Scanning electron microscopy of the collagen fibre architecture of the rabbit incisor periodontium

No association between periodontitis, preterm birth, or intrauterine growth restriction: experimental study in Wistar rats

BACKGROUND

The biologic plausibility of the possible association between periodontitis and adverse pregnancy outcomes has been assessed with the use of different experimental models. However, most experimental studies did not induce periodontitis in the animals but promoted an acute microbial challenge with selected periodontal pathogens or their products subcutaneous or intravenous or intraamniotic. The present study was then conducted to verify the biologic plausibility of such association by experimentally inducing periodontitis in Wistar rats.

OBJECTIVE

An experimental study on an animal model by the induction of periodontitis in 50% of sites and assessment of the presence of cytokines in the gingival tissue, serum, placenta, cord, and amniotic fluid was designed to test the null hypothesis that experimental periodontitis that is induced on rats does not result in adverse pregnancy outcomes.

STUDY DESIGN

Forty female Wistar rats were included in 2 groups: a periodontally healthy (without ligatures) and an experimentally induced periodontitis group (test, with ligatures). Forty-five days after the induction, the mating was initiated. Males were placed with females in the ratio of 1:2 for a period of 12 hours. The bodyweight of the female, from then on, was recorded daily. When the pregnancy was confirmed on day 20, laparotomy was performed. The amniotic fluid, placenta, umbilical cord, blood (serum) and maternal and gingival tissue samples were subjected to quantitative analysis for interleukin 1α, -6, -10, -4, -12p70, and -17a, tumor necrosis factor-α, and interferon-γ by multiplex methods. Mean scores, standard deviations, and standard errors for estimated measures were calculated. For cytokines analyses, the Mann-Whitney test was conducted to compare the concentration of the analytes from control and test groups in the different tissues samples. For comparison of cytokines reduction from gingival tissue to serum and from serum to placenta, the Wilcoxon Test was performed. Spearman's correlation was conducted among cytokines in the 5 different tissues that were evaluated.

RESULTS

The induced periodontitis in Wistar rats did not result in adverse outcomes of pregnancy. There were no statistically significant differences between groups in relation to prematurity, fetal, or birth weight. Regarding cytokines, there were no statistically significant differences in concentrations that were measured in each tissue between the groups with periodontitis and controls. Furthermore, all cytokine levels in the placenta, except interleukin-6, were diminished compared with the amniotic fluid or maternal serum, which suggested that the cytokines cannot easily be transferred via this tissue in maternal-fetal or fetomaternal direction. The fertility rate was reduced significantly in the group with periodontitis.

CONCLUSION

Periodontitis that is induced in rats is not a risk factor for preterm birth or low birthweight.

A constitutive model for the periodontal ligament as a compressible transversely isotropic visco-hyperelastic tissue

This study is devoted to the development of a non-linear anisotropic model for the human periodontal ligament (PDL). A thorough knowledge of the behaviour of the PDL is vital in understanding the mechanics of orthodontic tooth mobility, soft tissue response and proposed treatment plans. There is considerable evidence that the deformation of the PDL is the key factor determining the orthodontic tooth movement. The paper focuses on the biomechanical aspect of the behaviour of the PDL. In terms of continuous mechanics, the PDL may be treated as an anisotropic poro-visco-hyperelastic fibre-reinforced compressible material which is subject to large deformations and has an essentially non-linear behaviour. Furthermore, there are issues related to the non-linear tooth and PDL geometry. A new constitutive model for the PDL is proposed. The macroscopic continuum approach is used. The model is based on the non-linear large deformation theory, involving the Lagrangian description. The material is assumed to be compressible, visco-hyperelastic and transversely isotropic. A free-energy function is suggested that incorporates the properties. It also takes into account that the PDL behaves differently in tension and compression. The free-energy function and the associated constitutive equations involve several material parameters, which are to be evaluated from experimental strain-stress data available from the literature and the tooth movement experiments conducted by our team using novel optical motion analysis techniques.

A Transversally Isotropic Elasto-damage Constitutive Model for the Periodontal Ligament

A numerical formulation of an elasto-damage constitutive model was developed and implemented in a finite element software to investigate the biomechanical response of the periodontal ligament (PDL). The mathematical framework accounts for the description of large strains, anisotropy and inelastic phenomena. The anisotropic mechanical response is caused by the spatial orientation of the sub-structures of the tissue, such as collagen fibres. Inelastic behaviour, induced by high level strains, is modelled by means of damage models. In vitro experimental testing on PDL samples from pigs was performed to obtain tensile stress-strain curves. A finite element analysis is presented in order to define a general numerical approach. A comparison of numerical and experimental data is provided in order to show the reliability and effectiveness of the formulation assumed.

