A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors

Distinct post-translational features of type I collagen are conserved in mouse and human periodontal ligament

BACKGROUND AND OBJECTIVE

Specifics of the biochemical pathways that modulate collagen cross-links in the periodontal ligament (PDL) are not fully defined. Better knowledge of the collagen post-translational modifications that give PDL its distinct tissue properties is needed to understand the pathogenic mechanisms of human PDL destruction in periodontal disease. In this study, the post-translational phenotypes of human and mouse PDL type I collagen were surveyed using mass spectrometry. PDL is a highly specialized connective tissue that joins tooth cementum to alveolar bone. The main function of the PDL is to support the tooth within the alveolar bone while under occlusal load after tooth eruption. Almost half of the adult population in the USA has periodontal disease resulting from inflammatory destruction of the PDL, leading to tooth loss. Interestingly, PDL is unique from other ligamentous connective tissues as it has a high rate of turnover. Rapid turnover is believed to be an important characteristic for this specialized ligament to function within the oral-microbial environment. Like other ligaments, PDL is composed predominantly of type I collagen. Collagen synthesis is a complex process with multiple steps and numerous post-translational modifications including hydroxylation, glycosylation and cross-linking. The chemistry, placement and quantity of intermolecular cross-links are believed to be important regulators of tissue-specific structural and mechanical properties of collagens.

MATERIAL AND METHODS

Type I collagen was isolated from several mouse and human tissues, including PDL, and analyzed by mass spectrometry for post-translational variances.

RESULTS

The collagen telopeptide cross-linking lysines of PDL were found to be partially hydroxylated in human and mouse, as well as in other types of ligament. However, the degree of hydroxylation and glycosylation at the helical Lys87 cross-linking residue varied across species and between ligaments. These data suggest that different types of ligament collagen, notably PDL, appear to have evolved distinctive lysine/hydroxylysine cross-linking variations. Another distinguishing feature of PDL collagen is that, unlike other ligaments, it lacks any of the known prolyl 3-hydroxylase 2-catalyzed 3-hydroxyproline site modifications that characterize tendon and ligament collagens. This gives PDL a novel modification profile, with hybrid features of both ligament and skin collagens.

CONCLUSION

This distinctive post-translational phenotype may be relevant for understanding why some individuals are at risk of rapid PDL destruction in periodontal disease and warrants further investigation. In addition, developing a murine model for studying PDL collagen may be useful for exploring potential clinical strategies for promoting PDL regeneration.

Mechanical characterization of bovine periodontal ligament

This study is part of a research program that aims to develop a constitutive three-dimensional model of the periodontal ligament (PDL) through the identification of pertinent material parameters. As part of this program, bovine PDL was utilized to establish stress-strain responses under tensile and compressive loading conditions. Fresh bovine molars were secured, frozen and prepared to appropriate dimensional specifications. Bar-shaped specimens that comprised portions of dentine, PDL and bone were produced. Push-pull tests were conducted using a specifically constructed loading machine. Full range monotonic stress-strain diagrams were generated. The effect of a rate increase on cyclic S-E diagrams was also determined. The influence of specimen thickness was expressed in terms of modulus of elasticity, strength, uniaxial maximizer strain, and strain energy density. The overall load-response was heavily hysteretic in compression. On the tensile side, after a steep rise, the curve tended to flatten out asymptotically. Variations in rate that spanned four orders of magnitude had no effect on reciprocal load responses. The E-modulus was in the 4-8 MPa range, the strength of the PDL was 1-2 MPa, the maximizer strain was at 45-60% and the strain energy density ranged between 0.3 and 0.4 MPa.

The effect of functional occlusal forces on orthodontic tooth movement and tissue recovery in rats

The influence of physiologic occlusal forces on tissue response to orthodontic tooth movement and its subsequent recovery was investigated in rats. The mandibular incisors were subjected to tipping and intrusive loads for 2 weeks. In 27 animals, the teeth remained in occlusion, while the incisors were shortened out of occlusion in 29. In each group, some rats were killed at the end of the tooth movement process, and the rest were kept alive for a 13-week recovery period. The subgroups were matched with corresponding controls. Tooth intrusion and eruption were monitored throughout the experiment; then histomorphometric and histopathologic evaluations were performed on the incisors and their periodontal tissues. Tooth movement was barely affected by the functional forces. However, immediately upon 2-week load application, damage to the pulp was more extensive in the occluding teeth; this might be due to the combined intrusive vectors of functional and mechanical loads. The effect of the physiologic function was expressed primarily during the recovery period, in which the eruptive function and distorted periodontal ligament space returned to normal significantly faster in the occluding teeth. This was also true for the healing of dental and periodontal lesions. The incidence of tooth resorption was also reduced. Normal occlusal function is imperative for rapid reconstitution of the damage caused by orthodontic forces.

