Augmented baseplates yield optimum outcomes when compared with bone graft augmentation for managing glenoid deformity during reverse total shoulder arthroplasty: a retrospective comparative study.

PURPOSE: The purpose of this study was to compare the outcomes of primary reverse total shoulder arthroplasty (rTSA) using glenoid bone grafting (BG rTSA) with primary rTSA using augmented glenoid baseplates (Aug rTSA) with a minimum 2-year follow-up. METHODS: A total of 520 primary rTSA patients treated with 8° posterior glenoid augments (n = 246), 10° superior glenoid augments (n = 97), or combined 10° superior/8° posterior glenoid augments (n = 177) were compared with 47 patients undergoing glenoid bone grafting for glenoid bone insufficiency. The mean follow-up was 37.0(±16) and 53.0(±27) months, respectively. Outcomes were analyzed preoperatively and at the latest follow-up using conventional statistics and stratification by minimum clinically important difference (MCID) and substantial clinical benefit (SCB) thresholds where applicable. Radiographs were analyzed for baseplate failure, and the incidences of postoperative complications and revisions were recorded. RESULTS: The glenoid Aug rTSA cohort had greater improvements in patient-reported outcome measures (PROMs) and range of motion when compared with the BG rTSA group at a minimum of 2-year follow-up, including Simple Shoulder Test, Constant score, American Shoulder and Elbow Surgeons score, University of California Los Angeles score, Shoulder Pain and Disability Index score, shoulder function, Shoulder Arthroplasty Smart score, abduction, and external rotation (P < .05). Patient satisfaction was higher in the Aug rTSA group compared with the BG rTSA group (P = .006). The utilization of an augmented glenoid component instead of glenoid bone grafting resulted in approximately 50% less total intraoperative time (P < .001), nearly 33% less intraoperative blood loss volume (P < .001), approximately 3-fold less scapular notching (P < .01), and approximately 8-fold less adverse events requiring revision (P < .01) when compared with the BG rTSA cohort. Aside from SCB for abduction, the Aug rTSA cohort achieved higher rates of exceeding MCID and SCB for every PROM compared with BG rTSA. More specifically, 77.6% and 70.2% of the Aug rTSA achieved SCB for American Shoulder and Elbow Surgeons and Shoulder Pain and Disability Index vs. 55% and 48.6% in the BG rTSA, respectively (P = .003 and P = .013). CONCLUSION: The present midterm clinical and radiographic study demonstrates that the utilization of an augmented baseplate for insufficient glenoid bone stock is superior as judged by multiple PROMs and range of motion metrics when compared with bone graft augmentation at minimum 2-year follow-up. In addition, when analyzed according to MCID and SCB thresholds, the use of augmented baseplates outperforms the use of glenoid bone grafting. Complication and revision rates also favor the use of augmented glenoid baseplates over glenoid bone grafting. Long-term clinical and radiographic follow-up is necessary to confirm that these promising midterm results are durable.

Opioid requirement after rotator cuff repair is low with a multimodal approach to pain.

BACKGROUND: Current practices may aim to blunt rather than understand postoperative pain. Perhaps the most common serious complication of arthroscopic rotator cuff repair (ARCR) is persistence of opiate medication intake. Patients still receive upwards of 80 oxycodone 5 mg pills, or 600 morphine milligram equivalents (MMEs), leading more than 20% of opioid-naïve subjects to continue to fill opioid prescriptions beyond 180 days after surgery. Developing evidence-based guidelines for narcotic prescription after ARCR presents an opportunity for orthopedic surgeons to address the opioid epidemic. PURPOSE: The purpose of this study was (1) to prospectively determine the requirements for opiate medications after ARCR, and (2) to create an evidence-based guideline for postoperative prescription, in contrast to the anecdotal or expert panel recommendations that currently exist. We further investigated whether a liposomal bupivacaine (LB) interscalene never block (ISNB) would reduce pain and opiate consumption compared with standard bupivacaine ISNB (control) for ARCR. METHODS: The study enrolled 100 patients who underwent primary ARCR surgery. Patients were provided with postoperative "pain journals" to document their daily pain on a numerical rating scale, satisfaction with pain management using the Likert scale, and track their daily oxycodone 5 mg pill consumption during the 14-day postoperative period. Enrolled patients were further randomized to receiving an LB (experimental) or standard bupivacaine (control) ISNB. RESULTS: A total of 77% of all patients required fewer than 15 pills postoperatively. The LB group consumed an average of 1.7 fewer pills (13.0 MMEs) on postoperative day (POD) 1 (P = .02) and reported statistically lower pain during PODs 1 and 2 (P = .01 and P = .006), as well as cumulatively throughout the study period (P = .03). In addition, LB patients remained opioid-free at a higher rate (44% vs. 15% in controls, P = .03). CONCLUSION: With a multimodal approach, the majority of patients undergoing ARCR can manage postoperative pain with 15 or fewer oxycodone 5 mg tablets (112.5 MMEs) and maintain a high degree of satisfaction. The addition of an LB ISNB may further reduce the consumption of postoperative narcotics compared with a standard ISNB. This study provides evidence that may be used for surgeon guidelines in the effort to reduce opioid prescriptions after ARCR.

