The metopic suture: Natural history.

The metopic suture (MS) is one of the main sutures of the calvaria; premature closure is responsible for trigonocephaly, while persistence (metopism) is considered a normal variant. The ages of onset and completion of MS closure and prevalence of metopism in normal children are poorly documented. We studied the pattern of MS closure on 3D-CT scans of 477 children admitted for head trauma since 2012. We also studied the prevalence of trigonocephaly and the sex ratio in our clinical series of patients with all types of synostosis diagnosed during the last 4 decades. In the majority of children, MS closure started at 4 months and was complete at 9 months. The prevalence of metopism was stable after 1 year of age, at 5.1%; it was more than twice as frequent in girls (F/M ratio 2.1, non-significant). Our trigonocephaly series and the literature show a steady increase in prevalence over recent decades. During the same period, the prevalence of metopism decreased steadily. Data from comparative anatomy and paleoanthropology suggest that postnatal MS persistence in our species results from the risk of dystocia caused by the closed pelvis associated with bipedalism. The increasing incidence of trigonocephaly appears to parallel the fall in prevalence of metopism. The increasing use of cesarean section may have eliminated a potent selection factor in favor of postnatal persistence of the MS.