Third molar agenesis in modern humans with and without other tooth agenesis

Abstract

Background The number of teeth in the human dentition attracts special interest both from developmental and evolutionary aspects. The present case-control study focused on the formation of third molars in modern humans aiming to shed more light in this long lasting enigma.Methods For this reason, we investigated third molar formation in a sample of 303 individuals with tooth agenesis other than in third molars (agenesis group) and compared it to a sex and age matched control group of 303 individuals without tooth agenesis other than in third molars.Results The prevalence of third molar agenesis in the agenesis group was 50.8%, which is significantly higher than the 20.5% in the control group (p < 0.001). The chance of a missing third molar in the agenesis group was increased by 38.3% (p < 0.001), after controlling for the other tooth agenesis factor. When considering the percentages of the number of missing third molars per individual, a clear tendency towards more missing third molars was evident in the agenesis group compared to the control group. The frequency of bilaterally missing third molars in the agenesis group was 29% in the upper as well as in the lower jaw, which is about three times higher than the frequency of unilaterally missing third molars (p < 0.001). In the control group, bilaterally missing third molars occurred in 8.6% in the upper and 8.9% in the lower jaw.Conclusion The present results indicate that genetic factors involved in certain tooth agenesis also affect the entire dentition as a whole. Furthermore, the third molars are more vulnerable to factors involved in other tooth agenesis and they are more often affected as a total. These findings seem to be associated with the evolutionary trend in humans towards reduced molar number.