Defining the Access Routes to Intracochlear Drug Delivery Based on Temporal Bone Histology 3-Dimensional Models

Introduction: Optimizing inner ear access is necessary for the genetic and regenerative treatment of sensorineural hearing loss. This is a descriptive histological study, with three-dimensional modeling of the microanatomy of minimally invasive transcanal access routes to the scala tympani through the round window, and suggested form and dimensions of injection needles for inner ear drug delivery.

Three-dimensional models of eight human temporal bone histological sections were created to measure key anatomical relationships and optimal needle trajectories.

To spare the inner ear structures and reach the greatest depth in the scala tympani, the optimal trajectory via the round window is achieved with a needle leaned on and bent at the superiormost point of the round window niche operculum. The length of the needle's medial part is between 1.375 mm (1.2-1.6 mm) and 3.49 mm (2.8-4.3 mm), with a bend angle of 147° (123-167°). The penetration point of the round window membrane is its vertical axis center, reducing the risk of damage to the inner ear structure and avoiding the crista fenestra. The crista may shift the penetration point inferiorly when visible and not found behind the round window membrane. In all eight temporal bones, the crista never reached the center of the round window membrane, with the horizontal distance between the crista edge and the center being 0.55 mm (0.4-0.75 mm).

The data reveal the form and size factors for optimal transcanal access to the inner ear for structure- and function-preserving drug delivery.