Development and validation of a predictive model for outcomes in shoulder arthroplasty: a multicenter analysis of nearly 2000 patients.

BACKGROUND: Guiding expectations following shoulder arthroplasty is important in improving patient satisfaction. The purpose of this study was to develop a predictive model to calculate 2-year American Shoulder and Elbow Surgeons (ASES) scores in shoulder arthroplasty patients from a comprehensive set of preoperative patient factors and types of arthroplasty performed. METHODS: This retrospective multicenter study included 1947 shoulder arthroplasties performed from 2010 to 2015 at 3 high-volume centers. Twenty-six variables were evaluated for an association with 2-year ASES scores, and variables with P < .20 in our pair-wise analysis were used to develop a predictive model. The prediction root-mean-square error was calculated. External validation was performed using data from 233 patients who underwent shoulder arthroplasty performed by a separate shoulder surgeon at a center not involved with creation of the predictive model. RESULTS: A total of 1947 patients were analyzed, and their data were used to construct the predictive model. Variables most associated with 2-year ASES scores were patient age, preoperative ASES score, disability, chronic obstructive pulmonary disease, alcohol use, anatomic vs. reverse total shoulder arthroplasty, and primary vs. revision shoulder arthroplasty. By use of cross validation, the prediction error was 20.1, the proportion of variance explained was 25.3%, the mean absolute error was 15.9, and the C statistic for the linear regression model was 0.66. After external validation, the mean difference between predicted and actual 2-year ASES scores was 12.7 points, within the accepted minimal clinically important difference after shoulder arthroplasty. DISCUSSION: Data from nearly 2000 shoulder arthroplasties allowed the development and validation of a model to predict 2-year ASES scores following shoulder arthroplasty. The model was accurate within the minimal clinically important difference in 85% of patients.

Local injection of liposomal bupivacaine combined with intravenous dexamethasone reduces postoperative pain and hospital stay after shoulder arthroplasty.

BACKGROUND: Alternative techniques have been developed to address pain after shoulder arthroplasty and are well documented. We evaluated the effect of adding intraoperative liposomal bupivacaine and intravenous dexamethasone during shoulder arthroplasty. METHODS: We retrospectively reviewed 2 consecutive cohorts undergoing elective shoulder arthroplasty. The 24 patients in cohort 1 and the 31 patients in cohort 2 received perioperative multimodal management with preoperative and postoperative intravenous and oral narcotics, gabapentin, nonsteroidal anti-inflammatory drugs, acetaminophen, and single-injection interscalene block. Cohort 2 also received 8 to 10 mg of intravenous dexamethasone intraoperatively after the skin incision and liposomal bupivacaine injected at surgery. Patients who did and did not use preoperative narcotics were analyzed together and separately. We evaluated hospitalization length of stay, narcotic use, and visual analog scale pain before and after the change in the perioperative protocol. RESULTS: Cohort 1 was hospitalized longer (2 vs. 1 day; P < .001), required more narcotics on postoperative day 1 (21.0 vs. 10.0 mg; P < .001) and days 0 and 1 cumulatively (30.5 vs. 17.5 mg; P = .001), and had more pain on postoperative days 0 (6.5 vs. 3.5; P < .001) and 1 (7.5 vs. 3.5; P < .001) than cohort 2. In patients using preoperative narcotics, cohort 2 had less pain on postoperative day 1 (3.5 vs. 7.0; P = .006), less cumulative narcotic use (20 vs. 58.5 mg; P = .03), and shorter hospitalization (1 vs. 2 days; P = .052) than cohort 1. CONCLUSION: These changes to the perioperative shoulder arthroplasty protocol decreased hospitalization length of stay, narcotic requirement, and pain.

