The round window region is of critical importance in the anatomy of the middle ear. The aim of this paper is to describe its anatomy from an endoscopic point of view, emphasizing structures that have important surgical implications, in particular the fustis and the subcochlear canaliculus.
The fustis, a smooth bony structure that forms the floor of the round window region, is a constant and important structure. It seems to indicate the round window membrane and the correct position of scala tympani. A structure connecting the round window region to the petrous apex, named the subcochlear canaliculus, is also described. A retrospective review of video recordings of endoscopic dissection and surgical procedures, carried out between June 2014 and February 2015, was conducted across two Tertiary university referral centers. A total of 42 dissections were analyzed in the study. We observed the fustis in all the cases and we identify two different anatomical conformations. The subcochlear canaliculus was found in 81.0 %, with a pneumatization direct to the petrous apex in 47.7 %. Conformation and limits of the round window niche may influence the surgical view of the round window membrane. Endoscopic approaches allow a very detailed view, which enables a comprehensive exploration of the round window region. Accurate knowledge of the anatomical relationships of this region has important advantages during middle ear surgery.
The round window chamber is a three-dimensional space lying between the round window niche and round window membrane. Between the fustis and the finiculus a subcochlear canaliculus is often seen, which is a tunnel that connects the round window chamber with the petrous apex via a series of pneumatized cells. Three different types of subcochlear canaliculus have been described: type A has a large tunnel to the petrous apex detectable endoscopically; type B has a small tunnel present, with a connection to the petrous apex not detectable endoscopically; type C has no connection between the round window chamber and the petrous apex. The fustis (from latin, ‘‘club’’) is a smooth bony structure, which forms the floor of the round window chamber and seems to indicate the entrance to the round window niche. The structure links the styloid proeminence with the basal turn of the cochlea.
During the last 10 years, the knowledge of the middle ear anatomy, and specifically the retrotympanum, has been significantly improved with the help of the endoscopic approach. The traditional microscopic approach rarely allows for clear visualization of the retrotympanic spaces.