Development of the helicoidal plane

In this study of thebelicoidal occlusal plane the relationships between tooth wear, the transverse slopes of mandibular molars and dental arch breadths were examined in 74 pre-contemporary Australian Aboriginal skulls. With increasing age and tooth wear the orientation of the mandibular occlusal surfaces increased towards the buccal. The differential occlusal orientation from first to third molars, present at eruption, tended to increase progressively with tooth wear. These functionally induced changes, together with regional differences in relative breadths of the maxillary and mandibular dental arches, are important in the development of abelicoidal occlusal plane.     

Development and evolution of the helicoidal plane of dental occlusion

The helicoidal plane of dental occlusion is a composite feature involving axial inclination of teeth and effects of dental attrition. Recent studies disagree on its distribution and significance in hominoid primates. The distribution, development, and functional basis of the helicoidal plane are investigated here, based on quantitative analysis of dental morphology and attrition in 667 human and 60 chimpanzee dentitions. Helicoidal planes are nearly universal in the human and chimpanzee dentitions studied. Increasing axial inclination of molars from M1 to M3 is primarily responsible for the helicoidal plane, although attrition acts to increase its expression. In hominoids, increased molar axial tilt appears to be associated with facial shortening and dental reduction. Population and species comparisons suggest a functional relationship with cranial structure. Progressive axial tilt of molars producing a helicoidal plane is found consistently in mammals with cheek teeth positioned partly under the cranium, as in hominids, pongids, some cebids, macropodids, ursids, and sciurids. Facial shortening is an important trend in hominid evolution and axial inclination of molars might be expected to show progressive change from Australopithecus afarensis to recent Homo sapiens.     

Relationships between attrition and lingual tilting in human teeth

Associations between occlusal dental attrition and the lingual tilting of human teeth were investigated in two aboriginal California populations. A literature survey suggests lingual tilting is related geographically and temporally to the helicoidal occlusal plane in humans; if true, lingual tilting should be expected throughout the Homo osteological record. Buccal alveolar abscesses, exposed pulp chambers, extent of tooth attrition, angle of lingual tilt, and amount of overjet were observed for lingually tilted teeth. These attributes were analyzed statistically, leading to a conclusion that lingual tilting is slight and infrequent unless tooth attrition is pronounced. It is suggested that lingual tilting is due in part to masticatory stress. A feedback model considers lingual tilting as one conspicuous manifestation of a more complex “severe attrition syndrome,” the initial stimulus for which may derive from heavy occlusal tooth wear. Untested possibilities should be examined–e.g., age-related changes, the impact of the helicoidal occlusal plane, and the role of masticatory forces.     

Functional patterns of molar occlusion in platyrrhine primates

Mechanico-functional features of molar form were studied in Callithrix, Alouatta, Pithecia and Cebus. Molars of Callithrix and Alouatta are adapted to loading foods under relatively high occlusal pressure; those of Pithecia and Cebus, under relatively low occlusal pressure. General functional considerations suggest that these taxa are adapted to insectivorous, folivorous, frugivorous and omnivorous diets, respectively. The physical properties of foods, principally mechanical strength and deformability, determine the selective pressures involved in the evolutionary adaptation of molar form. A dietary classification based upon percentages of foods eaten does not always reflect morphological adaptations. Homologous parts of teeth and homologous parts of the masticatory cycle do not always subserve equivalent functions. The relevance of functional occlusal analysis for deciphering phylogeny and explaining evolutionary grades is stressed.     

Functional relationships between dental attrition and the helicoidal plane

Several quantitative techniques were used to describe and analyze the development of the helicoidal curve of the dentition in a pre-contact sample of 326 native Indians from British Columbia. Results were compared with studies of occlusal patterns in other populations, and anatomical-functional features of the helicoidal pattern were abstracted and generalized. The features that define the helicoidal pattern were found to develop with attrition. The pattern itself, and the complex which produces it, probably have been selected for because they produce an occlusion with an efficient cutting edge and a total complex that provides maximum resistance to the masticatory forces. Studies focusing on the developmental nature of occlusal patterns in early hominid populations would be beneficial in developing a model of the evolution of the helicoidal pattern in modern man.     

Molar occlusion and jaw mechanics of the Eocene primate Adapis

Wear facets on molars of the Eocene primate Adapis magnus are described. Striations on these wear facets indicate three separate directions of mandibular movement during mastication. One direction corresponds to a first stage of mastication involving orthal retraction of the mandible. The remaining two directions correspond to buccal and lingual phases of a second stage of mastication involving a transverse movement of the mandible. The mechanics of jaw adduction are analysed for both the orthal retraction and transverse stages of mastication. During the orthal retraction stage the greatest component of bite force is provided by the temporalis muscles acting directly against the food with the mandible functioning as a link rather than as a lever. A geometrical argument suggests that during the transverse stage of mastication bite force is provided by the temporalis muscles of both sides, the ipsilateral medial and lateral pterygoid muscles, and the contralateral masseter muscle.     

