Wormian bones go by many names including accessory bones of the skull, intrasutural bones or sutural bones. Wormian bones of the skull are formed from additional and separate centres of ossification of the cranium, located near or in the cranial sutures that lead to extra isolated bone segments in the cranium.
These small accessory bones tend to vary in size and also tend to be irregular in shape.
Accessory bones of the skull are found in various areas, usually arising in fontanelles and/or adjacent or along the sutures of the skull. The most common location that these accessory bones are found is at the lambdoid and lambdoid (posterior) fontanelle, with some arising at the masto-occipital suture. Sometimes a large isolated interparietal bone is found at lambda, called the Inca bone or Goethe’s ossicle.
The second most common location is at the coronal suture of the skull with the remaining locations at any other sutures and fontanelles. A single or a number of pterion ossicles may be found between the sphenoid angle of the parietal bone and the greater wing of the sphenoid bone. Pterion ossicle(s), again, vary in sizes but are usually symmetrical in shape.
Causes of accessory bones
The presence of accessory bones in the skull is not a rare occurrence but its existence and the type of accessory bone is related to cranial deformations. There are several theories relating to the incidence of accessory bones. Cultural cranial deformation, which can lead to pressure on the cranium, has been linked to the presence of the accessory bones (i.e. an external influence).
Another possibility is that it could be genetically linked, and another hypothesis is that it could be due to rapid enlargement of the cranium (an environmental stressor situation), where the incidence and number of accessory bones is commonly seen in hydrocephalic skulls where the cranium quickly expands. In this situation, the presence of accessory bones may be a result of an adaptational response.
Although the cause of accessory bones is not definitive or well known, there are many diseases that are associated with their presence. Most commonly, accessory bones of the skull have been found with disorders of the central nervous system. Congenital diseases and anomalies that are associated with accessory bones include (but are not limited to):
- cleidocranial dysplasia
- Menke’s Syndrome
- osteogenesis imperfecta
- Down’s syndrome
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