Outcomes of Metal Hypersensitivity in Knee Arthroplasty: A Systematic Review of the Literature

Background: Metal implant hypersensitivity remains a subject of ongoing debate in the orthopaedic community. With the global rise in total knee arthroplasty (TKA) procedures per annum and an increasing prevalence of reported metal allergies in the general population, understanding the clinical relevance of metal implant hypersensitivity in knee arthroplasty patients is essential. This systematic review evaluates the relationship between metal hypersensitivity, diagnostic testing, and postoperative outcomes following TKA. Methods: A literature search was conducted using the Ovid, MEDLINE, and Embase databases for studies published between 1990 and December 2024. Studies assessing diagnosis, management, or outcomes of metal hypersensitivity in TKA were included. Both prospective and retrospective studies were considered. The search yielded 554 records, with an additional 25 studies identified through manual reference screening. Following full-text assessment of 58 articles and application of predefined inclusion and exclusion criteria, 24 studies were included. Data extraction focused on study design, patient demographics, implicated metals, diagnostic methods, interventions, and outcomes. Patient Reporting Items for Systematic Reviews & Meta Analysis (PRISMA) guidelines were followed. Results: A pooled cohort of 5615 knee arthroplasty cases was analyzed, with a mean patient age of 65.35 years and a female predominance (60.55%). Among these, 1165 patients had a documented history of metal hypersensitivity. Twenty studies reported follow-up durations, with a mean follow-up of 27.59 months. Patient symptoms, diagnostic test results, patient-reported outcome measures, and revision rates were evaluated. Conclusions: The available literature demonstrates significant heterogeneity and limited comparability across studies. Current evidence suggests that self-reported metal allergy or positive skin patch/lymphocyte transformation tests do not reliably predict poorer pain or functional outcomes following TKA. The clinical significance of metal hypersensitivity remains uncertain, highlighting the need for standardized diagnostic criteria and further high-quality research.