Confocal laser scanning-microscopic observations on the three-dimensional distribution of oxytalan fibres in mouse periodontal ligament

The purpose was to improve confocal laser scanning-microscopic (CLSM) techniques to observe the three-dimensional (3-D) distribution of oxytalan fibres in mouse periodontal ligament and to clarify the 3-D relation between those fibres and blood vessels. As aldehyde fuchsin is a contrast agent and the specific wavelength affects the depth of penetration, CLSM reflectance imaging of oxytalan stained with aldehyde fuchsin after oxidization provides strong contrast. Oxytalan fibres, whose precise roles have not yet been clarified, are connective tissue fibres present in human periodontal ligament in addition to collagen fibres. Despite many studies on their arrangement and biomechanical characteristics, their 3-D distribution in relation to other structures has never been reported. Mandibular first molars of mice were sectioned mesiodistally, pre-oxidized by monopersulphate compound (Oxone), stained with aldehyde fuchsin and examined by CLSM. CLSM images clearly revealed the 3-D distribution, and relation of oxytalan fibres to blood vessels and other structures, as well as their branching patterns in the periodontal ligament. The marked anatomical correlations between the direction, distribution and branching patterns of oxytalan fibres and blood vessels suggest that they interact to perform a specialized physiological role in the periodontal ligament.

Comparison of biomechanical properties of the incisor periodontal ligament among different species

BACKGROUND

The aim of this study was to obtain a more precise understanding of the mechanical properties of the periodontal ligament in continuously erupting incisors by comparing the shear stress-strain relations among teeth from four closely related species.

METHODS

Four species of experimental animals (mice, hamsters, rats, and rabbits) were used. Transverse sections of the left mandibular incisors were cut from the incisal, middle, and basal regions of each incisor. The tooth was pushed out of the alveolar bone in an extrusive direction at 5 mm/min using a materials testing machine. The maximum shear stress, maximum shear strain, tangent modulus, and failure strain energy density were estimated from the resulting stress-strain curve. Polarized light microscopic observations of collagen fibers were also made.

RESULTS

All the biomechanical measures tended to decrease from the incisal toward the basal regions in all species. There were large species differences, especially in the incisal region, with the greatest maximum shear stress and failure strain energy density in hamsters. The greatest tangent modulus and the smallest maximum shear strain were observed in mice. The birefringent fiber architectures of the periodontal ligaments in the four species appeared to be similarly organized; the incisal periodontal ligament appeared to have more organized and thicker collagen fibres than did the middle and basal ligaments in the four species.

CONCLUSIONS

These results suggest that the regional differences in the biomechanical properties of the periodontal ligament depend upon the developmental stages of the periodontal collagen fibers that may be related to the general arrangement, diameters, and densities of the collagen fiber bundles and the fiber insertions into the alveolar bone and cementum. The species differences in the biomechanical properties may be due to differences in the width of the periodontal ligament and the waviness as well as the strength and stiffness of the periodontal collagen fibers.

Changes in the shape and orientation of periodontal ligament fibroblasts in the continuously erupting rat incisor following removal of the occlusal load

One of the main theories which attempts to explain the phenomenon of tooth eruption suggests that periodontal ligament (PDL) fibroblasts move actively and pull the tooth with them out of its socket. To find further support for this theory, we determined the changes in the shape and orientation of PDL fibroblasts induced by a transition from impeded to unimpeded eruption. We measured nuclear area, elongation (length-to-width ratio), and orientation (angulation in relation to the eruption axis) of PDL fibroblasts in impeded (functionally loaded) and unimpeded (hypoloaded) rat incisors. The mean cross-sectional nuclear area did not differ between fibroblasts in the two groups. In contrast, unimpeded eruption resulted in a marked increase in the mean nuclear elongation (from about 2 to 2.56) and a significant increase in the mean nuclear orientation (from 25.6 to 14.0 degrees). Bivariate analysis suggested that these changes occurred in the same cells. Analysis of nuclear elongation and orientation at various distances from the cementum toward the alveolar bone revealed a profile of both parameters, such that cells located 20 to 80 microns away from the cemental surface were more elongated and more frequently oriented toward the eruption axis, while cells at 0 to 20 and 80 to 100 microns were more round/oval and had a greater angulation with the eruption axis. These findings, together with other observations of changes in cell number, number of microtubules, and migration velocity which occur on the shift to unimpeded eruption, support the theory of active movement of PDL fibroblasts as an important component of tooth eruption.

