The glenohumeral joint, or shoulder joint, is a multiaxial synovial ball-and-socket joint that involves articulation between the glenoid fossa of the scapula and the head of the humerus. Due to the very limited interface of the humerus and scapula, it is the most mobile joint of the human body.
The glenoid fossa is shallow and contains the glenoid labrum which deepens it and aids in stability. With 120 degrees of unassisted flexion, the glenohumeral joint is the most mobile joint in the body.
Scapulohumeral rhythm helps to achieve further range of movement. The scapulohumeral rhythm is the movement of the scapula across the thoracic cage in relation to the humerus. This movement can be compromised by anything that changes the position of the scapula. This could be an imbalance in the muscles that hold the scapula in place which are the upper and lower trapezium. This imbalance could cause a forward head carriage which in turn can affect the range of movements of the shoulder.
The rotator cuff muscles of the shoulder produce a high tensile force, and help to pull the head of the humerus into the glenoid fossa.
The glenohumeral joint has a loose capsule that is lax inferiorly and therefore is at risk of dislocation inferiorly. The long head of the biceps brachii muscle travels inside the capsule to attach to the supraglenoid tubercleof the scapula.
Because the tendon is inside the capsule, it requires a synovial tendon sheath to minimize friction.
A number of bursae in the capsule aid mobility. Namely, they are the subdeltoid bursa (between the joint capsule and deltoid muscle), subcoracoid bursa (between joint capsule and coracoid process of scapula, coracobrachial bursa (between subscapularis muscle and tendon of coracobrachialis muscle), subacromial bursa (between joint capsule and acromion of scapula), and the subscapular bursa (between joint capsule and tendon of subscapularis muscle, also known as subtendinous bursa of subscapularis muscle). The bursa are formed by the synovial membrane of the joint capsule. An inferior pouching of the joint capsule between teres minor and subscapularis is known as the axillary recess.
The shoulder joint is a muscle dependent joint as it lacks strong ligaments.
- Superior, middle, and inferior glenohumeral ligaments
- Coracohumeral ligament
- Transverse humeral ligament
- Suprascapular nerve
- Axillary nerve
- Lateral pectoral nerve
- Branches of the anterior and posterior circumflex humeral and suprascapular arteries
The capsule can become inflamed and stiff, with abnormal bands of tissue (adhesions) growing between the joint surfaces, causing pain and restricting movement of the shoulder, a condition known as frozen shoulder or adhesive capsulitis.