The shoulder, renowned for its remarkable flexibility, facilitates a wide array of movements such as forward flexion, abduction, adduction, external and internal rotation, and 360-degree circumduction. This extensive mobility, however, renders it the most insecure joint in the body. Stability is primarily maintained by the supporting ligaments, muscles, and tendons.
Bones of the Shoulder
The shoulder is a complex ball-and-socket joint comprising three primary bones: the humerus, scapula, and clavicle.
Humerus (Upper Arm Bone)
The top end of the humerus forms the ball of the shoulder joint. This ball-like structure fits into the glenoid cavity of the scapula, allowing for a wide range of motion. The humerus is crucial for all arm movements, and any injury to this bone can significantly impact mobility and function.
The Scapula (Shoulder Blade): Anchor for Shoulder Movement and Stability
The scapula, or shoulder blade, is a flat, triangular bone that forms the posterior (back) part of the shoulder girdle. It acts as a crucial anchor for many muscles and ligaments that support shoulder stability, mobility, and arm function. Several of the rotator cuff muscles — including the supraspinatus, infraspinatus, subscapularis, and teres minor — all originate from specific surfaces of the scapula.
These muscles are commonly involved in shoulder injuries and can explore these in more detail on the rotator cuff tear page.
Key Bony Landmarks of the Scapula:
Acromion:
A bony projection that extends over the shoulder joint and connects with the clavicle to form the acromioclavicular (AC) joint. It provides protection to the underlying rotator cuff tendons, particularly the supraspinatus, and serves as a site of deltoid muscle attachment.
Spine of the Scapula:
A prominent ridge that runs across the back of the scapula, dividing it into the supraspinous fossa (above the spine) and infraspinous fossa (below the spine). These fossae serve as the origin points for the supraspinatus and infraspinatus muscles, respectively.
Subscapular Fossa
The subscapular fossa is a broad, slightly concave surface on the anterior (front) aspect of the scapula. It serves as the origin site for the subscapularis muscle, which is the largest and strongest of the rotator cuff muscles. This fossa provides a smooth area for the muscle to lie against the rib cage, allowing efficient gliding during shoulder movement. Because of its location and relationship to the thoracic wall, the subscapular fossa is not externally visible but plays a vital role in shoulder function and stability.
Coracoid Process:
A hook-like structure on the front of the scapula. It serves as an attachment point for several ligaments and muscles, including the short head of the biceps, coracobrachialis, and pectoralis minor. It also lies near the subscapularis, which originates from the anterior surface of the scapula.
Glenoid Cavity (or glenoid fossa):
A shallow, cup-like socket that articulates with the head of the humerus to form the glenohumeral joint. This joint is stabilised by the surrounding rotator cuff muscles, all of which help keep the humeral head centred during movement.
Clavicle (Collar Bone)
The S-shaped clavicle connects the scapula to the sternum, forming the acromioclavicular and sternoclavicular joints. It also protects vital nerves and blood vessels passing from the spine to the arms. The clavicle acts as a strut, maintaining the shoulder blade in a position where it can function effectively.
The articulating bones are cushioned by articular cartilage, which minimises friction and acts as a shock absorber. This smooth, white tissue covers the ends of the bones, allowing them to glide over each other without causing damage.
Additionally, the glenoid labrum, a ring of fibrous cartilage, enhances the glenoid cavity’s depth and surface area, securing the humeral head more effectively. The labrum acts like a suction cup to hold the ball of the joint in place, adding an extra layer of stability.
Soft Tissues in the Shoulder
Shoulder Ligaments
A ligament is a strong, flexible band of connective tissue that connects one bone to another. It helps stabilise joints, guide normal movement, and prevent excessive motion that could cause injury. In the shoulder, ligaments keep the bones aligned while still allowing a wide range of movement.
Coracoclavicular Ligaments
This ligament forms an arch over the shoulder joint, protecting the humeral head and rotator cuff tendons from direct trauma.
Acromioclavicular Ligaments
This ligament strengthens the acromioclavicular joint, helping to maintain the alignment of the clavicle and the scapula.
Coracoacromial Ligaments
These ligaments are essential for stabilising the clavicle and preventing dislocation of the acromioclavicular joint. They consist of two parts: the conoid and trapezoid ligaments.
Glenohumeral Ligaments
A set of three ligaments (superior, middle, and inferior) that form a capsule around the shoulder joint, preventing dislocation and ensuring stability. These ligaments are critical in maintaining the shoulder’s range of motion while preventing excessive movement that could lead to injury.
Summary
Shoulder Muscles and Tendons
The rotator cuff, consisting of four muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), forms a sleeve around the shoulder joint, providing stability and facilitating movement. These muscles work together to keep the humeral head cantered in the glenoid cavity during arm movements.
Deltoid Muscle
The deltoid, the largest and strongest muscle in the shoulder, covers the rotator cuff and is responsible for lifting the arm and giving the shoulder its rounded shape. It plays a vital role in abduction, flexion, and extension of the shoulder.
Biceps Tendons
Two tendons (long head and short head) connect the bicep muscle to the shoulder, allowing for flexion and supination of the forearm.
Rotator Cuff
These four tendons (supraspinatus, infraspinatus, teres minor, and subscapularis) attach the humerus to the rotator cuff muscles, ensuring stability and mobility. They are crucial for maintaining shoulder stability during dynamic movements.
Major Nerves of the Shoulder and Arm
Nerves, which transmit signals between the brain and muscles, pass through the shoulder from the neck, forming the brachial plexus. This network of nerves controls muscle movements and sensations in the shoulder, arm, and hand.
The Brachial Plexus
The brachial plexus he brachial plexus is a network of nerves that originates from the spinal cord in the neck (C5 to T1 nerve roots). Globally it supplies the shoulder, arm, forearm, and hand with motor and sensory innervation. It branches into several major nerves, including the musculocutaneous, axillary, radial, ulnar, and median nerves, which innervate different parts of the arm and hand.
Major Blood Vessels of the Shoulder and Arm
Arteries
Oxygenated blood is supplied to the shoulder region by the subclavian artery, which becomes the axillary artery in the armpit and the brachial artery down the arm. These arteries ensure that muscles receive the oxygen and nutrients needed for optimal function.
Veins
veins include the axillary vein, which drains into the subclavian vein; the cephalic vein, which runs along the upper arm; and the basilic vein, located near the triceps muscle. These veins are responsible for returning de-oxygenated blood to the heart for purification.
Shoulder Anatomy Summary
Understanding the shoulders' anatomy can provide insights into potential injuries and the importance of seeking professional care. My physiotherapy services in Northampton are designed to address various shoulder issues, offering treatments like sports massage and specialised shoulder physiotherapy.
At Chris Heywood Physio, I recognise the frustrations with long NHS waiting lists so I am ready to provide immediate and effective care.
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A Quick Note From Chris
It won’t surprise many of you to know that I don’t get any financial reward for writing and sharing these pages. I do it because I genuinely want to help people better understand their conditions and feel more in control of their recovery.
All I ask is that you don’t plagiarise or claim this work as your own — and if you’ve found it helpful, please consider sharing it with friends, family, or anyone else who might benefit from it. Follow my blog for regular updates on new topics, pages and future projects.
Thanks for reading — and for taking your health seriously.
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