EVALUATION OF ORTHODONTIC RELAPSE USING THE CUBIC SPLINE FUNCTION

A sample of 72 orthodontically treated patients was reexamined many ye ars out of retention. They had been treated either by the extraction o f four premolars or without extractions. The average number of years b etween the end of treatment and the taking of follow-up records was 20 , with a range of 12 to 35 years, Some conventional measurements were studied such as intertooth widths, arch perimeter, and incisor irregul arity, In addition, a new method for comparison of arch form at differ ent stages of treatment, which uses the cubic spline function, was use d, Cases were grouped into extraction and nonextraction, and statistic s were used to test the differences between the two groups. Correlatio ns between the spline variables and conventional variables were comput ed, and multiple regression analysis was carried out using the spline Variables as dependent variables, Same treatment and relapse changes w ere independent of whether the case was treated with extractions or no t, whereas other trends were unique to one treatment group. The correl ation analysis revealed strong relationships between variables that me asured changes during the same treatment stage. There were also modera te correlations between some of the spline variables and the tradition al measurements, Multiple regression analysis was used to account for changes in some spline variables, however, the usefulness of the model as a predictor is limited.