyadjustingaretroversion pinrelativetotheaxisoftheforearm.

Measurements

Oneach ofthe40 cadavershoulders,we testedthe eight above-described glenoid component combinations, each with ﬁve different degrees of humeral component retro-version(0◦,10◦,20◦,30◦,and40◦). Allthemeasurements

Figure1 Anatomicspecimens.A.Therotationmeasurementdevicewasscrewedtothehumeralintra-medullarythreadednail beforebeingsecuredtothemetallicholderarm.B.36-mmhumeralinsertoppositea29-mmbaseplatewitha7-mm-thicklateralised spacer.

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Figure2 Sequentialadjustmentstoglenoidcomponent posi-tioning(fromlefttoright).A andB.Insertionofadrillguide throughtheadjustmentV towardsthetargetofthecentring guideinsertedintotheglenoidbaseplate.Positionwasadjusted usingthe various screws. The objectivewas 0◦ onthe spirit level.C.Anatomic viewof acoaptedshoulderimplantready formeasurementofrotationmotionranges.

Figure 3 Method used to evaluate the degree ofhumerus abductionbasedonthegleno-metaphysealangle.

were made using the plane of the scapula as the refer-ence. Neutral rotation was defined as the forearm being perpendiculartotheplaneofthescapulawhentheelbow was in 90◦ of flexion. The maximum internal and exter-nalrotationROMsweremeasuredwiththehumerusin20◦ of abduction. The abduction angle was defined based on the gleno-metaphyseal angle, which reflects the position of the glenosphere relative tothe humeral implant. In a studycomparingpatientswithandwithoutscapular notch-ingafterRSA,themeangleno-metaphysealanglewas46.9◦ inpatientswithnotching,indicating20◦ofhumerus abduc-tion,and37.5◦ inpatientswithoutnotching,indicating52◦ ofhumerusabductionwithastrictlyverticalorientationof theglenoid[13](Fig.3).Weintentionallychosethelower abductionvaluetoenabledetectionofimpingements dur-ingarmrotation.ThemaximumrotationROMsweredefined astherangestoposterior oranteriorimpingement.When noimpingementoccurred withthegreatestmotionranges allowedbythemeasurementdevice,wearbitrarilyassigned avalueof200◦tointernalorexternalrotation,as appropri-ate.

Toassess the influence of theglenoid combinationson rotationROM,weplacedthehumeralcomponentinnative retroversion.Weassessedtheinfluenceofhumeral retrover-siononrotationROM byobtaining measurementswiththe five degreesof retroversion for each of the eightglenoid combinations.

Statisticalanalysis

The dataweredescribedasthemeanvalueswiththe95% conﬁdenceintervals(95%CIs).

To comparerotation ROMsacross glenoid implantsand retroversions,weusedtheWilcoxonrank sumtest.Values ofPlowerthan0.05wereconsideredsigniﬁcant.All statis-ticalanalyses were performed usingStatView4.1 (Abacus ConceptsInc.,Berkeley,CA,USA).

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Results

Inﬂuenceofglenoidmodularityoninternaland externalrotationmotionrange

The internal and external rotation ROMs were smallest with the centred 36-mm glenosphere (31.1±10.2◦ and 33.8±11.1◦,respectively,P<0.0001versusallother gleno-sphere sizes). Both rotation ROMs were greater with the 42-mm glenospheres, and the best ranges were obtained when a 7-mm or 10-mm lateralised spacer was added (78.8±8.6◦ and 99.3±6.3◦, respectively; P<0.001 ver-sus allother glenospheresizes). Thus,the increaseswere about46◦forinternalrotationand66◦forexternalrotation (P<0.0001). Witha given amount of lateralisation (7-mm spacer),the factorthat hadthelargestinﬂuence wasthe diameteroftheglenosphere(P<0.0001).The increasesin internalandexternalrotationwiththe42-mmglenosphere wereabout 20◦ and24◦ comparedtothe ranges obtained usingthe36-mmglenosphere(Table1).

