The coronal suture is the joint that connects the two uppermost bones of the cranium: the frontal and parietal bones. It is formed by the union of the two parietal bones, those to either side of the top of the skull, with the frontal bone, which is situated in the forehead. The coronal suture is a type of joint known as a fibrous joint, which is held together by dense collagen tissue. Though joints are often associated with movement, this type of joint allows little to no movement.
Like a crack in a sidewalk, the coronal suture extends from one temple laterally across the top of the skull to the other temple. As the edges of these bones are irregular, it does not form a perfectly straight line, and yet these bones fit tightly together like puzzle pieces. This is because the bones do not meet at birth. There is a slight gap between them to permit a small amount of movement as the baby pushes through the birth canal. As the skull grows and develops, the bones gradually close until they are nearly fused.
Once this happens, a matrix of collagen fibers known as Sharpey’s fibers holds the bones tightly together so that almost no movement may occur. The exception is that which gives the skull the elasticity required to endure the forces of expansion and contraction, as when there is swelling around the brain. For this lack of movement the coronal suture is also classified as a synarthrosis joint, as opposed to a diarthrosis like most movable joints of the body.
Another characteristic that separates the coronal suture and other fibrous joints from the diarthroses is the absence of synovial fluid. Diarthroses like the hip, shoulder, and knee joints feature a joint capsule between the bones that contains a sac of synovial fluid. This fluid lubricates the joint and helps eliminate friction between the adjoining bones as they move. Since the frontal and parietal bones do not truly move against each other and are essentially one bone, no synovial fluid is contained in the suture between them.
Occasionally, deformities are present between the cranial bones. These may occur when the skull bones close too quickly, resulting in a skull that is too tall and narrow, a condition known as oxycephaly, or one that is asymmetrical, a condition known as plagiocephaly. Conversely, deformities known as fontanelles may be seen when the bones close too slowly or not at all. In infants they are known as “soft spots” and will typically disappear as the bones eventually come together. In rare cases they are permanent, as with cleidocranial dystosis, a congenital disorder.