A quantitative assessment of the effects of axially directed extrusive loads on displacement of the impeded and unimpeded rabbit mandibular incisor

Tissue Engineering for Periodontal Ligament Regeneration: Biomechanical Specifications

The periodontal biomechanical environment is very difficult to investigate. By the complex geometry and composition of the periodontal ligament (PDL), its mechanical behavior is very dependent on the type of loading (compressive versus tensile loading; static versus cyclic loading; uniaxial versus multiaxial) and the location around the root (cervical, middle, or apical). These different aspects of the PDL make it difficult to develop a functional biomaterial to treat periodontal attachment due to periodontal diseases. This review aims to describe the structural and biomechanical properties of the PDL. Particular importance is placed in the close interrelationship that exists between structure and biomechanics: the PDL structural organization is specific to its biomechanical environment, and its biomechanical properties are specific to its structural arrangement. This balance between structure and biomechanics can be explained by a mechanosensitive periodontal cellular activity. These specifications have to be considered in the further tissue engineering strategies for the development of an efficient biomaterial for periodontal tissues regeneration.

Mechanical strength and viscoelastic response of the periodontal ligament in relation to structure

The mechanical strength of the periodontal ligament (PDL) was first measured as force required to extract a tooth from its socket using human specimens. Thereafter, tooth-PDL-bone preparations have extensively been used for measurement of the mechanical response of the PDL. In vitro treatments of such specimens with specific enzymes allowed one to investigate into the roles of the structural components in the mechanical support of the PDL. The viscoelastic responses of the PDL may be examined by analysis of the stress-relaxation. Video polarised microscopy suggested that the collagen molecules and fibrils in the stretched fibre bundles progressively align along the deformation direction during the relaxation. The stress-relaxation process of the PDL can be well expressed by a function with three exponential decay terms. Analysis after in vitro digestion of the collagen fibres by collagenase revealed that the collagen fibre components may play an important role in the long-term relaxation component of the stress-relaxation process of the PDL. The dynamic measurements of the viscoelastic properties of the PDL have recently suggested that the PDL can absorb more energy in compression than in shear and tension. These viscoelastic mechanisms of the PDL tissue could reduce the risk of injury to the PDL.

Differential Gene Expression of Periodontal Ligament Cells After Loading of Static Compressive Force

BACKGROUND

Compressive force is an important mechanical stimulus on the periodontal ligament (PDL) and is closely related to therapeutic tooth movement. In this study, early or late response genes related to the compressive stress in PDL cells were evaluated. Particularly, the expression of interleukin (IL)-6, IL-8, and alkaline phosphatase (ALP) was studied.

METHODS

The primary cultured cells from PDL were grown in a three-dimensional collagen gel, and received a continuous static compressive force (1.76 g/cm(2)). The expressed genes were screened by cDNA microarray assays for 2 or 12 hours after the initiation of the mechanical force application. The genes of interest that showed significant changes in expression in the cDNA microarray assay were analyzed further by quantitative reverse transcriptase polymerase chain reaction (RT-PCR), enzyme-linked immunoabsorbent assays (ELISA), and ALP assays.

RESULTS

ALP, IL-6, and IL-8 were selected among the genes that significantly changed expression (/M/ >0.7) and subsequently were confirmed by quantitative RT-PCR. The secreted protein concentrations for IL-6, IL-8, and ALP activity were measured at 72 hours after application of continuous static compressive force. The protein level of IL-6 was significantly increased at 72 hours (P <0.001), but there was no significant change in IL-8 (P >0.05). ALP activity was decreased approximately 41.5% compared to the control (P = 0.015).

CONCLUSIONS

Considering that IL-6 is a potent osteoclast activator and the compressive side of PDL during orthodontic tooth movement shows the resorption of calcified tissue, the changed expression of IL-6 and ALP in response to the static compressive force in PDL cells may contribute to the orthodontic tooth movement or alveolar bone remodeling.

