Shoulder Instability

What is it?

Normal shoulder anatomy

Your shoulder is made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula) and your collarbone (clavicle).

The head or ball of your upper arm bone fits into a shallow socket (glenoid) in your shoulder blade. It is like a golf ball sitting on a golf tee and liable to fall off. To help prevent this, strong connective tissues form a ligament system (labrum and capsule) which keeps the head of the upper arm bone centered in the glenoid socket. Shoulder stability also relies on contraction of the strong tendons and muscles that surround the head and socket (rotator cuff).

The shoulder has the largest range of movement of any joint in the body. The ability to move the arm in all directions comes at the expense of stability. Instability occurs when the ball is forced out of its shallow socket as a result of sudden injury or due to a loose joint. Once it has dislocated it is vulnerable to recurrent episodes.

Why does it occur?

There are three main types of instability

1. Traumatic structural

This is the most common type and occurs when the shoulder is forced out of its socket due to a fall or direct impact. This often occurs as a result of injury during a rugby match, surfing or falling off a horse. The force is so great that it tears the labrum and ligaments off the socket (Bankart tear). The joint then becomes unstable and repeated dislocations are common.

Bony Bankart: When the head is driven out it can knock off the bony edge of the glenoid. This results in gross instability and recurrent dislocations are inevitable.
Hill Sachs: A dent in the back of the humeral head which occurs during dislocation as the humeral head impacts against the front of the glenoid.
Engaging Hill Sachs: If the dent occurs on the joint surface of the humeral head, it will engage with the glenoid as the arm is rotated. The missing bone will allow it to fall of the glenoid.

2. Atraumatic dislocation

This occurs when the shoulder is loose and dislocates with only a small force, such as reaching out for an object or when turning in bed. It will often clunk back in without the need to go to casualty. This type of dislocation occurs in ‘lax’ joints i.e., those who are able to bend their knees and elbows the wrong way. This joint laxity usually remains trouble free, however, if the muscles around the shoulder start to pull in one direction more than the other, it is enough for a lax joint to become unstable. Other patients have a repetitive strain to the capsule which lengthens it. The capsule in the shoulder becomes stretched, so the centralising of the head in the glenoid socket becomes less precise. Referral for appropriate physiotherapy is the initial form of management. The physiotherapist should look at posture, and the way in which the muscles and shoulder joint are moving, aiming to restore the balance. Treatment can manage the problem as long as the exercises and advice is continued, but in some cases there is only minimal benefit. For these patients, surgical intervention is indicated.

3. Muscle patterning

Muscle Patterning refers to inappropriate recruitment of the muscles of the shoulder joint e.g. Latissimus Dorsi, Pectoralis Major, Anterior or Posterior Deltoid. This results in uncontrolled translation of the humeral head and often subluxation or dislocation of the joint. This unbalanced muscle action is involuntary and ingrained. Patients with muscle patterning essentially have a muscle recruitment sequencing problem that results in abnormal force couples which destabilises the joint.

What are the symptoms?

Common symptoms of chronic shoulder instability include:

Pain caused by shoulder injury
Repeated shoulder dislocations
The persistent feeling of the ball coming out of its socket

How is it diagnosed?

A detailed history and examination is essential to determine the extent of the injury and appropriate treatment modality. X-rays and magnetic resonance imaging (MRI) can be used to confirm the suspected diagnosis.

How is it treated?

Non-surgical treatment

Prior to consideration for surgery, all non-operative measures should be exhausted. Pain relief and modification of activities may alleviate symptoms initially. Physiotherapy in the early stages will delay the onset and extent of any stiffness. Strengthening shoulder muscles and working on shoulder control can increase stability. Your physiotherapist will design a home exercise program for your shoulder (See shoulder exercises and rehabilitation – Shoulder Stabilisation PDF).

If the pain continues, steroid injections may reduce the pain caused by inflammation in the joint. This can be very effective as the hydrocortisone acts as a powerful anti-inflammatory. Following an injection, the joint should be rested for 24 hours and the sticky plaster should remain on to prevent anything from entering the injection site.

Surgical treatment

Shoulder instability and dislocation occur when the shoulder capsule is stretched or torn, and/or when the labrum is detached from the glenoid. Shoulder arthroscopy also known as keyhole surgery, is used as an alternative to making large incisions. Advanced technologies have made it possible to use 4mm (1/4″) incisions, allowing the surgeon to perform the procedure accurately and safely. These small incisions lead to less pain and scarring after surgery, which means faster recovery time and being discharged from hospital the same day.

A telescope is used with a fibre optic light source attached to a camera. The image from the camera is projected on to a high resolution monitor with great magnification. A full inspection of the joint and the bursa is performed to confirm the diagnosis and to check for any further problems. Special surgical instruments are then used through further small incisions to perform the operation (See video – SLAP lesion repair).