The shoulder joint (glenohumeral joint) is a ball and socket joint between the scapula and the humerus. It is the major joint connecting the upper limb to the trunk. Show It is one of the most mobile joints in the human body, at the cost of joint stability. In this article, we shall look at the anatomy of the shoulder joint and its important clinical correlations. Structures of the Shoulder JointArticulating SurfacesThe shoulder joint is formed by the articulation of the head of the humerus with the glenoid cavity (or fossa) of the scapula. This gives rise to the alternate name for the shoulder joint – the glenohumeral joint. Like most synovial joints, the articulating surfaces are covered with hyaline cartilage. The head of the humerus is much larger than the glenoid fossa, giving the joint a wide range of movement at the cost of inherent instability. To reduce the disproportion in surfaces, the glenoid fossa is deepened by a fibrocartilage rim, called the glenoid labrum. [caption id="attachment_8650" align="aligncenter" width="294"] Fig 1 - The articulating surfaces of the shoulder joint.[/caption]Joint Capsule and BursaeThe joint capsule is a fibrous sheath which encloses the structures of the joint. It extends from the anatomical neck of the humerus to the border or 'rim' of the glenoid fossa. The joint capsule is lax, permitting greater mobility (particularly abduction). The synovial membrane lines the inner surface of the joint capsule, and produces synovial fluid to reduce friction between the articular surfaces. To reduce friction in the shoulder joint, several synovial bursae are present. A bursa is a synovial fluid filled sac, which acts as a cushion between tendons and other joint structures. The bursae that are important clinically are:
There are other minor bursae present between the tendons of the muscles around the joint, but this is beyond the scope of this article. [caption id="attachment_8651" align="aligncenter" width="422"] Fig 2 - The major bursae of the shoulder joint.[/caption]LigamentsIn the shoulder joint, the ligaments play a key role in stabilising the bony structures.
The other major ligament is the coracoacromial ligament. Running between the acromion and coracoid process of the scapula it forms the coraco-acromial arch. This structure overlies the shoulder joint, preventing superior displacement of the humeral head. [caption id="attachment_8652" align="aligncenter" width="301"] Fig 3 - The ligaments of the shoulder joint. The transverse humeral ligament is not shown on this diagram[/caption]MovementsAs a ball and socket synovial joint, there is a wide range of movement permitted:
Mobility and StabilityThe shoulder joint is one of the most mobile in the body, at the expense of stability. Here, we shall consider the factors the permit movement, and those that contribute towards joint structure. Factors that contribute to mobility:
Factors that contribute to stability:
[caption id="attachment_8653" align="aligncenter" width="700"] Fig 4 - The rotator cuff muscles, which act to stabilise the shoulder joint.[/caption]NeurovasculatureThe shoulder joint is supplied by the anterior and posterior circumflex humeral arteries, which are both branches of the axillary artery. Branches of the suprascapular artery, a branch of the thyrocervical trunk, also contribute. Innervation is provided by the axillary, suprascapular and lateral pectoral nerves. [start-clinical] Clinical Relevance: Common InjuriesDislocation of the Shoulder JointClinically, dislocations at the shoulder are described by where the humeral head lies in relation to the glenoid fossa. Anterior dislocations are the most prevalent (95%), although posterior (4%) and inferior (1%) dislocations can sometimes occur. Superior displacement of the humeral head is generally prevented by the coraco-acromial arch. An anterior dislocation is usually caused by excessive extension and lateral rotation of the humerus. The humeral head is forced anteriorly and inferiorly – into the weakest part of the joint capsule. Tearing of the joint capsule is associated with an increased risk of future dislocations. 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. Indeed, so-called ‘reverse Hill-Sachs lesions’ (impaction fracture of anteromedial humeral head) and ‘reverse Bankart lesions’ (detachment of posteroinferior labrum) can be seen in posterior dislocations. The axillary nerve runs in close proximity to the shoulder joint and around the surgical neck of the humerus, and so it can be damaged in the dislocation or with attempted reduction. Injury to the axillary nerve causes paralysis of the deltoid, and loss of sensation over regimental badge area. [caption id="attachment_10761" align="aligncenter" width="330"] Fig 5 - Anterior dislocation of the shoulder joint.[/caption]Rotator Cuff TendonitisThe rotator cuff muscles have a very important role in stabilising the glenohumeral joint. They are often under heavy strain, and therefore injuries of these muscles are relatively common. The spectrum of rotator cuff pathology comprises tendinitis, shoulder impingement and sub-acromial bursitis. Tendinitis refers to inflammation of the muscle tendons – usually due to overuse. Over time, this causes degenerative changes in the subacromial bursa and the supraspinatus tendon, potentially causing bursitis and impingement. The characteristic sign of supraspinatus tendinitis is the ‘painful arc’ – pain in the middle of abduction between 60-120 degrees, where the affected area comes into contact with the acromion. This sign may also suggest a partial tear of supraspinatus. [end-clinical] |