The Kinematics of Proal Chewing in Rats

Abstract

Chewing kinematics are well-documented in several mammal species with fused mandibular symphyses, but relatively understudied in mammals with an unfused symphysis, despite the fact that more than half of extant Mammalia have an unfused mandibular symphysis. The Wistar brown rat (Rattus norvegicus) is widely used in human health research, including studies of mastication or neurological studies where mastication is the output behavior. These animals are known to have unfused mandibular symphyses and proal jaw (rostrocaudal) motion during occlusion, but the lack of high resolution, 3-dimensional analysis of rat chewing leaves the functional significance of symphyseal mobility unknown. We used biplanar fluoroscopy and the X-ray reconstruction of moving morphology workflow to quantify chewing kinematics in 3 brown rats, quantifying overall jaw kinematics, including motions about the temporomandibular joint and unfused mandibular symphysis. During occlusion, the teeth and the mandibular condyle translate almost exclusively anteriorly (proal) during occlusion, with little motion in any other degrees of freedom. At the symphysis, we observed minimal flexion throughout the chew cycle. Overall, there are fundamental differences in jaw kinematics between rats and other mammals and therefore rats are not an appropriate proxy for ancestral mammal jaw mechanics. Additionally, differences between humans and rat chewing kinematics must be considered when using rats as a clinical model for pathological feeding research.

Fig. 1

Marker constellation location and coordinate system orientation. Four tantalum beads were surgically implanted into each of the following bones of 3 rats (Rattus norvegicus): cranium, left hemimandible, and right hemimandible seen from lateral right (A), anterior (B), and lateral left (C) views. An ACS was aligned (see methods for information on alignment) using the upper tooth row occlusal plane (created in Maya; plane) and positioned in the centroid of the second lower molar (D). Lower molar tooth translations were measured relative to the cranium via this ACS. The temporomandibular JCS was positioned at the centroid of the mandibular condyle (E; sphere; following Brainerd et al. 2010 and Menegaz et al. 2015). Symphysis coordinate system was placed at the centroid of the symphysis with ACS axes attached to left and right hemimandibles (F). Note that the depicted posture in (D–F) represents the “zero” position of the joint coordinate systems (i.e. fixed and mobile axes overlapping).

Fig. 2

Chew summary data. Chew cycles were cropped and spline-interpolated to summarize 6 degrees of motion throughout the chew cycle. Measurements were extracted from coordinate systems at the TMJ for rotations (A–B) and translations (C–D) of the mandible and translations (E–F) of the lower second molar. Panels A, D, and E are summarized from an individual rat. Panels B, D, and F are summary data of the absolute amplitude of motions across 3 individuals. Gray shaded area = occlusal phase, dashed lines = 95% confidence intervals, A–P translation during occlusion = yellow lines/red asterisks. Coordinate system axes: anteroposterior (X; red; triangle trace); superoinferior (Y; green; star trace); and transverse (=mediolateral; right; Z; blue; circle).

Fig. 3

Hemimandibles move anteriorly during occlusion. Anterior (A) and lateral (B and C) visualization of tooth movement illustrates mediolateral translation during open and close, but only anteriorly-directed movement during occlusion. Each data point (black) represents one % of the chew cycle. Red (§) = start, mouth open, yellow (¶) = occlusion, and green (#) = end, mouth open.

Fig. 4

Summary kinematics of the mandibular symphysis. Rotational (A) and translational (B) data were cropped and spline-interpolated to summarize 6 degrees of motion to summarize the whole chew cycle. Y-axis scaling is based on rotations/translations of the TMJ (Fig. 2A and C). Coordinate system axes: anteroposterior (X; red; triangle trace); superoinferior (Y; green; star trace); and transverse (=mediolateral; right; Z; blue).