Neurological Surgery No Shinkei Geka Volume 24, Issue 2 (February 1996)

Anatomical aspects of posterior fossa affecting lateral suboccipital approach:evaluation by bone-window CT Iwao YAMAKAMI 1 , Akira YAMAURA 1 , Junichi ONO 1 , Takao NAKAMURA 2 1Department of Neurosurgery, Chiba University School of Medicine 2Department of Kasori Hospital, Chiba University School of Medicine Keyword: cerebellopontine angle , computed tomography , lateral suboccipital approach , posterior fossa , complication pp.157-163
Published Date 1996/2/10
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Analyzing the bone-window CT of the posterior fos-sa, the authors investigated three anatomical aspects of the posterior fossa affecting the lateral suboccipital approach (LSA). The high-resolution 1.5mm-slice bone-window CT images of the posterior fossa in 40 patients with the cerebellopontine angle tumor were re-viewed regarding three anatomical aspects: 1) the in-ternal 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 in-ner table of the occipital bone was 9.6±3.3mm (max: 17mm, min: 3mm). The posterior surface of the pe-trous 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 lineA 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° (max: 61°, min: 0°) in the left side, and 28±12° (max: 56°, min: 0°) in the right side. The “petrous angle”, made by the cranial sagittal line and (the posterior half of) the pos-terior surface of the petrous bone, was 61.8±5.8° (max: 75°, min: 47°) in the left side, and 62.7±7.0 (max: 75°, min: 46°) in the right side.

In the patient with a prominent IOC, the LSA with a unilateral suboccipital craniotomy may induce the com-pression of the cerebellar hemisphere by the brain re-tractor 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 contra-laterally. 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 con-siderable variation among individuals. Since these ana-tomical aspects affect the difficulty and the postopera-tive complication of the LSA, it can not be overlooked to evaluate them preoperatively by the bone-window CT and plan the surgical approach.

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Neurological Surgery 脳神経外科
24巻2号 (1996年2月)
電子版ISSN 1882-1251 印刷版ISSN 0301-2603 医学書院