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Chapter 380outstanding biocompatibility properties. The Ti-oxide layer limits metal ion release more than the passivating layers on stainless steel or CoCr alloys. As a result, the cell/tissue response is improved, resulting in very little adverse tissue reaction, and close apposition between the implant and bone is established.(38,48) Moreover, Ti alloys have a low density; hence, Ti outperforms any other implant material when considering the specific strength.(48) Ti was first used as a material for TMJ implants in 1976 in the AO/ASIF-TMJ prosthesis and has since been used in many different alloys.(6) Commercially pure (cp) Ti (98.8–99.6 wt % Ti) and Ti6Al4V(89.0–91.0 wt % Ti, 5.5–6.5 wt % Al, and 3.5–4.5 wt % V) are currently the most commonly used Ti materials for implant applications in orthopedic surgery, and they are used in the two FDA-approved TMJ implants.(48) Ti can adopt two different crystal structures: a closely-packed hexagonal structure known as the α-phase, which is stable at low temperatures or in the presence of certain alloying elements such as Al, oxygen (O), and nitrogen (N); and a body-centered cubic structure knows as the β-phase, which is stable above 883°C but can also be preserved at lower temperatures depending on the presence of certain alloying elements, such as V, niobium (Nb), Mo, and Ta. Ti6Al4Vconsists of a mixture of both the α- and β-phase and, as such, can be subjected to thermomechanical processing, which improves its mechanical properties, such as tensile strength and fatigue strength.(48) However, the passive film of Ti6Al4Vis known to be more susceptible to corrosion, as the V2O5 in the film can dissolve, producing openings in the film and exposing the underlying Ti alloy.(19) The unalloyed cp Ti consists of α-Ti, which has a lower strength and fatigue resistance than the alloy, but because of the absence of alloying elements in the protective oxide layer, cp Ti is much more corrosion resistant and, hence, more biocompatible. This is most likely the rationale for the selection of materials for the Biomet/Lorenz and TMJ Concepts prostheses. For the fixation screws, a high strength and fatigue resistance is more critical than biocompatibility; therefore, Ti6Al4Vis used in both systems. For the ramal surface finish in the Biomet/Lorenz system and the backing of the UHMWPE fossa in the TMJ Concepts system, optimal osseointegration is the goal, so excellent biocompatibility Nikolas de Meurechy NW.indd 80 05-06-2024 10:14