Bulletin of the Hospital for Joint Diseases 2013;71(4):284-93

Transcription

1 284 Impact of Inferior Glenoid Tilt, Humeral Retroversion, Bone Grafting, and Design Parameters on Muscle Length and Wrapping in Reverse Shoulder Arthroplasty Christopher P. Roche, M.S., M.B.A., Phong Diep, B.S., Matthew Hamilton, Ph.D., Lynn A. Crosby, M.D., Pierre-Henri Flurin, M.D., Thomas W. Wright, M.D., Joseph D. Zuckerman, M.D., and Howard D. Routman, D.O. Abstract Purpose: This study quantifies the ability of humeral retroversion, glenoid tilt, bone graft, and varying prosthesis design parameters to restore anatomic muscle length and deltoid wrapping with reverse shoulder arthroplasty. Methods: A computer model simulated abduction and internal and external rotation for a normal shoulder, the RSP reverse shoulder, the Equinoxe reverse shoulder, and the Grammont reverse shoulder when implanted using various implantation methods. The length of eight different muscles and the deltoid wrapping angle were quantified to evaluate the ability of each implantation method and design to restore anatomic muscle tensioning. Results: Each reverse shoulder shifted the center of rotation medially and inferiorly relative to the normal shoulder and caused a corresponding shift in the position of the humerus. Each reverse shoulder elongated each head of the deltoid and shortened the internal and external rotators relative to the normal shoulder. The surgical techniques and prosthesis designs, which resulted in a more lateral humeral position, were associated with more deltoid wrapping and Christopher P. Roche, M.S., M.B.A., Phong Diep, B.S., and Matthew Hamilton, Ph.D., are employed by Exactech, Inc., Gainesville, Florida. Lynn A. Crosby, M.D., is in the Department of Orthopaedic Surgery, Georgia Regents University, Augusta, Georgia. Pierre- Henri Flurin, M.D., is at the Bordeaux-Merignac Clinique du Sport, Merignac, France. Thomas W. Wright, M.D., is in the Department of Orthopaedics and Rehabilitation, University of Florida, Gainesville, Florida. Joseph D. Zuckerman, M.D., Department of Orthopaedic Surgery, Hospital for Joint Diseases, NYU Langone Medical Center, New York, New York. Howard D. Routman, D.O., is with Atlantis Orthopaedics, Palm Beach Gardens, Florida. Correspondence: Joseph D. Zuckerman, M.D., Professor and Chairman, NYU Hospital for Joint Diseases, Department of Orthopaedic Surgery, 301 East 17th Street, 14th Floor, New York, New York 10003; joseph.zuckerman@nyumc.org. better tensioning of the anterior and posterior shoulder muscles. Conclusions: Muscle tensioning and deltoid wrapping can be substantially altered by surgical implantation methods using the Grammont reverse shoulder. However, the results of this study demonstrate that more anatomic muscle tensioning and improved deltoid wrapping are achieved using alternative prosthesis designs that better restore the lateral position of the humerus. The reverse shoulder inverts the anatomic articulations making the glenoid convex and the humerus concave, creating a fixed fulcrum that prevents the humerus from migrating superiorly. Inverting the concavities shifts the center of rotation (CoR) inferiorly and medially and changes the position of the humerus. The magnitude of change in the center of rotation and position of the humerus has important implications on muscle tensioning, range of motion, and stability. 1-5 Significant efforts have been made to refine the surgical implantation method and recommend prosthesis design modifications to reduce both the type of complications and their associated rates While many of these complications are inter-related, most of the mitigating efforts have prioritized the reduction of the scapular notching rate. Some recommendations have been widely accepted, such as inferiorly shifting the glenosphere as recommended by Nyfeller and coworkers, 4 while others have been more controversial, such as increasing glenosphere thickness (independent of glenosphere diameter) to lateralize the CoR off the face of the glenoid as recommended by Frankle and associates. 3,12 Similar controversy exists regarding recommendations to reduce instability (and notching) by inferiorly tilting the glenosphere or by changing the prosthesis humeral neck angle. 1,5,16-19 Humeral orientation is also controversial; some recommend placing the humerus in 0 to ver- Roche CP, Diep P, Hamilton M, Crosby LA, Flurin PH, Wright TW, Zuckerman JD, Routman HD. Impact of inferior glenoid tilt, humeral retroversion, bone grafting, and design parameters on muscle length and deltoid wrapping in reverse shoulder arthroplasty. Bull Hosp Jt Dis. 2013;71(4):

