Long-term changes in arch form after orthodontic treatment and retention.

The purpose of this study was to evaluate the long-term stability of orthodontically induced changes in maxillary and mandibular arch form. Dental casts were evaluated before treatment, after treatment, and a minimum of 10 years after retention for 45 patients with Class I and 42 Class II, Division 1 malocclusions who received four first premolar extraction treatment. Computer generated arch forms were used to assess changes in arch shape over time. Buccal cusp tips of first molars, premolars, and canines plus mesial, distal, and central incisal aspects of incisors were marked, photocopied, and digitized in a standardized manner. An algorithm was used to fit conic sections to the digitized points. The shape of the fitted conics at each time period was described by calculating the parameter eccentricity; a small value represented a more rounded shape and a larger value represented a more tapered shape. Findings demonstrated a rounding of arch form during treatment followed by a change to more tapered. Arch form tended to return toward the pretreatment shape after retention. The greater the treatment change, the greater the tendency for postretention change. However, individual variation was considerable. The patient's pretreatment arch form appeared to be the best guide to future arch form stability, but minimizing treatment change was no guarantee of postretention stability.