[Root resorption as a result of orthodontic treatment]. / Wortelresorptie als gevolg van orthodontische behandeling.

Apical root resorption is a biological process induced when orthodontic force is exerted on a tooth and local necrosis of the periodontal ligament occurs. Macrophages remove the necrotic tissue. In this way, differentiating osteoclasts can both attach to the now available dental surface and can then provoke root resorption. There is considerable uncertainty among dental practitioners on how to deal with clinically relevant apical root resorption (bigger/equal 2 mm) during or after orthodontic treatment. To increase understanding and to improve the quality of care, the Dutch Association of Orthodontists has developed a clinical practice guideline. Recommendations have been formulated for the diagnosis of apical root resorption, possible risk factors and treatment management in order to respond adequately to this problem in practice.

[Mechanisms of tooth eruption]. / Mechanismen van tanderuptie.

Tooth eruption is of the utmost importance for the normal development of the dentition and the face. Since the 1980s, it has been known that the tooth germ itself is not essential for facilitating the processes that make tooth eruption possible. For that reason, recent research on the regulatory mechanisms of tooth eruption has focused mainly on the enamel organ and the dental follicle. Different regulatory mechanisms act on the occlusal and the apical sides of an erupting tooth. On the occlusal side osteoclast differentiation is stimulated. This leads to the development of an eruption canal, a process in which macrophages and matrix metalloproteases also play an important role. On the apical side the most important factors are the transcription factor RUNX2 and the bone morphogenic protein 2. They are responsible for the deposition of trabecular bone in that area. Many regulatory mechanisms which are involved in tooth eruption are also active in other developmental processes. This explains that certain syndromes can also have an effect on the tooth eruption process.

Local variations in turnover of periodontal collagen fibers in rats.

BACKGROUND AND OBJECTIVE: The exact cause of orthodontic relapse is still unclear, although it is often suggested to be caused by periodontal collagen fibers. We hypothesize that long-lived collagen fibers in the periodontium cause relapse. The aim was to determine the half-life of periodontal collagen fibers around rat molars. MATERIAL AND METHODS: Thirty weanling rats were repeatedly injected with (3) H-proline, and autoradiography of histological sections was performed at 1, 4, 8, 15, 22, 29, 36, 57, 78 and 113 d after labeling. Grain densities determined in specific areas of the periodontium were used to calculate collagen half-life. RESULTS: The half-life (t(½) ) was found to decrease from the supra-alveolar region to the apical periodontal ligament region. It was longer in the supra-alveolar region (1.39 ± 0.14 wk) compared with the deeper regions (p < 0.05). The t(½) of the upper periodontal ligament region (0.78 ± 0.20 wk) was longer than that of the inter-radicular periodontal ligament region (0.42 ± 0.07 wk, p < 0.05). The t(½) of the apical periodontal ligament region was 0.61 ± 0.15 wk. CONCLUSION: The data indicate that long-lived collagen fibers do not exist in the soft tissues of the periodontium, and are probably not responsible for relapse. The differences in collagen half-life might be caused by local variations in compressive strain induced by normal function.

The biomechanical behaviour of the hyalinized periodontal ligament in dogs during experimental orthodontic tooth movement.

During orthodontic tooth movement, the mechanical behaviour of the extracellular matrix of the periodontal ligament (PDL) determines the cellular processes involved in turnover of the PDL and alveolar bone. This mechanical behaviour is the basis for finite element (FE) models and FE analyses. Five young adult male beagle dogs were used to test the null hypothesis that the mechanical behaviour of the PDL is identical in normal and hyalinized PDL. Therefore, tooth transposition was measured after standardized force application by super-elastic nickel titanium (NiTi) coil springs, exerting a constant force of 100 cN for 5 hours in both conditions. A rapid transposition during the first few seconds was found. However, it was significantly less for hyalinized than for non-hyalinized PDL. Subsequently, a short-lived creep movement was found for hyalinized PDL, while creep persisted at the non-hyalinized sides (analysis of variance and Tukey's multiple comparisons post hoc tests). The results showed substantial biomechanical differences between hyalinized and non-hyalinized PDL at different time points (Mann-Whitney). This indicates that FE models in the study of long-term orthodontic tooth movement, which are based solely on the characteristics of normal PDL should be reconsidered.

