Histological evaluation of the effect of transseptal fibre resection on the rate of physiological migration of rat molar teeth

Three-dimensional analysis of the physiologic drift of adjacent teeth following maxillary first premolar extractions

We assessed the three-dimensional (3D) pattern of the physiologic drift of the remaining adjacent teeth after premolar extraction due to orthodontic reasons and the associated factors. Data were collected from 45 patients aged 17.04 ± 5.14 years who were scheduled to receive a fixed appliance after maxillary premolar extraction. Seventy-five drift models were obtained and digitalized via 3D scanning. The average physiologic drift duration was 81.66 ± 70.03 days. Angular and linear changes in the first molars, second premolars, and canines were measured using the 3D method. All the examined teeth had tipped and moved towards the extraction space, leading to space decreases. Posterior teeth primarily exhibited significant mesial tipping and displacement, without rotation or vertical changes. All canine variables changed, including distal inward rotation and extrusion. The physiologic drift tended to slow over time. Age had a limited negative effect on the mesial drift of posterior teeth, whereas crowding had a limited positive effect on canine drift. Thus, the mesial drift of molars after premolar extraction may lead to molar anchorage loss, particularly among younger patients. The pattern of the physiologic drift of maxillary canines can help relieve crowding and facilitate labially ectopic canine alignment, whereas canine drift is accelerated by more severe crowding.

Establishment of an orthodontic retention mouse model and the effect of anti-c-Fms antibody on orthodontic relapse

Orthodontic relapse after orthodontic treatment is a major clinical issue in the dental field. However, the biological mechanism of orthodontic relapse is still unclear. This study aimed to establish a mouse model of orthodontic retention to examine how retention affects the rate and the amount of orthodontic relapse. We also sought to examine the role of osteoclastogenesis in relapse using an antibody to block the activity of M-CSF, an essential factor of osteoclast formation. Mice were treated with a nickel-titanium closed-coil spring that was fixed between the upper incisors and the upper-left first molar to move the first molar in a mesial direction over 12 days. Mice were randomly divided into three groups: group 1, no retention (G1); group 2, retention for 2 weeks (G2); and group 3, retention for 4 weeks (G3). In G2 and G3, a light-cured resin was placed in the space between the first and second molars as a model of retention. Orthodontic relapse was assessed by measuring changes in the dimensions of the gap created between the first and second molars. To assess the activity and role of osteoclasts, mice in G3 were injected with anti-c-Fms antibody or PBS, and assessed for changes in relapse distance and rate. Overall, we found that a longer retention period was associated with a slower rate of relapse and a shorter overall amount of relapse. In addition, inhibiting osteoclast formation using the anti-c-Fms antibody also reduced orthodontic relapse. These results suggest that M-CSF and/or its receptor could be potential therapeutic targets in the prevention and treatment of orthodontic relapse.

Distribution of epithelial cells and their relationship to immunocompetent cells in rat molars: a confocal and transmission electron microscope study

This study sought to investigate the distribution of cytokeratin (CK)-immunopositive cells and their relationship to immunocompetent ED1- and OX6-immunopositive cells in rat periodontium by immunohistochemistry and electron microscopy. CK-immunopositive cells were generally distributed along the surface of the tooth root. They could also be found between root dentin and cementum, in the perivascular space, and close to or in the alveolar bone lacunae. ED1-immunopositive cells exhibited a compact shape with small processes and were widely distributed in the periodontium. Few sections demonstrated an intimate relationship between the CK- and ED1-immunopositive cells close to the cementum, in the perivascular space, and close to or in the alveolar bone. Numerous OX6-immunopositive cells with long branching processes were widely distributed in the periodontal ligament, surrounding and holding CK-immunopositive cells in the cell clusters, close to the cementum. Transmission electron microscopy revealed OX6-immunopositive cells that extended their cytoplasmic processes, which contained vesicles and occasionally lysosomes in between the epithelial cells. This study demonstrates the close relationship between the epithelial cells and the immunocompetent cells in a rat periodontium, indicating a functional interrelationship. It is possible that in a non-inflammatory periodontium, the epithelial cells act not independently, but through interaction with immunocompetent cells.