Reduction of maxillary molars in Homo sapiens sapiens: a different perspective.

Crown and cusp areas, and buccolingual and mesiodistal diameters of maxillary molars of complete upper tooth rows (30 males, 30 females) were analysed in order to quantify changes in size and shape from the first to the third molar. Uni- and multivariate analyses revealed the mesial cusps, in particular the protocone (mesiolingual cusp), to be more stable than the other cusps. Although there is a gradient in size from the first to third molar, shape changes were found to be marked. Overall, the findings are in keeping with the field theory and the hypotheses of environmental constraints on later developing teeth. However, not all of the results could be entirely explained by these concepts. Functional aspects seem to account for the relative stability of the protocone and the buccolingual crown diameter. It appears that this functional complex is relatively stable despite the overall reduction of tooth size, which is probably secondary to processes occurring in the jaws and the cranium. This finding may have implications for studies on tooth reduction between populations of different time periods.     

Relationships between age and dental attrition in Australian aboriginals

Tooth wear scores (ratios of exposed dentin to total crown area) were calculated from dental casts of Australian Aboriginal subjects of known age from three populations. Linear regression equations relating attrition scores to age were derived. The slope of the regression line reflects the rate of tooth wear, and the intercept is related to the timing of first exposure of dentin. Differences in morphology between anterior and posterior teeth are reflected in a linear relationship between attrition scores and age for anterior teeth but a logarithmic relationship for posterior teeth. Correlations between age and attrition range from less than 0.40 for third molars (where differences in the eruption and occlusion of the teeth resulted in different patterns of wear) to greater than 0.80 for the premolars and first molars. Because of the generally high correlations between age and attrition, it is possible to estimate age from the extent of tooth wear with confidence limits of the order of +/- 10 years.     

Model for Molecular Interactions in Helicoidal Cavities

Abstract

A Fourier transform procedure using curvilinear coordinates is developed to determine the interaction potential between a molecule and a helicoidal cavity. When the molecular orientation is free inside the cavity, straightforward criteria for the applicability of the continuum approximation and the molecular confinement along the cavity axis are obtained. At low temperature, the molecule does not rotate freely, being generally trapped in potential wells outside the helix axis and numerical calculations are required to verify whether these criteria hold. An application to N₂ trapped in an AlPO₄-5 cavity shows that the molecule center of mass is confined close to the cavity axis with the molecular axis collinear to the cavity axis.     

Changes in articular eminence morphology with dental function

Measurements of mandibular fossa depth and slope of the articular eminence were obtained for human skeletal samples chosen to represent a wide spectrum of subsistence strategies and oral function: hunter-gatherers (Eskimos, Australians), American Indians dependent to a variable extent on maize agriculture, and early twentieth century American whites and blacks. In the Eskimo and Australian samples, a generalized and steady increase in fossa depth and slope was observed with increasing functional age (tooth wear) through wear level 5 (of 8), followed by a levelling off or slight decrease in fossa depth in later wear levels on the anterior teeth and a sharp decrease in fossa depth in later wear levels on the molars. Although much less consistent due in part to extensive and early molar loss, patterns of variation in the remaining samples were characterized overall by a decrease in fossa depth and slope with increasing wear, especially on the molars. Further, in those samples with high incidences of posterior tooth loss, fossa depth was routinely less and the eminence more gently sloped in subsamples having experienced molar loss than in subsamples retaining all their molars. These data provide evidence that the temporomandibular joint (TMJ) undergoes continuous morphological alteration throughout adult life, and that these alterations are probably mediated by dental function. Moreover, they suggest that differences in patterning of such alterations may exist among human groups with contrasting patterns of tooth use.     