The biomechanical properties of the healing periodontium of replanted rat mandibular incisors

One of the most important aspects in tooth replantation seems to be restoration of the tooth support function of the healing periodontal ligament (PDL). We examined the support function, as measured by the mechanical properties, of the healing PDL at 7, 14, and 21 days after replantation of the left mandibular incisor in rats. From each dissected left mandible, a transverse section(650 microm in thickness) of the incisor was cut through an axis near the labial alveolar crest. Each section was intrusively loaded at a rate of 5 mm min(-1), and the shear stress-strain curve for the PDL was analyzed. Mechanical measures of the healing PDL showed gradual improvement after replantation. By 21 days, the mechanical strength returned to 53% of the control value; the extensibility, to 85%; the stiffness, to 61%; and the toughness, to 52%. The healing PDL exhibited reattachment of fibers in the middle region of the PDL, and the birefringent collagen fibers appeared to have regained the functional orientation by 14 days. The ratios occupied by the birefringent collagen fibers in the tooth-related, middle, and bone-related areas of the healing PDL gradually improved and returned to 78, 51, and 48% of the respective control values by 21 days. These results suggest that the support function of the healing PDL is gradually restored and that the biomechanical restoration is closely related to the reorganization and reorientation of collagen fiber bundles in replanted rat incisors.

Alveolar bone Sharpey fibers of the rat incisor in normal and altered functional conditions examined by scanning electron microscopy

The morphology and the area density of Sharpey fibers in the socket of the rat incisor under normo-, hyper-, and hypofunctional conditions were evaluated using scanning electron microscopy. Sharpey fibers appeared either as dome-shaped projections, when highly mineralized, or as depressions when less mineralized. Near the alveolar crest, most of the fibers were fully mineralized and arranged in compact longitudinal rows. Toward the basal end of the socket, the rows became interrupted, forming islets of gradually smaller size and number. The density of the Sharpey fibers was higher (P < 0.01) in the mesial and distal faces than in the lingual face in most of the socket length. In normofunctional conditions, in all faces the density decreased 70 to 90 times from the crestal toward the basal region of the socket (P < 0.01). The degree of mineralization of the Sharpey fibers also decreased steadily in the same direction, indicating that, for support, the periodontal ligament matures from basal to incisal and is fully developed only in the crestal region. In hyper- and hypofunctional conditions, the same distribution was observed. The area density of the Sharpey fibers in the hyperfunctional condition showed a slight increase at the basal levels of the socket mesial and distal faces (P < 0.01 or P < 0.05). In hypofunctional incisors, the density decreased significantly (P < 0.01) at the mesial and distal faces in all regions of the socket, and at the lingual face, the decrease (P < 0.05) was restricted to the incisal regions. This may be one of the factors for the weakening of the periodontal ligament in hypofunctional incisors.

The effects of intrusive loading on axial movements of impeded and unimpeded rat incisors: estimation of eruptive force

Axial movements of impeded and unimpeded rat mandibular incisors were measured following application and removal of intrusive loads of 1, 2, 5, 10 and 20 mN in a stepwise order at intervals of 1h on erupting teeth. The tooth movements were recorded by a displacement detector under artificial respiration with halothane anaesthesia. The loading and unloading procedures brought about the tooth movements in two steps: an initial rapid movement immediately after application or removal of the load and a subsequent slow and gradual movement. The initial rapid intrusive or extrusive tooth movements were significantly greater in the unimpeded than in the impeded teeth at the same load. The forces to stop extrusive tooth movements, estimated from the formula of regression lines showing correlations between the intrusive loads from 0 to 5 mN and tooth movements (microm/30 min), were 4.2 mN in the impeded and 2.9 mN in the unimpeded incisors. We suggest that repeated shortenings of the rat incisor did not cause an increase in the eruptive force and that changes in the resistance of the periodontal ligament predominantly regulate the axial movement of the rat incisor.

Axial movements of rat mandibular incisors measured under artificial respiration with halothane anaesthesia

The axial movements of rat incisors were recorded continuously for over 20 h. The rats were anaesthetized with halothane delivered by intratracheal intubation using an artificial respirator. A haemostatic clamp was used to immobilize the jaw and the displacement detector exhibited high resolution, good linearity and low drift. The previously impeded and unimpeded eruption rates (the average extrusive movements) were 406 and 548 microns/24 h, respectively. The previously unimpeded rate was lower than the normal unimpeded rates obtained by other investigators. It appears that the eruptive movement of the previously unimpeded incisor was inhibited by halothane anaesthesia. Circadian rhythm was not observed in the eruption rates of previously impeded or unimpeded incisors. Following the i.p. injection of a lethal dose of pentobarbital sodium, the incisors retracted. The average intrusive movement was significantly greater in the previously impeded group (29 microns) than in the previously unimpeded group (12 microns) at 30 min after the lethal injection. It is assumed that the elimination of the pushing force at death may cause the stretched periodontal fibres to recoil, thereby pulling the tooth in an intrusive direction. It is also likely that the mechanical strength of the ligament is reduced in the previously unimpeded group by repeated shortening of the incisors; therefore the force needed to retract previously unimpeded incisors may be less than that needed to retract previously impeded incisors.

