Osteoclast clearance from periodontal tissues during orthodontic tooth movement

THE EFFECT OF BISPHOSPHONATE RISEDRONATE HYDROGEL ON ALKALINE PHOSPHATASE AND OSTEOCLASTS DURING RELAPSE MOVEMENT

Objectives: To analyze the effect of risedronate hydrogel on enzyme alkaline phosphatase (ALP) and osteoclast/osteoblast ratio during tooth relapse movement Materials and methods: The research design is experimental with time series. The lower incisors of 75 guinea pigs are distally moved using open coil spring. The guinea pigs were divided into three groups: without risedronate (group A; n = 25); given 250 µmol/L of risedronate hydrogel (group B; n = 25), and given 500 µmol/L of risedronate hydrogel (group C; n = 25). Risedronate were applied intrasulcularly in the mesial part of the gingival sulcus every 3 days. After 14 days of stabilization, the open coil spring was removed (bisphosphonate administration was continued). The relapsed teeth and ALP levels on days 0, 3, 7, 14, and 21 were measured. The osteoclast/osteoblast ratio was measured by hematoxylin and eosin staining. ANOVA test was used to determine the difference in the three groups and their interactions with concentration and time. Results: There was a significant difference in osteoclast/osteoblast ratio on day 3 (p = 0.019, p

Involvement of Noncoding RNAs in the Differentiation of Osteoclasts

As the most important bone-resorbing cells, osteoclasts play fundamental roles in bone remodeling and skeletal health. Much effort has been focused on identifying the regulators of osteoclast metabolism. Noncoding RNAs (ncRNAs) reportedly regulate osteoclast formation, differentiation, survival, and bone-resorbing activity to participate in bone physiology and pathology. The present review intends to provide a general framework for how ncRNAs and their targets regulate osteoclast differentiation and the important events of osteoclastogenesis they are involved in, including osteoclast precursor generation, early differentiation, mononuclear osteoclast fusion, and multinucleated osteoclast function and survival. This framework is beneficial for understanding bone biology and for identifying the potential biomarkers or therapeutic targets of bone diseases. The review also summarizes the results of in vivo experiments and classic experiment methods for osteoclast-related researches.

The intrasulcular application effect of bisphosphonate hydrogel toward osteoclast activity and relapse movement

Introduction

Orthodontic relapse occurs after orthodontic treatment and shifting of teeth to unfavorable positions. Bisphosphonates’ effects on bone resorption and relapse prevention have been extensively investigated. However, topical administration, which results in local effect, is still a problem.

Objective

This study aimed to investigate the effect of risedronate with gelatin hydrogel as a carrier to prevent relapse movement by inhibiting osteoclast activity.

Methods

Lower incisors of 75 guinea pigs were moved distally using an orthodontic appliance until ±3 mm length. Gelatin hydrogel was fabricated to obtain a semisolid controlled release of 250 (Bis-CR250) and 500 mmol/L risedronate (Bis-CR500) and then applied intrasulcularly into the mesial subperiosteal area of 50 guinea pigs (25 in each group) every 3 days; the rest were the control (Bis-CR000). After 14 days of stabilization, the apparatus was removed. The distance decrease between incisors and the osteoclast number with TRAP staining at 0, 3, 7, 14, and 21 days were measured. ANOVA was used to determine the differences among the different time and experimental groups.

Results

Both treatments showed significantly less relapse movement compared to the control (p < 0.05) at 14 and 21 days. Bis-CR500 more effectively inhibited the relapse movement than Bis-CR250 on day 21, indicating a dose dependency in the inhibition. Both treatments showed less osteoclast numbers than control (p < 0.05).

Conclusion

Controlled release of bisphosphonate risedronate with a topically administered gelatin hydrogel has shown to be effective in decreasing the tooth relapse movement and osteoclast activity.

