Understanding glenohumeral motion in normal and pathologic states requires the precise measurement of shoulder joint kinematics. Multiple studies have linked abnormal shoulder joint kinematics with various shoulder disorders including secondary impingement, rotator cuff tears, glenohumeral osteoarthritis, labral injury, and glenohumeral instability.
Although shoulder pathology is associated with abnormal kinematics, there is little detailed information about baseline values that can provide reference points for the restoration of normal shoulder kinematics. The most commonly studied parameter for glenohumeral rotation is scapulohumeral rhythm, defined as the ratio between glenohumeral elevation and upward scapulothoracic rotation, which was first reported to be 2:1 by Inman. Two types of abnormalities in scapulohumeral rhythm have been recognized in shoulders with pathologic conditions: (1) increased rhythm exacerbating the likelihood of secondary impingement by biomechanically decreasing the volume of the subacromial space, and (2) decreased rhythm serving as a compensatory method that potentially avoids impingement symptoms and improves rotator cuff function.
There is considerable variation in the magnitude of physiologic glenohumeral translations reported in the literature. Normal in vivo glenohumeral translations ranging from 0.3 to 2.6 mm in the superior-inferior direction have been demonstrated by means of dynamic measurements involving fluoroscopy and static measurements involving radiography. Normal in vitro superior shoulder translations of 2.0 to 5.7 mm have been reported. Increased mean in vivo glenohumeral translations of approximately 1.5 mm in patients with symptomatic rotator cuff tears, impingement syndrome, and biceps tenodesis have been reported, with individual increases of up to 6 to 8.9 mm.
In recent years, biplane fluoroscopy has emerged as a highly accurate and precise method to measure in vivo three-dimensional kinematics; this method allows measurement of glenohumeral joint motion to within fractions of a millimeter. Clinically, measurement of the full range of arm elevation is important because arm elevation is a common motion during activities of daily living and athletic activities. It is uncommon for shoulder kinematics in fluoroscopic studies to be reported in all three of the standard planes of arm elevation: abduction (coronal plane elevation), scaption (scapular plane elevation), and forward flexion (sagittal plane elevation). As a result, the relative effect of the plane of elevation on glenohumeral translation and scapulohumeral rhythm remains unknown. Our purpose was to measure three-dimensional glenohumeral translations and rotations during abduction, scaption, and forward flexion in healthy subjects. Our hypothesis was that glenohumeral translations and scapulohumeral rhythm would change with the plane of elevation.