ObjectiveTo compare the mid-term effectiveness of arthroscopic shoulder capsular release combined with acromiohumeral distance (AHD) restoration in the treatment of diabetic secondary stiff shoulder and primary frozen shoulder. Methods A retrospective analysis was conducted on clinical data of 22 patients with diabetic secondary stiff shoulder (group A) and 33 patients with primary frozen shoulder (group B), who underwent arthroscopic 270° capsular release combined with AHD restoration treatment. There was no significant difference between the two groups in gender, age, affected side, disease duration, and preoperative AHD, shoulder flexion range of motion, abduction range of motion, American Shoulder and Elbow Surgeons (ASES) score, visual analogue scale (VAS) score, and Constant score (P>0.05). Only the difference in the internal rotation cone rank and external rotation range of motion between the two groups showed significant differences (P<0.05). The improvement in shoulder pain and function was evaluated by using VAS score, ASES score, and Constant score before operation and at last follow-up. Active flexion, abduction, external rotation range of motion, and internal rotation cone rank were recorded and compared. AHD was measured on X-ray films.Results All patients were followed up 24-92 months (median, 57 months). There was no significant difference in follow-up time between group A and group B (P>0.05). No fractures or glenoid labrum tears occurred during operation, all incisions healed by first intention, and no complication such as wound infection or nerve injury was observed during the follow-up. At last follow-up, there were significant improvements in active flexion, abduction, external rotation range of motion, internal rotation cone rank, AHD, VAS score, ASES score, and Constant score when compared with preoperative ones in both groups (P<0.05). Except for the difference in change in external rotation range of motion, which had significant difference between the two groups (P<0.05), there was no significant difference in other indicators between the two groups (P>0.05). ConclusionArthroscopic capsular release combined with AHD restoration can achieve good mid-term effectiveness in the treatment of diabetic secondary stiff shoulder and primary frozen shoulder. However, the improvement in external rotation range of motion is more significant in the patients with diabetic secondary stiff shoulder.