2 285 sion ; whereas, others recommend 20 to 40 retroversion 22 to reduce impingement, facilitate (internal or external) rotation, and improve stability. Some have recommended bone grafting a worn glenoid to improve stability and have reported that bone grafting worn glenoids can reduce the scapular notching rate 23,24,27,28 with the Grammont reverse shoulder. These observations led Boileau and colleagues 29 to controversially recommend using bone graft with the Grammont reverse shoulder in non-worn glenoids to lateralize the CoR, improve muscle tension and deltoid contour, and reduce impingement. The magnitude of controversy related to such basic implantation methods and prosthesis design principles reflects the level of uncertainty and lack of consensus among orthopaedic surgeons using this type of prosthesis. To this end, a computer model quantified changes in muscle length and deltoid wrapping when implanting the Grammont reverse shoulder in each of the aforementioned surgical technique recommendations during two different motion simulations. These measures of muscle tensioning and deltoid wrapping are compared to that of the normal shoulder and two different alternative reverse shoulder designs for each type of motion to evaluate the ability of surgical technique modifications to restore the anatomic tensioning of the muscles in the shoulder using the Grammont reverse shoulder prosthesis. Methods A 3-D computer model was developed in Unigraphics (Siemens PLM; Plano, TX, USA) and used to simulate abduction in the scapular plane and internal and external rotation for the normal shoulder and three commercially-available reverse shoulder designs with more than 6 years of clinical history. The 36 mm x 18 mm Grammont (Depuy Orthopaedics, Inc; Warsaw, IN, USA) has a humeral neck angle of 155, a center of rotation 0 mm lateral to the glenoid fossa, and a humeral stem and liner medial offset of 9.8 mm. The 32 mm x 26 mm RSP (DJO Surgical; Austin, TX, USA) has a humeral neck angle of 135, a center of rotation 10 mm lateral to the fossa, and a humeral stem and liner medial offset of 10.9 mm. The 38 mm x 21 mm Equinoxe (Exactech, Inc; Gainesville, FL, USA) has a humeral neck angle of 145, a center of rotation 2 mm lateral to the fossa, and a humeral stem and liner medial offset of 20.8 mm. Each reverse shoulder was geometrically modeled and implanted in a 3-D digitized scapula and humerus; a 3-D digital clavicle and ribcage were also included (Pacific Research Laboratories, Inc; Vashon Island, WA). The digital humerus and scapula were assembled to simulate a normal shoulder, functioning as the control in this analysis; the humeral head was centered on the glenoid and offset by 4 mm from the center of the glenoid to account for the thickness of the cartilage and labrum. Eight muscles were simulated as three lines from its origin on the scapula or clavicle to its insertion on the humerus: anterior deltoid (yellow), middle deltoid (dark green), posterior deltoid (magenta), subscapularis (light green), infraspinatus (dark blue), teres major (red), teres minor (cyan), and the clavicular portion of the pectoralis major (orange) (Fig. 1). To characterize the biomechanical impact of humeral retroversion on each muscle, the Grammont was implanted according to the manufacturer s recommendations so that the glenoid baseplate aligns with the inferior glenoid rim as the humeral component was successively oriented at 0, 20, and 40 retroversion (Fig. 2). To characterize the biomechanical impact of glenosphere tilt on each muscle, the Grammont was implanted along the inferior glenoid rim at 0 and 15 inferior tilt (with 20 humeral retroversion). To characterize the biomechanical impact of bone grafting a non-worn glenoid on each muscle, the Grammont was Figure 1 Computer model of eight muscles simulated as three lines from origin to insertion, anterior (left) and posterior (right) views of the 36 mm Grammont reverse shoulder (0 tilt, 20 humeral retroversion) at 15 abduction. Color figure available online at www. nyuhjdbulletin.org.

