summary TKA Metal Hypersensitivity is a complication of TKA that may lead to persistent knee pain and stiffness as a result of an allergic reaction to the metallic components. Diagnosis involves careful patient history, and ruling out infection or aseptic loosening. Patch testing may be helpful in diagnosis. Treatment generally involves component exchange to a hypoallergenic femoral component with all-polyethylene tibial component. Epidemiology Incidence rare Etiology Pathophysiology Type IV - delayed-type cell mediated hypersensitivity mediated by T cells reaction to nickel found in cobalt-chromium alloys metal ions deposit in local tissue, must react with proteins to initiate hypersensitivity biggest link may be metal ions/hypersensitivity as a causative initiator of early implant loosening Presentation History may have history of dermatitis, eczema, other signs of atopic hypersensitivity Symptoms vague persistent pain persistent, unremitting effusion/swelling stiffness Physical exam document range of motion Studies Evaluation patch testing (limited evidence for direct link to diagnose hypersensitivity) lymphocyte transformation test (LST) T-lymphyocyte rich immunohistopathology negative work-up for chronic/acute infection (must be ruled out) Diagnosis exact definition is controversial some argue combination of a positive patch test, positive immunohistopathology, and relief of symptoms upon implant exchange is only way to confirm diagnosis others argue that metal hypersensitivity is a diagnosis of exclusion, only arrived upon when infection and aseptic loosening is ruled out Treatment Operative implant exchange indications persistent symptoms affecting quality of life technique hypoallergenic femoral component with all-polyethylene tibial component if possible at time of surgery, chronic inflammatory synovitis typically present