2.1.4. Design of Screws

Unlike hip or knee joints, the bony anatomy of mandibular ramus and temporal glenoid fossa do not afford the use of modular stock components for TMJ TJR that can be stabilized initially with press-fitting or cementation [2]. Therefore, TMJ devices have to rely only on screws for initial fixation and stabilization of their components. Clinicians have underlined the need for improved methods of internal fixation of prosthetic TMJ devices to minimize or eliminate implant loosening and joint failure [1]. The position of inserted screws was more important than the number of screws for stable fixation of the condylar TMJ prosthesis [20]. Our methodology of designing patient-specific total TMJ prostheses based on anatomically accurate 3D models provides realistic and accurate options in deciding positions of fixation screws for the prostheses. The positions of predrilled screw holes in the prostheses can be selected to avoid unintentional injury to delicate structures in the vicinity while ensuring stable fixation of the devices. Moreover, unlike stock TMJ implants, the custom-designed TMJ prostheses do not have any unused screw holes which may act as stress-risers under functional in-vivo loading post-implantation.

**Figure 19.** Patient-specific design of a fossa implant with circular articular surface/hole in the inferior face of the device (**A**). The device has a novel feature; perforated medial notches (**B**) protruding into host bone at implantation. Each perforated notch is designed to fit into surgically created mating grove in the host bone, thereby maximizing device stability by allowing ingrowth of bone into the prosthesis after implantation (**C**,**D**). The notches also provide a mode for load transfer between the prosthesis and native bone, thereby reducing the amount of load and resultant stress acting on the fixation screws. The surgeons can be provided with custom-designed templates and cutting guides to accurately cut the slots in native bone to accommodate perforated notches. **Figure 19.** Patient-specific design of a fossa implant with circular articular surface/hole in the inferior face of the device (**A**). The device has a novel feature; perforated medial notches (**B**) protruding into host bone at implantation. Each perforated notch is designed to fit into surgically created mating grove in the host bone, thereby maximizing device stability by allowing ingrowth of bone into the prosthesis after implantation (**C**,**D**). The notches also provide a mode for load transfer between the prosthesis and native bone, thereby reducing the amount of load and resultant stress acting on the fixation screws. The surgeons can be provided with custom-designed templates and cutting guides to accurately cut the slots in native bone to accommodate perforated notches.

Motion of implanted TMJ prosthesis under load can cause degeneration of the surrounding bone, which may lead to further device loosening and possible failure [1]. Screws may loosen with time and function, requiring replacement. For long-term success of the TMJ implants, forces from the implant to the bone and vice versa must occur without relative motion or without intermittent loading [2,21]. The use of bone screws with sharp threads in TMJ implants prevents movement between the screw head and prosthesis [15]. The custom-designed screws of our TMJ prostheses system provide optimal fixation through locking mechanism—a unique feature not currently offered by any of the US FDA-approved TMJ TJR devices (see Figures 22 and 23). Threads on the screw-head surface provide high grade fixation by firmly engaging with the matching threads in the

screw hole of either condylar/ramal or fossa implants. The possibility of movement between the screws and prosthesis can be eliminated by using such locked screws. In addition to the surgical condylectomy guides, our methodology also provides the surgeons with customized screw-drill-guides and templates for the TMJ prostheses. These drill guides are intended to create a hole of preferred dimension (diameter and depth), and at the accurate location and orientation for each screw as prescribed by the surgeon. *Materials* **2022**, *15*, x FOR PEER REVIEW 17 of 34

**Figure 20.** Shape outline of the patient-specific total TMJ prosthesis. Ramal component of the prosthesis is extended anteriorly up to the chin (**A**–**F**) to support mandibular host bone and graft (with aesthetic dental implant) after removal of the imaginary tumor (shown in red) in the left mandibular body/molar region. **Figure 20.** Shape outline of the patient-specific total TMJ prosthesis. Ramal component of the prosthesis is extended anteriorly up to the chin (**A**–**F**) to support mandibular host bone and graft (with aesthetic dental implant) after removal of the imaginary tumor (shown in red) in the left mandibular body/molar region.