The effect of rate of eruption on periodontal ligament glycosylaminoglycan content and enamel formation in the rat incisor

The rate of eruption of rat mandibular incisors was either increased by cutting one tooth out of occlusion or eliminated by means of pinning. The effects of such changes in eruption rate on the sulphated glycosylaminoglycan content of the periodontal ligaments was analysed. The length of the enamel secretory zone and the composition of the developing enamel matrix protein was also compared. Sulphated glycosylaminoglycan content of the periodontal ligament increased fourfold (P < 0.001) during accelerated eruption but decreased to a corresponding extent (P < 0.001) in the absence of eruption, when compared with controls. The length of the enamel secretory zone was also significantly reduced in the immobilised teeth, although the protein content was similar compared with controls. The results demonstrate the differential response to varied eruption rates of the periodontal ligament and enamel, particularly in respect of the extracellular matrix. The data are consistent with the view that the ground substance of the periodontal ligament plays a role in the generation of the eruptive force.

The effects of cytoskeletal inhibitors on the collagen gel contraction by dog periodontal ligament fibroblasts in vitro

Periodontal ligament fibroblasts (PLF) were incorporated into small collagen gel discs (100 microliters, dia 10.0 mm), cultured in media containing various concentrations of cytochalasin D or colchicine and the diameters of the discs then measured. Cytochalasin D (3.1-800 nM) inhibited contraction of the gel discs in a dose-dependent manner and 800 nM of cytochalasin D completely inhibited contraction. This complete inhibition was still observed when cell density in the gel was increased. Although colchicine at a concentration of 0.98-15.7 nM also inhibited the contraction in a dose-dependent manner, contraction at 50% of the control level was still observed in the presence of a high concentration (0.25 mM, 100 micrograms/ml). These results strongly indicate that actin filaments are more intimately involved in PLF-mediated collagen gel contraction than microtubules. Although it is suggested that PLF are involved in tooth eruption, earlier studies have shown that microtubule inhibitors inhibit eruption of rat incisors whereas the microfilament inhibitor, cytochalasin B, has no effect. Thus, the PLF theory alone seems insufficient to explain the mechanism of tooth eruption.

The effects of orthodontic forces on the mechanical properties of the periodontal ligament in the rat maxillary molars

Previous studies have shown that the mechanical strength of the periodontal ligament decreased markedly after the application of an orthodontic force to the rat mandibular first molar. However, the effects of orthodontic forces on the mechanical properties of the periodontal ligament of the other teeth in the same dental arch have not been examined. Furthermore, a detailed analysis of the mechanical parameters has not been made. The purpose of the present study was to analyze mechanical parameters such as maximum shear load, maximum deformation, elastic stiffness, and failure energy in shear, estimated from the load-deformation curves obtained by extracting the rat maxillary molars from their sockets after application of an orthodontic force for 5 days. It was found that the mechanical properties of the periodontal ligament of the rat maxillary molars varied considerably among control teeth in the same dental arch. Decreases in the mechanical properties of the periodontal ligament of the experimental teeth were greatest in the first, less in the second, and least in the third molars. Significant correlations were found between the maximum load and the failure energy and between the maximum deformation and the failure energy in all teeth. It was concluded that orthodontic force affected not only the mechanical properties of the periodontal ligament of the teeth to which the force had been applied but also the mechanical properties of distant teeth.

The effect of age on the mechanical properties of the periodontal ligament in the incisor teeth of growing young rats

Mechanical properties of the periodontal ligament of the incisor have been examined by pushing the tooth out of its surrounding alveolar bone in sections of the mandibles of rats from 3 to 24 weeks of age. The maximum load, failure energy in shear and elastic stiffness estimated from the load-deformation curves, showed a tendency to increase with age from 3 to 12 weeks but the maximum deformation did not. Age-related changes were not appreciable in all mechanical measures estimated from the stress-strain curves. It is suggested that increases in the maximum load and elastic stiffness from the load-deformation curves with age are closely related to those in the size of teeth or in the area of the periodontal ligament facing cementum. It is also suggested that qualitative and quantitative changes in the supporting tissues or in the collagen fibres/unit area of the periodontal ligament are minor from 3 to 24 weeks of age in the rat mandibular incisor.