Inﬂuenceofhumeralretroversiononinternaland externalrotationmotionranges

Mean native humerus retroversion was 17.25◦ (range, 0—40◦).Maximalinternalrotationwitheachoftheglenoid componentcombinationsdecreasedsignificantlyashumeral component retroversion increased (P<0.0001). In con-trast,maximalexternalrotationincreasedsignificantlywith humeral component retroversion (P<0.0001). With native retroversion,bothinternalandexternalrotationROMswere closely similar to those obtained at 20◦ of retroversion. Thechangesinbothinternalandexternalrotationinduced byvarying the degreeof humeral componentretroversion weresimilarwitheachoftheglenoidcomponent combina-tions.ClassificationoftheimplantsbasedonrotationROM produced the same orderas during thepart of the study involvingchangesintheglenoidcomponent(Fig.4).

Discussion

Inﬂuenceofglenoidmodularityoninternaland externalrotationmotionrange

Lossofrotationwiththeelbowbythesideisthemainsource offunctionalimpairmentafterRSA[2].Themaintechnical methodthathasbeensuggestedtoimproverotationROMs isglenospherelateralisationviaeitherbonegraft interposi-tion[11]orachangeinimplantdesign[8].Othermethods consistineccentricglenospherepositioningortiltingofthe glenosphere.Weevaluatedtheimpactofthesevariantson rotationROMandriskofimpingement.

Ourresultsindicatethatthelarger42-mm glenosphere producesconsiderablygreaterrangesofinternaland exter-nal rotation. The increases were largest when we used a lateralised spacer replicating the effects of the BIO-RAD procedure.Thecombinationofa10-mmlateralisedspacer (adding to the 3mm supplied by the 42-mm glenosphere comparedtothe36-mmglenosphere)anda9-mm inferior

Figure 4 Mean motion ranges in degrees with the arm abductedat20◦,with theﬁvehumeral component retrover-sionvaluestestedforeachoftheglenoidcombinations.Panel A:internalrotation.PanelB:externalrotation.Althoughthe dataareindependent,theyareshownascontinuouscurvesto improvereadabilityandcomparability.However,thetrendsare preserved.

overhangsigniﬁcantlyimprovedbothinternalandexternal rotation.

Nopreviousbiomechanicalstudiesassessedtheinﬂuence ofglenoidcomponentmodularity oninternal andexternal rotationROMsand onthe occurrenceofanteriorand pos-teriorimpingement,respectively.Theavailabledatacome fromretrospectivecase-seriesstudiesofpatientsmanaged with RSA. With 8.5-mm lateralisation, external rotation increasedby15◦andinternalrotationwiththeelbowbythe sideby1pointontheConstantscoreafteramean follow-upof36months[9,10].Whenanautologousbonegraft7or 10mminthicknesswasimplanted(BIO-RSA),external rota-tionimprovedby10◦andinternalrotationby1.4pointonthe Constantscoreafter28months[11].Useofa42-mm gleno-spherewasassociatedwitha13◦ improvementinexternal rotationwiththe elbowby the side aftera mean follow-upof12months,whereasrotationROMdecreased whena 38-mm glenosphere wasimplanted [14]. Internal rotation decreasedby fourvertebrallevelswiththe42-mm gleno-sphereandincreasedtothe sameextent withthe 38-mm glenosphere.However,thesubgroupsizesinthisstudywere

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J.

Berhouet

et

al.

Table1 Maximalrangesofinternalandexternalrotationwiththearmabductedat20◦,witheightdifferentglenoidcomponentcombinationsandwithnativeretroversion. Thedataaremeanswiththeir95%conﬁdenceintervals.

GS36mm GS36mm+10◦ tilt GS 36mm+2mm eccentric GS42mm GS 36mm+5-mm LS GS 36mm+7-mm LS GS 42mm+7-mm LS GS 42mm+10-mm LS MaxER 31.1 (10.2) 46.3 (10.3) 56.3 (11.5) 68.3 (10.9) 51 (10.6) 56.5 (9) 76.5 (9.1) 78.8 (8.6) MaxIR 33.8 (11.1) 51.3 (11.2) 62.3 (11.3) 74 (10) 55.6 (11) 68.5 (9.3) 92.6 (7.4) 99.3 (6.3)

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smallandunequal,andthethicknessofthehumeralinserts wasvariable.

