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, 248 (2), 205-13

Stereological Analysis of Bone Architecture in the Pig Zygomatic Arch

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Stereological Analysis of Bone Architecture in the Pig Zygomatic Arch

S Teng et al. Anat Rec.

Abstract

Background: Stereological analysis of trabecular bone structure may reveal information about regional variations in stress distribution, especially in areas like the zygomatic arch in which those variations are difficult to assess mechanically. This study investigates regional differences in trabecular orientation, thickness, and density in the zygomatic and squamosal bones of pigs.

Methods: Zygomatic arches were serially sectioned frontally (n = 4), horizontally (n = 4), or parasagittally (n = 4), at a thickness of 0.8 mm. Sections were viewed under a stereomicroscope; video-images were digitized and analyzed with an automated program.

Results: All regions were anisotropic. Predominant orientation of trabeculae differed between and within bones. Three main patterns were seen. Anteriorly, zygomatic trabeculae were mainly arranged vertically and anteroposteriorly (relative to the occlusal plane). Posteriorly, including the jaw joint region, the squamosal featured primarily mediolateral trabeculae. In the midsection of the arch, where the two bones overlap, the trabeculae displayed a predominantly anteroposterior orientation with a secondary mediolateral peak. Trabeculae were typically 0.3-0.4 mm wide and occupied 40-50% of the area of the sections with few regional variations.

Conclusions: Trabecular bone in the pig zygomatic arch is arranged orthogonally, relative to the occlusal plane. In conjunction with information from strain gauge recording, these data suggest that the zygomatic bone is bent in the parasagittal plane whereas the squamosal is bent out-of-plane. The mediolateral trabeculae in the posterior regions are consistent with a cantilever effect at the jaw joint.

Figures

Fig. 1
Fig. 1
Right zygomatic arch, showing test regions and sectioning planes. A: Lateral and slightly superior view. The zygomatic bone is hatched and the squamosal bone is white. B: Superior view. Note that the squamosal bone is L-shaped in this view. The transversely oriented posterior medial region forms the articular eminence of the jaw joint. The abbreviations are those used in Figure 2–Figure 4.
Fig. 2
Fig. 2
Right zygomatic arch sectioned in the horizontal plane, showing regions examined. The arrow points anteriorly; lateral is to the right. Sa, anterior region of the squamosal bone, consisting mainly of woven compact bone; Spl and Spm, posterolateral and posteromedial regions of the squamosal; Za, anterior region of the zygomatic bone, mostly occupied by the maxillary sinus (asterisk); Zm and Zp, middle and posterior regions of the zygomatic bone.
Fig. 3
Fig. 3
Trabecular orientation frequency distributions. A: frontal sections; B: horizontal sections; and C: sagittal sections. S, squamosal (dotted lines and open symbols); Z, zygomatic (solid lines and symbols); a, anterior; i, inferior; m, middle; p, posterior; pl, posterolateral; pm, posteromedial; s, superior. Note a tendency for orthogonal orientation (0/180° or 90°) relative to the occlusal plane in all planes of section.
Fig. 4
Fig. 4
Comparison of anisotropy. Results from each plane are indicated by the hatching pattern of the bars. ANOVAs show there are differences (*: p < 0.05) among planes in all regions except the anterior zygomatic. Abbreviations above each bar give the most frequent orientation of trabeculae (see Fig. 3). A–P, anteroposterior; M-L, mediolateral; V, vertical. Trabeculae in the middle-superior zygomatic region were oblique in frontal section, as were those in the middle-inferior zygomatic region in sagittal section. Other abbreviations as in Figure 3.
Fig. 5
Fig. 5
Summary of the trabecular architecture found in the zygomatic arch. The lines show the predominant orientation observed in each plane of section (F, frontal; H, horizontal; S, sagittal). The length of the lines is proportional to the degree of anisotropy. The scale bar indicates a value of 1.0. Except for the middle zygomatic region, trabeculae were oriented orthogonally, i.e., vertically, anteroposteriorly, mediolaterally. In the middle zygomatic region, two exceptions were seen (asterisks in A and B, and dashed line in A; for further details see Fig. 3 and text). A: Lateral/superior view, showing anteroposteriorly oriented (visible in H and S planes) and vertically oriented (visible in F and S planes) trabeculae. B: Horizontal section, showing anteroposteriorly oriented and mediolaterally oriented (visible in F and S planes) trabeculae. The two bones of the arch have been separated for clarity
Fig. 6
Fig. 6
Suggested loading of the posterior portion of the squamosal bone (articular eminence). Vertical loading from the condyle (C) causes cantilever bending (b) in the eminence, which is mounted on the braincase (bc). This loading condition could result in mediolateral stress within the eminence. Note the angulation of the jaw joint surface is 15–25° relative to the occlusal plane.

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