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BASIC SCIENCE

Factors affecting the stability of reverse shoulderarthroplasty: a biomechanical study

Allison L. Clouthier, MASca,b, Markus A. Hetzler, BASca,b, Graham Fedorak, MDb,c,J. Tim Bryant, PhDa,b, Kevin J. Deluzio, PhDa,b, Ryan T. Bicknell, MD, MSc, FRCSCb,c,*

aMechanical and Materials Engineering, Queen’s University, Kingston, ON, CanadabHuman Mobility Research Centre, Kingston, ON, CanadacDivision of Orthopaedic Surgery, Department of Surgery, Kingston General Hospital, ON, Canada

Background: Despite the success of reverse shoulder arthroplasty (RSA) in treating patients with painfulpseudoparalytic shoulders, instability is a common complication and currently the factors affectingstability are not well understood. The objective of this study was to investigate a number of factors aswell as the interactions between factors to determine how they affect the stability of the prosthesis.These factors included: active arm posture (abduction and abduction plane angles), loading direction, gle-nosphere diameter and eccentricity, and humeral socket constraint.Methods: Force required to dislocate the joint, determined using a biomechanical shoulder simulator, wasused as a measure of stability. A factorial design experiment was implemented to examine the factors andinteractions.Results: Actively increasing the abduction angle by 15� leads to a 30% increase in stability and use of aninferior-offset rather than a centered glenosphere improved stability by 17%. Use of a more constrainedhumeral socket also increased stability; but the effect was dependent on loading direction, with a 88%improvement for superior loading, 66% for posterior, 36% for anterior, and no change for inferior loading.Abduction plane angle and glenosphere diameter had no effect on stability.Conclusion: Increased glenohumeral abduction and the use of an inferior-offset glenosphere were found toincrease the stability of RSA. Additionally, use of a more constrained humeral socket increased stability foranterior, posterior, and superior loading. These identified factor effects have the potential to decrease therisk of dislocation following RSA.Level of evidence: Basic Science Study, Biomechanical Study.� 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.

Keywords: Reverse shoulder arthroplasty; stability; dislocation; simulator; factorial

Reverse shoulder arthroplasty (RSA) has been success-ful in treating those suffering from massive rotator cuff tearwith glenohumeral arthritis, a condition that results in

uests: Ryan T. Bicknell, MD, MSc, FRCSC, Nickle 3, 76

ngston, ON K7L 2V7, Canada.

ss: rtbickne@yahoo.ca (R.T. Bicknell).

ee front matter � 2013 Journal of Shoulder and Elbow Surgery

/10.1016/j.jse.2012.05.032

a painful, functionally limited shoulder.7,15 This conditionis not adequately managed using conventional shoulderarthroplasty21,24; yet, RSA has been shown to reduce painand improve function for these patients.4,6 With thisprocedure, the ‘‘ball-and-socket’’ anatomy of the shoulderjoint is reversed, allowing a more constrained joint to beimplanted and increasing the deltoid moment arm, amongother biomechanical advantages.

Board of Trustees.

Figure 1 Glenohumeral joint simulator. Force was provided tothe anterior (AD), middle (MD), and posterior (PD) deltoid cablesby pneumatic actuators. A material testing machine was used asa linear actuator to impose a displacing force, which could beapplied anteriorly, posteriorly, superiorly, or inferiorly through theuse of pulleys. Optical marker triads on the humerus and scapulawere used to capture shoulder kinematics.

440 A.L. Clouthier et al.

The complication rate associated with RSA, however,remains high, with rates in clinical studies ranging from 8%to 55%.11,13,28,29,31 Often, the most common complicationis cited as being instability of the shoulder.6,7,9,33 This isconcerning as management of instability is difficult anddislocation often results in a loss of shoulder function andthe need for additional surgery.10

Cadaveric andnoncadaveric shoulder simulators thatmodelmuscle loading and bony geometry have been used to examineshoulder biomechanics, including shoulder injuries,25 musclefunction,1 and traditional arthroplasty.27 The use of suchsimulators in investigations of RSA stability has been limited.However, a recent cadaveric study found that humeralcomponent version and humeral cup thickness have minimaleffect on stability.18 This contradicts results found in a biome-chanical study that used the prosthesis components alone,which found that having the humeral component in someanteversion improved intrinsic stability of the prosthesis.12

This discrepancy indicates the need to examine reverseshoulder biomechanics in a physiological model to betterunderstand behavior of the prosthesis in vivo. In addition,moststudies have examined single factors separately, neglectinginteractions between factorswhich could provide better insightinto the complicated nature of the instability issue.

