Analyzing center of rotation during opening and closing movements of the mandible using computer simulations

The traditional hinge axis theory for guiding clinical procedures in dentistry and dental articulators has been challenged by the concept of an instantaneous center of rotation (ICR), which is becoming more prevalent in modern explanations of mandibular movement. The purpose of this study was to analyze traditional hinge axis theory using three-dimensional computer simulations and to compare it with ICR. Three-dimensional computational models that reproduced the traditional pantograph tracing method were created to simulate the opening and closing movements of the jaw. Models of the bones, muscles and ligaments were combined to create a dynamic representation using ArtiSynth, a biomechanical simulation toolkit. The mandibular motion is constrained based on contact between the articular eminence and the mandibular condyle, and is limited by spring-like ligaments, as well as passive properties of the skeletal muscles. To estimate the center of rotation according to the traditional axis theory, markers on the pantograph were traced during mandibular opening and closing movement. The ICR was computed at each time step throughout the simulation. To locate a single hinge axis in simulation, the point about which the mandible seems to rotate during early opening and terminal closing was determined. The estimated center of rotation was inconsistent with the ICR, yet motion was found to be well approximated by a pure rotation. The inconsistency suggests that the use of the ICR position for the clinical dental procedures has its limitations.