The glenohumeral joint is a synovial multiaxial spheroidal joint between the roughly hemispherical head of the humerus and the shallow glenoid fossa of the scapula (Fig. 46.15). Notable for its relative lack of bony constraint, the joint possesses three degrees of freedom. Its static and dynamic stability depends on the surrounding muscular and soft tissue envelope more than on its shape and ligaments: effective function is achieved by a complex interaction between the articular and soft tissue restraints. It is the most mobile joint in the body and the most frequently dislocated.
The articular surfaces are reciprocally curved and are really ovoids. (Here, as in the hip, where ovoid surfaces are almost spherical they are often termed spheroidal.) The area of the humeral convexity exceeds that of the glenoid concavity such that only a small portion opposes the glenoid in any position (Fig. 46.16). The remaining capitular articular surface is in contact with the capsule, so that contact on the glenoid fossa is much more uniformly distributed over its entire articular surface. The radius of curvature of the glenoid fossa in the coronal plane is greater than that of the humeral head, and is deepened by a fibrocartilaginous rim, the glenoid labrum (Fig. 46.13, Fig. 46.17). Both articular surfaces are covered by hyaline cartilage, which is thickest centrally and thinner peripherally over the humerus, and the reverse in the glenoid cavity. In most positions, their curvatures are not fully congruent, and the joint is loose-packed. Close packing (full congruence) is reached with the humerus abducted and laterally rotated ( Fig. 5.61).
The glenoid labrum is a fibrocartilaginous rim around the glenoid fossa. It is triangular in section and varies in size and thickness; its base is attached to the margin of the fossa and its free inner edge projects as a continuation of the curve of the glenoid. It blends above with two fasciculi from the long tendon of biceps. The labrum deepens the cavity, may protect the bone, and probably assists lubrication. Its attachment is sometimes partly deficient anterosuperiorly, in which case synovial membrane may protrude through the gaps.Fibrous capsule
A fibrous capsule envelops the joint (Fig. 46.14, Fig. 46.16). It is attached medially to the glenoid neck outside the glenoid labrum, and encroaches on the coracoid process to include the attachment of the long head of biceps. The capsule often extends and attaches to the base of the coracoid and to the body of the scapula, forming anterior and posterior recesses. Laterally, it is attached to the anatomical neck of the humerus, i.e. near the articular margin, except inferomedially, where it descends more than 1 cm on the humeral shaft. It is so lax that the bones can be distracted for 2 or 3 cm, which accords with the very wide range of movement possible at the glenohumeral joint. However, such unnatural separation requires relaxation of the upper capsule by abduction.
The fibrous capsule is supported by the tendons of supraspinatus (above), infraspinatus and teres minor (behind), subscapularis (in front) and by the long head of triceps (below). The rotator interval is a medially based triangular area of uncovered capsule between the superior edge of subscapularis and the anterior edge of supraspinatus as these tendons pass on either side of the base of the coracoid: it may represent an area of weakness that increases instability in some shoulders. Triceps is separated from the capsule by the axillary nerve and posterior circumflex humeral vessels as they pass back from the axilla (Fig. 46.14). The capsule is least supported inferiorly, and subjected to the greatest strain in full abduction, when it is stretched tightly across the humeral head. It is strengthened anteriorly by extensions from the tendons of pectoralis major and teres major.
