Text

Objectives and questions: Instability remains a known complication after reverse shoulder arthroplasty (RSA), particularly with the traditional medialized–distalized (Grammont) design. Modern design modifications incorporating lateralization aim to reduce instability. This study investigated the incidence and patterns of postoperative instability after primary RSA.

Material and methods: All patients undergoing primary RSA between 2006 and 2023 were identified from a single-institution registry. Prosthetic designs were classified as medialized–distalized (MD), lateralized (L), or lateralized–distalized (LD). Dislocations, their timing, direction, and treatment were analyzed.

Results: Among 2,080 primary RSAs, 18 instability events occurred in 17 patients (overall incidence <1%). Instability was most frequent with the MD design (1.4%), lower with the L design (0.7%), and absent with the LD design. Dislocations followed a time-dependent pattern: early dislocations occurred predominantly anteriorly and were related to insufficient soft-tissue tension; intermediate dislocations were inferior and associated with secondary osseous events (i.e. acromion/spine fractures etc.); late dislocations were posterior and frequently accompanied by advanced scapular notching. Early dislocations occurred up to 2.5 years postoperatively (n = 7). Three patients underwent immediate revision surgery and achieved stability, while conservative treatment resulted in persistent instability in two out of four patients. Intermediate dislocations (n = 6) occurred between 2.5 and 8.5 years postoperatively. All patients reported a preceding adverse event, such as acromial fractures (n = 2), scapular spine fractures (n = 3), or traumatic rupture of the posterior deltoid (n = 1), occurring on average 2–3 years prior to dislocation (range: 2 months to 6 years postoperatively). Here stability was achieved after conservative treatment after acromial fractures and if the spina fracture had healed successfully after ORIF and increase in glenosphere diameter (n=1). Late dislocations (n = 4) occurred at 8.5 – 13 years postoperatively. One patient was stable after revision, whereas one remained unstable after conversion to hemiarthroplasty. Both others were lost to follow-up shortly after dislocation.

Discussion and conclusions: Postoperative instability after primary RSA is rare and strongly influenced by implant design and time from surgery. Modern lateralized and lateralized–distalized designs demonstrate lower instability rates than the classical Grammont design. Distinct temporal dislocation patterns suggest different underlying mechanisms, with soft-tissue insufficiency, secondary osseous compromise, and polyethylene wear as likely contributors. These findings support the continued use of contemporary RSA designs to reduce instability risk.