The shoulder or scapulo-humeral joint (Articulatio scapulo-humeralis) is
formed by the junction of the distal end of the scapula with the
proximal end of the humerus. The =articular surfaces= are: (1) On the
scapula, the glenoid cavity; (2) on the humerus, the head. Both surfaces
are approximately spherical and similar in curvature, but the humeral
surface is much more extensive than that of the scapula.
The =joint capsule= is ample enough to allow the bones to be drawn apart
about an inch (ca. 2 to 3 cm.); but this requires a very considerable
amount of force unless air is admitted into the joint cavity. The
fibrous layer (or capsular ligament) is not attached to the margin of
the joint surfaces, but at a distance of one to two centimeters from it.
It is strengthened in front by two diverging elastic bands, which arise
on the scapular tuberosity and end on the lips of the bicipital groove.
A pad of fat is interposed between the capsule and the tendon of the
biceps.
Ligaments are absent from this joint, but the muscles and tendons around
it afford remarkable security, so that dislocation seldom or never
occurs. The large extent of the head of the humerus is also of
importance in this regard.
The principal muscles which are attached around the joint and act as
ligaments are: externally, the supraspinatus, infraspinatus, and teres
minor; internally, the subscapularis; in front, the biceps and
supraspinatus; behind, the triceps.
=Movements.=—While it is a typical enarthrosis in structure, and capable
of the various movements of the ball-and-socket joint, the chief normal
movements are flexion and extension. In the position of rest the angle
formed between the scapula and humerus posteriorly is about 110° to
120°; in flexion it is reduced to about 80°, and in extension it is
increased to about 145°. Adduction and abduction are very restricted,
the former being limited chiefly by the infraspinatus, the latter by the
subscapularis and the low insertion of the superficial pectoral muscles.
Rotation is somewhat freer, but does not exceed 33°, when all the
muscles are removed (Franck).