Accelerated eruption of rat lower incisor. Relationship between impeded and unimpeded eruption rates, rate of attrition, tooth length, and production of dentin and enamel

The present investigation studies the effects of persistently cutting one lower rat incisor out of occlusion. Within four days, the rate of eruption of the cut (unimpeded) incisor increased to 216% and that of the uncut (impeded) contralateral to 136% of the baseline rate. While the former remained high, the latter decreased gradually to about 90% within three weeks. The rate of attrition of the impeded incisor increased to 233% of the baseline rate within two days, then fell abruptly, and remained at a slightly lower level than the rate of eruption. Accordingly, the length of the erupted part of the impeded incisor decreased initially, but increased gradually after about four days. Measurements made on SEM micrographs of the series of transverse tooth segments obtained when cutting the incisor out of occlusion, showed that growth-related increase in mesiodistal tooth width was arrested from the 10th segment, dentin thickness decreased gradually to about 50% in the 12th segment, and enamel thickness, after an initial increase, decreased to about 80% in the 11th segment. The present study provides experimental evidence that accelerated eruption affects morphogenesis and histogenesis of the rat lower incisor. An impeded incisor, especially the contralateral, may not serve as an ideal control.

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 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.

Long-term effects of orthodontic forces on the morphology of the rat-incisor socket and its location in the mandible

The effect of orthodontic force application on the rat-incisor socket and mandible was studied on roentgenograms. A mean linguo-intrusive force of 19 +/- 0.6 g was applied continuously to the shortened left lower incisor for a period of two (group A) and four weeks (group B). A third group of rats, subjected to shortening of the left mandibular incisor only, served as a hypofunctional control (group C). A fourth group of normal rats constituted the intact control (group D). After a recovery period of three months, the animals were killed, and standardized roentgenograms of the cleaned mandibles were taken. Socket and mandibular dimensions were measured on magnified tracings of the roentgenograms. Comparison of groups A and B with the control groups, on the one hand, and of group C with group D, on the other, facilitated isolation of the hypofunctional factor. In groups A and B, the orthodontic forces caused changes in the parameters affected (i.e., socket area, alveolar bone thickness, mandibular dimensions) and not affected by hypofunction (i.e., socket angulation and location, anterior-socket length). The former findings implies modification of the adaptive capability of dental structures to functional demands. It is concluded that mechanical loading of the incisor for two to four weeks causes long-lasting changes in the socket and its surrounding bone.

A comparison of the biomechanical properties of the periodontal ligaments of erupting and erupted teeth of non-continuous growth (ferret mandibular canines)

Extrusive loads of 0.01-0.2 N were applied to the permanent mandibular canine teeth of two groups of ferrets. In one group, aged approx. 7 weeks, the tooth had only recently emerged into the mouth. In the second group, comprising adult animals, the tooth had fully erupted and had been in function for a considerable time. Biphasic, viscoelastic-like responses followed both the application and removal of the load in both groups. However, there was significantly more mobility for all phases of the loading and recovery cycles in the erupting teeth. The data were compared with previous findings for continuously growing teeth. They highlight the marked periodontal changes which take place when tooth erupts into the mouth, provide some evidence against the generation of a tractional eruptive force, and show some differences in response between teeth of limited or continuous growth.

The intrusive mobility of the incisor tooth of the guinea pig

The pattern of mobility of the mandibular incisor with intrusive loads was consistent with the view that the periodontal tissues are viscoelastic. A significant time-dependency of the response was observed. The pattern of mobility was similar to that reported for axially directed extrusive loads and for horizontal loads.

