This study introduces a new TMJ prosthesis, which is totally different from the commercially available Zimmer Biomet and TMJ Concepts prostheses as for the design perspective, in addition to the manufacturing process. The clinical application of stock Zimmer Biomet is more common than the customized one. The stock Zimmer Biomet includes the fossa and mandibular components, in which the former is made of a grade of UHMWPE with a spherical articulating surface and a planar bony surface and the latter is fabricated with a single Co–Cr–Mo alloy accompanied by an oblate condylar head and planar mandibular handle (Fig. 1a) [10, 24]. TMJ Concept is a custom-made prosthesis, consisting of the fossa and mandibular components. The fossa is constructed of a pure titanium custom-made sheet with a welded mesh that interfaced with the dense UHMWPE articulating surface, and the mandibular component is constructed of 2 basic materials: Ti6Al4 V alloy coated with Co–Cr–Mo alloy head (Fig. 1b) [22, 23]. Particularly, the presented prosthesis in the current study is also a customized one which includes three parts: the fossa, condylar head, and mandibular handle components. The fossa component is the first innovative part of this prosthesis, which is a patient-specific design with single UHMWPE, which could match the anatomy of the fossa, zygomatic arch, and articular eminence very well. Moreover, the condylar head and mandibular handle components are constructed from Co–Cr–Mo and Ti6Al4 V alloys, respectively, which are connected together by the machine taper connection mechanism-a common connecting method for different metal materials in orthopedic prostheses (Fig. 1c). Based on the design principle, it is much easier and faster for the processes of manufacture and implantation in clinical application.
The 3D printing, as an additive manufacture method for the TMJ prosthesis, showed the second innovative point of this prosthesis, which is more consistent with the trend of medical development compared with the commercial Zimmer Biomet and TMJ Concepts prostheses manufactured by the conventional Casting Co–Cr–Mo alloy and wrought Ti6Al4 V alloys, respectively [10, 11, 22, 24]. As well known, a wide variety of 3D printing technologies have been advocated the medical field over the last three decades, especially for the fabrication of hip and knee joint prostheses in Orthopedic surgery . Reviewing the kinds of literature, just one TMJ implantation device published in 2017 presented the use of 3D printing technique to fabricate the TMJ prosthesis for patients requiring joint replacement surgery [31, 32]. Its mandibular component with an oblate condylar head (same with TMJ Biomet) was fabricated from only titanium alloy by the 3D printing machine, while the condylar head of our prosthesis is constructed of Co–Cr–Mo alloy by 5-axis milling device, and only the mandibular handle component of our prosthesis constructed of titanium alloy by 3D printing. Theoretically, according to the development of the TMJ prosthesis, the functioning surfaces of TMJ prosthesis should have low wear, flow, and fatigue coefficients [9, 33, 34], therefore, the Co–Cr–Mo alloy has the prominent merits for use as the condylar head and may even show better outcomes in long-term follow-ups. In any case, both 3D printing prostheses have proved that modern 3D printing technology has enabled the more sophisticated, flexible, and automated production of TMJ prosthesis directly from CAD data.
The study is a prospective self-control research with very strict inclusion and exclusion criteria. Only patients diagnosed as unilateral end-stage TMJ osteoarthrosis in combination with a stable occlusion relationship have been recruited. Since John Murray Carnochan first reported an alloplastic TMJ reconstruction in the 1840s, most papers presented for TMJ prosthesis were retrospective researches [9, 13,14,15, 33, 34]. Limited literature is available addressing the clinical application of Zimmer Biomet or TMJ Concepts prostheses with the prospective design [10, 11, 22,23,24]. In addition, all these articles usually had a wide range of inclusion criteria, which included degenerated or resorbed joints, ankylosis, trauma, failed autogenous grafts, and other end-stage TMJ pathologies [10, 11, 22,23,24]. In fact, the wide inclusion criteria made the results offset, which eventually confuses the reader and also being difficult to understand and interpret the outcomes well for every specific type of TMJ diseases. As stated above, the results in the study would be more accurate and credible than before.
Clinically, in order to confirm the safety of the prosthesis in clinical use, we recorded the surgical complications, occlusion relationship, CT check after surgery, and some laboratory indices, including liver, and kidney function tests, routine blood, urine, and stool tests to determine if any related complications occurred postoperatively. From the comprehensive results (no severe maxillofacial complication occurrence, no other systematic organ damages, no displacement, breakage, and loosening of the prosthesis, and excellent bone contact with host bone), we realized that the safety of the prosthesis has been verified. However, Mercuri et al. reported the serum metal levels in patients who underwent different maxillofacial implanted metallic objects. The results showed the possibility of the increases of the metal levels including the cobalt, titanium, or chromium in the bloodstream after dental implant placement, orthognathic surgery using rigid metal fixation plates and screws, and total TMJ prosthesis. But they did not elucidate the clinical symptoms resulting from the metal level increases . Nevertheless, we will also concentrate on the metal level analysis and possibly relative clinical discomforts for our patients in future follow-up to further confirm the safety of the TMJ prosthesis.
Moreover, completely postoperatively subjective and objective indices have been measured to confirm the efficacy of the prosthesis in 12 patients without missing case or in compliance for more than one year following surgery. The methods of follow-up, which have been widely used in previous studies for the clinical applications of Zimmer Biomet or TMJ Concepts products, were referred to the common criteria confirmed by Kent et al. in 1993 [22, 23, 33]. Based on these criteria or method, our study showed an average of 90.7% decrease in pain, 70.8% improvement in mandible function, 79.9% improvement in diet, and 32.8% increase in MIO at 1 year after surgery. From 1993 to 2017, there were many studies that evaluated the postoperative efficacy of TMJ prostheses (mainly TMJ Biomet and Concepts) using the same criteria. Their results showed 48–78.1% decrease in pain, 51–60% improvement in mandible function, 51.5–69.5% improvement in diet, and 23.9–66% increase in MIO [10, 11, 22,23,24]. The improvements in pain, mandible function, and diet in our study were more obvious compared to other studies. This could be related to the inclusion criteria. In our study, patients with TMJ osteoarthrosis were included only, usually presenting with periarticular pain. Meanwhile, the mandible functions, diet, and MIO have been limited due to pain. In other studies, the included patients were usually recorded with different types of TMJ pathologies (osteoarthritis, ankylosis, idiopathic condylar resorption, and so on) together, so that the evaluated indices have been influenced by each other. For lateral and forward movements, and opening mouth deviation, we found some negative outcomes, including the significant limitations of the mandible forward movement and lateral movement to the normal side, and the deviation to the operated side when mouth opening. These were attributed to the attachment loss of the lateral pterygoid muscle, which usually helps the mandible move forward and contralaterally. Meanwhile, the finding of the attachment loss between this muscle and the prosthesis in postoperative CT images can further verify this reason. Actually, these negative results have been found and explained in previous studies [11, 23]. But they just found the limitation to the contralateral joint side. There were no self-control results to show how serious the problems were, and no postoperative CT to check the attachment of the muscle. Therefore, the evidence in this study was more persuasive because of being prospectively self-control project, where only unilaterally operated patients were included. As a result, the efficacy of the new prosthesis has been confirmed based on the subjective and objective indices.