The incidence of radiographic aseptic loosening of the humeral component in reverse total shoulder arthroplasty.

BACKGROUND: The reverse total shoulder arthroplasty (RTSA) has been used in the treatment of complex shoulder problems. The incidence of aseptic loosening of the humeral component has not been previously reported. METHODS: This is a multicenter, retrospective, blinded, case-control radiographic review of 292 patients to determine the rate of humeral stem loosening. There were 177 cemented and 115 press-fit humeral components. Radiographs were critiqued for radiolucent lines adjacent to the humeral stem based on the method described by Gruen et al. RESULTS: The overall rate of loosening was 0.74%. No radiographic loosening occurred in the press-fit group (115 stems). In the cemented group (177 stems), 2 shoulders (1.18%) were identified with radiographically loose stems. No loosening occurred in the press-fit group. No statistically significant difference was found in humeral stem loosening when the press-fit group and the cemented group were compared (P = .198). DISCUSSION: Our study indicates the cemented or press-fit RTSA system will result in a low incidence of radiolucent lines and radiographic loosening. Compared with historical survivorship of conventional anatomic total shoulder arthroplasty, RTSA shows a lower rate of radiographic stem loosening at a mean of 38.46 months. CONCLUSIONS: The RTSA has a low incidence of humeral stem loosening at midterm. These results underscore the importance of careful selection of patients to provide the benefits of this surgical technique. Press-fit fixation may provide a lower risk to stem loosening.

Preoperative external rotation deficit does not predict poor outcomes or lack of improvement after reverse total shoulder arthroplasty.

INTRODUCTION: The purpose was to compare postoperative outcomes and functional improvement between patients with preoperative aER deficits vs. preserved aER function. RESULTS: There were 115 patients in the <0° aER group and 314 in the ≥30° aER group. Preoperative patients in the <0° group were worse for all measures except subjective pain while post-operatively, they had significantly greater improvement for all measures of motion. Postoperatively, both groups achieved comparable scores for forward elevation, pain, SST and ASES. CONCLUSION: This study demonstrates that patients with a complete aER deficit can recover substantial and comparable function after RTSA.

Patient-reported outcomes of reverse total shoulder arthroplasty: a comparative risk factor analysis of improved versus unimproved cases.

BACKGROUND: The purpose of this study was to compare characteristics of patients who reported to be subjectively unimproved vs. improved after reverse total shoulder arthroplasty. METHODS: Data were derived from a prospective registry of patients who underwent reverse total shoulder arthroplasty with a minimum 2-year follow-up. Patients were asked to rate their subjective satisfaction and then divided into those who were unchanged or worse (unimproved group [UG]) vs. better or much better (improved group [IG]). The groups were compared for differences in demographic characteristics, preoperative factors, functional outcomes, and complications. RESULTS: There were 1425 patients in the IG and 134 patients in the UG. Patients in the IG were more likely to have a diagnosis of osteoarthritis. Patients in the UG were more likely to have coronary artery disease and diabetes and to have undergone prior surgery. No differences in implant configuration were found between groups. Preoperative measures for patients in the UG were worse for pain and function but not for range of motion. The outcomes in patients in the UG were worse for all postoperative measures, as well as for preoperative-to-postoperative improvement. Of the patients in the UG, 48% continued to have moderate to severe pain postoperatively. The complication rate was significantly higher in the UG. DISCUSSION: Up to 8.5% of patients rate themselves as unimproved after surgery. These patients are more likely to have certain comorbidities and to have undergone prior surgery. Although outcomes were significantly worse for all measures in the UG, improvement occurred in all measures despite patients subjectively being worse or unchanged. Residual pain and difficulty sleeping play a substantial role in subjective assessment of overall outcome.

Effect of Tuberosity Healing on Clinical Outcomes in Elderly Patients Treated With a Reverse Shoulder Arthroplasty for 3- and 4-Part Proximal Humerus Fractures.

