Shoulder blade

Structure of the shoulder blade (Scapula)

The shoulder blade, Scapula, is a flat three-sided bone whose margins, Margo medialis (1), Margo lateralis (2), and Margo superior (3), are separated by angles, Angulus superior (4), Angulus inferior (5) and the sloped Angulus lateralis (6).
The anterior surface, the Facies costalis, is flat and slightly hollowed (Fossa subscapularis). It sometimes shows well-formed line musculares. The facies posterior is divided by the shoulder bones, spina scapulae (7), into a smaller fossa supraspinata (8) and a larger fossa infraspinata (9). The spina scapulae begins with a triangular field (trigonum spinae) on the medial side, increases laterally in height and ends with a flattened extension, the shoulder height, acromion (10).

Close to the lateral end is an oval articular surface for connection to the clavicle, the Facies articularis clavicularis.

The Angulus acromii is an easily palpable bone point that marks the point where the lateral edge of the Acromion merges into the Spina scapulae. The lateral angulus is the location of the acetabulum, the cavitas glenoidalis (11). At its upper margin a small hump is visible, the Tuberculum supraglenoidale (12). Below the Cavitas is the Tuberculum infraglenoidale (13). After cavitas glenoidalis the neck of the scapula lies, collum scapulae.
Above the Cavitas glenoidalis rises the raven beak process, Processus coracoideus (14). It bends at right angles to lateroventral and ends flattened. Together with the acromion it protects the underlying joint. Medial from the base of processus coracoideus is found in margo superior an incision, incisura scapulae (15).
The bony socket Cavitas glenoidalis (11) of the Articulatio humeri, a ball joint, is considerably smaller than the head of the humerus. The socket is enlarged by a fibrous cartilaginous joint lip, Labrum glenoidale. The surface of the Cavitas glenoidalis is approx. 6qcm. The weight of the upper extremity is about 4 kg. Since there are no stronger ligaments, the muscles surrounding the joint must secure it. This is why we speak of a muscle-secured joint. The so-called "rotator cuff" is part of this muscular securing and especially strengthens the joint capsule.

The Caput humeri is approximately spherical. The synovial joint capsule is attached to the labrum glenoidale of the scapula.
Movements in three degrees of freedom are possible. It is called abduction and adduction, whereby the resting position of the caput humeri in the scapula plane is assumed. There is the anteversion, the lifting of the arm forward and its counter-movement, the retroversion. By a rotatory component, a compound movement, the circumduction or the circling, results with the participation of the aforementioned movements, whereby the arm practically describes a cone mantle.

Abduction movements always result in a co-motion of the scapula; excessive co-motion of the scapula occurs with an abduction of more than 90 degrees (elevation).

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