There exists a large variety of factors which maycontribute to stability of RSA. These are based on clinicalresults indicating that, for example, the reverse shouldermay be more prone to dislocation in certain directions2 orthat some arm positions may lead to instability.22 Otherfactors, such as compressive force and humeral socketdepth,16 have been found to have an effect on the intrinsicstability of the prosthesis components but have not beenexamined in a physiological model that accounts for theeffect of active constraints, such as muscle loading, andpassive constraints, such as bony geometry and soft tissuebulk. Additionally, some factors, such as glenosphereeccentricity19 and diameter,13 have been theorized to affectstability but have not yet been investigated. None of thesefactors have been examined in a model that includes bonygeometry and active muscle loading.

Therefore, the objective of this study was to determinethe effect of loading direction, active arm posture, gleno-sphere diameter, glenosphere eccentricity, and humeralsocket constraint on stability and the interactions betweenthese factors in a simulator that includes active muscleloading and bony geometry.

Materials and methods

Joint simulator

Stability testing was performed using a custom glenohumeral jointsimulator (Fig. 1). Delta XTEND (DePuy Inc., Warsaw, IN, USA)components were implanted in Sawbones (Pacific ResearchLaboratories, Inc., Vashon, WA, USA) with both the glenosphere

and humeral component in neutral version. The scapula was fixed tothe simulator frame while the humerus was free to move. The 3heads of the deltoid (anterior, middle, and posterior deltoid) weremodeled using aircraft cable and muscle forces were applied usingpneumatic actuators (Bimba Manufacturing Company, UniversityPark, IL, USA) and were recorded using in-line load cells (Trans-ducer Techniques, Inc., Temecula, CA, USA). Muscle insertionpoints and paths were determined from previous anatomicalstudies,8,20,26 and the cables were connected to the humerus using3-degree-of-freedom ball joints. As the rotator cuff is oftencompromised in patients who receive this surgery, a worst casescenario was assumed and no rotator cuff muscles were included inthe model. A 3.6-kg weight was attached to the distal humerus torepresent the weight of the arm.32

A cable wrapped around the humeral component at theprosthesis-bone interface connected to a material testing machine(Instron, Norwood, MA, USA) applied the displacing force to thehumeral head. This location of force application has been usedpreviously14,18 and was chosen as it is the closest to the center ofrotation that the force can be applied, thereby minimizing appliedmoments. Through the use of pulleys, the displacement could bedirected in 1 of 4 primary directions relative to the scapula(anterior, posterior, superior, or inferior). Displacements wereapplied at 100 mm/min.

Motion tracking was performed using an Optotrak Certussystem (Northern Digital Inc., Waterloo, ON, Canada). Shoulderangles were calculated based on the coordinate systems recom-mended by the International Society of Biomechanics,30 as shownin Figure 2. Shoulder angles were calculated based on a YXYEuler sequence, resulting in abduction, abduction plane, andinternal rotation angles. The abduction angle measures gleno-humeral elevation with 0� representing a resting arm. Theabduction plane angle is similar to horizontal flexion and repre-sents the plane that abduction occurred in, with 0� being abductionin the scapular plane. Finally, internal rotation represents rotationabout the long axis of the humerus.

Experimental design

Six factors were analyzed in this screening experiment: loadingdirection, abduction angle, abduction plane angle, socket depth,

Figure 2 Coordinate systems used. ME, medial epicondyle; LE,lateral epicondyle; GH, glenohumeral joint center; AA, acromionangle; TS, trigonum spinae; IA, inferior angle. The origin of thehumeral coordinate system is at GH; Yh is the line from GH to themidpoint of LE and ME; Xh is normal to the plane formed by ME,LE, and GH pointing anteriorly; and Zh is the cross-product of Xh

and Yh. The origin of the scapular coordinate system is at AA; Zs isthe line pointing from TS to AA; Xs is normal to the plane formedby AA, TS, and IA; and Ys is the cross-product of Zs and Xs.

Stability of reverse shoulder arthroplasty 441

glenosphere diameter, and glenosphere eccentricity. It was possibleto modify each of these factors without reinstallation of the pros-thesis, thus eliminating any confounding variation in the orientationof the implant in the bone. Each factor was tested at a low and highlevel, representing the range of variation for the factor, with theexception of loading direction which had 4 levels. The angles werechosen to be the extremes of the shoulder’s range of motion (ROM)without impingement of the humeral socket on the scapula, andwere achieved by applying experimentally derived forces to thedeltoid cables. For component-related factors, high and low levelswere chosen based on the options available in the Delta XTENDsystem. The factors and levels of each factor are shown in Figure 3.

