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Scapulothoracic motion interface
The scapulothoracic motion interface is the site of movement between the scapula (shoulder blade) and the thorax (chest wall). The scapula moves around the chest in many daily and athletic activities. This motion does not take place between layers of muscles which separate the scapula from the ribs.
Scapula gliding across the thorax
Normally the scapula moves across the thorax gliding on a scapulothoracic motion interface. The deep surface of this interface consists of the ribs and their covering musculature. The superficial surface of the interface consists of the scapular border along with the serratus muscles. There are no generally accepted conventions for describing the position of the scapula on the thorax. Terms such as protraction retraction and winging are useful in describing types of movement but do not lend themselves to the definition of positions. A method of describing scapulothoracic positions and motions is needed to help us understand how the scapula functions in motions such as a golf swing or pushing a heavy load.
Some insight into scapulothoracic motion was gained by studying eleven patients with glenohumeral arthrodeses. Each patient had electromagnetic sensors attached to his thorax and humerus. Because all had solid glenohumeral fusions their humerothoracic and scapulothoracic motions were equal. Starting from a position where the scapula was flat against the chest wall these subjects averaged 47 +/- 13 degrees of maximal elevation in the plus 90 degree thoracic plane and 22 +/- 8 degrees of maximal elevation in the minus 90 degree thoracic plane. The total arc of scapular rotation about its medial reference line was 55 +/- 20 degrees. It is apparent that the scapulothoracic joint is able to make major contributions to shoulder motion.
Factors limiting scapulothoracic motion
Movement of the scapula on the chest wall is limited by the motion allowed at the sternoclavicular and acromioclavicular joints by the coracoclavicular ligaments by the compliance of the scapula's musculotendinous attachments and by the geometry of the scapulothoracic motion interface.
Scapulothoracic range of motion can be abnormally limited by such factors as sternoclavicular arthritis acromioclavicular arthritis contracture rib or scapular fracture post-traumatic scarring tumor dislocation or other factors disrupting the scapulothoracic motion interface.
Movie
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| Motion of the shoulder |
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