Posterior Shoulder Instability

Posterior Shoulder Instability

Relatively rare incidence, reported to be 2-4%

Pathoanatomy

Static stabilizers:
Studied in capsular cutting experiments
Schwartz et al., 1988 – posterior inferior capsule is the primary posterior restraint, with anterior superior capsule and superior glenohumeral ligaments providing secondary restraint.
Warren et al. 1984 – posterior dislocation occurred only after the anterior superior capsule was incised
Harryman et al., 1992 – posterior subluxation is resisted by an intact rotator interval capsule.
Intra-articular negative pressure

Dynamic stabilizers:
It is assumed that the rotator cuff will act to stabilize the humeral head in flexion, adduction, and internal rotation
Blasier et al, 1997 – all cuff muscles contributed to posterior stability, but the subscapularis was most significant.

– unlike anterior dislocators, capsular detachment is uncommon – 10% (Bigliani, 1995)

Hottya et al., AJR, Sept 1998
MR evaluation of 4 acute posterior dislocations
found posterior capsular disruption, partial tears of teres minor, labral tears and fraying

Classification Noble, Morin in Shoulder Injuries in the Athlete, Hawkins ed. 1996

Acute posterior dislocation
With impression defect in humeral head
Without impression defect in humeral head (rare)

Chronic posterior dislocation
Locked (missed) with impression defect

Recurrent posterior subluxation
Voluntary
Habitual (willful)
Muscular control (not willful)

Involuntary
Positional (demonstrable)
Nonpositional (not demonstrable).

Acute Traumatic Posterior Dislocation

extremely rare to not have an impression defect in the humeral head
indirect violent trauma, including seizures, electrical shock, MVA, or a posteriorly directed blow on the forward, outstretched extremity with the arm in flexion, adduction, and internal rotation.
both acute posterior subluxations and dislocations reduce spontaneously, making it difficult to distinguish the two.
the arm is held in fixed adduction and internal rotation, with no external rotation ability.
the humeral head may be palpable posteriorly, with an empty glenoid fossa.
treatment is by closed reduction – performed by flexion and adduction with longitudinal and lateral traction
no real consensus on how long it should be immobilized for, if at all
recurrence rate is felt to be uncommon

Chronic Posterior Dislocation (Locked/Missed)

a posterior dislocation in the presence of an impression defect with the humeral head remaining subluxed posteriorly
if presenting late (missed), pain will be diminished, but functional deficit will be the chief complaint, particularly the inability to externally rotate the arm. (may lead to the diagnosis of “frozen shoulder”)
the key to diagnosis is the internal rotation deformity
treatment depends on a variety of factors: duration of dislocation, size of impression defect, presence of changes in the glenoid,

Size of Defect Duration of Dislocation Treatment

20% 50% 50% > 6 months Hemiarthroplasty (0o retroversion)

Subscapularis transfer – McLaughlin procedure, Neer & Foster modification

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