Date of Award

Spring 1993

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Dentistry

First Advisor

Ferguson, Donald J.

Second Advisor

Kittleson, Russell T.

Third Advisor

Cariapa, Vikram

Abstract

Long-term stability is a primary goal in orthodontic treatment Though several post-treatment and post-retention studies have investigated relapse of overbite, interincisal angle, and arch width, depth and length, few of these studies have focused on the behavior of the curve of Spee and its relation to other postretention mandibular arch dimensional changes. The purpose of this study was to evaluate the post-retention behavior of the curve of Spee and determine the strength relationship between curve of Spee changes and changes in selected mandibular arch dimensions, i.e., overbite, interdental widths of canines, premolars and molars, as well as arch circumference. Twenty four randomly selected cases treated by the extraction of premolars in the permanent dentition were observed at approximately 6 years postretention. Pre-treatment, post-treatment and post-retention study models were measured three-dimensionally on a milling machine. Corresponding lateral cephalometric radiographs were traced and evaluated. Paired t-tests and Pearson's correlations were used to identify statistically significant results at a probability level < 0.05. The following conclusions were reached: 1. Premolar extraction was associated with a decrease in arch circumference and arch circumference continues to decrease post-retention. 2. lnter-incisal angle did not change significantly in the predominantly Angle Class I first premolar extraction sample. 3. The strength of relationship between curve of Spee change and overbite change post-retention was not strong enough to be able to predict on an individual case basis. 4. The canines, retracted into extraction sites during treatment, were stable at this retracted position. Inter-canine width, expanded during therapy, decreased to near its pre-treatment dimension during the post-retention period. 5. The premolars underwent the greatest amount of relative extrusion, or leveling, during treatment and demonstrated a slight tendency to relapse post-retention. 6. During treatment, premolars moved anteroposteriorly in the direction of the extraction site while inter-premolar width remained constant. Post-retention, inter-premolar width decreased. 7. During treatment, first and second molars were extruded, moved mesially and constricted. Post-retention, molars showed little change vertically and continued their mesial migration. Width of the first molars continued to decrease while second molar width remained stable. 8 Orthodontic leveling in the mandibular arch remained stable over time.

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