There are usually two or three openings in the capsule: below the coracoid process, connecting the joint to a bursa behind the tendon of subscapularis (anterior); between the humeral tubercles, transmitting the long tendon of biceps and its synovial sheath; connecting the joint to a bursa under the tendon of infraspinatus (posterior and inconstant).Ligaments
The ligaments associated with the glenohumeral joint are the glenohumeral (superior, middle and inferior), coracohumeral and transverse humeral.Glenohumeral ligaments
Three glenohumeral ligaments, only visible from within the joint, reinforce the capsule anteriorly and inferiorly (Fig. 46.18). They do not act as traditional ligaments (which carry a pure tensile force along their length), but become taut at varying positions of abduction and humeral rotation, acting as ???check-reins'. Moreover, they do not have the strength characteristics of the ligaments at the knee. The superior glenohumeral ligament passes from the supraglenoid tubercle, just anterior to the origin of the long head of biceps, to the humerus near the proximal tip of the lesser tubercle on the medial ridge of the intertuberculous groove, the fovea capitis. It forms an anterior cover around the long head of biceps, and is part of the rotator interval. Together with the coracohumeral ligament it is an important stabilizer in the inferior direction, helping to keep the humeral head suspended (the coracohumeral ligament is more robust than the superior glenohumeral ligament). The middle glenohumeral ligament arises from a wide attachment below the superior glenohumeral ligament, along the anterior glenoid margin as far as the inferior third of the rim, and passes obliquely inferolaterally, enlarging as it does, to attach to the lesser tubercle deep to the tendon of subscapularis, with which it blends. The width and thickness of this ligament may be as much as 2 cm and 4 mm respectively. It provides anterior stability at 45?? and 60?? abduction. The thicker and longer inferior glenohumeral ligament complex is a hammock-like structure with anchor points on the anterior and posterior sides of the glenoid. It arises from the anterior, middle and posterior margins of the glenoid labrum, below the epiphysial line, and passes anteroinferiorly to the inferior and medial aspects of the neck of the humerus. The anterior, superior edge of the inferior ligament is thickened as the superior band, and the diffuse thickening of the anterior part of the capsule to which it is attached is known as the axillary pouch. The anterior band of the inferior glenohumeral ligament is thought to be the primary static anterior stabilizer of the abducted and externally rotated glenohumeral joint. (For further details consult Burkart & Debski 2002.)
The coracohumeral ligament is attached to the dorsolateral base of the coracoid process and extends as two bands, which blend with the capsule, to the greater and lesser tubercles (Fig. 46.14). Portions of the coracohumeral ligament form a tunnel for the biceps tendon on the anterior side of the joint. The rotator interval is reinforced by the coracohumeral ligament. It also blends inferiorly with the superior glenohumeral ligament.Transverse humeral ligament
The transverse humeral ligament is a broad band which passes between the humeral tubercles, and is attached superior to the epiphysial line (Fig. 46.14). It converts the intertubercular sulcus into a canal, and acts as a retinaculum for the long tendon of biceps.Synovial membrane
The synovial membrane lines the capsule and covers parts of the anatomical neck. The long tendon of biceps traverses the joint in a synovial sheath which continues into the intertubercular sulcus as far as the surgical neck of the humerus (Fig. 46.14, Fig. 46.16).Bursae Many bursae adjoin the shoulder joint.
They are usually found between the tendon of subscapularis and the capsule, communicating with the joint between the superior and middle glenohumeral ligaments; on the superior acromial aspect; between the coracoid process and capsule; between teres major and the long head of triceps: anterior and posterior to the tendon of latissimus dorsi. The subacromial bursa, between deltoid and the capsule, does not communicate with the joint cavity but is prolonged under the acromion and coracoacromial ligament, and between them and supraspinatus: it appears to be attached, together with the subdeltoid fascia, to the acromion. Bursae sometimes occur behind coracobrachialis and between the tendon of infraspinatus and the capsule, occasionally opening into the joint.Vascular supply
The glenohumeral joint is supplied by branches from the anterior and posterior circumflex humeral, suprascapular and circumflex scapular vessels.Innervation
The glenohumeral joint is innervated mainly from the posterior cord of the brachial plexus. The capsule is supplied by the suprascapular nerve (posterior and superior parts), axillary nerve (anteroinferior) and the lateral pectoral nerve (anterosuperior).Factors maintaining stability
The articulation between the relatively large humeral head and the shallow glenoid fossa allow a wide range of movement at the expense of providing an unstable bony complex. The anterior joint capsule is strong but lax. A variety of additional factors help to increase the stability of the joint: the glenoid labrum deepens the concavity of the articulating glenoid, the glenohumeral ligaments act as static stabilizers in certain positions, and there is a negative pressure within the joint. The coracoacromial arch (coracoid, acromion and coracoacromial ligament) prevents upward dislocation of the humerus. The tendons of subscapularis, supraspinatus, infraspinatus and teres minor fuse with the lateral part of the joint capsule to form the ???rotator cuff'. These short muscles collectively produce a compressive force during active glenohumeral movements which maintains congruent contact between the head of the humerus and the glenoid fossa, helps to resist skid, and checks excessive translation. The rotator cuff also provides strong lateral stability and prevents this part of the lax capsule from being nipped during joint movements. The long head of biceps offers additional superior support. The long head of triceps offers inferior support which is particularly important when the shoulder is abducted. However, the glenohumeral joint is least stabile inferiorly when the shoulder is fully abducted.Movements at the shoulder (glenohumeral) joint
The shoulder is capable of any combination of swing and spin over a very wide range. Laxity of the capsule, and a humeral head which is large relative to a shallow glenoid fossa, afford a wider range of movement than at any other joint. Flexion???extension, abduction???adduction, circumduction and medial and lateral rotation all occur at the shoulder. Although the majority of the movement of the shoulder occurs at the glenohumeral joint, there is a varying contribution from the scapulothoracic articulation in most directions, most significantly in abduction, and excluding lateral rotation.