The natural history of the helicoidal occlusal plane and its evolution in early Homo

In modern man the pitch of the occlusal plane may vary along the tooth-row. When anterior cheek-teeth show a plane sloping upward palatally, whilst that on posterior cheek-teeth slopes upward buccally, there results a twisted or helicoidal occlusal plane (Ackermann). Several hypotheses have been proposed for the structural basis of the helicoidal occlusal plane. Campbell's proposal ('25) has gained widest acceptance, namely that the helicoid results from anteroposterior differences in upper and lower alveolar arch width. In the early 1960s, while studying the Olduvai hominids assigned to Homo habilis, the author noted changing occlusal slopes along the tooth-row and a slight helicoid, although these featues had not been noted in other early hominids. Subsequently, Wallace showed a total absence of the helicoid from South African australopithecines, and its presence in Swartkrans Homo, SK 45 and SK 80. Recent studies confirm the presence of the helicoid in all available specimens of H. habilis, including Stw 53 found at Sterkfontein in 1976. Hence, this trait may distinguish between Australopithecus and early Homo. Measurements of the maxillary arch widths have shown that, whereas in Australopithecus arch widths increase to a maximum at M3, in early Homo maxillary arch widths are greatest at M2. The decline in posterior maxillary arch width is part of a general reduction of that region. Thus despite striking elongation of premolars and M1 in early Homo, M2 and M3 are mesiodistally abbreviated. It is hypothesized that the onset of posterior arch reduction, with the appearance of a helicoid, was a structural and functional concomitant of the transition from the presumed australopithecine ancestor to H. habilis.     

Palaeoecology of cave bears as evidenced by dental wear analysis: a review of methods and recent findings

Abstract

The study of dental wear was first used years ago to infer the palaeoecology of fossil mammals and in particular their diet. Results depend predominantly on the scale of the analysis used. Analyses of dental macrowear, mesowear or microwear do not provide the same type of dietary information, be it about the seasonal, annual or lifetime diet. This contribution focuses on emblematic species, cave bears (Ursidae), in particular Ursus spelaeus spelaeus. Methods used by previous researchers to infer their dietary preferences and thus their palaeoecology are reviewed and compared. This review is complemented by an analysis of several specimens of cave bears from the Goyet cave in Belgium, using dental microwear texture analysis (DMTA), a methodology widely applied for reconstructing palaeodiets. Three main conclusions are drawn here: (1) DMTA is the method that provides the most precise palaeobiological inferences; (2) during the pre-dormancy period, cave bears show dietary flexibility; (3) dental wear alone might be not sufficient to provide a complete reconstruction of the cave bear palaeodiet.     

Individual tooth macrowear pattern guides the reconstruction of Sts 52 (Australopithecus africanus) dental arches

The functional restoration of the occlusal relationship between maxillary and mandibular tooth rows is a major challenge in modern dentistry and maxillofacial surgery. Similar technical challenges are present in paleoanthropology when considering fragmented and deformed mandibular and maxillary fossils. Sts 52, an Australopithecus africanus specimen from Sterkfontein Member 4, represents a typical case where the original shape of the dental arches is no longer preserved. It includes a partial lower face (Sts 52a) and a fragmented mandible (Sts 52b), both incomplete and damaged to such an extent to thwart attempts at matching upper and lower dentitions. We show how the preserved macrowear pattern of the tooth crowns can be used to functionally reconstruct Sts 52's dental arches. High‐resolutiondental stone casts of Sts 52 maxillary and mandibular dentition were mounted and repositioned in a dental articulator. The occlusal relationship between antagonists was restored based on the analysis of the occlusal wear pattern of each preserved tooth, considering all dental contact movements represented in the occlusal compass. The reconstructed dental arches were three‐dimensional surface scanned and their occlusal kinematics tested in a simulation. The outcome of this contribution is the first functional restoration of A. africanus dental arches providing new morphometric data for specimen Sts 52. It is noteworthy that the method described in this case study might be applied to several other fossilspecimens. Am J Phys Anthropol, 2013. © 2013 Wiley Periodicals, Inc.     

Patterns of dental wear and the role of the canine in Cercopithecinae.

The importance of dental wear patterns in understanding masticatory functions in primates has long been appreciated. However, studies of wear patterns among populations of nonhuman primates are few. The purpose of this investigation is to establish the developmental aspects of dental wear in the Cercopithecinae and to describe certain relevant morphological traits. Studies were made of dental casts from 200 primate specimens of Macaca nemestrina, Macaca mulatta, and Papio cynocephalus. These casts were taken at four-month intervals, beginning at two years of age and continuing over a period of six to seven years. The wear pattern starts with the rounding and eventual flattening of the protoconid and protocone of the erupted first molars. Once this stage is reached, the hypoconid and metaconid of the mandibular, and the hypocone and paracone of the maxillary molars are rounded and eventually flattened. This pattern is maintained until the cusp tips are removed and the dentin exposed, however, the entoconid and metacone are not subjected to significant wear at this stage. Analysis of these dental casts and museum specimens has provided data on the development of dental wear during the maturation of these primates. The distribution of forces acting upon the teeth produce diagnostic patterns of wear, which provide evidence of the force location and magnitude. In examining the data, the hypothesis of canine guidance and its limitation of mandibular motion was evaluated. Specimens whose canines were removed demonstrate that the canines play no significant role in the development or maintenance of dental wear planes.     

