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

Abstract

Posterior shoulder instability (PSI) is less common than anterior shoulder instability, accounting for 2-12% of total shoulder instability cases. However, a much higher frequency of PSI has been recently indicated, suggesting that PSI accounts for up to 24% of all young and active patients who are surgically treated for shoulder instability. This differentiation might be explained due to the frequent misinterpretation of vague symptoms, as PSI does not necessarily present as a recurrent posterior instability event, but often also as mere shoulder pain during exertion, limited range of motion, or even as yet asymptomatic concomitant finding. In order to optimize current treatment, it is crucial to identify the various clinical presentations and often unspecific symptoms of PSI, ascertain the causal instability mechanism, and accurately diagnose the subgroup of PSI. This review should guide the reader to correctly identify PSI, providing diagnostic criteria and treatment strategies.

Keywords: classification; pathomechanisms; posterior shoulder instability; therapy.

Figure 1

The ABC classification of PSI. Copyright © 2024 Elsevier Inc. Figure reproduced with permission from Moroder et al. 2024 (7), ‘SECEC Didier Patte Prize 2023: The ABC of Posterior Shoulder Instability.’ Journal of Shoulder and Elbow Surgery (https://doi.org/10.1016/j.jse.2023.11.019).

Figure 2

Progression from an acute posterior shoulder subluxation (type A1) to an acquired static PSI (type C2) in a middle-aged male over the course of three years. Copyright © 2024 Elsevier Inc. Figure reproduced with permission from Moroder et al. 2024 (7), ‘SECEC Didier Patte Prize 2023: The ABC of Posterior Shoulder Instability.’ Journal of Shoulder and Elbow Surgery (https://doi.org/10.1016/j.jse.2023.11.019).

Figure 3

(A) Arthro MRI of a patient with structural PSI (type B2). A typical posterior labral tear without medialization but with a small cartilage defect is illustrated. (B) In order to detect a PSI type B2, the O'Brien test can be performed with the arm in 90° forward flexion and elbow fully extended. The arm is then horizontally adducted and internally rotated, so the thumb faces downward. The examiner applies a downward force to the arm against resistance by the patient (68). Copyright © 2024 Elsevier Inc. Figure reproduced with permission from Moroder et al. 2024 (7), ‘SECEC Didier Patte Prize 2023: The ABC of Posterior Shoulder Instability.’ Journal of Shoulder and Elbow Surgery (https://doi.org/10.1016/j.jse.2023.11.019).

Figure 4

Axial posttraumatic CT (A) and MRI (B) scans after a bike accident with traumatic posterior shoulder dislocation and reposition under analgesia (type A2). (C₁, C₂) 3D anatomical reconstructions of the humeral head demonstrating a reverse Hill–Sachs defect. (D) Arthroscopic view of the retrograde disimpaction of the RHSL combined with posterior capsulolabral repair within a few days after the traumatic event.

Figure 5

Example of motion-activated neuromuscular electric stimulation with shoulder pacemaker to activate hypoactive muscle groups during concentric (A), eccentric (B), and functional (C) training (e.g. throwing motion). Figure reproduced from Moroder et al. 2020 (15), ‘Shoulder-Pacemaker Treatment Concept for Posterior Positional Functional Shoulder Instability: A Prospective Clinical Trial.’ American Journal of Sports Medicine48: 2097–2104. (https://doi.org/10.1177/0363546520933841).

Figure 6

Axial (A) and coronal (B) MRI in a patient with structural dynamic posterior shoulder instability (type B2), showing a grade IV chondral defect of the posterior glenoid as well as a posterior labral lesion. Arthroscopy confirmed the lesions (C). First, a posterior capsulolabral repair was performed (D). Then, autologous cartilage was used for an arthroscopic repair using a minced cartilage procedure (E). (AutoCart, Arthrex, Naples, FL, USA).

Figure 7

Arthroscopic views from the anterosuperior portal of a right shoulder with the patient in lateral decubitus position. (A–F) Both suture limbs of each anchor are passed through the posterior capsulolabral complex using a suture lasso and tied to form mattress stitches, reattaching and shifting the posterior capsulolabral complex anteriorly and at the same time covering the posterior cartilage defect. Figure reproduced from Moroder et al. 2022 (62), ‘Arthroscopic Posterior Articular Coverage and Shift (PACS) Procedure for Treatment of Preosteoarthritic Constitutional Static Posterior Shoulder Instability (Type C1).’ American Journal of Sports Medicine50:3617–3624. (https://doi.org/10.1177/03635465221124851).

Figure 8

Chronic locked posterior shoulder dislocation (type C2) visible on AP radiograph (A) and axial CT scan (B) with preserved elevation capacity of the affected shoulder (C, D). © 2021 Thieme. Figure reproduced from Ruttershoff et al. 2023 (65), ‘Joint Preserving Treatment of Chronic Locked Posterior Shoulder Dislocation by Means of Combined Humeral Allograft Reconstruction and Posterior Glenoid Autograft Augmentation.’ Zeitschrift für Orthopädie und Unfallchirurgie161:290–296. (https://doi.org/10.1055/a-1651-0943).

Figure 9

(A) Postoperative CT scan right after the surgery demonstrating anatomical reduction of the former chronic posterior locked humeral head after fresh-frozen femoral allograft reconstruction of the humeral defect and posterior glenoid augmentation with a harvested tricortical bone autograft from the ipsilateral iliac crest. One-year postoperative follow-up with radiographic (B) and clinical (C) evaluation. Figure modified from Ruttershoff et al. 2023 (65), ‘Joint Preserving Treatment of Chronic Locked Posterior Shoulder Dislocation by Means of Combined Humeral Allograft Reconstruction and Posterior Glenoid Autograft Augmentation.’ Zeitschrift für Orthopädie und Unfallchirurgie 161:290–296. (https://doi.org/10.1055/a-1651-0943).