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.

Relationship between enamel formation and eruption rate in rat mandibular incisors

The relationship between the formation of dental enamel and tooth eruption was investigated. Rat mandibular incisor eruption rate was accelerated by maintaining incisors out of occlusion. Rate of eruption, enamel thickness, secretory zone length and matrix breakdown were measured. Eruption rate increased by 120% in experimental teeth but enamel secretion increased by only 90%. There were no obvious differences between control and experimental teeth in final enamel thickness or in the molecular weight distribution of the enamel matrix proteins.

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.

Long-term effect of loading on the fibroblast population of the periodontal ligament in the rat lower incisor

The number and density of the cells in the tooth-related periodontal ligament (t-PDL) were related to the recovery of eruptive potential 2.5 months after load removal. Four left incisors in which eruption returned to normal (group A) and six in which eruption remained grossly impaired after a two-week application of latero-intrusive loads of 18.5 +/- 0.69 (group B) were compared to four control rats with unimpeded eruption. The incisors were demineralized, embedded in paraplast and cut into 6 micron transverse serial sections from which a three-dimensional reconstruction of the PDL was made by computerized histomorphometry, and the fibroblast population counted. The t-PDL volumes in controls and in groups A and B reached 1.15, 1.05 and 1.53 mm3, respectively; the estimated number of cells in the same PDL volumes were 2244 X 10(3), 1.659 X 10(3) and 2094 X 10(3) cells, respectively. The mean cell count/segment of PDL and the calculated number of cells/unit of periodontal tissue were significantly lower (p less than 0.001) in the formerly stressed, lateral periodontium of both experimental groups. In group B, these values were also decreased in the mesial periodontium.

A transmission electron-microscope stereological study of the blood vessels, oxytalan fibres and nerves of mouse-molar periodontal ligament

Mandibular, mesiobuccal ligament was sectioned from the alveolar crest to the apex at predetermined levels. Data collected using standard point counting procedures was analysed for main effects due to animal, side of mouth, depth and zone across the ligament. Statistically-significant variations only occurred between different lateral thirds with the middle third containing the most oxytalan fibres and the bone third the greatest vascular and neural volumes. Stereology showed that the ligament was 3-7 times more vascular than other connective tissues. Eighty-eight per cent of the periodontal blood volume was enclosed in vessels with a mean lumenal diameter of 20.9 microns and characterized by a thin endothelial lining and few perivascular cells. These vessels had a surface density of 125.1 cm2/cm3. Oxytalan fibres had a length density of 1258 X 10(3) cm/cm3 and a mean caliper diameter of 0.7 micron. Furthermore, 78 per cent of fibres were adjacent to cells, 14 per cent within principal collagen fibres and 8 per cent in the walls of blood vessels. The length density of nerves within the ligament was 255.9 X 10(3) cm/cm3. Unmyelinated axons constituting 95 per cent of periodontal nerve fibres had a mean caliper diameter of 1.4 micron.

The effects of preventing movement of the rat incisor on the structure of its periodontal ligament

A quantitative, ultrastructural study was made on the periodontal ligaments of rat mandibular incisors immobilized for 18 days. Fibroblasts and their organelles (i.e. endoplasmic reticulum, mitochondria, microtubules, microfilament bundles, lysosomes, intracellular collagen profiles and intercellular contacts), oxytalan fibres, collagen fibrils and ground substance were quantified. There was re-orientation of tissue at the lateral borders of the ligament and an apparent increase in size of epithelial cell rests. Within the matrix, the mean collagen-fibril diameter decreased and the amount of ground substance increased. No change occurred in oxytalan fibres. Within the cells, effects were seen only in cell contacts and in the relative volume of intracellular collagen, lending no support to the view that fibroblast activity generates the force responsible for tooth eruption.

Changes in collagen fibril diameters in a lathyritic connective tissue

Collagen fibril diameters were measured in molar and incisor periodontal ligaments of adult rats given aminoacetonitrile (0.15% in their drinking water). Fibrils were also measured in the periodontal ligaments of pair-fed control animals and animals fed ad libitum. The lathyrogen produced a significant change in the distribution of fibril diameters in both molars and incisors and an increase in mean fibril diameter in incisors relative to pair-fed controls. The reduced food intake of the pair-fed control group produced a significant decrease in mean fibril diameter in incisors relative to the ad libitum control group. These results that lathyrogens may affect collagen fibril diameters in an adult connective tissue in a manner similar to that previously shown for a fetal tissue.