Rate of orthodontic tooth movement after changing the force magnitude: an experimental study in beagle dogs.

OBJECTIVES: To study a possible dose-response relation between force magnitude and rate of orthodontic tooth movement by altering forces during bodily orthodontic tooth movement. SETTING AND SAMPLE POPULATION: Eight young adult beagle dogs were used. The experiments were carried out in the Central Animal Facility, and all analyses were conducted in the Department of Orthodontics and Oral Biology, Radboud University Nijmegen Medical Centre. MATERIALS AND METHODS: Orthodontic appliances were placed exerting a reciprocal force on the mandibular second premolars and first molars. A force of 10 or 300 cN was randomly assigned to each side of the dogs. After 22 weeks, all forces were changed to 600 cN. Based on intra-oral measurements, tooth movement rates were calculated. RESULTS: The premolars showed no difference in the rates of tooth movement with 10 or 300 cN. Replacing 10 for 600 cN increased the rate, but replacing 300 for 600 cN did not. Molars moved faster with 300 than with 10 cN, and changing both forces to 600 cN increased the rate of tooth movement. Data from all teeth were pooled considering their relative root surfaces, and a logarithmic relation was found between force and rate of tooth movement. CONCLUSIONS: Only in the very low force range, a positive dose-response relation exists, while in higher force ranges, no such relation could be established.

Orthodontic force stimulates eNOS and iNOS in rat osteocytes.

Mechanosensitive osteocytes are essential for bone remodeling. Nitric oxide, an important regulator of bone remodeling, is produced by osteocytes through the activity of constitutive endothelial nitric oxide synthase (eNOS) or inducible nitric oxide synthase (iNOS). We hypothesized that these enzymes regulate the tissue response to orthodontic force, and therefore we investigated eNOS and iNOS expression in osteocytes during orthodontic force application. The upper rat molars were moved mesially by NiTi coil springs (10 cN, 120 hrs) in a split-mouth design. Immunohistochemical staining revealed that, in the tension area, eNOS-positive osteocytes increased from 24 hrs on, while iNOS-positive osteocytes remained largely constant. In the compression area, iNOS-positive osteocytes increased after 6 hrs, while eNOS- positive osteocytes increased after 24 hrs. This suggests that eNOS mediates bone formation in the tension area, while iNOS mediates inflammation-induced bone resorption in the compression area. Both eNOS and iNOS seem to be important regulators of bone remodeling during orthodontic force application.

Mechanobiology of tooth movement.

This review describes the mechanical and biological signalling pathways during orthodontic tooth movement and provides an update of the current literature. A theoretical model is introduced to elucidate the complex cascade of events after the application of an orthodontic force to a tooth. In this model, the events are divided into four stages: matrix strain and fluid flow, cell strain, cell activation and differentiation, and remodelling. Each stage is explained in detail and discussed using recent literature.

Influence of panel composition on aesthetic evaluation of adolescent faces.

