Ultrasound-Guided Intra-Articular vs Gluteal Intramuscular Corticosteroid Injection for Frozen Shoulder

Inclusion Criteria (all must be met): 1. Symptom duration ≤9 months; clinically diagnosed unilateral primary (idiopathic) frozen shoulder (including diabetes-associated frozen shoulder). 2. Age 18 to 65 years. 3. Worst shoulder pain in the past 24 hours (NRS-WP, 0-10) ≥4. 4. Passive range of motion (ROM) limitation of the affected shoulder: compared with the contralateral normal shoulder, a decrease of ≥30° in at least two of the following three directions: forward flexion, abduction, and external rotation with the arm at the side. 5. Radiographic exclusion of other shoulder pathology: Affected-shoulder anteroposterior (AP) radiograph excludes glenohumeral osteoarthritis, calcific tendinopathy, and other structural abnormalities; and axillary view and/or scapular Y (outlet) or transthoracic view excludes glenohumeral dislocation. Exclusion Criteria (any of the following): 1. Secondary frozen shoulder due to thyroid disease, cardiovascular disease, stroke, radiotherapy, prior neurosurgical or breast surgery, etc. (diabetes excluded from this item); or frozen shoulder secondary to major shoulder trauma requiring medical care (e.g., fracture, dislocation, rotator cuff tear). 2. Confirmed or highly suspected full-thickness or massive rotator cuff tear causing functional pseudoparalysis (e.g., positive Jobe test with marked strength loss). 3. Local infection of the affected shoulder or any contraindication to shoulder injection (e.g., uncorrected bleeding risk/coagulopathy). 4. Contraindication to corticosteroid therapy (e.g., poorly controlled diabetes, systemic infection). 5. Prior manipulation under anesthesia, arthroscopic capsular release, or open surgical release for frozen shoulder on the affected shoulder. 6. Any corticosteroid treatment via any route within the past 3 months (including shoulder injection). 7. Requires long-term or intermittent corticosteroid use for other conditions (e.g., autoimmune disease such as rheumatoid arthritis, acute asthma exacerbations). 8. Bilateral frozen shoulder, or contralateral history of frozen shoulder not yet fully recovered. 9. Conditions significantly affecting upper-limb function assessment or ability to perform rehabilitation (e.g., marked limb disability). 10. Pregnant or breastfeeding. 11. Lacks the cognitive ability to comply with study procedures. 12. Does not reside in the region served by the participating hospital(s) (anticipated inability to complete follow-up). 13. Compensation claims (e.g., work-related injury, traffic accident) or ongoing legal disputes/litigation.