Date of Award


Document Type


Degree Name

Master of Dental Science (MDS)



Research Advisor

Edward F. Harris, PhD


Quinton C. Robinson, DDS, MS Richard A. Williams, DDS, MS


Orthodontics, Malocclusion, Tooth movement, Dental arch form, Extraction treatment, Premolar extraction


Malocclusions can involve arch-size tooth-size discrepancies that have to be resolved by premolar extractions. Treatment planning in such cases usually involves removing first (P1) or second premolars (P2). Choosing an extraction pattern is a learned skill that incorporates many diagnostic factors. This study compared the amounts of in-treatment tooth movement in two common extraction patterns (4/4 and 5/5) in order to gain insight into the criteria used for differential diagnosis and treatment planning. Objective: A method of cast analysis was developed to measure the anteroposterior and transverse changes in tooth positions during orthodontic treatment. Data were used (1) to assess how the extraction space was used to resolve the malocclusion and (2) to identify pretreatment occlusal variables associated with the amounts of orthodontic tooth movement. Methods: Dental casts of 90 orthodontic cases with Class I malocclusions from American white adolescents were digitized (70 landmarks) at the start and end of treatment to capture the in-treatment tooth movements. A battery of 20 pretreatment occlusal variables (overjet, overbite, arch length, etc.) was used as predictor variables, and the in-treatment changes in each tooth’s position were the outcomes. Multivariate linear regression was used to find pretreatment characteristics that predicted which premolars had been extracted. Results: Cases treated with P1 extractions exhibited (1) greater incisor irregularity, (2) greater overjet, (3) more frequent interdental spacing, (4) longer arch lengths (1-3 and 1-6), (5) greater overbite, and (6) broader arch widths (3-3 and 6-6). Predictably, the occlusal variables in combination were more predictive than when studied univariately. Only about half the extraction space was used to resolve anterior problems in the average P1 case, and less was used in the average P2 case. Conclusions: Choosing an extraction pattern appropriate to a malocclusion is a learned skill. This study quantifies some of those diagnostic characteristics that specialists use to choose a premolar extraction pattern suited to a particular malocclusion.