3 286 muscle at each degree of motion; each average muscle length at each degree of motion was compared as a percentage of the corresponding muscle length of the normal shoulder. To clarify, a positive percentage indicates elongation of the muscle relative to the normal shoulder, whereas a negative percentage indicates shortening of the muscle relative to the normal shoulder. The angle of abduction in which the middle deltoid stops wrapping around the greater tuberosity was also quantified as a measure stability (e.g., less deltoid wrapping implies reduced humeral head compression into the glenoid) for the normal shoulder, and the Grammont, RSP, and Equinoxe reverse shoulders. Figure 2 Representative computer model image of scapular view of the 36 mm Grammont 0 tilt, 0 retro (top), 36 mm Grammont 0 tilt, (middle), and 36 mm Grammont 0 tilt, 40 retro (bottom) reverse shoulders at 30 abduction, scapula made transparent to permit visualization of the glenoid component. Color figure available online at implanted with a 29 mm x 10 mm cylindrical bone graft along the inferior glenoid rim at 0 and 15 inferior tilt as the humeral component was successively oriented at 0, 20, and 40 retroversion (Fig. 3). Finally, to characterize the biomechanical impact of prosthesis design on each muscle, the 32 mm RSP and 38 mm Equinoxe reverse shoulders were implanted identically to the 36 mm Grammont by positioning the glenoid baseplate along the inferior glenoid rim with 0 tilt while orienting the humeral component at version (Fig. 4). After assembly, two motions were simulated: 1. abduction and 2. internal and external rotation. To simulate abduction, the humeral component was abducted from 0 to 80 in the scapular plane relative to a fixed scapula. To simulate internal and external rotation, the humeral component was rotated 40 internally and 40 externally with the arm at 0 abduction. For each simulated motion, muscle lengths were measured as the average length of the three lines representing the Results Each reverse shoulder, regardless of design or surgical implantation method, shifted the CoR medially and inferiorly relative to the normal shoulder (Table 1). This shift in the CoR caused a medial shift of the humerus and a decrease in the middle deltoid wrapping angle for all reverse shoulder prostheses (Table 2). For each simulated motion, each reverse shoulder elongated each head of the deltoid, shortened the internal rotators (subscapularis and teres major, with the exception of the pectoralis major which was elongated), and shortened the external rotators (infraspinatus and teres minor) relative to the normal shoulder (Tables 3, 4, and 5). In general, the surgical techniques and designs which resulted in a more lateral humeral position were associated with more deltoid wrapping and better tensioning of the anterior and posterior shoulder muscles. Specifically in abduction (Table 3), the Grammont reverse shoulder with 0 tilt and 0 humeral retroversion with graft configuration had the most lateral humeral position and was associated with the most deltoid wrapping and best muscle tension compared to the other Grammont configurations analyzed. Conversely, the Grammont reverse shoulder with 15 tilt and 20 humeral retroversion without graft configuration had the most medial humeral position and was associated with the smallest deltoid wrapping and worst muscle tension. The RSP reverse shoulder shifted the humerus more lateral than all Grammont reverse shoulders except the 0 tilt and 0 retroversion with graft configuration and had more deltoid wrapping than all Grammont reverse shoulders except the 0 tilt and 0 with graft and version configurations. The