The cochlea is a small, spiral bone structure in the inner ear that plays a critical role in hearing. It contains three types of bones: (1) the petrous portion of the temporal bone, which is the trabecular bone that houses the cochlea; (2) the modiolus, which is the central conical axis of trabecular bone; and (3) the osseous spiral lamina (OSL), which is a thin layer of bone composed of two plates of compact bone that protrudes from the modiolus and separates part of the two fluid-filled canals that run through the cochlea, the scala vestibuli, and the scala tympani, which, like the canals, makes two and three-quarter turns around the modiolus [23, 24].

The bony structures of the inner ear were superficially described in early anatomical studies, but imaging demonstration has only been possible more recently [1, 2, 16]. Anatomical descriptions of the bony inner ear are usually restricted to the modiolus [16, 25] and use histopathological techniques. Although histopathology remains the gold standard for cytological evaluation, the specimen preparation process involves the physical destruction of the specimen and creates typical anisotropic resolution (0.5 × 0.5 × 100 µm) if translated into 3D [26,27,28].

Surrounded by the dense cochlear bone, under around 2.5-µm resolution microCT, a trabecular looking modiolus stands in the middle of the cochlea, projecting itself to form the two bony plates that determine the shape of the OSL. Although the OSL consists of two plates of compact bone, the porous nature of these plates and the presence of numerous bony pillars make it more similar as a whole to a trabecular bone. In fact, trabecular bone is by definition a hierarchical, spongy, and porous lattice structure that provides the framework for the soft, highly cellular bone marrow filling the intertrabecular spaces. At a microstructural scale, trabecular bone architecture is organized to optimize load transfer [29]. Similarly, the OSL system as a whole can be thought of as trabecular bone, with two individual plates connected by bony pillars on the inside and the habenular openings at its end. The OSL plates have a lace-like appearance enfolding the fibers that will later form the cochlear nerve. The radial middle portion of the basal turn of the OSL (both plates) has a more porous appearance than its edges, similarly to Raufer’s description [6]. This surface reduction is congruent with the sparse distribution of the bony pillars in the middle of the OSL. Although habenular openings were described by Küçük et al. [15] as tunnels, internal inspection of the structure indicates that the openings do not behave like tunnels. On the contrary, they convey the impression of a cave and appear to have a structural support function, joining the plates together on the lateral end, at the insertion point of the bridge and limbus regions. The bony pillars could have a similar support function, keeping the plates together while providing stability that can have a potentially significant influence on the spread of excitation during its vibration. However, more investigation on this possibility is needed.

The lace-like appearance of the OSL plates turns out to be a common characteristic, and it was observed in the measured specimens. Porosity analysis demonstrated a considerable higher percentage of pores on both vestibular plate VP and tympanic plate TP on the basal turn if compared to previous study [6]. Unlike the work of Raufer et al. [6], which was limited to considering different portions of the basal turn and suprabasal turn (between 1 and 12 mm of longitudinal distance from the base), in this study, we also calculated porosity relative to the middle turn and the apex. In these cases, we found a constant presence of porosity in both TP (mid, 61%; apex, 56%) and VP (mid, 50%; apex, 61%). Moreover, while the TP appears to be more porous than VP in the basal and middle turns, although this difference is less pronounced than in previous descriptions [6], in the apex, we find more porosity in VP than in TP (61% vs. 56%). With regard to our study results, we believe that the volumetric reconstructions used in our measurements provide a more realistic depiction of the OSL anatomy. Since 3D investigation provides more information than conventional 2D studies, this technique seems to be more suitable for demonstrating the porosity of the structure. The difference in methodology can also be responsible of the main differences found in the analysis of OSL width compared to previous studies as described in Fig. 10. The major discrepancy was found at the basal turn where, besides the use of different methodologies, both studies lack in samples group size (our pilot study n = 2; previous study n = 1). Further investigation will have to be carried out to provide reliable statistics especially on distances closer to the base. The combination of a highly porous trabecular bone and the fluid filled cochlea creates a new approach for hearing mechanical studies. Evidently, future kinetic cochlea studies must consider the OSL and the cochlear partition as one mobile structure, the differences in porosity from base to apex, and the effects of the fluid dynamics in their vibration analysis calculations. This modeling work can help support recent theories regarding the influence of different material properties on the motion of the cochlear partition and contribute to the understanding of low-frequency and bone conductive hearing [30]. As a result, it will be useful in the development of clinical interventions for the preservation of hearing.

Another possible clinical consequence of the OSL’s porosity is that it can potentially play a role in manufacture of new implant technology, direct inner ear drug delivery, and stem cell delivery. Secondary to hair cell loss, spiral ganglion’s neuron degeneration is one of the major causes of sensorineural hearing loss. The use of perilymph as carrier for inner ear drug delivery is common, but it is inefficient due to the difficulty in reaching stable therapeutic concentrations throughout the cochlea [31,32,33,34,35,36,37]. The spiral ganglion’s neurons are located in Rosenthal’s canal which runs along the OSL, and there is increasing evidence of their permeability to small molecules and neurotrophic genes [38,39,40,41,42]. The difficulty of drugs in crossing the blood–labyrinth barrier has been reported [33, 43, 44]. In the future, drug-eluted cochlear implants could potentially take advantage of the OSL’s porosity to enable drug distribution directly to the spiral ganglion [45].

Geometrical reconstruction of the OSL revealed a cantilever structure that should be considered in future mathematical models for studying hearing mechanics. Although it should be noted that the voxel size used in this pilot study may be considered relatively large to obtain accurate measurements, especially in the OSL thickness range, the thickness and width we measured are consistent with previous studies [6, 7]. However, in contrast to earlier findings [6], our porosity measurements demonstrated the presence of pores in both VP and TP (up to 69%), as well as random bony pillars distribution throughout the cochlea in both radial and longitudinal directions. Additionally, it was possible to observe the habenular openings through a new perspective; rather than the previously described “tunnel”, our study showed a “cave”. Furthermore, the connection between the cochlear partition and the OSL that we have demonstrated in 3D should encourage others to consider that they possess a “one flexible structure” morphology. In this article, we present three of four factors (i.e., geometry) to be considered in future studies of hearing mechanics: the width, thickness, and stiffness (porosity) of the OSL. The inter-plate bony pillars (including its thickness and related stiffness) should also be considered as a factor and added to the mathematical models. Gathering information on these variables by applying 3D methodology described here to a larger sample size will provide a better understanding of hearing mechanics.