The oval window region has recently been identified as a potential route for drug diffusion into the inner ear. Locally applied gadolinium and trimethylphenylammonium (TMPA) have been shown to directly diffuse into the vestibule through the oval window region. Given the potential importance of the oval window region in the diffusion of substances into the inner ear, this work aimed to use micro-CT to obtain a data set of anatomical characteristics of the annular ligament of the stapes in the human temporal bone, a region thus far poorly studied. Twenty-one temporal bones were micro-dissected to preserve the otic capsule and allow perfusion of fixative stains into the inner ear.
The specimens were scanned with micro-CT methods to provide 3D reconstructions and measurement. The 3D reconstructions were able to demonstrate an undisturbed stapes footplate and annular ligament from which measurements could be taken. This study found a wide variance in the volumes and thicknesses of the stapedial ligaments. There was a positive correlation between the size of the stapes footplate and the annular ligament.
The mechanism for drug diffusion from the middle to inner ear had been widely accepted as occurring through the round window into the scala tympani with local communication to the scala vestibuli (Salt et al. 2003; Salt and Ma 2001). Recently, the oval window region has been identi- fied as another potential route. A study (King et al. 2011) calculated through the use of high-resolution MR imaging that 90 % of the intravestibular gadolinium (Gd) that had been applied to the middle ear communicated directly into the vestibule in the region of the stapes and thus the oval window.
A subsequent study (Salt et al. 2012) used uptake of trimethylphenylammonium (TMPA) to qualitatively conclude that 30–40 % entered the vestibule in the vicinity of the stapes. This research has heightened interest in the morphology of the stapes as a potential vehicle for drug delivery in the clinic. Now it appears that the size of the footplate, its thickness, and that of the surrounding annular ligament might influence the diffusion of therapeutic agents through the oval window niche (Tanaka and Motomura 1981; Zou et al. 2005, 2012). Here, we use highresolution micro-CT imaging of the intact cadaveric temporal bone to explore morphological features of the stapes and annular ligament that might influence drug delivery via the oval window. The major and most obvious advantage of the micro-CT method is the nondestructive nature of measurements of the stapes footplate and annular ligament with both in situ.