Excessive range of movement
Instability of the shoulder
Definition
Glenohumeral instability is defined as ‘the inability to maintain the humeral head centred in the glenoid fossa’.1 Recurrent instability may be characterized as dislocation, subluxation or apprehension. Dislocation is the complete separation of the articular surfaces without spontaneous relocation. Subluxation is defined as symptomatic translation of the humeral head without complete separation of the articular surfaces. Apprehension refers to the fear that the shoulder will subluxate. Instability can also be characterized according to the direction of the (sub)luxation: anterior, posterior, inferior and superior. Subluxations can also be classified according to their cause: they may be traumatic, atraumatic or voluntary. Around 95% of (sub)luxations are anterior. Most have a traumatic origin, although instability can also result from relatively small but repeated movements of the arm as may occur in swimmers, volleyball players or baseball pitchers.2 Derangement of the labrum, intracapsular ligaments or joint capsule are regarded as the main underlying causes of chronic instability.3 Exceptionally, osseous changes such as a fracture of the glenoid are found.
Factors in glenohumeral stability
A considerable range of mobility is essential at the glenohumeral joint. As a consequence, the shoulder has less bony and ligamentous stability than any other diarthrodial joint. There is a considerable incongruence between the two articulating surfaces with only 25–30% of the humeral head being covered by the glenoid fossa. This enables a wide range of positions without neck-rim contact.4 Furthermore, the glenohumeral joint does not have strong isometric articular ligaments that provide stability while the joint is flexed around a defined anatomical axis as is the case in knee, elbow and ankle joints. Instead, the glenohumeral ligaments have a stabilizing function only at the extremes of motion and play no part in most functional positions of the joint.5
In spite of all this, the normal shoulder joint is quite stable and able to centre the humeral head in the glenoid cavity throughout most of the arc of movement.6 This is achieved by a set of mechanisms which classically are divided into static factors (those that do not require the expenditure of energy by muscles) and dynamic factors (requiring muscle energy).
Static factors
Joint surfaces
Recent studies have demonstrated that, although the bony surfaces of the joint are largely incongruent (flat glenoid and round humerus), congruence is restored by the difference in thickness of cartilage. Glenoid cartilage was found to be the thickest at the periphery and thinnest centrally, whereas humeral articular cartilage was thickest centrally and thinnest peripherally (Fig. 1). This leads to a merely uniform contact between humeral head and glenoid surface throughout shoulder motion.7
Fig 1 Glenoid cartilage is thickest at the periphery (1); humeral cartilage is thickest centrally (2). The glenoid labrum increases the depth of the glenoid socket and the surface area for the humeral head (3).
The glenoid labrum contributes to stability by increasing the depth of the glenoid socket (Fig. 1) and so increasing the surface area for the humeral head.8,9
Cohesion–adhesion forces
Joint surfaces, wet with joint fluid, are held together by the molecular attraction of the fluid to itself and to the joint surfaces. This is called the adhesion–cohesion mechanism (compare with two wet microscope slides pressed together) and is a very strong stabilizing factor in the glenohumeral joint.
Negative pressure
There is minimal (less than 1 cc) free fluid in the normal shoulder joint. The normal shoulder is sealed by the capsule so that external fluid cannot enter it. Osmotic action by the synovium removes free fluid, keeping a slightly negative pressure within the normal joint. The negative intra-articular pressure creates a vacuum that pulls the humerus against the glenoid.10 The limited joint volume effect is reduced if the joint is ‘vented’ (opened to the atmosphere) or when the capsular boundaries of the joint are very compliant.11 This will lead to the production of a ‘sulcus sign’ (see below): under attempted traction on the arm, the flexible capsule is dragged into the joint.
Capsule and ligaments
The joint capsule is large, loose and redundant (the capacity of the glenohumeral joint capsule is larger than that of the humeral head). Capsule and ligaments alone cannot prevent glenohumeral translation when the joint is in most of its range of movement. Recent studies show that the main importance of the capsule for stability lies in the feedback mechanism of the proprioceptive innervation and contraction of the rotator cuff.12,13
Dynamic factors
Both shoulder girdle and rotator cuff muscles contribute to ‘dynamic’ glenohumeral stability. The muscles of the shoulder girdle (trapezius, rhomboideus, latissimus dorsi, serratus anterior and levator scapulae) stabilize the scapula as a firm platform for movement of the humeral head.14 The ‘rotator cuff’ (supraspinatus, infraspinatus, subscapularis and the long head of the biceps) serves three stabilizing functions:
• Dynamic ligaments. 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.15,16
• By contracting together, the rotator cuff muscles and the biceps press the humeral head into the glenoid fossa (Fig. 2), locking it into position and thus providing a secure scapulohumeral link for upper extremity function.17
Fig 2 By contracting together the rotator cuff muscles and the biceps press the humeral head into the glenoid fossa, locking it in position.
