Glenohumeral instability is the tendency of the glenohumeral joint to sublux or dislocate due to loss of its normal functional or anatomical stabilizers.
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Clinical presentation
Glenohumeral instability can be divided into:
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static
lack of alignment at rest position, which can be depicted using diagnostic imaging studies
causes include chronic rotator cuff tear and severe osteoarthritis (OA)
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dynamic: lack of alignment during movement or weight-bearing; its etiology can be
traumatic (TUBS, Traumatic Unilateral dislocations with a Bankart lesion requiring Surgery): most commonly due to episode(s) of anterior joint dislocation and typically associated injuries
atraumatic (AMBRI, atraumatic, multidirectional, bilateral, rehabilitation, and occasionally requiring an inferior capsular shift): associated with increased capsular laxity, glenohumeral hypermobility and spontaneous dislocation
Glenohumeral instability can also be categorized according to the pattern of instability:
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anterior
by far the most common, accounting for up to 95% of all cases
also known as TUBS (see above)
most frequently due to prior anterior shoulder dislocation
usually results from anterior glenolabral injury, particularly from disruption of the anterior band of the inferior glenohumeral ligament (IGHL) e.g. Bankart lesion
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posterior
rare
also most frequently due to posterior shoulder dislocation
usually results from posterior glenolabral injury, particularly from disruption of the posterior band of the inferior glenohumeral ligament (IGHL) e.g. reverse Bankart lesion, or disruption of the posterior labrum and/or glenoid rim e.g. Bennett lesion
associated with glenoid dysplasia and retroversion
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multidirectional
also known as AMBRI (see above)
usually not due to previous dislocation, but rather congenital joint capsule laxity
often bilateral
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superior
usually associated with multidirectional
As a result of this greater mobility, a number of secondary changes may become evident, including:
subacromial spur formation
hypertrophy of the greater tuberosity
coracoacromial ligament hypertrophy
These changes, in turn, may lead to shoulder impingement.
Risk factors
previous traumatic dislocation (especially anterior shoulder dislocation)
athletes (throwing, swimming, tennis)
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congenital
medial anterior shoulder capsular insertion (type III)
absent or small glenohumeral ligaments
congenital laxity of capsule or ligaments
Radiographic features
Plain radiograph
Typical bone injuries may be visible, especially in case of anterior instability, but often the radiographs look normal.
CT
CT is superior in visualizing bony injuries of the humeral head and glenoid rim in case of traumatic instability. In atraumatic instability, the findings are often non-specific.
MRI
MRI and arthrographic studies are very accurate in showing chondral and labral injuries (such as Bankart lesion, ALPSA, GLAD and HAGL, as well as their counterparts in posterior instability). Visualization of the Perthes lesion may be improved by the use of the ABER position. In multidirectional instability, a circumferential labral tear is often present.
Treatment and prognosis
In general, both anterior and posterior instability requires surgical repair and strengthening of the capsule.
Multi-directional instability is usually treated conservatively with rotator cuff strengthening exercises 2.