• Stefan Greiner, Christan Schmidt, Christian König, Carsten Perka, and Sebastian Herrmann.
• Center for Musculoskeletal Surgery, Charité-University Medicine Berlin, Charitéplatz 1, D-10117 Berlin, Germany. stefan.greiner@charite.de
• Clin. Orthop. Relat. Res. 2013 Mar 1; 471 (3): 940-6.

BackgroundHumeral 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/PurposesThe 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.MethodsLateralized 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.ResultsThe 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).ConclusionsAfter lateralized RSA, the subscapularis and teres minor maintained their length and rotational moment arms, their flexion forces were increased, and abduction capability decreased.Clinical RelevanceOur findings could explain clinically improved rotation in lateralized RSA in comparison to nonlateralized RSA.

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