Anterior Fontanel Bone

GENESIS

On very rare occasions, the anterior fontanel will ossify into a bony plate that may be slightly elevated in relation to the rest of the cranium. It is believed that they arise from the formation of abnormal cranial ossification centers. This may occur due to decreased growth-stretch tensile forces across the anterior fontanel, and can be present at birth or appear later in infancy. It is sometimes seen with multiple suture synostosis and can also occur in otherwise normal infants (similar to Wormian bones, see Chapter 41 ), in which case it is considered a normal variant. It is thought to be associated with other anatomical anomalies such as osteogenesis imperfecta, rickets, and other bone dysplasias, with only one report in the literature where there was a recurrence in siblings. The anterior fontanel normally closes between 4 and 26 months, with 90% closing between 7 and 19 months and 42% closing before 12 months. The area of the anterior fontanel can be calculated and compared to published standards of infant groups based on gestational age at birth or chronological age. Delayed closure of the fontanel has been associated with a variety of pathologic conditions such as osteogenesis imperfecta, hypophosphatasia, cleidocranial dysplasia, and various other skeletal dysplasias. An ossified anterior fontanel does not carry the same significance. In one cohort of 11 patients with anterior fontanel bones referred to a craniofacial center, 5 had craniosynostosis (1 sagittal, 3 metopic, 1 bicoronal), 1 had acrocallosal syndrome, and 5 were isolated. All patients in the surgical cohort had good postoperative results, and the others were followed and had normal development. This can also be referred to as a bregmatic Wormian bone and has been reported with metopic synostosis. According to Oostra et al., supernumerary bones that result from normal, nonfused ossification centers are likely to be different from bones deriving from additional centers within the sutures and fontanels.