Research paperMicro-channels in the mastoid anatomy. Indications of a separate blood supply of the air cell system mucosa by micro-CT scanning
Highlights
► Micro-channels were found in the mastoid bone using micro-CT via volume rendering. ► 2D quantitative analysis on a ROI helped to determine their diameter and density. ► Their properties suggest that they contain a separate abundant blood supply to the mastoid mucosa.
Introduction
The middle ear (ME) of the human temporal bone contains both the mastoid and the tympanum. Whereas the mastoid consists of a larger series of connected air filled cells divided by bony septae, the tympanum can be considered a single air cell which contains the ossicles. These structures are important for the transfer of sound pressures from the ear canal and tympanic membrane (TM) to the inner ear; thus, the function of the tympanum mainly relates to sound conduction. However, the physiology of the mastoid is not entirely understood although its structural properties may indicate more functions. The numerous bony septae separating the mastoid into smaller air cells may imply an acoustic function by reducing resonances and increasing hearing sensitivity for low frequencies (Fleischer, 2010). More well known aspects of the mastoid function are related to ME pressure, gas exchange, and otitis media (Sadé and Ar, 1997; Doyle, 2000).
The mastoid cellular structure greatly enhances its surface area-to-volume ratio (AV-ratio); based on the analysis of clinical CT scannings the AV-ratio is on average 16 cm−1 (Park et al., 2000). Moreover, the mastoid mucosa is relatively thin with a flat cuboidal surface epithelium and a rich superficial vascular supply; the distance from its surface to the underlying vascular structures is smaller than found for the mucosa of the tympanum (Ars et al., 1997). These structural findings suggest that mastoid gas exchange is favored both by a high AV-ratio as well as a short mucosal diffusion distance and high perfusion (Ars et al., 1997; Park et al., 2000).
We have previously investigated methods of image analysis on clinical CT scannings of the mastoid in order to describe methods for semi-automatic determination of its surface area and volume; however, we found that their resolutions may be too low for detection of the smallest air cells. Consequently, we supplemented these investigations by applying micro-CT scanning on a smaller series of temporal bones. By first visual inspection of these data we found numerous micro-channels which have not been described earlier in the literature. The current study reports on the structure of these channels, because they are likely to represent a separate blood supply for the mastoid mucosa. This may have important implications for the understanding of the ME physiology including its pressure regulation and gas exchange.
Section snippets
Data preparation
Three human temporal bones were used in this study; they were preserved in alcohol after removal of their surface soft tissues and kept in our laboratory for otosurgical training purposes. Scanning of the bone specimens was performed using a custom-built micro-CT scanner available at the Department of Physics and Astronomy, Ghent, Belgium (Masschaele et al., 2007).
Before the scanning process, each bone specimen was carefully placed in a plastic cup on a computer-controlled rotation table in
Quantitative analysis based on 2D images
The three mastoid bones appeared with normal pneumatization of the air cell system without sclerotic changes seen in diseased mastoids (Fig. 1). Thus, the three bones were considered to represent normal ME's. Table 3 depicts the diameter and the density of the channels; their mean diameter was 158 μm (SD = 74 μm) with a range between 40 and 440 μm. The number of channels within the ROI's of the three specimens varied from 23 to 35 (mean = 27), while their density varied between 64 and 97 cm−2
Discussion
The image analysis of the mastoid carried out in this study was based on micro-CT scanning. This modality greatly improves the resolution of images, and it has already been applied in ME research modeling the anatomy of ME itself as well as the ossicular chain (Lee et al., 2010; Salih et al., 2012); however, micro-CT scannings applied in research of the mastoid has not been reported earlier.
Similar to high-resolution clinical scannings of the temporal bone, the present micro-CT scanning images
Conclusions
The current study demonstrated notable micro-channels in the temporal bone which connected the outer surface directly to the mastoid air cell system, but they also formed a network between the air cells; further, more channels were connected to the individual air cells. According to their dimensions and structure, these channels were hypothesized to represent a separate vascular supply for the mastoid mucosa. This may altogether indicate that the mastoid mucosa is well vascularized which may
Acknowledgments
The Spar Nord Foundation, The Obel Family Foundation, and The Research Administration at Aalborg Hospital, Aarhus University Hospital, generously supported this study. Giedrius Lelkaitis, M.D., Department of Pathology, and Simona Padurariu, M.Sc.D., Department of Otolaryngology, Head and Neck Surgery, Aalborg Hospital, are acknowledged for providing the histological specimen in Fig. 7 and revising the manuscript.
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