Applied anatomy of the shoulder
Introduction
The main function of the joints of the shoulder girdle (Fig. 1) is to move the arm and hand into almost any position in relation to the body. As a consequence the shoulder joint is highly mobile, where stability takes second place to mobility, as is evident from the shape of the osseous structures: a large humeral head lying on an almost flat scapular surface. Stability is provided mainly by ligaments, tendons and muscles; the bones and capsule are of secondary importance.
Fig 1 A global view of all five joints of the shoulder girdle: 1, glenohumeral joint; 2, acromioclavicular joint; 3, sternoclavicular joint; 4, subacromial joint or subacromial gliding mechanism; 5, scapulothoracic gliding mechanism.
The function of the shoulder girdle requires an optimal and integrated motion of several joints. In fact five ‘joints’ of importance to ‘shoulder’ function can be distinguished:1
• The acromioclavicular joint (2)
• The sternoclavicular joint (3)
• The subacromial joint or subacromial gliding mechanism (4): the space between the coracoacromial roof and the humeral head, including both tubercles. This is the location of the deep portion of the subdeltoid bursa
• The scapulothoracic gliding mechanism (5): this functional joint is formed by the anterior aspect of the scapula gliding on the posterior thoracic wall.
Optimal mobility also requires an intact neurological and muscular system.
It is not our intention to give a complete anatomical review, which can be obtained from anatomy texts. Only those structures which are of specific clinical importance will be focused on.
Bones
Osseous structures of interest are the scapula, humerus and clavicle. Neither the vertebral column nor the thoracic cage is discussed here (see chapters on the anatomy of the cervical and thoracic spine).
Scapula
The scapula is a thin sheet of bone that functions mainly as a site of muscle attachment (Figs 2–3, see Putz, Figs. 286, 287, 288). Its medial border is parallel to the spine, the lateral and superior borders are oblique. It has a superior, a lateral and an inferior angle. The inferior angle corresponds to the interspinal level between the spinous processes of T7 and T8.
Fig 2 Posterior view of the scapula: 1, coracoid process; 2, acromion; 3, glenoid; 4, infraspinal fossa; 5, scapular spine; 6, supraspinal fossa.
Fig 3 Anterior view of the scapula: 1, acromion; 2, coracoid process; 3, glenoid fossa; 4, anterior surface.
The scapula contains four processes: the acromion, the coracoid, the spine and the articular process (the glenoid).
The dorsum of the scapula is convex. It is divided by its spine into two fossae: the supraspinal and infraspinal fossa, containing the corresponding muscles. The scapular spine runs from the junction of the upper and middle third of the medial border, where it is rather flat, and corresponds to the level of the third thoracic spinous process. Laterally it becomes more prominent and meets the acromion at a right angle posteriorly. This angle is easily palpable and is one of the main bony landmarks at the shoulder. The acromion turns further anteriorly and covers part of the humeral head.
The coracoid process is found at the anterior aspect of the scapula. The tip points outwards and is easily palpated in the lateral part of the subclavian fossa. Further down, on the anterior aspect of the scapula, is a large concavity which contains the subscapularis muscle.
At the lateral angle, just beyond the neck of the scapula, is the glenoid fossa. This has a rather shallow surface, which is directed anterolaterally and slightly cranially tilted. It is approximately one-quarter the size of the humeral head and this, plus its shallow concavity, makes the joint both very mobile and vulnerable to (sub)luxations.
The ventral surface of the scapula is flat and covered with the attachment of the subscapularis muscle, except for the medial border and inferior angle where the serratus anterior muscle is inserted.
Humerus
The articular surface of the humeral head points in a medial, posterior and slightly caudal direction and is separated from the major and minor tuberosities by its anatomical neck.
When the arm is hanging down the side with its anterior aspect facing the body, the greater tuberosity lies laterally and the lesser tuberosity anteriorly. They are separated from each other by the bicipital sulcus (Fig. 4).
Clavicle
The clavicle joins the sternum to the acromion. At its medial end it has a forward convexity whereas its lateral end is rather more concave (see Putz, Fig. 284).
The joint capsules of both the sternoclavicular and the acromioclavicular joints are reinforced by several ligaments.
The clavicle has many muscular and ligamentous attachments. The insertion of the coracoclavicular ligament is of practical importance. It is found laterally on the inferior aspect of the clavicle, and just medial to it is the origin of the subclavius muscle.
The clavicle gives support to the shoulder girdle by acting as a strut between scapula and sternum. Due to its S-shape, the outer end describes a much larger rotation during arm elevation than its inner end. Therefore, lesions of the acromioclavicular joint ligaments are more frequent than are lesions of the sternoclavicular joint ligaments.
