To determine which anatomical parameters of the round window (RW) influence the surgical approach for cochlear implantation and in which plane they should be measured.

Sixty-six patients (40 females and 26 males; mean age: 60.4±17.7 [26–84] years) who underwent preoperative ultra-high resolution CT underwent cochlear implantation between February 2020 and March 2024. Two radiologists reviewed the anatomical parameters of the RW niche (type, minimal width), membrane (size, inclination) and bony overhang (size, inclination) in the axial, parasagittal and Stenvers planes.

The RW membrane size in the Stenvers plane was the most reproducible and the only difference between patients requiring a promontory cochleostomy approach and those with an easy RW approach, with an optimal cut-off value of 1.55mm for the need for a cochleostomy. The RW membrane size in the parasagittal and Stenvers planes were highly correlated. The minimum width and type of the RW niche and the size and inclination of the RW bony overhang did not influence the surgical approach and had variable interobserver agreement.

The size of the RW membrane should be measured in the Stenvers plane. Other anatomical parameters of the RW do not influence the surgical approach.