What is the Normal Anatomy of the Shoulder Joint?
The shoulder is a 'ball-and-socket' joint. The 'ball' at the top of the upper arm bone (the humerus) fits neatly into the 'socket', called the glenoid, which is part of the shoulder blade (scapula). The shoulder is the most flexible joint in the body, making it the most susceptible to instability and injury.
Movements of the Shoulder Joint
The wide range of movement of the shoulder joint includes:
- Forward flexion
- External rotation
- Internal rotation
- 360-degree circumduction
Bones of the Shoulder Joint
The three bones that come together to form the shoulder joint include:
- Collarbone (clavicle)
- Shoulder blade (clavicle)
- Upper arm bone (humerus)
Shoulder Joint Cartilage
The articulating ends of the shoulder bones are covered by smooth tissue called articular cartilage, which allows the bones to slide over each other without friction, enabling smooth movement. The 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.
Shoulder Joint Ligaments
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: These are a group of 3 ligaments that form a capsule around the shoulder joint, connecting the head of the arm bone to the glenoid cavity of the shoulder blade. The capsule forms a water-tight sac around the joint. Glenohumeral ligaments play a very important role in providing stability to the otherwise unstable shoulder joint by preventing dislocation.
Shoulder Joint Muscles
The rotator cuff is the main group of muscles in the shoulder joint and is comprised of 4 muscles. The rotator cuff forms a sleeve around the humeral head and glenoid cavity, providing additional stability to the shoulder joint while enabling a wide range of mobility. The deltoid muscle forms the outer layer of the rotator cuff and is the largest and strongest muscle of the shoulder joint.
Shoulder Joint Tendons
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 deeper muscles of the rotator cuff. These tendons provide more stability and mobility to the shoulder joint.
Nerves of 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 of the arm pass through the shoulder joint from the neck. These nerves form a bundle at the region of the shoulder called the brachial plexus. The main nerves of the brachial plexus are the musculocutaneous, axillary, radial, ulnar and median nerves.
Blood Vessels of the Shoulder Joint
Blood vessels travel alongside 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.