Background Secondary frozen shoulder after traumatic anterior shoulder instability is rare. The therapeutic management and clinical outcome of this condition are not well known. This study aimed to investigate the characteristics of such rare cases and verify treatment outcomes. Methods We reviewed the cases of 12 patients with secondary frozen shoulder after anterior shoulder dislocation or subluxation between April 2007 and March 2018. All patients underwent physical therapy along with an intra-articular injection. Patients with refractory stiffness received arthroscopic mobilization. The range of motion, Rowe score, and University of California, Los Angeles score were evaluated at the first and final visits. A telephone survey was performed to determine the long-term outcomes including recurrent instability, the Oxford Shoulder Score, and the Oxford Instability Score. Results The mean age of patients at the first visit was 42.5 years. Two patients underwent surgical treatment, which revealed scar-like tissue of the anteroinferior capsule. The range of motion, Rowe score, and University of California, Los Angeles score significantly improved at a mean follow-up of 15 months. At a mean follow-up of 82 months, the telephone survey revealed recurrent instability in 1 patient who was conservatively treated; the average Oxford Shoulder Score and Oxford Instability Score were 46.4 and 43.2, respectively. Conclusions The average patient age observed in this study was higher than the known peak age of traumatic anterior shoulder instability occurrence. Less activity, loss of capsule elasticity, or scarring after a capsular tear may lead to stiffness after traumatic anterior shoulder instability. Conservative treatment can be used as the first-line therapy, followed by effective arthroscopic mobilization when conservative treatment fails.
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