Predictors of Third Molar Impaction: A Systematic Review and Meta-analysis

The objective of this meta-analysis was to evaluate the prevalence of third molar (M3) impaction worldwide in individuals ≥17 y, from either sex, who had undergone oral radiography and presented with no orofacial syndromes or defects. We performed a literature search using PubMed, ISI Web of Science, and Google Scholar and retrieved English and non-English articles from any period for review. We included studies reporting M3 impaction prevalence based on radiographic examination. Risk of bias was assessed regarding individuals with craniofacial syndromes, prior extraction of permanent teeth, multiple effect size estimates, and studies conflating lack of eruption with impaction. Our search yielded 49 studies involving 83,484 individuals. Worldwide M3 impaction prevalence was found to be 24.40% (95% confidence interval [95% CI]: 18.97% to 30.80%). The odds of M3 impaction in the mandible were 57.58% (95% CI: 43.3% to 68.3%, P < 0.0001) higher than in the maxilla, but we did not detect any difference in the odds of impaction between men and women (18.62%, 95% CI: -4.9% to 48.0%, P = 0.12). Mesioangular impaction was most common (41.17%, 95% CI: 33.8% to 49.0%), followed by vertical (25.55%, 95% CI: 20.0% to 32.0%), distoangular (12.17%, 95% CI: 9.1% to 16.0%), and horizontal (11.06%, 95% CI: 8.3% to 14.6%). Impaction of 1 (42.71%, 95% CI: 30.0% to 56.5%) or 2 (29.64%, 95% CI: 19.5% to 42.3%) M3s was much more common than 3 (12.04%, 95% CI: 7.2% to 19.3%) or 4 (8.74%, 95% CI: 5.2% to 14.5%). There were small differences among impaction prevalence depending on geographic region (F test, P = 0.049). Selection bias was evident because individuals had to undergo radiographic examination to be included in the analysis. The subgroup analysis by sex was underpowered. Worldwide M3 impaction prevalence is lower than previously reported. The percentage of individuals with impacted M3s is much smaller than the percentage that undergoes clinical treatment for M3 problems.