2023 年 29 巻 2 号 p. 91-99
Abnormal paths of the incisal point would have shown the presence of a temporomandibular joint dysfunction (TMD). However, it is difficult to estimate the condylar movement based on the incisal movement. There was no functional or biomechanical basis for clinically using the maximum opening distance as an indicator of condyle gliding. Therefore, it is necessary to use a six-degree-of-freedom jaw tracking device to observe and assess the condylar movement.
The purpose of this study was to propose a new analytical method to evaluate mandibular functions by observing TMJ kinematic parameters in a post-treatment TMD patient.
A 56-year-old male TMD patient had mouth opening limitation with configuration change in his left condyle. To assess jaw function of the post-treatment TMD patient, his jaw movement characteristics were compared to those of a healthy volunteer with the almost same range of mouth opening.
An upper incisal point and a left arbitrary condyle axis point (L-AP) were used as reference points. In addition, two points (L-rAP and L-CP) were used to analyze the left condylar movement. The L-rAP was below the L-AP with the same range of and opposite direction of movement in antero-posterior direction. The L-CP was the midpoint between the L-AP and L-rAP. Movement trajectories of reference points were investigated.
The trajectory of the left condylar movement in the volunteer was a downward-convex curve, whereas that in the TMD patient was an upward-convex curve. In the volunteer, the ratio of rotational and translational movements was appropriately coordinated during opening-closing movement. In contrast, in the TMD patient, the rotational movements around the L-CP mainly occurred. This rotational movement caused the forward movement of the left condyle, thus avoiding the left side deviation of the mandible and the limitation of the opening. The adaptive changes in jaw movement characteristics were occurred in the post-treatment TMD patient. These findings suggested that, in terms of kinematics, the L-CP was a reference point that resembles the least motion axis (LMA) rather than the instantaneous center of rotation.
As a result of this study, it is suggested that the mandibular TMJ kinematic parameters in the TMD patient were different from those in the volunteer, and it was suggested that establishing three reference points, including a point located away from the condylar point, was as useful as the method using the LMA for analyzing the movement of the entire mandible and for identifying the characteristics of the condylar movement.