Variations inpatientselection criteriafor RSA,surgical approaches,implantationtechnique,andprosthesisdesign are obstacles to comparisons of case-series studies. The study reports sometimes fail to provide important infor-mationsuch asthecondition of the rotatorcuffmuscles, particularly the teres minor, and the degree of humeral implantretroversion.Themainlimitationsofourstudyare thevariabilityincadavershoulderanatomyandtheabsence of a morphological analysis of each scapula. In addition, possiblevariationsinprosthesisimplantationandthe exten-sivesofttissueresectionduringpreparationofthecadaver specimens may have inﬂuenced the measured ROMs and the reliabilityof ourimpingement assessments.In partic-ular,whentheshoulderhasbeendetachedfromthetorso, internalrotationcannotbeevaluatedbasedonthe hand-to-spinemanoeuvre,whichalsorequiresretropulsion.Thus,it canbedifﬁculttodeterminewhetherROMdifferencesare ascribablesolelytodifferences inimplant geometry. Nev-ertheless, a detailed standardised protocol was used for implantationofall 40prosthesesinourstudy.Thedegree of detail and standardisation of this protocol is a major strengthofourwork.

We found betterrotationROMs withthe42-mm gleno-sphere, particularly when a lateralised spacer was used. However,thiscombinationraises anumberofproblemsin everyday practice. Obtaining sufﬁcient exposure toallow its implantation can prove challenging. The considerable bulk of this prosthesis in the joint cavity puts the sur-rounding soft tissues under tension. Excessive tension of thesub-scapularisor teresminormusclesmaycause func-tionalalterationsresponsibleforlossofinternalorexternal rotation[4].Thisfactorisamongtheexplanationsput for-wardbyDeWilde[15]toexplainthelossof rotationseen with a 42-mm glenosphere. Nevertheless, greater gleno-spherediameterclearlyexertsamajorinﬂuenceonrotation ROM.Asuseofthe42-mmglenosphereisnotalways feasi-ble, availabilityof an intermediateglenosphere diameter might be useful. Thus, a 39-mm glenosphere might con-stitutean acceptablecompromise whenimplantationof a 42-mm glenosphere raises insurmountable technical prob-lems.

Inﬂuenceofhumeralretroversiononinternaland externalrotationmotionrange

Inourstudy,increasingthedegree ofhumeral component retroversion improved external rotation while decreasing internal rotation in similar measure. As expected, the opposite occurred when the degree of retroversion was decreased. At eachof the tested retroversionvalues, the inﬂuenceoftheglenospherewasthesameasatthenative retroversionvalue:rotationROMwasgreatestwiththe 42-mmglenosphere,particularlywhenalateralisedspacerwas used.Thiscombination decreased theriskof anteriorand posteriorimpingement.

As with data reported for inferior impingement, our results regarding rotation ROM can be explained by the anatomyofthescapularpillar[16—18].Withhigh retrover-sionvalues,posteriorimpingementoccurslateandtherange

ofexternalrotationishigh.Inthissituation,thehumerusis displacedanteriorly,awayfromthescapularpillar,whichis inamoreposteriorlocation.Anteriorimpingement,in con-trast,occursearlier.Withthe42-mmglenosphere,weeven foundinafewcasesthatanteriorimpingementonthe cora-coidprocessoccurredbeforeimpingementonthescapular pillarcoulddevelop.Thelossofinternalrotationprobably explainstheearlyinferiorimpingementseenwith30◦or40◦ ofhumeralretroversion.Thisistheworstsituationinterms offunctionand subsequentclinical course,asthepatient hasboth ROM limitationand a high risk of early scapular notching.

