Head and Neck Imaging

Keros' classification

describes the olfactory fossa. Lateral to the lamina cribrosa, the ethmoid bone is covered superiorly by the frontal bone. This frontal bone covering (called fovea ethmoidalis) and the lamina cribrosa can be at more or less the same level, or a height difference may exist. The very thin, more or less sagittally orientated bone connecting the horizontal lamina cribrosa and fovea ethmoidalis is called the lateral lamella of the lamina cribrosa; between the midline and the lateral lamella is the olfactory fossa.

  • Type I: olfactory fossa is 1-3 mm deep, lateral lamella is nearly nonexistent;

     

  • Type II: olfactory fossa is 4-7 mm deep; and

     

  • Type III: olfactory fossa is 8-16 mm deep (Fig.1).

    Especially in type III, the ethmoid roof is significantly higher than the anterior cranial fossa, increasing the surgical risk of intracranial entry during functional endoscopic sinus surgery.

    • RH

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    Fig.1

    Coronal CT images through sinonasal cavities at the level of the crista galli. Lamina cribrosa (arrow); fovea ethmoidalis (arrowhead). Type I (a), type II (b, olfactory fossa depth about 6 mm), type III (c, olfactory fossa depth about 9 mm).
    Keros' classification, Fig.1 (a)
    Keros' classification, Fig.1 (b)
    Keros' classification, Fig.1 (c)