Doctoral Dissertation: A Three-Dimensional Analysis of Age-Related Change in the Adult Craniofacial Skeleton

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Most skeletal research assumes that the human skull does not change in shape or size after age 20. Indeed, most studies work under the assumption that aging plays little or no role in adult cranial variation, and adults of any age can be used equally in a study of human cranial form. Contrary to this assumption, many scientists have found significant change in the adult skull long after sexual maturation. This poses potential problemsfor hundreds of clinical, paleoanthropological, and forensic studies that donot consider age-related variability in skull form in their choice of humancomparative samples. A hypothetical example from the medical realm demonstrates this most clearly, though this is not itself medical research. Some clinical studies compare the shape of normal and abnormal adult skulls to better understand the nature of genetic diseases affecting skull development. If the adult skull changes with age, a comparison between a normal and abnormal sample of 60 yr. olds might display extensive differences in skull shape and size. The same comparison using samples of normal and abnormal 20 yr. olds, on the other hand, might show few or no differences. Depending on the age-composition of comparative samples, doctors may refrain from treating skull regions that are actually pathological, or alternatively, attempt to correct an individual's skull that only appears to be abnormal. Researchers would never think of comparing non-age-matched juveniles, yet this is common practice with adults. If adult cranial change exists, any study of the skull that does not consider variation due to age in their samples cannot be sure of the validity or unbiased character of their results. The proposed study will rigorously test assumptions of adult cranial stability using three-dimensional methods. Multiple skeletal samples of known age and sex, differing stages of tooth-loss, and from different geographic regions will be examined for systematic patterns of age-related change in the skull. Three-dimensional coordinate data will be collected from 115 landmarks on the skull and mandible. These data will be analyzed using Euclidean Distance Matrix Analysis. A pilot study using this method supports the presence of systematic change in the adult skull and documents sex differences in the patterns of this change that were not detected in previous studies. These pilot data demonstrate the critical need to consider adult age-related change in analyses of human skull form, and the essential role the proposed study has in the restructuring and reinterpretation of previous works.