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motion at both the sterno-clavicular joint at the inner and the acromio-clavicular joint at the outer end of the clavicle. Any disorder of the scapula, the sterno-clavicular joint, the acromio-clavicular joint, or any of the surrounding soft tissues will affect function of the shoulder joint.
82.2 The gleno-humeral joint is the articulation between the ball shaped head of the humerus and the saucer or shield-shaped, glenoid surface of the scapula. The articulating bone surfaces are covered with a tough and resilient hyaline cartilage (gristle) surface.
82.3 The joint surface of the humeral head is hemispherical and is covered with a tough hyaline cartilage (gristle) surface which is believe to impart some shock absorbing qualities to the bone. The surrounding parts of the upper humerus provide insertion for a number of broad tendons that pass from the scapula and the chest wall to the upper arm.
82.4 The much smaller glenoid is also covered in hyaline cartilage and its shallow saucer shaped surface is bounded and is slightly deepened by a fibro-cartilage (tougher gristle) rim.
82.5 Contact The shape and mismatch in size of these opposing joint surfaces is such that only 25% of the humeral joint surface is in contact with the glenoid at any one time. This allows great freedom of movement but confers little stability on the joint which depends instead on negative pressure within its lining and on the strong surrounding ligaments and muscles to keep the bone surfaces opposed.
Figure 30: The Shoulder joint: a view of the glenoid cavity with humerus removed
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