Enamel hypoplasias, defined as developmental defects in enamel thickness, are easily studied and provide an indelible indicator of periods of stress during tooth crown development (prenatally to 12 months for deciduous teeth, and birth to 7 years for permanent teeth; Figure 2.6). They have been increasingly favored as indicators of stress in studies of precontact and historic skeletal populations. In the vast majority of cases, defects found in archaeological materials fit a chronologic pattern and appear to be the result of systemic metabolic stress (Rose et al. 1985). Thus, they are frequently referred to as chronologic or linear enamel hypoplasias (LEHs), reflecting the linear and chronologic nature of the defects caused by systemic stress at a specific point in time. Although enamel hypoplasias due to systemic stresses are common and easily discerned from defects due to nonsystemic factors, it is difficult to attribute a more exact cause to these defects.
Goodman and Rose (1990) have proposed a threshold model for considering the etiology of LEH development as the additive consequences of four factors: (1) unknown etiological factors, (2) underlying nutritional status, (3) disease, and (4) tooth susceptibility. Although the exact nature of the physiological stress cannot be deduced, the model illustrates how one or several factors can come together to place an individual at risk for developing dental defects. In general, the data suggest that many precontact populations are stressed to some degree, and when compared with contemporary populations from underdeveloped areas, they often have similar or higher frequencies of defects. The chronology of defects suggests that the postweaning period is particularly stressful. Variability in rates within and between populations provides evidence that this indicator is sensitive to stress differences across age, sex, and temporally divided subgroups and between populations.
The data are generally presented both as “chronologies” of defects and as overall frequencies of defects per tooth. Because most teeth exhibited some degree of attrition, one or more developmental periods were frequently recorded as missing. Therefore, to increase sample size, the tooth-specific rates are based on the frequency of defects per middle and cervical third.
LEH data can also be used to estimate the age of weaning in many populations. Because weaning is a traumatic process to the infant, LEHs can often form during this time because of disruptions in growth of the enamel. The age of weaning can be used to discuss issues of interbirth interval as well. Typically a woman will not begin to ovulate until breastfeeding of one infant ceases. If weaning occurs around the age of 1, for example, a woman would be able to become pregnant again around that time, giving an interbirth interval of approximately 2 years.