Regional structural characteristics of bovine periodontal ligament samples and their suitability for biomechanical tests

Abstract

Mechanical testing of the periodontal ligament requires a practical experimental model. Bovine teeth are advantageous in terms of size and availability, but information is lacking as to the anatomy and histology of their periodontium. The aim of this study, therefore, was to characterize the anatomy and histology of the attachment apparatus in fully erupted bovine mandibular first molars. A total of 13 teeth were processed for the production of undecalcified ground sections and decalcified semi-thin sections, for NaOH maceration, and for polarized light microscopy. Histomorphometric measurements relevant to the mechanical behavior of the periodontal ligament included width, number, size and area fraction of blood vessels and fractal analysis of the two hard-soft tissue interfaces. The histological and histomorphometric analyses were performed at four different root depths and at six circumferential locations around the distal and mesial roots. The variety of techniques applied provided a comprehensive view of the tissue architecture of the bovine periodontal ligament. Marked regional variations were observed in width, surface geometry of the two bordering hard tissues (cementum and alveolar bone), structural organization of the principal periodontal ligament connective tissue fibers, size, number and numerical density of blood vessels in the periodontal ligament. No predictable pattern was observed, except for a statistically significant increase in the area fraction of blood vessels from apical to coronal. The periodontal ligament width was up to three times wider in bovine teeth than in human teeth. The fractal analyses were in agreement with the histological observations showing frequent signs of remodeling activity in the alveolar bone - a finding which may be related to the magnitude and direction of occlusal forces in ruminants. Although samples from the apical root portion are not suitable for biomechanical testing, all other levels in the buccal and lingual aspects of the mesial and distal roots may be considered. The bucco-mesial aspect of the distal root appears to be the most suitable location.

Fig. 1

Schematic drawing illustrating the sectioning of the mandibular first molar into four horizontal sections (a–d) of about 1 cm thickness.

Fig. 2

Low power light micrographs showing (A) the mesial and (B) the distal roots of the first mandibular molar. At all root levels, the horizontal cross-section area was larger at the distal than at the mesial root, the buccal bone plate was thicker than the lingual, and the bone in the interradicular septum was highly trabecular. The six different locations investigated were distal root–buccal aspect (db), distal root–distal aspect (dd), distal root–lingual aspect (dl), mesial root–buccal aspect (mb), mesial root–mesial aspect (mm), and mesial root–lingual aspect (ml). The periodontal ligament (PDL) was rich in blood vessels (C), and cells and connective tissue fibers (D). A–C = ground sections; D = thin section. AB, alveolar bone; C, cementum; D, dentin; P, pulp.

Fig. 3

One example showing consecutive pictures (A) before and (B) after all visible blood vessels in the periodontal ligament (PDL) were manually selected and filled with a white background color to ease their detection in the subsequent automated measuring process. AB, alveolar bone; C, cementum. A, B = ground sections.

Fig. 4

Thin sections without (A,B) and with (C,D) prior maceration illustrating the insertion (arrows) of principal periodontal ligament (PDL) fibers into the alveolar bone (AB) and the cementum (C). The maceration process has removed the cells and some extracellular matrix constituents but not the collagen fibers.

Fig. 5

Ground (A, C–F) and thin (B) sections illustrating the remodeling activity of the alveolar bone (AB) and the cementum (C). At most sites (A, B, D), the surface contour was smoother in cementum than in bone. The irregular and jagged surface contour of the alveolar bone, the marked differences in the staining intensity between bone matrix compartments (A), and the presence of osteoclasts (arrows) were indicative of a high bone remodeling activity. In contrast, the cementum surface revealed only sparse resorption lacunae (C). However, soft tissue channels (asterisks) were frequently observed in the apical cementum (D). These soft tissue channels contained blood vessels (BV). The presence of both Howship's lacunae (arrowheads) and newly deposited cementum matrix (NC) indicated remodeling activity in the depth of the cementum layer (E, F). D: dentin; PDL: periodontal ligament.

Fig. 6

Ground sections illustrating the six locations investigated: (A) dl, (B) dd, (C) db, (D) ml, (E) mm, and (F) mb. Large variations in periodontal ligament (PDL) width, area fraction of blood vessels (BV), number and size of blood vessels, amount and orientation of collagen fibers, and surface contour of both the alveolar bone (AB) and the cementum (C) may be seen.

Fig. 7

Percent area fraction of blood vessels for the four root levels, a (apical) to d (coronal).

Fig. 8

Periodontal ligament width for the four root levels, a (apical) to d (coronal).

Fig. 9

Ground sections viewed under polarized light showing non-uniform fiber density and orientation. The periodontal ligament (PDL) fiber system connecting the cementum (C) with the surrounding alveolar bone (AB) was either straight (A), angulated (B), or arranged in a criss-cross pattern (C, D). In contrast to the root levels b–d (A, B), the apical periodontal ligament presented a low fiber density and a lower degree of fiber organization (C, D). D, dentin.