Orthodontic Relapse And Mandibular Cortical Thickness- A Case-Control Study

Objectives: Dental relapse is a return of crowding after orthodontic straightening. Risk factors for relapse are not clear. The goal of this case-control study was to evaluate whether decreased mandibular bone quantity increases the risk for greater lower incisor crowding post-retention. Methods: The study sample was selected from the post-retention database at the University of Washington. The lower incisor irregularity index (Little) was measured on all casts obtained at least 10 years post-retention. Cases were defined as individuals with an irregularity index greater than 6, while controls were defined as individuals with an irregularity index smaller than 3. Subjects needed good quality radiographs obtained at the same time as the post-retention models in order to measure bone quantity. Mandibular cortical thickness measured below the mental foramen on either a panoramic radiograph or a lateral cephalogram was used as a marker of mandibular bone quantity. Logistic regression analyses were used to relate the irregularity index to cortical thickness and to adjust for potentially confounding variables (age and gender). A Spearman Correlation was done to check the reliability of the investigator’s radiograph measurements. Results: 21 cases and 99 controls were identified (on average 16 years post-retention, mean age 32, 69% female). The mandibular cortical thickness was smaller in the cases than in the controls (4.36mm versus 4.68mm), which was statistically significant (p<0.05) when measured on the lateral cephalogram. For every 1mm increase in mandibular cortical thickness (measured on the lateral cephalogram) there was a 53% decreased risk for having post-retention irregularity greater than 6mm (95% confidence interval 0.23-0.94). Both radiographs exhibited a 0.98 Spearman Correlation Coefficient. Conclusion: Dental relapse post-retention is greater in individuals with a thinner mandibular cortex. This indicates that decreased mandibular bone quantity may be a factor associated with increased dental relapse. Supported by NIH/NIDCR #T35DE07150 and NIH/NIDCR K24DE00433.