The shoulder is the most flexible joint in the body, enabling a wide range of movements including, forward flexion, abduction, adduction, external rotation, internal rotation, and 360-degree circumduction.
Although the shoulder has the least constraint of any joint in the body, the support of ligaments, muscles and tendons function to provide the required stability and normal function.
The shoulder is a ball and socket joint made up of three bones, namely the humerus, scapula, and clavicle.
The end of the humerus, or upper arm bone, forms the ball of the shoulder joint. An irregular shallow cavity in the scapula called the glenoid cavity forms the socket for the head of the humerus to fit in. The two bones together form the glenohumeral joint, which is the main joint of the shoulder.
The scapula is a flat triangular shaped bone that forms the shoulder blade. It serves as the site of attachment for most of the muscles that provide movement and stability to the joint. The scapula has four bony processes - acromion, spine, coracoid and glenoid cavity. The acromion and coracoid process serve as attachment sites for ligaments and tendons.
The clavicle bone, or collarbone, is an S-shaped bone that connects the scapula to the sternum or breastbone. It forms two joints: the acromioclavicular joint, where it articulates with the acromion process of the scapula, and the sternoclavicular joint where it articulates with the sternum, or breast bone. The clavicle also forms a protective covering for important nerves and blood vessels that pass under it from the spine to the arms.
The ends of all articulating bones are covered by smooth tissue called articular cartilage, which allows the bones to slide over each other without friction for smooth movement. Articular cartilage reduces pressure and acts as a shock absorber during movement of the shoulder bones.
Extra stability to the glenohumeral joint is provided by the glenoid labrum, a ring of fibrous cartilage that surrounds the glenoid cavity. The glenoid labrum increases the depth and surface area of the glenoid cavity to provide a more secure fit for the half-spherical head of the humerus.
Ligaments are the thick strands of fibers that connect one bone to another. The ligaments of the shoulder joint include
- Coraco-clavicular ligaments: These ligaments connect the collarbone to the shoulder blade at the coracoid process
- Acromio-clavicular ligament: This connects the collarbone to the shoulder blade at the acromion process
- Coraco-acromial ligament: It connects the acromion process to the coracoid process
- Glenohumeral ligaments: Three glenohumeral ligaments are formed by thickened portions of the shoulder capsule, which is the deepest layer of soft tissue connecting the ball and socket in the shoulder joint. The glenohumeral ligaments play a very important role in providing stability to the otherwise unstable shoulder joint by preventing shoulder dislocation.
The rotator cuff is a group of four muscles that originate on the shoulder blade (scapula) and insert as tendons onto the top of the arm bone (humerus). They form the deepest muscle-tendon layer in the shoulder. The rotator cuff is essential for rotating and raising the arm at the shoulder.
The deltoid muscle is the largest and strongest muscle of the shoulder, functioning to raise the arm forward, backward, and to the side.
Tendons are strong tissues that join muscle to bone allowing the muscle to control the movement of the bone or joint. Two important groups of tendons in the shoulder joint are the biceps tendons and rotator cuff tendons.
Biceps tendons are the two tendons that join the biceps muscle of the upper arm to the shoulder. They are referred to as the long head and short head of the biceps.
Rotator cuff tendons are a group of four tendons that join the head of the humerus to the muscles of the rotator cuff. These tendons provide stability and mobility to the shoulder joint.
Nerves carry messages from the brain to muscles to direct movement (motor nerves) and send information about different sensations such as touch, temperature and pain from the muscles back to the brain (sensory nerves).
The nerves passing from the spinal cord in the neck down to the arm form a bundle at the region of the shoulder called the brachial plexus. The main nerves arising from the brachial plexus are the musculocutaneous, axillary, radial, ulnar and median nerves.
Blood vessels travel along with the nerves to supply blood to the arms. Oxygenated blood is supplied to the shoulder region by the subclavian artery that runs below the collarbone. As it enters the region of the armpit, it is called the axillary artery and further down the arm, it is called the brachial artery. The main veins carrying de-oxygenated blood back to the heart for purification include:
- Axillary vein: this vein drains into the subclavian vein
- Cephalic vein: this vein is found in the upper arm and branches at the elbow into the forearm region. It drains into the axillary vein.
- Basilic vein: this vein runs opposite the cephalic vein, near the triceps muscle. It drains into the axillary vein.