Phase II jaw movements and masseter muscle activity during chewing in Papio anubis

Abstract

It was proposed that the power stroke in primates has two distinct periods of occlusal contact, each with a characteristic motion of the mandibular molars relative to the maxillary molars. The two movements are called phase I and phase II, and they occur sequentially in that order (Kay and Hiiemae [1974] Am J. Phys. Anthropol. 40:227-256, Kay and Hiiemae [1974] Prosimian Biology, Pittsburgh: University of Pittsburgh Press, p. 501-530). Phase I movement is said to be associated with shearing along a series of crests, producing planar phase I facets and crushing on surfaces on the basins of the molars. Phase I terminates in centric occlusion. Phase II movement is said to be associated with grinding along the same surfaces that were used for crushing at the termination of phase I. Hylander et al. ([1987] Am J. Phys. Anthropol. 72:287-312; see also Hiiemae [1984] Food Acquisition and Processing, London: Academic Press, p. 257-281; Hylander and Crompton [1980] Am J. Phys. Anthropol. 52:239-251, [1986] Arch. Oral. Biol. 31:841-848) analyzed data on macaques and suggested that phase II movement may not be nearly as significant for food breakdown as phase I movement simply because, based on the magnitude of mandibular bone strain patterns, adductor muscle and occlusal forces are likely negligible during movement out of centric occlusion. Our goal is to better understand the functional significance of phase II movement within the broader context of masticatory kinematics during the power stroke. We analyze vertical and transverse mandibular motion and relative activity of the masseter and temporalis muscles during phase I and II movements in Papio anubis. We test whether significant muscle activity and, by inference, occlusal force occurs during phase II movement. We find that during phase II movement, there is negligible force developed in the superficial and deep masseter and the anterior and posterior temporalis muscles. Furthermore, mandibular movements are small during phase II compared to phase I. These results suggest that grinding during phase II movement is of minimal importance for food breakdown, and that most food breakdown on phase II facets occurs primarily at the end of phase I movement (i.e., crushing during phase I movement). We note, however, that depending on the orientation of phase I facets, significant grinding also occurs along phase I facets during phase I.

Para-masticatory wear facets and their functional significance in hunter–gatherer maxillary molars

“…The lower inclination found in phase II wear facets is probably linked to the high magnitudes of occlusal and masticatory forces exerted during normal mastication. In fact, during the end of phase I, the food bolus is exposed to crushing (forces exerted perpendicularly to the occlusal surface) and grinding (a combination of forces exerted parallel and normally to the occlusal surface) action, where most of the food breakdown occurs, and consequently large occlusal forces are expected (Kay and Hiiemae, 1974;Wall et al, 2006;Krueger et al, 2008). Recent studies on masticatory movements of Papio anubis suggest that little food processing occurs during phase II (Wall et al, 2006).…”

Spatial and temporal ecological diversity amongst eocene primates of france: Evidence from teeth

“…The power stroke in primates can be divided into two distinct phases: Phase I which is associated with a shearing motion, and Phase II which is associated with grinding (e.g., Kay, 1977). On the basis of the study of the jaw during Phase I and II activity, Wall et al (2006) indicate Phase II movements may not be as significant for food breakdown as Phase I. Indeed, adductors experience little activity during Phase II of chewing in primates.…”

“…In this context, the legitimacy of microwear patterns on Phase II facets has to be reconsidered. Food breakdown on Phase II facets occurs principally at the end of Phase I movement (i.e., crushing during Phase I movement; Wall et al, 2006). Microwear patterns can therefore still reflect diet, as scratches and pits can still form on Phase II facets as a result of activity during Phase I.…”

Function of pretribosphenic and tribosphenic mammalian molars inferred from 3D animation

“…They named the sequential components phases I and II. Wall et al (2006) summarize the phases of Hiiemäe and Kay (1972) as follows: Phase I is an upward, anterior, and medial movement of the lower molars that terminates at maximum intercuspation or in centric occlusion. This is followed by phase II, an anterior, medial, and slightly downward movement of the molars followed by jaw opening.…”

“…Due to the presence of insignificant mandibular strain in the late stage of the power stroke, some authors suggested that occlusal forces are low or negligible during phase II in the tribosphenic molar pattern (Hiiemäe 1984;Hylander et al 1987). This means that in comparison to phase I, only a small percentage of total food breakdown occurs during phase II (Wall et al 2006). However, the several small contact areas that can be identified for a longer power stroke suggest a more versatile way of food breakdown of longer duration for the basic tribosphenic molar.…”