Locating the shoulder joint center in relation to the humeral epicondyles: a novel prediction technique

Hahn, Michael
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In 3-dimensional (3-D) modeling of whole-body biomechanics it is crucial to establish reliable segment endpoints for the calculation of inertial segment parameters. Segment endpoints are commonly defined by joint centers, which have commonly been located relative to external bony landmarks. This technique does not apply to the shoulder, due to the amount of soft tissue surrounding the joint. Attempts have been made to relate the shoulder joint center (SJC) to the acromion process, but there is some doubt whether these methods hold throughout the shoulder's full range of motion (ROM). The purpose of this study was to develop a set of linear regression equations to predict the 3-D location of the SJC relative to the epicondyles of the humerus throughout full ROM. A sample of 42 skeletal specimens was measured for the regression model and a separate set of 4 specimens was used to validate the equations. The equations were then validated using volunteer subjects throughout shoulder flexion/extension and abduction/adduction. The movements were recorded with planar video and 3-D real-time motion analysis. The video collection was hand digitized and used as a comparison for the prediction technique. This comparison required the 3-D position of the predicted SJC to be projected onto the plane of video collection. Vectors were then calculated from the epicondyles to the SJC at 100 intervals. To determine SJC coordinate variance, total excursion and centroid values were calculated. A repeated measures ANOVA was used for statistical comparison. It was hypothesized that the prediction technique would not be significantly different in locating the SJC throughout the shoulder's ROM. It was found that the prediction method caused the SJC to lie significantly more proximal than the digitized SJC in flexion and abduction. The excursion and centrold results of the prediction technique indicated that the SJC traveled more medially in abduction and more anteriorly in flexion than planar digitizing could account for. While these findings do not support the hypothesis, the prediction technique developed in this study may provide a more legitimate representation of SJC movement and thus allow more accurate modeling of the upper extremities.

Health and human performance, Exercise and sport science