OBJECTIVES: To evaluate tuberosity union rate and clinical outcome after 3- and 4-part proximal humerus fractures in the elderly. DESIGN: Retrospective, multicenter database cohort study. SETTING: Level I and Level II trauma centers. PATIENTS: Fifty-five patients older than 65 years had insertion of reverse shoulder arthroplasty (RTSA) for OTA/AO 11-B and 11-C proximal humerus fractures. INTERVENTION: Treatment with RTSA using a dedicated low profile onlay fracture stem using variable tuberosity fixation. MAIN OUTCOME MEASURES: Constant score, the American Shoulder and Elbow Surgeons score, Shoulder Pain and Disability Index score, University of California at Los Angeles score, Simple Shoulder Test score, visual analog pain score, shoulder function score, active range of motion, external rotation (ER)-specific tasks and position, rate of greater tuberosity healing, effect of tuberosity healing on overall clinical metrics, incidence of humeral lucency, and scapular notching. RESULTS: Eighty-three percent of the greater tuberosities that were repaired united. Greater tuberosity union resulted in greater active ER (P = 0.0415). There was a statistically significant difference in the ability to do ER-type activities between the 2 cohorts reflected in the ability to position one's hand behind their head with the elbow forward (P = 0.002) and comb their hair (P < 0.001). CONCLUSION: The use of a low profile onlay fracture stem in RTSA for acute 3- and 4-part proximal humerus fractures in the elderly can result in a high tuberosity union rate. Greater tuberosity healing significantly influences ER and ER-type activities that are not apparent by analysis of the overall metrics studied. LEVEL OF EVIDENCE: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Subscapularis-Sparing Approaches to Total Shoulder Arthroplasty: Ready for Prime Time?

To minimize the risk of subscapularis failure after shoulder replacement, a method of preserving the subscapularis while allowing access to the glenohumeral joint was developed. Only the inferior 30% to 50% of the subscapularis tendon is detached from the humerus, leaving the superior aspect attached to the lesser tuberosity. This subscapularis-sparing, minimally invasive approach to the glenohumeral joint was evaluated in 43 subjects with a minimum 2-year follow-up and subscapularis strength equal to the opposite side. The approach provided adequate exposure for shoulder replacement surgery and decreased risk of postoperative failure of the subscapularis tendon.

Isolated Rhabdomyolysis of the Infraspinatus Muscle Following the CrossFit "Sissy Test": A Report of Two Cases.

CASE: Following the completion of a CrossFit-style challenge (the "Sissy Test"), 2 patients presented with severe pain and swelling over the posterior aspect of the scapula. Magnetic resonance imaging demonstrated isolated edema of the infraspinatus muscle in both patients; the compartments were compressible. Neurovascular checks and observation of range of motion were performed. The patients were treated nonoperatively and were discharged with the diagnosis of overuse syndrome with rhabdomyolysis of the infraspinatus muscle. CONCLUSION: With marked increase in the popularity of extreme fitness, monitoring for rhabdomyolysis and potential renal dysfunction is essential.

Stemless and Short Stem Humeral Components in Shoulder Arthroplasty.

Humeral-sided arthroplasty design evolution continues to be supported by the published literature in the transition away from traditional stemmed devices. Early studies have shown not only absence of failure with these newer shorter and stemless designs but also equivalence in terms of early outcomes compared to traditional stemmed devices with the benefits of shorter operative time, less blood loss, easier revision, and the potential to reduce stress shielding and periprosthetic fractures. We will review the literature available on the different designs of both short stem and stemless humeral devices.

Reverse Shoulder Arthroplasty Prosthesis Design Classification System.

Multiple different reverse total shoulder arthroplasty (rTSA) prosthesis designs are available in the global marketplace for surgeons to perform this growing procedure. Subtle differences in rTSA prosthesis design parameters have been shown to have significant biomechanical impact and clinical consequences. We propose an rTSA prosthesis design classification system to objectively identify and categorize different designs based upon their specific glenoid and humeral prosthetic characteristics for the purpose of standardizing nomenclature that will help the orthopaedic surgeon determine which combination of design configurations best suit a given clinical scenario. The impact of each prosthesis classification type on shoulder muscle length and deltoid wrapping are also described to illustrate how each prosthesis classification type impacts these biomechanical parameters.

Optimizing Deltoid Efficiency with Reverse Shoulder Arthroplasty Using a Novel Inset Center of Rotation Glenosphere Design.

