Effects of the glenohumeral joint center on the role of the middle deltoid : implications for reverse total shoulder arthroplasty.

For shoulders with rotator cuff damage, the reverse shoulder replacement medializes the center of glenohumeral joint rotation, lending larger moment arms to undamaged muscles like the deltoid. The aim of this thesis was to create and use a computational model of the glenohumeral joint with an adjustable joint center to investigate and quantify the role of its position on middle deltoid moment arms for coronal plane abduction. The model confirmed that muscle moment arms increase as the center of rotation is shifted medially or inferiorly from the anatomical position. Specifically, medial translation of the glenohumeral center increased moment arms at low abduction angles while inferior shifts increased moment arms at high angles. Excursions of the middle deltoid were found to increase linearly as the joint center was moved medially or inferiorly: 1 mm medial translation or 1.33 mm inferior translation caused about 1 mm of increase in muscle excursion.