Shoulder Instability

Shoulder instability is abnormal translation of the humeral head on the glenoid, ranging from subluxation to frank dislocation, and is either traumatic or atraumatic.

Most dislocations are traumatic, and around 97% of those are anterior. Incidence peaks in young men aged 15 to 29, with a second peak in older adults after low-energy falls. The risk of recurrence falls as age at first dislocation rises, and almost 90% of recurrences happen within 2 years.

The glenohumeral joint trades bony congruity for mobility. The static stabilisers are the labrum, capsule and glenohumeral ligaments; the inferior glenohumeral ligament complex is the main restraint to anterior translation in abduction and external rotation. The rotator cuff and scapular muscles are the dynamic stabilisers.

Classification:

• •TUBS (traumatic, unidirectional, Bankart lesion, surgery often needed) and AMBRI (atraumatic, multidirectional, bilateral, rehabilitation, inferior capsular shift if surgery) are the older model
• •The Stanmore (polar) classification: Type I traumatic structural, Type II atraumatic structural (capsular laxity, often with hypermobility), Type III muscle patterning (non-structural). Many patients sit between types

Lesions after traumatic anterior dislocation:

• •Bankart lesion: avulsion of the anteroinferior labrum; variants include a bony Bankart (glenoid rim fracture) and ALPSA (anterior labroligamentous periosteal sleeve avulsion, healed medially)
• •Hill-Sachs lesion: impaction fracture of the posterosuperolateral humeral head
• •HAGL (humeral avulsion of the glenohumeral ligament): easily missed if only the labrum is assessed
• •Rotator cuff tear: increasingly common with age; around 40% of people over 40 have one after a traumatic anterior dislocation

Atraumatic instability reflects capsular laxity, often with generalised hypermobility (Beighton score), or abnormal muscle patterning, usually without a discrete labral tear. Posterior instability is uncommon and classically follows a seizure, electric shock or high-energy trauma.