A sutural bone is an accessory bone formation found within the skull, representing an anatomical variation in the human cranium. These irregular, isolated pieces of bone develop where the flat bones of the skull come together at their joints, known as sutures. They are not part of the standard 22 bones that compose the human skull but are instead extra ossicles that appear in addition to the normal bony structure. Studying these formations is important in fields like anatomy and radiology because they can sometimes be confused with skull fractures in medical imaging.
What Exactly Are Sutural Bones?
Sutural bones are small, supernumerary ossicles that develop within the fibrous joints connecting the cranial bones. Their formation results from additional, isolated centers of intramembranous ossification that fail to merge completely with adjacent cranial bones during development. This process creates a separate bone piece situated directly within the suture line.
These bones are highly variable in size and shape, often appearing polygonal or round, ranging from a few millimeters to over a centimeter. They may occur on one side of the skull or symmetrically. Sutural bones are also frequently referred to as Wormian bones, named after the Danish anatomist Ole Worm. For most people, the presence of one or two small sutural bones is considered a benign anatomical finding.
The Primary Location in the Cranium
Sutural bones are found almost exclusively in the skull, specifically along the lines where the cranial plates articulate. The most common site for their occurrence is the Lambdoid suture, which runs along the back of the head, connecting the parietal bones to the occipital bone. This suture is frequently involved due to its complex, interlocking nature.
While the Lambdoid suture is the dominant location, sutural bones can appear in other cranial joints. These include the sagittal suture (top midline of the head), the coronal suture (near the forehead), or the squamosal suture (over the temple area).
Specific Accessory Bones
Larger, more distinct sutural bones are sometimes identified at the former sites of the fontanelles (soft spots on an infant’s skull). A prominent example is the Inca bone (os incae), a large accessory bone that forms at the Lambda, the junction of the Lambdoid and Sagittal sutures. Another specific location is the Pterion, a complex junction on the side of the skull, where a small epipteric bone (pterion ossicle) can sometimes be found.
When Sutural Bones Indicate More Serious Conditions
While a few small sutural bones are often normal variations, the discovery of numerous, large, or unusually shaped ones can be clinically significant. A high number is often defined as more than ten bones or those exceeding a certain size (e.g., 6 millimeters by 4 millimeters). In these instances, they serve as a potential marker for an underlying genetic or skeletal disorder.
These conditions often involve abnormal bone development or delayed ossification, which creates space for the accessory centers to form. Specific congenital disorders associated with excessive sutural bones include Osteogenesis Imperfecta (brittle bone disease) and Cleidocranial Dysostosis. They are also observed in conditions affecting skeletal development more broadly, such as Down Syndrome, Pyknodysostosis, and Hypothyroidism. The presence of these bones in a mosaic-like pattern can alert medical professionals to investigate for associated pathologies.