Shoulder Anatomy

Better known as the collarbone, the clavicle connects the shoulder to the rest of the skeleton and acts as a strut. At its lateral (outer) end it forms the acromioclavicular joint. At its medial (inner) end, it forms the sternoclavicular joint.

Because it is the only bony link between the arm and the trunk, the clavicle holds the shoulder blade out and back, away from the chest wall. This strut effect is what gives the arm its working arc; when the clavicle is fractured or the joint at either end is disrupted, the shoulder tends to drop forward and down and overhead reach becomes difficult.

The clavicle sits directly under the skin along its whole length, so a change in its shape after an injury is usually easy to see and feel.

The glenoid is the part of the scapula (shoulder blade) that forms the shoulder joint with the humerus. It is a shallow cup that is deepened by a rim of fibrocartilage called the labrum.

Compared with the hip, the glenoid socket is very shallow. It covers only about a quarter to a third of the humeral head. That shallowness is the trade-off that buys the shoulder its range of movement, and it means stability depends heavily on the labrum, the capsule and ligaments, and the surrounding muscles rather than on bone.

The labral rim also acts as the anchor point for the capsule and ligaments. When the shoulder dislocates, the labrum is commonly torn away from the front of the glenoid, and a rim of bone can be lost with it. The amount of bone remaining is one of the things assessed on imaging when planning stabilisation surgery.

The humerus is the upper arm bone that runs between the elbow and the shoulder joint.

Its upper end forms the ball of the shoulder, the humeral head, which is covered in smooth articular cartilage where it meets the glenoid. Just below the head are two bony bumps, the greater and lesser tuberosities, which are where the rotator cuff tendons attach. The groove between them carries the long head of the biceps tendon.

Those attachment points matter clinically: a fracture through the tuberosities changes where the cuff pulls from, and the shape and version of the humeral head are measured when planning a shoulder replacement.

The rotator cuff is a highly specialised arrangement of four main tendons in the shoulder which provides an extraordinary range of motion overhead with precise control. The rotator cuff is formed by the tendons of the subscapularis, supraspinatus, infraspinatus and teres minor muscles.

Each has a different job. Subscapularis runs across the front of the joint and rotates the arm inwards. Supraspinatus runs over the top and helps lift the arm away from the body. Infraspinatus and teres minor run across the back and rotate the arm outwards.

Working together, they do something more subtle than moving the arm: they hold the humeral head centred on the glenoid while the much larger deltoid muscle lifts it. Without that centring effect the head rides upwards and the mechanics of the joint change. This is why a cuff tear can cause weakness and loss of control rather than only pain, and why rehabilitation after cuff surgery focuses on restoring that balance.

A bursa is a potential space filled with a film of fluid, associated with joints to allow smooth gliding movement between structures. The subacromial bursa lies between the acromion (the point of the shoulder) and the rotator cuff, allowing the cuff to slide smoothly under the acromion when you raise your arm over your head.

In a healthy shoulder the bursa is barely more than two moist surfaces sliding on each other. If the cuff is irritated or the space is tight, the bursal lining thickens and becomes inflamed (subacromial bursitis) and the gliding surface becomes a source of pain, typically felt on the outer upper arm and worst when reaching overhead or lying on that side at night.

Part of the scapula (shoulder blade), the acromion is a projection of bone that curves over the top of the glenohumeral joint. It provides a scaffold for attachment of the deltoid muscle and forms the acromioclavicular joint with the clavicle.

The undersurface of the acromion forms the roof of the space the rotator cuff passes through. The shape of that undersurface varies between people (flat, curved or hooked) and bone spurs can form along its front edge over time. Where the space is tight, the cuff and bursa can be compressed against it during overhead movement.