Equinoxe reverse shoulder design had the most lateral humeral position, most deltoid wrapping, tensioned the three heads of the deltoid and the pectoralis more than the RSP and Grammont reverse shoulders, and better restored the anatomic tension of the subscapularis, infraspinatus, teres major, and teres minor, regardless of the surgical implantation method. Similar trends were observed in internal and external rotation (Tables 4 and 5). For both simulated motions, decreasing humeral retroversion with the Grammont increased the tension of the posterior shoulder muscles and decreased the tension of the anterior shoulder muscles. Specifically in abduction (Table 3), as

4 287 Figure 3 Representative computer model image of scapular view of the 36 mm Grammont 0 tilt, (left), 36 mm Grammont 15 tilt, (middle), and 36 mm Grammont 0 tilt, with a 29x10 mm graft (right) reverse shoulders at 0 abduction, rib cage removed to permit visualization of humeral position. Color figure available online at Figure 4 Scapular view of the Grammont (left), RSP (middle), and Equinoxe (right) reverse shoulders at 0 abduction, rib cage removed to permit visualization of humeral position. Color figure available online at the Grammont was implanted from 20 to 0 retroversion, the tension of the subscapularis, teres major, and pectoralis major decreased (by 4.1%, 1.7%, 1.1%, respectively) while the tension of the teres minor and infraspinatus increased (by 8.9% and 3.8%, respectively). As the Grammont was implanted from 20 to 40 retroversion, the tension of the subscapularis, teres major, and pectoralis major increased (by 4.0%, 2.2%, 1.5%, respectively), while the tension of the teres minor and infraspinatus decreased (by 9.3% and 4.4%, respectively). These differences are more pronounced with the arm at the side (Tables 4 and 5); most notably, the tension of the teres minor increased in internal (9.5%) and external (13.8%) rotation in 0 retroversion and decreased in internal (10.8%) and external (13.3%) rotation when implanted in 40 retroversion, respectively. Conversely, the tension of the subscapularis decreased in internal (5.6%) and external

5 288 Table 1 Change in Center of Rotation for Each Reverse Shoulder Relative to Normal Shoulder Medial Shift in Center of Rotation Inferior Shift in Center of Rotation 36 Grammont, 0 tilt, 28.3 mm 8.0 mm 36 Grammont, 15 tilt, 31.0 mm 7.7 mm 36 Grammont, 0 tilt, 0 retro 28.3 mm 8.0 mm 36 Grammont, 0 tilt, 40 retro 28.3 mm 8.0 mm 0 tilt, 0 retro 19.2 mm 8.0 mm 0 tilt, 19.2 mm 8.0 mm 0 tilt, 40 retro 19.2 mm 8.0 mm 15 tilt, 21.9 mm 10.2 mm 32 mm RSP, 0 tilt, 20.0 mm 6.9 mm 38 Equinoxe, 0 tilt, 27.1 mm 4.5 mm Table 2 Medial-Lateral Position of the Humerus and Its Impact on Wrapping Distance from Lateral Coracoid to Lateral Greater Tuberosity with Humerus Abducted at 0 Angle of Abduction which Middle Stops Wrapping Greater Tuberosity Distance from Bottom of Acromion to Top of the Greater Tuberosity Normal Shoulder 56.2 mm mm 36 Grammont, 0 tilt, 34.7 mm mm 36 Grammont, 15 tilt, 32.0 mm mm 36 Grammont, 0 tilt, 0 retro 36.3 mm mm 36 Grammont, 0 tilt, 40 retro 32.6 mm mm 0 tilt, 0 retro 45.4 mm mm 0 tilt, 43.8 mm mm 0 tilt, 40 retro 41.7 mm mm 15 tilt, 41.1 mm mm 32 mm RSP, 0 tilt, 44.5 mm mm 38 Equinoxe, 0 tilt, 47.1 mm mm (5.2%) rotation in 0 retroversion and increased in internal (6.0%) and external (4.4%) rotation when implanted in 40 retroversion, respectively. Implanting the Grammont glenosphere with 15 inferior tilt decreased the tension of each muscle by 0.1% to 3.1% relative to the normal shoulder. Specifically in abduction (Table 3), implanting the Grammont glenosphere with 15 inferior tilt decreased the tension