Lateralized Reverse Shoulder Arthroplasty Maintains Rotational Function of the Remaining Rotator Cuff
Center for Musculoskeletal Surgery, Charité-University Medicine Berlin, Charitéplatz 1, D-10117, Berlin, Germany, .Clinical Orthopaedics and Related Research (Impact Factor: 2.77). 12/2012; 471(3). DOI: 10.1007/s11999-012-2692-x
BACKGROUND: Humeral rotation often remains compromised after nonlateralized reverse shoulder arthroplasty (RSA). Reduced rotational moment arms and muscle slackening have been identified as possible reasons for this impairment. Although several clinical studies suggest lateralized RSA may increase rotation, it is unclear whether this is attributable to preservation of rotational moment arms and muscle pretension of the remaining rotator cuff. QUESTIONS/PURPOSES: The lateralized RSA was analyzed to determine whether (1) the rotational moment arms and (2) the origin-to-insertion distances of the teres minor and subscapularis can be preserved, and (3) their flexion and abduction moment arms are decreased. METHODS: Lateralized RSA using an 8-mm resin block under the glenosphere was performed on seven cadaveric shoulder specimens. Preimplantation and postimplantation CT scans were obtained to create three-dimensional shoulder surface models. Using these models, function-specific moment arms and origin-to-insertion distances of three segments of the subscapularis and teres minor muscles were calculated. RESULTS: The rotational moment arms remained unchanged for the middle and caudal subscapularis and teres minor segments in all tested positions (subscapularis, -16.1 mm versus -15.8 mm; teres minor, 15.9 mm versus 15.3 mm). The origin-to-insertion distances increased or remained unchanged in any muscle segment apart from the distal subscapularis segment at 0° abduction (139 mm versus 145 mm). The subscapularis and teres minor had increased flexion moment arms in abduction angles smaller than 60° (subscapularis, 2.7 mm versus 8.3 mm; teres minor, -6.6 mm versus 0.8 mm). Abduction moment arms decreased for all segments (subscapularis, 4 mm versus -11 mm; teres minor, -3.6 mm versus -19 mm). CONCLUSIONS: After lateralized RSA, the subscapularis and teres minor maintained their length and rotational moment arms, their flexion forces were increased, and abduction capability decreased. CLINICAL RELEVANCE: Our findings could explain clinically improved rotation in lateralized RSA in comparison to nonlateralized RSA.
Article: Reverse Shoulder Arthroplasty[Show abstract] [Hide abstract]
ABSTRACT: The reverse shoulder arthroplasty is considered to be one of the most significant technological advancements in shoulder reconstructive surgery over the past 30 years. It is able to successfully decrease pain and improve function for patients with rotator cuff-deficient shoulders. The glenoid is transformed into a sphere that articulates with a humeral socket. The current reverse prosthesis shifts the center of rotation more medial and distal, improving the deltoid's mechanical advantage. This design has resulted in successful improvement in both active shoulder elevation and in quality of life.
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ABSTRACT: The reverse shoulder replacement, recommended for the treatment of several shoulder pathologies such as cuff tear arthropathy and fractures in elderly people, changes the biomechanics of the shoulder when compared to the normal anatomy. Although several musculoskeletal models of the upper limb have been presented to study the shoulder joint, only few of them focus on the biomechanics of the reverse shoulder. This work presents a biomechanical model of the upper limb, including a reverse shoulder prosthesis, to evaluate the impact of the variation of the joint geometry and position on the biomechanical function of the shoulder. The biomechanical model of the reverse shoulder is based on a musculoskeletal model of the upper limb, which is modified to account for the properties of the DELTA¯ reverse prosthesis. Considering two biomechanical models, which simulate the anatomical and reverse shoulder joints, changes in muscle lengths, muscle moment arms, and muscle and joint reaction forces are evaluated. The muscle force sharing problem is solved for motions of unloaded abduction in the coronal plane and unloaded anterior flexion in the sagittal plane, acquired using video-imaging, through the minimization of an objective function related to muscle metabolic energy consumption. After the replacement of the shoulder joint, significant changes in the length of the pectoralis major, latissimus dorsi, deltoid, teres major, teres minor, coracobrachialis, and biceps brachii muscles are observed for a reference position considered for the upper limb. The shortening of the teres major and teres minor is the most critical since they become unable to produce active force in this position. Substantial changes of muscle moment arms are also observed, being consistent with the literature. As expected, there is a significant increase of the deltoid moment arms, and more fibers are able to elevate the arm. The solutions to the muscle force sharing problem support the biomechanical advantages pointed to the reverse shoulder design and show an increase in activity from the deltoid, teres minor and coracobrachialis muscles. The glenohumeral joint reaction forces estimated for the reverse shoulder are up to 15% lower than those in the normal shoulder anatomy. The data presented here complements previous publications, which altogether allow researchers to build a biomechanical model of the upper limb including a reverse shoulder prosthesis.
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ABSTRACT: 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.
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