Prenatal rotational field involving deciduous lateral incisors.

In an earlier radiographic study of dental development in the third-trimester fetus, considerable variability was obsersed in the degree of rotation of the deciduous lateral incisors (VAN DER LINDEN, MACNAMARA and Buxi, J Dent Res 51: 71, 1972) Accordingly, we became interestedl in the extent of lateral-incisor rotation in still younger specimens, the age at onset of lateralincisor rotation, and the degree of correspondence between the jaws. Ouir subject material consisted of 18 anatomically normal human specimens, in tthe 16to 26-week-ol( age range; the specimens were carefully positioned during histologic sectioning to provide serial 15-micrometer (jim) frontal sections with minimal skewing (Illustration). As givern in the table, 64 of the 72 deciduous lateral incisors were rotated (89%) and only eight of these incisors (11%) failed to show axial rotation. Analyzing the upper and lower jaws separately, 31 of 36 lateral incisors were rotated in 18 maxillas (86%) and 33 of 36 mandibular lateral incisors were also rotated (92%). Axial rotation of deciduous incisors was therefore the rule in both jaws as early as reliable judgments could be made. In the maxilla, lateral incisors tended to be iotated in the lateral or mesiolabial direction in 24 of 36 incisors, or 61%. Similarly, in the mandible, the majority of lateral incisors were rotated in the lateral (mesiolabial) direction, that is, 78%. Chi-square tests showed no difference between the jaws in the prevalence of lateral (mesiolabial) rotation, medial (mesiolingual) rotation, and in the proportion of unrotated incisors. The majority of fetuses showed mesiolabial rotation in both jaws. The data indicate that the majority of the deciduous lateral incisors develop in a rotated position during the second and third trimesters and most of them rotated in the lateral or mesiolabial direction. Clearly, there is the effect of a rotational field affecting both jaws