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Stock or patient-specific TMJ replacement1214Extra advantages of patient-fitted prosthesesCustom(ized) prostheses allow for controlled occlusal correction and proper mastication without having to opt for (simultaneous) orthognathic surgery.(11) In case of congenital anomalies with severe hypoplasia (e.g., hemifacial microsomia), the extended mandibular and fossa components also substitute missing bone and allow for proper facial rotation in conjunction with other facial osteotomies.(39) For defects due to trauma, osteomyelitis, or oncological resection, an extended TJR can substitute both the affected TMJ and the additional bony defect in the mandible or skull base, once again making further surgery (e.g., microvascular bone flaps) unnecessary.(39,40) One of the distinct advantages of CAD/CAMdesigned implants for these cases is the optimal aesthetic outcome (and consequent psychosocial integration), which would not be possible through the combination of a stock TMJ TJR and a second implant or autologous graft.(39) Screw position and length can be determined using a patient-fitted approach to prevent damage to the inferior alveolar nerve.(5,38) For simple TJR, the patient-fitted mandibular component can be inserted via a mini-retromandibular incision, diminishing the risk of lesioning the mandibular branch of the facial nerve, in a similar fashion as described by Biglioli et al.(41) for condylar fractures.Both FDA-approved systems (Zimmer Biomet and TMJ Concepts) manufacture at least parts (condylar head) in cobalt-chromiummolybdenum (CoCrMo) alloy. It should be noted that Zimmer Biomet also has a version of their stock prosthesis fully in Ti, yet this version is not FDA-approved, nor are there, as far as we are aware of, any post-operative results discussed in current literature. In a meta-analysis on orthopedic prostheses, about 10% of the population was found to be allergic to one or more components of the implants, usually nickel, of which they contain 1%. Other components are cobalt (62%–67%), chromium (27%–30%), and molybdenum (5%–7%).(42) In patients with a functioning prosthesis, the proportion of allergies rose to 23%, and in those with a failing prosthesis to 63%. It may be that the Nikolas de Meurechy NW.indd 121 05-06-2024 10:14