## 5 Discussion
Fabricated using a combination of liquid monomer activator and powdered polymer particles, it is considered non-toxic and biocompatible when heat polymerized [33]. Heat polymerized PMMA biomedical implants are specifically used for customized bone defect reconstruction of critical non-loading bearing areas, such as following cranioplasty [22]. Similar to the highly popular UHMWPE, PMMA too has orthopedic applications. While UHMWPE is predominantly used as an articulating interface in alloplastic joints, self-polymerizing PMMA is used as a bone cement for fixation [34]. In the present study pre-formed blocks of unsintered zirconia and heat polymerized PMMA were used to custom fabricate TMJ parts based on 3D digital modelling. The justification for selecting these three biomaterials was based on their availability, reported biocompatibility, prospect of CAD/CAM manufacturing, and mechanical properties, whichare either higher than or are comparable to that of cortical bone (Table 6) [35; 36; 37]. It must however be noted that the choice of material should be based mainly on the elastic modulus of the biomaterial in comparison to that of natural bone [17; 18; 19]. [PERSON] et al. reported that it is possible to modify the elastic properties of titanium implants manufactured using an additive process, without affecting their mechanical performance, by incorporating advanced porous and lattice structures [17]. In addition to fabrication of implants with Young's modulus closer to that of natural bone, porous structures also help minimize stress shielding effect [17]. The biomechanical simulation testing methodology used in the present study was developed indigenously, based on previously reported protocols. The cycling time and compression force applied during vertical and horizontal strokes were designed to simulate anatomic loading of TMJ during an approximate functional life of three years [3; 24]. No macroscopic structural damages were observed in the prosthetic condyle and glenoid fossa parts, following biomechanical simulation testing using different combinations (Table 1). Although, wear facets were evident macroscopically and through SEM in both the condyle and glenoid fossa specimens, no microscopic cracks could be appreciated (Figures 13 and 14). The most important findings which were quantified, were the reduction in weight of the prosthetic TMJ parts, when tested using different combinations. This was evaluated based on evidences in the literature which have reported functional wear of alloplastic joints under clinical conditions [38]. Incidentally, while weight reduction in the micro-scale was observed in all specimens evaluated in the study, it was generally greater in the glenoid fossa parts than in the condylar parts (Figure 11). Moreover, the degree of wear significantly differed in the prosthetic glenoid fossa parts, depending upon the type of condylar biomaterial articulated against (Table 5). The greatest wear in terms of mean reduction in weight of the prosthetic TMJ parts, were seen in acrylic parts, followed by EBM-Ti and zirconia parts. Furthermore, acrylic and EBM-Ti condyle and fossa parts exhibited greater wear in comparison to the zirconia fossa or condyle parts against which they were tested. [PERSON] et al. [21] based on a clinical series of eight patients treated with Groningen TMJ prosthesis, designed out of zirconia condyle and fossa parts, reported the use of a UHMWPE disc between the articulating surfaces. The purpose of the disc was to optimize the overall biomechanical properties of the joint considering the high compressive strength and hardness, and low tensile strength of zirconia (Table 6). The afore-mentioned factor could also be the reason behind greater wear observed among EBM-Ti and acrylic parts in the present study, when tested against zirconia. The use of a disc fabricated out of biomaterials with low hardness and high tensile strength, such as UHMWPE or PMMA might help prevent wear of the articulating surfaces associated with prosthetic zirconia TMJ parts [21].