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Biomaterials in TMJ replacement753This means that materials with an elastic modulus closer to that of bone are preferred over materials with a higher elastic modulus.(34,48,51) Also, as the size of an implant device becomes larger, it becomes stiffer, resulting in stress shielding. This problem can be overcome by making the device hollow or porous.(34,51) It should however be noted that up until this date, no issues concerning stress shielding with total TMJ implant devices have been reported, meaning that this should be considered rather a theoretical, than a practical consideration.FunctionalityIn addition to the host response to the materials used, premature material breakdown can also significantly decrease the longevity of an implant. For load-bearing implant applications, the materials must be mechanically sound and should be able to withstand the forces they are subjected to over a long period of time. With respect to TJR, high strength, excellent fatigue and wear resistance, and fracture toughness, are desired qualities.(13,34,48) Mechanical strengthIt is important to realize that the distribution of loads is very different for a TMJ device, compared to a hip or knee TJR, for instance.(52) A hip prosthesis has a functional load varying from 3.5 to 6 times the body weight and a rather constrained functional anatomy. Determining the functional load for a TMJ prosthesis, however, is far more difficult, as the biting forces measured at the molars (estimated at 265 N) differ greatly from those at the incisors (estimated at 60 to 160 N), and the functional anatomy of a TMJ prosthesis is less constrained. Thus, the functional load is dependent on the position of the lower jaw at any given moment. Furthermore, patients who are in need of a TMJ TJR will most likely exhibit altered TMJ function, resulting in different and often reduced functional loads.(13,18) Apart from the altered function, in many cases there is also an alteration in the anatomy of the joint, resulting once more in great alterations in the load the joint is subjected to, depending on the position of the lower jaw.(5,13,38,40)Nikolas de Meurechy NW.indd 75 05-06-2024 10:14