INTRODUCTION: Paul Grammont's hemispherical gleno sphere concept medializes the center of rotation (CoR) to the glenoid face to increase deltoid abductor moment arms and improve muscle efficiency. Reducing glenosphere thickness to less than half its spherical radius further medializes the CoR and offers the potential for even greater improvements in efficiency. To that end, this study quantifies deltoid abducttor moment arms for six different rTSA prostheses during scapular abduction from 0° to 140°. METHODS: A 3D computer model was developed in Uni graphics to quantify deltoid moment arms during scapular abduction for the normal anatomic shoulder, the 36 mm Grammont Delta III (Depuy, Inc.), 36 mm BIO-RSA ® (Tornier, Inc.), the 32 mm RSP® (DJO, Inc.), and the Equinoxe® rTSA (Exactech, Inc.) with three different glenosphere geometries: 38 mm x 21 mm, 46 mm x 25 mm, and the novel 46 mm x 21 mm. Each muscle was simulated as three lines from origin to insertion as the arm was elevated; positional data was exported to Matlab where the abductor moment arms were calculated for the anterior, middle, and posterior deltoid from 0° to 140° humeral abduction in the scapular plane using a 1.8:1 scapular rhythm. RESULTS: The 46 mm x 21 mm glenosphere had the larg est average abductor moment arms and also the largest efficiency for all three heads of the deltoid, having a 4.8% to 40.7% increase in the average deltoid efficiency relative to all other designs tested. The glenosphere design with the next most efficient deltoid was the 36 mm Delta III, which had the next most medialized CoR. The two least efficient designs were the BIO-RSA ® and the DJO RSP® , which had the most lateral CoR. DISCUSSION: These results provide new biomechanical insights on the impact of glenosphere geometry on deltoid abductor moment arms and demonstrate that subtle changes in rTSA prosthesis design can result in dramatic improvements. Increasing glenosphere diameter while also decreasing thickness to be less than half its spherical radius may minimize the muscle forces required to perform activities of daily living. Clinical follow-up is necessary to demonstrate a reduction in complications related to joint over-loading and also demonstrate greater increases in range of motion for patients with weak musculature.

Impact of Posterior Wear on Muscle Length with Reverse Shoulder Arthroplasty.

The use of reverse total shoulder arthroplasty (rTSA) in patients with posterior glenoid wear can be challenging. Implanting a baseplate in the correct version may require significant eccentric reaming, which further medializes the joint line and results in greater rotator cuff muscle shortening. To restore the joint line, bone graft may be required, though it is associated with additional risks. As an alternative solution, augmented glenoid baseplates offer the potential to restore the joint line and improve rotator cuff muscle tensioning without the need for eccentric reaming or supplemental bone graft. To that end, this computer analysis quantifies the rotator cuff muscle length for standard and augmented rTSA when used in a normal and posteriorly worn glenoid. These results demonstrate that shortening of the rotator cuff occurred for both the standard and posterior augmented reverse shoulder designs with additional muscle shortening occurring in scapula with posteriorly worn glenoids. More anatomic rotator cuff muscle tensioning was observed with augmented glenoid baseplates. The use of posterior augmented glenoid baseplates has the potential to improve stability and better restore active internal and external rotation, a current limitation of rTSA. However, clinical follow-up is necessary to confirm these favorable biomechanical results.

Indications, technique, and pitfalls of reverse total shoulder arthroplasty for proximal humerus fractures.

Management of proximal humerus fractures with hemiarthroplasty has been shown to yield unpredictable and inconsistent outcomes. Risk factors for clinical failure following hemiarthroplasty include postoperative tuberosity failure, advanced age, female sex, osteoporosis, and inability or unwillingness to participate in the extensive rehabilitation required. In this difficult-to-manage injury, reverse total shoulder arthroplasty (rTSA) for fracture has been shown to reliably restore elevation, even in cases of tuberosity failure, and to restore rotational movements if the tuberosity heals. In addition, a fracture-specific implant design has been suggested to improve the likelihood of tuberosity healing. Early results indicate that fractures of the proximal humerus that are indicated for arthroplasty should be considered for rTSA. Tuberosity integrity appears to impact rotational movement but not restoration of elevation.

The role of subscapularis repair in reverse total shoulder arthroplasty.

BACKGROUND: Controversy surrounds the role of the sub- scapularis (SSC) in reverse shoulder arthroplasty (rTSA) and the need for repair, if possible, at the conclusion of the procedure. QUESTIONS AND PURPOSE: Some investigators have concluded that an intact SSC is critical for stability; others have found no such correlation. What factors should be part of the decision-making matrix on SSC management for surgeons considering rTSA? FINDINGS: The data on management of the SSC in rTSA support a design-based approach. Researchers have shown that the SSC is critical to stability when the surgeon uses an implant with a medialized humeral component and medialized glenoid component. However, lateralized designs allow for more stability from horizontal deltoid compression and may not require repair of the SSC. In addition, SSC repair has been shown to increase the workload of the residual posterior rotator cuff and the deltoid in rTSA, both of which may have negative consequences on overall function. Lateralization from the glenoid component increases deltoid work, whereas lateralization from the humeral component maintains deltoid efficiency while improving stability. CONCLUSIONS: The need for SSC repair in rTSA can vary based on the implant selected. Humeral and glenoid offset influence the stability and kinematics of rTSA.

