Journal of Oral Rehabilitation 2000 27; 299 – 305 Kinematics of the human mandible for different head postures C. M. VISSCHER, J. J. R. HUDDLESTON SLATER, F. LOBBEZOO & M. NAEIJE Department of Oral Function, Section Craniomandibular Disorders, Academic Centre for Dentistry Amsterdam, Amsterdam, Netherlands SUMMARY The influence of head posture on move- relative to the path in a natural head posture. In a forward head posture, the movement path is shifted ment paths of the incisal point (IP) and of the posteriorly whereas during lateroflexion, it deviates mandibular condyles during free open – close move- ments was studied. Ten persons, without cran- to the side the head has moved to. Moreover, the intra-articular distance in the temporomandibular iomandibular or cervical spine disorders, partici- joint during closing is smaller with the head in pated in the study. Open close mandibular move- ments were recorded with the head in five pos- military posture and greater in forward head pos- ture, as compared to the natural head posture. Dur- tures, viz., natural head posture, forward head ing lateroflexion, the intra-articular distance on the posture, military posture, and lateroflexion to the right and to the left side, using the Oral Kinesiolog- ipsilateral side is smaller. The influence of head ic Analysis System (OKAS-3D). This study showed posture upon the kinematics of the mandible is probably a manifestation of differences in mandi- that in a military head posture, the opening move- ment path of the incisal point is shifted anteriorly bular loading in the different head postures. Introduction For decades, the kinematics of the human mandible have been the topic of investigation (Uhlrich, 1959; Gibbs et al., 1971; Goodson & Johansen, 1975; Gibbs & Lundeen 1982). Until recently, with most of the available mandibular movement recording instru- ments it was only possible to study the movements of a single point of the mandible, usually the lower incisal point. The movement paths of this point dur- ing free open – close and laterotrusive movements as well as during chewing are well documented (Posselt, 1952). More recently, six degrees of freedom jaw movement recording systems have become available. They make it possible to reconstruct the movement paths of any point of the lower jaw with respect to the upper jaw (Gibbs et al., 1971; Merlini & Palla, 1988; Pro ¨ schel et al., 1993; Yatabe et al., 1995). This offers the possibility to study the movements of the condyle within the temporomandibular joint as well. One of the interesting suggestions which has come from these condylar movement studies was that dur- ing free open – close movements, the kinematic centre of the condyle/disc complex would move closer to the articular eminence during opening than during closing. Yatabe et al. (1997) concluded that during opening, the condyle – disc complex moves along the articular eminence; during closing there is a small distance between the condyle – disc complex and the articular eminence: the intra-articular distance. This intra-articular distance appears to be dependent upon the mechanical load imposed upon the mandible dur- ing closure (Naeije & Lobbezoo, 1997; Huddleston Slater, Visscher & Naeije, 1998). A small mechanical resistance counteracting mandibular closure makes the intra-articular distance smaller, indicating that the condyle – disc complex is then also slightly pressed against the articular eminence. © 2000 Blackwell Science Ltd 299