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

Construction of hyperelastic model of human periodontal ligament based on collagen fibers distribution

OBJECTIVE

The human periodontal ligament (PDL) dominated by collagen fibers showed hyperelastic mechanical behavior under orthodontic force. Despite previous researches on the hyperelastic model of PDL, there were certain limitations because of the types of samples and the ignorance of distribution of collagen fibers. Therefore, the aim of this study was to quantify the effect of collagen fibers distribution of human PDL on its hyperelastic behavior.

METHODS

A total of 6 human PDL samples of root neck, root middle and root apex were cut from human maxillary central incisor and lateral incisor. The spatial angles of collagen fibers were observed by optical microscope, the hyperelastic model was constructed combined with the observation results. The quasi-static uniaxial tensile tests with displacement load 0.05 mm/min were carried out, and the test data were used to identify the parameters of model.

RESULTS

The mechanical behavior of human PDL conformed to the trend of hyperelastic materials, and greatly depended on the spatial angles of internal collagen fibers. The R² value statistical fit of the constitutive model to the data is excellent (R² > 0.98). This model could excellently describe the hyperelastic properties of human PDL.

SIGNIFICANCE

In this study, we quantitatively described the effect of spatial distribution of collagen fibers on the mechanical properties of human PDL. The accuracy of this model was verified by the uniaxial test data, and the relevant model parameters were acquired, which have certain reference value in subsequent researches on hyperelasticity of human PDL and clinical treatment.

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.

Approach towards the porous fibrous structure of the periodontal ligament using micro-computerized tomography and finite element analysis

The periodontal ligament (PDL) is a porous and fibrous soft tissue situated around the tooth, which plays a key role in the transmission of loads from the tooth to the alveolar bone of the mandible. Although several studies have tried to characterize its mechanical properties, the behaviour of this tissue is not clear yet. In this study, a new simulation methodology based on a material model which considers the contribution of porous and fibrous structure with different material model formulations depending on the effort direction is proposed. The defined material model was characterized by a non-linear approximation of the porous fibrous matrix to experimental results obtained from samples of similar species and was validated by rigorous test simulations under tensile and compressive loads. The global PDL response was also validated using the parameters of the characterization in a finite element model of full human canine tooth obtained by micro-tomography. The results suggest that the porous contribution has high influence during compression because the bulk modulus of the material depends on the ability of interstitial fluid to drain. On the other hand, the collagen fibres running along the load direction are the main responsible of the ligament stiffness during tensile efforts. Thus, a material model with distinct responses depending of the load direction is proposed. Furthermore, the results suggest the importance of considering 3D finite element models based of the real morphology of human PDL for representing the irregular stress distribution caused by the coupling of complex material models and irregular morphologies.

Functional cues in the development of osseous tooth support in the pig, Sus scrofa

Alveolar bone supports teeth during chewing through a ligamentous interface with tooth roots. Although tooth loads are presumed to direct the development and adaptation of these tissues, strain distribution in the alveolar bone at different stages of tooth eruption and periodontal development is unknown. This study investigates the biomechanical effects of tooth loading on developing alveolar bone as a tooth erupts into occlusion. Mandibular segments from miniature pigs, Sus scrofa, containing M(1) either erupting or in functional occlusion, were loaded in compression. Simultaneous recordings were made from rosette strain gages affixed to the lingual alveolar bone and the M(2) crypt. Overall, specimens with erupting M(1)s were more deformable than specimens with occluding M(1)s (mean stiffness of 246 vs. 944 MPa, respectively, p=0.004). The major difference in alveolar strain between the two stages was in orientation. The vertically applied compressive loads were more directly reflected in the alveolar bone strains of erupting M(1)s, than those of occluding M(1)s, presumably because of the mediation of a more mature periodontal ligament (PDL) in the latter. The PDL interface between occluding teeth and alveolar bone is likely to stiffen the system, allowing transmission of occlusal loads. Alveolar strains may provide a stimulus for bone growth in the alveolar process and crest.

Oscillatory shear loading of bovine periodontal ligament--a methodological study

This study examined the stress response of bovine periodontal ligament (PDL) under sinusoidal straining. The principle of the test consisted in subjecting transverse tooth, PDL and bone sections of known geometries to controlled oscillatory force application. The samples were secured to the actuator by support plates fabricated using a laser sintering technique to fit their contours to the tooth and the alveolar bone. The actuator was attached to the root slices located in the specimen's center. Hence the machine was able to push or pull the root relative to its surrounding alveolar bone. After determining an optimal distraction amplitude, the samples were cyclically loaded first in ramps and then in sinusoidal oscillations at frequencies ranging from 0.2 to 5 Hz. In the present study the following observations were made: (1) Imaging and the laser sintering technique can be used successfully to fabricate custom-made support plates for cross-sectional root-PDL-bone sections using a laser sintering technique, (2) the load-response curves were symmetric in the apical and the coronal directions, (3) both the stress response versus phase angle and the stress response versus. strain curves tended to "straighten" with increasing frequency, and (4) the phase lag between applied strain and resulting stress was small and did not differ in the intrusive and the extrusive directions. As no mechanical or time-dependent anisotropy was demonstrable in the intrusive and extrusive directions, such results may considerably simplify the development of constitutive laws for the PDL.

Relationship between the tooth eruption and regional blood flow in angiotensin II-induced hypertensive rats

OBJECTIVE

The mechanism of action of vasoactive drugs on tooth movement is unknown. The purpose of the present study was to measure simultaneously the axial movement of the mandibular incisor, regional blood flow at the base of the incisor, and systemic arterial blood pressure in angiotensin II-induced hypertensive rats to determine the possible cause of tooth displacement.

DESIGN

The measurements were made under artificial respiration with halothane anaesthesia. In the experimental animals, 2.5 microg of angiotensin II in 1 ml of Ringer's solution was infused at 0.83 ml/h for 12 h from the femoral vein. In the control animals, only Ringer's solution was infused.

RESULTS

Angiotensin II caused an increase of the mean arterial blood pressure from 86 to 119 mm Hg, and decreases of the eruption rate from 667 to 494 microm/24 h and the regional blood flow from 262 to 214 mV. There was a positive correlation between the eruption rate and regional blood flow, and a negative correlation between the blood pressure and regional blood flow.

CONCLUSION

These results suggest that angiotensin II caused constriction of the peripheral vascular smooth muscle resulting in an increase of arterial blood pressure and a decrease of regional blood flow, followed by a decrease of fluid volume and then a reduction of either the pressure within the socket or of the eruptive force. We assume that the regional vascular pressure within the socket plays an important role in determining the position of the rat incisor.

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.