THE JOINTS OF THE SHOULDER GIRDLE 63 THE JOINTS OF THE SHOULDER GIRDLE! JUDY MORRIS, B.APP.SC.(PHYSIO.), GRAD.DIP.MAN.THERAPY The long process of progression from quad­ muscle as a morphological component of del.. ruped to biped posture has demanded skeletal toid and indeed this muscle is absent in pri.. and muscular changes which have formed mate mammals such 8S the opossum. From a distinct trend in those forms in which the this position teres minor can now act on the forelimb has been freed for prehension. humerus in a downward direction.. The scapula has shifted from a high cervical Through the primates to man, subscapularis position which it occupies in the lower ani­ muscle decreases only slightly in size, hut with mals (e.g. amphibians) so that the neck and expansion of its origin there is an increase head are free. The development of orthograde in the number of fasciculi. As a result the forms has also been accompanied by an lowest fasciculi act like teres minor in a down­ anteroposterior flattening of the thoracic cage ward direction on the humerus. Infraspinatus with dorsal displacement of the scapula where muscle is similarly affected so that the it is in a better position to function as a base teres minor, subscapularis and infraspinatus to facilitate arm movement. muscles form an important functional group The scapula is suspended against the chest which acts as a depressor of the head of the wall by the muscles acting upon it. Therefore humerus as well as a rotator. The importance the increased functional demands of the pre.. of this action in relation to the total shoulder hensile limb has affected its shape profoundly, movement has been demonstrated electrornyo.. particularly in the relationship between length graphically (Inman et al., op. cit., Basmajian, and breadth of the bone. The inferior angle 1963). There is continuous activity in this of the scapula has extended in a caudal muscle group during both abduction and direction so that the vertebral horder is rela.. flexion. Without them the power of deltoid tively long. This €xtension of the infraspinous to move the arm from the side is much f ossa alters the relationship of the axillary diminished. border to the glenoid cavity and thus the The clavicle which is an essential requisite angle of action of its attached muscles (Inman for prehensile activities is absent in those et al., 1944; De Palma, 1973). animals who use their fore limbs for support In addition the progressive increase in size such as the dog and the horse, and is present and extent of the acromion process, coupled in animals even where there is only a partial with the distal migration of the deltoid in.. freeing of the fore limb e.g. the rat and mar.. sertion has greatly increased the leverage of supiaL Through its articulations at either end that muscle-particularly for activities above it provides a mechanism for increasing range the horizontal, a fundamental characteristic of at the glenohumeral joint, thereby allowing primates (Inman et al., Opt cit.). the wide range of motion essential for full The preceding skeletal changes emphasize elevation of the arm. the importance of the infraspinal muscles in De Palma (1957) in a study of 66 clavicles achieving the great range of motion possible found marked individual variations. He ob.. in the shoulder joint in man. With the ex­ served that the lateral third of the clavicle tension of the inferior angle of the scapula, exhibits varying degrees of anterior torsion. that part of the deltoid muscle which attaches His study revealed that there were three to it in lower animals follows it distally and definite types of clavicle, which showed the becomes separated from the main mass of following specific features. muscle as teres minor. Comparative ana· Type I showed the greatest amount of an.. tomical studies have revealed the teres minor terior torsion of the lateral third and the lReceived June, 1977 .. acromial end is flat and thin with a small Aust..! ..Physiother., XXIV, 2, June, 1978 64 THE AUSTRALIAN JOURNAL OF PHYSIOTHERAPY articulating surface. The plane of the acromio.. conoid tubercle on the under surface of the clavicular joint is directed down and inward, clavicle, just where it curves posteriorly on its the average angle being 16° from the vertical. lateral curve. The manner of its attachment At the sternal end, the plane of the sterno­ provides a mechanism capable of producing clavicular joint is not far from vertical and further outward rotation of the scapula. As the is directed down and outward with the average scapula rotates during abduction of the arm, 0 angle of 7.5 • the conoid process is displaced downwards. Type II illustrates less anterior torsion than Through the action of the ligament on the Type I, the acromial end is stouter and posterior curvature of the clavicle, that bone slightly more rounded. The plane of