Carpal instability refers to abnormal kinematics of the carpal bones due to injury of the intrinsic (intercarpal) or extrinsic (radiocarpal/ulnocarpal) ligaments. Mayfield classification of perilunate injuries describes progressive ligamentous failure from radial to ulnar: stage 1 (scapholunate ligament), stage 2 (lunocapitate), stage 3 (lunotriquetral), and stage 4 (perilunate dislocation with lunate volar displacement).
Specific patterns include: scapholunate (SL) instability — most common, from fall on outstretched hand (FOOSH), causing dorsal intercalated segment instability (DISI) with scaphoid flexion and lunate extension on lateral radiograph; lunotriquetral (LT) instability causing volar intercalated segment instability (VISI); midcarpal instability from extrinsic capsular ligaments; ulnar-sided wrist pain from triangular fibrocartilage complex (TFCC) tears with dynamic distal radioulnar joint (DRUJ) instability.
Diagnosis combines history (FOOSH mechanism), physical examination (Watson scaphoid shift test for SL, ballottement for LT), specific radiographs (scapholunate gap >3 mm 'Terry Thomas sign', cortical ring sign of scaphoid, dorsal intercalated segment instability), MRI for ligament integrity, and arthroscopy as gold standard. Untreated SL instability progresses through scapholunate advanced collapse (SLAC wrist) over years. Treatment depends on chronicity and reducibility: acute (<6 weeks) repair with bone anchors, subacute repair plus capsulodesis, chronic reconstruction (Brunelli, modified Brunelli, RASL), and salvage procedures (proximal row carpectomy, four-corner fusion, total wrist arthrodesis) for advanced arthritis.