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Perspectives in Human Biology

Editor: Charles E. Oxnard

Centre for Human Biology, Department of Anatomy and Human Biology, University of Western Australia, Nedlands, WA 6907, Australia, 1999.

Volume 4, Issue 3: Dento-facial variation in perspectiveNote 1

Grant Townsend and Jules Kaiser (Issue Editors)

Reviewed for naturalSCIENCE by

Stephen Molnar

Professor Emeritus of Anthropology, Washington University in St. Louis, St. Louis, MO 63130-4899, USA


This volume contains a collection of research reports presented at the 1997 Annual Conference of the Australian Society for Human Biology, held in conjunction with the Fifth World Academic Conference on Human Ecology. These 20 papers, preceded by brief introductory comments by the editors, cover a broad range of topics relating to dento-facial structures and cranial variation which are of considerable interest to students of the ever-expanding field of dental anthropology.

The papers describe methods ranging from the morphometrics of dental arches and crown dimensions to CAT scans and magnetic resonance imaging of the craniofacial skeleton. Computer analysis of facial profiles of lateral radiographs is given special attention in several of the papers, and the causes and variation of tooth wear are described in some detail. Also, the results from an ongoing multi-year twin study enable quantification of dental and facial asymmetry and provide a clearer understanding of the relative influences of genes and environment on a number of complex polygenic traits.

The volume leads off with a keynote address that critically discusses the use of dental complexes to establish population affinities. This paper highlights the long debate over phylogenetic comparisons between populations based on discrete dental traits, and the extent to which observed morphological varation is genetic or environmental in origin. The author urges caution in the application of population genetics to the study of the frequency of such traits and stresses the need for more and better genetic dental studies. It is encouraging that several of the conference papers help shed light on these questions of relative environmental and genetic influences.

The five papers that follow the keynote address report on various results from an ongoing Australian twin study that includes 519 pairs (245 monozygotic (MZ) and 274 dizygotic (DZ)). Comparisons of Carabelli's trait in both permanent and deciduous teeth show that expression of the trait is under strong genetic control, especially in the deciduous molars. Expression of this trait was higher in the MZ twins. Another study describes a high concordance of anterior spacing (primate or canine diastema) in primary dentition; it was higher in MZ than DZ twins, and lower in the mandible than in maxillary arches. These results are to be expected for monozygotic twins, but the next two papers on Handedness and Asymmetry failed to demonstrate any genetic influences. Non-right-handedness was higher in both MZ and DZ twins than in the general population (12-23% vs. 10%) but there were no differences between twin types and the degree of concordance was similar in both. Comparisons of asymmetry of deciduous dentition crown measurements (BL × MD) failed to confirm that twins were more symmetric than singletons. The fifth paper of this group uses a statistical approach to reveal the degree of genetic influence on dental variation. Crown sizes of the permanent teeth of 298 twin pairs were used to develop a biometric model that partitions genetic variance from environmental influence by comparison of covariance of twin pairs. This newer version of the heritability formula brings us closer to understanding the development of multilfactorial traits. We are still a long way, however, from identifying the degree of genetic influence. Differences in intrauterine environments account for major twin variations which obscure genetic factors.

In addition to the data derived from the Australian twin studies, analysis of dental models of subjects from a Finnish growth study that included persons with 47 chromosomes (XXY, Klinefelter's syndrome) demonstrated that the additional X chromosome had its greatest effect on intercuspal dimensions (maxillary premolars). These results are consistent with a model that postulates X chromosome influence on enamel thickness. The complexity of the interaction of oral dental structures and growth is further highlighted by a study of dental crown sizes of children with varying degrees of cleft lip with or without cleft palate. Comparisons of right and left deciduous teeth showed an increase in fluctuating asymmetry of teeth in the cleft region. Also, among cleft palate children, there was a reduction in dental crown dimensions of permanent teeth.

Three papers discuss occlusal tooth wear, and the comparative data gained from observations of Australian Aboriginal crania and from Euro-Australian clinical cases will be quite useful to anthropologists. The study of near-contemporary Aboriginal crania of populations from the Adelaide, South Australia area offers several valuable insights into the interrelationships between occlusal wear and craniofacial morphology. The observation of a negative correlation between cranial base length and molar wear offers convincing evidence of the influence of masticatory biomechanics on the distribution of occlusal loading. The next paper closely examines several clinical aspects of tooth wear found among clinical patients caused by abrasion, attrition, erosion or a combination of all three. The finer detail of microwear is described, and its differing forms and causes are illustrated by the microwear patterns produced on acrylic nightguards used to treat bruxism. It appears that attrition is universal and produces extensive wear even among contemporary populations. A third paper offers some much needed data on the tooth wear of deciduous teeth. Dental casts of 30 Australian aboriginal children, participants in a semi-longitudinal growth study at Yuendumu, Central Australia, were examined at age 4-5 years and again at age 10-11, and comparisons were made with Caucasian children of similar ages. Photographs and scoring methods accurately describe the rates of dentin exposure. Rates of wear were extensive in both groups but greater in the Caucasian group, probably due to the erosive nature of their diet. The wear processes of attrition and abrasion, however, predominated in Aboriginal children.

