Petrosquamous fissure

Last revised by Francis Deng on 13 Mar 2021

The petrosquamous fissure is the obliquely oriented fissure between the medial petrous part and lateral squamous part of the temporal bone.

Gross anatomy

The anterior (ventral) petrosquamous fissure is the medial continuation of the tympanosquamous fissure and petrotympanic fissure (together forming a Y configuration). It is oriented obliquely toward the greater wing of sphenoid where it splits into the sphenosquamosal suture and sphenopetrosal suture. This part of the fissure is posterior to the squamous part of temporal bone that is medial to the mandibular fossa, and anterior to the petrous part of temporal bone that is an inferior projection of the tegmen tympani known as the intertympanosquamosal crest or crista tegmentalis 2.

The middle petrosquamous fissure is difficult to delineate in the adult. On the intratympanic side, the fissure may be related to the cog 2. On the endocranial side, the fissure may show a groove for the petrosquamous sinus, a tributary of the sigmoid sinus 2.

At the posterior petrosquamous fissure, the mastoid tegmen projects inferiorly due to developmental apposition between squamous and petrosal parts, forming Koerner septum internally 2. Externally, the posterior petrosquamous fissure is also known as the squamomastoid suture.

Radiographic features


The anterior part of the fissure is seen on axial views as an anteromedially oriented cleft extending from the mandibular fossa toward the greater wing of the sphenoid 1

The middle part of the fissure may be seen on coronal views as a tiny defect in the tegmen tympani 1.

The posterior part of the fissure is seen as a vertically oriented site of apposition on the mastoid surface between the squamous part (squamomastoid) and petrosal part (petromastoid) 2.

The Koerner septum is a useful landmark for approximating the orientation of the petrosquamous fissure.

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Cases and figures

  • Figure 1: diagram - medial temporal bone
    Drag here to reorder.