• By contracting selectively, the rotator cuff muscles can resist displacing forces resulting from contraction of the principal shoulder motors.18 For example, when the pectoralis major and anterior deltoid muscles elevate and flex the shoulder, they tend to push the humeral head out the back of the glenoid fossa; this displacement is resisted by contraction of the subscapularis, infraspinatus and teres minor (Fig. 3). When the lateral deltoid initiates shoulder abduction, the supraspinatus and the long head of the biceps actively resist upward displacement of the humeral head relative to the glenoid fossa.
Fig 3 By contracting selectively the rotator cuff muscles can resist the displacing forces of the principal shoulder motors.
History
The history and detailed physical examination are both very important for diagnosis. Unfortunately symptoms are often vague and clinical findings subtle.19,20 The majority of patients mention a previous injury, usually a fall onto the outstretched arm in lateral rotation and abduction, which resulted in a luxation of the humeral head.21 However, it should not be assumed that there is also a record of manipulative reduction at that time because in most cases the humeral head only subluxates and immediately reduces spontaneously. The earlier in life this happens, the more likely is future transient subluxation.22 Sometimes, patients are encountered without a history of earlier displacement. This situation is called atraumatic instability and is more likely to be multidirectional. Atraumatic instability is more a syndrome that may arise from a set of pathogenic disorders: a flat glenoid, compliant capsule or weak neuromuscular control. It is often referred to as the AMBRII syndrome: Atraumatic, Multidirectional, Bilateral, with Rehabilitation directed at restoring neuromuscular control, as the most important therapeutic measure; in case surgery is necessary, it should include reconstruction of the rotator Interval and tightening of the Inferior capsule.23–25
An attack of subluxation is usually described as follows: the patient experiences a sudden paralysing pain and weakness of the arm during activity. Whatever is held may be dropped. Very often the pain is brought on by unguarded movements such as raising the arm during throwing, swimming or serving at tennis. The pain is only momentary and disappears spontaneously after a few moments, after which the patient is usually able to return to activities without much pain or problem. In advanced cases, however, and after repeated attacks, the shoulder may be felt to slip out and clunk back into place with increasing ease and in an increasing number of activities.26 Finally the shoulder may become uncomfortable even with the arm at rest and by then the acute symptoms are brought about with less forceful activities such as turning the steering wheel of a car or when the patient puts the arm in the sleeve of a coat.27
Functional examination
The basic functional examination may show some abnormal findings, typical for a possible minor instability: an enlarged range of movement (ROM) and a loose end-feel. Momentary subluxation can also appear as a kind of painful arc.28 During active elevation of the arm, the humeral head subluxates to a certain extent, blocks the movement for a while and slips back into place as the movement is continued. Recently it has been shown that a real painful arc, caused by impingement, may coexist in subluxation of the shoulder as a secondary phenomenon to the instability.29,30 An overlap seems to exist in the concepts of instability and impingement and for this reason when a painful arc is present minor shoulder instability must always be kept in mind.31,32 On passive lateral rotation, when performed firmly, a different type of end-feel may be encountered. First resistance is felt, which then is overcome and finally gives rise to a new ligamentous end-feel. When the arm is brought back to neutral position a click is often felt. This sequence of sensation – although not necessarily painful – may indicate anterior instability. The same occurs in posterior instability for passive medial rotation.
Accessory tests
If a minor instability is suggested after the history and the basic functional examination, the classic stress tests to challenge the stability of the joint in various directions must be performed. During these tests, both apprehension and palpable subluxations are sought. To differentiate normal laxity from pathological instability, both shoulders should be compared. The uninvolved arm is always examined first.
Apprehension test for recurrent anterior dislocation (Fig. 4)
This test can be performed with the patient either standing or supine. The fingers of the contralateral hand are placed on the anterior aspect of the humeral head, the thumb posteriorly. With the other hand the examiner brings the arm into full lateral rotation. In this position, anteriorly directed pressure is applied with the thumb to the posterior part of the humeral head.33 The test can be repeated with increasing degrees of abduction varying from 90° to full elevation. The test is regarded as positive if the patient judges that it provokes the same impression as the one felt when the shoulder moves out of place. Pain alone is not regarded as a positive apprehension sign. The examiner must be able to feel the humeral head subluxating or the patient looks and feels apprehensive and resists further motion. Thus, the apprehension is greater than the pain.34
Relocation test (Fig. 5)
