The asterion is defined as the junction of the lambdoid, parietomastoid, and occipitomastoid sutures. The asterion has been used as a landmark in lateral approaches to the posterior fossa. However, its reliability as a landmark comes into question considering the apparent variability of its position.
One hundred dried skulls were obtained for a study. A 2-mm drill hole was placed at the asterion on each side. The position of the drill hole on the inner surface of the skull was next determined and recorded.
The asterion was located over the posterior fossa dura in 32% on the right and 25% on the left. Its position was over the transverse or sigmoid sinus complex in 61% on the right and 66% on the left. The landmark was located above the transverse-sigmoid sinus complex in 7% on the right and 9% on the left.
The asterion is not a strictly reliable landmark in terms of locating the underlying posterior fossa dura. Its location is very often directly over the transverse-sigmoid sinus complex. Burr holes placed at the asterion may often open the bone directly over the sinus, leading to potential damage 1).
Point on the skull corresponding to the posterior end of the parietomastoid suture.
It is a so-called craniometric point on the exposed skull, just behind the ear, where three cranial sutures meet:
The lambdoid suture parieto-mastoid, and occipito-mastoid sutures,
or where three cranial bones meet:
Parietal bone, Occipital bone, and Mastoid portion of the Temporal bone.
In the adult, it lies 4 cm behind and 12 mm above the center of the entrance to the ear canal.