Molar morphology of Australian Aboriginals and Cook Islanders is described in detail in two papers. The first uses an interesting and useful application of Moiré contourography to detail morphology and cusp heights taken in 48 casts of females with unworn dentitions. This provides a description of relative cusp sizes (dm2) on the crowns and permanent molars. In the second study, components of mandibular molars of Cook Islanders, measured with sliding calipers, showed the second molars to be more variable than the first. The second molar trigonid and talonid components were smaller, and this pattern was the same for both sexes. These data should add to the range of variables encompassed in comparison of population dental complexes.

Comparative studes of dental and craniofacial morphology were offered in the next two papers. Dental arches of several Pacific Island populations were contrasted for size and shape with Japanese and Australian populations. The Fijian arches were the largest and characterized by wide V-shaped maxilla. Degrees of basicranial flexion were described for Maori and Moriori and compared with Indian skulls. The depth of the madibular (glenoid) fossa was related to the degree of flexion, which is greater in Indian and Caucasoid skulls. The authors postulate that this deepening of the fossa may be related to a higher frequency of temporomandibular joint (TMJ) problems. These observations fit with those made previously on early hominid fossils and suggest that a reduction of dentofacial robusticity has an important influence on temporomandibular form.

The final three papers, as well as two of the earlier ones, offer extensive descriptive data on facial contours, intracranial volume, and the major radiographic landmarks for both bone and soft tissue. Papers concerning the facial profile gave fine detail of the location and application of craniofacial landmarks for the study of facial asymmetry. These landmarks are familiar to most readers, but the computerized cephalometric system used to derive them from standard posterio-anterior radiographs will be a major help for students of craniofacial variability. Facial profiles are described in some detail in the next paper, in which special attention is given to the detail of measurement of soft tissue. The data for both of these papers were generated as part of ongoing craniofacial research to assist in facial reconstruction of children suffering from various developmental abnormalities.

CAT scans and magnetic resonance imaging (MRI) have become standard research tools over the past decade. The use of MRI to determine the TMJ relationship in children with various forms of malocclusion is one such application described in one of the final papers of the conference. The study offers considerable detail, expecially the shape of the condylar head, and records a number of subjects with steep articular eminences. This paper might be considered in conjunction with the one reporting glenoid fossa depth and craniofacial flexion, for this bears directly on TMJ problems.

CAT scan techniques are reviewed and mapping of landmarks for skull volume determinations is offered in the penultimate presentation of the conference. The need for population standards are stressed and this paper provides precise methods to guide their collection. The final paper measures intracranial volume of patients with craniosynostosis (premature fusion of cranial sutures). The assumption has been that treatment of such infants requires surgical intervention to increase skull volume. This paper tests this assumption, and its preliminary finding is that patients with certain syndromes of craniosynostosis have an intracranial volume within normal ranges, while those with certain other syndromes do not. Though these final papers are excellent, this reviewer thinks that the data presented, in fact the whole of the conference, would have benefited by some summary remarks by the editors to place the studies in a broader context of facial diversity.

This volume of conference papers suffers from some of the same problems shared by most symposium publications; uneven coverage of relevant background literature and a wide range of stylistic differences being the most common. But each paper has undergone a peer review and clearly presents the results from intensive and well-designed research. The most up to date methods and application of research tools (hardware and software) have been used and readers will benefit by careful attention to the detailed descriptions. These details will be equally useful to clinicians and researchers. Dental anthropologists who are more concerned with interpopulation variability will have to try a bit harder to fit these extensive data into a broader context of human diversity of craniofacial and dental morphology. But it will be rewarding because there is much of value in this volume, grounded on the years of research experience of the authors who have published extensively.

It is necessary to carefully consider the cautions offered in the keynote address; phylogenetic comparisons between populations must be well founded on knowledge of genetic influence. Complexes of dental traits occur in varying frequencies among smaller breeding populations, and are difficult to extend to the continental groupings we generally call races. Dental anthropologists have a special responsibility to approach questions of phylogeny and functional morphology with care. Some of the authors in this volume demonstrated such care, but there were others who neglected significant studies relevant to their reports. Nevertheless, there is much useful information reported here that describes well the functional complexity of human orofacial and craniofacial anatomy. It is up to the reader to place this array of data in the larger context of human diversity and adaptation.

Notes

1 To whom orders can be addressed: Maciej Henneberg, Department of Anatomical Sciences, Medical School, University of Adelaide, Adelaide, SA 5005, Australia. Email maciej.henneberg@adelaide.edu.au.

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