Clastic cells in orthodontic treatment: Translational challenges and recent advances

OBJECTIVES

Orthodontic treatment consists of numerous appliance activations that rely on stimulation of osteoclasts at alveolar bone sites. However, the action of osteoclast-like cells on dentin ("odontoclasts") is a pathological side effect of orthodontic treatment. The aim of this article is twofold: (a) To report preliminary results from ongoing cell culture experiments to identify unique markers of dentin resorption, and (b) To discuss our work using nanoparticle tracking analysis (NTA) and exosomes for developing biological fluid-based biopsies to monitor clastic cell activity.

SETTING AND SAMPLE POPULATION

Twelve healthy volunteers in permanent dentition.

MATERIAL AND METHODS

For the in vitro experiments, murine clastic cell precursors were cultured on dentin or bone slices for 7 days and phage-display biopanning was used to identify molecular surface differences between osteoclasts and odontoclasts. In the human study, gingival crevicular fluid (GCF) samples were collected using different tools and analysed for protein and exosome recovery.

RESULTS

Biopanning generated antibody fragments that were uniquely reactive to odontoclasts. Numerous nanoparticles in the size range of exosomes were detected in all of the human GCF samples.

CONCLUSIONS

Our results support that there are molecular differences between osteoclasts and odontoclasts. Emerging technologies may allow the use of exosomes in GCF as a clinical tool to detect markers of root resorption.

Tissue responses resulting from tooth movement surgically assisted by corticotomy and corticision in rats

OBJECTIVE

To compare the histological responses in corticotomy- and corticision-assisted tooth movement.

MATERIALS AND METHODS

Ninety Wistar rats were divided into three groups: C (control-tooth movement only), CT (tooth movement + corticotomy), and CI (tooth movement + corticision). Surgeries were performed on the vestibular and lingual cortical bone of the maxillary first molar. Tooth movement was carried out with nickel-titanium closed coil springs having a force of 30 g. The rats were sacrificed at 3, 14, and 28 days. To evaluate the number of osteoclasts and amount of root resorption, a tartrate-resistant acid phosphatase stain was used. Hematoxylin and eosin staining was performed for areas of hyalinization, and the organic bone matrix was stained with picrosirius.

RESULTS

The CT group showed a greater number of osteoclasts than did the C group on day 3 (P < .05). At the same time point, the CT and CI groups showed a delayed onset of organic bone matrix remodeling and a lower incidence of root resorption than did the C group (P < .05). There were also fewer hyalinization areas in the CI group than in the C group on day 3 (P < .05).

CONCLUSIONS

Corticotomy effectively increased bone resorption during the early stages of tooth movement, but this increase was not observed for corticision. The surgical procedures did not accelerate organic bone matrix remodeling. Corticotomies and corticisions decreased the risk of root resorption only during the early stages of movement. Corticision reduced the level of hyalinization, while corticotomy did not.

Accelerated orthodontic tooth movement following le fort I osteotomy in a rodent model

PURPOSE

In surgery-first accelerated orthognathic surgery, the clinical phenomenon of accelerated orthodontic tooth movement after osteotomy is a benefit compared with the conventional approach. However, because much of the literature on this phenomenon is based on empirical evidence and case reports, experimental animal-based studies are needed to verify and quantify this acceleration effect. The purpose of this prospective experimental study was to identify whether osteotomy procedures increase tooth movement.

MATERIAL AND METHODS

Le Fort I osteotomies were performed on the left maxillas in 15 male adult Sprague-Dawley rats. After surgery, a continuous force of 0.5 N was placed on the maxillary left first molar to move the tooth mesially. Another 15 rats had no surgery and served as controls. On days 1, 14, and 28, digital caliper measurements were taken to record tooth movement.

RESULTS

In the experimental group, the maxillary left first molars moved significantly more rapidly on days 14 and 28 (P < .05). Histologic findings showed more active alveolar bone remodeling.

CONCLUSION

Le Fort I osteotomy significantly accelerated the rate of orthodontic tooth movement. Histologically, more active and extensive bone remodeling was observed after osteotomy.