In analysis of shoulder movements it is preferable to refer humeral movement to the scapula, rather than to conventional anatomical planes (Fig. 46.19). When the arm hangs at rest the glenoid fossa faces almost equally forwards and laterally, and the humeral capitular and scapular (topographical) axes correspond, although the humerus, relative to the anatomical position, is medially rotated. Flexion carries the arm anteromedially on an axis through the humeral head orthogonal to the glenoid fossa at its centre. Abduction and adduction occur in a vertical plane orthogonal to that of flexion???extension; the axis is horizontal, through the humeral head, parallel with the glenoid plane. Pure abduction raises the arm anterolaterally in the plane of the scapula. However, when referred to the trunk, flexion and extension occur in the paramedian plane, and abduction and adduction in the coronal plane. In this sense, raising the arm vertically from flexion or raising it from abduction are both accompanied by humeral rotation in opposite directions. Whether ???scapular' or any other plane of abduction is described, these are selections from an infinite series. In scapular abduction points on the humeral surface pursue vertical cords but in rotation they are horizontal. In ???pure' flexion???extension, in a plane orthogonal to the scapula the axis of movement, and the notional ???mechanical axis', are regarded as projected from the centre of the glenoid cavity.
Glenohumeral abduction is approximately 90??, but angles up to 120?? have been reported. Some 60?? further abduction occurs at the sterno- and acromioclavicular articulations. Contralateral vertebral flexion also aids in bringing the arm to the vertical. During active elevation, movements at the glenohumeral and acromioclavicular joints are simultaneous, except in the initial few degrees, when most, often all, movement is glenohumeral. For every 15?? of elevation, glenohumeral movement is said to be 10?? and scapular movement 5??. During the initial stages of abduction, subscapularis, infraspinatus and teres minor counteract the strong upward component of pull of deltoid, which would otherwise cause the humeral head to slide up; the additive turning moments exerted by deltoid and supraspinatus about the shoulder joint can then abduct the arm.
In flexion, the humerus swings at right angles to the scapular plane and scapular rotation cannot increase the elevation (120??) obtainable in full flexion. If the fully flexed humerus is also abducted, elevation increases pro rata until, when the humerus reaches the scapular plane, i.e. when true abduction is reached, 180?? of elevation becomes possible. In medial or lateral rotation, the humerus revolves about one-quarter of a circle around a vertical axis; the range is greatest when the arm is pendent, and least when it is vertical. When assessing the rotational range at the glenohumeral joint, the forearm should be flexed to a right angle at the elbow: this will prevent the effects of superadded pronation or supination in the pendent limb. In circumduction, which is a succession of the foregoing movements, the distal end of the humerus describes the base of a cone, its apex at the humeral head. This glenohumeral movement can be much increased by scapular movements, e.g. in acts of slinging objects with force.
