Wormian Bones

GENESIS

Wormian bones are accessory bones that occur within cranial suture lines and are often considered to be a simple anatomical variant. Although they themselves do not cause any impairment, their significance as a clinical finding is variable. The prevalence varies by size and quantity. In a 2019 Greek study of 124 dry adult skulls, 74.7% had Wormian bones, most commonly located in the lambdoid suture (44.6%), followed in order of frequency by the coronal suture (39.8%), asterion (21% on the left and 15.3% on the right side), and parietomastoid suture (15.1% on the left and 13.9% on the right side). Individuals with few and small (<4 mm) Wormian bones are fairly common and individuals with many large (greater than 10 mm) Wormian bones ( Fig. 41.1 ) are extremely rare. To be considered pathologically significant, they must number more than 10 in number, be larger than 6 mm by 4 mm, and be arranged in a general mosaic pattern. In one study, the majority of children with an “excessive” number of Wormian bones had some abnormality of the central nervous system. Reported abnormalities ranged from gross malformations to minimal brain dysfunction, although this study may have been biased because it used a hospital-based population. Thus some individuals with many Wormian bones may have other anomalies and/or central nervous system dysfunction. The name Wormian bones is derived from a Danish anatomist named Olaus Worm who described these small irregular ossicles located within cranial sutures in a letter to Thomas Bartholin in 1643. They occur most commonly in the lambdoid sutures and within fontanels, and the pathogenesis of Wormian bones is thought to be related to variations in dural growth stretch along open sutures and within fontanels, causing ossification defects. Such sutural bones persist and are not incorporated into the adjacent bone during mineralization and maturation. Although the prevalence of Wormian bones in the general population varies from 8% to 15%, the true prevalence is around 14%. Males are more often affected than females, and differences among ethnic groups have been noted, with the highest incidence in Chinese individuals (80%). Ethnic variation in Wormian bones may suggest a possible genetic influence, but environmental influences could also play a role.

A positive correlation has been noted between the frequency of lambdoid Wormian bones and the degree of deformation observed in primitive cultures that practice fronto-occipital head binding, which suggests that these bones form in relation to changes in pressure along the lambdoid sutures. Note that the term “Inca bone” was initially used as a synonym for Wormian bones in deformed Peruvian skulls, which were mistakenly considered to be a racial trait. In rabbits with premature coronal synostosis, Wormian bones appeared in the coronal and sagittal sutures after the onset of cranial growth alterations induced by premature coronal synostosis, which suggests that Wormian bones form in relation to external factors. One study tabulated the frequency and location of large Wormian bones (>1 cm) in three-dimensional computed tomography scans from 207 cases of craniosynostosis and compared this data to control. Among cases of craniosynostosis, large Wormian bones were significantly more frequent (117 out of 207 three-dimensional computed tomography scans) than in control skulls (131 out of 485), with a 3.5-times greater odds of developing a Wormian bone with premature suture closure ( P < .001) ( Fig. 41.2 ). Several others have also since reported Wormian bones in craniosynostosis and other head shape deformities. Nondeformed crania have more Wormian bones than circumferentially deformed crania but have fewer Wormian bones than anteroposteriorly deformed crania. This may relate to variations in tension across the sutures, a hypothesis tested by O’Loughlin, who demonstrated that the frequency and location of Wormian bones vary depending on the type and degree of cranial deformation, with posteriorly placed Wormian bones appearing in greater numbers in deformed crania and with sagittal synostosis. The increased frequency of Wormian bones noted in Chinese infants might be related to their traditional supine infant sleeping position and the resultant pressure against their occiput. If so, an increased frequency of Wormian bones may soon be noted in other cultures that have adopted the supine sleep positioning to prevent sudden infant death syndrome (SIDS), and it might be expected that more Wormian bones will be associated with a higher Cephalic Index (biparietal diameter divided by the anteroposterior diameter multiplied by 100). One study in fact measured the Cephalic Index and counted the number of Worman bones in pre-Colombian purposefully deformed skulls and compared them to control skulls from a medical school anatomy course. They found that the higher the Cephalic Index (i.e., more brachycephalic), the more Wormian bones were found within the skull sutures.