The asterion is a term used for a point at the side of the human skull that serves as the junction for three types of sutures. These are lines of a stationary joint that can be found in the skull, among other places in the skeletal system. The asterion is also called a craniometric point, which signifies that it can be used as a measuring point in the skull.
Regarding its location, the asterion can be found behind the ear. Specifically, it is located about 1.6 inches (4.1 cm) behind and about 1.5 inches (3.8 cm) above the hearing organ. The three sutures that the asterion unifies are the lambdoid suture, the parieto-mastoid suture and the occipito-mastoid suture.
The lambdoid suture is perhaps the most crucial of the three features that the asterion joins. Named for its resemblance to the lambda, the 11th letter in the Greek alphabet, it connects the parietal and occipital bones. These are two of the three bones that the asterion is also responsible for joining. While the parietal bone forms the cranium's sides and roof, the occipital bone constitutes its rear and lower half. The asterion also connects the mastoid part of the third cranial part, the temporal bone, which helps the parietal bone in forming the skull's sides as well as comprising its base.
During a child's first years of life, when the bones are still forming, the lambdoid suture may close too quickly, which can result in a condition called plagiocephaly. This is when one side of the skull is more developed in the front, while the other side of it is more defined in the back, thus presenting a malformed, asymmetrical structure.
The sutures, however, also have their own functions. The parieto-mastoid suture is so named because it joins the mastoid section of the parietal bone with that of the temporal bone. Similarly, the occipito-mastoid suture joins the mastoid process of the occipital bone with that of the temporal bone.
The use of the term "asterion" for this part of the skull is rather unclear. Asterion is the name of a Greek mythological figure who served as one of three river gods. It is also the alternative name of the Beta Canum Venaticorum star. There is little or no correlation, however, between the aforementioned uses and the naming of this particular craniometric point.
The asterion has become important in neurosurgery. Surgeons use it as a measurement for safe entrance into the skull during operations, such as the removal of tumors like meningiomas. Another common operation is for treating stenosis, or narrowing, of the lambdoid suture.