Shoulder Anatomy

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. While this mobility is essential for daily activities, it also makes the shoulder vulnerable to injury. Understanding shoulder anatomy helps patients comprehend their conditions and treatment options.

Bones of the Shoulder

The shoulder joint is a ball-and-socket joint formed by three bones:

• Humerus - The upper arm bone, with its rounded head forming the "ball" that fits into the shoulder socket
• Scapula (Shoulder Blade) - A flat, triangular bone with the glenoid cavity (socket) that receives the humeral head. The scapula features important processes including the acromion, coracoid process, and spine that serve as attachment points for muscles and ligaments
• Clavicle (Collarbone) - An S-shaped bone connecting the shoulder blade to the breastbone (sternum), forming protective coverage for vital nerves and blood vessels

These bones form two major joints: the glenohumeral joint (main shoulder joint) and the acromioclavicular (AC) joint where the clavicle meets the acromion.

Soft Tissues and Cartilage

The articulating surfaces are covered with smooth articular cartilage that allows frictionless movement and shock absorption. The glenoid labrum, a ring of fibrous cartilage, surrounds the glenoid cavity to deepen the socket and provide a more secure fit for the humeral head, enhancing stability.

Ligaments of the Shoulder

Several strong ligaments provide stability to the inherently mobile shoulder joint:

• Glenohumeral Ligaments - Three ligaments (superior, middle, and inferior) that form a capsule around the shoulder joint, preventing dislocation
• Coracoclavicular Ligaments - Connect the clavicle to the coracoid process of the scapula
• Acromioclavicular Ligament - Connects the clavicle to the acromion
• Coracoacromial Ligament - Connects the acromion to the coracoid process, forming a protective arch over the rotator cuff

Rotator Cuff Muscles

The rotator cuff is the primary muscle group stabilizing the shoulder, consisting of four muscles:

• Supraspinatus - Initiates arm abduction (lifting away from body) and stabilizes the humeral head
• Infraspinatus - Externally rotates the arm
• Teres Minor - Assists with external rotation
• Subscapularis - Internally rotates the arm and prevents anterior dislocation

The Deltoid muscle forms the outer layer covering the rotator cuff and is the largest, strongest shoulder muscle responsible for major arm movements.

Tendons of the Shoulder

Tendons connect muscles to bones, enabling movement:

• Rotator Cuff Tendons - Four tendons corresponding to the rotator cuff muscles, providing stability and controlled motion
• Biceps Tendons - The long head and short head of the biceps attach at the shoulder, with the long head passing through the joint

Nerves of the Shoulder

The brachial plexus is a network of nerves passing through the shoulder region from the neck to the arm. Key nerves include:

• Axillary Nerve - Controls the deltoid muscle and provides sensation to the lateral shoulder
• Musculocutaneous Nerve - Innervates the biceps muscle
• Radial, Ulnar, and Median Nerves - Continue down the arm to control forearm and hand function

Blood Vessels

The subclavian artery supplies oxygenated blood to the shoulder, becoming the axillary artery as it enters the armpit region and then the brachial artery in the upper arm. Veins including the axillary, cephalic, and basilic veins return deoxygenated blood to the heart.