The shoulder is the most mobile joint in the body and as such, a complex arrangement of structures is required to stabilise the shoulder during movement. Unfortunately, these structures can be prone to injury and this can influence the stability of the shoulder.
The glenoid (shoulder socket) is a very flat cup and as such the shoulder is inherently unstable compared to the hip joint, where the socket fully envelops the ball of the hip joint. To deepen the shoulder socket, the glenoid has a rim of fibrous cartilage all the way around its periphery. This helps provide some stability but it is further reinforced with the aid of the shoulder capsule. This sac like pouch joins to the labrum and helps contain the shoulder on to its socket. To further complicate matters, there are also condensations of tissue within the capsule that make up the ligaments of the shoulder. It is the continuity of these ligaments that help maintain shoulder stability. However, as the ligaments are integral to the capsule, any injury to the capsule or to the labrum, can result in shoulder instability.
The classification of shoulder instability is complicated however The Stanmore Classification system developed at the Royal National Orthopaedic Hospital is well recognised as the best way to describe the types of instability that can develop.
Type I; this is usually caused by a traumatic event that affects what was fundamentally a normal shoulder. In such an injury, the labrum of the glenoid can become detached and when it does, the tension in the associated capsule is lost thereby allowing the shoulder to dislocate. This is the typical type of dislocation as might occur in a rugby player.
This type of injury can result in a number of injuries.
- Labral tear; if the labrum is injured a variety of injuries can occur. A minimal labral tear is termed a Perthe’s Lesion, however if the labrum is more significantly detached, it is referred to as a Bankart Tear. If the tear also results in an associated fracture to the anterior glenoid it is referred to as a Bony Bankart Lesion. Furthermore, when a labral tear also strips off the thick lining to the bony neck of the glenoid, (socket) it is referred to as a ALPSA lesion (Anterior Labral Periosteal Sleeve Avulsion).
- Hill-Sachs lesion; when the humeral head dislocates it can be damaged when it becomes ‘impaled’ on the rim of the bony glenoid. In such circumstances the shoulder is usually locked in a dislocated position and a hospital reduction is often necessary. As well as damaging the labrum, his type of injury can also cause damage to the glenoid rim. This is termed a Bony Bankart lesion. The damage to the humeral head is referred to as a Hill Sachs defect. If either of these bony injuries reaches a certain size, they may be implicated in further dislocations.
- HAGL lesion (Humeral Avulsion of Glenohumeral Ligament). This is where the ligaments that help support the humeral head are injured. Typically the injuries sustained in a dislocation affect those structures that are in close proximity to the shoulder joint and glenoid. However, a HAGL lesion typically results in injuries to the capsule where it attaches to the humeral neck.
Type II; this type of instability can occur following a traumatic event however the shoulder is usually more prone to dislocating as the tension in the capsulo-labral tissues is often much less than in a typical shoulder. Although stability can usually be maintained, if the normal restraining forces in the shoulder are compromised, a dislocation can occur. This type of injury typically affects gymnasts or swimmers. Although this type of dislocation can be associated with capsuolo-labral injuries, the extent of any soft tissue injury is usually less than in a Type I injury and most dislocations often relocate spontaneously or with minimal effort.
Type III; this is a more unusual type of dislocation where the muscles that support the shoulder overly influence the stability of the shoulder. In such circumstances the muscles of the shoulder can push or pull the shoulder out of joint. This is called ‘muscle patterning’ and although the instability can often start as a “party trick”, it can then develop into an uncontrolled mechanism of shoulder dislocation. Such a dislocation does not normally result in any damage to the shoulder and is usually not managed surgically.
- Subluxation; can occur when a shoulder partially dislocates, but then spontaneously relocates. This can result in a painless or painful clunk.
- Dislocation; the shoulder completely dislocates and remains in a dislocated position for a variable amount of time. This often results in pain, although pain may become less after multiple dislocations.
