Koichiro Komatsu

MECHANICAL RESPONSES OF THE PERIODONTAL LIGAMENT IN THE TRANSVERSE SECTION OF THE RAT MANDIBULAR INCISOR AT VARIOUS VELOCITIES OF LOADING IN VITRO

Stress-strain curves of the periodontal ligament (PDL) were obtained at various velocities of extrusive loading of 1, 10, 10(2), 10(3) and 10(4) mm/24 h in vitro. Significant increases of the maximum shear stress, tangent modulus and failure strain energy density were found with increases in the velocity of loading. The maximum shear strain increased from a velocity of 1 to 10 mm/24 h but decreased from 10 to 10(4) mm/24 h. It was shown histologically that the free surface of the PDL adhering to the cementum after mechanical testing was rough and irregular at higher velocities and rather smooth at lower velocities. These results showed that the mechanical properties and mode of failure of the rat incisor PDL were greatly dependent on the strain rate. It is possible that the PDL of the continuously erupting rat incisor has mechanical characteristics favourable for resisting weakly to slow and small eruptive forces but strongly to the fast and large occlusal forces as suggested previously [Chiba and Komatsu, The Biological Mechanisms of Tooth Eruption and Root Resorption (1988)].

The effect of velocity of loading on the biomechanical responses of the periodontal ligament in transverse sections of the rat molar in vitro

Stress-strain relations of this ligament in transverse sections of the mandibular first molar were examined over a wide range of velocities of loading from 1 to 10(4) mm/24 h. With increasing velocities, the maximum shear stress, tangent modulus and failure strain-energy density increased but the maximum shear strain decreased. The mechanical responses at the highest velocity for the molar ligament were compared with those previously found for the incisor ligament. Mechanical strength, stiffness and toughness were greater for the molar than for the incisor ligament; the molar ligament therefore has more extensible fibres or a different fibre arrangement. Comparison of the mechanical responses at the slowest velocity suggests that, though the stress level was greatly reduced (presumably because of stress relaxation), the fibre components of the molar ligament still reacted at this velocity. It is also suggested that the differences in the ratios of the mechanical measures in 10(4)-1 mm/24 h between the two types of teeth are due partly to their different periodontal fibre architectures.

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.

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

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.

Long-term effects of local pretreatment with alendronate on healing of replanted rat teeth.

BACKGROUND AND OBJECTIVE Our previous study showed that topical alendronate, an inhibitor of bone resorption, reduces root resorption and ankylosis for 21 d after replantation of rat teeth. The aim of the present study was to evaluate the long-term inhibitory effects of topical alendronate in the replanted teeth. MATERIAL AND METHODS The rat maxillary first molars were extracted, placed in saline containing 1 mm alendronate (alendronate group) or saline (saline group) for 5 min and then replanted. The maxillae were dissected at 60 and 120 d. Microcomputed tomography horizontal sections at three root levels were analyzed for root and bone resorption, ankylosis and pulp mineralization. RESULTS In the alendronate group at 60 and 120 d, the frequencies of resorption of roots and bone were lower than those in the saline group. The p values show statistical significances of lower frequencies in the alendronate group than in the saline group by chi-square test (see Table 1). Ankylosis and pulp mineralization occurred in the alendronate and saline groups. Bone marrow spaces were narrowed in conjunction with bone tissue expansion around the replanted teeth in the alendronate group. CONCLUSION The inhibitory effects of topical alendronate were retained on root and bone resorption, but not on ankylosis and pulp mineralization, in the replanted teeth for 4 mo. Alendronate might also stimulate bone formation around the rat replanted teeth.

Polarized light microscopic analyses of collagen fibers in the rat incisor periodontal ligament in relation to areas, regions, and ages

We prepared decalcified sagittal sections (20 μm thick) from the incisal, middle, and basal regions of the mandibular incisor of male Wistar rats aged 2, 6, 12, and 24 months, and examined the sections using polarized light microscopy. Most of the birefringent fibers appeared to run obliquely across the periodontal ligament. Birefringent fibers running parallel to the long axis of the incisor were also found in the intermediate area of the ligament. Similar fiber architecture was observed in all four age groups. Quantitative analysis showed that the retardation values of collagen were higher in the bone‐ and tooth‐related areas and lower in the intermediate area of the ligament. The values for the bone‐ and tooth‐related areas increased from the basal toward the incisal regions in all four age groups. Age‐related changes in the retardation values were found only in the incisal region of the incisor. In the incisal region, the values for the bone‐ and tooth‐related areas increased markedly from 2–24 months of age, whereas those for the intermediate area increased slightly but significantly with age. Our findings indicate that the degrees of molecular organization and alignment of collagen fibers in the bone‐ and tooth‐related areas of the ligament are higher than those in the intermediate area and increase near the incisal region and with age. It is also suggested that the collagen fibers in the intermediate area remain immature along the long axis of the incisor throughout the life span of the animal. Anat Rec 268:381–387, 2002.

