Design and Finite Element Analysis of Patient-Specific Total Temporomandibular Joint Implants

Trans-mastoid anchorage as a novelty in glenoid fossa reconstruction for hemifacial microsomia with agenesis of zygomatic arch and external acoustic meatus-Technical note with a case illustration

Total joint reconstruction (TJR) has become the most preferred method of reconstruction in recent years for hemifacial microsomia (HFM). This requires meticulous planning for the glenoid fossa and ramus prosthesis along with the arch in certain indications. TJR in a type V HFM is extremely challenging due to agenesis/hypolasia of the zygoma and arch which compromises anchorage of the glenoid prosthesis. Conventional options used for such indications incorporate extended designs for the fossa. However, the two designs used commonly are associated with limitations; (i) the temporal extension is overtly large and cannot be anchored in patients with thin temporal bone and (ii) the glenoid fossa incorporated into the arch prosthesis, where the fossa is not positioned on stable bone and occlusal loading is non-physiological; transmitted onto the arch prosthesis rather than the skull base. The authors propose the use of the mastoid process as an alternative for anchorage of fossa prosthesis. This technique offers stable anchorage while facilitating ideal positioning on the skull base for optimal masticatory load transmission. The case illustration depicts reconstruction of the TMJ, zygoma and the zygomatic arch in a 31-year-old man with type V HFM, with a three-year follow-up, with good clinical outcomes sans complications.

Computational modelling of the fossa component fixation associated with alloplastic total temporomandibular joint replacements

The alloplastic total temporomandibular joint (TMJ) replacement is a complex surgical approach to end-stage TMJ disorders. The fixation of TMJ prostheses remains a critical issue for implant design and performance. For the fossa component, it is generally considered to use fixation screws to achieve tripod stability. However, the fossa may still come loose, and the mechanism remains unknown. A computational framework, consisting of a musculoskeletal model for calculating muscle and TMJ forces, and a finite element model for the fossa fixation simulation, was developed. A polyethylene (PE) fossa with stock prosthesis design was analyzed to predict contact pressures at the fixation interfaces, and stresses/strains in the fossa implant and bone during the static loading of normal chewing bite and maximum-force bite. The predicted maximum von Mises stresses were 33 MPa and 44 MPa for the bone, 13 MPa and 28 MPa for the PE fossa, and 131 MPa and 244 MPa for the screws, for the normal and maximum bites, respectively; the peak minimum principal strain was in the range of -2514 ∼ -3545 με for the bone. The results show that the sufficient initial mechanical strength of the fossa component fixation can be established using the screws in combination with bone support. The functional loads applied through the prosthetic TMJ bearing can be largely transferred to supporting bone without causing high level stresses. Tightening fixation screws with a pretension of 100 N can reduce transverse load to the screws and help prevent screw loosening. Further research is recommended to accurately quantify the transverse load and its influence on screw loosening during dynamic loading, and the frictional properties at the bone-implant interface.

Validation of Experimental and Finite Element Biomechanical Evaluation of Human Cadaveric Mandibles

Background: Biomechanical analysis of human mandible is important not only to understand mechanical behavior and structural properties, but also to diagnose and develop treatment options for mandibular disorders. Therefore, the objective of this research was to generate analytical and experimental data on mandibles, construct custom 3D models, and compare the analytically derived maximum strains with strain gage data in five areas of interest for each mandible. Methods: We investigated the surface strains in the cadaveric human mandibles under different configurations of cyclic compressive loads in an experimental setting and compared these experimental strain data with results derived from computational finite element analysis (FEA), accurately replicating the experiments. Strains on the surface of each mandible were measured with strain gauges, and subsequently a subject-specific finite element (FE) volume mesh was generated from computed tomography (CT) scans of each mandible. Strain patterns of each mandible were derived from the FEA simulating the experimental setup and matched with the experimental data. Findings: Analysis of experimental data showed that strain as measured at the condylar locations was significantly different from those at other locations on the mandible, and that the sex and age of the subject did not have a significant correlation with the strain. Comparing the FE numerical predictions with the experimental data, we found a good statistical correlation and statistical agreement between in-vitro measurements and FE results. Interpretation: The study demonstrates that our methodology of generating subject-specific FE models is a valid and accurate, non-invasive method to evaluate the complex biomechanical behavior of human mandibles.