Complete deltoid muscle paralysis due to post-traumatic para-labral cyst, an unusual cause of quadrilateral space syndrome.

Quadrilateral space syndrome (QSS) is an uncommon cause of posterior shoulder pain and deltoid weakness, most often related to axillary nerve compression. We report a young adult male who presented with persistent pain and weakness of the right shoulder following trauma. Examination revealed deltoid atrophy and loss of abduction strength. MRI demonstrated a large inferior paralabral cyst compressing the quadrilateral space, while electromyography confirmed axillary nerve involvement. Initial management with arthroscopic labral repair and cyst decompression provided partial relief; however, persistent neurological deficit required secondary open quadrilateral space decompression. The patient achieved complete recovery of range of motion and deltoid strength, with no cyst recurrence at the 2.5-year follow-up. This case underscores the importance of considering QSS in patients with unexplained posterior shoulder pain and weakness, highlights paralabral cysts as a rare but treatable cause, and demonstrates the value of staged management and long-term follow-up in preventing chronic axillary neuropathy.