[Anatomical aspects of posterior fossa affecting lateral suboccipital approach: evaluation by bone-window CT]

Analyzing the bone-window CT of the posterior fossa, the authors investigated three anatomical aspects of the posterior fossa affecting the lateral suboccipital approach (LSA). The high-resolution 1.5 mm-slice bone-window CT images of the posterior fossa in 40 patients with the cerebellopontine angle tumor were reviewed regarding three anatomical aspects: 1) the internal occipital crest (IOC), 2) the posterior surface of the petrous bone, and 3) the "petrous angle". The IOC was sometimes prominent and protruded profoundly into the posterior fossa. The height of IOC from the inner table of the occipital bone was 9.6 +/- 3.3mm (max : 17 mm, min : 3mm). The posterior surface of the petrous bone was convex to the posterior fossa in the most cases; the zenith of the prominence was the porus acusticus. The convexity of the posterior surface in the CT image was objectively evaluated by the "porus angle" made by two lines of A and B; the line A was the posterior half of the posterior surface of the petrous bone, and the line B was the anterior half of it. The "porus angle" in 40 cases was 28 +/- 14 degrees (max : 61 degrees, min : 0 degrees) in the left side, and 28 +/- 12 degrees (max : 56 degrees, min : 0 degrees) in the right side. The "petrous angle", made by the cranial sagittal line and (the posterior half of) the posterior surface of the petrous bone, was 61.8 +/- 5.8 degrees (max : 75 degrees, min : 47 degrees) in the left side, and 62.7 +/- 7.0 (max : 75 degrees, min : 46 degrees) in the right side. In the patient with a prominent IOC, the LSA with a unilateral suboccipital craniotomy may induce the compression of the cerebellar hemisphere by the brain retractor and the prominent IOC, and develop cerebellar contusion. Such a postoperative cerebellar complication can be avoided by a large suboccipital craniotomy with the resection of the prominent IOC extending contralaterally. The severe convexity of the posterior surface of the petrous bone, i.e. the large "porus angle", makes it difficult to get the view of the petroclival region in the LSA. The larger is the "petrous angle", the less cerebellar compression is necessary for the approach to the cerebellopontine angle by the LSA ; the large "petrous angle" is advantageous to the approach. The three anatomical aspects of the posterior fossa, i.e. the IOC, the posterior surface of the petrous bone ("porus angle"), and the "petrous angle", show a considerable variation among individuals. Since these anatomical aspects affect the difficulty and the postoperative complication of the LSA, it can not be overlooked to evaluate them preoperatively by the bone-window CT and plan the surgical approach.