Posterior fossa measurements. I. The normal size of the posterior fossa.

The posterior fossa area was defined as that situated below Twining's line while the one above was called the supratentorial area. A ratio has been calculated: Posterior fossa area/Supratentorial area X 100 = posterior fossa ratio. Above 7 years this was (mean) 16.5% +/- 2 and below 7 years 14.0% +/- 4, with the ranges (13.2-20.2) and (9.0 - 19.4), respectively. When the ratio falls below 13% and 9% respectively, this implies therefore a malformation of the posterior fossa, i.e. too flat a space. Two formulas for calculation of the areas mentioned as well as two other ratios are also introduced.