Th17-cells in atopic dermatitis stimulate orthodontic root resorption

OBJECTIVE

The aim of this study was to investigate how atopic dermatitis (AD) contributes to root resorption during orthodontic tooth movement.

MATERIALS AND METHODS

Atopic dermatitis model mice and wild-type mice were subjected to an excessive orthodontic force (OF) to induce movement of the upper first molars. The expression levels of the tartrate-resistant acid phosphatase (TRAP), IL-17, IL-6, and RANKL proteins were determined in the periodontal ligament (PDL) by an immunohistochemical analysis. Furthermore, the effects of the compression force on co-cultures of CD4(+) cells from AD patients or healthy individuals and human PDL cells were investigated with regard to the levels of secretion and mRNA expression of IL-17, IL-6, RANKL, and osteoprotegerin.

RESULTS

The immunoreactivities for TRAP, IL-17, IL-6, and RANKL in the AD group were found to be significantly increased. The double immunofluorescence analysis for IL-17/CD4 detected immunoreaction. The secretion of IL-17, IL-6, and RANKL, and the mRNA levels of IL-6 and RANKL in the AD patients were increased compared with those in healthy individuals.

CONCLUSION

Th17 cells may therefore be associated with the deterioration of root resorption of AD mice, and may explain why AD patients are more susceptible to root resorption than healthy individuals when an excessive OF is applied.

Tisssue responses in corticotomy- and osteotomy-assisted tooth movements in rats: histology and immunostaining

INTRODUCTION

The purpose of this histologic study was to examine underlying cellular responses to corticotomy- and osteotomy-assisted tooth movements.

METHODS

Thirty-six rats were divided into 5 groups: corticotomy-assisted tooth movement (CO + TM), sham corticotomy without tooth movement (CO alone), osteotomy-assisted tooth movement (OS + TM), sham osteotomy without tooth movement (OS alone), and unassisted tooth movement (TM alone). Standard orthodontic springs were activated to produce mesial tooth movement. The rats were killed at 3, 21, and 60 days after activation for osteoclast and blood vessel counts, and immunostaining with proliferating cell nuclear antigen (PCNA), transforming growth factor beta 1 (TGF beta 1), vascular endothelial growth factor (VEGF), and osteocalcin were performed.

RESULTS

The CO + TM group had significantly more osteoclasts at 3 days (P <0.005) compared with the OS + TM group. The alveolar bone surrounding the dental roots was replaced with multicellular tissue at 21 days in the CO + TM group but was intact in the OS + TM group with the exception of a distal distraction site. At day 21, immunostaining with PCNA, TGF beta 1, VEGF, and osteocalcin occurred at the mesial border of bone in the CO + TM group, whereas a diffuse pattern was observed in the distal distraction sites at 21 and 60 days in the OS + TM group.

CONCLUSIONS

Corticotomy-assisted tooth movement produced transient bone resorption around the dental roots under tension; this was replaced by fibrous tissue after 21 days and by bone after 60 days. Osteotomy-assisted tooth movement resembled distraction osteogenesis and did not pass through a stage of regional bone resorption.

Biologic response of rapid tooth movement with periodontal ligament distraction

INTRODUCTION

Length of treatment is a complaint of many orthodontic patients. The purpose of this study was to evaluate the security and feasibility of rapid tooth movement with periodontal ligament distraction.

METHODS

Eight male beagles, aged 13 to 16 months, were used in this study. Extraction of the mandibular second premolar and alveolar surgery to reduce the osteal resistance on the mesial side of the extraction socket were performed on the experimental side. Then a device was placed to distract the first premolars distally on the experimental side; on the control side, the first premolars were distalized with nickel-titanium coil springs. The beagles were killed in the first, second, fourth, and eighth weeks after orthodontic force application.

RESULTS

The first premolar on the experimental side moved more rapidly than that on the control side (P <0.05). Histologic data indicated that more new bone was deposited on tension area of the experimental side than on the control side. Active and extensive bone resorption in the compressive area and bone deposition in the tension area were observed on the experimental side.

