The shoulder is the most flexible joint in the body making it the most susceptible to instability and injury. It is a 'ball-and-socket' joint. A ‘ball' at the top of the upper arm bone, humerus, fits neatly into a 'socket’, called the glenoid, which is part of the shoulder blade, scapula.
The shoulder joint is made up of several bones and soft tissues. It has three bones, the collarbone (clavicle), scapula, and humerus.
The humerus provides attachment to the muscles of the upper arm. Scapula is the bone that connects the upper arm bone with the collarbone. It is a flat bone which is roughly triangular in shape and provides attachment to the muscles of back and neck.
The clavicle is an S-shaped short bone that connects the shoulder girdle to the body (trunk). It supports the shoulder in a functional position with the axial skeleton so that the arm has maximum range of movement. It also protects major underlying nerves and blood vessels as they pass from the neck to the axilla.
The coracoid process is the extension of the scapula around the shoulder joint at the front portion of the scapula. The acromial process is the extension of scapula around the shoulder joint at the back that forms a roof, acromion.
Glenoid is the smooth shallow depression at the end of scapula that forms the socket of the shoulder joint.
The soft tissues of the shoulder joint include:
- Rotator cuff – A group of 4 tendons make up the rotator cuff which holds the head of the humerus in the socket.
- Biceps tendon – The biceps tendon is a long cord-like structure which attaches the biceps muscle to the shoulder and helps stabilize the joint.
- Coraco-clavicular ligament – This ligament connects the clavicle with the coracoid process of the scapula.
- Acromio-clavicular ligament – It connects the clavicle with the acromion process.
- Glenoid labrum – The glenoid labrum is a ring of fibro cartilage surrounding the cavity of the scapula for stabilization of the shoulder joint.
- Articular cartilage or the capsule – It is a capsule that surrounds the shoulder joint and helps to keep the ball and socket normally aligned.