Initial third molar development is delayed in jaws with short distal space: An early impaction sign?

The multifactorial aetiology of third molar (M3) impaction remains puzzling. While short retromolar jaw space is a well-established risk factor in young adults, space alone does not explain how impaction develops. Here we investigated in children and adolescents a potential new factor, delayed M3 development. We hypothesized that two important impaction correlates - lack of jaw space, and delayed M3 development - are also positively correlated with each other, and may together help suggest from early ages subsequent possible M3 impaction.

We studied 689 M3 regions on retrospective CBCT images of 179 Canadian patients aged 8-13 years, using multiple linear mixed-effects (LME) modeling to predict M3 development (using Demirjian's stages) in both upper and lower jaws based upon available distal jaw space while also accounting for age, sex, jaw, and oral quadrant.

Lack of distal jaw space was associated with less-developed M3s in upper and lower jaws (t = 3.209, p = .001). If space availability fell below a given threshold, absence of M3 development was predicted. Upper M3s developed more than lower M3s for each additional millimetre of retro-M1 space gained. Between-sex differences were not found.

Retromolar jaw space and M3 development are two positive correlates of M3 impaction that co-associate as early as childhood. Our results suggest that delayed initial M3 development merits investigation as a potential prognosticator for late M3 eruption relative to when jaw growth ceases - an outcome known to increase the likelihood of impaction.