One replicate of a half-fraction factorial design experiment23

was carried out to examine these 6 factors in 64 trials. All factorswere varied concurrently, resulting in 32 trials performed at eachlevel for all factors except loading direction, for which 16 trialswere performed at each level. Based on implementation of a halffraction, as opposed to a full, factorial design, there was aliasingbetween the following 2-factor interactions: loading direction/abduction angle and abduction plane/eccentricity, loading direc-tion/abduction plane and abduction/eccentricity, and loadingdirection/eccentricity and abduction/abduction angle. As a resultof this, if one of these interactions is significant, then its aliasedinteraction will show significance as well, whether or not it is trulysignificant. If any aliasing issues are encountered, however, aliasingof interactions can be eliminated through completion of the otherhalf of the factorial experiment.

Data analysis

The displacing force required to dislocate the shoulder joint wasused as a measure of stability for this investigation. Significantfactors were determined using a normal probability plot of effectestimates23 for the 6 factors and all 2-factor interactions. Thesewere verified using a 6-way ANOVA and post hoc Tukey testswhere P < .05 was considered significant.

Results

Force to dislocate the shoulder was used as a measure ofstability for this study. To determine the repeatability of thisvalue, the shoulder was dislocated 5 times in each direction,with the same arm posture and components each time.Based on the methods of Bland and Altman,3 it wasdetermined that the force to dislocate in 2 replicate trialswould be within 14.1 N of one another 95% of the time.

Four active shoulder postures were used in this experi-ment (2 levels of abduction, 2 levels of abduction plane).The muscle forces applied and resulting shoulder angles foreach posture used are shown in the Table. Standard devi-ations were found to be less than 0.4 N for force inputs and4.5� for shoulder angles.

It was determined that abduction (P < .0001), socketdepth (P < .0001), and glenosphere eccentricity (P < .001)had a significant effect on force to dislocate the joint. Inaddition, the loading direction/socket depth interaction wasdeemed significant (P < .001).

Actively increasing glenohumeral abduction from 45� to60� was found to increase force to dislocate by 59.5 N(30.5%) (Fig. 4). Changing from a centered to an inferior-offset glenosphere increased force to dislocate by 35.3 N(17.0%). Force to dislocate was not affected by changes inthe abduction plane or the diameter of the glenosphere(P > .05). Increasing the socket depth of the humeralcomponent by changing from a high mobility cup toa retentive cup resulted in an increase in force to dislocatethe joint as well; but it was dependent on loading direction(Fig. 5). The increase in stability occurred for superior,posterior, and anterior loading, but not inferior. Force todislocate was increased by 140.8 N (88.0%), 109.6 N(66.3%), and 70.5 N (35.9%) for superior, posterior, andanterior loading, respectively. Also contributing to thisinteraction was the result that when using a retentivehumeral cup, force to dislocate was 69.1 N (29.8%) greaterfor superior loading than inferior loading. No other 2-factorinteractions were found to be significant.

Discussion

Instability is often the most common complication followingRSA; therefore, understanding how to decrease the risk ofdislocation is crucial for improving outcomes of this surgery.This study determined which factors influence force todislocate in RSA and how they interact with one another.

Increased glenohumeral abduction significantly improvedstability independent of all other factors investigated. Thiseffect was expected as increased abduction was achieved viaincreased muscle forces, thus producing a larger joint reac-tion force. In a previous study of the intrinsic stability ofRSA, compressive force was found to be the most importantof the 3 factors affecting stability that were studied.16 It isdifficult to make recommendations for risky arm postures

Figure 3 Factors and levels investigated.

Table I Forces applied to each muscle and resulting shoulder angles for low and high levels of the abduction and abduction planefactors. Standard deviations are shown in brackets (n ¼ 16 for each posture)

Factor level (�) Muscle force (N) Shoulder orientation (�)

Abduction plane Abduction Anterior deltoid Middle deltoid Posterior deltoid Abduction plane Abduction Internal rotation

�10 45 30.02 (0.09) 65.13 (0.15) 50.07 (0.19) �13.27 (3.35) 44.40 (3.25) 7.12 (2.66)�10 60 30.01 (0.10) 99.97 (0.07) 49.94 (0.17) �6.19 (3.00) 61.58 (3.17) 2.39 (2.50)55 45 70.05 (0.11) 65.14 (0.35) 29.93 (0.10) 61.72 (4.35) 44.60 (3.48) �9.10 (4.40)55 60 70.14 (0.29) 100.09 (0.30) 30.08 (0.28) 53.20 (2.86) 63.55 (4.47) �2.67 (2.74)

442 A.L. Clouthier et al.

based on this as consistently maintaining high degrees ofabduction is not practical. However, as this effect is likelyattributed, in part, to the increased deltoid forces, it reiteratesthe suggestion that has been made previously that increasingdeltoid tension may help to stabilize the joint.5,15 Care mustbe taken not to over-tension the deltoid, however, as thiscauses excess strain on the acromion which may lead to itsfracture as well as the potential for pain and a limitedROM.13

This increase in stability based on larger muscle forces alsoindicates that active rehabilitationmay bemore beneficial forpatients than passive rehabilitation, as the muscle activitycould reduce the risk of instability.