The proteoglycan contents of the temporomandibular joint disc influence its dynamic viscoelastic properties

The collagen fibers and proteoglycans in the disc of temporomandibular joint provide resistance to various loadings. Thus far, however, the role of the proteoglycans in determining the viscoelastic properties of the disc has not been investigated. In the present study the hypothesis was tested that the viscoelastic behavior of the disc decreases by the removal of proteoglycans. In 32 bovine discs, dynamic tensile tests with a wide range of frequencies were performed. Before testing, specimens were treated with different concentrations of alpha-amylase to remove proteoglycans. As the frequency increased from 0.1 to 100 Hz, the disc became more viscoelastic. Increasing the concentration of alpha-amylase significantly decreased its viscoelasticity. It was concluded that proteoglycans play an important role in determining the viscoelastic properties of the disc and, therefore, give the disc a greater capacity for distributing and reducing stresses.

Biomechanical and morphological studies on the periodontal ligament of the rat molar after treatment with alpha-amylase in vitro

Biomechanical properties and morphological features of the rat molar periodontal ligament were examined after treatment with alpha-amylase. Treatment with alpha-amylase induced dose-dependent decreases in the maximum shear stress, tangent modulus, and failure strain-energy density of the periodontal ligament; in addition, it weakened the alcian-blue staining of the periodontal ligament and exposed periodontal collagen fibrils as revealed by scanning electron microscopy. Azan staining and polarized microscopic observations of the periodontal collagen fibers were not markedly different between the control and alpha-amylase treated specimens. These results suggest that decreases in the strength of the periodontal ligament due to alpha-amylase digestion are largely due to removal of interfibrillar substances such as acid glycosaminoglycans and neutral polysaccharides from the periodontal ligament. It is also suggested that the interaction of the interfibrillar substances with collagen fibrils is involved the biomechanical properties of the periodontal ligament.

Changes in the fibre arrangement of the rat incisor periodontal ligament in relation to various loading levels in vitro

The relationship between the fibre arrangement of the periodontal ligament and the load-deformation behaviour was investigated at various loading levels. Transverse sections of the rat incisor were loaded in the eruptive direction in vitro and the deformation fixed at predetermined loads. Sections were prepared at these deformation levels. The periodontal ligaments were examined by polarized-light and scanning-electron microscopy. At the initial ("toe') part of the load-deformation curve, the periodontal fibres were gradually pulled and bent towards the direction of loading; their wavy pattern and periodic dark and bright bands became indistinct. At the next, linear part of the curve, the running direction of the fibres changed gradually until they were straightened and stretched. At the yielding part of the curve, the periodontal fibres began to rupture. Ruptured fibres adhering to the bone surface returned to their original obliquity and showed periodic dark and bright bands. The fibrous components of the rat incisor periodontal ligament thus transmit forces to bone at the linear part of the curve when the tooth is axially loaded.

The fibrous architecture of the rat periodontal ligament in cryosections examined by scanning electron microscopy

Cryosections through the incisor and molar teeth of the rat mandible were examined, with and without hyaluronidase pretreatment, in the scanning electron microscope. In the fully erupted molar teeth the fibres of the periodontal ligament were organized into bundles which crossed the space from the alveolus to the cementum and inserted into the associated mineralized tissues. In the erupting incisor teeth, three distinct zones were evident. The outer alveolar and cemental zones were composed of coarse fibre bundles which inserted into the adjacent mineralized tissues, while the middle zone was composed of collagenous laminates running along the axis of the tooth. These observations confirm a proposed model for the structure of the erupting periodontal ligament and suggest that the method used will provide further information about the role of the ligament in tooth support and eruption.

An immunohistochemical study examining the role of collagen type VI in the rodent periodontal ligament

The distribution of collagen types I and VI has been examined in the periodontal ligaments of rat incisor and molar teeth using cryosections and immunohistochemical staining procedures. The stain for collagen type I was uniform in all the ligaments examined. Uniform staining for collagen type VI was evident only in the ligament of the fully erupted molars, and the stain was absent from the mid-zone of the ligaments of the erupting molars and incisors. The staining pattern of the collagen type VI antibodies is consistent with previous reports which have suggested that the removal of collagen type VI precedes the degradation of major banded collagen fibrils in remodelling connective tissues. The removal of collagen type VI from the mid-zone of the incisor ligament may precede the remodelling events which facilitate tooth eruption.

