Instability

♦ The fundamental principle or essence of the shoulder is concavity compression. Stability of the shoulder is the condition in which a balanced centralizing joint reaction force (CJRF) exists to maintain concavity compression of the glenohumeral joint whatever the position of the limb and hand.♦ Instability is a symptom. It can be defined as the condition of symptomatic abnormal motion of the joint. It refers to a perturbation of concavity compression. It is not a diagnosis.♦ Instability is the result of perturbations of structural factors and non-structural factors.♦ The clinical syndrome of instability is a disturbance of one or more of these factors in isolation or together. The relative importance of each factor to the syndrome can change over time. The relationship between these factors is described by the Stanmore triangle.♦ Both structural and non-structural factors can be perturbed by arrested or incomplete development (dysplasia) or by injury (disruption).♦ The aim of treatment is the restoration of (asymptomatic) stable motion by restoration of the CJRF and so restoration of the condition of concavity compression.♦ Management follows simple principles: surgery should be undertaken within the context of a well-considered rehabilitation program largely centred around optimizing rotator cuff function.♦ Failures of management are often due to failure of or incomplete diagnosis, failure of healing, inadequate attention to patient- and pathology- specific rehabilitation programs, or insufficient attention to lifestyle considerations.♦ Disrupted anatomy is restored, preferably by anatomic operations with predictably good outcomes. Dysplastic anatomy is augmented, often by non-anatomic operations with less predictable outcomes. Revision stabilizations are generally nonanatomic, and have higher failure rates.