A computerized analysis of the shape and stability of mandibular arch form

To determine whether a particular ideal orthodontic arch form could be identified, the mandibular dental casts of 30 untreated normal cases, 30 Class I nonextraction cases, and 30 Class II nonextraction cases were examined. Following computerized digitizing and the use of a mathematic function called polynomial of the fourth degree, arch forms were generated for each sample and then compared to 17 commercially produced arch forms. Results showed that no particular arch form predominated in any of the three samples. A shape representing a combination of the "Par" and "Vari-Simplex" arch forms approximated to only 50% of the cases in the three samples. The remaining 50% of the cases displayed a wide variety of arch forms. Cases that had changes in arch form during nonextraction treatment frequently were not stable; almost 70% showed significant long-term posttreatment changes. Customizing arch forms appears to be necessary in many cases to obtain optimum long-term stability because of the great individual variability in arch form found in this study.