A Wormian Bone in a Neonate

July 31, 2012

Wormian bones (anterior fontanellar bones) are extra islands of bone within the calvarial sutures of the skull.

A full-term, 39-week gestational-age male was born to a 30-year-old gravida 3 para 3 mother. The mother had received routine prenatal care during an uncomplicated pregnancy; prenatal ultrasonography showed a normal fetus. The infant was born by cesarean section (the mother had previously had a C-section); his delivery and postnatal course were unremarkable. Two older siblings were healthy without any bony or calcium abnormalities. 

During the infant’s admission examination in the newborn nursery, an abnormality was noted on palpation of the cranium. Instead of the normally soft and flat anterior fontanelle, a bony mass occupied the fontanellar space. This structure was hard and diamond-shaped and felt similar to the other fully calcified cranial bones. It was separated from the frontal and parietal bones by a rim of what felt like normal fontanellar or sutural connective tissue.

The coronal, sagittal, metopic, and lambdoid sutures were of normal size and shape and were ballotable. The posterior fontanelle was also of normal size, shape, and position. The infant had a normal frontal-occipital circumference. The rest of the physical examination findings were normal as well.

Skull radiographs showed a wormian bone occupying the anterior fontanel (Figure 1): it was identified as an anterior fontanellar bone. It was located in the central region of the normal anterior fontanellar space and appeared radiographically similar to the other cranial bones. No craniosynostosis was seen.

Discussion
Wormian bones are extra islands of bone within the calvarial sutures of the skull1(Figure 2) and represent independent ossification centers.1 They are most commonly located within the lambdoid sutures. Anterior fontanellar bones, a specific type
Figure 1.
of wormian bone, are also known as bregmatic bones. Absence of the anterior fontanelle due to an anterior fontanellar bone has been reported previously in the literature.1-4 A retrospective review of surgical reports from British Columbia looked at 100 patients younger than 1 year with isolated sagittal craniosynostosis. The authors report 4 cases of a wormian bone replacing the anterior fontanelle in this group of patients.1 A case series in the United Kingdom over a 3-year period identified 11 patients with anterior fontanellar bones; 5 of these patients also had craniosynostosis. The 6 patients with anterior fontanellar bones without craniosynostosis had normal head growth and development; there was no progression to craniosynostosis during the follow-up period.2

An older case series from the United States noted 35 children with anterior fontanellar bones.3 The authors noted that anterior fontanellar bones did not affect the normal growth of the skull and should be considered a normal variant.3 An autopsy report of a 66-year-old woman noted a supernumerary cranial bone in the area of the anterior fontanelle with an otherwise normal cranial size and shape,4 further evidence of the lack of long-term effects of an anterior fontanellar bone.

The etiology of wormian bones is controversial.5 Rabbits with experimentally induced craniosynostosis are prone to develop wormian bones; the theory is that increased pressure on the sutures leads to their formation.6  However, in the absence of craniosynostosis, the cause of wormian bones remains unclear and
Figure 2.

tends to be attributed to the vague category of “genetic.” It has not been reported that wormian bones lead to future craniosynostosis.

Anterior fontanellar bones are a rare anatomic variant. Consider the diagnosis when an infant lacks an anterior fontanelle. It would be reasonable to get skull radiographs if an anterior fontanellar bone is suspected, to rule out craniosynostosis. Given the apparently benign natural history of anterior fontanellar bones, only routine follow-up to monitor head growth is necessary. Referral to a craniofacial specialist would be warranted, as usual, for children with abnormal head shape or growth.

Our patient came to the clinic for a routinely scheduled visit at 2 weeks but was lost to follow-up. Progression to craniosynostosis would not be expected in this patient, however, and he should not have any sequelae as a result of this anterior fontanellar bone.

References
1. Agrawal D, Steinbok P, Cochrane DD. Pseudoclosure of anterior fontanelle by wormian bone in isolated sagittal craniosynostosis. Pediatr Neurosurg. 2006;42:135-137.
2. Woods RH, Johnson D. Absence of the anterior fontanelle due to a fontanellar bone. J Craniofac Surg. 2010;21:448-449.
3. Girdany BR, Blank E. Anterior fontanel bones. Am J Roentgenol Radium Ther Nucl Med
1965;95:148-153.
4. Barberini F, Bruner E, Cartolari R, et al. An unusually wide human bregmatic Wormian bone: anatomy, tomographic description, and possible significance. Surg Radiol Anat. 2008;30:683-687.
5. O'Loughlin VD. Effects of different kinds of cranial deformation on the incidence of wormian bones. Am J Phys Anthropol. 2004;123:146-155.
6. Burrows AM, Caruso KA, Mooney MP, et al. Sutural bone frequency in synostotic rabbit crania. Am J Phys Anthropol. 1997;102:555-663.