CONCLUSIONS

These results suggest that the periodontal ligament can be rapidly distracted without complications. The rapid orthodontic tooth movement by distracting the periodontal ligament cannot be emulated by current conventional orthodontic concepts and methods.

Anabolic effect of intermittent PTH(1-34) on the local microenvironment during the late phase of periodontal repair in a rat model of tooth root resorption

This study examined the histological changes and possible effects of intermittent parathyroid hormone (PTH) (1-34) treatment during the early and late phase of periodontal repair in a rat model of tooth root resorption. In a total of 70 animals, which either received intermittent PTH(1-34) systemically or sham injections for up to 70 days after discontinuation of an orthodontic force, histological characteristics were correlated to time-dependent distinct expression patterns of osteoprotegerin and receptor activator of nuclear factor kappaB ligand by PDL cells in the former compression and tension side of tooth movement by means of immunohistochemistry and histomorphometrical analysis. The balance of these key regulators of bone remodeling was demonstrated to be shifted in favor of hard tissue repair by intermittent PTH administration, which was demonstrated to exert anabolic effects in several cell culture and animal experiments as well as in humans, in the late phase of repair. These data indicate a role for PDL cells as potent regulators of periodontal repair by modifying the local microenvironment and point to the anabolic potential of an intermittent PTH administration to support these reparative processes.

Osteoclast differentiation during experimental tooth movement by a short-term force application: an immunohistochemical study in rats

OBJECTIVE

The origin of osteoclasts responsible for bone resorption during orthodontic tooth movement is not yet clear. Their precursors may reside within the periodontal ligament (PDL) or could be recruited from the circulation or the bone marrow. The aim of this study was to investigate the spatial and sequential distribution of osteoclast precursors during experimental tooth movement by using three differentiation markers: receptor for macrophage colony stimulating factor (c-Fms), receptor activator of nuclear factor-kappaB (RANK), and calcitonin receptor (CTR).

MATERIAL AND METHODS

Six-week-old Wistar rats were used. Elastic bands were inserted between the upper 1st and 2nd molars for 1, 2, 3, and 6 days. Immunohistochemical staining for c-Fms, RANK, or CTR was performed on parasagittal sections and positive cells were counted.

RESULTS

Before force application, many c-Fms+ and a few RANK+ precursors were present in the bone marrow. No c-Fms+ osteoclast precursors were observed in the PDL. After force application, the number of RANK+ but not c-Fms+ precursors increased rapidly in the PDL. In bone marrow, the number of c-Fms+ and RANK+ precursors also increased rapidly, as did multinuclear c-Fms+, RANK+, and CTR+ cells. Subsequently, the number of c-Fms+, RANK+, and CTR+ multinuclear cells in the PDL increased. After 6 days, the expression profiles tended to return to baseline levels.

CONCLUSION

Osteoclast precursors differentiate within the bone marrow and then migrate into the PDL during early tooth movement.

Cellular and extracellular factors in early root resorption repair in the rat

The aim of this study was to investigate the role of extracellular matrix components, such as collagen type I, fibronectin, and osteopontin (OPN) during cementum repair following experimentally induced tooth movement, and to characterize the cells taking part in the regenerative process. The upper right first molars were moved mesially in 21 three-month-old male Wistar rats using a coil spring with a force of 0.5 N. After 9 days, the appliance was removed and the animals were killed in groups of three immediately after withdrawal of the force and 5, 7, 10, 12, 14, and 17 days later. Three rats served as non-experimental control animals. The maxillae were prepared and processed for histological analysis. Together with the disappearance of the multinucleated odontoclasts from the resorption lacunae, signs of repair were visible 5 days after the release of the orthodontic force. The first signs of cementum repair were seen on day 10. The newly produced cementum was of the acellular extrinsic fibre type (AEFC) and reattachment was achieved with the principal periodontal ligament (PDL) fibres orientated almost perpendicular to the root surface. The initial interface formed between the old and new cementum, as well as the new AEFC, was characterized by a strong immunoreaction with OPN and collagen I antibody, but only a weak immunoreaction with the fibronectin antibody. Only a small number of mononuclear cells, which were involved in the repair process, showed a positive immunoreaction with the osteoblastic lineage markers runt-related transcription factor 2 and osteocalcin. These same cells stained sparsely with muscle segment homeobox homologue 2, but not with the E11 antibody. Thus, most of the cells associated with this reparative activity on the surface of the lacunae were differentiated PDL cells of the fibroblastic phenotype. Cells with a defined osteoblastic phenotype seemed to be of minor importance in this repair process.

