HISTORY: 16-year-old male presenting with left thumb injury. Patient presented due to thumb injury during basketball 5 weeks prior to presentation. He reported a basketball was thrown extremely hard and hit him on the lateral side of the thumb, pushing his thumb towards his 5th digit. Despite pain, he was able to finish the game. He had noticeable swelling when the game ended. Despite icing, he continued to have swelling in addition to pain with pinching items or texting. This led him to the urgent care 1 week prior to presentation. X-rays were obtained at the urgent care and he was sent to sports medicine clinic. PHYSICAL EXAMINATION: Examination of left hand showed noticeable deformity at MCPJ, with proximal phalanx appearing subluxed on MC. Flexible deformity on radial aspect of the thumb MCP joint. Tenderness to palpation over the radial side of the MCP joint where there is a prominence. No swelling or bruising. Full range of motion of the thumb without limitation. Strength 4/5 with thumb flexion, extension, abduction, adduction. Pain with thumb manual muscle strength testing. Stable to UCL stress at 0 and 30 degrees. RCL laxity at 0 and 30 degrees without clear endpoint. Normal thumbs up and okay sign. Normal thumb to pinky opposition. Normal grip. Neurovascularly intact. DIFFERENTIAL DIAGNOSIS: 1.Radial collateral ligament tear 2.Flexor pollicis brevis tear 3.1st proximal phalanx fracture 4. Sesamoid fracture TESTS AND RESULTS: XR Left Hand Impression: No fractures identified. Subluxation of thumb at MCP joint. MRI Left Hand Impression: Complete disruption of radial collateral ligament of the MCP joint of the thumb with ulnar subluxation of the phalanx compared to the metacarpal. Significant joint fluid. Ulnar deviation of the flexor policis longus and sesamoids. Some marrow edema of the distal metacarpal as well as subchondral bone of first phalanx. FINAL/WORKING DIAGNOSIS: Chronic radial collateral ligament rupture at metacarpal level TREATMENT AND OUTCOMES: 1.Surgical radial collateral ligament repair 2.Initially immobilized with thumb spica splint 3.Casted from 1-5 weeks post operatively 4.Out of cast at 5 weeks post op out; into OT formed hand-splint and range of motion exercises started 5.Follow up planned 9 weeks post-op. Work with OT to achieve full, painless ROM and normal strength prior to returning to basketball