Obtaining Glenoid Positioning Data from Scapular Palpable Points In Vitro

Both clinical and biomechanical problems affecting the shoulder joint suggest that investigators should study force transmission into and out from the scapula. To analyze force transmission between the humeral head and the glenoid, one must know the position of the glenoid. Studies have analyzed the position of the scapula from the positions of three palpable points, but the position of the glenoid relative to three palpable points has not been studied. Dry scapulae ( ) were subjected to X-rays and a critical angle, (which relates the plane determined by the three palpable points on the scapula to a plane containing the glenoid center and the first two palpable points) was calculated. The mean value for was degrees. The obtained allows us to determine the position of the glenoid from three palpable points. This information could be used in calculation of forces across the shoulder joint, which in turn would allow optimizing the choice of strengthening exercises. 1. Introduction Both clinical and biomechanical problems affecting the shoulder joint suggest that investigators should study force transmission into and out from the scapula [1]. One clinical problem is the wearing away of the posterior part of the glenoid in patients who need total shoulder replacement [2]. Another problem is dyskinesis, (e.g., abnormal scapular motion) due to various clinical entities such as internal derangement of the shoulder, acromioclavicular instability, and fractured clavicle [1, 3]. In addition, movements of the scapula are subjected to extensive variations, which in itself influence the interaction of glenoid and humeral head [4]. Some authors postulate that dyskinesis causes swimmer’s shoulder and impingement syndrome [5, 6]. The biomechanical problem is finding the location of the glenoid. Since the glenoid is always centered about the humeral head [7], the problem becomes finding two coordinates of the glenoid, for example, Euler angles. To the best of our knowledge, there are no studies dealing with this problem. This is mainly because current techniques lack the ability to locate, in three dimensions, the angle between the glenoid center and the force vector from the humeral head. This angle is needed in the calculation of force transmission across the shoulder and to make more definite the diagnosis of dyskinesia. To solve these problems there is a need to define the position of the glenoid. We define the center line as a line running from the point at which the spine and medial border meet (O) through the center of the glenoid (G) to a point midway between the