The objective of this study was to evaluate the influence of professional background, age, gender, and geographical region of panel members on their evaluation of the facial aesthetics of adolescents, and to assess the optimal panel size for epidemiological studies on facial aesthetics. A panel of 76 adult laymen from two different regions (Belgium and The Netherlands) and a panel of 89 orthodontists from the same two regions, evaluated photographic sets (one frontal, one three-quarter smiling, and one lateral view) of 64 adolescents (32 boys, 32 girls) on a visual analogue scale (VAS) in relation to a reference set of photographs. The effects of the characteristics of the panel members on the VAS scores for boys and girls separately, as well as their interactions, were evaluated by multilevel models. The adolescents entered the model as a random effect and four characteristics of the panel members were included in the model as fixed effects. The multilevel model with main effects and first-order interactions revealed that laymen rated adolescents as more attractive than orthodontists. This finding was significant for all laymen, except for older males, and Belgian laymen, when rating girls. Older panel members rated boys significantly more attractive than younger panel members. Males rated adolescents more attractive than females. The latter was significant for all male subgroups, except for the lay male subgroup. There were regional differences. Based on the intraclass correlation coefficient, a panel of seven randomly selected laymen and/or orthodontists is sufficient to obtain reliable results in the aesthetic evaluation of adolescent faces, using photographs and a VAS.

CMT-3 inhibits orthodontic tooth displacement in the rat.

OBJECTIVE: Orthodontic tooth movement requires extensive remodeling of the periodontal ligament (PDL) and the alveolar bone. Osteoclasts resorb bone, allowing teeth to migrate in the direction of the force. Matrix metalloproteinases (MMPs) are able to degrade the extracellular matrix of the periodontal tissues. Chemically modified tetracyclines (CMTs) can inhibit MMPs, but lack antimicrobial activity. We hypothesize that CMT-3 will decrease the rate of orthodontic tooth movement in the rat. DESIGN: Eighteen Wistar rats received a standardized orthodontic appliance at one side of the maxilla. During 14 days, three groups of six rats received a daily dose of 0, 6 or 30mg/kg CMT-3, and tooth displacement was measured. Thereafter, osteoclasts were counted on histological sections using an ED-1 staining. Multi- and mononuclear ED-1-positive cells in the PDL were also counted. In addition, sections were stained for MMP-9. RESULTS: CMT-3 significantly inhibited tooth movement (p=0.03) and also decreased the number of osteoclasts at the compression sides in the 30mg/kg group (p<0.05). Significantly more mono- than multinuclear ED-1-positive cells were present in the PDL, but no significant differences were found between the dosage groups. Osteoclasts in the 30mg/kg group seemed to contain less MMP-9 than in the control. CONCLUSIONS: CMT-3 inhibits tooth movement in the rat, probably by reducing the number of osteoclasts at the compression side. This might be due to induction of apoptosis in activated osteoclasts or reduced osteoclast migration. Reduced MMP activity by CMT-3 might also directly inhibit degradation of the organic bone matrix.

Biomechanical behaviour of the periodontal ligament of the beagle dog during the first 5 hours of orthodontic force application.

The aim of this study was to describe the mechanical behaviour of the periodontal ligament (PDL) in response to loading with different forces for a period of 5 hours. Seven young adult male beagle dogs (age 1.0-1.5 years) were used. After extractions and placement of implants, custom-made appliances on both sides of the mandible were used to measure the displacement of the second premolars. Tooth displacement was measured during 5 hours of force application. Each dog underwent two measurement sessions. One premolar was moved with a force of 100 cN in the first session and with 50 cN in the second. The contralateral premolar was moved with forces of 100 and 300 cN, respectively. Time-displacement curves showed a rapid instantaneous response lasting only a few seconds followed by a slowly decreasing creep displacement. The instantaneous response demonstrated a large individual variability, caused by both a dog and a force effect. Differences in tooth and PDL anatomy and in the orientation of the periodontal fibres are probably important in this respect. The individual variability faded after the first seconds of tooth displacement, when the viscoelastic properties of the periodontal fibres became more pronounced. The force effect was non-linear for the first minute. Higher forces did not lead to proportionally larger displacements. The non-linearity decreased in the second response. The PDL is a complex material that might be considered as a non-linear fibre-reinforced poroviscoelastic material.

Fluid shear stress inhibits TNFalpha-induced osteocyte apoptosis.

