Hill-Sachs lesions (impaction fracture of posterolateral humeral head against anteroinferior glenoid) and Bankart lesions (detachment of antero-inferior labrum with or without an avulsion fracture) can also occur following anterior dislocation. Tearing of the joint capsule is associated with an increased risk of future dislocations. The humeral head is forced anteriorly and inferiorly – into the weakest part of the joint capsule. Superior displacement of the humeral head is generally prevented by the coraco-acromial arch.Īn anterior dislocation is usually caused by excessive extension and lateral rotation of the humerus. Anterior dislocations are the most prevalent (95%), although posterior (4%) and inferior (1%) dislocations can sometimes occur. Biceps tendon – it acts as a minor humeral head depressor, thereby contributing to stability.Ĭlinical Relevance: Common Injuries Dislocation of the Shoulder JointĬlinically, dislocations at the shoulder are described by where the humeral head lies in relation to the glenoid fossa.Ligaments – act to reinforce the joint capsule and form the coracoacromial arch.It deepens the cavity and creates a seal with the head of humerus, reducing the risk of dislocation. Glenoid labrum – a fibrocartilaginous ridge surrounding the glenoid cavity.The resting tone of these muscles act to compress the humeral head into the glenoid cavity. Rotator cuff muscles – surround the shoulder joint, attaching to the tuberosities of the humerus, whilst also fusing with the joint capsule.A commonly used analogy is the golf ball and tee. Bony surfaces – shallow glenoid cavity and large humeral head – there is a 1:4 disproportion in surfaces. Here, we shall consider the factors the permit movement, and those that contribute towards joint structure. The shoulder joint is one of the most mobile in the body, at the expense of stability. Circumduction (moving the upper limb in a circle) – produced by a combination of the movements described above.External rotation (rotation away from the midline, so that the thumb is pointing laterally) – infraspinatus and teres minor.Internal rotation (rotation towards the midline, so that the thumb is pointing medially) – subscapularis, pectoralis major, latissimus dorsi, teres major and anterior deltoid.Adduction (upper limb towards midline in coronal plane) – pectoralis major, latissimus dorsi and teres major.Past 90 degrees, the scapula needs to be rotated to achieve abduction – that is carried out by the trapezius and serratus anterior.The middle fibres of the deltoid are responsible for the next 15-90 degrees.The first 0-15 degrees of abduction is produced by the supraspinatus.Abduction (upper limb away from midline in coronal plane):.Biceps brachii weakly assists in forward flexion. Flexion (upper limb forwards in sagittal plane) – pectoralis major, anterior deltoid and coracobrachialis.Extension (upper limb backwards in sagittal plane) – posterior deltoid, latissimus dorsi and teres major.The shoulder joint is an extremely mobile joint, with a wide range of movement possible: This resists superior displacement of the humeral head. Coracoacromial ligament – extends between the acromion and coracoid process of the scapula, forming an arch-like structure over the shoulder joint (coracoacromial arch).It holds the tendon of the long head of the biceps in the intertubercular groove. Transverse humeral ligament – extends between the two tubercles of the humerus.It supports the superior part of the joint capsule. Coracohumeral ligament – extends from the base of the coracoid process to the greater tubercle of the humerus.They act to stabilise the anterior aspect of the joint. Glenohumeral ligaments (superior, middle and inferior) – extend from the humerus to the glenoid fossa, reinforcing the joint capsule.Ligaments play an important role in stabilising the shoulder joint: The synovial membrane lines the inner surface of the joint capsule and produces synovial fluid to reduce friction between the articular surfaces. The joint capsule is lax – permitting greater mobility (particularly abduction). It extends from the anatomical neck of the humerus to the border or ‘rim’ of the glenoid fossa. The joint capsule is a fibrous sheath which encloses the structures of the joint. Fig 1 – The articulating surfaces of the shoulder joint.
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