A longitudinal assessment of sex differences in the growth of the mandibular retromolar space

Growth of the retromolar space, typically measured in dentistry as the space posterior to the second permanent molars (Zelić & Nedeljković, 2013), allows for the eruption of the second and particularly the third permanent molars. Third molars are the most common impacted teeth (Andreasen, 1997, Bishara, 1999). Kahl et al.(1994) have demonstrated that 97.4% of impacted teeth had insufficient space for eruption. Other scholars have confirmed a positive correlation between a larger retromolar space and eruption of lower third molars (Ghougassian and Ghafari, 2014, Svendsen and Björk, 1988) while a smaller retromolar space is associated with impaction (Al-Gunaid et al., 2019, Huang et al., 2022, Zelić and Nedeljković, 2013). Modern humans generally have reduced mandibular dimensions, which may be seen to include the mandibular retromolar space. In fact, earlier hominins such as Neandertals and Middle Pleistocene Homo even had retromolar space behind the erupted mandibular third molars; in paleoanthropology, it is this space behind the third molars that is called the “retromolar space” (Franciscus & Trinkaus, 1995). While this trait (space behind the third molars) is seen in paleolithic Homo sapiens (Franciscus and Trinkaus, 1995, Nara et al., 1998, Trinkaus, 2007), it is arguably less common in recent or extant modern humans (though it does still occur in some populations; De la Cova, 2016). Prior research suggests a connection between mandibular size/shape (including corpus length) and dietary shifts through time (Carlton & Van Gerven, 1977; Katz et al., 2017; May et al., 2018), which may be at least partially responsible for this decrease in retromolar space.

Retromolar space growth patterns have been examined in detail by relatively few studies (Chen et al., 2010, Ganss et al., 1993, Ghougassian and Ghafari, 2014, Richardson, 1987, Zelić and Nedeljković, 2013). Richardson evaluated retromolar space growth and found that from ages 13–18, the average total molar space increased by 4 mm: the posterior molar space at the anterior border of the ramus increased by approximately 2 mm and the lower first permanent molar mesialized by approximately 2 mm (Richardson, 1987). There is ongoing debate regarding the age retromolar space growth ceases, with some studies contending it ceases by 16 years of age (Chen et al., 2010, Ganss et al., 1993, Niedzielska et al., 2006) and others finding growth through at least 18 years of age (Shiller, 1979, Zelić and Nedeljković, 2013).

Retromolar space variation by sex and ancestry is also unclear. Current literature is split on whether there are sex differences in retromolar space. Some scholars have found smaller retromolar spaces (Ghougassian & Ghafari, 2014), lower rate of growth (Chen et al., 2010), and/or earlier cessation of growth of the retromolar space (Chen et al., 2010) in females. In contrast, Zelić and Nedeljković (2013) found no sex differences in retromolar space or third molar eruption levels.

Few studies have looked specifically at the retromolar space growth pattern, and most were either cross-sectional studies (Ghougassian and Ghafari, 2014, Zelić and Nedeljković, 2013) or studies with small sample sizes (Richardson, 1987: n = 51; Chen et al., 2010: n = 28). Longitudinal studies allow a thorough evaluation of the potential patterns and trends that are unavailable in a cross-sectional study (e.g., Richardson, 1987; Chen et al., 2010)). Potential sex differences are an important consideration which were not included in all prior studies (for example, Richardson, 1987). Our study thus represents the largest longitudinal study of sex differences in retromolar space growth, and the only one to apply longitudinal statistical analyses to this question.

The current study employs a large, retrospective longitudinal dataset to examine growth of the retromolar space. The aims of this study were to examine net growth and growth velocity of the retromolar space longitudinally across males and females, both in comparison to chronological age and skeletal maturation.

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