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The impact of scapular notching on reverse shoulder glenoid fixation
J Shoulder Elbow Surg (2013) 22, 963-970
www.elsevier.com/locate/ymse
The impact of scapular notching on reverse shoulder glenoid fixation Christopher P. Roche, MSa, Nicholas J. Stroud, MSa, Brian L. Martin, BSa, Cindy A. Steiler, AAa, Pierre-Henri Flurin, MDb, Thomas W. Wright, MDc, Matthew J. DiPaola, MDd, Joseph D. Zuckerman, MDe,* a
Exactech, Gainesville, FL, USA Bordeaux-Merignac Clinic, Bordeaux-Merignac, France c Department of Orthopedic Surgery, University of Florida, Gainesville, FL, USA d Department of Orthopedic Surgery, Wright State University, Dayton, OH, USA e Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, New York, NY, USA b
Background: Scapular notching is a well-documented complication of reverse shoulder arthroplasty. The effect of scapular notching on glenoid fixation is unknown. Materials and methods: This study dynamically evaluated reverse shoulder glenoid baseplate fixation and assessed the effect of scapular notching on fixation in composite scapulae. A cyclic test was conducted to simulate 55 of humeral abduction in the scapular plane as a 750-N axial load was continuously applied to induce a variable shear and compressive load. Before and after cyclic loading, a displacement test was conducted to measure glenoid baseplate displacement in the directions of the applied static shear and compressive loads. Results: For the scapulae without a scapular notch, glenoid baseplate displacement did not exceed the generally accepted 150-mm threshold for osseous integration before or after cyclic loading in any component tested. For the scapulae with a scapular notch, glenoid baseplate displacement exceeded 150 mm in 2 of the 7 samples before cyclic loading and in 3 of the 7 samples after cyclic loading. The average pre-cyclic glenoid baseplate displacement in the direction of the shear load was significantly greater in scapulae with a scapular notch than those without a scapular notch both before (P ¼ .003) and after (P ¼ .023) cyclic loading. Conclusions: Adequate glenoid baseplate fixation was achievable in most cases in scapulae with a severe scapular notch; however, the fact that this micromotion threshold was not met in all scapulae with a notch is concerning and implies that severe notching may play a role in initial glenoid baseplate stability. Level of evidence: Basic Science Study, Biomechanical Study. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Reverse shoulder arthroplasty; glenoid fixation
No Investigational Review Board approval was required for this biomechanical evaluation using composite scapulae. *Reprint requests: Joseph D. Zuckerman, MD, NYU Hospital for Joint Diseases, Department of Orthopaedic Surgery, 301 E 17th St, 14th Flr, New York, NY 10003, USA. E-mail address: [email protected] (J.D. Zuckerman).
Reverse shoulder arthroplasty is a viable treatment option for patients with rotator cuff tear arthropathy and other degenerative diseases of the glenohumeral joint in which the fixed fulcrum created by the inverted articular curvatures (ie, the convex glenoid and the concave
1058-2746/$ - see front matter Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2012.10.035
964 humerus) provide the stability necessary for shoulder function.4,14,18,48,49,52 Because of significant improvements in pain and function for certain patients in which there is no other treatment option, reverse shoulders are also used in revision and salvage procedures where soft tissue and bone are often severely compromised.4,12,15,28,33,49,52,54 Despite these successes, reverse shoulder arthroplasty is by no means a perfect solution. Concerns remain due to high complication rates54 and deteriorating functional results with longer follow-up.18 Scapular notching has been reported to occur in w70% of patients with reverse shoulder designs that have a medialized center of rotation (>20% of which are large notches of grade 3 or 4) and has been demonstrated to correlate with radiolucent lines and clinical outcomes.4,6,7,26,27,32,33,46-49,51-54 The effect of scapular notching on glenoid fixation is unknown and particularly worrisome given that glenoid loosening was the primary failure mode of historical (eg, Grammont predecessor) reverse shoulder prostheses whose center of rotation was laterally offset relative to the glenoid fossa.4,13,16,17,40 Short-term and medium-term clinical outcome studies have reported aseptic glenoid loosening rates between 0% and 12% with modern reverse shoulder arthroplasty designs,15,18,48,52 with an average rate of 5%.54 A recent study by Guery et al18 projected the reverse shoulder glenoid loosening survival curve to be 84% at 10 years. Shortterm glenoid loosening rates may increase when used in conjunction with glenoid wear, particularly if the glenoid prosthesis is superiorly tilted or lateralized using bone graft. Long-term loosening rates may be underestimated due to the widespread prevalence of scapular notching as a complication; with longer follow-up, a number of asymptomatic patients with notching may eventually develop some form of glenoid loosening. These and other concerns led Sirveaux et al48 to project a survival curve with failure defined as revision at 5, 7, and 8 years to be 91.3%, 74.6%, and 29.8%, respectively.48 It is currently unknown whether and how many patients with asymptomatic scapular notching will eventually develop symptomatic loosening long-term. Additional clinical follow-up is required to better understand the implications of scapular notching on the reverse shoulder glenoid loosening rate. Among the numerous potential factors influencing the reverse shoulder glenoid-loosening rate are mechanical impingement, follow-up, bone quality, bone stock, initial fixation strength, and heightened patient demands. Because the reverse shoulder glenoid component is uncemented, aseptic glenoid loosening can occur due to insufficient initial fixation. The generally accepted prosthesis displacement threshold necessary to promote osseous integration and longterm fixation is 150 mm; prosthesis displacements >150 mm may result in fibrous in-growth instead.8,23,42 Poor initial fixation is likely to result in screw fatigue failure and aseptic glenoid loosening. Many recent reverse shoulder biomechanical studies aim to better understand the factors related to glenoid loosening
C.P. Roche et al. and demonstrate a widespread desire to reduce the glenoidloosening rate.10,11,20-22,44,45 This study used a dynamic glenoid loosening test method that simulates the physiologic loading conditions typical of reverse shoulder arthroplasty. This test method has been used in other studies to demonstrate significant differences in displacement between different numbers of screws, different screw configurations, variable (eg, medialized/lateralized) center of rotation, and different densities of bone substitute substrates.44,45 The purpose of this study was to quantify glenoid fixation in composite scapulae with and without a scapular notch and evaluate the null hypothesis that a grade 4 scapular notch has no impact on the fixation of a reverse shoulder glenoid implant.
