Diagnosis and treatment of posterior shoulder instability based on the ABC classification

Key points

• Posterior shoulder instability (PSI) accounts for 2–12% of shoulder instability cases, but recent data suggest up to 24% among young, active patients treated surgically; it often presents with vague symptoms rather than clear dislocations.
• The ABC classification distinguishes first-time (A1 subluxation, A2 dislocation), dynamic (B1 functional, B2 structural) and static PSI (C1 constitutional, C2 acquired).
• Non-operative treatment is recommended in A1, B1 and C1, while surgery may be considered in A2, B2 and C2 depending on individual characteristics and functional demand.
• Diagnosis rests on history and targeted examination completed by MRI; CT is used to quantify extensive bone loss.
• For functional instability (B1), a shoulder pacemaker approach, with motion-triggered neuromuscular electrical stimulation during physiotherapy, gave better results than physiotherapy alone in a randomized trial.

Overview

This EFORT Open Reviews instructional lecture reviews the diagnosis and treatment of PSI using the ABC classification developed by the authors' group, which sorts patients by the nature of the instability and its mechanism. The authors argue that PSI is underdiagnosed because it may appear only as pain on exertion, limited motion or an incidental finding. It is a review, not a graded guideline.

First-time instability (group A)

Group A covers a single event less than three months earlier. Acute first-time traumatic PSI has an incidence of 1.1 per 100,000 per year, almost 20 times lower than anterior instability, with peaks at 20–49 years and above 70 years. A1 subluxations are treated conservatively in most cases; young age, posterior decentering and a higher gamma angle are risk factors for recurrence. In A2, a posterior fracture-dislocation should be reduced openly and fixed, mostly with plates; a reverse Hill-Sachs lesion with a gamma angle above 90° should be addressed surgically, if possible within a few days, and after more than two weeks options include the McLaughlin procedure, lesser tuberosity transposition or allograft filling.

Dynamic instability (group B)

B1 functional instability, caused by abnormal muscle activation, affects up to 3% of a young, active population, mostly female teenagers and young adults; physiotherapy is recommended and surgery only as a last resort because results are unpredictable. A flattened or convex glenoid was found in 60% of cases and is linked to poorer non-surgical results. B2 structural instability, mostly in young men after trauma or repetitive microtrauma, responds well to arthroscopic posterior labral repair, but posterior glenoid bone loss above 11% may require bone grafting.

Static instability (group C) and diagnosis

C1 constitutional instability is often found incidentally; arthroscopic posterior articular coverage and shift (PACS), posterior open-wedge osteotomy and bone block procedures improve symptoms but none reliably recenters the joint. C2 acquired instability affects about 7% of brachial plexus birth lesions and may follow seizures, electrocution or major trauma; treatment aims to restore the joint surfaces, with arthroplasty in advanced osteoarthritis. The O'Brien and forced internal rotation tests are highly sensitive, the show-me test suits B1, and the Jerk and Kim tests assess B2; radiographs should include true anterior-posterior, axillary and scapular Y views, and MRI is the gold standard for soft tissue injuries.

Frequently asked questions

Why is posterior instability often missed?

It frequently presents as pain on exertion, limited range of motion or an asymptomatic finding rather than as recurrent dislocation.

Is surgery appropriate for functional instability?

Only as a last resort; outcomes are unpredictable and may worsen symptoms, so physiotherapy, including the shoulder pacemaker concept, comes first.

When should a reverse Hill-Sachs lesion be treated surgically?

The review advises surgery when the gamma angle is above 90°, to reduce the risk of recurrent instability.

Source

Paksoy A, Akgün D, Lappen S, Moroder P. Diagnosis and treatment of posterior shoulder instability based on the ABC classification. EFORT Open Rev 2024;9(5):403-412. doi:10.1530/EOR-24-0025. Open access. Summary prepared by Medpresso from the original publication; it is not a substitute for the full text or for medical advice.