Date of Award


Document Type


Degree Name

Master of Dental Science (MDS)



Research Advisor

Edward Harris, Ph.D.


Gregory Hutchins, D.D.S. Quinton Robinson, D.D.S., M.S.


First Premolar Extraction, Palatal Rugae, Canine Retraction, Extraction Decisions


The most common method of resolving substantial tooth size-arch length discrepancies (TSALD) is to treat patients with the extraction of premolars. Not all of the extraction space, however, is needed to eliminate the patient's TSALD. Also, it is typical that some of the extraction space will be lost because the molars are used as anchorage units to retract the anterior teeth. OBJECTIVE: This study developed a method to measure changes in the positions of the canines and second premolars relative to the palatal rugae. These data were used to assess how the extraction space was used in resolving the malocclusion. Data were also collected on the severity of the malocclusion—namely, pretreatment overjet, overbite, incisor irregularity, and TSALD—to test whether these were predictive in the extraction decision. The goal was to better understand first premolar extraction changes and decisions. METHODS: Pretreatment and posttreatment dental casts were available for 50 adolescent North American whites with Class I malocclusion who were treated with first-premolar extractions. Sixty-four landmarks were recorded in all three planes of space with a MicroScribe G2X 3D digitizing system and distances were computer-generated using trigonometric algorithms. RESULTS: Removing a first premolar gave an average of 7.3 mm of space in each quadrant of which 3.8 mm (51%) was consumed by canine retraction, while 3.6 mm (49%) was used as the second premolar moved mesially. Changes in the transverse dimension after premolar extraction treatment were minor, though second premolar width diminished about 1 mm as these teeth were moved into a narrower segment of the arch (P < 0.01). The extent of incisor irregularity had a significant positive association with the amount of canine retraction (r = 0.33; P = 0.02). CONCLUSION: A four first premolar extraction pattern in Class I patients provides space that is consumed almost equally by the retraction of canines and the mesial movement of the buccal segments in the maxilla. Multivariately, the two significant variables associated with increased canine retraction in these patients were transverse canine change (i.e., buccally displaced canines) and a high level of incisor irregularity.