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Biomaterials in TMJ replacement713immunological response), which can form organometallic complexes by binding to cells or proteins. These complexes can subsequently cause allergic sensitization by being processed by antigen-presenting cells (APCs), such as macrophages, B-cells, or dendritic cells. APCs present the processed allergen to T-helper cells, which in turn activate either B-cells or cytotoxic T-cells. B-cells are responsible for type I hypersensitivity reactions, which are characterized by the production of IgE antibodies and activation of mast cells and basophils. Cytotoxic T-cells cause type IV hypersensitivity, also known as the delayed-type hypersensitivity reactions. APCs cause sensitization of TH1-cells, which release cytokines when stimulated; the cytokines, in turn, activate both macrophages and cytotoxic T-cells, resulting in cellular damage.(18,45,46) It should be noted that the particle size of the debris plays an important role in the risk of hypersensitization. While UHMWPE-particles vary in size from <10 µm to >100 µm, most metal debris is around 1–4 µm. While the larger particles cannot be processed by APCs, the smaller metal wear debris can.(18) As a result, while metal-on-metal TMJ prostheses produce less wear, the incidence of metal hypersensitivity is higher than with metal-on-UHMWPE prostheses. This was clearly demonstrated by the results of a study by Wolford et al.(16), in which prosthesis removal because of metal hypersensitivity or device failure was required for 33% of Christensen prostheses but only 3% of TMJ Concepts prostheses.As a result of hypersensitivity reactions to metals, patients can develop both local and systemic symptoms. Local symptoms vary from skin dermatitis, erythema, and urticaria to TMJ or myofascial pain, facial swelling, muscular spasms, headaches, earaches, tinnitus, and vertigo. Systemic reactions include depression, fibromyalgia/chronic fatigue, neurologic or gastrointestinal problems, vasculitis, cardiac instability, and even end-organ failure and death (in the most extreme cases). Furthermore, the local reaction can lead to loosening of the implant, resulting in failure.(18,45,46)Both Sidebottom et al.(47) and Hussain et al.(46) advise performing a patch test in all patients who are scheduled for a TMJ TJR to prevent Nikolas de Meurechy NW.indd 71 05-06-2024 10:14