Composition and metabolism of the extracellular matrix in the periodontal ligament of impeded and unimpeded rat incisors

Potential differences were studied in the composition and metabolism of the connective tissue components (collagen, non-collagenous proteins and glycosaminoglycans) of the periodontal ligament (PDL) as a result of alterations in the rate of eruption. The amount of collagen, glycosaminoglycan (GAG) or the water content of the tissue was the same in the unimpeded and impeded situation. The only difference was in the amount of structural non-collagenous proteins: 33 per cent increase in the unimpeded PDL. The rate of turnover of insoluble collagen, structural non-collagenous proteins and the sulphated GAG, studied in vivo by administration of [3H]-proline or [35S]-sulphate, did not differ significantly in both situations: their extremely short half-lives were respectively 9.7, 3.5 and 1.7 days. Increase in the rate of eruption and hence in the remodelling process does not result in alterations in the metabolic activity of the PDL components. Their metabolism is probably already at maximal rate and may permit the more rapid rate of eruption when the opposing force of the antagonists is diminished during the non-occlusional state of the incisors.

Ultrastructural analysis of the microfibrillar component of mouse and human periodontal oxytalan fibers

Microfibrillar populations were quantified for mouse and human periodontal ligament oxytalan fibers in cross-section. Ten mouse fibers (Group 1) and ten human fibers (Group 2) were randomly selected from representative levels of the ligament and digitized for the total fiber area (TFA) and the microfibrillar component area (MFA). A third group of specific mouse oxytalan fibers (RER) was also evaluated. Quantified fiber dimensions varied markedly as did the percentage microfibrillar area for each group. Staining revealed morphologically different components within each fiber. Mouse and human microfibrils were composed of varying numbers of microfilaments. Analysis of covariance showed that the structural relationship between MFA and TFA was linear for each of the three groups. Slopes for the Group 1 mouse and human microfibrillar components were the same (p greater than 0.1) but different (p less than 0.05) for the RER group. Intercepts for all three groups were significantly different at the 1% level. The relevance of these species specific findings is discussed.

Changes in axial mobility of undisturbed teeth and following sustained intrusive forces in adult monkeys (Macaca fascicularis)

The axial mobility and changes in rest position were studied over 3-h periods. No loads were applied to 10 teeth. All displayed reduction in mobility; eight became extruded. Other teeth were displaced into their sockets to a sustained level by a screw for 3 h. Mobility decreased for those teeth which were intruded a distance equivalent to less than 800 mN force, but increased following greater intrusion. It is argued that vascular changes played no part; it is more likely that aggregation of proteoglycans occurred in the periodontal ground substance when there was little or no displacement of the tooth. Displacement would have generated tension in the collagen fibres of the periodontal ligament, leading to smaller proteoglycan molecules being formed and a consequent increase in mobility.

Long-term effect of intrusive loads of varying magnitude upon the eruptive potential of rat incisors

The present investigation deals with the functional recovery of the periodontium following application of intrusive loads of varying magnitudes to continuously growing rat incisors. The daily rate of eruption of these teeth served as an indicator of the functional capacity of their periodontal ligament. The left mandibular incisor was shortened to prevent occlusion in thirty experimental and ten control rats. The experimental animals were divided into three groups and constant, calibrated axial loads of 8 Gm./cm.2 (light), 15 Gm./cm.2 (medium), and 26 Gm./cm.2 (heavy) were applied to their incisors by means of closed-coil springs for a period of 2 weeks. The daily eruption rate of all teeth was measured for 80 days following removal of the springs. In all experimental animals, severe and long-lasting impairment of the eruptive potential occurred. The least impairment was seen in the group with the light loads, their eruption rate being fully restored 64 days after removal of the loads, by which time 165 percent of all dental and periodontal tissues was renewed. In the medium-load group, renewal of the tissues reached 140 percent, while a gross impediment in normal eruption rate persisted throughout the entire experimental period. The teeth subjected to heavy loads received a diminished impact of the axially directed loads due to the strong horizontal component of force; consequently, they behaved like the incisors of the light-load group. The light loads proved to cause the least impairment and were the most beneficial to functional recovery.