Radiographic and histological methodologies in estimating the chronology of crown development in modern humans and great apes: a review, with some applications for studies on juvenile hominids

There has been a burgeoning of interest in the last decade on growth studies in hominids. These studies have relied heavily on dental development, and have compared juvenile hominids to modern human and ape standards, which are usually established using radiographic data. There has been considerable discussion on the most appropriate methods of deriving population standards from radiographs, but very little on the accuracy of the radiographic image itself. Previous histological and dissection studies have shown that age at onset of mineralization is overestimated, and age at crown completion is underestimated using radiographs. This study considers the process of X-ray absorbence by mineralized tissues and the formation of radiographic images of developing teeth. Following tooth initiation a critical mass of mineral is required for the tooth to register superimposed on the absorbence of alveolar crypt bone, which accounts for the late identification of tooth initiation. Determination of completion of crown growth depends upon the identification of the last formed enamel at the cervix. Recognition of this key stage is difficult as crown growth slows towards the cervix, and the last secreted enamel may take months to attain full mineralization levels due to the prolonged maturation process. Morphological and geometric factors have a significant influence on the imaging of the completed crown. The last formed enamel is located on the buccal face, where enamel thins progressively to nothing. X-ray absorption by enamel at the cervix becomes insignificant, and may be counterbalanced by increased dentine absorption. Approximal enamel in contrast is clearly visualized once maturation is complete. However, developmentally this enamel face initiates later, and is completed much earlier than buccal enamel. All of the radiographic estimates of crown completion times are based upon interpretations of approximal enamel completion. These considerations suggest that the human population standards in current usage may not represent true anatomical and chronological stages of crown development, and care should be taken in referring juvenile hominids to these radiological standards.