The shoulder joint (or glenohumeral joint from Greek glene, eyeball, + -oid, 'form of', + Latin humerus, shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. It involves an articulation between the glenoid fossa of the scapula (shoulder blade) and the head of the humerus (upper arm bone). Due to the very loose joint capsule that gives a limited interface of the humerus and scapula, it is the most mobile joint of the human body. Structure The shoulder joint is a ball-and-socket joint between the scapula and the humerus. The socket of the glenoid fossa of the scapula is itself quite shallow, but it is made deeper by the addition of the glenoid labrum. The glenoid labrum is a ring of cartilaginous fibre attached to the circumference of the cavity. This ring is continuous with the tendon of the biceps brachii above. Spaces Significant joint spaces are: The normal glenohumeral space is 4–5 mm. The normal subacromial space in shoulder radiographs is 9–10 mm; this space is significantly greater in men, with a slight reduction with age. In middle age, a subacromial space less than 6 mm is pathological, and may indicate a rupture of the tendon of the supraspinatus muscle. The axillary space is an anatomic space between the associated muscles of the shoulder. This space transmits the subscapular artery and the axillary nerve. Capsule The shoulder joint has a very loose joint capsule, which can sometimes predispose the shoulder to dislocate. The "U shaped" dependent portion of the axillary part of the capsule located between the anterior and posterior bands of inferior glenohumeral ligament is called "axillary pouch". Synovium extends below the long head of biceps and subscapularis tendon to form subscapular bursa. Therefore, long head of biceps is extrasynovial and intracapsular, attaching to supraglenoid tubercle. Bursae A number of small fluid-filled sacs known as synovial bursae are located around the capsule to aid mobility: Between the joint capsule and the deltoid muscle is the subacromial-subdeltoid bursa. Between the capsule and the acromion is the subacromial bursa. The subcoracoid bursa is between the capsule and the coracoid process of the scapula. The coracobrachial bursa is between the subscapularis muscle and the tendon of the coracobrachialis muscle. Between the capsule and the tendon of the subscapularis muscle is the subscapular bursa, this is also known as the subtendinous bursa of the