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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. [ABSTRACT FROM AUTHOR]

Introduction: Paul Grammont's hemispherical glenosphere 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 abductor moment arms for six different rTSA prostheses during scapular abduction from 0° to 140°. Methods: A 3D computer model was developed in Unigraphics 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 largest 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Background: Controversy surrounds the role of the subscapularis (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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Introduction: A computer model quantified glenoid baseplate 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 cm³), the 32 mm RSP the second most (38.0 cm³), and the Equinoxe the least (31.7 cm³). The 32 mm RSP reamed away the most glenoid bone (3.7 cm³), the Delta III the second most (3.6 cm³), and the Equinoxe the least (3.3 cm³), with greater bone removed with inferior tilt. The Equinoxe had the most glenoid baseplate surface contact (501.3 mm²), the RSP had the second most (386.0 mm²), and the Delta III had the least (360.6 mm²), with less surface contact occurring with medial wear. Discussion and Conclusions: Different rTSA design parameters and preparation techniques impact the amount of glenoid surface contact area and the amount of humeral and glenoid bone removed when implanting each device. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 better 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. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to validate a reverse shoulder computer impingement model and quantify the impact of implant position on scapular impingement by comparing it to that of a radiographic analysis of 256 patients who received the same prosthesis and were followed postoperatively for an average of 22.2 months. Methods: A geometric computer analysis quantified anterior and posterior scapular impingement as the humerus was internally and externally rotated at varying levels of abduction and adduction relative to a fixed scapula at defined glenoid implant positions. These impingement results were compared to radiographic study of 256 patients who were analyzed for notching, glenoid baseplate position, and glenosphere overhang. Results: The computer model predicted no impingement at 0° humeral abduction in the scapular plane for the 38 mm, 42 mm, and 46 mm devices when the glenoid baseplate cage peg is positioned 18.6 mm, 20.4 mm, and 22.7 mm from the inferior glenoid rim (of the reamed glenoid) or when glenosphere overhang of 4.6 mm, 4.7 mm, and 4.5 mm was obtained with each size glenosphere, respectively. When compared to the radiographic analysis, the computer model correctly predicted impingement based upon glenoid baseplate position in 18 of 26 patients with scapular notching and based upon glenosphere overhang in 15 of 26 patients with scapular notching. Conclusions: Reverse shoulder implant positioning plays an important role in scapular notching. The results of this study demonstrate that the computer impingement model can effectively predict impingement based upon implant positioning in a majority of patients who developed scapular notching clinically. This computer analysis provides guidance to surgeons on implant positions that reduce scapular notching, a well-documented complication of reverse shoulder arthroplasty. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]