Mechanisms of tooth eruption and orthodontic tooth movement

Teeth move through alveolar bone, whether through the normal process of tooth eruption or by strains generated by orthodontic appliances. Both eruption and orthodontics accomplish this feat through similar fundamental biological processes, osteoclastogenesis and osteogenesis, but there are differences that make their mechanisms unique. A better appreciation of the molecular and cellular events that regulate osteoclastogenesis and osteogenesis in eruption and orthodontics is not only central to our understanding of how these processes occur, but also is needed for ultimate development of the means to control them. Possible future studies in these areas are also discussed, with particular emphasis on translation of fundamental knowledge to improve dental treatments.

Decrease in the number and apoptosis of alveolar bone osteoclasts in estrogen-treated rats

BACKGROUND AND OBJECTIVE

Bone is a mineralized tissue that is under the influence of several systemic, local and environmental factors. Among systemic factors, estrogen is a hormone well known for its inhibitory function on bone resorption. As alveolar bone of young rats undergoes continuous and intense remodeling to accommodate the growing and erupting tooth, it is a suitable in vivo model for using to study the possible action of estrogen on bone. Thus, in an attempt to investigate the possibility that estrogen may induce the death of osteoclasts, we examined the alveolar bone of estrogen-treated rats.

MATERIAL AND METHODS

Fifteen, 22-d-old female rats were divided into estrogen, sham and control groups. The estrogen group received estrogen and the sham group received corn oil used as the dilution vehicle. After 8 d, fragments containing alveolar bone were removed and processed for light microscopy and transmission electron microscopy. Sections were stained with hematoxylin and eosin and tartrate-resistant acid phosphatase (TRAP)-an osteoclast marker. Quantitative analysis of the number of TRAP-positive osteoclasts per mm of bone surface was carried out. For detecting apoptosis, sections were analyzed by the Terminal deoxynucleotidyl transferase-mediated dUTP Nick-End Labeling (TUNEL) method; TUNEL/TRAP combined methods were also used.

RESULTS

The number of TRAP-positive osteoclasts per mm of bone surface was significantly reduced in the estrogen group compared with the sham and control groups. TRAP-positive osteoclasts exhibiting TUNEL-positive nuclei were observed only in the estrogen group. In addition, in the estrogen group the ultrastructural images revealed shrunken osteoclasts exhibiting nuclei with conspicuous and tortuous masses of condensed chromatin, typical of apoptosis.

CONCLUSION

Our results reinforce the idea that estrogen inhibits bone resorption by promoting a reduction in the number of osteoclasts, thus indicating that this reduction may be, at least in part, a consequence of osteoclast apoptosis.

