Scapulothoracic Joint

Original Editor - Venus Pagare

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Description

The Scapulothoracic (ST) “joint” is not a true anatomic joint as it has none of the usual joint characteristics (union by fibrous, cartilaginous, or synovial tissues). It is an articulation of the scapula with the thorax which depends on the integrity of the anatomic AC and SC joints. The SC and AC joints are interdependent with the ST joint because the scapula is attached by its acromion process to the lateral end of the clavicle & through the AC joint; the clavicle, in turn, is attached to the axial skeleton at the manubrium of the sternum through the SC joint. Any movement of the scapula on the thorax must result in movement at either the AC joint, the SC joint, or both; that is, the functional ST joint is part of a true closed chain with the AC and SC joints and the thorax.

Resting position of the Scapula

Normally, the scapula rests at a position on the posterior thorax approximately 2 inches from the midline, between the second through seventh ribs (scapula extends from the level of T2 spinous process to T7 orT9 spinous process, depending on the size of the scapula) and the medial border is about 6 cms lateral to the spine. The scapula also is internally rotated 30° to 45° from the coronal plane, is tipped anteriorly approximately 10° to 20° from vertical, and is upwardly rotated 10° to 20° from vertical. The magnitude of upward rotation has as its reference a “longitudinal” axis perpendicular to the axis running from the root of the scapular spine to the AC joint.  If the vertebral or medial border of the scapula is used as the reference axis, the magnitude of upward rotation at rest is usually described as 2° to 3° from vertical. lthough these “normal” values for the resting scapula are cited, substantial individual variability exists in scapular rest position, even among healthy subjects.   

Motions Available

• Movements of scapulothoracic joint are a very important element of shoulder kinesiology. The wide range of motion available to the shoulder joint is due to the large movement available at ST joint.  Observation and measurement of individual SC and AC joint motions are more difficult than observing or measuring motions of scapula on the thorax. Consequently, ST position and motions are described and measured far more frequently than are the SC and AC joint motions upon which ST motions are dependent.
  
• The motions of the scapula from the resting or reference position includes three rotations that occur at the AC joint. These are upward/downward rotation, internal/external rotation, and anterior/posterior tipping. Of these three AC joint rotations, only upward/downward rotation is readily observable at the ST, and it is therefore considered to be a “primary” scapular motion. Internal/external rotation and anterior/posterior tipping are normally difficult to observe and are therefore considered to be “secondary” scapular motions. The scapula also has available translatory motions of scapular elevation/depression and protraction/retraction. The linkage of the scapula to the AC and SC joints prevents scapular motions both from occurring in isolation and from occurring as true translatory motions. Instead, scapular motions on the thorax must occur in combinations, such as the simultaneous upward rotation, external rotation, and posterior tipping that occur when the arm is abducted.

1. Elevation & Depression
   Commonly described as translatory motions in which the scapula moves upward (cephalad) or downward (caudally) along the rib cage from its resting position.
   These motions at ST joint occurs as a composite of SC and AC joint rotations.
   The motion of shrugging the shoulders which involves scapular elevation occurs as a result of scapula following the path of elevating clavicle about the SC joint and requires subtle adjustments in anterior/posterior tipping and internal/external rotation at the AC joint to maintain the scapula in contact with the thorax.
   Depression of the scapula at the ST joint occurs as the action reverse to that of elevation.
   

Scapulothoracic Stability

Stability of the scapula on the thorax is provided by the structures that maintain integrity of the linked AC and SC joints. The muscles that attach to both the thorax and scapula maintain contact between these surfaces while producing the movements of the scapula. In addition, stabilization is provided through the ST musculature by pulling or compressing the scapula to the thorax. 
The ultimate functions of scapular motion are to orient the glenoid fossa for optimal contact with the maneuvering arm, to add range to elevation of the arm, and to provide a stable base for the controlled motions between the humeral head and glenoid fossa. The scapula, with its associated muscles and linkages, per-forms these mobility and stability functions so well that it serves as a premier example of dynamic stabilization in the human body.

Closed and Open Packed Position

 Because it is not a true joint, it does not have a capsular pattern nor a close packed position.

Resting Position

The resting position of this joint is the same as  for the acromioclavicular joint i.e arm by side.