Posterior Shoulder Instability

Posterior instability accounts for 2-5% of shoulder dislocations, and it matters out of proportion to that because most acute posterior dislocations are missed at first presentation. The figure usually quoted, 50-79%, is a range drawn from older case series rather than a single prospective study or one validated number, so the safe exam answer is "the majority", with the range offered if pressed.

Why it is missed. The AP film can look almost normal, because the humeral head is still roughly concentric in that projection. Suspect a posterior dislocation after a seizure or electrocution, or whenever the shoulder will not externally rotate, and always get an axillary or modified axial view.

Who. The rate is higher in patients with epilepsy, and after a seizure the dislocation is often bilateral. Athletes are the other group: overhead athletes, backhanders and linemen. Males are affected more often than females.

Mechanism. Four routes lead to a posterior shoulder:

• Seizure or electrocution - massive muscle contraction, in which the internal rotators (subscapularis, pectoralis, latissimus dorsi) overpower the external rotators
• Trauma - a posterior force with the arm in flexion, adduction and internal rotation, the FAIR position of risk
• Sport - repetitive microtrauma in the overhead athlete
• Voluntary - with a psychological component

The mirror of anterior instability. Each feature of posterior instability has an anterior counterpart.

The posterior restraints. The posterior glenoid rim gives bony stability, and the fibrocartilaginous posterior labrum deepens the socket. The capsular restraints are the posterior capsule and the posterior band of the IGHL, with infraspinatus and teres minor, the external rotators, as the muscular stabilisers. The posterior capsule is thinner than the anterior, so its stabilisation is less robust.

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Why the FAIR position is the position of risk. Flexion, adduction and internal rotation place the humeral head against the posterior capsule and labrum.

The lesions. The posterior labral tear, the reverse Bankart, is the key lesion for surgical stabilisation. On the humeral side the McLaughlin lesion, or reverse Hill-Sachs, is an impaction fracture of the anteromedial humeral head, and it can engage on the posterior glenoid. A reverse bony Bankart adds posterior glenoid bone loss, and recurrent instability stretches the posterior capsule into laxity.

Not every posterior instability is a reverse Bankart. Recognising the full spectrum matters because the site of the lesion dictates where the repair must be placed: a glenoid-sided labral repair will fail if the true lesion is on the humeral side.

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Posterior instability is classified three ways: by the type of instability, by the size of the humeral head defect, and by how long the shoulder has been out.

By type. The type sets the treatment approach.

The Stanmore polar framework (structural / muscular-patterning / atraumatic) increasingly guides whether to operate, replacing the rigid traumatic-versus-voluntary dichotomy.

By size of the McLaughlin lesion. Defect size dictates treatment strategy:

• Small, under 20% - stable after reduction, with a good prognosis; may do well without specific treatment
• Moderate, 20-40% - may engage
• Large, over 40% - high risk of engaging and re-dislocation

The threshold for engagement is also quoted as greater than 25-40% of the articular arc, at which the defect may engage and lock on the posterior glenoid.

By chronicity. Chronic dislocations have worse outcomes.

• Acute - within 6 weeks; closed reduction may be attempted
• Chronic locked - more than 6 weeks; usually needs open surgery
• Recurrent - repeated episodes; surgical stabilisation, after a failed rehabilitation trial if the instability is atraumatic

History. Ask about the mechanism: a seizure, an electrocution, or trauma in the FAIR position. The pain is posterior, the patient cannot externally rotate, and there may have been previous episodes. A voluntary dislocator can demonstrate the instability.

Examination. The arm is held adducted and in internal rotation. Blocked external rotation is the key finding, and inability to externally rotate should raise suspicion of a locked posterior dislocation. The jerk and Kim tests are specific for posterior labral lesions:

• Jerk test - patient supine, arm at 90 degrees of flexion and internal rotation; the examiner applies an axial load and adducts the arm across the body. Posterior subluxation followed by a clunk as it reduces when the arm is returned to neutral is positive for a posterior labral tear
• Kim test - patient seated, arm at 90 degrees of abduction; a posterior force is applied with elevation, and posterior subluxation is positive
• Posterior apprehension - loading in the FAIR position

Radiographs. Take an AP, an axillary and a scapular Y view. The axillary view is essential and diagnostic, showing the posterior position of the head that the AP can hide.

Two further signs:

• Rim sign - increased space between the humeral head and the glenoid
• Through sign - overlap of head and glenoid on the Y view

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CT. CT characterises bone loss and quantifies the McLaughlin lesion, and it is essential for planning bone grafting or arthroplasty. It gives three things:

• Posterior glenoid bone loss as a percentage
• McLaughlin lesion size as a percentage of the arc
• 3D reconstructions for surgical planning

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MRI. MRI shows the posterior labral tear (reverse Bankart), posterior capsular stretching or disruption, and the McLaughlin lesion as marrow oedema or a defect. It also picks up associated rotator cuff injuries, especially in older patients. MR arthrography enhances labral visualisation.

