Keros classification of olfactory fossa

Dr Craig Hacking ◉ ◈ and Dr Mark Holland et al.

The Keros classification is a method of classifying the depth of the olfactory fossa.

The ethmoid labyrinth is covered by the fovea ethmoidalis of the frontal bone and separates the ethmoidal cells from the anterior cranial fossa.

The very thin, horizontal cribriform plate (lamina cribrosa) of the ethmoid bone is bounded laterally by the vertical lateral lamella. The lateral lamella joins the cribriform plate to the fovea ethmoidalis.

In adults, the olfactory recess is a variable depression in the cribriform plate that medially is bounded by the perpendicular plate and laterally by the lateral lamella. It contains olfactory nerves and a small artery.

The depth of the olfactory fossa is determined by the height of the lateral lamella of the cribriform plate. Keros in 1962¹, classified the depth into three categories.

type 1: has a depth of 1-3 mm (26.3% of population)
type 2: has a depth of 4-7mm (73.3% of population)
type 3: has a depth of 8-16mm (0.5% of population)

The type 3 essentially exposes more of the very thin cribriform plate to potential damage from trauma, tumor erosion, CSF erosion (in benign intracranial hypertension) and local nasal surgery or orbital decompression ².

Thin bone in the skull base, especially the cribriform plate, is susceptible to erosion, encephalomeningocoele herniation and CSF leaks

The most common sites of erosion or defects in the skull base are