Materials and methods Fourteen 38-mm Equinoxe reverse shoulder implants (Exactech Inc, Gainesville, FL, USA) were tested in a fourth-generation composite/dual density scapula (Pacific Research, Inc, Vashon, WA, USA) with a 1.63 g/cm3 ‘‘cortical’’ shell and a 0.27 g/cm3 ‘‘cancellous’’ interior structure. This substrate provides a representative substitute for the density, strength, and modulus of glenoid cortical and cancellous bone in the recipient patient population for reverse shoulder arthroplasty.1,25,30,38 A custom cutting jig was designed to create a Nerot-Sirveaux48 grade 4 scapular notch (n ¼ 7) in the dual-density composite scapulae (Fig. 1 and Fig. 2) The scapular notch was cut to approximate the profile of the humeral liner as it articulates around the center of rotation of the 38-mm glenosphere. The fixation of these notched scapulae was compared with that of composite scapulae without a scapular notch (ie, Nerot-Sirveaux48 grade 0; n ¼ 7) before and after cyclic loading. For the notched and non-notched scapulae, initial fixation of the glenoid baseplate was achieved using 4 (1 superior, 3 inferior), 4.5- 30mm polyaxial locking compression screws and a press-fit tapered cage peg; because the inferior screw is often fractured in a grade 4 scapular notch, a shorter (18-mm) inferior screw was used in the notched scapulae (the other 3 screws were 30 mm long). The 18mm inferior screw was selected because it is the shortest length offered and only had 1 thread engaged in the medial portion of the notched scapular neck, minimizing its fixation contribution. After assembly, each composite scapulae was cut and potted with bone cement. This reverse shoulder glenoid-loosening test was conducted in 2 phases. The first phase is the displacement test, which measures the fixation of the reverse shoulder glenoid baseplate in the composite scapulae before and after the application of 10,000 cycles of dynamic loading for 55 at 0.5 Hz. In the displacement test, the axial test machine (Instron Corp, Norwood, MA, USA; resolution of 1 mm) and 3 digital indicators (Mitutoyo, Kawasaki, Japan; resolution of 1 mm) quantify the glenoid baseplate displacement relative to the composite scapula as a compressive (433 N) and shear (357 N) load is applied. The compressive axial load is applied perpendicular to the reverse glenoid baseplate and the shear load is applied parallel to the face of the glenoid baseplate along its superior/inferior axis (Fig. 3) The specific combination of compressive and shear load was based on the findings of Poppen et al,43 who described the
Reverse shoulder glenoid fixation and scapular notching
965
Figure 1 Posterior views of the (A) non-notched (grade 0) composite scapula and the (B) grade 4 scapular notch composite scapula with cut guide. positioned in the biaxial testing apparatus and aligned along the superior/inferior axis of the glenoid baseplate. A 750-N axial load is constantly applied through the center of the humeral liner as the glenosphere/glenoid baseplate/composite scapula are rotated about the humeral component with a stepper motor to create a sinusoidal angular displacement profile encompassing an arc of 55 at 0.5 Hz for 10,000 cycles (Fig. 4) This loading profile over the 55 arc of abduction in the scapular plane would induce a maximum shear load of 346 N at each extreme of rotation (27.5 and þ27.5 ) and a maximum compressive load of 750 N at the midpoint of the arc of rotation (ie, 0 ; Fig. 5). The components are cooled with a continuous jet of air, with no lubrication. Statistical analysis was performed by means of a 2-tailed unpaired Student t test (significance defined as P < .05) to compare prosthesis displacements relative to each scapulae (notched and non-notched) in the directions of the shear and compressive loads before and after cyclic loading.
Results Figure 2
Image of grade 4 scapular notch composite scapula.
maximum shear load in the shoulder to be 0.42 body weight (BW) at 60 abduction, where the corresponding compressive load was 0.51 BW (at 60 abduction). We assumed a BW of 86.7 kg (191 lb), the average weight of a man in the United States.9 This yielded a shear load of 357 N (0.42 86.7 kg) and a compressive load of 433 N (0.51 86.7 kg) during the displacement test. The 55 arc of motion used in the cyclic test simulates w25 to 80 of abduction in the scapular plane relative to a fixed scapula; the displacement test is performed at the center of this 55 arc of motion. Considering that the Poppen et al43 60 abduction measurements included scapula-thoracic motion, the displacement test is performed on the glenoid component at the approximate orientation of this peak shear load. Digital indicators are used to subtract any compliance of the test construct. The second phase is the cyclic test. The cyclic test simulates the primary motion of reverse shoulder arthroplasty; that is, the abduction motion generated by the deltoid.4,16,17,35 The humeral liner and glenosphere/glenoid baseplate/composite scapula are
For the composite scapulae without a scapular notch (eg, grade 0), glenoid baseplate displacement did not exceed the generally accepted 150-mm threshold for osseous integration before (maximum shear and compression displacement was 99 and 113 mm, respectively) or after (maximum shear and compression displacement was 103 and 102 mm, respectively) cyclic loading in any component tested. For the composite scapulae with a scapular notch (eg, grade 4), glenoid baseplate displacement exceeded 150 mm in 2 of the 7 samples before cyclic loading (maximum shear and compression displacement was 153 and 150 mm, respectively) and in 3 of the 7 samples after (maximum shear and compression displacement was 253 and 181 mm, respectively) cyclic loading. The average pre-cyclic glenoid baseplate displacement in the directions of the shear and compression loads was 66 19 and 78 27 mm, respectively, for the composite scapulae without a scapular notch and 114 28 and 99 42 mm, respectively, for the composite scapulae with
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C.P. Roche et al. with a scapular notch. The average glenoid baseplate displacement in the direction of the shear load was significantly greater in the composite scapulae with a scapular notch than in the composite scapula without a scapular notch before (P ¼ .003) and after (P ¼ .023) cyclic loading. The average glenoid baseplate displacement in the direction of the compression load was not statistically different in the composite scapulae with a scapular notch than in the composite scapula without a scapular notch both before (P ¼ .277) and after (P ¼ .292) cyclic loading.
Discussion
Figure 3 Displacement test performed to measure initial glenoid baseplate fixation before and after cyclic loading as a 357-N shear load and a 433-N compressive load are applied to the glenoid baseplate.