The effect of corticosteroids and cyclophosphamide on the eruption of resected incisor teeth in the rat

It has been observed that there may be two mechanisms making resected rat incisors erupt during their phase of normal eruption rates, only one of which works during their slow phase of eruption. The effects of cyclophosphamide and corticosteroids on the eruption of non-resected teeth may also be consistent with the presence of two eruptive mechanisms, one responsive to cyclophosphamide and corticosteroids, the other not, or less, responsive to them. To see whether this previous work has, in different experiments, revealed the same pair of mechanisms, the effects of cyclophosphamide and corticosteroids on resected incisors were investigated. Cortisone accelerated their eruption and cyclophosphamide slowed it when they were erupting at normal rates, but neither compound had any effect during the initial slow phase.

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.

In-vitro mechanics of intrusive loading in porcine cheek teeth with intact and perforated root apices

Cyclic intrusive loads were applied to deciduous cheek teeth and monitored by radiography in dissected mandibles in three successive states: (A) intact, (B) vertically bisected in a bucco-lingual plane, (C) with perforation of the apices by lateral drilling of the mandible subsequent to (B). Loading was in a compression cage in an Instron mechanical testing-machine utilizing cross-head speeds of 0.5 and 5 mm/min, leading to loading rates of approximately 2 and approximately 20 N/s. Peak loads were approximately 100 N, causing an intrusion of approximately 200 microns. Load/recovery curves for each tooth in the three states were recorded and the data treated assuming: (a) a simple shear on the periodontal ligament, (b) tensile loading on the fibres of the collagen fraction of the ligament alone. Treatment (b) gave the best agreement with published mechanical data on isolated mammalian periodontal ligament. The small effect of apicectomy on the mechanical behaviour indicates that a mobile fluid-like support mechanism is unlikely to operate for major loads in vitro and tensile support by the collagen fibres of the periodontal ligament is the most likely dominant mechanism.

Extrusive mobility of teeth in adult monkeys (Macaca fascicularis)

Extrusive mobility of 15 teeth in 4 adult macaques was compared with intrusive mobility of the same teeth. The character of the load displacement and recovery records was similar; the qualitative response to repeated 4 N thrusts, 4 N loads sustained for 30 s and residual loading with 50, 30 and 10 mN, was the same whether the force was applied in an intrusive or extrusive direction. Fast and slow loading rates (100 and 1 Ns-1) both caused greater intrusion than extrusion at 4 N (p less than 0.001). Thus the mechanisms of tooth support are similar for intrusive and extrusive force. The oblique orientation of the principal fibres of the ligament may allow a greater degree of initial intrusive movement, before tension is generated, than occurs in axially-aligned fibres under extrusive force. Extrusive loading may be a convenient method for studying tension in the periodontal ligament.

The mobility of the lathyritic rabbit mandibular incisor in response to axially-directed extrusive loads

Axially-directed extrusive loads of 0.01-0.1 N were applied to the lathyritic mandibular incisors of 12 rabbits. Tooth position was monitored continuously with a variable capacitance displacement transducer. Compared with pair-fed controls, lathyritic teeth showed markedly reduced resistance to extrusive loading, indicating that the periodontal collagen has a role in supporting the tooth. The pattern of tooth mobility, however, was little affected. The results may have implications for the eruptive mechanism. That eruption rates can appear unchanged by lathyrogens might not preclude an effect of the drug upon the eruptive force, assuming that there is a concomitant decrease in the resistance of the periodontal tissues to eruption. The teeth of rabbits in the pair-fed control group showed increased resistance to extrusive loading compared with animals fed ad libitum, indicating that changes in food and water intake may also influence the mechanical properties of the periodontium.

Interactions between thyroxine, hydrocortisone and cyclophosphamide in their effects on the eruption of the rat mandibular incisor

The effects on the unimpeded eruption rates of the rat mandibular incisor of daily doses of thyroxine (0.5 mg/kg) and hydrocortisone (12.5 mg/kg) and of a single injection of cyclophosphamide (40 mg/kg), either alone or in combination, were studied over approximately 15 days. Both the thyroxine and hydrocortisone alone produced increases in eruption, whereas a decrease was recorded with cyclophosphamide alone. When thyroxine was combined with hydrocortisone, the increase in eruption rate was greater than that with thyroxine or hydrocortisone alone. When cyclophosphamide was combined with hydrocortisone, there was no difference compared with cyclophosphamide alone. These results obtained with drug combinations are consistent with eruption having a multifactorial basis.

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.

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

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.