the rotates on its long axis. Without this move.. acromioclavicular joint makes a greater angle ment, abduction of the arm would be severely with the vertical than Type I. The average restricted. Inman (op. cit.) showed that by fixing the clavicle with internal pins and angle is 26.1 0 and the angle of the plane with manually preventing its rotation, elevation of the sternoclavicular joint is slightly greater, 0 the average being 10.9°. the arm was limited to 110 • Type III demonstrates the least amount of torsion. Its acromial end is stont and rounded Sternoclavkular Joint presenting an almost completely circular articular surface. The plane of the acromio­ 1 clavicular joint averages 36.1 0 and the sterno­ clavicular joint 13.9 0 from the verticaL In a clinical study (De Palma op. cit.) of the relationship between painful acromio­ clavicular joints (due to degenerative changes) and the three types of clavicles described, it was found that the majority of patients pos­ sessed clavicles as in Type I. The plane of the joint in Type I is such that with shoulder movement there is greater shearing force act­ ing on the articular surfaces than there is with the other two types. In addition, the articular surfaces of the joints in Type I are smaller and this may well he a predisposing factor to degenerative changes in the articular cartilage. T The acromioclavicular joint is characterized Acromioclavicular Joint by the variability and shape of clavicular facets and the intra-articular disc (Moseley, 1968; De Palma op_ cit.)" The articular sur­ faces permit motion in all directions including rotation of the clavicle and this joint is the true centre of rotation of the scapula. The Three clavicle types, showing variation in the plane capsule is a rather loose structure reinforced of the acromioclavicular and sternQclavicular joints. by superior and inferior acromioclavicular (From De Palma, 1957.) ligaments and superiorly by the intertwining tendinous attachments of trapezius and del.. The inferior aspect of the joint bears a close toid. An important component of the joint relationship to the coracoacromial ligament is the coracoclavicular ligament which binds and the subacromial bursa. For this reason the scapula to the clavicle. This consists of degenerative changes in the joint with inferior two tough bands, the trapezoid and conoid lipping of the clavicle can give rise to signs ligaments, which are directed upwards and and symptoms simulating periarthritic lesions laterally to insert into the trapezoid line and of the shoulder (De Palma op. cit.). Aust./.Physiother., XXIV, 2, June, 1978 THE JOINTS OF THE SHOULDER GIRDLE 65 Movement of the joint occurs around three by upward or downward motion while axes: retraction is always associated with up­ ward motion of the clavicle. 1. Vertical Axis which allows movement of scapula on the outer end of the clavicle. 3. Longitudinal Axis, which allows rotation It enables the vertebral horder of the of the clavicle associated with elevation scapula to move away from the chest wall and backwards motion. The reverse rota.. i.e. winging of the scapula and its cor­ tion is not possible from the resting rection. position. 2. Anteroposterior Axis which movement of De Palma observed that the sternoclavicular the scapula on this axis enables the lower joint was a much sounder anatomical stIue.. angle of the scapula to rotate forward on ture than the acromioclavicular joint. He the chest wall, facilitating elevation of attributed this to the tough intra-articular the humerus by tilting the glenoid fossa disc which seems capable of protecting the upwards. articular surfaces from severe degenerative 3. Frontal Axis which allows the lower angle changes for many decades. of the scapula to tilt away from the chest In contrast, the acromioclavicular joint wall. exhibits rapid and extensive degenerative The sternoclavicular joint is the only hony changes beginning as early as the second attachment of the entire upper limb to the decade. The intra-articular disc provides no axial skeleton. The joint surface on the ster­ protection to the articular surfaces. While num is concave from above downward, but degenerative changes do not necessarily pro­ slightly convex from front to back. The joint duce symptoms, they do make the joint vul­ surface on the medial end of the clavicle is nerable to minor injuries and strains. not reciprocally shaped. This incongruity of The scapulothoracic j oint is not a true joint, fit is compensated by the interposition of a hut is a riding of the scapula on the posterior fibrocartilaginous disc which divides the joint thoracic cage. The scapula lies in a plane into two compartments. This disc acts as a about midway between frontal and sagittal shock ahsorber of forces transmitted to the planes, and its bed of loose areolar tissue aids joint along the clavicle from the shoulder.