The peculiar relation of the long head of biceps to the shoulder joint may serve several purposes. By its connection with both the shoulder and elbow, the muscle harmonizes their actions as an elastic ligament during all their movements. It helps to prevent the humeral head impinging on the acromion when deltoid contracts and to steady it in movements of the arm. In paralysis of supraspinatus it may also help initiate abduction of the arm, particularly when the humerus is laterally rotated.Muscles producing movements
The muscles which produce movements at the glenohumeral joint are principally deltoid, pectoralis major, latissimus dorsi and teres major. These long muscles all converge on the humerus, acting at mechanical advantage on a joint which, as a result of glenoid shallowness and capsular laxity, is relatively unstable. The long muscles are counteracted by the rotator cuff, a group of short muscles (subscapularis, supraspinatus, infraspinatus and teres minor) which are attached nearer to the joint, and which centre the head of the humerus in the glenoid fossa through the midrange of motion, when the capsuloligamentous structures are lax.Flexion
Pectoralis major (clavicular part), deltoid (anterior fibres) and coracobrachialis assisted by biceps. The sternocostal part of pectoralis major is a major force in flexion forwards to the coronal plane from full extension.Extension
Deltoid (posterior fibres) and teres major, from the dependent position. When the fully flexed arm is extended against resistance, latissimus dorsi and the sternocostal part of pectoralis major act powerfully until the arm reaches the coronal plane.Abduction
Deltoid. Initially its effect is mainly upward and, unless opposed, this would displace the humerus upwards. Subscapularis, infraspinatus and teres minor exert downward traction and so apply an opposing force: together with deltoid they constitute a ???couple' to produce abduction in the scapular plane. Supraspinatus assists in effecting and maintaining this movement, but its precise role is controversial.Medial rotation
Pectoralis major, deltoid (anterior fibres), latissimus dorsi, teres major and, with the arm pendent, subscapularis.Lateral rotation
Infraspinatus, deltoid (posterior fibres) and teres minor. Lateral rotation is important for clearance of the greater tubercle and its associated tissues as it passes under the coracoacromial arch, as well as for relaxation of the capsular ligamentous constraints.Rotator cuff disease
The subacromial space is defined inferiorly by the humeral head, and superiorly by the anterior edge and inferior surface of the anterior third of the acromion, coracoacromial ligament and acromioclavicular joint, forming the coracoacromial arch. It is occupied by the supraspinatus tendon, subacromial bursa, tendon of the long head of biceps brachii, and the capsule of the shoulder joint. Rotator cuff disease is a painful condition with a multifactorial aetiology in which severe or chronic impingement of the rotator cuff tendons on the undersurface of the coracacromial arch is often a significant factor. The cuff normally impinges against the coracoacromial arch when the humerus is abducted, flexed and internally rotated. This is known as the impingement position. The supraspinatus tendon is anatomically affected most by the impingement, which interestingly also coincides with an area of reduced vascularity in this tendon. Severe impingement can be caused by thickening of the coracoacromial arch, by inflammation of the cuff from disorders such as rheumatoid arthritis, or as a result of prolonged overuse in the impingement position, e.g. in cleaning windows. When associated with a tendinopathy from age-related degenerative changes within the tendon, impingement may be associated with partial or complete tears of the cuff. Clinically, this condition causes tenderness over the anterior portion of the acromion, and pain which typically occurs on abducting the shoulder between 60?? and 120?? (the painful arc).Glenohumeral joint dislocations
The glenohumeral joint is the most frequently dislocated joint in the body. It is most unstable anteroinferiorly, which explains why the vast majority of dislocations are anterior, and occur when the arm is forced backwards when it is in abduction, external rotation and extension. Clinically, a dislocated shoulder loses its normal contour, and the acromion process, rather than the greater tubercle, becomes the most lateral bony structure. The axillary nerve and artery may be injured during dislocation, and this can lead to inability to abduct the shoulder as a result of paralysis of deltoid together with an area of anaesthesia over the distal part of the muscle (sometimes referred to as the ???badge area' of skin), as well as ischaemic changes in the limb. Posterior dislocation is rare and typically occurs when violent movements produce marked internal rotation and adduction, e.g. in epileptic seizures or electric shock.