Shoulder Multidirectional Instability

This condition is moderately common, occurs typically on both sides and is a non-traumatic condition which interferes with the function of the shoulder. The laxity of the shoulder capsule and thereby the lack of its inherent ligamentous restrictions is the underlying problem causing these difficulties. With this laxity there is an excess of mobility in the shoulder joints in every joint direction. Patients may complain of instability, with the feelings that the shoulder will partly or wholly come out of joint at times. If this instability is not obvious to the patient they may complain only of pain when they present.

Conservative treatment is the first line of management for this condition, with physiotherapy treatment consisting of strengthening of the muscular parts of the scapular stability and rotator cuff systems. Once conservative treatment has been attempted and not been successful then consideration can be given to surgery. Surgery can tighten up the shoulder capsule, increasing the strength of the static stabilisers. Typically surgery has been done in open technique but arthroscopic technique is become more prevalent.

How common this pathology is in the overall population is not clear and it is much more common to have instability of the shoulder from traumatic events such as incidents which lead to shoulder dislocation. In this field there are several different classifications, TUBS stands for:

* Trauma involved in the cause

* Unidirectional instability – only in a single direction

* Bankart lesion (damage to the rim around the shoulder socket)

* Surgery – is a common requirement

A single or repetitive dislocation of a shoulder joint traumatically can lead to the generalised instability problem described in TUBS.

The instability type which is multidirectional is given the acronym AMBRI which stands for:

* Atraumatic – there was no accident or injury to explain the onset

* Multidirectional instability – laxness in all joint movements

* Bilateral – both shoulders are typically affected

* Rehabilitation – this is the initial treatment process

* I refers to the technical types of surgery and where they are performed.

The shoulder joint exhibits a high level of joint mobility to allow it to participate in placing the hand in many potential places in space, in front of the eyes so we can see what we are doing. This mobility is at the cost of stability, so the shoulder fails to be sufficiently stable under certain conditions.

Thinking about the stability of the shoulder it is helpful to concentrate on a few concepts. The idea of balance is related to the way the head of the humerus centres itself on the socket accurately. The main muscles responsible for maintaining this anatomical alignment are those of the rotator cuff, keeping the joint in line as the larger movement muscles do their actions. If an imbalance or weakness develops in the muscles of the scapula or the rotator cuff then the balance can be disturbed. A cartilage rim around the socket, the glenoid labrum, deepens the socket and the muscles compress the two parts together, enhancing stability.

An upward movement of the humeral head on the socket is undesirable and this tendency is resisted by the compressive force of the rotator cuff and by the curve of the upper socket area. The joint surfaces have some adhesion as they are wetted by the synovial fluid, with air being pressed out of the joint by the tight fit of the rounded ball and the depth of the socket, creating a degree of suction effect to enhance stability. Some amount of negative pressure which develops in a tight joint also adds to the effect. The stability which is improved by these effects is in the mid-range of the joint’s movement, where there is least stability from the ligaments.

The joint capsule acts passively to hold back excessive movement of the shoulder and keep it within safe limits, with thickened areas of the capsule developed into the ligaments of the shoulder, the most important of which is the inferior glenohumeral ligament. This does not mean that the muscles, the dynamic stabilisers of the shoulder system, are not very important in the normal function of the shoulder. Physiotherapy concentrates on strengthening and re-educating the rotator cuff and scapular stability muscles.