Figure 4 Cyclic test performed to simulate 55 abduction in the scapular plane for 10,000 cycles at 0.5 Hz as a 750-N load is applied.
a scapular notch. The average post-cyclic glenoid baseplate displacement in the directions of the shear and compression loads was 58 42 and 83 22 mm, respectively, for the composite scapulae without a scapular notch and 134 65 and 105 46 mm, respectively, for the composite scapulae
Shoulder surgeons have recently turned to reverse shoulder arthroplasty as a solution for many previously difficult-totreat conditions. Reverse shoulder arthroplasty is most commonly used for rotator cuff tear arthropathy; however, indications have expanded to include comminuted proximal humeral fractures in the elderly, tumors, proximal humeral malunions and nonunions, and failed anatomic total shoulder replacement.4,31,34,36,37 Although the exact usage is difficult to know, a steep increase has occurred in procedures performed in the United States since the device was cleared by the U.S. Food and Drug Administration in March 2004. Joshi et al24 reported that 2652 reverse shoulder arthroplasty procedures were performed in the United States in 2005 and 15,200 in 2008 and projected 31,584 reverse shoulders will be performed in the United States in 2012.24 Given this steep increase in usage and that the long-term survival of these implants is unknown, the number of reverse shoulder arthroplasty revision surgeries is expected to increase in the coming years. In many of these revision surgeries, the shoulder surgeon can expect that the scapular bone will be compromised due to the high prevalence of scapular notching as a complication.4,6,7,32,33,46-49,51-54 Severe scapular notching may potentiate screw loosening and loss of glenoid baseplate fixation in the initial postoperative revision period. In this case, the shoulder surgeon will be faced with a choice between conversion of the failed reverse shoulder arthroplasty to a hemiarthroplasty or attempting to revise to a new, reverse shoulder. The inferior scapula is a difficult area in which to graft bone, and little information is available to guide shoulder surgeons in this clinical scenario. This study attempted to help clarify whether initial fixation and stability is possible in a representative bone model of a severely notched scapula and demonstrated that in most cases, the generally accepted 150-mm displacement threshold for osseous integration was met before (5 of 7 scapulae) and after (4 of 7 scapulae) cyclic loading in the scapula bone model with a grade 4 scapular notch. In the scapula bone model without a scapular notch, glenoid baseplate displacement did not exceed the generally accepted 150-mm threshold before or after cyclic loading in
Reverse shoulder glenoid fixation and scapular notching
Figure 5
967
Shear and compressive reaction forces during the cyclic test (27.5 ).
any component tested. In addition, the glenoid baseplate motion associated with the non-notched scapulae in the direction of the applied shear load was significantly less before (P ¼ .003) and after (P ¼ .023) cyclic loading than that of the scapulae with a scapular notch. Glenoid baseplate fixation was achievable in most cases in scapulae with a severe scapular notch; however, that this micromotion threshold was not met in all scapulae with a notch is concerning and implies that severe notching may play a role in initial glenoid baseplate stability. Therefore, the results of our study lead us to reject the null hypothesis and conclude that a large scapular notch does affect glenoid fixation in reverse shoulder arthroplasty. The testing scenario in this study used 4 screws to achieve fixation. The glenoid baseplate used in this study permits a total of 6 polyaxial locking compression screw positions. It is unclear whether adding more screws to the construct could potentially mitigate the increased micromotion observed in a severely notched scapula scenario. Future work should evaluate this potential strategy given that a previous study demonstrated significant differences in glenoid baseplate displacement between different numbers of screws and different screw configurations in a 0.24 g/cm3 polyurethane bone substitute model.44 These fixation results may not be applicable to all reverse shoulder glenoid baseplates because not all offer 4 polyaxial locking compression screws or have as large of a glenoid baseplate surface area. A few study design considerations warrant discussion. For the cyclic test, the 750-N axial load we used to simulate a worst-case loading condition is deemed appropriate because the loading of reverse shoulder arthroplasty is expected to be less than that of an anatomic total shoulder arthroplasty due to a variety of factors:
First, the reverse shoulder is primarily used in elderly, low-demand patients. Most of these patients have compromised rotator cuff musculature, resulting in a lower joint reaction force. Additionally, there are inherent design features of the reverse shoulder itself, such as the medially shifted center of rotation that increases the deltoid abductor moment arm requiring the deltoid to generate less torque to elevate the arm, and the distally shifted center of rotation that elongates the deltoid and improves its resting tension and tone.5,19,29,39,41,50 Although 10,000 cycles is low relative to hip and knee testing and less than the 100,000 cycles recommended by the ASTM F 2028-08 anatomic total shoulder glenoid loosening test, this value is justified because this test does not simulate prosthesis wear-related failure modes but instead the limited period in which initial fixation of a noncemented glenoid prosthesis is attained.2,3,21 In the displacement test, the shear and compressive loads were applied directly to the glenoid baseplate to ensure that there was no motion between the glenosphere and glenoid baseplate. Finally, the use of the 150-mm threshold established in previous studies was assumed to be translatable to describe the initial fixation of the glenoid in reverse shoulder arthroplasty. Numerous studies10,11,20-22 have attempted to evaluate the initial fixation of a reverse shoulder glenoid baseplate. Each study was limited in how it simulated the loading conditions of reverse shoulder arthroplasty because they failed to dynamically simulate abduction,10,11,20-22 used too dense of a glenoid substrate (eg, 0.48 g/cm3 polyurethane blocks),20 tested it for an insufficient number of cycles (eg, 500 or 1000),10,11,20 or applied too extreme
968 a combination of shear and compressive load for a reverse shoulder.20-22 The simple test method used in this study attempted to simulate the function of a reverse shoulder and evaluate its fixation in a composite scapula. Previous reverse shoulder glenoid loosening studies20-22 attempted to apply ASTM F 2028-08 to reverse shoulders; however, the use of ASTM F 2028-08 is inappropriate because the reverse shoulder glenoid and humeral articular components are semiconstrained and conforming and only permit rotation; whereas, the anatomic glenoid and humeral components are unconstrained and nonconforming and permit both rotation and translation. The cyclic test used in this study attempted to simulate this semiconstrained, conforming articulation by only rotating (not translating) the glenosphere about the humeral liner in the scapular plane. Our reverse shoulder glenoid loosening study has some limitations. First, we used a composite scapula model rather than matched pair cadaveric specimens to reduce variables related to cortical and cancellous density, cortical thickness, and scapular morphology and ensure that the created scapular notch was identical in each tested specimen. Second, loading in the glenohumeral joint is complex, and we used a constant 750-N axial load during the cyclic test to simulate these loading conditions, which induced a variable compression and shear load depending on the angle of articulation (Fig. 5). This loading profile is more physiologically relevant than the extreme 756-N compressive and 756-N shear load combinations previously used by Harman et al,20 Hoenig et al,21 and Hopkins et al.22 Terrier et al50 recently compared the muscle forces in both the anatomic and reverse shoulder arthroplasty designs and found that the forces associated with the reverse shoulder varied between 30% and 50% less than that of the anatomic shoulder, depending on the presence of different rotator cuff muscles (only a supraspinatus tear vs no rotator cuff, respectively). Terrier et al50 reported the maximum force in an anatomic shoulder was 0.86 BW, the maximum force in a reverse shoulder with no rotator cuff was 0.42 BW, and the maximum force in a reverse shoulder with only a supraspinatus tear was 0.62 BW; these values are similar to what was reported by Kontaxis et al.29 Finally, we used digital indicators to measure glenoid baseplate displacement before and after cyclic loading but did not measure displacement during the cyclic test.