Bone tissue can adapt to orthodontic load. Mechanosensing in bone is primarily a task for the osteocytes, which translate the canalicular flow resulting from bone loading into osteoclast and osteoblast recruiting signals. Apoptotic osteocytes attract osteoclasts, and inhibition of osteocyte apoptosis can therefore affect bone remodeling. Since TNF-alpha is a pro-inflammatory cytokine with apoptotic potency, and elevated levels are found in the gingival sulcus during orthodontic tooth movement, we investigated if mechanical loading by pulsating fluid flow affects TNF-alpha-induced apoptosis in chicken osteocytes, osteoblasts, and periosteal fibroblasts. During fluid stasis, TNF-alpha increased apoptosis by more than two-fold in both osteocytes and osteoblasts, but not in periosteal fibroblasts. One-hour pulsating fluid flow (0.70 +/- 0.30 Pa, 5 Hz) inhibited (-25%) TNF-alpha-induced apoptosis in osteocytes, but not in osteoblasts or periosteal fibroblasts, suggesting a key regulatory role for osteocyte apoptosis in bone remodeling after the application of an orthodontic load.

[Dissertations 25 years after date 9. How is tooth eruption regulated?]. / Proefschriften 25 jaar na dato 9. Hoe wordt tanderuptie gereguleerd?

A lot of attention has been paid to the questions of how and why teeth erupt. In the past many theories were developed, all of which showed mechanistic characteristics and suggested that a certain structure exerts force on the tooth germ to initiate its eruption. The dominant theory considered the collagenous fibres or the fibroblasts within the periodontal ligament to be the primary moving force in the eruption process. However, most research was done on continuously erupting incisors of rodents or lagomorphs, an experimental model with serious drawbacks. Because dogs, like humans, have teeth with limited eruption, 25 years ago research was carried out on tooth eruption in beagles. One of the most important conclusions of this study was that the periodontal ligament is not the primary moving force in tooth eruption, as its development only begins at the end of the eruption process. In subsequent years several others have focused their research on tooth eruption in beagles. The current state of knowledge in this field can be summarized as follows: the reduced enamel epithelium and the dental follicle control bone deposition and resorption around an erupting tooth germ, enabling its occlusal movement; the periodontal ligament develops only after its emergence in the oral cavity, and is thus not important in the eruption process; the tooth itself does not play a role in the regulation of its eruption.

Objective measures as indicators for facial esthetics in white adolescents.

The objective of this study was to examine the contribution of objective measures representing anterior-posterior and vertical characteristics, dental esthetics, or their combination that are used in daily orthodontic practice in the assessment of facial esthetics. A panel of 78 laymen evaluated facial esthetics of 32 boys and 32 girls, stratified over the four Angle classes, on a visual analogue scale. The relation between the objective parameters and facial esthetics was evaluated by backward multiple regression analysis. Dental esthetics as expressed by the Aesthetic Component of the Index of Orthodontic Treatment Need (AC/IOTN) appeared to be the most important indicator for facial esthetics. A new parameter, the "horizontal sum" was found to be a reliable variable for the anterior-posterior characteristics of the patient. Addition of this newly defined parameter to the AC/IOTN improved the prognostic value from 25% to 31%.

Myofibroblasts in palatal wound healing: prospects for the reduction of wound contraction after cleft palate repair.

The surgical closure of orofacial clefts is considered to impair maxillary growth and dento-alveolar development. Wound contraction and subsequent scar tissue formation, during healing of these surgical wounds, contribute largely to these growth disturbances. The potential to minimize wound contraction and subsequent scarring by clinical interventions depends on the surgeon's knowledge of the events responsible for these phenomena. Fibroblasts initiate wound contraction, but proto-myofibroblasts and mature myofibroblasts are by far the most important cells in this process. Myofibroblasts are characterized by their cytoskeleton, which contains alpha-smooth-muscle actin. Additionally, their contractile apparatus contains bundles of actin microfilaments and associated contractile proteins, such as non-muscle myosin. This contractile apparatus is thought to be the major force-generating element involved in wound contraction. After closure of the wound, the myofibroblasts disappear by apoptosis, and a less cellular scar is formed. A reduction of contraction and scarring might be obtained by inhibition of myofibroblast differentiation, stimulation of their de-differentiation, stimulation of myofibroblast apoptosis, or impairment of myofibroblast function. In this review, we will discuss all of these possibilities, which ultimately may lead to a better outcome of cleft palate surgery.

