Effects on temporomandibular joint caused by orthodontic intermaxillary elastics

Abstract

The Finite element method was used to investigate whether variations of intermaxillary elastics would develop harmful effects on the healthy temporomandibular joint. The biomechanical behaviour of computed dynamic mouth opening and closing without and with seven configurations of orthodontic intermaxillary elastics were analysed. We developed a finite element model with a masticatory system based on anonymised CT and MRI scans using Mimics and MSC.Marc/Mentat software. Subjects were individu-als with normal occlusion and without temporomandibular disorder. The two six-degree-of-freedom temporomandibular joints consisted of articular cartilage, disc, dis-cal ligament and retrodiscal lamina. Twelve pairs of muscles approximated by Hill-type point-to-point actuators were employed to move the mandible. <br /> The material properties of cartilaginous tissues were considered as Mooney-Rivlin ma-terial model. The simulated kinematic behaviour of mouth opening and closing without orthodontic intermaxillary elastics was consistent with numerous in vivo studies. The compressive stress was mainly at the intermediate zone of the disc and the anterior slope of the condylar process on full mouth opening. With around 1.25 N medium in-termaxillary elastics initial load at intercuspation, the distribution of the maximum prin-cipal stresses in the discs of the models with and without elastics at full mouth opening were quite similar. <br /> Compared with the simulation without elastics, the tensile and compressive stresses in the discs with different intermaxillary elastics were not significantly larger. In some sim-ulations with elastics, the tensile or compressive stress was significantly smaller, but there was no regulation of the difference, according to the value of the left and right sides. It is the same with the stresses in the condylar cartilage, and they were compa-rable with the stresses in the discs. This is consistent with previous studies. In general, the movement of the mandible without and with seven configurations of elastics was highly alike in pattern and magnitude. This also supported the resemblance of the dis-tribution of stresses on the surface of the discs and condylar cartilage. <br /> In the results of non-symmetrical elastics simulations, molar traction and anterior teeth traction, there was obvious displacement in the negative direction on the x-axis, before 0.06s. This means that the mandible moved toward the anchorage side at the early stage of mouth opening, but this was corrected after a few steps as the amplitude of the mouth depressor muscles increased. <br /> The finite element model of TMJ including the masticatory system helped us to observe and analyse the biomechanical behaviour of the joint during functional loading. Ortho-dontic intermaxillary elastics did not significantly influence the distribution and magni-tude of the stress in cartilaginous tissues, and the kinematic behaviour of the mandible. In general, the present finite element model can help to analyse the relationship be-tween temporomandibular joint and therapeutic approaches in the clinic. Nevertheless, due to the individual diversities, the complex material properties of the cartilaginous tissues and dynamic masticatory behaviour, the computed result is only advocated as a reference for the clinic treatment.