Conclusions The incidence of revision reverse shoulder arthroplasty is projected to increase over time. Severe scapular notching can be a challenging clinical scenario that does seem to have an effect on glenoid baseplate fixation in a minority of severely notched composite scapulae
C.P. Roche et al. tested. Further study will help clarify whether rescue strategies exist to mitigate this issue.
Disclaimer C.P.R., N.J.S., B.L.M., and C.A.S. are employees of Exactech Inc. P.-H.F., T.W.W., and J.D.Z. receive royalties from Exactech Inc. The other authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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Reverse shoulder glenoid fixation and scapular notching 14. Frankle M, Siegal S, Pupello D, Saleem A, Mighell M, Vasey M. The reverse shoulder prosthesis for glenohumeral arthritis associated with severe rotator cuff deficiency. A minimum two-year follow-up study of sixty patients. J Bone Joint Surg Am 2005;87:1697-705. http://dx.doi. org/10.2106/JBJS.D.02813 15. Frankle MA, Teramoto A, Luo Z-P, Levy JC, Pupello D. Glenoid morphology in reverse shoulder arthroplasty: classification and surgical implications. J Shoulder Elbow Surg 2009;18:874-85. http:// dx.doi.org/10.1016/j.jse.2009.02.013 16. Grammont P, Baulot E. Delta shoulder prosthesis for rotator cuff prosthesis. Orthopedics 1993;16:65-8. 17. Grammont P, Trouilloud P, Laffay JP, Deries X. Etude et realisation d’une nouvelle prothese d’epaule. Rhumatologie 1987;39:407-18. 18. Guery J, Favard L, Sirveaux F, Oudet D, Mole D, Walch G. Reverse total shoulder arthroplasty. Survivorship analysis of eighty replacements followed for five to ten years. J Bone Joint Surg Am 2006;88: 1742-7. http://dx.doi.org/10.2106/JBJS.E.00851 19. Halder AM, Zhao KD, Odriscoll SW, Morrey BF, An KN. Dynamic contributions to superior shoulder stability. J Orthop Res 2001;19: 206-12. 20. Harman M, Frankle M, Vasey M, Banks S. Initial glenoid component fixation in ‘‘reverse’’ total shoulder arthroplasty: a biomechanical evaluation. J Shoulder Elbow Surg 2005;14(1 Suppl S):162-167S. http://dx.doi.org/10.1016/j.jse.2004.09.030 21. Hoenig MP, Loeffler B, Brown S, Peindl R, Fleischli J, Connor P, et al. Reverse glenoid component fixation: is a posterior screw necessary? J Shoulder Elbow Surg 2010;19:544-9. http://dx.doi.org/10.1016/j.jse. 2009.10.006 22. Hopkins AR, Hansen UN, Bull AMJ, Emery R, Amis AA. Fixation of the reversed shoulder prosthesis. J Shoulder Elbow Surg 2008;17:97480. http://dx.doi.org/10.1016/j.jse.2008.04.012 23. Jasty M, Bragdon C, Burke D, O’Connor D, Lowenstein J, Harris WH. In vivo skeletal responses to porous-surfaced implants subjected to small induced motions. J Bone Joint Surg Am 1997;79: 707-14. 24. Joshi D. Reverse shoulder replacement soaring popularity. Orthopedics this Week. Volume 5, Issue 22. July 21, 2009. pp23-26. 25. Kalouche I, Crepin J, Abdelmoumen S, Mitton D, Guillot G, Gagey O. Mechanical properties of glenoid cancellous bone. Clin Biomech (Bristol, Avon) 2010;25:292-8. http://dx.doi.org/10.1016/j.clinbiomech.2009.12.009 26. Karelse ATJA, Bhatia DN, De Wilde LF. Prosthetic component relationship of the reverse Delta III total shoulder prosthesis in the transverse plane of the body. J Shoulder Elbow Surg 2008;17:602-7. http://dx.doi.org/10.1016/j.jse.2008.02.005 27. Kempton LB, Balasubramaniam M, Ankerson E, Wiater JM. A radiographic analysis of the effects of glenosphere position on scapular notching following reverse total shoulder arthroplasty. J Shoulder Elbow Surg 2011;20:968-74. http://dx.doi.org/10.1016/j.jse. 2010.11.026 28. Klein SM, Dunning P, Mulieri P, Pupello D, Downes K, Frankle MA. Effects of acquired glenoid bone defects on surgical technique and clinical outcomes in reverse shoulder arthroplasty. J Bone Joint Surg Am 2010;92:1144-54. http://dx.doi.org/10.2106/ JBJS.I.00778 29. Kontaxis A, Johnson GR. The biomechanics of reverse anatomy shoulder replacement–a modelling study. Clin Biomech (Bristol, Avon) 2009;24:254-60. 30. Lehtinen JT, Tingart MJ, Apreleva M, Warner JJP. Total, trabecular, and cortical bone mineral density in different regions of the glenoid. J Shoulder Elbow Surg 2004;13:344-8. http://dx.doi.org/10.1016/ S1058274604000291 31. Lenarz C, Shishani Y, McCrum C, Nowinski RJ, Edwards TB, Gobezie R. Is reverse shoulder arthroplasty appropriate for the treatment of fractures in the older patient? Early observations. Clin Orthop Relat Res 2011;469:3324-31. http://dx.doi.org/10.1007/s11999-0112055-z
969 32. Levigne C, Boileau P, Favard L, Garaud P, Mole D, Sirveaux F, et al. Scapular notching in reverse shoulder arthroplasty. J Shoulder Elbow Surg 2008;17:925-35. http://dx.doi.org/10.1016/j.jse.2008.02.010 33. Levigne C, Garret J, Boileau P, Alami G, Favard L, Walch G. Scapular notching in reverse shoulder arthroplasty: is it important to avoid it and how? Clin Orthop Relat Res 2011;469:2512-20. http://dx.doi.org/ 10.1007/s11999-010-1695-8 34. Levy J, Frankle M, Mighell M, Pupello D. The use of the reverse shoulder prosthesis for the treatment of failed hemiarthroplasty for proximal humeral fracture. J Bone Joint Surg Am 2007;89:292-300. http://dx.doi.org/10.2106/JBJS.E.01310 35. Mahfouz M, Nicholson G, Komistek R, Hovis D, Kubo M. In vivo determination of the dynamics of normal, rotator cuff-deficient, total, and reverse replacement shoulders. J Bone Joint Surg Am 2005; 87(Suppl 2):107-13. http://dx.doi.org/10.2106/JBJS.E.00483 36. Martin TG, Iannotti JP. Reverse total shoulder arthroplasty for acute fractures and failed management after proximal humeral fractures. Orthop Clin North Am 2008;39:451-7. vi. http://dx.doi.org/10.1016/j. ocl.2008.06.006 37. Mavrogenis AF, Mastorakos DP, Triantafyllopoulos G, Sakellariou VI, Galanis EC, Papagelopoulos PJ. Total scapulectomy and constrained reverse total shoulder reconstruction for a Ewing’s sarcoma. J Surg Oncol 2009;100:611-5. http://dx.doi.org/10.1002/jso.21340 38. Mimar R, Limb D, Hall RM. Evaluation of the mechanical and architectural properties of glenoid bone. J Shoulder Elbow Surg 2008; 17:336-41. http://dx.doi.org/10.1016/j.jse.2007.07.024 39. Mura N, O’Driscoll SW, Zobitz ME, Heers G, Jenkyn TR, Chou S-M, et al. The effect of infraspinatus disruption on glenohumeral torque and superior migration of the humeral head: a biomechanical study. J Shoulder Elbow Surg 2003;12:179-84. http://dx.doi.org/10.1067/mse.2003.9 40. Neer CS. Shoulder reconstruction. Philadelphia, PA: Saunders; 1990. 