Recording scapular motion using an acromion marker cluster.

Disorders of the shoulder complex can be accompanied by changes in the movement pattern of the scapula. However, scapular motion is difficult to measure. A possible non-invasive method for dynamic three-dimensional kinematic measurement of the human scapula is the use of a marker cluster placed on the flat part of the acromion. A small light-weight acromion marker cluster (AMC) is presented in this study. In order to assess validity, kinematics obtained with the AMC were compared to simultaneous scapula locator (SL) recordings in a series of postures. The test/retest variability of replacement of the AMC, was also assessed. Measurement errors appeared to be sensitive for the plane of movement, the degree of humerus elevation, and replacement of the AMC. The AMC generally under-estimated scapula motion, compared to the SL. Some significant differences were found between the two methods, although the absolute differences were small (maximum mean difference 8.4 degrees in extreme position). In humerus forward flexion and abduction the maximum mean differences were 6 degrees or lower. In conclusion, the AMC is a valid method of measuring scapular movement during arm elevation that could be used in shoulder pathologies. Placement and planes of movement should be carefully considered and elevation of the humerus should not exceed 100 degrees.