The Anatomy and Anatomical Variations of the Round Window Prechamber and Their Implications on Cochlear Implantation: An Anatomical, Imaging, and Surgical Study

THIEME 288 Original Research The Anatomy and Anatomical Variations of the Round Window Prechamber and Their Implications on Cochlear Implantation: An Anatomical, Imaging, and Surgical Study Ahmed Mohamed Mehanna1 Moustafa Mohamed Abdelnaby1 1 Department of Otolaryngology, Alexandria University, Midan al Khartoum, Alexandria, Egypt 2 Department of Radiodiagnosis, Faculty of Medicine of Alexandria University, Midan al Khartoum, Alexandria, Egypt Mohamed Eid2 Address for correspondence Dr. Ahmed Mohamed Mehanna, Department of Otolaryngology, Alexandria University, Midan al Khartoum, Alexandria, Egypt 11511 (e-mail: ). Int Arch Otorhinolaryngol 2020;24(3):288–298. Abstract Keywords ► Cochlear Implantation ► facial nerve ► middle ear ► cochlea received November 23, 2018 accepted September 7, 2019 published online December 13, 2019 Introduction Over the last decades, there has been a tremendous increase in the number of cochlear implant recipients and, consequently, there is a recent increase of interest in the proper understanding of the anatomy of the round window (RW), which is the most important anatomical land mark during cochlear implant surgery. Objectives The present study was undertaken to assess the detailed surgical and radiological anatomy of the RW prechamber; its shape, directions, measurements, common anatomic variations, and its relationships with different surrounding structures as related to cochlear implantation. Methods A total of 20 cadaveric specimens of human temporal bone were microscopically dissected for the anatomical assessment of the measurements of the RW and its relation to surrounding structures in the tympanum. A total of 20 patients were subjected to cochlear implantation, and a radiological and surgical assessment of the anatomy of their RW prechambers was performed. Results The distances between the RW and the facial canal (FC), the jugular fossa (JF), the carotid canal (CC), and the oval window (OW) were measured. Among the cases subjected to cochlear implantation, the infracochlear tunnel was studied radiologically; the lengths of the anterior and posterior pillars were assessed, and the relation with the direction at which the RW faces was statistically analyzed. Conclusions Proper understanding of the topographic anatomy of the RW, including its direction of opening and the distances from different adjacent structures in the tympanum, is essential for a successful cochlear implantation surgery, since it can help decision-making before the surgery and is useful to avoid many complications, such as misplaced electrode and iatrogenic injury to the surrounding structures. DOI https://doi.org/ 10.1055/s-0039-1698783. ISSN 1809-9777. Copyright © 2020 by Thieme Revinter Publicações Ltda, Rio de Janeiro, Brazil The Round Window Prechamber Introduction Over the last decades, there has been a tremendous increase in the number of cochlear implant recipients, and, consequently, there is a recent increase of interest in the proper understanding of the anatomy of the round window (RW), which is the most important anatomical landmark, not only in cochlear implant surgery but also in other otologic procedures. A thorough and proper understanding of the anatomy of the RW is now considered essential and mandatory for the practicing otolaryngologist rather than for an experimental scientist.1 The round window membrane (RWM) is normally hidden under a boney overhang, termed the RW niche (RWN), which is formed by a posterior pillar (postis posterior), the tegmen, and the anterior pillar (postis anterior). The RWM with its niche is termed the RW prechamber. The membrane itself is located at the end of the scala tympani anterolateral to the hook region.2 Insertion of the electrode array either through the RWM or by means of creation of a cochleostomy has been debated for a long time a; however, both approaches depend on the meticulous identification of the RW. The early reports of cochlear implantation emphasized insertion through the RWM; this approach was replaced by drilling a cochleostomy anteroinferior to the RWM. The crista fenestra, which is preserved in the first approach, usually obscures the vision of the surgeon during RW insertion. Recent researches emphasized the residual hearing preservation and a soft atraumatic surgery. Round window membrane insertion with preservation of the crista fenestra is now the insertion site of choice for many surgeons, as it is claimed that the electrode will be kept in place by this bone; therefore, if the electrode is inserted through the cochleostomy, the point of first contact to the basilar membrane is 5 mm far anterior, so the contact length toward the modiolus is 5 mm shorter.3–5 The position of the RWN, its shape, and the direction that its opening faces are variable among individuals. The awareness of possible anatomical variations of the RW, as it relates to the different anatomical structures in the tympanum, can help with the decision-making process before surgery regarding the type, the length, and the site of insertion of the cochlear implant electrode array and it, subsequently, minimizes the rate of complications and misplacement.6,7 Materials and Methods We performed a prospective study on 20 cadaveric specimens and on 20 patients subjected to cochlear implantation at the main university hospital in Egypt, to assess the detailed surgical and radiological anatomy of the RW prechamber; its shape and direction, measurements, common anatomic variations, and the relationships with the different surrounding structures as related to cochlear implantation. The present study was performed after approval from the ethical committee for research at the university hospital and from the affiliated hospitals. Mehanna et al. A) A total of 20 cadaveric specimens of human temporal bones with no available data about name and sex were fixed in 10% formalin and then were microscopically dissected as following: cortical mastoidectomy was performed, followed by posterior tympanotomy, then the tympanic membrane and the posterior meatal skin were removed, for assessment of the following parameters: I) Round window morphology: 1) RW visibility through the posterior tympanotomy and through the external auditory canal. 2) RW shape; oval, rounded, triangular, pear-shaped or quadrangular. 3) The direction of the RW opening; posterior, if it faces completely to the posterior mesotympanum; inferior, if it faces completely down; and, lastly, posterinferior if between both sites. 4) The presence or absence of a well-defined infracochlear air cell track. 5) In all the dissected temporal bones, full visualization of the RWM was achieved after complete removal of the bony overhangs. The crista fenestra was properly identified as a sharp bony crest located in the anterior and inferior borders of the niche with variable shapes and variable degrees of narrowing to the access of the scala tympani of the basal turn of the co (...truncated)