Virtual image of the incus from a saline lens: an aid to safe dissection of the mastoid antrum.

Dissection of the antrum requires judicious visualization of the tegmen, horizontal canal, incus, and facial nerve. Inadvertent contact with the incus by the power drill may cause significant acoustic trauma. An effective "early warning device" accurately predicting exposure of the incus may be constructed with the saline irrigation fluid. The formation of the meniscus at the antrum permits visualization of the image of the incus around the corner of the bony canal wall overhang. A thin-lens model of this phenomenon has been created for first-order analysis of the optics, and it provides useful criteria for using this image. Image minification, displacement, and distortion were analyzed. A more complex model was then created to assess the impact of changing meniscus shape. The latter helped in understanding an added "swinging-door effect" noted as the meniscus was altered. The model predicted accurately the phenomena as observed in the temporal bone preparation. Illustrative examples of the optics and photographic demonstrations of the phenomena are presented.