Joints and intracapsular ligaments
Glenohumeral joint
The glenohumeral joint (Fig. 5) is a ball-and-socket between humeral head and glenoid fossa. There is a remarkable geometrical relationship between glenoid and head which is responsible for the considerable mobility of the joint but is also an important predisposing factor to glenohumeral instability. First, the large spherical head of the humerus articulates against the small shallow glenoid fossa of the scapula (only 25–30% of the humeral head is covered by the glenoid surface). Second, the bony surfaces of the joint are largely incongruent (flat glenoid and round humerus). However, the congruence is greatly restored by the difference in cartilage thickness: glenoid cartilage is found to be the thickest at the periphery and thinnest centrally, whereas humeral articular cartilage is thickest centrally and thinnest peripherally. This leads to a uniform contact between humeral head and glenoid surface throughout shoulder motion.
The labrum is a fibrous structure that forms a ring around the periphery of the glenoid (see Putz, Fig. 298). It acts as an anchor point for the capsuloligamentous structures and for the long head of the biceps. It further contributes to stability of the joint by increasing the depth of the glenoid socket, enlarging the surface area and acting as a load-bearing structure for the humeral head.
The synovial membrane of the joint capsule is mainly attached to the labrum, covering its inner surface, and at the anatomical neck of the humerus. It forms a sleeve around the intra-articular and inter-tubercular part of the long tendon of the biceps.
The fibrous portion of the capsule is very lax and has several recesses, depending on the position of the arm. At its caudal end it forms the axillary recess, which allows normal elevation of the arm. Very often adhesions form here.
The joint capsule is large, loose and redundant: the capacity of the glenohumeral joint capsule is larger than that of the humeral head to allow full and free range of motion of the shoulder.
At the anterior portion of the capsule three local reinforcements are present: the superior, medial and inferior glenohumeral ligaments (Fig. 6). These contribute, together with the subscapularis, supraspinatus, infraspinatus and teres minor muscles, to the stability of the joint.
Fig 6 The glenohumeral joint capsule and ligaments: superior (1), medial (2) and inferior (3) glenohumeral ligaments.
The supraspinatus, infraspinatus, teres minor and subscapularis tendons reinforce the superior, posterior and anterior capsule. By virtue of the blending of their tendons with the glenohumeral capsule and ligaments, selective contraction of the cuff muscles can adjust the tension in these structures, producing ‘dynamic’ ligaments.
Subacromial space
The suprahumeral gliding mechanism consists of the coracoacromial arch (see below) on one side and the proximal part of the humerus, covered by the rotator cuff and the biceps tendon on the other. Both parts are separated by the subacromial bursa that acts as a joint space (Fig. 7). Investigators point to the importance of contact and load transfer between the rotator cuff and the coracoacromial arch in the function of the normal shoulder. Normally there is no communication between the bursa and the glenohumeral joint space but it may be established by rupture of the rotator cuff.
Acromioclavicular joint
The acromioclavicular joint (Fig. 8) is the only articulation between the clavicle and the scapula. It contains a disc which usually has a large perforation in its centre. The capsule is thicker on its superior, anterior and posterior surfaces than on the inferior surface. The anteroposterior stability of the acromioclavicular joint is controlled by the acromioclavicular ligaments and the vertical stability is controlled by coracoclavicular ligaments (conoid and trapezoid).
Sternoclavicular joint
The only osseous connection between the skeleton of the trunk and the upper limb is formed by the clavicle. Its medial end lies in contact with the superolateral angle of the sternal manubrium and with the medial part of the cartilage of the first rib to form the sternoclavicular joint (Fig. 9, see Putz, Fig. 285). In both the vertical and anteroposterior dimensions, the clavicular portion is larger than the opposing manubrium and extends superiorly and posteriorly relative to the sternum. The prominence of the clavicle enables its palpation. The sternoclavicular joint is mobile along all axes and almost every movement of the scapula and the arm is associated with some movement at this joint.
The capsule of the joint is thin at its inferior aspect but is reinforced anteriorly by the anterior sternoclavicular ligament, posteriorly by the posterior sternoclavicular ligament and cranially by the interclavicular ligament. The latter courses further above the sternum, to which it is firmly attached. Just lateral to the inferior part of the capsule is the costoclavicular ligament. This ligament connects the medial end of the clavicle to the cartilage of the first rib.
An intra-articular meniscus, firmly attached at its circumference to the joint capsule, to the clavicle and to the cartilage of the first rib, divides the joint into two separate cavities.
Behind the left joint is the common carotid artery, behind the right joint the innominate (brachiocephalic) vein. Both structures must be kept in mind when infiltrating on either side.
