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Chapter 376FatigueBesides the static mechanical strength, also the dynamic fatigue strength plays an essential role in determining the durability of an implant. Fatigue refers to the progressive structural damage resulting from repeated cyclic loading. It is important to realize that any material inherently contains small defects or can develop micro-cracks because of stress concentrations, which will enlarge if the applied load exceeds a certain threshold (fatigue strength, endurance limit). As a result, the implant material becomes weaker and can fracture unexpectedly, even when loading conditions remain well below its static mechanical strength. As stated earlier, the stiffness of an object can be reduced by increasing its porosity. While these surface irregularities allow for tissue ingrowth with better fixation of the implant, they also form stress zone hotspots where micro-cracks can form.(48)WearMaterials with low wear resistance promote the formation of wear debris, which can cause implant loosening and inflammatory or allergic reactions. Furthermore, a lower wear resistance limits the total life span of the implant, which must be taken into consideration when treating younger and more active patients.(48) The rate at which wear appears can be influenced by several factors, such as the surface roughness and the geometry of the articulating surfaces.(34)In hip implants, the total wear volume is 20–100 times lower with metalon-metal CoCr implants than with metal-on-UHMWPE implants.(48) However, several issues regarding these findings deserve comment. First, there is a clear difference in loading between the TMJ and the hip joint. Whereas the hip is a concentric, rotational joint in which the articulation is semi-constrained, the TMJ can perform both rotational and translational movements, with a total contact area smaller and less congruent than that found in the hip joint. The absence of foreign body giant cell reactions in metal-on-UHMWPE TMJ systems, which can be found in metal-on-UHMWPE hip prosthesis, is a clear indicator of this difference in loading.(18,34) Mercuri et al.(41) state that because of these anatomical differences, the TMJ is not suited for metal-on-metal TJR. Nikolas de Meurechy NW.indd 76 05-06-2024 10:14