Strain in the braincase and its sutures during function

Bone geometry in cercopithecoid mandibles

This study explores the relation between cortical bone geometry in the mandibular corpus and in vivo masticatory stress patterns and dietary specialization in cercopithecoid primates. Cortical bone distribution in the mandibles of three species of Old World monkeys (Macaca fascicularis, Procolobus badius, Lophocebus albigena) was measured by computed tomography. The arrangement of bone within sections was quantified as (1) the ratio of cortical area to the enclosed (total) subperiosteal area; (2) the ratio of orthogonal second moments of area; and (3) size-adjusted measures of cortical area and regional thickness. Cross-sectional geometry differed among samples, but consistent patterns of cortical thinning and bone area were found within individual sections. This consistency was despite the marked differences in diet and feeding behavior that distinguish the three taxa. Lingually thin and basally thick cortical bone was found in the three monkeys; previously published data suggest that this pattern may be stereotypical among anthropoid primates. It is hypothesized that the interactive effects of shear, bending and torsion produce eccentric loads in corpus sections, which are mirrored by this asymmetrical arrangement of cortical bone. When interpreted against existing data for other primate groups, these results are consistent with the hypothesis that masticatory-loading profiles are broadly similar across anthropoids despite the distinctive occlusions found among the suborder. Understanding of the impact of diet on jaw morphology is, therefore, not improved by considerations of cortical bone distribution, i.e. the inference of diet from jaw form is best predicated on considerations of relative corpus size rather than cross-sectional geometry.

Jaw muscles and the skull in mammals: the biomechanics of mastication

Among non-mammalian vertebrates, rigid skulls with tight sutural junctions are associated with high levels of cranial loading. The rigid skulls of mammals presumably act to resist the stresses of mastication. The pig, Sus scrofa, is a generalized ungulate with a diet rich in resistant foods. This report synthesizes previous work using strain gages bonded to the bones and sutures of the braincase, zygomatic arch, jaw joint, and mandible with new studies on the maxilla. Strains were recorded during unrestrained mastication and/or in anesthetized pigs during muscle stimulation. Bone strains were 100-1000 micro epsilon, except in the braincase, but sutural strains were higher, regardless of region. Strain regimes were specific to different regions, indicating that theoretical treatment of the skull as a unitary structure is probably incorrect. Muscle contraction, especially the masseter, caused strain patterns by four mechanisms: (1) direct loading of muscle attachment areas; (2) a compressive reaction force at the jaw joint; (3) bite force loading on the snout and mandible; and (4) movement causing new points of contact between mandible and cranium. Some expected patterns of loading were not seen. Most notably, strains did not differ for right and left chewing, perhaps because pigs have bilateral occlusion and masseter activity.

In vivo surface strain and stereology of the frontal and maxillary bones of sheep: implications for the structural design of the mammalian skull

Does the skull of the sheep behave as a tube or as a complex of independent bones linked by sutures? Is the architecture within cranial bones optimized to local strain alignment? We attempted to answer these questions for the sheep by recording from rosette strain gauges on each frontal and maxillary bone and from single-axis gauges on each dentary of five sheep while they fed on hay. Bone structure was assessed at each rosette gauge site by stereological analysis of high-resolution radiographs. Structural and strain orientations were tested for statistical agreement. Ranges of strain magnitudes were +/-1200 mu epsilon on the mandible, +/-650 mu epsilon on the frontals, and +/-400 mu epsilon on the maxillae. Each gauge site experienced one strain signal when on the working (chewing) side and a different one when on the balancing (nonchewing) side. The two signals differed in mode, magnitude, and orientation. For example, on the working side, maxillary gauges were under mean compressive strains of -132 mu epsilon (S.D., 73.3 mu epsilon), oriented rostroventrally at 25 degrees -70 degrees to the long axis of the skull. On the balancing side, the same gauges were under mean tensile strains of +319 mu epsilon (S.D., 193.9 mu epsilon), at greater than 65 degrees to the cranial axis. Strain patterns on the frontals are consistent with torsion and bending of the whole skull, indicating some degree of tube-like mechanical behavior. Frontal and maxillary strains also showed a degree of individual loading, resulting from modulation of strains across sutures and local effects of muscle activity. The sheep skull seems to behave as a tube made of a complex of independent bones. Structural orientation was in statistically significant agreement with the orientation of working-side compressive principal strain epsilon 2, even though principal tensile strains may be as large or larger. Cranial bone architecture in sheep is not optimized to both strain signals it experiences.