A measuring system for facial aesthetics in Caucasian adolescents: reproducibility and validity.

A new measuring system to judge facial aesthetics in young Caucasians is presented. The system uses sets of three photographs (one frontal, one three-quarter smiling, and one lateral) as a stimulus. Scores are performed on a visual analogue scale (VAS) with separate sets of reference photographs for girls and boys. The choice of the reference photographs was based on a panel evaluation of facial aesthetics of 40 boys and 40 girls from the archive of the orthodontic department. Reproducibility of the new measuring system was tested on a series of photographic sets (one frontal, one three-quarter smiling, and one lateral view) of 64 patients, using a panel of 78 adult laymen and 89 professionals. The panel members assessed these sets of photographs on a VAS, in relation to the reference sets. The system was shown to be reproducible. Although the intra-observer reproducibility was low, the reliability coefficient was excellent (Cronbach's alpha > or = 0.98). Validity was tested by comparing the scores on the new scales with those of the three-quarter smiling photographic views on an earlier published scale. The correlation between the ratings on the new measuring system and the earlier published scale was 0.82 for laymen and 0.77 for professionals. The new system is simple and flexible in its use, and reproducible and valid for assessing facial aesthetics in young Caucasians. The system can be used in further investigations on the evaluation of facial aesthetics.

Incidence and severity of root resorption in orthodontically moved premolars in dogs.

OBJECTIVES: To study treatment-related factors for external root resorption during orthodontic tooth movement. DESIGN: An experimental animal study. SETTING AND SAMPLE POPULATION: Department of Orthodontics and Oral Biology, University Medical Centre Nijmegen, The Netherlands. Twenty-four young adult beagle dogs. EXPERIMENTAL VARIABLE: Mandibular premolars were bodily moved with continuous or intermittent controlled orthodontic forces of 10, 25, 50, 100, or 200 cN according to standardized protocols. At different points in time histomorphometry was performed to determine the severity of root resorption. OUTCOME MEASURE: Prevalence of root resorptions, defined as microscopically visible resorption lacunae in the dentin. Severity of resorption was defined by the length, relative length, depth, and surface area of each resorption area. RESULTS: The incidence of root resorption increased with the duration of force application. After 14-17 weeks of force application root resorption was found at 94% of the root surfaces at pressure sides. The effect of force magnitude on the severity of root resorption was not statistically significant. The severity of root resorption was highly related to the force regimen. Continuous forces caused significantly more severe root resorption than intermittent forces. A strong correlation (0.60 < r < 0.68) was found between the amount of tooth movement and the severity of root resorption. CONCLUSIONS: Root resorption increases with the duration of force application. The more teeth are displaced, the more root resorption will occur. Intermittent forces cause less severe root resorption than continuous forces, and force magnitude is probably not decisive for root resorption.

Growth stimulation of mandibular condyles and femoral heads of newborn rats by IGF-I.

Primary and secondary cartilage differ in embryonic origin and are generally considered to have a different mode of growth. However, few experimental studies exist that directly compare the two types of cartilage and their growth regulation. The regulation of cartilage growth is a complex mechanism involving growth factors like insulin-like growth factor-I (IGF-I). The purpose of this study was to compare the growth of mandibular condyles of 4-day-old rats with that of femoral heads in vitro and to analyze the effects of IGF-I. Explants were cultured for up to 2 weeks with 0, 5, and 25 ng/ml IGF-1. Both, 5 and 25 ng/ml IGF-I significantly stimulated growth of the mandibular condyles while only 25 ng/ml IGF-I stimulated growth of the femoral heads. IGF-I increased glycosaminoglycan synthesis of both condylar and femoral cartilage. However, only the DNA synthesis of the mandibular condyles was significantly increased by IGF-I while that of the femoral heads was not affected. It is concluded that IGF-I stimulates growth of both secondary condylar cartilage and primary femoral cartilage. The mandibular condyle appears to be more sensitive to IGF-I than the femoral head, which may partly be due to the different developmental stage.