The acute dislocation. Four steps, in order:

1. Diagnose - high suspicion with a seizure or electrocution; get the axillary view and check whether the dislocation is locked
2. Size the defect - CT to assess the humeral head defect
3. Closed reduction - traction and gentle external rotation, which may succeed if the dislocation is less than 6 weeks old
4. Immobilise - brace in neutral or external rotation, avoiding internal rotation

Neutral versus external-rotation bracing after reduction or repair lacks robust evidence; the fixed point is keeping the arm out of internal rotation.

Recurrent instability. Atraumatic instability is treated conservatively first, with physical therapy, rotator cuff and scapular strengthening, and activity modification. If that fails, surgery addresses each lesion present: posterior labral (reverse Bankart) repair, posterior capsulorrhaphy, and any bone loss. Voluntary dislocators are the exception: surgical outcomes are poor, so surgery is avoided and psychological counselling is part of their care.

The McLaughlin lesion. The key is to fill or address the engaging defect, and its size decides how:

• Under 20% - conservative, or soft-tissue treatment only
• 20-40% - the McLaughlin procedure, which transfers subscapularis into the defect, or an osteochondral allograft
• Over 40% - osteochondral allograft or hemiarthroplasty

Arthroscopic posterior labral repair. The patient is in the beach chair or lateral decubitus position, with a standard posterior viewing portal, an anterior portal and a posterolateral working portal. Capsular plication is added for laxity.

1. Mobilise the labrum from the glenoid
2. Prepare a bleeding bone bed on the glenoid rim
3. Place 2-4 suture anchors on the glenoid face
4. Shuttle the sutures through the labrum
5. Tie the knots to reduce the labrum to the glenoid

Posterior capsulorrhaphy. Indicated for capsular laxity or a patulous capsule, and often combined with labral repair. It can be done arthroscopically or open, shifting the redundant capsule superiorly with plication stitches through the capsule. Thermal capsular shrinkage has been abandoned because of chondrolysis and capsular necrosis, and is mentioned in a viva only to be rejected.

Posterior glenoid bone loss. Loss greater than 25-30% requires bone grafting. The exact loss at which soft-tissue repair fails is less certain: it is often quoted as 20-25%, is not well defined, and remains debated. Posterior bone grafting is less common than anterior bone loss surgery, and the graft options are:

• Iliac crest bone graft
• Distal clavicle autograft
• Fresh frozen allograft

The graft is placed through an open posterior approach and fixed to the glenoid with screws. Posterior bone-block techniques are less standardised than the anterior Latarjet, with no high-level comparative data on open versus arthroscopic blocks.

Posterior bone loss (a reverse bony Bankart) is addressed by a bone block. A separate structural problem is excessive glenoid retroversion or posteroinferior dysplasia / hypoplasia, which biomechanically directs the head posteriorly and is a recognised cause of soft-tissue repair failure.

Measuring version. Version is measured on axial CT, commonly by the Friedman method, which uses a line from the medial scapular border to the glenoid centre as the reference axis. The normal glenoid is slightly retroverted, around 2 to 7 degrees. Pathological retroversion is generally taken as more than about 10 degrees, and it both causes instability and predicts failure of an isolated labral repair.

Dysplasia. Posteroinferior glenoid dysplasia, a deficient, rounded or hypoplastic ("lazy-J") posteroinferior glenoid, is a developmental contributor identified on CT or MRI.

Posterior glenoid (opening-wedge) osteotomy. It is reserved for instability driven by excessive retroversion, often quoted as more than about 10 to 15 degrees, where soft-tissue work alone is likely to fail. It is technically demanding with a historically high complication rate, so it is used selectively and in specialist centres:

• Graft-site fracture
• Avascular necrosis
• Intra-articular penetration
• Over-correction producing anterior subluxation

Recurrence after posterior stabilisation runs at 5-15% in experienced hands. It is worse with voluntary instability, bone loss and unrecognised pathology.

The FAIR position stresses the posterior repair, so it is avoided during early rehabilitation and range of motion is progressed to full gradually.

By type. Traumatic recurrent instability has good outcomes with surgery, with 85-90% stability. Atraumatic instability may respond to physiotherapy, with surgery if it fails. A large McLaughlin lesion carries a worse prognosis and may need arthroplasty if it engages.

Throwing athletes. They consistently return to their pre-injury level less often, so counselling and graded return-to-throw protocols are emphasised.