The aim of orthodontic treatment is to provide the patient with a good static and functional occlusion. During research that was conducted to complete a seminar on the static and functional aspects of occlusion, the author discovered that there might be a discrepancy between the goals of an ideal static occlusion, and the goals of an ideal functional occlusion. An ideal static occlusion seemed to require a flat mandibular plane and a minimal amount of overbite after active orthodontic treatment, whereas an ideal functional occlusion required a curved mandibular plane and an overbite of 4 mm to prevent cusp interferences during functional mandibular movements. The rationale behind the excessively flat mandibular plane and minimal overbite after orthodontic treatment is to compensate for the tendency of the bite to deepen during the period following orthodontic treatment. This tendency to relapse causes uncertainty about the stability of orthodontic treatment. Little research has been dedicated to examining the long-term stability of the leveled curve of Spee. In addition, there seems to be a considerable amount of controversy surrounding the long-term stability of overbite correction after orthodontic treatment. The aim of this study was to evaluate the stability of the curve of Spee and the overbite following orthodontic treatment. In addition, the relationship between the curve of Spee and the presence of anterior guidance after a period of orthodontic retention, was examined. The relationship between the overbite and the presence of anterior guidance was also examined, and the results were used to predict an ideal value for the overbite to avoid possible dental cusp interferences. Standardized digital photographs of the dental casts of 40 subjects were taken at three different stages: before treatment (T1), after orthodontic treatment (T2), and three years (mean) post-treatment. Accurate electronic measurement of the curve of Spee, using computer software, was completed for all three stages. The overbite was measured with a dial caliper. Clinical evaluation of the functional occlusion, with special reference to anterior guidance, was performed on all the subjects. Statistical analysis was carried out in search of statistical significant changes between the various stages, and possible correlations between the different variables. The results indicated that the leveling of the curve of Spee is a stable treatment procedure. The overbite was less stable than the curve of Spee, and nearly half the amount of overbite correction obtained during treatment, relapsed in the three years (mean) post-treatment. No relationship was found between the curve of Spee and the presence of anterior guidance at T3. A highly significant relationship was found between the overbite and the presence of anterior guidance. Subjects with a small overbite seemed to be predisposed to posterior interferences during mandibular protrusion. An overbite of not less than 3mm was found to be a desirable feature after orthodontic retention in order to reduce potentially interfering contacts. More research is necessary to clarify the relationship between dental interferences and temporomandibular disorders (TMD). / Dissertation (MChD (Orthodontics))--University of Pretoria, 2004. / Orthodontics / unrestricted