In previous studies, the value of humeral component retroversion is rarely reported and, when it is known, it varies across authors. In the seminal description of RSA byGrammontetal.[3],the humeralcomponentwas pos-itionedin 30◦ ofretroversion. Valentietal.[10] used20◦ ofretroversiontooptimisethefunctionoftheglenoidand humeralcomponentsbyaligningthemalongthesameaxis. Humeralcomponentretroversionseemstohavelittleimpact on implant alignment, however. In a computed tomogra-phystudy ofnormalshoulders,DeWildeetal. [19]found 8◦ of divergence between the glenoid and humeral axes. Karelse etal. [20] conducted a radio-clinical study of 30 RSAs and observed that gleno-humeral divergence values ofup to34◦ hadno effectoninternal rotation.Thus, no soundbasis existsfor recommendinga specificfixed posi-tionappropriateforall situations.Atthe endof the2006 SOFCOTsymposium,Walch[3]arguedinfavourofusing lit-tleornohumeralcomponentretroversion,asthissituation wasassociated withbenefits on activities of daily living, strength,theabsoluteandweightedConstant’sscores,and passiveanteriorelevation.However,noeffectwasfoundon therateofnotchingafterameanfollow-upof52months. Morerecently,Stephensonetal.[21]reportedthat20◦ to 40◦ofretroversionproducedthegreatestmaximalrotation ROMs without scapular impingement. The measurements wereperformedwiththearmabductedto60◦intheplane ofthescapula.

Thus,therecommendationsvaryandtheoptimaldegree ofretroversionofthehumeralcomponentremainsunclear. Themainfunctionallimitationthataffectseveryday activ-itiesafter RSA is loss of internal rotation,which induces majorimpairments, particularly for dressing and perineal care. In theory, the hand can be placed on the spine if internalrotationis at least100◦, providedretropulsionof theshouldercanbeachievedsimultaneously[22].Everyday activitiesrarelyrequiremorethan90◦ ofexternalrotation withtheelbowbytheside.Inourstudy,thebestbalance betweeninternalandexternal rotationwasobtained with 0◦ to20◦ ofhumeral retroversion. Themeannative retro-versionvalueof17.5◦seemedtoproducethebestresult.As mentionedabove, thisretroversion value wasalso associ-atedwiththegreatestrangetoinferiorimpingement.Thus, withthe 42-mm glenosphereand the native retroversion, therewas a good balance between the maximal ROMs in internal and external rotation, which were 69◦ and 74◦, respectively.Inferiorimpingementoccurredat6◦of eleva-tion.With 40◦ of retroversion, internal rotationwas only 48◦ whereasexternal rotationincreased to86◦, and infe-riorimpingementoccurredearlier,at7◦ ofelevation.This wastheleastfavourablesituation.With0◦ofretroversion,

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inferiorimpingement alsooccurred at 6◦ ofelevation but therotationROMswerenotwellbalanced,asinternal rota-tion increased to 81◦ but external rotation decreased to 60◦. Finally, the only drawbackof using the native retro-version value is limitation of arm elevation in the plane of the scapula. The range of this movement is greater whenretroversiontendstowards0◦.However,cautionisin orderregardingthispoint,asthescapula-thoracicjointwas blockedinourstudy.Thisjointplaysakeyroleinarm ele-vationafterRSA[23].Patientsreceivingfollow-upafterRSA rarelycomplainaboutlossofarmelevation.

Aﬁnalargumentinfavour ofusingthenative retrover-sion value for positioning the humeral component is that itproducesan optimalmatchbetween theprostheticand nativeepiphyses. Withtheextreme retroversionvaluesof 0◦or40◦,wefoundamismatchbetweentheprosthetic epi-physisandproximalhumerus,indicatingariskoffractureof thelesserandgreatertuberosity,respectively.

Conclusion

A 42-mm glenosphere with a 7-mm or 10-mm lateralised spacer produced the best internal and external rotation ROMswiththearmin20◦ofabduction.Anintermediate-size glenospheremeasuring39mmindiametermightconstitute agoodcompromisewhen implantationof a42-mm gleno-sphereraisesmajortechnicalchallenges.

Replicatingthenative humeralretroversion value, usu-ally between 10◦ and 20◦, seems the best option for obtaining a good balance between internal and external rotationmotionranges.

Disclosure

of

interest

BerhouetJulien:none. GaraudPascal:none.

FavardLuc:consultantforTornierInc.

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