Access to Round Window Niche via Posterior Tympanotomy and Impact of Drilling Its Overhangs: A Cadaveric Descriptive Study

Abstract

We intended to study the morphological parameters of round window region and assess the gain in exposure achieved by drilling the round window niche overhang. The Exposure of the round window membrane (RWM) is of prime importance to carry out atraumatic electrode insertion for cochlear implantation. The anatomy of round window has been a subject of considerable debate in literature. Fifty-one Formalin preserved adult cadaveric temporal bones were micro-dissected to carry out an ‘optimal’ posterior tympanotomy to expose the round window region. The bony overhangs of round window niche (RWN) were next drilled to achieve maximal possible exposure the RWM without violating the annulus of the same. The exposure was classified as per St Thomas’ Hospital classification. The round window could not be visualized in 3 bones (5.9%). The commonest morphology of RWN was dome shaped, found in 18 (37.5%) and that of the RWM was oval shaped, found in 14 (29.2%) bones. Pre drilling 41 bones had a > 50% exposure of RWM while post drilling > 50% exposure could be achieved in all the bones except the 3 bones in which RWN could not be visualized to begin with. The drilling of the RWN overhangs exposed RWM in entirety in 91.7% of bones with a visible morphology of RWN pre drilling. RWN and RWM exhibit varied morphology. Drilling of the round window niche overhangs can considerably enhance the exposure of RWM.