Anatomy of the Round Window Region With Relation to Selection of Entry Site Into the Scala Tympani

Abstract

Objectives/Hypothesis The aim of cochlear implantation is to safely insert an electrode array into the scala tympani (ST) while avoiding damage to surrounding structures. There is disagreement on the optimal way of entering the ST—the round window (RW) approach versus cochleostomy. Regardless of the chosen approach, it is vital to understand the regional anatomy, which is complex, difficult to conceptualize, and rarely dissected in temporal bone courses. The goal of this study was to examine the anatomy of the RW to gain more in‐depth knowledge on the local relationships of the anatomical structures and propose an approach for entering the ST in cochlear implant surgery tailored to the encountered anatomy. Study Design: Cadaveric prevalence study and expert opinion with literature review. Methods Cadaveric temporal bone dissection (n = 13) by the first author assessing the RW anatomy. Results The round window membrane (RWM) and the osseous spiral lamina (OSL) are curved structures, each with a horizontal and a vertical part. The two horizontal portions are very closely apposed. The relationship between the OSL and the RWM determines the best site for a cochleostomy, which if required is best placed anteroinferiorly to the RWM. The distance between the oval window inferior margin and the RW membrane is less than 2 to 3 mm. The ST initially extends inferiorly and medially to the RW. Conclusions The findings of our dissection have implications for cochlear implant surgery in aiming to avoid trauma to the OSL and basilar membrane and aid decision making in choosing the safest surgical approach. Level of Evidence 5. Laryngoscope, 131:E598–E604, 2021

In vivo endoscopic study of the variations of anatomical structures in the hidden spaces of the middle ear during endoscopic ear surgery

“…Yoda et al in 2011 showed that 4.9% of normal temporal bones had a false RW membrane in their study on 12 cadavers [21]. False RW membrane is a significant morphologic variation in the human RW [11]. False RW membrane may block the drug from coming into contact with the round window membrane [21].…”

“…The round window region has a special interest in the middle ear due to its extreme surgical importance [11]. Yoda et al in 2011 showed that 4.9% of normal temporal bones had a false RW membrane in their study on 12 cadavers [21].…”

“…Subcochlear canaliculus: it is a pneumatized bony channel that connects the round window chamber with the petrous apex and lies inferior to the cochlea. It was classified as type A: present and type B: absent [11].…”

“…The round window niche is another important compartment that has a special focus due to surgical importance in cochlear implantation and cholesteatoma affection [11]. The round window niche is an anatomical bony structure that forms the entrance of the cochlear membrane and closes the scala tympani, it has a triangular shape that is made by the posterior pillar, tegmen, and anterior pillar and is confluent to the retrotympanum inferiorly and lies between the subiculum posteriorly and the finiculus anteriorly [12].…”

Image-Based Planning of Minimally Traumatic Inner Ear Access for Robotic Cochlear Implantation

“…The scala tympani, the favored intra-cochlear lumen for implant placement, can be accessed through a RW or extended RW approach or a RW-related cochleostomy ( 12 , 13 ). A favorable trajectory directed into the scala tympani, without targeting the osseous spiral lamina and the lateral wall of the basal portion, must pass through the canonus of the niche ( 14 ). Removal of the canonus (canonectomy) or creation of an opening in the canonus (canonostomy) may cause trauma to the hook region, where the osseous spiral lamina, the spiral ligament and the basilar membrane fuse ( 10 ).…”