41. Parsons IM, Apreleva M, Fu FH, Woo SLY. The effect of rotator cuff tears on reaction forces at the glenohumeral joint. J Orthop Res 2002; 20:439-46. http://dx.doi.org/10.1016/S0736-0266(01)00137-1 42. Pilliar RM, Lee JM, Maniatopoulos C. Observations on the effect of movement on bone ingrowth into porous-surfaced implants. Clin Orthop Relat Res 1986;208:108-13. 43. Poppen NK, Walker PS. Forces at the glenohumeral joint in abduction. Clin Orthop Relat Res 1978;135:165-70. 44. Roche C, Flurin PH, Wright T, Crosby LA, Hutchinson D, Zuckerman JD. Effect of varying screw configuration and bone density on reverse shoulder glenoid fixation following cyclic loading, Transactions of the annual meeting of the Orthopaedic Research Society. San Francisco, CA: Orthopaedic Research Society; 2008. p. 1553. 45. Roche C, Steffens J, Flurin P-H, Wright TW, Crosby LA, Zuckerman JD. Reverse shoulder glenoid loosening test method: an analysis of fixation between two different offset glenospheres, Transactions of the annual meeting of the Orthopaedic Research Society. Long Beach, CA: Orthopaedic Research Society; 2011. p. 563. 46. Seebauer L. Reverse prosthesis through a superior approach for cuff tear arthropathy. Techn Shoulder Elbow Surg 2006;7:13-26. 47. Simovitch RW, Zumstein MA, Lohri E, Helmy N, Gerber C. Predictors of scapular notching in patients managed with the Delta III reverse total shoulder replacement. J Bone Joint Surg Am 2007;89:588-600. http://dx.doi.org/10.2106/JBJS.F.00226 48. Sirveaux F, Favard L, Oudet D, Huquet D, Walch G, Mole D. Grammont inverted total shoulder arthroplasty in the treatment of glenohumeral osteoarthritis with massive rupture of the cuff. Results of a multicentre study of 80 shoulders. J Bone Joint Surg Br 2004;86: 388-95. http://dx.doi.org/10.1302/0301-620X.86B3.14024 49. Stechel A, Fuhrmann U, Irlenbusch L, Rott O, Irlenbusch U. Reversed shoulder arthroplasty in cuff tear arthritis, fracture sequelae, and revision arthroplasty. Acta Orthop 2010;81:367-72. http://dx.doi.org/ 10.3109/17453674.2010.487242 50. Terrier A, Reist A, Merlini F, Farron A. Simulated joint and muscle forces in reversed and anatomic shoulder prostheses. J Bone Joint Surg Br 2008;90:751-6. http://dx.doi.org/10.1302/0301-620X.90B6.19708
970 51. Vanhove B, Beugnies A. Grammont’s reverse shoulder prosthesis for rotator cuff arthropathy. A retrospective study of 32 cases. Acta Orthop Belg 2004;70:219-25. 52. Werner CML, Steinmann PA, Gilbart M, Gerber C. Treatment of painful pseudoparesis due to irreparable rotator cuff dysfunction with the Delta III reverse-ball-and-socket total shoulder prosthesis. J Bone Joint Surg Am 2005;87:1476-86. http://dx.doi.org/10.2106/JBJS.D. 02342
C.P. Roche et al. 53. Wierks C, Skolasky RL, Ji JH, McFarland EG. Reverse total shoulder replacement: intraoperative and early postoperative complications. Clin Orthop Relat Res 2008;467:225-34. http://dx.doi.org/10.1007/ s11999-008-0406-1 54. Zumstein MA, Pinedo M, Old J, Boileau P. Problems, complications, reoperations, and revisions in reverse total shoulder arthroplasty: a systematic review. J Shoulder Elbow Surg 2011;20:146-57. http://dx. doi.org/10.1016/j.jse.2010.08.001
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The impact of scapular notching on reverse shoulder glenoid fixation Christopher P. Roche, MSa, Nicholas J. Stroud, MSa, Brian L. Martin, BSa, Cindy A. Steiler, AAa, Pierre-Henri Flurin, MDb, Thomas W. Wright, MDc, Matthew J. DiPaola, MDd, Joseph D. Zuckerman, MDe,* a
Exactech, Gainesville, FL, USA Bordeaux-Merignac Clinic, Bordeaux-Merignac, France c Department of Orthopedic Surgery, University of Florida, Gainesville, FL, USA d Department of Orthopedic Surgery, Wright State University, Dayton, OH, USA e Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, New York, NY, USA b
Background: Scapular notching is a well-documented complication of reverse shoulder arthroplasty. The effect of scapular notching on glenoid fixation is unknown. Materials and methods: This study dynamically evaluated reverse shoulder glenoid baseplate fixation and assessed the effect of scapular notching on fixation in composite scapulae. A cyclic test was conducted to simulate 55 of humeral abduction in the scapular plane as a 750-N axial load was continuously applied to induce a variable shear and compressive load. Before and after cyclic loading, a displacement test was conducted to measure glenoid baseplate displacement in the directions of the applied static shear and compressive loads. Results: For the scapulae without a scapular notch, glenoid baseplate displacement did not exceed the generally accepted 150-mm threshold for osseous integration before or after cyclic loading in any component tested. For the scapulae with a scapular notch, glenoid baseplate displacement exceeded 150 mm in 2 of the 7 samples before cyclic loading and in 3 of the 7 samples after cyclic loading. The average pre-cyclic glenoid baseplate displacement in the direction of the shear load was significantly greater in scapulae with a scapular notch than those without a scapular notch both before (P ¼ .003) and after (P ¼ .023) cyclic loading. Conclusions: Adequate glenoid baseplate fixation was achievable in most cases in scapulae with a severe scapular notch; however, the fact that this micromotion threshold was not met in all scapulae with a notch is concerning and implies that severe notching may play a role in initial glenoid baseplate stability. Level of evidence: Basic Science Study, Biomechanical Study. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Reverse shoulder arthroplasty; glenoid fixation
No Investigational Review Board approval was required for this biomechanical evaluation using composite scapulae. *Reprint requests: Joseph D. Zuckerman, MD, NYU Hospital for Joint Diseases, Department of Orthopaedic Surgery, 301 E 17th St, 14th Flr, New York, NY 10003, USA. E-mail address: [email protected] (J.D. Zuckerman).
