The assessment of crowding without the need to record arch perimeter. Part I: Arches with acceptable alignment

A simplified, mathematically determined technique for calculating arch perimeter (the overlap method) is described and its validity in determining an accurate assessment of crowding is tested. Indices, expressed in millimetres of crowding or spacing, were computed for both the whole arch and the labial segment alone. Study casts of 36 individuals with clinically acceptable lower arches were used to validate the method. The degree of crowding or spacing calculated was compared with a "clinical' assessment of each arch, in which the irregularity was measured directly using a steel ruler. Various calculation schemes were tested. Depending on exactly how the overlap was determined, the results varied slightly. Although the arches showed acceptable alignment, it was preferable to include a strategy for normalizing the positions of any rotated teeth before the overlaps were calculated. Repositioning any bucco-lingually displaced teeth into the line of the arch, however, was not useful. For the complete arch good agreement with the clinical assessment was reached on 31 occasions and for the labial segment, all but one appraisals were within 0.5 mm of each other. In the remaining instances (five complete arches and one labial segment), the degree of crowding or spacing was between 0.5 and 1 mm of the clinical assessment. Considering that clinical measurement of minor degrees of crowding and spacing cannot be precise, these results were considered acceptable. The method was easy to use, relying only on the recording of mesio-distal tooth widths and was acceptably reproducible. The technique would therefore appear to provide a valid yet simple research tool with which to record the degree of crowding. Its ability to cope with irregular and crowded arches will be the subject of a subsequent review.