Scientific Articles   |    
Moment Arms of the Shoulder Musculature After Reverse Total Shoulder Arthroplasty
David C. Ackland, PhD1; Sasha Roshan-Zamir, MBBS(Hons), GradDipAnat2; Martin Richardson, MBBS, MS, FRACS, FAOrthA3; Marcus G. Pandy, PhD1
1 Department of Mechanical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia. E-mail address for D.C. Ackland: dackland@unimelb.edu.au. E-mail address for M.G. Pandy: pandym@unimelb.edu.au
2 1 Wanbrow Avenue, Balwyn North, Victoria 3103, Australia. E-mail address: s_rz@netspace.net.au
3 Suite 7.5, 32 Erin Street, Richmond, Victoria 3121, Australia. E-mail address: orthovic@bigpond.com
View Disclosures and Other Information
Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.

Investigation performed at the Department of Mechanical Engineering, The University of Melbourne, Parkville, Victoria, Australia

Copyright ©2010 American Society for Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2010 May 01;92(5):1221-1230. doi: 10.2106/JBJS.I.00001
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Reverse total shoulder arthroplasty is known to increase the moment arm of the middle subregion of the deltoid during shoulder abduction; however, at present, comprehensive data regarding the shoulder muscle moment arm through the full range of abduction and flexion are not available. The purpose of this study was twofold: (1) to measure the instantaneous moment arms of thirteen subregions of major muscles spanning the glenohumeral joint during abduction and flexion of the shoulder after reverse total shoulder arthroplasty and (2) to compare these data with the muscle moment arms previously measured preoperatively in the anatomical shoulders.


Reverse total shoulder arthroplasty was performed on eight entire cadaveric upper extremities. The specimens were mounted onto a dynamic testing apparatus, and the instantaneous abductor/adductor and flexor/extensor moment arms of subregions of the deltoid, latissimus dorsi, pectoralis major, teres major, and subscapularis muscles (a total of thirteen subregions) were measured with use of the tendon excursion method. These muscle moment arms were compared with those measured preoperatively in the anatomical shoulders.


Reverse total shoulder arthroplasty resulted in significant increases in the abductor moment arms of the anterior subregion of the deltoid (mean increase = 10.4 mm; 95% confidence interval = 7.5 to 13.3 mm) and the middle subregion of the deltoid (mean increase = 15.5 mm; 95% confidence interval = 10.8 to 20.3 mm) as well as recruitment of the posterior subregion of the deltoid as an abductor. The superior subregion of the pectoralis major (the clavicular fibers) and anterior subregion of the deltoid were the most effective flexors and had a substantial potential to initiate flexion. The adductor and extensor moment arms of the teres major, latissimus dorsi subregions, and inferior and middle subregions of the pectoralis major increased substantially after the arthroplasty. The subscapularis subregions behaved as extensors, abductors, and adductors after the arthroplasty; this was in contrast to their roles in the anatomical shoulder, in which they were mainly flexors and adductors.


Reverse total shoulder arthroplasty increases the moment arms of the major abductors, flexors, adductors, and extensors of the glenohumeral joint, thereby reducing muscle effort during common tasks such as lifting and pushing.

Clinical Relevance: 

The results suggest that surgeons should attempt to preserve as much of the pectoralis major and subscapularis as possible during reverse total shoulder arthroplasty as these muscles may contribute substantially to shoulder mobility and stability, respectively.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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