Lateralized Reverse Shoulder Arthroplasty Maintains Rotational Function of the Remaining Rotator Cuff
ABSTRACT 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.Orthopedic Clinics of North America 07/2013; 44(3):389-408. DOI:10.1016/j.ocl.2013.03.010
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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.Journal of Biomechanical Engineering 09/2013; 135(11). DOI:10.1115/1.4025325
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ABSTRACT: Background: External rotation can be compromised after reverse total shoulder arthroplasty (RTSA). A functional teres minor (TM) is relatively common in patients with posterosuperior tears of the rotator cuff, and its function should be enhanced for better postoperative results. The aim of this study was to investigate how the version of humeral fixation can affect the TM rotational moment arm and muscle length as well as impingement after RTSA. Methods: A 3-dimensional shoulder model was used to describe RTSA. Four humeral fixation versions were tested: +20 degrees, 0 degrees, -20 degrees, and -40 degrees (+, anteverted; -, retroverted). TM rotational moment arm and length as well as impingement-free range of motion were calculated for a set of 3 simple clinical motions: (1) scapula plane abduction (0 degrees-150 degrees); (2) internal/external rotation with the arm in adduction; and (3) internal/external rotation with the arm in abduction. Six common activities of daily living were also evaluated. Results: An anteverted fixation maximized TM moment arms, but it also resulted in very short muscle length (compared with normal) and increased inferior impingement. In contrast, 40 degrees humeral retroversion resulted in the longest TM muscle length, but it also showed the smallest moment arms and increased anterior impingement in some of the activities of daily living. Conclusions: Even if TM external rotation moment arm is higher in RTSA than in a normal shoulder, the decreased length could impair its force generation. The 0 degrees and 20 degrees retroversion was the optimum compromise between sufficient TM length and moment arm with minimum impingement. (C) 2015 Journal of Shoulder and Elbow Surgery Board of Trustees.Journal of Shoulder and Elbow Surgery 10/2014; 24(4). DOI:10.1016/j.jse.2014.08.019