ABC classification of posterior shoulder instability

Abstract

Posterior glenohumeral instability (PGHI) is a commonly under- and misdiagnosed pathology owing to its variety of clinical presentations. In order to facilitate diagnosis and treatment, the simple yet comprehensive ABC classification for PGHI is based on the underlying pathomechanical principles and current standard of treatment. Three main groups of PGHI are distinguished based on the type of instability: A (first time), B (dynamic), C (static). Two subtypes further differentiate these groups in terms of their specific pathomechanism and provide a guideline in the choice of appropriate treatment: A (1, subluxation; 2, dislocation), B (1, functional; 2, structural), C (1, constitutional; 2, acquired). While conservative treatment is warranted in most patients with type 1 PGHI (A1, B1, C1), surgical treatment should be considered on an individual basis in patients with type 2 PGHI (A2, B2, C2), while keeping in mind that the different groups and subtypes can overlap, co-exist, or even progress from one to another over time. Of course the necessity for surgical treatment depends on the extent of the structural defects, on the severity of symptoms, on the chronicity, as well as on patient-specific functional demand, age, and health status. Nonetheless, the ABC classification helps to correctly diagnose the type of PGHI and provides a guideline for the generally recommended type of treatment.

Keywords: ABC Classification; Functional instability; Locked posterior dislocation; Posterior shoulder instability; Posterior subluxation.

Fig. 1

The ABC classification of posterior shoulder instability

Fig. 2

First-time posterior subluxation (A1). Magnetic resonance image of a young male patient who sustained an acute posterior shoulder subluxation with small reverse Hill–Sachs lesion and minor posterior capsulolabral damage during sports participation without major trauma

Fig. 3

First-time posterior dislocation (A2) X‑ray (a) and computed tomography image (b) of an acute locked posterior shoulder dislocation with large reverse Hill–Sachs defect as well as magnetic resonance scan (c) of a reduced acute posterior shoulder dislocation with large Hill–Sachs defect and posterior Bankart lesion

Fig. 4

Structural dynamic posterior instability (B2). Dynamic structural posterior shoulder instability with posterior bony Bankart lesion visible on magnetic resonance (MR) (a) and computed tomography (b) images. MR arthrography of a patient with posterior capsular redundancy and posterior labral damage as an example of combined structural deficiency and structural damage (c)

Fig. 5

Constitutional static posterior instability (C1). Static constitutional posterior shoulder instability with increased posterior humeral head translation, congenital convex-shaped glenoid, and increased retroversion

Fig. 6

Acquired static posterior instability (C1). Static acquired posterior shoulder instability due to chronic locked posterior shoulder dislocation with large reverse Hill–Sachs defect (a) and due to untreated posterior glenoid rim impaction fracture with de-centered humeral head and progressed degenerative changes (b)