(0917) Defining Transcanal Transtympanic Access for Intracochlear Drug Delivery Based on Human Temporal Bone Histological Sections

Introduction: Novel therapies for hearing loss require minimally invasive access to the inner ear. This presentation aims to describe the form and size factors needed for minimally invasive transtympanic drug delivery to the scala tympani (ST) through the round window membrane (RWM) with preservation of inner ear structures.

Methods: Micro-anatomical 3D models were created based on eight human temporal bone histological sections. The distances, angles, and relationships between the external auditory canal (EAC), otic capsule, RWM, and intra-cochlear structures were defined, targeting the ST. A compatible needle for drug delivery is defined.

Results: The optimal trajectory to the ST via a transcanal trans-RWM approach is achieved with a needle that is angled at its distal end such that the angle abuts the superior most point of the round window niche operculum. The angled part of the needle was found to have a minimum length of 1.375 mm (1.2-1.6 mm) and a maximum of 3.49 mm (2.8-4.3 mm) in order to pierce the RWM but not intra-cochlear structures. Based on the possible trajectories from the EAC, the optimal angle is 147 degrees (123-167). The suggested penetration point of the RWM is the center of its vertical axis, accounting for the distance to the basilar membrane, the depth of insertion in the ST, and the vector in relation to the shape of the ST.
The crista fenestra may shift the penetration point. The crista's surface area occupied 17%-38% (mean 25.7%) of the RWM's area but never reached the center of the RWM. The measured horizontal distance between the edge of the crista and the RWM is 0.55 mm (0.4-0.75 mm). Access from the EAC is variable, and can be achieved through its inferior half without risking the chorda tympani nerve.

Conclusions: The data provides the form and size factors for optimal transcanal, transtympanic access to the inner ear for structure preservation drug delivery. The optimal penetration point is the center of the RWM’s vertical axis, approached with a needle which abuts the superior most point of the RWN operculum and is angled at 147 degrees. There are variable approach trajectories from the EAC, most from its inferior half.