Osteoclast distribution within the rat interdental septum coincident to experimental tooth movement using light forces

There is little quantitative information concerning the number and distribution of bone-matrix resorbing osteoclasts (BMRO) within the adjacent alveolar bone coincident to tooth movement. We moved the right first maxillary molar tooth anteriorly in 40 female rats (E), the left side was untreated, serving as an internal control (IC). Forty female age and weight matched rats were untreated, serving as external controls (EC). BMRO were identified on periosteal and endosteal surfaces of the interdental septum from 1-5 days after initial force application using the MTB-322 antibody. The number of BMRO at periosteal surfaces of E was greater in IC and EC from 2-5 days (P < 0.001). Similarly, the number of BMRO at endosteal surfaces was greater from 1-5 days (P < 0.001). The number of BMRO at periosteal and endosteal surfaces was greater within E than within EC from 1-5 days (P < 0.001). Outcome data from IC were often significantly different from EC. Our data suggest that relatively low forces increase the number of BMRO, which are not uniformly distributed onto both periosteal and endosteal surfaces. Thus, the interdental septum resorbs at the alveolar wall and within spaces between the trabeculae, which, taken together, results in net removal of bone from areas of compression. These data also suggest that experimental tooth movement produces significant differences in the number and distribution of BMRO within IC and EC. Thus, EC groups should be included in studies of tooth movement.

Inhibition of tooth movement by osteoprotegerin vs. pamidronate under conditions of constant orthodontic force

The undesired movement of anchor teeth, and relapse of previously moved teeth, are major clinical problems in orthodontics. Dental implants are increasingly used to preserve anchorage, but these are costly and require invasive surgical procedures. An alternative strategy for maintaining anchorage may be the use of biological inhibitors of osteoclastic bone resorption. In the present study, we investigated the relative efficacy of pamidronate vs. osteoprotegerin (OPG) in inhibiting bone resorption and tooth movement, using a new orthodontic model in mice in which maxillary molars are moved for prolonged periods by near-constant, clinically relevant forces. Osteoclast influx to compression sites was initiated on day 3, was maximal on day 4, and persisted until at least day 12 after force application. Tooth movement paralleled osteoclast numbers. Minimal osteoclast apoptosis was observed, suggesting that recruitment, rather than programmed cell death, is a critical regulatory mechanism under conditions of constant force. Osteoclasts were reduced at compression sites by both OPG (95%) and pamidronate (70%); tooth movement was more dramatically inhibited by OPG (77% vs. 34%). Our findings indicate that constant orthodontic force regulates the recruitment, activation, and viability of osteoclasts, and that OPG could have clinical utility in preventing undesired tooth movement.

Acceleration of orthodontic tooth movement by alveolar corticotomy in the dog

INTRODUCTION

Tooth movement and alveolar bone reaction after corticotomies have not been thoroughly examined. In this study, the effects of corticotomies on orthodontic tooth movement and alveolar bone reaction were investigated in dogs.

METHODS

Corticotomies were performed on the cortical bone of the mandibular left third premolar region in 12 male adult beagles. The third premolars on the left experimental side and on the right sham side were moved mesially with a continuous force of 0.5 N.

RESULTS

Tooth movement velocities from 0 to 1 week and from 1 to 2 weeks after the corticotomies were significantly faster on the experimental side than on the sham side. Hyalinization of the periodontal ligament appeared only at 1 week after the corticotomies on the experimental sides, whereas it was observed from 1 to 4 weeks after the corticotomies on the sham sides. Tartrate-resistant-acid-phosphatase positive cells of the experimental side tended to work vigorously at an early time on the alveolar wall and in the bone marrow cavities.

CONCLUSIONS

Orthodontic tooth movement increased for at least 2 weeks after the corticotomies. This might be brought about by rapid alveolar bone reaction in the bone marrow cavities, which leads to less hyalinization of the periodontal ligament on the alveolar wall.

Cytokine Profiles in Crevicular Fluid During Orthodontic Tooth Movement of Short and Long Durations

BACKGROUND

Orthodontic treatment induces a distortion of the extracellular matrix of the periodontium, resulting in alterations in cytoskeletal configuration. Cytokines are known to facilitate this process by inducing cellular proliferation, differentiation, and stimulation of periodontal remodeling. The aim of the present study was to measure a panel of proinflammatory cytokines (PICs) in crevicular fluid (GCF) samples during tooth movement of short and long durations.

