Factors affecting the stability of reverse shoulder arthroplasty: A biomechanical study

Mechanical and Materials Engineering, Queen's University, Kingston, ON, Canada
Journal of shoulder and elbow surgery / American Shoulder and Elbow Surgeons ... [et al.] (Impact Factor: 2.37). 08/2012; 22(4). DOI: 10.1016/j.jse.2012.05.032
Source: PubMed

ABSTRACT BACKGROUND: Despite the success of reverse shoulder arthroplasty (RSA) in treating patients with painful pseudoparalytic shoulders, instability is a common complication and currently the factors affecting stability are not well understood. The objective of this study was to investigate a number of factors as well as the interactions between factors to determine how they affect the stability of the prosthesis. These factors included: active arm posture (abduction and abduction plane angles), loading direction, glenosphere diameter and eccentricity, and humeral socket constraint. METHODS: Force required to dislocate the joint, determined using a biomechanical shoulder simulator, was used as a measure of stability. A factorial design experiment was implemented to examine the factors and interactions. RESULTS: Actively increasing the abduction angle by 15° leads to a 30% increase in stability and use of an inferior-offset rather than a centered glenosphere improved stability by 17%. Use of a more constrained humeral socket also increased stability; but the effect was dependent on loading direction, with a 88% improvement for superior loading, 66% for posterior, 36% for anterior, and no change for inferior loading. Abduction plane angle and glenosphere diameter had no effect on stability. CONCLUSION: Increased glenohumeral abduction and the use of an inferior-offset glenosphere were found to increase the stability of RSA. Additionally, use of a more constrained humeral socket increased stability for anterior, posterior, and superior loading. These identified factor effects have the potential to decrease the risk of dislocation following RSA.

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    ABSTRACT: The purpose of this study is to demonstrate that inferior inclination of the glenosphere is a protecting factor from joint dislocation in reverse total shoulder replacement. The hypothesis is that an average of 10° of inferior inclination of the glenoid component would determine a significant inferior rate of dislocation as compared to neutral inclination. A retrospective case (dislocation)-control (stability of the implant) study was performed. Inclusion criteria were the homogeneity of the prosthetic model and availability of pre- and postoperative imaging of the shoulder, including antero-posterior and axillary X-ray views. Glenoid and glenosphere inclination were calculated according to standardized methods. Difference in between the angles determined the inferior tilt. Thirty-three cases fit the inclusion criteria. Glenoid and glenosphere inclination measured, respectively, 74.1° and 83.5°. The average tilt of the glenosphere measured 9.4°. The average tilt in stable patients was 10.2°. Tilt in patients with atraumatic dislocation measured, respectively, -6.9° (superior tilt) and 2.4°, while it was 8.3° for the patient with traumatic instability. The association between the tilt of glenosphere and atraumatic dislocation was significant. A 10° inferior tilt of the glenoid component in reverse shoulder arthroplasty is associated with a reduced risk of dislocation when compared to neutral tilt.
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    ABSTRACT: Postoperative instability continues to be one of the most common complications limiting outcomes of reverse shoulder arthroplasty (RSA). The optimal management of this complication remains unknown. The purpose of this study was to evaluate the outcomes of patients with postoperative dislocation after RSA managed with closed reduction. All patients who were treated with a closed reduction for dislocation after RSA in the period between May 2002 and September 2011 were identified and retrospectively reviewed. Final outcomes including recurrent instability, need for revision surgery, American Shoulder and Elbow Surgeons outcome score, and range of motion were evaluated. A total of 21 patients were identified. Nearly 50% of cases (10 of 21) had previous surgery, with 80% (8 of 10) of these being previous arthroplasty. The average time to first dislocation was 200 days, with 62% (13 of 21) occurring in the first 90 days. At average follow-up of 28 months, 62% of these shoulders remained stable (13 of 21), 29% required revision surgery (6 of 21), and 9% remained unstable (2 of 21). The average American Shoulder and Elbow Surgeons score was 68.0 for patients treated with closed reduction for instability and 62.7 for those treated with revision surgery (P = .64). This study shows that an initial dislocation episode after RSA with use of this implant can be successfully managed with closed reduction and temporary immobilization in more than half of cases. Given that outcomes after revision surgery are not different from those after closed treatment, we would continue to recommend an initial attempt at closed reduction in the office setting in all cases of postoperative RSA dislocation. Copyright © 2014 Journal of Shoulder and Elbow Surgery Board of Trustees. Published by Elsevier Inc. All rights reserved.
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