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Chapter 5134experiment, reporting on histological findings of the peri-articular tissues, no evaluation of the amount of wear was reported on (12). Several more studies evaluated either the histological reaction of the peri-articular tissues or wear pattern in explanted TMJ TJR after, yet as far as we are aware of, this is the first study to report on TMJR wear results through an in vivo experiment (9,14). While wear can be evaluated via either in vitro or in vivo testing, in vivo testing is preferred for TMJ replacements for at least three reasons. Firstly, there is evidence from hip joint prostheses that in vivo wear rates are much higher than those evaluated by in vitro testing, risking underestimation of the total wear rate (15). Secondly, the TMJ makes rotational and anteroposterior as well as mediolateral translative movements. Mimicking in vivo scenarios in an in vitro testing environment that captures the specific degrees of freedom in movement that occur during mastication would be extremely difficult. Thirdly, the amount of force to which the TMJ is subjected remains uncertain (14,16), which limits the ability to create a reliable in vitro experimental environment. When evaluating potential in vivo animal models, the primate TMJ is most similar to a human’s, yet their daily mastication rate is rather low. In addition, several major ethical issues and cost of care prohibit the use of primates for this type of research. It is for said reason that several different animal models, such as the pig, goat and sheep model, have been investigated and proven to be reliable and relatable in vivo experimental models for TMJ investigations. While having both their advantages (the anatomically and biomechanical resemblance to the human TMJ) and limitations (the more outspoken laterotrusive movements) both the goat and sheep model are considered the ‘gold standard’ in large animals (17,18). Further, sheep spend on average 4 hours per day eating at a rate of 128 mastication cycles per minute and an average of 8 to 9 hours per day ruminating at a rate of 100 cycles per minute (19). Due to this high daily mastication rate, exceeding that of goats, the total duration of an in vivo evaluation of implant wear can be conducted over a shorter time frame than in humans or other species.After developing a novel patient-specific additively manufactured (AM, also referred to as 3D-printed) titanium (Ti) alloy TMJ replacement Nikolas de Meurechy NW.indd 134 05-06-2024 10:14