METHODS

Twelve patients (11 to 27 years of age) participated in this study: six patients each for tooth movement of short and long duration. GCF sampling was done at different times, ranging from 24 hours to 4 months after force application. The profiles of PICs were analyzed with a multiplex technique.

RESULTS

PICs were elevated significantly in the early stage of tooth movement but at different time points. Interleukin (IL)-1beta and -6 and tumor necrosis factor-alpha (TNF-alpha) reached significant levels at 24 hours; IL-8 reached a significant elevation at 1 month. During the linear stage of tooth movement, all cytokines were diminished to their baseline levels. The results demonstrated that IL-1beta, -6, and -8 and TNF-alpha play a significant role during the early stage of tooth movement but not during the linear stage.

CONCLUSIONS

Once the microenvironment of periodontal tissue is activated by an orthodontic force, several key PICs are produced to trigger a cascade of cellular events. The periodontal system stabilizes at a new physiological homeostasis as indicated by the downregulation of the early-phase PICs.

Effects of human relaxin on orthodontic tooth movement and periodontal ligaments in rats

INTRODUCTION

The rate-limiting step in orthodontic treatment is often the rapidity with which teeth move. Using biological agents to modify the rate of tooth movement has been shown to be effective in animals. Relaxin is a hormone present in both males and females. Its main action is to increase the turnover of fibrous connective tissues. Thus, relaxin might increase the amount and rate of tooth movement through its effect on the periodontal ligament (PDL). The purpose of this study was to measure the effect of relaxin on orthodontic tooth movement and PDL structures.

METHODS

Bilateral orthodontic appliances designed to tip maxillary molars mesially with a force of 40 cN were placed in 96 rats. At day 0, the animals were randomized to either relaxin or vehicle treatment. Twelve rats in each group were killed at 2, 4, 7, and 9 days after appliance activation. Cephalograms were taken at appliance placement and when the rats were killed. Tooth movement was measured cephalometrically in relation to palatal implants. Fractal analysis and visual analog scale assessments were used to evaluate the effect of relaxin on PDL fiber organization at the tension sites in histologic sections. The in-vitro testing for PDL mechanical strength and tooth mobility was performed by using tissue from an additional 20 rats that had previously received the same relaxin or vehicle treatments for 1 or 3 days (n = 5).

RESULTS

Both groups had statistically significant tooth movement as functions of time. However, relaxin did not stimulate significantly greater or more rapid tooth movement. Fractal and visual analog scale analyses implied that relaxin reduced PDL fiber organization. In-vitro mechanical testing and tooth mobility assessments indicated that the PDL of the mandibular incisors in the relaxin-treated rats had reduced yield load, strain, and stiffness. Moreover, the range of tooth mobility of the maxillary first molars increased to 130% to 170%, over vehicle-treated rats at day 1.

CONCLUSIONS

Human relaxin does not accelerate orthodontic tooth movement in rats; it can reduce the level of PDL organization, reduce PDL mechanical strength, and increase tooth mobility at early time points.

Cellular, molecular, and tissue-level reactions to orthodontic force

Remodeling changes in paradental tissues are considered essential in effecting orthodontic tooth movement. The force-induced tissue strain produces local alterations in vascularity, as well as cellular and extracellular matrix reorganization, leading to the synthesis and release of various neurotransmitters, cytokines, growth factors, colony-stimulating factors, and metabolites of arachidonic acid. Recent research in the biological basis of tooth movement has provided detailed insight into molecular, cellular, and tissue-level reactions to orthodontic forces. Although many studies have been reported in the orthodontic and related scientific literature, a concise convergence of all data is still lacking. Such an amalgamation of the rapidly accumulating scientific information should help orthodontic clinicians and educators understand the biological processes that underlie the phenomenon of tooth movement with mechanics (removable, fixed, or functional appliances). This review aims to achieve this goal and is organized to include all major findings from the beginning of research in the biology of tooth movement. It highlights recent developments in cellular, molecular, tissue, and genetic reactions in response to orthodontic force application. It reviews briefly the processes of bone, periodontal ligament, and gingival remodeling in response to orthodontic force. This review also provides insight into the biological background of various deleterious effects of orthodontic forces.

