Jaw movement is a complex motion as translational (anterior-posterior, left-right, and upper-lower) and rotational (around each axis) motion of mandible in the three-dimensional space. The movement of the mandible observed from the maxilla is called mandibular movement, while the movement of the maxilla observed from the mandible is called complementary mandibular movement. Complementary mandibular movement is often not clearly distinguished from mandibular movement, even though it is frequently used in clinical practice, as in the case of articulator with incisal guide table attached to the mandible and condylar-type articulators.

In the present review, the authors addressed Gothic arch drawing in the horizontal plane, border movement trajectory in the sagittal plane, as well as the configuration of the articulator and its correspondence to jaw movement, and organized the mandibular movements and complementary mandibular movements so that the relationship and differences between them can be properly understood. In addition, in order to reproduce jaw movement as numerical model in the coming CAD/CAM era, the concepts of kinematic axis as a model of mandibular movement, complementary kinematic axis as a model of complementary mandibular movement, and intermaxillary axis as a model common to both mandibular movement and complementary mandibular movement are presented. Moreover, in order for readers to be able to calculate the mandibular and complementary mandibular pathways, and the respective axes at arbitrary positions using actual jaw movement data, the authors have carefully described the calculation procedure using Excel and devised a way for readers to experience jaw movement analysis.

Jaw movement is a 6-degree-of-freedom movement consisting of translational and rotational motion in three directions, but the present review focuses on 2-degree-of freedom and 3-degree-of-freedom jaw movements. It is hoped that this review will be the first step for readers who are learning to understand jaw movement.

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.

Digastric muscle (Dig) is composed of anterior belly (Ant-Dig) and posterior belly (Post-Dig) innervated by different motor neurons. Ant-Dig reflex is known to be a jaw-opening reflex and numerous studies have reported modulation of Ant-Dig reflex responses in functions (Lund and Olsson, 1983; Yamamura et al., 1998; Yamada et al., 2013). However, the Post-Dig reflex and its contribution to orofacial function have never been investigated before.

This is the first time to reveal the nature and modulation of Post-Dig reflex during rest, chew, lick, and swallow. We investigated how the reflex was evoked and modulated in the functions.