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A quantitative method for the evaluation of three-dimensional structure of temporal bone pneumatization - PubMed

A quantitative method for the evaluation of three-dimensional structure of temporal bone pneumatization

Cheryl A Hill et al. J Hum Evol. 2008 Oct.

Abstract

Temporal bone pneumatization has been included in lists of characters used in phylogenetic analyses of human evolution. While studies suggest that the extent of pneumatization has decreased over the course of human evolution, little is known about the processes underlying these changes or their significance. In short, reasons for the observed reduction and the potential reorganization within pneumatized spaces are unknown. Technological limitations have limited previous analyses of pneumatization in extant and fossil species to qualitative observations of the extent of temporal bone pneumatization. In this paper, we introduce a novel application of quantitative methods developed for the study of trabecular bone to the analysis of pneumatized spaces of the temporal bone. This method utilizes high-resolution X-ray computed tomography (HRXCT) images and quantitative software to estimate three-dimensional parameters (bone volume fractions, anisotropy, and trabecular thickness) of bone structure within defined units of pneumatized spaces. We apply this approach in an analysis of temporal bones of diverse but related primate species, Gorilla gorilla, Pan troglodytes, Homo sapiens, and Papio hamadryas anubis, to illustrate the potential of these methods. In demonstrating the utility of these methods, we show that there are interspecific differences in the bone structure of pneumatized spaces, perhaps reflecting changes in the localized growth dynamics, location of muscle attachments, encephalization, or basicranial flexion.

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Figures

Fig. 1
Fig. 1

Cross-sectional slices representing the male mastoid region showing pneumatization in a) Gorilla, b) Pan, c) Papio, and d) Homo. These slices are from the posterior view, with the lateral edge of the mastoid region to the left. Gorilla, Pan, and Papio specimens are extensively pneumatized with the air cell tract extending into the squamous portion as well as the mastoid and petrous portions. Pan specimens also have pneumatized zygomatic processes. Pneumatization is not as extensive in Homo specimens—the air cell tract does not infiltrate the squamous and zygomatic portions. Sediment is evident in the Homo specimen (black arrow). White arrows: occipitomastoid suture; (P) parietal; (EAM) external auditory meatus; (MP) mastoid process.

Fig. 2
Fig. 2

Selection of the correct threshold is important to prevent the incorrect estimation of the parameters used in this study: (left) original scan data; (middle) overestimated threshold; (right) underestimated low threshold. The SVD method places 8,000 points (gray dots) within the VOI. From these points 2,049 random orientations are drawn between the point and the nearest interface between bone and air (gray lines). If the threshold is too high or low, the amount of bone will be estimated incorrectly.

Fig. 3
Fig. 3

Transparent three-dimensional reconstructions from HRXCT scans of (A) Gorilla, (B) Pan, (C) Papio, and (D) Homo temporal bones showing the mastoid air cell tract and trabecular bone, viewed from the lateral side (Z: zygomatic process). These representations show the selected regions of interest (white box) extending from the edge of the external auditory meatus to the edge of the expansion of the mastoid air cell tract. Since the selected regions of interest included cortical bone, sensory, and vascular structures, the point where orthogonal X, Y, and Z planes intersect in the center of the ROI was selected as the center of the VOI (dark gray sphere). The largest possible VOI was placed within the center of the cropped ROI representing the mastoid region. These VOIs were used to calculate the select parameters in Quant3D. Amira software was used to make the 3D reconstructions.

Fig. 4
Fig. 4

A ternary diagram plotting the isotropy and elongation indices for each individual in the sample. The relationship between these indices for each specimen provides information about the shape of the structures within the VOI. Isotropic structures cluster near the top, having high isotropy indices and low elongation indices (the degree of anisotropy will be 1.0). At the other extremes, bony deposits can have more plate-like (lower left) or rod-like structure (lower right). ■ male Homo, □ female Homo, ▲ male Gorilla, △ female Gorilla, ● male Pan, ○ female Pan, ⋆ male Papio, ☆ female Papio.

Fig. 5
Fig. 5

Graphs plotting trabecular thickness (top) and bone volume fractions (bottom) against slice thickness. Bone volume fractions (r = 0.191, p = 0.312) and anisotropy (data not shown; r = 0.337, p = 0.068) do not significantly increase with larger slice thicknesses. Regression analyses indicate that trabecular thickness significantly increases with slice thickness (r = 0.468, p = 0.009). Removal of Gorilla from the regression shows that the association is no longer significant (r = 0.379, p = 0.082).

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