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Chapter 374Furthermore, a good fit is needed as well.(6,12,13,34,38) This, however, is quite difficult, as patients in need of a total TMJ prosthesis often have a deformed TMJ, caused either by prior surgical procedures or the nature of their joint pathology.(12,13) Several earlier designs made use of PMMA to both fixate the prosthesis and achieve a better fit, but this approach was abandoned because of the risks of thermal trauma to the surrounding tissue during in situ curing of the polymer, as well as fragmentation under functional loading.(6,12,13,38) Alternatively, more flexible materials that can be easily adapted to the form of the patient’s skull will allow for a good fit, although micro-motions can occur because of their flexible nature.(12,34) Manufacturers of modern FDA-approved stock TMJ TJR devices have tried to overcome fit problems by providing their device components in different sizes and shapes, among which the surgeon can select the best fit for the individual patient. Despite these attempts, the surgeon will often be forced to ‘make the components fit’, as stated by Mercuri et al.(13), either by altering the patient’s anatomy to the prosthesis via reshaping the bone; shimming the component with autogenous bone, bone substitute, or alloplastic cement; or bending the device components.(5,12,13,38) On the contrary, custom TMJ TJR devices, such as the TMJ Concepts system, are designed and manufactured to the patient’s anatomy, so no or only very little alteration is needed during implantation.(9,13,37,38)Stress shieldingBesides achieving good fixation and fit with primary stability, the elastic modulus (E value) also plays an important role in preventing micromotion and assuring good stability. Bone has an elastic modulus of 4–30 GPa, depending on the type of bone and direction of measurement.(34,48,51) Materials with a lower elastic modulus, such as titanium alloys (55–112 GPa), are more flexible, while the elastic modulus of Vitallium is 218 GPa, resulting in a less deformable material. The importance of the elastic modulus becomes clear in the process of stress shielding. As the elastic modulus of a material increases, it takes more force to deform the material and the underlying bone will be ‘shielded’ from stress. According to Wolff’s law, which states that bone will remodel itself in accordance to the loads it is subjected to, this would lead to a loss in bone density and weakening of the bone. As a result, bone resorption and implant loosening can be seen. Nikolas de Meurechy NW.indd 74 05-06-2024 10:14