Bent and Kuhn Classification of Frontal Sinus Cells:

• Type 1: A single air cell above the agger nasi cell
• Type 2: Two or more air cells above the agger nasi cell
• Type 3: A single large cell above the agger nasi cell that extends into the frontal sinus
• Type 4: Cells are entirely contained within the frontal sinus

References:

Bent, J. P., Cuilty-Siller, C., & Kuhn, F. A. (1994). The Frontal Cell as a Cause of Frontal Sinus Obstruction. American Journal of Rhinology, 8(4), 185-192. https://doi.org/10.2500/105065894781874278