Normal alignment

The scapula lies against the posterior thoracic cage overlying the second to the seventh ribs inclusive, its medial border parallel to, and about three fingers breadth from, the middle of the back. It lies flat against the thorax, rotated 30 degrees anterior to the frontal plane, in contact with the curvature of the ribcage.

The scapulothoracic articulation

The scapula moves by gliding over the underlying ribs and muscles of the posterolateral surface of the thoracic cage. The specific movements involved are elevation, depression, medial and lateral rotation. Scapular elevation and depression have an approximate linear range of 10–12 cm and these movements are usually accompanied by some degree of rotation. With scapular elevation the glenoid fossa tends to face more upwards and, with depression, downwards.

The scapula rotates around an axis located just below the medial end of its spine over a range of about 60 degrees. This involves a lateral displacement of its inferior angle of 10–12 cm, and a medial displacement of the superolateral angle of 5–6 cm.

Normal alignment

When viewed from the side, less than one-third of the humeral head should normally protrude in front of the acromion process. The antecubital crease at the elbow joint faces anteriorly during neutral rotation whilst the olecranon process faces posteriorly and the palm of the hand faces the side of the body. The proximal and distal ends of the humerus should be in the same vertical plane when viewed from the side, the front or the back (Sahrmann 2002, p. 198).

Normal alignment

The shoulders should be positioned slightly below the horizontal axis through T1 in the front and back view. In the side view, the plumb line should bisect the acromion (Sahrmann 2002, p. 194).

The glenoid fossa is a shallow, saucer-shaped articular depression at the lateral border of the scapula, deepened only slightly by a fibrous rim. The ball and socket arrangement of the glenohumeral joint allows movement to occur around innumerable axes intersecting at the head of the humerus. This makes it the most mobile joint in the body and its movements are described as flexion, extension, abduction, adduction, and medial and lateral rotation, but, like the whole pectoral girdle, this inherently unstable arrangement requires the support of powerful musculature.

Scapulohumeral rhythm

When upper limb movement also begins to involve scapular rotation, the glenoid fossa travels in an arc of which the clavicle forms the radius and the relative positions of clavicle and scapula change at both the acromio- and sternoclavicular joints.

During the first 60 degrees of shoulder flexion and 30 degrees of shoulder abduction the movement of the scapula is highly variable. Inman et al (1994) termed this the setting phase. After the setting phase the humerus and scapula move in a constant ratio. A ratio of 2 degrees of glenohumeral motion for 1 degree of scapulothoracic motion results in 120 degrees of glenohumeral joint motion and 60 degrees of scapula motion at the completion of shoulder flexion. More recent studies have reported variability in the exact timing of that motion (Sahrmann 2002, p. 202).