Assessment of the value of anorganic bone additives in sinus floor augmentation: a review of clinical reports.

In order to objectively assess the value of anorganic bone additives in sinus floor augmentation, a review of the English literature till May 2002 was carried out. The studies or parts of studies had to include at the least two patients, with a follow-up of at least 3 months, whilst histomorphometric data ought to be available of biopsies taken from the core of the graft. Only 12 studies fulfilled these criteria. Firm conclusions could not be drawn because there were substantial differences in histomorphometric techniques, healing periods and ratio autogenous bone anorganic additives, but it seemed that autogenous bone without anorganic additives resulted in the highest amount of bone after a 4-6 months healing period, while hydroxyapatite and bovine bone mineral, used as a bone substitute, gave the lowest amount of bone. No correlation between bone volume and time of graft healing could be found. The consequences, however, for implant placement and survival are at present, not fully understood. A plea is made for systematic prospective clinical and experimental studies to assess the various bone additives and ratios of autogenous bone to bone additives in composite grafts analysed with an adequate histomorphometric technique. Part of this study was presented at the 15th ICOMS, Durban, South Africa, 19-24 May 2001.

Soft tissue reaction to microgrooved poly-L-lactic acid implants loaded with transforming growth factor beta(3).

In both normal and disturbed wound healing, the generation of large, contracting scars can raise serious functional and cosmetic problems. A possible strategy to minimize or avoid the generation of scar tissue surrounding an implant is to apply transforming growth factor-beta(3) (TGF-beta(3)) to the implant. TGF-beta(3) (0, 1, or 2.5 microg) was freeze-dried onto poly-L-lactic acid (PLA) microgrooved substrates (width, 10 microm; depth, 1 microm) and implanted subcutaneously on the backs of rats for 2 and 8 weeks. After sacrifice, implants and surrounding tissue were histologically processed. Light microscopic and histomorphometric evaluation of capsule thickness, capsule quality, and implant-tissue interface was performed. In addition, we stained for alpha-smooth muscle actin (SMA), collagen, and ED-1 (a monocyte-macrophage marker). All implants were surrounded by a fibrous capsule. Capsules of the implants loaded with 1 or 2.5 microg of TGF-beta(3) showed significantly higher capsule quality. This meant that capsules were more mature compared with implants without TGF-beta(3). However, no significant differences were found in terms of thickness of the capsules or quality of the interface. Finally, apparently significant differences were also found in the expression of alpha-SMA, when comparing the various growth factor concentrations at both implantation points. In conclusion, the use of microgrooved PLA substrates with TGF-beta(3) did not lead to an overall improvement of periimplant tissue healing.

Age effect on orthodontic tooth movement in rats.

Orthodontic procedures seem to be more time-consuming in adults than in juveniles. This might be related to delay in the initial tissue response or to a slower turnover of the bone and periodontal ligament in adults. To study this problem, we studied orthodontic tooth movement in two groups of 30 rats, aged 6 wks and 9-12 mos, respectively. At one side of the maxilla, 3 molars together were moved mesially with a standardized orthodontic appliance delivering a force of 10 cN. The other side served as a control. The results showed a faster initial tooth movement in juvenile than in adult animals. However, once tooth movement had reached the linear phase, the rate of tooth movement was the same in both groups. The results indicate that, besides a delay in the onset of tooth movement in adult animals, tooth movement could be equally efficient in adults once it had started.