[Morphological and clinical study of acoustic tumor with respect to enlargement of internal auditory canal: mechanism of bone destruction].

The mechanism of widening of the internal auditory canal (IAC) by an acoustic tumor is still unknown. Degree of IAC enlargement was expressed as "IAC ratio' calculated from the measurement of bilateral horizontal diameters from bone-imaging CT. Comparing IAC ratio to radiological findings of tumors, such as size, solidity, mode of attachment to pyramidal bone and the presence of ventricular dilatation, we found a positive correlation between the ratio and the volume of solid tumors (especially of smaller ones) and more marked destruction of IAC in association with ventricular dilatation. Preoperative degrees of hearing ability or facial nerve function didn't correlate with that of IAC enlargement. It is our hypothesis that the mechanism involved in the destruction of IAC is as follows: (1) The repeated strokes of the tumor caused by brain pulsation gradually erode the bony wall of IAC (especially, that of the posterior wall). The apparently less destruction of IAC in cases with mostly cystic large tumors might be due to the restriction of this pulsatile movement. (2) Increased CSF pressure on the site has some influence on the mechanism of bone erosion as also does solidity of the tumor.