Reverse shoulder arthroplasty is a viable treatment option for patients with rotator cuff tear arthropathy and other degenerative diseases of the glenohumeral joint in which the fixed fulcrum created by the inverted articular curvatures (ie, the convex glenoid and the concave
1058-2746/$ - see front matter Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2012.10.035
964 humerus) provide the stability necessary for shoulder function.4,14,18,48,49,52 Because of significant improvements in pain and function for certain patients in which there is no other treatment option, reverse shoulders are also used in revision and salvage procedures where soft tissue and bone are often severely compromised.4,12,15,28,33,49,52,54 Despite these successes, reverse shoulder arthroplasty is by no means a perfect solution. Concerns remain due to high complication rates54 and deteriorating functional results with longer follow-up.18 Scapular notching has been reported to occur in w70% of patients with reverse shoulder designs that have a medialized center of rotation (>20% of which are large notches of grade 3 or 4) and has been demonstrated to correlate with radiolucent lines and clinical outcomes.4,6,7,26,27,32,33,46-49,51-54 The effect of scapular notching on glenoid fixation is unknown and particularly worrisome given that glenoid loosening was the primary failure mode of historical (eg, Grammont predecessor) reverse shoulder prostheses whose center of rotation was laterally offset relative to the glenoid fossa.4,13,16,17,40 Short-term and medium-term clinical outcome studies have reported aseptic glenoid loosening rates between 0% and 12% with modern reverse shoulder arthroplasty designs,15,18,48,52 with an average rate of 5%.54 A recent study by Guery et al18 projected the reverse shoulder glenoid loosening survival curve to be 84% at 10 years. Shortterm glenoid loosening rates may increase when used in conjunction with glenoid wear, particularly if the glenoid prosthesis is superiorly tilted or lateralized using bone graft. Long-term loosening rates may be underestimated due to the widespread prevalence of scapular notching as a complication; with longer follow-up, a number of asymptomatic patients with notching may eventually develop some form of glenoid loosening. These and other concerns led Sirveaux et al48 to project a survival curve with failure defined as revision at 5, 7, and 8 years to be 91.3%, 74.6%, and 29.8%, respectively.48 It is currently unknown whether and how many patients with asymptomatic scapular notching will eventually develop symptomatic loosening long-term. Additional clinical follow-up is required to better understand the implications of scapular notching on the reverse shoulder glenoid loosening rate. Among the numerous potential factors influencing the reverse shoulder glenoid-loosening rate are mechanical impingement, follow-up, bone quality, bone stock, initial fixation strength, and heightened patient demands. Because the reverse shoulder glenoid component is uncemented, aseptic glenoid loosening can occur due to insufficient initial fixation. The generally accepted prosthesis displacement threshold necessary to promote osseous integration and longterm fixation is 150 mm; prosthesis displacements >150 mm may result in fibrous in-growth instead.8,23,42 Poor initial fixation is likely to result in screw fatigue failure and aseptic glenoid loosening. Many recent reverse shoulder biomechanical studies aim to better understand the factors related to glenoid loosening
C.P. Roche et al. and demonstrate a widespread desire to reduce the glenoidloosening rate.10,11,20-22,44,45 This study used a dynamic glenoid loosening test method that simulates the physiologic loading conditions typical of reverse shoulder arthroplasty. This test method has been used in other studies to demonstrate significant differences in displacement between different numbers of screws, different screw configurations, variable (eg, medialized/lateralized) center of rotation, and different densities of bone substitute substrates.44,45 The purpose of this study was to quantify glenoid fixation in composite scapulae with and without a scapular notch and evaluate the null hypothesis that a grade 4 scapular notch has no impact on the fixation of a reverse shoulder glenoid implant.
