Conservative treatment of acute traumatic posterior shoulder dislocations (Type A) is a viable option especially in patients with centred joint, low gamma angle, and middle or old age

Purpose Purpose of this study was to evaluate the mid- to long-term outcome after conservatively treated first-time posterior shoulder dislocations and to determine structural defects associated with failure. Methods In this multi-centric retrospective study, 29 shoulders in 28 patients with first-time acute posterior shoulder dislocation (Type A1 or A2 according to the ABC classification) and available cross-sectional imaging were included. Outcome scores as well as radiological and magnetic resonance imaging were obtained at a mean follow-up of 8.3 ± 2.7 years (minimum: 5 years). The association of structural defects with redislocation, need for secondary surgery, and inferior clinical outcomes were analysed. Results Redislocation occurred in six (21%) shoulders and nine shoulders (31%) underwent secondary surgery due to persistent symptoms. The posttraumatic posterior glenohumeral subluxation was higher in the redislocation group compared to the no redislocation group; however, statistical significance was not reached (61.9 ± 12.5% vs. 50.6 ± 6.4%). Furthermore, a higher adapted gamma angle was observed in the failed conservative treatment group versus the conservative treatment group, similarly without statistically significant difference (97.8° ± 7.2°, vs. 93.3° ± 9.7°). The adapted gamma angle was higher than 90° in all patients of failed conservative therapy and the redislocation group. An older age at the time of dislocation showed a significant correlation with better clinical outcomes (SSV: r = 0.543, p = 0.02; ROWE: r = 0.418, p = 0.035 and WOSI: r = 0.478, p = 0.045). Posterior glenohumeral subluxation after trauma correlated with a worse WOSI ( r = − 0.59, p = 0.02) and follow-up posterior glenohumeral decentring ( r = 0.68, p = 0.007). The gamma angle ( r = 0.396, p = 0.039) and depth of the reverse Hill–Sachs lesion ( r = 0.437, p = 0.023) correlated significantly with the grade of osteoarthritis at follow-up. Conclusion Conservative treatment is a viable option in patients with an acute traumatic posterior shoulder dislocation with good outcome after mid- and long-term follow-up especially in patients with centred joint, low gamma angle, and middle or old age. Level of evidence IV.