of the anterior deltoid (0.8%), middle deltoid (1.0%), posterior deltoid (1.0%), subscapularis (2.2%), infraspinatus (2.2%), teres major (1.9%), teres minor (3.5%), and the clavicular portion of the pectoralis major (0.8%) relative to the Grammont with 0 tilt. These differences are more pronounced with the arm at the side (Tables 4 and 5); most notably, the tension of the teres minor, teres major, subscapularis, and infraspinatus decreased in internal (4.2%, 2.8%, 2.5%, and 2.2%, respectively) and external (4.9%, 2.7%, 2.3%, and 2.4%, respectively) rotation, relative to the Grammont with 0 tilt. Using bone graft with the Grammont reverse shoulder in a non-worn glenoid increased the tension of each muscle. Specifically in abduction (Table 3), the Grammont with graft increased the tension of the anterior deltoid (2.5%), middle deltoid (3.1%), posterior deltoid (3.2%), subscapularis (7.4%), infraspinatus (7.4%), teres major (6.4%), teres minor (12.1%), and the clavicular portion of the pectoralis major (2.6%) relative to the Grammont without graft. These improvements are more pronounced with the arm at the side (Tables 4 and 5); most notably, the tension of the teres minor, teres major, subscapularis, and infraspinatus increased in internal (14.0%, 9.3%, 8.2%, and 7.5%, respectively) and external (16.8%, 9.0%, 7.5%, and 8.1%, respectively) rotation, relative to the Grammont without graft. Discussion Inverting the anatomic concavities with reverse shoulder arthroplasty fundamentally changes the position of the CoR relative to the normal shoulder and causes a corresponding shift in the position of the humerus, which has implications on deltoid wrapping and muscle tensioning. Each reverse shoulder elongated each head of the deltoid, shortened the

6 289 Table 3 Average Muscle Length Relative to Normal Shoulder as Each Reverse Shoulder is Abducted in the Scapular Plane from 0 to 80 Ant. Mid Post. Subscap Infraspin Teres Major Teres Minor Pec Major 36 Grammont, 0 tilt, 4.7% 4.8% 1.7% -11.2%* -12.8%* -11.0%* -20.5% 2.2% 36 Grammont, 15 tilt, 3.9% 3.8% 0.7% -13.2%* -14.7%* -12.7%* -23.2% 1.3% 36 Grammont, 0 tilt, 0 retro 4.5% 4.9% 1.9% -14.8%* -9.5% -12.5%* -13.5%* 1.0% 36 Grammont, 0 tilt, 40 retro 5.1% 4.8% 1.5% -7.6% -16.6%* -9.1% -27.7% 3.7% 0 tilt, 0 retro 7.0% 8.5% 5.2% -8.2% -3.1% -6.8% -4.1% 3.6% 0 tilt, 7.2% 7.8% 5.0% -4.6% -6.4% -5.4% -10.9%* 4.8% 0 tilt, 40 retro 7.6% 7.7% 4.8% -1.0% -10.2%* -3.5% -18.1%* 6.3% 15 tilt, 7.3% 7.7% 4.5% -6.9% -8.9% -7.5% -14.7%* 4.7% 32 RSP, 0 tilt, % 7.0% 4.6% -3.9% -5.6% -4.5% -9.7% 3.6% retro 38 Equinoxe, 0 tilt, 7.3% 8.2% 6.3% 0.0% -1.6% -1.1% -3.5% 5.1% *Muscle shortening > 10%; Muscle shortening > 20%. Table 4 Average Muscle Length Relative to Normal Shoulder as Each Reverse Shoulder is Internally Rotated from 0 to 40 with the Arm at 0 Abduction Ant. Mid Post. Subscap Infraspin Teres Major Teres Minor Pec Major 36 Grammont, 0 tilt, 13.4% 15.6% 9.7% -18.7%* -19.7%* -22.0% -32.0% 5.6% 36 Grammont, 15 tilt, 13.3% 15.5% 9.2% -20.7% -21.5% -24.1% -34.9% 5.1% 36 Grammont, 0 tilt, 13.2% 15.7% 9.8% -23.3% -17.1%* -23.3% -25.6% 4.6% 0 retro 36 Grammont, 0 tilt, 13.6% 15.5% 9.4% -13.8%* -23.1% -19.6%* -39.4% 7.1% 40 retro 13.3% 15.7% 11.3% -16.6%* -11.1%* -15.9%* -16.1%* 6.0% 0 tilt, 0 retro 0 tilt, 13.5% 15.6% 11.2% -12.0%* -13.7%* -14.7%* -22.6% 7.2% 0 tilt, 40 retro 13.8% 15.4% 10.9% -7.1% -16.9%* -12.4%* -29.9% 8.7% 15 tilt, 14.6% 16.7% 11.7% -14.2%* -15.9%* -17.2%* -25.9% 7.8% 32 RSP, 0 tilt, 20 retro 12.4% 14.7% 10.7% -10.8%* -12.6%* -13.1%* -21.0% 5.8% 38 Equinoxe, 0 tilt, 15.4% 18.4% 14.5% -8.5% -11.7%* -10.4%* -19.1%* 7.5% *Muscle shortening > 10%; Muscle shortening > 20%; Muscle shortening > 30%.

