The mechanical failure of a temporomandibular joint (TMJ) replacement system can stem from various factors, with one notable cause being the unnatural kinematics and loading observed in prosthetic joint function. Given that,. the primary objective of the study was to investigate whether including functionally graded materials (FGMs) at the site of the prosthetic joint was feasible for achieving a greater resemblance to natural mandibular kinematics, while simultaneously reducing the reaction force exerted on the prosthetic joint. To this end, an FGM-based TMJ implant was designed with the intention of reducing joint reaction forces and enhancing joint flexibility to achieve a more natural kinematic behavior.

In this study, patient-specific finite element models of the intact and implanted mandible were simulated for two different biting tasks: incisal biting (INC) and left group biting (LGF). To incorporate FGMs into the implant design, an initial implant design was voxelized (with a voxel size of 0.25mm x 0.25mm x 0.25mm), allowing each voxel to be assigned a specific material. Subsequently, 10 voxel layers of the same size were added onto the implant head/prosthetic condyle. To investigate the impact of FGMs on mandibular biomechanics, a comprehensive analysis was performed on five distinct implant designs. These designs were differentiated solely by the functional gradient in material properties across the additional 10 voxel layers on top of the implant head. Each voxel (layer) was assigned a value of ρ, which represented the volume fraction of the hard phase.

The five distinct implant designs were as follows: (1) The first design was labelled as the "Polymeric Implant" consisting solely of the hard phase with a volume fraction of ρ=100%. It served as the reference for comparisons with other designs. (2) The second design featured an abrupt hard-soft connection without a gradient. This design was specifically examined to demonstrate the presence of stress concentrations at discrete interfaces between the hard and soft materials in layered composites. It was named "Implant with abrupt hard-soft connection without a gradient". (3) The third design was characterized by an FGM comprising ten discrete values of the volume fraction of the hard phase, ranging from ρ=0% at the surface layer to ρ=90% at the 10th layer with an increment of 10%. This design was designated as "FGM, ρ= [0, 10,…, 90]". (4) The fourth design encompassed an FGM featuring nine distinct values of the volume fraction of the hard phase, ranging from ρ=10% at the two surface layers to ρ=90% at the 10th layer with an increment of 10%. It was referred to as "FGM, ρ= [10, 20,…, 90]". (5) The fifth design comprised an FGM incorporating eight discrete values of the volume fraction of the hard phase, ranging from ρ=20% at the three surface layers to ρ=90% at the 10th layer with an increment of 10%. This design was named "FGM, ρ= [20, 30,…, 90]". The primary objective of investigating the fourth and fifth designs was to analyse the influence of a relatively less soft material at the surface (outer layer) of the implant head.

The biomechanical performance of an implant design was determined by utilizing normalized cross-correlation (similarity) and Euclidean distance (closeness) measurements between the displacement fields of the intact and implanted mandible. The performance of an implant design for a specific biting task was assessed by calculating the average of the percentage errors in each of these quantities. Additionally, the reaction forces on the prosthetic joint were compared between the implanted models.

The results showed that the second design, which featured an abrupt hard-soft connection without a gradient, was not viable. This was due to the presence of stress concentrations, stress increase up to more than ninefold was observed, at discrete interface between materials of contrasting hardness. This phenomenon could result in crack initiation and delamination, significantly compromising the overall performance and durability of the design. The study also revealed that material properties distribution on the surface of the implant head had a crucial impact on regulating mandibular displacements. Notably, greater displacements were observed when the outer layer of the implant head featured relatively softer material.

In comparison to the polymeric implant, “FGM,ρ=[0,10,…90]” implant exhibited an average performance increase of 19% for INC and a 3% performance decrease for LGF regarding mandibular displacement. Additionally, it reduced the joint reaction forces on the prosthetic joint by 8% and 12% during INC and LGF, respectively.

Moreover, the "FGM, ρ=[0,10,…90]" implant increased the mandibular range of motion by 20% during INC and 88% during LGF. Furthermore, it contributed to more symmetrical mandibular movement during INC, while the concept of symmetry was not applicable in the context of LGF.

These results have provided valuable insights for future research and development of TMJ implants, potentially leading to the creation of implants with improved long-term behavior.