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Chapter 372allergic reactions and the risk of early rejection of the TMJ TJR. If patch testing is inconclusive, further assessment using lymphocyte transformation testing can be considered.(45) Corrosion With respect to metals, the physiological situation is an extremely aggressive environment. The presence of both salts, especially chloride [Cl-] anions, and proteins significantly facilitates (electro)chemical reactions leading to dissolution of metal ions into the body. These released ions can accumulate around the implant, causing cytotoxicity and inflammation or be transported throughout the body, resulting in systemic effects, eliciting hypersensitivity, disturbing trace metal ion levels and their concomitant biochemical reactions, and even producing carcinogenic effects.(19,48) Metallic biomaterials, such as CoCr or Ti alloys, owe their biocompatibility to the formation of a thin, yet protective, oxide film during the early stages of corrosion (passivation). This layer is immediately rebuilt when damaged (re-passivation) because of its thermodynamic stability and acts as a barrier against the diffusion of metal ions from the bulk metal into the surroundings, effectively limiting the uniform corrosion rate. However, in addition to their main components, Cr2O3 (CoCr alloys) or TiO2 (Ti alloys), these layers also incorporate small amounts of sub-oxides and oxides of other alloying elements present in the base metal. Such alterations in the passive film can make the implant more sensitive to corrosion, a concern that has initiated the development of new alloys.Moreover, different forms of localized corrosion can still endanger the longevity of an implant and should be taken into account during the implant design process. Examples of these deteriorative processes are the accelerated corrosion in shielded sites, such as underneath screw heads (crevice corrosion); corrosion due to highly localized de-passivation of the protective oxide layer, especially in the presence of Cl- ions (pitting corrosion); corrosion due to electrical contact between dissimilar metals, such as implants versus fixation screws (galvanic corrosion); and corrosion induced by micro-motions due to cyclic loading at the implantbone interface (fretting corrosion).(19,48) Nikolas de Meurechy NW.indd 72 05-06-2024 10:14