Design rationale for a posterior/superior offset reverse shoulder prosthesis.

INTRODUCTION: A computer model quantified and compared muscle lengths and moment arms of two different reverse shoulder arthroplasty humeral tray designs during two different motions. METHODS: The computer model simulated internal/ external rotation and abduction in the scapular plane for the normal shoulder, the 38 mm non-offset, and the 38 mm posterior-superior offset reverse shoulders. Muscle lengths were directly measured for seven muscles during each motion. External rotation moment arms were calculated for five muscles during each motion. RESULTS: The offset tray shifted the humerus posteriorly and superiorly relative to the non-offset tray. The more superior humeral position with the offset tray elongated the deltoid 1.0% to 3.8% less and caused each muscle to convert from an adductor to abductor earlier in abduction. The more posterior humeral position with the offset tray better restored the anatomic muscle tension, decreased the internal rotation capability (e.g., moment arm) of the subscapularis and teres major by 7.1 mm and 9.5 mm, and increased the external rotation capability of the posterior deltoid, infraspinatus, and teres minor by 1.3 mm, 8.6 mm, and 7.8 mm. DISCUSSION AND CONCLUSIONS: The offset humeral tray bet- ter restored the anatomic muscle tensioning and increased the external rotation moment arms relative to the non-offset humeral tray, which has positive implications on strength, stability, and motion.

Effect of prosthesis design on muscle length and moment arms in reverse total shoulder arthroplasty.

The purpose of this article is to compare the effect reverse shoulder design philosophy has on the lengths and moment arms of the external rotators of the shoulder (i.e. posterior deltoid, infraspinatus, and teres minor). In this study, a single bone model was used to normalize the origin and insertion of each muscle. Four different commercially available designs were virtually implanted into the bones. The assemblies oriented with the arm at the side (0° of abduction) and articulated to 45° internal rotation and 45° external rotation. The muscle length and moment arm for each muscle were analyzed and compared to the anatomic shoulder. The results indicate that all the external rotators are shortened relative to the anatomic shoulder, but the MGLH design shortened the muscles the least. The moment arm analysis showed the teres minor and infraspinatus moment arms increased relative to the normal shoulder in external rotation for all designs. The moment arms are sensitive to the lateral offset of the stem relative to the center of rotation. The MGLH design had the largest moment arm during the external rotation range of motion due to its large humeral offset.

Comparison of bone removed with reverse total shoulder arthroplasty.

INTRODUCTION: A computer model quantified glenoid base - plate surface contact area and the amount of humeral and glenoid bone removed by three different reverse shoulder prosthesis designs when implanted with various techniques. METHODS: The computer model quantified differences in glenoid baseplate contact area and the cortical and cancellous humeral and glenoid bone removed to implant the 36 mm Depuy Delta III, 32 mm neutral DJO RSP, and the 38 mm Exactech Equinoxe reverse shoulders when each was implanted along the inferior glenoid rim of a normal and 10 mm medially eroded scapula in 20° of humeral retroversion. The impact of inferior glenoid tilt was also quantified. RESULTS: The Delta III resected the most overall humeral bone (47.4 cm 3 ), the 32 mm RSP the second most (38.0 cm 3 ), and the Equinoxe the least (31.7 cm 3 ). The 32 mm RSP reamed away the most glenoid bone (3.7 cm 3 ), the Delta III the second most (3.6 cm 3 ), and the Equinoxe the least (3.3 cm 3 ), with greater bone removed with inferior tilt. The Equinoxe had the most glenoid baseplate surface contact (501.3 mm 2 ), the RSP had the second most (386.0 mm 2 ), and the Delta III had the least (360.6 mm 2 ), with less surface contact occurring with medial wear.

Impact of inferior glenoid tilt, humeral retroversion, bone grafting, and design parameters on muscle length and deltoid wrapping in reverse shoulder arthroplasty.

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 better tensioning of the anterior and posterior shoulder muscles. CONCLUSIONS: Muscle tensioning and deltoid wrapping can be substantially altered by surgical implantation meth- ods 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.