Enamel thickness and the helicoidal occlusal plane

In the present study 38 unworn maxillary molars (M¹ = 16, M²= 12, M³ = 10) of modern humans from a Slavic necropolis were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Five slightly worn M¹s and one slightly worn M³ were also used thus increasing the total sample to 44, but measurements made on the worn areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether changes in these dimensions in anteroposterior direction can be related to the helicoidal occlusal plane. Uni- and multivariate analyses revealed that the distribution of enamel thickness within and between maxillary molars corresponds to a helicoidal occlusal wear pattern. Enamel thickness along the occlusal basin increases from anterior to posterior, which may lead to rapid development of a reverse curve of Monson in first molars when compared to posterior teeth. However, although these overall differences together with the serial, especially delayed eruption pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between molars indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under the Phase I wear facet of the paracone, i. e., the lingual slope of the paracone, than under the Phase II facet of the protocone, i. e., the buccal slope of that cusp. These proportional differences are most pronounced in M³, as evidenced by uni- and multivariate statistics. It thus appears that the pattern of enamel thickness distribution from M¹ to M³ follows a trend towards providing additional tooth material in areas that are under greater functional demands, that is, corresponding to a lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In addition, the heights of the dentine horns in anteroposterior direction change in a way that lends support to the hypothesis that the axial inclination of teeth could be one of the most important factors for the development of the helicoidal occlusal plane. Finally, the changes in morphology and enamel thickness distribution from first to third upper molars found in this study suggest that molars could be “specialized” in their function, i. e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly. © 1994 Wiley-Liss, Inc.     

Specialized wear facets and late ontogeny in mammalian dentitions

Abstract

Many types of wear facets can be found in mammalian teeth. Some are related to the initial surface, others use the cross-section of the enamel as the main tool. In primary occlusal surfaces facets mark the gradual wear, that are related to a relatively late ontogeny. Facets in teeth with secondary occlusal surfaces, however, represent specific arrangements of crests of enamel and dentine. Such facets require some initial wear to become fully functional. The tooth morphology guarantees such facets to be effective for a long period of time. Therefore they can be discriminated as specialized facets. From the different types of facets three specialized ones were selected, blade facets, rasp-facets, and nipper-facets, because they ate widely distributed, function differently, and are comparable with mechanical tools. They are long lasting and differ in the amount of exposed dentine. The amount of dentine is used to differentiate phases during late ontogeny, the part of the life history, when teeth are exposed to wear. Consequently the various types of facets can be related to different ontogenetic phases. The relevant phases are prolonged at the cost of other ontogenetic phases. Therefore, the various specialized wear facets represent heterochronies within the ontogeny of teeth.     

Preliminary examination of non-occlusal dental microwear in anthropoids: Implications for the study of fossil primates

Most studies of microscopic wear on non-human primate teeth have focused on the occlusal surfaces of molars. Recent analyses of the buccal surfaces of human cheek teeth have demonstrated an association between diet and dental microwear on the these surfaces as well. In the current study, we examine microwear on both the buccal and lingual surfaces of non-human primate molars to assess the potential of these surfaces to reveal information concerning anthropoid feeding behaviors. We compare frequency of microwear occurrence in 12 extant and 11 fossil anthropoid species. Among the living primates, the occurrence of microwear on non-occlusal surfaces appears to relate to both diet and degree of terrestriality. The implications of this research for the inference of feeding behaviors and substrate use in fossil cercopithecoids are discussed. © 1996 Wiley-Liss, Inc.     

Form and patterning of anterior tooth wear among aboriginal human groups

Form and severity of dental attrition was assessed in aboriginal human groups including hunter-gatherers (Eskimos, Australians) and those with dependence to a varying degree on food production (Southwest U.S. and Ohio American Indians). Wear on anterior teeth was both relatively and absolutely greater in the hunter-gatherers, as indicated by comparisons of wear on anterior and posterior teeth which come into occlusion at roughly the same time. Distinct differences in form of anterior wear were also apparent: The hunter-gatherers exhibited steadily increasing incidences of labially rounded wear with greater functional age, while the food-producing groups showed little or no rounding but instead high frequencies of heavily cupped wear (especially in those with premature loss of posterior teeth). These differences were attributed to nonmasticatory utilization of the front teeth in hunter-gatherers and to employment of the anterior teeth in masticatory (grinding) activities necessitated by large-scale molar loss in food producers.