Time course analysis of influence of food hardness on head posture and pitching of head during masticatory movement.

The purpose of the present study was to investigate the relationship between mastication and head posture using foods with different degrees of hardness. A total of 12 healthy, dentulous volunteers participated in the study. Each participant was required to chew two types of gummy candy with two levels of hardness while sitting upright. Measurements were conducted using an optoelectric jaw-tracking system with 6 degrees of freedom (Gnatho-Hexagraph II JM-2000®). The horizontal plane perpendicular to the direction of gravitational force served as the reference plane. Analysis of the gradient of the Frankfurt plane (head posture) and pitching of the head during masticatory movement was conducted. The influence of the type of test food on these parameters was evaluated during mastication. During stable mastication, the gradient of the Frankfurt plane was 4.66 degrees on average, close to the horizontal plane. The time course of the Frankfurt plane gradient revealed a tendency toward dorsal flexion during the first to middle phases of mastication, and a tendency toward ventral flexion during the middle to last phases, regardless of the hardness of the test food. The participants were divided into two groups based on change in head posture during chewing. The results showed while there was no change in head posture in the group with marked pitching of the head, head posture did change in the group with little pitching.