Glenosphere eccentricity also had an impact on force todislocate. One explanation for this may be that, in vivo, aninferior-offset glenosphere may increase deltoid tension. Inaddition, one mode that has been suggested for dislocation isimpingement causing a levering mechanism leading todislocation of the joint.13Another benefit of the inferior-offsetglenosphere is that it helps to prevent inferior impingement.

Humeral socket depth was found to have a large effect onstability that was dependent on loading direction. Thesignificant effect of humeral socket depth was expected asthe deeper socket options exist specifically to increase thestability of the joint, which has previously been found to havea significant effect on stability.16 However, because of theimplementation of a factorial experiment design that allows

2-factor interactions to be investigated, the dependence of thiseffect on direction of dislocation was identified. This result isbased, in part, on the unconstrained nature of humerus duringdislocations. Because of this, the applied displacing forceoften caused the humerus to rotate prior to subluxation;therefore, in directions that allow greater ROM (ie, inferior,anterior), the humeral cup was rotated to a position where itcovers less of the glenosphere, resulting in the increasedsocket depth becoming less effective. This reflects what couldhappen in vivo and indicates that although increased socketdepth adds stability to the joint in general, it does depend onthemode of dislocation. It should also be noted that there is anassociated trade-off between mobility and stability, as theretentive cups are more prone to impingement on the scapulaand have a reduced theoretical ROM17; therefore, the decisionof humeral socket depth must be a patient specific one.

The remaining factors, abduction plane and glenospherediameter, had no impact on force to dislocate. The minimaleffect of glenosphere diameter agrees with results foundpreviously.16 The lack of effect of abduction plane is likelyattributed to the unconstrained humerus in the experimentalsetup; however, it was felt that an unconstrained humerusbetter represented how dislocations may occur in vivo.

This was the first study to investigate the effects ofshoulder orientation and glenosphere eccentricity on thestability of RSA and the first to examine the effects of

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Figure 4 Force to dislocate by factor. Modifying abduction, socket depth, and glenosphere eccentricity created significant changes inforce to dislocate the shoulder. An asterisk indicates P < .05.

Figure 5 Interaction between loading direction and socketdepth. Increasing the socket depth from a high mobility cup toa retentive cup caused significant increases for all loading direc-tions except inferior loading. There was also a significant differ-ence in force to dislocate between superior and inferior loadingwhen using a retentive cup. An asterisk indicates P < .05.

Stability of reverse shoulder arthroplasty 443

glenosphere diameter and humeral socket constraint usingan anatomical shoulder model, which is critical for under-standing the biomechanics of the prosthesis under theinfluence of biological factors such as muscle loading andbony geometry. Another strength of this study lies in itsfactorial design which allowed interactions between factorsto be analyzed along with main effects.

There are some limitations associated with this experi-mental setup. Although this simulator models a more phys-iological setting than some simulators by including activemuscle loading, the effect of soft tissue bulk and ligamentousstructures are still not accounted for, and these likely have an

influence on the stability as well. Therefore, further researchshould be undertaken in cadaver models to determine howthese and other factors impact stability with various softtissue structures in place aswell as othermuscles crossing theglenohumeral joint. Additionally, these experiments werecarried out using a Grammont-style prosthesis, and it ispossible that different results could be encountered for otherimplant designs. Another limitation is that dislocation wasmodeled by applying a force to the humeral head. This maynot fully replicate all possible mechanisms of dislocation;however, it is believed to give a good representation ofstability while simplifying and focusing the experiment.Finally, the aliasing of some 2-factor interactions was alsoa potential limitation; but, as none of the aliased interactionswere significant, any such issues were avoided.

Future directions for this work include closer examinationof these factors and others in a cadaver model that includesthe passive constraint created by soft tissue structures. It isalso critical that interactions between factors be examined.None of these factors operates in isolation in vivo; it is,therefore, essential to understand interactions betweenfactors to fully understand the problem of instability in RSA.Fully understanding howvarious factors affect the stability ofRSA will provide knowledge that can be implementedintraoperatively to improve the results of this procedure.

Conclusion

Changes in abduction plane angle and glenospherediameter have no effect on the stability of reverse shoulderarthroplasty. However, active glenohumeral abductionincreases the stability indicating that increased deltoidtension and active rehabilitation may decrease the risk ofdislocation for this prosthesis. Using an inferior-offset

444 A.L. Clouthier et al.

glenosphere also increases stability and increasedhumeral socket depth improves stability for all loadingdirections, with the exception of inferior loading.

Disclaimer

The authors, their immediate families, and any researchfoundations with which they are affiliated have notreceived any financial payments or other benefits fromany commercial entity related to the subject of this article.

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