During the normal use of the shoulder the humeral head is centred within the glenoid and the ccoracoacromial arch. When the shoulder cannot maintain this centred position during use it becomes unstable. An unstable shoulder prevents normal function of the arm. Shoulder instability is not the same as joint laxity. Joint laxity is merely one end of a spectrum of collgagen elasticity variation.
The concavity of the glenoid and corocoacromial arch, along with the passive and active forces that press the humeral head both into the glenoid and the coracoacromial arch, maintain the head in its centred position. This concavity compression mechanism is dependent on the integrity of the glenoid, the coracacromial arch, and constraining ligaments of the shoulder,but mainly muscular compression of the glenohumeral joint Loss of any of these elements due to developmental, degenerative, traumatic or iatrogenic factors may compromise the ability of the shoulder to centre the humeral head in the glenoid.
Joint capsule and ligaments
The joint capsule and associated ligaments act as check reins to rotation and function only at the extremes of motion when they come under tension. During the mid range of motion the capsule and ligaments are lax and therefore allow the humeral head to be passively translated during physical assessments such as the sulcus and drawer tests.
They prevent humeral rotation beyond the point where the muscles are effected and ultimately prevent the rotator cuff muscles from being overstretched. They also substitute for muscle forces when no muscle is present , for example , the Coracohumeral ligament and rotator interval capsule that lie between the supraspinatus and subscapularis, the most powerful of the rotator cuff muscles, provides a compressive force in abduction. The inferior Glenohumeral ligament lies beneath the glenohumeral joint and provides a compressive force when the arm is fully abducted. Furthermore this IGHL sling (“or hammock”) moves anteriorly when the shouldr is abducted and externally rotated, helping to prevent antro-inferior subluxation/dislocation.
A ball sitting on a flat table has no tendency to centre itself. Even a slight displacing force causes it to slide or roll. If the table has a concavity the ball will sit at the base of the concavity. The deeper the concavity the more force it takes to move the ball out of it. The stability is increased if a greater force presses the ball into this concavity – this mechanism is known as concavity compression.
The glenoid concavity has three components;
The osseous glenoid, is slightly concave; the articular cartilage, which is thicker at the periphery and thinner in the centre and thus makes the concavity deeper; and the glenoid labrum (triangular in shape) which further deepens the glenoid concavity. The glenoid labrum optimizes the surface area for glenohumeral contact and creates a conforming seal with the head of the humerus.
The rotator cuff muscles compress the head of the humerus into the glenoid concavity (link to rotator cuff anatomy). The important characteristic of the rotator cuff muscles is that they function in almost any position of the glenohumeral joint. Other muscles such as the deltoid, long head of biceps, triceps, latissimus dorsi, teres major and pectoralis major can contribute in certain positions. For example, when the arm is elevated to 90 degrees the deltoid becomes a strong compressor. A large tear or incompetence of the rotator cuff can result in an instability in the direction of the affected tendon. For example massive superior cuff deficiency is commonly associated with progressive superior displacement of the humeral head.
TO ILLUSTRATE THE PRINCIPLE MUSCLES PROVIDING STABILITY TO THE SHOULDER
The coracoacromial arch
The principal of concavity compression applies to the ball and socket joint between the proximal humeral convexity and the coracoacromial arch. The primary compressor of this articulation is the deltoid. Compression into the arch results when the arm presses down, such as to rise from a chair or using crutches.
Adheson cohesion suction cup
Adhesion cohesion is the process by which the wettable joint surfaces adhere to each other because of the adhesive properties of the water molecules, “liquid tension”.
This allows the joint surfaces to glide whilst staying congruous. The glenohumeral suction cup relies on the effect of the non compliant centre, ie the glenoid and the more flexible periphery , the labrum. As a result the glenoid can stick to the humeral head like a toy arrow to a wall. The suction mechanism is enhanced by the negative intra articular pressure.
Generally shoulders will become unstable for one of two reasons. In the first instance for structural reasons, the capsulolabral structures may have been damaged by a major injury or already deficient, predisposing the shoulder to develop instability after only a mild force or repetitive micro-trauma, eg the overhead athlete. The second reason is the development of unbalanced muscle recruitment around the shoulder.
A most useful system of shoulder instability classification has been described by Ian Bayley at the Royal National Orthopaedic Hospital. He recognises the fundamental issue that a combination of pathologies may co-exist and that with time cases will move from one group to another.
In his system of shoulder classification (which is used for anterior dislocations, posterior dislocations, sub-luxations and complete dislocations), three basic polar groups are defined.
I Traumatic Structural
III Habitual non-structural (muscle Patterning)
Patients will be classified into a polar group or more commonly somewhere on the line joining two polar groups