Immunohistochemical evaluation of osteoclast recruitment during experimental tooth movement in young and adult rats

OBJECTIVE

Orthodontic tooth movement starts slower in adults than in juveniles, but the rate of tooth movement in later phases is the same in both age groups. The hypotheses to be tested are that these phenomena are related to slower osteoclast recruitment in adults than in juveniles, but that in later phases the osteoclast numbers are the same in both age groups.

DESIGN

Standardized orthodontic tooth movement was performed in two groups of 30 rats, aged 6 weeks and 9-12 months, respectively. All maxillary molars at one side were together moved mesially by a continuous force of 10 cN. The other side served as a control. After 1, 2, 4, 8 and 12 weeks, groups of animals were killed. After ED1 staining osteoclast numbers at the mesial and distal sides of selected roots were counted.

RESULTS

At the compression sides osteoclast numbers increased in both age groups. In young rats, a maximum was reached at 2 weeks, in adults at 4 weeks. In later phases of tooth movement, the number of osteoclasts in the adults was approximately twice as high as in the juveniles, while the rate of tooth movement was the same. A positive correlation between the rate of tooth movement and osteoclast numbers was found only in young rats.

CONCLUSIONS

Orthodontic forces induce faster osteoclast recruitment in young than in adult rats and more osteoclasts are needed to achieve a certain rate of tooth movement in adult than in young rats.

Apoptosis in the regenerate produced by mandibular osteodistraction in the mature rat*

AUTHORS: Liu ZJ, Anderson MW, Gu GM, King GJ.

OBJECTIVE

Little is known about the contribution of apoptosis to the formation of the regenerate in distraction osteogenesis. This study investigated the role of apoptosis in the regulation of osteogenesis in relation to mandibular distraction rate and recovery time.

METHODS

One hundred and twenty-nine 3-month rats received unilateral mandibular ramus osteotomy and distraction device placement. After 3-day latency, these were distracted for 5 days. The slow group was distracted a total of 1 mm (0.2 mm/day), the moderate 2 mm (0.4 mm/day), the rapid 3 mm (0.6 mm/day), and the sham was not distracted (0.0 mm/day). Eight to nine rats from each group were killed at 6 (mid-distraction), 10 (early-consolidation), 24 (mid-consolidation) and 38 (late-consolidation) days following device placement. Baseline data were obtained from an additional eight rats killed at day 3 (end of latency). Sagittal sections (7 mum) of the harvested hemi-mandibles were embedded in paraffin, double-stained with the DeadEnd(TM) Colorimetric TUNEL system for apoptotic cells and tartrate-resistant acid phosphatase for osteoclasts. Cell counts of apoptotic cells and osteoclasts (apoptotic or otherwise) were preformed at 40x magnification using a calibrated grid at the middle regions of the caudal, central and rostral parts of the distraction gap. Counting reproducibility was verified as <13%.

RESULTS

Counts from all three regions were combined because no significant regional difference was found. The majority of the fibrous matrix regenerate was seen at days 6 and 10 while osseous regenerate occurred mainly at days 24 and 38. Significantly higher levels of apoptotic activity were seen at day 24. Apoptotic osteoclasts also peaked at day 24. Apoptotic cell numbers in the slow and moderate groups most closely followed the pattern of the sham in which the apoptosis activity peaked at days 10 or 24. However, those in the rapid group showed delayed peaks at days 24 or 38.

CONCLUSIONS

Chalmers Street, Surry Hill, NSW 2010, Australia. gshe6437@mail.usyd.edu.au The transition from fibrous to osseous regenerate during mandibular distraction osteogenesis is accompanied by an increase in cell clearance via apoptosis. A slow to moderate distraction rate allows for the most typical pattern of bone healing while a rapid rate prolongs the healing process.