Improved methods for immunohistochemical detection of BrdU in hard tissue

Bromodeoxyuridine (BrdU) is used to label synthesizing DNA and to chase label-retaining cell (LRC). As stem cells divide slowly in adult tissues, they can be visualized as LRCs. In order to identify LRCs in hard tissue, we examined optimal conditions of fixation, demineralization, and DNA denaturation/antigen retrieval for immunohistochemistry of BrdU in hard tissues including bone, tooth, and periodontal ligament. Mice were subcutaneously injected with BrdU (50 microg/g body weight) twice a day from the postnatal day 11 to day 15 and sacrificed at 2 h after the last injection. Dissected maxillae were fixed (Bouins solution or 4% paraformaldehyde), demineralized (Morses solution or EDTA), and embedded in paraffin. Antigen retrieval procedures were performed before incubation with primary antibody. When sections were treated with HCl for DNA denaturation, the staining intensity of BrdU positive cells was not affected by difference of fixatives. Higher sensitivity was obtained by demineralization with Morse than with EDTA. Although heat-induced antigen retrieval techniques in citrate buffer (pH 6.0) showed as well or better sensitivity than acid pretreatment, heating caused tissue damage specifically to tooth dentine and the surrounding tissue. When the LRCs at four weeks after the last injection of BrdU were compared, much more LRCs were observed in specimen demineralized with Morse than with 10% EDTA. Our data suggest that demineralization with Morse with Bouin fixative plus HCl pretreatment gives rise to the optimal results for BrdU immunodetection in hard tissue.

Synchronous recording of load-deformation behaviour and polarized light-microscopic images of the rabbit incisor periodontal ligament during tensile loading.

Tooth-periodontal ligament-bone segments were cut in the form of rectangular prisms (1.5 mm wide, 0.65 mm thick, and long enough to allow anchorage of the bone and tooth-end portions in a stretching jig) from the mandibular incisors of 10 rabbits. The experimental set-up enabled simultaneous recording, on video, of the changing image brightness under polarizing optics together with extension across the periodontal ligament. Specimens were stretched until failure at a velocity of 0.5 mm/min. The tensile load-deformation curve of the ligament exhibited an initial, non-linear region that was followed by a linear region, a subsequent yielding region preceding the maximum point, and a final descending region. Gradual increases in the intensity of birefringence in the linear and yielding regions indicated that stress concentrations occur in the supporting fibres attached to mineralized tissues. In the final descending region of the curve, progressive breakages of individual fibre bundles occurred, mainly in the middle zone of the ligament. Analysis of the polarized light-microscopic images showed that the increases in brightness and area of birefringent collagen fibre bundles occurred in parallel with the stress generated. These results suggest that the collagen fibre bundles became aligned with the direction of loading and the intensity of their birefringence increased according to the applied tensile force.

Constitutive modeling of the non-linear visco-elasticity of the periodontal ligament

A non-linear visco-elastic constitutive model is adopted to describe the relaxation phenomena of the periodontal ligament (PDL). The introduction of a non-linear formulation of visco-elasticity is necessary because experimental data from the literature referring to animal models show that the relaxation rate depends on the level of strain applied. In particular, the percentage of relaxation increases with decrease of the applied strain. The constitutive model is consistent with the non-linear elastic behavior of the PDL in the case of high rate loading and large strains attained by the tissue. A hyperelastic formulation is adopted for the elastic behavior of the PDL and this formulation is developed adopting suitable measures of stress and strain. The anisotropy of the tissue induced by specific spatial orientation of collagen fibers is included in the model. With respect to recent numerical formulation proposed to describe the non-linear visco-elasticity of the PDL, the proposed model has the advantage of being more consistent with the micro-structural configuration of the tissue and the large strains it can undergo. The results obtained show that a reasonable description of the PDL relaxation phenomena can be obtained by assuming that relaxation times are independent of strain, whereas the relative stiffness results are dependent on strain applied through an exponential function.