7 290 Table 5 Average Muscle Length Relative to Normal Shoulder as Each Reverse Shoulder is Externally Rotated from 0 to 40 with the Arm at 0 Abduction Ant. Mid Post. Subscap Infraspin Teres Major Teres Minor Pec Major 36 Grammont, 0 tilt, 13.6% 15.7% 10.1% -17.3%* -21.0% -21.6% -36.9% 6.8% 36 Grammont, 15 tilt, 13.5% 15.7% 9.6% -19.1%* -22.9% -23.7% -40.0% 6.2% 36 Grammont, 0 tilt, 0 retro 13.2% 15.7% 10.4% -21.6% -15.7%* -24.4% -28.2% 5.1% 36 Grammont, 0 tilt, 13.6% 15.7% 9.7% -13.6%* -25.4% -18.5%* -45.3% 8.5% 40 retro 13.4% 15.7% 11.9% -15.4%* -10.4%* -17.3%* -17.6%* 6.6% 0 tilt, 0 retro 0 tilt, 13.8% 15.7% 11.6% -11.0%* -14.6%* -14.5%* -26.4% 8.3% 0 tilt, 40 retro 14.2% 15.6% 11.1% -7.4% -19.0%* -11.4%* -34.7% 10.0% 14.9% 16.8% 12.1% -13.0%* -16.9%* -17.0%* -30.0% 8.8% 15 tilt, 32 RSP, 0 tilt, % 14.7% 11.0% -10.1%* -13.6%* -13.2%* -24.7% 7.4% retro 38 Equinoxe, 0 tilt, 16.6% 18.3% 14.3% -8.5% -12.4%* -12.3%* -22.4% 11.4% *Muscle shortening > 10%; Muscle shortening > 20%; Muscle shortening > 30%. internal rotators (with the exception of the pectoralis major which was elongated) and shortened the external rotators relative to the normal shoulder. Surgical techniques and implant designs that lateralized the humerus closer to its anatomic position were associated with improved deltoid wrapping and more anatomic muscle tensioning of the internal and external rotators. These changes in muscle length have the potential to dramatically alter the length-tension relationship of each muscle relative to their normal physiologic function. 2,30-36 elongation between 10% and 20% has been suggested to improve its resting tone and tension, increase strength, and improve the overall stability of the joint; however, increased deltoid elongation also modifies the normal deltoid contour, decreases its wrapping angle around the greater tuberosity, and creates cosmetic concerns. 1-3,37 De Wilde and coworkers reported that the Grammont and RSP reverse shoulders elongated the deltoid when the arm was at 0 abduction by 16.4% and 13.0%, respectively. 2 Similarly, Jobin and colleagues reported that the average deltoid elongation of three different reverse shoulders was 17.0% when the arm was at 0 abduction. 33 We reported a maximum elongation of the middle head of the deltoid between 14.7% and 18.4% at 0 abduction depending upon reverse shoulder design, these results are in agreement with those presented by both De Wilde and Jobin and colleagues. 2,33 The functional effect of shortening the anterior and posterior rotator cuff is unknown. Shortening of the rotator cuff by as much as 45.3% was observed in this study. This magnitude of muscle shortening may be an explanation for the limited improvements in active internal and external rotation reported with the Grammont reverse shoulder relative to designs that lateralize the humerus more, 1,3,6,8,9,18,38 for why subscapularis repair is necessary for stability with reverse shoulder designs having a medialized humerus, 39,40 and for why patients with reverse shoulders are reported to have a different scapulo-humeral rhythms and specifically more scapular motion than in normal shoulders. 41 Implanting the Grammont in more or less humeral retroversion asymmetrically tensions the anterior and posterior shoulder muscles and slightly impacts the deltoid wrapping angle, with a few more degrees of wrapping occurring with less humeral retroversion. Impingement-free external rotation is reported to increase with more humeral retroversion and impingement-free internal rotation is reported to increase with less humeral retroversion. 15,20-22 While more passive external rotation may be achieved with the humerus oriented in more retroversion, our results demonstrate that the tension of the external rotators decreases, while the tension of the internal rotators increases, potentially negating the functional improvements. Based upon this tradeoff of impingement-free motion and muscle tensioning, we recommend implanting the humerus in version. Implanting the Grammont glenosphere with 15 inferior

8 291 tilt slightly medialized the humerus, decreased the middle deltoid wrapping angle, and decreased the tension of each muscle. Achieving inferior glenosphere tilt requires eccentric reaming of the inferior glenoid which has the advantage of removing bone that could impinge with the humeral liner but the disadvantage of removing cortical bone which may be necessary for fixation. 4,16,42 This removal of structural bone potentially reduces the glenoid baseplate contact area, decreasing the surface area available to distribute the resulting shear loads, and ultimately may increase the risk of glenoid loosening. 42 Based upon the improvements in muscle tension and conservation of bone, we recommend implanting the glenoid with 0 tilt. Implanting the Grammont with bone graft in a non-worn glenoid lateralized the humerus and improved muscle tensioning and deltoid wrapping best compared to the other Grammont configurations analyzed. Using bone graft increased the deltoid wrapping angle by an average of 17.3 and substantially increased the tension of each muscle relative to the Grammont without graft. However, using bone graft failed to restore that anatomic middle deltoid wrapping angle and the anatomic tension of the anterior and posterior shoulder muscles. Bone grafting a non-worn glenoid increases the risk of complications (e.g. graft resorption, impingement with screws in the graft, fracture of the graft, etc). 