- Apprehension; one can often sense when the shoulder is about to dislocate. This an result in apprehension such that the arm is used in more limited way for fear of dislocating the shoulder.
- Pain; pain can be caused by the event of a subluxation or dislocation or as a result of the damage that occurs as a result of the instability. If the surface of the humeral head (ball) or of the glenoid (socket) is damaged, then it can cause pain in its own right. Minor instability can also result in too much movement of the ball across the socket, which in turn can result in shoulder impingement pain.
- Degenerative change; if the cartilage surface of the glenoid (socket) or the humeral head (ball) have been damaged as a result of the instability, then degenerative changes to the joint can occur. This is referred to as instability arthropathy.
- Weakness; Following recurrent dislocations the shoulder can feel weak as it is not possible for the unstable joint to fully optimise the surrounding musculature. However, rotator cuff tendon tears can also occur following dislocations. This commonly occurs in the older population groups (>30yrs) and can be missed if not specifically looked for at the time of assessment.
- Nerve injury; In some dislocations, particularly involving high energy accidents, or when the shoulder has been dislocated for a long period of time, the nerves around the shoulder can be injured. The axillary nerve, which is essential for normal shoulder function, is most commonly affected. Nerve injuries can also be a significant source of pain.
Causes & Risk factors
There are numerous causes and risk factors for recurrent shoulder instability. The more common ones include;
- Hypermobility or collagen disorders such as Ehlers –Danlos Syndrome
- Abnormal orientation (version) of the glenoid (socket)
- Lax ligaments or a very lax capsule
- Contact sports
- Repetitive overhead sports or activities; swimming/gymnastics.
- Falling onto an outstretched hand which is forced upwards and outwards
- Age of first dislocation; the younger you are the more likely it will recur
The diagnosis of shoulder instability is made clinically following a history and examination.
Imaging can also be used to help determine if there is any damage to the shoulder that will predispose the shoulder to recurrent instability. It is also helpful when planning and surgical reconstruction.
- X-rays; can be used to look for bony damage to the glenoid or humeral head. These would include Hill-Sachs defects and bony Bankart lesions.
- MR arthrogram; is ideal for looking at the integrity of the capsulo-labral tissues. Just before the MRI is undertaken, contrast is injected into the shoulder which then highlights the soft tissue injuries.
- CT / CT arthrogram; CT scans are particularly helpful when considering cases of instability where associated bony injuries have been sustained.
There are a number of treatments available for recurrent instability, however the treatment has to be specific for the nature of the instability. The options include;
- Physiotherapy; exercises to improve core strength, the shoulder blade position as well as the strength of the rotator cuff are all important in optimising the muscles that aid shoulder stability. Physiotherapy has an essential role in Types II and III instability and may be the sole treatment necessary to manage the instability in the long term.
- Arthroscopic shoulder stabilisation; if physiotherapy has failed to address the stability or if the there is an underlying capsulo-labral injury, reconstructive surgery may be necessary. This is usually the case in Type I injuries and in those Type II injuries that have failed to respond to physiotherapy treatment. An arthroscopic (key-hole) procedure can be undertaken during which the damaged capsulo-labral structures are reconstructed such that they are re-attched to the glenoid, from which they have become detached. If the capsulo-labral structures have remained intact but instead, have been stretched, they can be tightened / plicated to improve the stability. An arthroscopic stabilisation can be performed as a ‘key-hole’ procedure through 3 or 4 small incisions. A sling or brace is used to support the shoulder for 4-6 weeks thereafter.
- Bone augmentation procures. In cases where significant bony damage has occurred to either the humeral head, the glenoid or both, an operation aimed at reconstructing the bony injury might be needed. There are a number of such operations and include; the Laterjet procedure., the Edin-Hibernate procedure and the bony reconstruction of a deficient glenoid using distal tibial allograft. The indications for each of these operations are varied and each operation is tailored to the structural injury of the patient.