In vitro measurement of regional differences in the mechanical properties of the periodontal ligament in the rat mandibular incisor

Mechanical testing was performed in various regions of the ligament at a velocity of intrusive loading of 5 mm/min. The perimeters of the lingual cementum and socket wall and the sectional area of the ligament were measured radiographically. Load-deformation curves were transformed into stress-strain curves using the area and width of the lingual part of the ligament. The mechanical properties of the ligament differed markedly along the long axis of the incisor; mechanical measures from the load-deformation and stress-strain curves decreased gradually from the incisal towards the basal regions. It is suggested that these regional differences in mechanical properties were much more marked than those in teeth of limited growth or in other connective tissues, and that the nature of the collagen fibres, together with surrounding cells, ground substance and blood vessels, may be important in determining such differences.

In vitro mechanics of the periodontal ligament in impeded and unimpeded rat mandibular incisors

Load-deformation curves and/or the stress-strain curves were obtained by the mechanical testing of transverse sections of impeded and unimpeded rat incisors in vitro to evaluate more precisely the mechanical properties of the periodontal ligament. Perimeters of the lingual dentine and socket wall and the sectional area of the periodontal ligament were measured radiographically. An extension rate of 5 mm/min in an extrusive direction was used. The average maximum shear load, the elastic stiffness (tan beta) and the failure energy in shear in the impeded group were respectively 6.4, 6.0 and 5.7 times those in the unimpeded group. The average maximum deformation was similar. The average maximum shear stress, the elastic stiffness (tan alpha) and the failure strain energy density in the impeded group were respectively 7.2, 8.1 and 6.5 times those in the unimpeded group. The average maximum strain in the unimpeded group was significantly greater (1.4 times) than that in the impeded group. Mechanical responses of the periodontal ligament seem to be very sensitive to internal and external disturbance of the tooth: changes in such properties may be detectable even when other techniques fail to reveal differences. Our method provides a useful tool for such mechanical analysis.

Continuous monitoring of the position of the ferret mandibular canine tooth to enable comparisons with the continuously-growing rabbit incisor

A variable capacitance displacement transducer, previously employed for continuously monitoring movements of the rabbit mandibular incisor, was modified for use with the ferret mandibular canine. Comparing this tooth of limited growth with the tooth of continuous growth, similarities were seen in the reactions to extrusive loads and the responses at death. For the tooth of limited growth, however, less creep was seen with extrusive loading and hexamethonium did not produce changes in tooth position.

Resorption of permanent teeth

The varying extent of permanent tooth resorption attributable to different types of orthodontic appliances is a continuing cause for concern. This paper reviews recent researches which contribute to an improved understanding of the origin and behaviour of osteoclasts; and considers the structure and behaviour of the periodontal ligament and its relationship to the cementum and bundle bone of the tooth socket. Permanent tooth resorption is identified as a normal phenomenon. A simple explanation is offered as to why under normal circumstances the bone of the tooth socket is more likely to be resorbed than the tooth itself. With this knowledge treatment procedures can be selected which will reduce tooth resorption risks to a minimum.

Development of dentition and dermal skeleton in embryonic Scyliorhinus canicula

Serial sections ranging from very young embryos to hatched juveniles and whole embryos of Scyliorhinus show that dentition and dermal skeleton belong to two independent secondary developmental fields that differ both developmentally and structurally. The development of the dentition starts very early, with a thickening of the ectoderm in the region of the mouth (stage 04), the invagination of the dental lamina (stage 18), and the formation of the germs of the first generation (stage 20). Tooth replacement movements start only near the end of embryogenesis (stage 35). Scale germs, on the other hand, first begin to form at stage 24. Scales erupt shortly before the animal hatches (stage 43). Only one scale generation is formed during embryogenesis. The forces which erupt the scales may come from fluid pressures in vacuoles of the fibrous layer of the dermis. Those which erupt the teeth probably also result from similar fluid pressures. The crown and upper part of the base of scales and teeth are formed by cells of the inner dental epithelium which are differentiated from the ectoderm. They are also formed by odontoblasts which are derived from the vascular layer of the dermis. However, the basal plates of scales and teeth containing the anchoring fibers are formed by osteoblasts, which are derived from the fibrous layer of the dermis.

Collagen fibre architecture in the periodontal ligament

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