Measuring Scapular Movement Using Three-Dimensional Acromial Projection

Abnormal scapular kinematics are associated with a number of shoulder pathologies, including, but not limited to, shoulder impingement syndrome [1, 2], rotator cuff tendinopathy [3], rotator cuff tears [4], shoulder instability [5], and adhesive capsulitis [6, 7]. A variety of methods have been proposed to quantify scapular movement. These have progressed from initial attempts utilizing two-dimensional radiographs to modern experimental designs creating digital maps from high-speed computerized imaging [8]. In 1944, Inman et al utilized two-dimensional analysis of radiographs to report a 2:1 relationship between glenohumeral elevation and upward rotation of the scapula [9]. This relationship was termed the scapulohumeral rhythm, and recent studies have reported similar rhythm ratios [10–12]. Despite being able to capture broad motions, these initial methodologies fail to account for “out of plane” movements and provide inaccurate definitions for scapular motions [13]. To address these issues, three-dimensional techniques including three-dimensional radiographic analysis [14], three-dimensional digitization techniques [15–17], and three-dimensional electromagnetic-based measurements [18, 19] were developed. However, none thus far has been clinically accepted as a model for shoulder biomechanics. Existing noninvasive approaches to tracking scapular motion have limitations. Because of the scapula’s complex 3-dimensional kinematics and its mobility relative to the overlying skin, tracking the scapula using surface markers can be difficult and can produce systematic error [20]. Karduna et al present two noninvasive methods for measuring scapular orientation using surface cutaneous markers [21]. The first is a custom adjustable jig that attaches to the scapular spine [21–23]. The second method consists of applying an electromagnetic sensor to the skin over the acromion to track scapular motion [21]. While both methods offer reasonable validity in measuring scapular motion, with less than ten-degree error measuring between 30 and 120-degrees of humerothoracic elevation, the acromial projections were found to more accurately measure upward rotation of the scapula [21]. Although it is difficult to track the scapula using cutaneous methods, the advantages of this technique over other previous methodologies are ease of use and the noninvasive approach [24]. The use of an electromagnetic sensor on an acromion cluster has been found to be an effective method of tracking the scapula as validated by a recent study using bone pins [25]. Unfortunately, electromagnetic systems are limited both by the number of channels that can be simultaneously recorded, by requiring close proximity of the participant to the system’s receiver, and by requiring that the participant be “wired” to the device. Video motion capture systems, in contrast, allow for simultaneous collection of additional segmental data (e.g., simultaneous capture of the humerus, thorax and neck), while reducing the proximity requirements. These factors ultimately led to the use of video motion capture in measuring shoulder kinematics. The purpose of this investigation was to assess the accuracy of a method for measuring the three-dimensional dynamics of the shoulder complex utilizing video motion capture and reflective cutaneous markers affixed to the skin over the acromion process. This would provide an easy-to-use, noninvasive model for assessing shoulder biomechanics for use in studies of shoulder injury.