Shoulder Instability
What Is the Anatomy of the Shoulder That Maintains Stability?
The shoulder is a shallow ball-and-socket joint formed by the head of the humerus (upper arm bone) and the glenoid of the scapula (shoulder blade). Stability is provided by both static and dynamic structures:
Static stabilisers include the labrum (a rim of cartilage that deepens the socket) and the joint capsule/ligaments.
Dynamic stabilisers include the rotator cuff muscles and the deltoid, which work together to center the humeral head within the socket during movement.
Because the socket is shallow and allows extensive motion, the shoulder relies heavily on these soft tissue structures for stability.
How does the Shoulder Dislocate?
The shoulder can dislocate when the humeral head is forced out of the glenoid socket, most commonly during sports, falls, or sudden trauma. The majority of dislocations occur in an anterior direction (the humeral head moves forward), often when the arm is positioned in abduction and external rotation.
Less commonly, the shoulder can dislocate or become unstable posteriorly (from the back) due to repetitive trauma (such as pushing in sports), significant trauma, electrical injuries, or seizures. Inferior dislocations occur from high-energy trauma and are associated with significant neurovascular injuries. Lastly, patients with ligamentous laxity can have chronic multidirectional instability and may dislocate their shoulder voluntarily or with low-energy mechanisms (such as rolling in bed).
Once a dislocation occurs, the stabilising tissues can become stretched or the bone can wear away, increasing the risk of recurrent instability.
What structures get injured after a Shoulder Dislocation?
A dislocation may damage several structures that normally stabilise the shoulder:
The labrum can tear away from the glenoid, known as a Bankart lesion.
The capsule and ligaments may stretch or tear.
The humeral head may sustain a compression fracture on its back surface (Hill–Sachs lesion).
In some cases, bone loss occurs from the glenoid rim.
The rotator cuff or nearby nerves (especially the axillary nerve) can also be injured.
What is the long-term consequence of having recurrent Shoulder Instability?
Repeated dislocations can lead to progressive damage to the labrum, bone, and cartilage. Over time, this can result in:
Recurrent shoulder instability.
Decreased confidence or fear of movement.
Progressive glenoid bone loss.
Early-onset arthritis.
Poor shoulder function or pain even with routine activities.
How is Shoulder Instability diagnosed?
Diagnosis is based on a combination of clinical history, examination, and imaging:
History of dislocations, subluxations, or sensations of the shoulder “slipping out.”
Physical examination to assess joint laxity, muscle strength, and apprehension signs.
X-rays are used to rule out fractures and assess bone loss.
MRI is used to evaluate labral tears, capsular injury, and rotator cuff pathology.
CT scans may be used to measure bone loss in patients being considered for surgery.
What Are the Indications for Treating Shoulder Instability Non-Operatively?
Non-operative management is appropriate for patients with:
Older or low-demand patients with a low risk of recurrence.
In-season athletes who wish to avoid surgery to continue playing.
Patients preferring to avoid surgery.
Patients with multidirectional instability (voluntary dislocators) who are poor surgical candidates.
Non-operative treatment includes physiotherapy focusing on rotator cuff and scapular stabilisation, activity modification, and proprioceptive training.
What Are the Indications for Treating Shoulder Instability Operatively?
Surgery is indicated in the following situations:
Recurrent dislocations or subluxations despite rehabilitation.
First-time dislocators in young patients.
Glenoid or humeral bone loss compromising stability.
High-demand or contact athletes at risk of repeat dislocation.
Failure of non-operative management to restore confidence and stability.
Surgical options include arthroscopic labral repair, capsular plication, or bone augmentation procedures such as a Latarjet or bone graft reconstruction depending on the degree of bone loss.