Materials and methods Fourteen 38-mm Equinoxe reverse shoulder implants (Exactech Inc, Gainesville, FL, USA) were tested in a fourth-generation composite/dual density scapula (Pacific Research, Inc, Vashon, WA, USA) with a 1.63 g/cm3 ‘‘cortical’’ shell and a 0.27 g/cm3 ‘‘cancellous’’ interior structure. This substrate provides a representative substitute for the density, strength, and modulus of glenoid cortical and cancellous bone in the recipient patient population for reverse shoulder arthroplasty.1,25,30,38 A custom cutting jig was designed to create a Nerot-Sirveaux48 grade 4 scapular notch (n ¼ 7) in the dual-density composite scapulae (Fig. 1 and Fig. 2) The scapular notch was cut to approximate the profile of the humeral liner as it articulates around the center of rotation of the 38-mm glenosphere. The fixation of these notched scapulae was compared with that of composite scapulae without a scapular notch (ie, Nerot-Sirveaux48 grade 0; n ¼ 7) before and after cyclic loading. For the notched and non-notched scapulae, initial fixation of the glenoid baseplate was achieved using 4 (1 superior, 3 inferior), 4.5- 30mm polyaxial locking compression screws and a press-fit tapered cage peg; because the inferior screw is often fractured in a grade 4 scapular notch, a shorter (18-mm) inferior screw was used in the notched scapulae (the other 3 screws were 30 mm long). The 18mm inferior screw was selected because it is the shortest length offered and only had 1 thread engaged in the medial portion of the notched scapular neck, minimizing its fixation contribution. After assembly, each composite scapulae was cut and potted with bone cement. This reverse shoulder glenoid-loosening test was conducted in 2 phases. The first phase is the displacement test, which measures the fixation of the reverse shoulder glenoid baseplate in the composite scapulae before and after the application of 10,000 cycles of dynamic loading for 55 at 0.5 Hz. In the displacement test, the axial test machine (Instron Corp, Norwood, MA, USA; resolution of 1 mm) and 3 digital indicators (Mitutoyo, Kawasaki, Japan; resolution of 1 mm) quantify the glenoid baseplate displacement relative to the composite scapula as a compressive (433 N) and shear (357 N) load is applied. The compressive axial load is applied perpendicular to the reverse glenoid baseplate and the shear load is applied parallel to the face of the glenoid baseplate along its superior/inferior axis (Fig. 3) The specific combination of compressive and shear load was based on the findings of Poppen et al,43 who described the
Reverse shoulder glenoid fixation and scapular notching
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Figure 1 Posterior views of the (A) non-notched (grade 0) composite scapula and the (B) grade 4 scapular notch composite scapula with cut guide. positioned in the biaxial testing apparatus and aligned along the superior/inferior axis of the glenoid baseplate. A 750-N axial load is constantly applied through the center of the humeral liner as the glenosphere/glenoid baseplate/composite scapula are rotated about the humeral component with a stepper motor to create a sinusoidal angular displacement profile encompassing an arc of 55 at 0.5 Hz for 10,000 cycles (Fig. 4) This loading profile over the 55 arc of abduction in the scapular plane would induce a maximum shear load of 346 N at each extreme of rotation (27.5 and þ27.5 ) and a maximum compressive load of 750 N at the midpoint of the arc of rotation (ie, 0 ; Fig. 5). The components are cooled with a continuous jet of air, with no lubrication. Statistical analysis was performed by means of a 2-tailed unpaired Student t test (significance defined as P < .05) to compare prosthesis displacements relative to each scapulae (notched and non-notched) in the directions of the shear and compressive loads before and after cyclic loading.
Results Figure 2
Image of grade 4 scapular notch composite scapula.
maximum shear load in the shoulder to be 0.42 body weight (BW) at 60 abduction, where the corresponding compressive load was 0.51 BW (at 60 abduction). We assumed a BW of 86.7 kg (191 lb), the average weight of a man in the United States.9 This yielded a shear load of 357 N (0.42 86.7 kg) and a compressive load of 433 N (0.51 86.7 kg) during the displacement test. The 55 arc of motion used in the cyclic test simulates w25 to 80 of abduction in the scapular plane relative to a fixed scapula; the displacement test is performed at the center of this 55 arc of motion. Considering that the Poppen et al43 60 abduction measurements included scapula-thoracic motion, the displacement test is performed on the glenoid component at the approximate orientation of this peak shear load. Digital indicators are used to subtract any compliance of the test construct. The second phase is the cyclic test. The cyclic test simulates the primary motion of reverse shoulder arthroplasty; that is, the abduction motion generated by the deltoid.4,16,17,35 The humeral liner and glenosphere/glenoid baseplate/composite scapula are
For the composite scapulae without a scapular notch (eg, grade 0), glenoid baseplate displacement did not exceed the generally accepted 150-mm threshold for osseous integration before (maximum shear and compression displacement was 99 and 113 mm, respectively) or after (maximum shear and compression displacement was 103 and 102 mm, respectively) cyclic loading in any component tested. For the composite scapulae with a scapular notch (eg, grade 4), glenoid baseplate displacement exceeded 150 mm in 2 of the 7 samples before cyclic loading (maximum shear and compression displacement was 153 and 150 mm, respectively) and in 3 of the 7 samples after (maximum shear and compression displacement was 253 and 181 mm, respectively) cyclic loading. The average pre-cyclic glenoid baseplate displacement in the directions of the shear and compression loads was 66 19 and 78 27 mm, respectively, for the composite scapulae without a scapular notch and 114 28 and 99 42 mm, respectively, for the composite scapulae with
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C.P. Roche et al. with a scapular notch. The average glenoid baseplate displacement in the direction of the shear load was significantly greater in the composite scapulae with a scapular notch than in the composite scapula without a scapular notch before (P ¼ .003) and after (P ¼ .023) cyclic loading. The average glenoid baseplate displacement in the direction of the compression load was not statistically different in the composite scapulae with a scapular notch than in the composite scapula without a scapular notch both before (P ¼ .277) and after (P ¼ .292) cyclic loading.
Discussion
Figure 3 Displacement test performed to measure initial glenoid baseplate fixation before and after cyclic loading as a 357-N shear load and a 433-N compressive load are applied to the glenoid baseplate.
Figure 4 Cyclic test performed to simulate 55 abduction in the scapular plane for 10,000 cycles at 0.5 Hz as a 750-N load is applied.
a scapular notch. The average post-cyclic glenoid baseplate displacement in the directions of the shear and compression loads was 58 42 and 83 22 mm, respectively, for the composite scapulae without a scapular notch and 134 65 and 105 46 mm, respectively, for the composite scapulae
Shoulder surgeons have recently turned to reverse shoulder arthroplasty as a solution for many previously difficult-totreat conditions. Reverse shoulder arthroplasty is most commonly used for rotator cuff tear arthropathy; however, indications have expanded to include comminuted proximal humeral fractures in the elderly, tumors, proximal humeral malunions and nonunions, and failed anatomic total shoulder replacement.4,31,34,36,37 Although the exact usage is difficult to know, a steep increase has occurred in procedures performed in the United States since the device was cleared by the U.S. Food and Drug Administration in March 2004. Joshi et al24 reported that 2652 reverse shoulder arthroplasty procedures were performed in the United States in 2005 and 15,200 in 2008 and projected 31,584 reverse shoulders will be performed in the United States in 2012.24 Given this steep increase in usage and that the long-term survival of these implants is unknown, the number of reverse shoulder arthroplasty revision surgeries is expected to increase in the coming years. In many of these revision surgeries, the shoulder surgeon can expect that the scapular bone will be compromised due to the high prevalence of scapular notching as a complication.4,6,7,32,33,46-49,51-54 Severe scapular notching may potentiate screw loosening and loss of glenoid baseplate fixation in the initial postoperative revision period. In this case, the shoulder surgeon will be faced with a choice between conversion of the failed reverse shoulder arthroplasty to a hemiarthroplasty or attempting to revise to a new, reverse shoulder. The inferior scapula is a difficult area in which to graft bone, and little information is available to guide shoulder surgeons in this clinical scenario. This study attempted to help clarify whether initial fixation and stability is possible in a representative bone model of a severely notched scapula and demonstrated that in most cases, the generally accepted 150-mm displacement threshold for osseous integration was met before (5 of 7 scapulae) and after (4 of 7 scapulae) cyclic loading in the scapula bone model with a grade 4 scapular notch. In the scapula bone model without a scapular notch, glenoid baseplate displacement did not exceed the generally accepted 150-mm threshold before or after cyclic loading in
Reverse shoulder glenoid fixation and scapular notching
Figure 5
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Shear and compressive reaction forces during the cyclic test (27.5 ).
