Purpose: The aim of this study was to analyze the mandibular movement of a patient with square mandible using the 4-dimensional (4D) analyzing system, and to suppose the cause of square mandible from characteristics of the movements. Method: Subjects were a healthy volunteer and a patient with square mandible. A volunteer was a 26-year old female who had no missing teeth and no morbid findings in clinical examination. A patient was a 37-year old female. Diagnostic imaging scarcely depicted any disc derangement, but a severely limited jaw opening was noted. As well, her facial appearance showed a characteristic square mandible facial configuration.
The skull and mandible were reconstructed into a 3-dimensional (3D) bone model from the CT data and mandibular movements were recorded by a measurement device of MM-JI-E (Shofu Inc.). The bone model and the mandibular movements were combined using the 4D analyzing system.
The origin and halt of each masticatory muscle were positioned on the surface of the 3D bone model, and connected together with a string respectively. In this system, the color of the string was designed to be passively changed in accordance with mandibular movements.
Results and Discussion: In this case, the cause of opening limitation was not the opening muscle itself but contraction of the temporal muscle. Therefore, it was supposed that the load extended to the masseter muscle from the temporal muscle depended on the masseter muscle, and resulted in muscle hypertrophy or angle hyperplasia.
