Third molar removal is a very common surgical procedure but there is no consistent estimate for the frequency of third molar impaction with estimates varying from 3-57%.
The aim of this review was to assess the worldwide prevalence of third molar (M3) impaction and the demographic and morphologic factors that increase susceptibility to this pathology.
Searches were conducted in the Google Scholar, PubMed, and Thomson Reuters ISI Web of Science databases with no limits on language or year of publication. Studies with participants with no orofacial syndromes or defects ≥17 y old with diagnosis of M3 impaction based on radiographic examination were considered.
Outcome measures of interest included; the proportion of individuals with impaction of at least 1 M3; the odds ratio of M3 impaction rate for a) mandible versus maxilla and b) men versus women, and 3) the proportion of impacted M3s exhibiting various angulations of impaction.
- 49 studies involving 83,484 individuals were included.
- Worldwide M3 impaction prevalence was found to be 24.40% (95% CI;18.97% to 30.80%).
- Odds of M3 impaction in the mandible were 57.58% (95%CI; 43.3% – 68.3%, P < 0.0001) higher than in the maxilla.
- No difference in the odds of impaction between men and women was detected (18.62%, 95% CI; –4.9% to 48.0%, P = 0.12).
- Mesioangular impaction was most common 41.17%, (95%CI: 33.8%-49.0%), followed by vertical 25.55% (95% CI; 20.0%-32.0%),
- Distoangular 12.17%(95%CI; 9.1%-16.0%), and Horizontal 11.06%(95%CI; 8.3%-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.
The authors concluded:
We demonstrated that the average worldwide rate of M3 impaction is 24.40% (95% CI: 18.97% to 30.80%; Fig. 2) but that effect sizes are highly heterogeneous and driven by biological variables. Subgroup analyses showed no differences in M3 impaction rate between sexes and only slight differences in frequency among geographic regions. M3 impaction is much more likely in the mandible than the maxilla, and mesioangular impaction is the most frequently seen orientation. Understanding the distribution of M3 impaction frequencies is an important first step for understanding the ultimate causes of impaction and for clinicians making decisions on treatment protocols.
This review has searched 3 large databases for relevant studies. Although there is a lack of detail about whether there was independent selection for inclusion or any indication of whether formal quality assessment of the selected studies was undertaken. The authors discuss the issue of whether the decision to include only those studies using radiographs created any sampling bias. For example if the included patients were only those seeking treatment there is a potential that this might lead to overestimation of impaction frequency. However, the authors also highlight that their overall estimate for M3 impaction is lower than the estimates obtained from the majority of other studies.
Carter K, Worthington S. Predictors of Third Molar Impaction: A Systematic Review and Meta-analysis. J Dent Res. 2015 Nov 11. pii: 0022034515615857. [Epub ahead of print] Review. PubMed PMID: 26561441.