Enamel thickness and growth rates in modern human permanent first molars over a 2000 year period in Britain

Objectives: This study explores variation and trends in first molar enamel thickness and daily enamel secretion rates over a 2000 year period in Britain. Methods: Permanent first molars (n=89) from the Roman, Anglo-Saxon, and Medieval periods, as well as modern day Britain, were analysed using standard histological methods. Relative enamel thickness (RET) and linear measurements of cuspal and lateral thickness were calculated for mesial cusps. Daily secretion rates (DSRs) were calculated for inner, mid, and outer enamel regions in both cuspal and lateral enamel. Significant differences and trends were identified between samples using non-parametric statistical tests. Results: Enamel thickness differed between some populations, but no temporal trends were identified. Early Anglo-Saxon molars had significantly thinner RET than both Late Anglo-Saxon (p<0.00) and Medieval (p<0.00) molars. Lateral enamel from the Roman molars was significantly thinner than the modern day sample (p=0.04). In contrast, DSRs slowed significantly from the more ancient to the modern day samples in every comparison except the mid lateral enamel region. Discussion: This study presents the first evidence for a gradual slowing in the daily rate that enamel is secreted in molars over the past 2000 years in Britain. However, this trend was not matched by a change in enamel thickness, which remained fairly consistent over this time period. These findings suggest that modern human molars of similar enamel thickness, from different modern and ancient populations, formed at different rates.