The Shoulder Joint

Key points

• • It is a multiaxial ball-and-socket joint; the shallow glenoid cavity, deepened only slightly by the fibrocartilaginous glenoid labrum, trades stability for a very wide range of movement.
• • Its main active stabilisers are the four rotator cuff muscles (supraspinatus, infraspinatus, teres minor and subscapularis, remembered as SITS), which pull the humeral head into the socket.
• • The capsule is weakest below, where no cuff muscle reinforces it, so the head most often dislocates in the anteroinferior direction.
• • The axillary nerve winds around the surgical neck of the humerus just below the joint, so it can be injured by a dislocation or a fracture there, numbing the skin over the deltoid.
• • The tendon of the long head of biceps runs through the joint and along the intertubercular groove, contributing to the dynamic stability of the joint.
• • The supraspinatus tendon and the subacromial bursa lie under the acromion, where they can be irritated in subacromial impingement to give a painful arc of movement.