Loading of the temporomandibular joint: anatomical and in vivo evidence from the bones

'Loading of the TMJ' is usually understood to mean a compressive force applied to the articular surfaces of the jaw joint. Theoretical models of jaw mechanics can be manipulated to support either the presence or the absence of loading, depending on the assumed contraction patterns of the muscles and the assumed occlusion. This paper synthesizes a series of studies on jaw joint function using pigs as substitutes for humans. Bone strain (deformation) was directly measured on the lateral surfaces of the condylar neck and the squamosal (equivalent to the human temporal) bone. Chewing strains indicate that loading does occur and is not light. The peak strains on the condyle are indeed primarily compressive, but the situation is dynamic. Small tensile strains can occur during chewing, and protrusive splints may decrease the strain resulting from muscle stimulation. The squamosal bone is even more surprising, in that the major strain is tensile. The most likely explanation for this finding is that the squamosal bone is bent under the load. Thus, the two elements of the TMJ are deformed in different ways by the same movements and muscle activities. Internal bony architecture reflects these differences. The condyle is filled with fine, vertically oriented bony trabeculae. The articular eminence has thick cortices and trabeculae oriented approximately transversely. In conclusion, the TMJ is loaded, but the situation is complex. The largest forces seen by the condyle are compressive, and they arise from muscle contraction. These same forces serve to bend the squamosal bone.

In vivo function of the craniofacial haft: the interorbital "pillar"

The craniofacial haft resists forces generated in the face during feeding, but the importance of these forces for the form of the craniofacial haft remains to be determined. In vivo bone strain data were recorded from the medial orbital wall in an owl monkey (Aotus), rhesus macaques (Macaca mulatta), and a galago (Otolemur) during feeding. These data were used to determine whether: the interorbital region can be modeled as a simple beam under bending or shear; the face is twisting on the brain case during unilateral biting or mastication; the interorbital "pillar" is being axially compressed during incisor loading and both axially compressed and laterally bent during mastication; and the interorbital "pillar" transmits axial compressive forces from the toothrow to the braincase. The strain data reveal that the interorbital region cannot be modeled as a anteroposteriorly oriented beam bent superiorly in the sagittal plane during incision or mastication. The strain orientations recorded in the majority of experiments are concordant with those predicted for a short beam under shear, although the anthropoids displayed evidence of multiple loading regimes in the medial orbital wall. Strain orientation data corroborate the hypothesis that the strepsirrhine face is twisted during mastication. The hypothesis that the interorbital region is a member in a rigid frame subjected to axial compression during mastication receives some support. The hypothesis that the interorbital region is a member in a rigid frame subjected to lateral bending during mastication is supported by the epsilon1/absolute value epsilon2 ratio data but not by the strain orientation data. The timing of peak shear strains in the medial orbital wall of anthropoids does not bear a consistent relationship to the timing of peak shear strain in the mandibular corpus, suggesting that bite force is not the only external force influencing the medial orbital wall. Strain orientation data suggest the existence of two distinct loading regimes, possibly associated with masseter or medial pterygoid contraction. Regardless of the loading regime, all taxa showed low strain magnitudes in the medial orbital wall relative to the anterior root of the zygoma and the mandibular corpus. The strain gradients documented here and elsewhere suggest that, in anthropoids at least, local effects of external forces are more important than a single global loading regime. The low strain magnitudes in the medial orbital wall and in other thin bony plates around the orbit suggest that these structures are not optimally designed for resisting feeding forces. It is hypothesized that their function is to provide rigid support and protection for soft-tissue structures such as the nasal epithelium, the brain, meninges, and the eye and its adnexa. In contrast with the face of Otolemur, which appears to be subjected to a single predominant loading regime, anthropoids may experience different loading regimes in different parts of the face. This implies that the anthropoid and strepsirrhine facial skulls might be optimized for different functions.

Mammalian feeding and primate evolution: An overview

Most of the papers included in this volume are derived from presentations in a symposium on Mammalian Feeding at the 65th Annual Meetings of the American Association of Physical Anthropologists in North Carolina in 1996. The aims of this symposium were to gather together the preeminent researchers on mammalian mastication and document the state of research in that field. The symposium emphasized in vivo studies of mammalian feeding because of a paucity of recent reviews of this field, but included morphometric and modeling papers as well. Subsequently the papers were revised, and were submitted in spring 1998 for publication, pending the outcome of peer review. Copyright 2000 Wiley-Liss, Inc.