Second molar distal surface caries in the context of mandibular third molar absence and impactions
Authors:   , Tödtling Verena 
Published: March 2023

In chapter 1 We address the growing concerns relating the increasing frequency of distal surface caries (DSC) in mandibular second molars.

In chapter 2 we compared the scientific evidence of effects/consequences of removal of third molars with conservative management of asymptomatic disease-free impacted third molars by conducting a Cochrane systematic review. We found that the presence of third molars may be associated with increased risk of periodontitis affecting the adjacent second molar in the long term. However, there is insufficient evidence to demonstrate a difference in DSC risk.

In chapter 3 we conducted an additional systematic review of epidemiological studies to assess the prevalence of DSC. A meta-analysis of 6 DSC prevalence studies and a subgroup analysis of 3 studies concerning various third molar angulations were performed. Prevalence subtotals were 20% for prospective and 15% for retrospective studies on a molar level in a population referred to hospital care. A subgroup analysis of 1296 patients (1666 molars) yielded DSC prevalence rates among mesial 36% and 22% with horizontal impactions. We concluded that European-based studies suggest that about 1 in every 4 patients referred to hospital for a third molar assessment may be affected by DSC and that convergent third molar impactions pose a significantly greater risk.

In chapter 4 a systematic review was conducted to gain a greater insight into the incidence rates of DSC on second permanent molars. Two incidence studies were included in this systematic review. The DSC incidence was reported in 1 study with a 25-year follow-up as relative risk adjacent to erupted (RR = 2.53), soft tissue impacted (RR = 0.83) and bony impacted third molars (RR = 1.44) in comparison to when the third molar was absent. The second study reported a DSC incidence of 100 surface-years (1% of all sites) with an 18-month follow-up period. We concluded that both cohort studies indicated that DSC incidence was higher in the male population when third molars were erupted.

Chapter 5 assesses how retention of impacted third molars is associated with plaque stagnation and the development of DSC. We have investigated the composition of dental plaque on the distal surface of the mandibular second molar. Using short read sequencing of the bacterial 16S rRNA gene, we compared the microbiome of these surfaces at four impaction angulations and where the third molar is missing. Amplicon-Sequence-Variants (ASVs) associated with Veillonella were significantly more abundant at angulations with convergent angulations. Detection of Veillonella at increased abundance in convergent impactions, and distinctive profiles at horizontal impactions, may partly elucidate associations of convergent angles with DSC.

In Chapters 6 and 7 we determine the prevalence of DSC when examining patients for routine dental check-ups and compare the prevalence in Manchester with the population in Bucharest. Radiographs involving 1012 and 251 patients from the Manchester and Bucharest population respectively were evaluated. The overall prevalence of DSC was 63.9% and 19.9% in the Manchester and Bucharest population respectively. Partially erupted mesio-angularly impacted mandibular third molars and third molars with compromised molar-to-molar contact points, loss of lamina dura of ≥ 2mm, male gender, increasing age and a higher modified decayed, missing, filled tooth score were identified as risk factors. The UK population which is governed by very strict third molar guidelines had a statistically significant (p < 0.001) higher DSC prevalence and DSC was cumulative with increasing age in comparison to the Romanian population sample in which patients are treated with preventative third molar removal which presented with increased DSC prevalence in younger age groups and is a characteristic of third molar prophylactic removal in a population.