24,27-29 Because placing bone graft behind the glenoid plate lateralizes the CoR relative to the native glenoid, it subjects the bone graft-native glenoid interface to increased torque and shear that could compromise healing and slow rehabilitation, which is reported to take at least 6 months to incorporate. 12,27,43 While Boileau and coworkers 29 reported that only 2% of autografts did not fully incorporate using the Grammont in a non-worn glenoid, this follow-up was relatively short, humeral head autograft was available and used in every case, and it should be viewed relative to the historical glenoid loosening rate associated with the Grammont reverse shoulder (only 5%). 11 Hill and associates reported much higher long-term autograft resorption rates in the glenoid with total shoulder arthroplasty (5 of 17 cases). 44 Additionally, the graft resorption rate (and costs) may increase if humeral head autograft is not available, requiring iliac crest autograft, allograft, or a hybrid graft to be used as a substitute. 23,24,27,45,46 Due to these additional risks, we do not recommend bone grafting a non-worn glenoid; these risks are unnecessary given that both alternative reverse shoulder designs evaluated in this study achieved as good or better humeral lateralization, deltoid wrapping, and muscle tensioning without the use of bone graft. We only recommend grafting the glenoid with reverse shoulder arthroplasty in cases of severe glenoid wear in which it is necessary to obtain adequate fixation or lateralize the joint line to obtain stability. 26 Restoring the anatomic position of the humeral tuberosities is important to tension any remaining rotator cuff muscles in a more natural physiologic manner and offers the potential to better restore rotational strength. Without adequate tensioning, the anterior and posterior rotator cuff cease to function independently as humeral rotators and together as a transverse force couple to impart joint stability 30,31,34 ; they act instead as only a simple tether of the humerus to prevent dislocation. Future work should seek to optimize these reverse shoulder design parameters to better tension all of the muscles in the shoulder girdle as a collective unit rather than just elongating the deltoid or lengthening the humerus for stability and function. 35,36 This study has some limitations. The digital bone models are of one size and do not account for anatomic variations. We measured the length of eight muscles as the average of three lines from a fixed origin on the scapula or clavicle to its insertion on the humerus as it was abducted and internally and externally rotated. The computer model limited rotation of the scapula and did not simulate wrapping of each muscle around the humerus or scapula. While the degree of abduction where the middle deltoid stopped wrapping the greater tuberosity was quantified, it was done so visually based upon when the muscle line ceased to intersect the humeral bone. Because the wrapping of each muscle was not modeled, it is likely that the individual muscle lengths at low elevation may be slightly underestimated in each situation. Finally, it is unknown if the normal shoulder is the best reference as the collapsed condition of the glenohumeral joint resulting from the pathologies in which the reverse shoulder is indicated may result in muscle remodeling, altering the origin to insertion distance of each muscle. 47 Conclusion The reverse shoulder is geometrically and biomechanically different than the normal shoulder. Muscle tensioning and deltoid wrapping can be altered by different surgical implantation methods. While varying humeral retroversion, glenosphere tilt, and using graft behind the glenoid baseplate of a non-worn glenoid offer the potential to improve deltoid wrapping and muscle tensioning with the Grammont reverse shoulder, each is associated with tradeoffs that could adverse impact outcomes. Minor differences in prosthesis design parameters (less than 10 mm of glenoid and humeral offset and 10 of humeral neck angle) relative to the Grammont reverse shoulder were observed to dramatically improve deltoid wrapping and resulted in more anatomic tensioning of the anterior and posterior rotator cuff muscles, without requiring any compromising modification to the surgical implantation method. Disclosure Statement Christopher P. Roche, M.S., M.B.A., Phong Diep, B.S., and Matthew Hamilton, Ph.D., are employed by Exactech, Inc., Gainesville, Florida. Lynn A. Crosby, M.D., Pierre-Henri Flurin, M.D., Thomas W. Wright, M.D., Joseph D. Zuckerman, M.D., and Howard D. Routman, D.O., receive royalties from Exactech, Inc., Gainesville, Florida.

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