any component tested. In addition, the glenoid baseplate motion associated with the non-notched scapulae in the direction of the applied shear load was significantly less before (P ¼ .003) and after (P ¼ .023) cyclic loading than that of the scapulae with a scapular notch. Glenoid baseplate fixation was achievable in most cases in scapulae with a severe scapular notch; however, that this micromotion threshold was not met in all scapulae with a notch is concerning and implies that severe notching may play a role in initial glenoid baseplate stability. Therefore, the results of our study lead us to reject the null hypothesis and conclude that a large scapular notch does affect glenoid fixation in reverse shoulder arthroplasty. The testing scenario in this study used 4 screws to achieve fixation. The glenoid baseplate used in this study permits a total of 6 polyaxial locking compression screw positions. It is unclear whether adding more screws to the construct could potentially mitigate the increased micromotion observed in a severely notched scapula scenario. Future work should evaluate this potential strategy given that a previous study demonstrated significant differences in glenoid baseplate displacement between different numbers of screws and different screw configurations in a 0.24 g/cm3 polyurethane bone substitute model.44 These fixation results may not be applicable to all reverse shoulder glenoid baseplates because not all offer 4 polyaxial locking compression screws or have as large of a glenoid baseplate surface area. A few study design considerations warrant discussion. For the cyclic test, the 750-N axial load we used to simulate a worst-case loading condition is deemed appropriate because the loading of reverse shoulder arthroplasty is expected to be less than that of an anatomic total shoulder arthroplasty due to a variety of factors:
First, the reverse shoulder is primarily used in elderly, low-demand patients. Most of these patients have compromised rotator cuff musculature, resulting in a lower joint reaction force. Additionally, there are inherent design features of the reverse shoulder itself, such as the medially shifted center of rotation that increases the deltoid abductor moment arm requiring the deltoid to generate less torque to elevate the arm, and the distally shifted center of rotation that elongates the deltoid and improves its resting tension and tone.5,19,29,39,41,50 Although 10,000 cycles is low relative to hip and knee testing and less than the 100,000 cycles recommended by the ASTM F 2028-08 anatomic total shoulder glenoid loosening test, this value is justified because this test does not simulate prosthesis wear-related failure modes but instead the limited period in which initial fixation of a noncemented glenoid prosthesis is attained.2,3,21 In the displacement test, the shear and compressive loads were applied directly to the glenoid baseplate to ensure that there was no motion between the glenosphere and glenoid baseplate. Finally, the use of the 150-mm threshold established in previous studies was assumed to be translatable to describe the initial fixation of the glenoid in reverse shoulder arthroplasty. Numerous studies10,11,20-22 have attempted to evaluate the initial fixation of a reverse shoulder glenoid baseplate. Each study was limited in how it simulated the loading conditions of reverse shoulder arthroplasty because they failed to dynamically simulate abduction,10,11,20-22 used too dense of a glenoid substrate (eg, 0.48 g/cm3 polyurethane blocks),20 tested it for an insufficient number of cycles (eg, 500 or 1000),10,11,20 or applied too extreme
968 a combination of shear and compressive load for a reverse shoulder.20-22 The simple test method used in this study attempted to simulate the function of a reverse shoulder and evaluate its fixation in a composite scapula. Previous reverse shoulder glenoid loosening studies20-22 attempted to apply ASTM F 2028-08 to reverse shoulders; however, the use of ASTM F 2028-08 is inappropriate because the reverse shoulder glenoid and humeral articular components are semiconstrained and conforming and only permit rotation; whereas, the anatomic glenoid and humeral components are unconstrained and nonconforming and permit both rotation and translation. The cyclic test used in this study attempted to simulate this semiconstrained, conforming articulation by only rotating (not translating) the glenosphere about the humeral liner in the scapular plane. Our reverse shoulder glenoid loosening study has some limitations. First, we used a composite scapula model rather than matched pair cadaveric specimens to reduce variables related to cortical and cancellous density, cortical thickness, and scapular morphology and ensure that the created scapular notch was identical in each tested specimen. Second, loading in the glenohumeral joint is complex, and we used a constant 750-N axial load during the cyclic test to simulate these loading conditions, which induced a variable compression and shear load depending on the angle of articulation (Fig. 5). This loading profile is more physiologically relevant than the extreme 756-N compressive and 756-N shear load combinations previously used by Harman et al,20 Hoenig et al,21 and Hopkins et al.22 Terrier et al50 recently compared the muscle forces in both the anatomic and reverse shoulder arthroplasty designs and found that the forces associated with the reverse shoulder varied between 30% and 50% less than that of the anatomic shoulder, depending on the presence of different rotator cuff muscles (only a supraspinatus tear vs no rotator cuff, respectively). Terrier et al50 reported the maximum force in an anatomic shoulder was 0.86 BW, the maximum force in a reverse shoulder with no rotator cuff was 0.42 BW, and the maximum force in a reverse shoulder with only a supraspinatus tear was 0.62 BW; these values are similar to what was reported by Kontaxis et al.29 Finally, we used digital indicators to measure glenoid baseplate displacement before and after cyclic loading but did not measure displacement during the cyclic test.
Conclusions The incidence of revision reverse shoulder arthroplasty is projected to increase over time. Severe scapular notching can be a challenging clinical scenario that does seem to have an effect on glenoid baseplate fixation in a minority of severely notched composite scapulae
C.P. Roche et al. tested. Further study will help clarify whether rescue strategies exist to mitigate this issue.
Disclaimer C.P.R., N.J.S., B.L.M., and C.A.S. are employees of Exactech Inc. P.-H.F., T.W.W., and J.D.Z. receive royalties from Exactech Inc. The other authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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