Shoulder
Shoulder Anatomy
The shoulder is one of the most complex joint of the body formed by the articulation of three bones – the Humerus, the Clavicle and the Scapula.
The shoulder joint is a complex of four joints. The glenohumeral joint, which is the main ball and socket joint, the AC joint, the sub acromial joint (space) and the scapula thoracic joint (space). Pain and clinical problems can arise from either of these joints.
The bones of the shoulder joint are supported by ligaments, muscles, nerves and blood vessels.
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Bones
The Humerus is the main big bone of the shoulder joint. The humeral head is fairly big compared to the size of the socket. The Humerus has the articular surface, the greater (GT) and the lesser (LT) tuberosities. The GT and LT provide the attachment for the rotator cuff muscles.
The clavicle (collar bone) is an ‘S’ shaped bone lying horizontally across the front of the chest. This is the only long bone in body which lies horizontally. This provides attachment to muscles and along with the scapula forms the A-C joint at one end and the sternum (breast bone) to form the S-C joint (sterno-clavicular). The clavicle is the only bony connection of the arm to the main body. It also provides a protection to the nerves and blood vessels as they pass form the chest / neck into the arm.
The scapula is a flat bone on the chest wall. The scapula has 4 parts – the coracoid, the acromion, the spine and glenoid cavity. The coracoid offers attachment to muscles and acts as a ‘light house’ when undertaking any surgery to the shoulder. The acromion provides attachment to muscles and the Coraco-Acromial ligament which provides stability to the shoulder joint. The glenoid forms the socket of the shoulder joint.
The Joint
The shoulder is a ball and socket type of joint. The socket (glenoid) is quite shallow which makes it one of the most mobile joints of the body. The ball is about 3 – 4 times bigger than the socket which allows the movements, but this also makes it prone to dislocation easily compared to the other joints. The socket is deepened by a thickened fibrous cartilage – called the ‘Labrum’ which provides increased stability.
Ligaments
The shoulder joint has a lining (capsule) all around the ball and socket which encloses the whole of the joint and forms a watertight sac all around. The gleno-humeral ligaments are parts of the capsule which are thickened and provide stability to the joint. They are the Superior, Middle and Inferior.
There are numerous ligament which help stabilise the joint. The Coraco-Acromial ligament –connects the acromion to the coracoid and is important in rotator cuff problems. The Coraco-Clavicular ligaments are strong ligaments which help stabilise the clavicle to the scapula. The Acromio-clavicular ligaments support the AC joint.
Muscles around Shoulder
The rotator cuff is a network of four muscles and tendons that form a covering around the top of the upper arm bone (Humerus). These muscles form a cover around the head of the Humerus. The rotator cuff helps to lift and rotate the arm and stabilize the ball of the shoulder within the joint. The four muscles of the cuff are called ‘SITS’ (supraspinatus, infraspinatus, subscapularis, and teres minor muscles). These are attached to the scapula (shoulder blade) on the back through a single tendon unit. The unit is attached on the side and front of the shoulder on the greater tuberosity of the Humerus. There is a bursa (sac) between the rotator cuff and acromion that allows the muscles to glide freely when moving.
The deltoid is the large outer layer of muscle which is the strongest muscle of the shoulder joint. There are other muscles like the trapezius, serratus, sterno-clavicular mastoid which connect the spine and shoulder blade.
The clavicle (collar bone) is an ‘S’ shaped bone lying horizontally across the front of the chest. This is the only long bone in body which lies horizontally. This provides attachment to muscles and along with the scapula forms the A-C joint at one end and the sternum (breast bone) to form the S-C joint (sterno-clavicular). The clavicle is the only bony connection of the arm to the main body. It also provides a protection to the nerves and blood vessels as they pass form the chest / neck into the arm.
The scapula is a flat bone on the chest wall. The scapula has 4 parts – the coracoid, the acromion, the spine and glenoid cavity. The coracoid offers attachment to muscles and acts as a ‘light house’ when undertaking any surgery to the shoulder. The acromion provides attachment to muscles and the Coraco-Acromial ligament which provides stability to the shoulder joint. The glenoid forms the socket of the shoulder joint.
Tendons of the shoulder
Tendons are strong fibrous tissue which connect the muscle to the to the bone. Around the shoulder two important group of tendons which are the biceps tendon and the rotator cuff tendons.
Biceps, as the name stands has two heads, the long head and the short head. The long head of biceps starts from within the socket and can cause pain in the shoulder. The short head of biceps attaches to the coracoid process.
The rotator cuff tendons can often be damaged due to degeneration or trauma.
Nerves of the Shoulder
Nerves are like electric cables which carry messages from brain to the muscles (motor) and from skin, muscles and joints back to the brain (sensory). The main Nerves of the shoulder are from the brachial plexus. The brachial plexus is protected by the clavicle in the front. The three nerves of the arm are Median, Radial and Ulnar nerves
Blood Vessels of the Shoulder
Blood vessels travel along with the nerves to supply oxygenation to the arm. The main artery is the brachial artery which lies behind the clavicle and changes its name to axillary artery as it goes into the armpit. The main veins of the shoulder are the Cephalic vein, Basilic vein and the axillary vein.