RANKL, osteopontin, and osteoclast homeostasis in a hyperocclusion mouse model

The biological mechanisms that maintain the position of teeth in their sockets establish a dynamic equilibrium between bone resorption and apposition. In order to reveal some of the dynamics involved in the tissue responses towards occlusal forces on periodontal ligament (PDL) and alveolar bone homeostasis, we developed the first mouse model of hyperocclusion. Swiss-Webster mice were kept in hyperocclusion for 0, 3, 6, and 9 d. Morphological and histological changes in the periodontium were assessed using micro-computed tomography (micro-CT) and ground sections with fluorescent detection of vital dye labels. Sections were stained for tartrate-resistant acid phosphatase, and the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteopontin (OPN) was analyzed by immunohistochemistry and real-time polymerase chain reaction (PCR). Traumatic occlusion resulted in enamel surface abrasion, inhibition of alveolar bone apposition, significant formation of osteoclasts at 3, 6 and 9 d, and upregulation of OPN and RANKL. Data from this study suggest that both OPN and RANKL contribute to the stimulation of bone resorption in the hyperocclusive state. In addition, we propose that the inhibition of alveolar bone apposition by occlusal forces is an important mechanism for the control of occlusal height that might work in synergy with RANKL-induced bone resorption to maintain normal occlusion.

Tooth wear and the "design" of the human dentition: a perspective from evolutionary medicine

Worn, flat occlusal surfaces and anterior edge-to-edge occlusion are ubiquitous among the dentitions of prehistoric humans. The concept of attritional occlusion was proposed in the 1950s as a hypothesis to explain these characteristics. The main aspects of this hypothesis are: 1) the dentitions of ancient populations in heavy-wear environments were continuously and dynamically changing owing to life-long attritional tooth reduction and compensatory tooth migration, 2) all contemporary humans inherit these compensatory mechanisms, and recent reduction in wear severity has resulted in failure to develop attritional occlusion, and 3) this failure leads to an increased frequency of various dental problems in modern societies. Because of the potential significance of this concept, we review and synthesize relevant works and discuss attritional occlusion in the light of current knowledge. Available evidence, on balance, supports the first and second points of the hypothesis. As noted by many workers, the human dentition is basically "designed" on the premise that extensive wear will occur, a conclusion that seems reasonable when one realizes that humans evolved in heavy-wear environments until relatively recently. Some dental problems in contemporary societies appear to reflect the disparity between the original design of our dentition and our present environment, in which extensive wear no longer occurs, but this possibility still needs further investigation.

Effects of sympathectomy on experimentally induced pulpal inflammation and periapical lesions in rats

The role of sympathetic nerves in bone physiology is largely unknown. Recent studies have shown a correlation between sympathectomy and bone remodeling. The present experiments were aimed to study the effects of unilateral sympathectomy on bilateral experimentally induced pulpitis and periapical lesions in the rat maxilla and mandible. Adult male Sprague-Dawley rats were used. Experimental rats (n=11) had the right superior cervical ganglion surgically removed (SCGx) and control rats (n=5) had sham surgery. Pulpal inflammation and periapical bone lesions in the maxilla and mandible were created 14 days later in both experimental and control rats by exposing the dental pulp in the first and second molars and leaving them open to the oral microflora. The rats were perfused 20 days thereafter and the jaws processed for immunohistochemistry with neuropeptide Y (NPY) and ED1 as primary antibodies. Sympathectomy resulted in an almost complete loss of NPY-immunoreactive (IR) fibers in the right SCGx jaws. In the non-sympathectomized (non-SCGx) left side and in the control rats, sprouting of NPY-IR fiber was observed in the inflamed pulp tissue adjacent to reparative dentin formation and in the apical periodontal ligament of the partially necrotic first molars. Significantly more ED1-IR osteoclasts were found in the resorptive lacunae lining the periphery of the periapical lesions on the SCGx side compared with the non-SCGx side (P<0.04) and the controls (P<0.03). The size of the periapical lesions were larger on the SCGx side compared with the non-SCGx side (P<0.03) in the mandible, but not in the maxilla. We conclude that inflammation causes sprouting of NPY-IR nerve fibers and that unilateral removal of the SCG increases both the area of the periapical lesions and the number of osteoclasts in the inflamed region.

Localization and density of myeloid leucocytes in the periodontal ligament of normal rat molars

The phenotypic distribution and density of macrophage-associated antigen-expressing cells in the periodontal ligament (PDL) of normal rat mandibular first molars was evaluated by immunohistochemistry, and an attempt made to identify dendritic cells (DCs) by immunoelectron microscopy. Cells immunopositive to ED1 (a general macrophage marker) were widely distributed throughout the PDL and were most common around blood vessels. A small number of T lymphocytes and OX62 (anti-veiled cells and gammadelta T cells)-positive DC-like cells were also found. The relative density of cells immunopositive to ED9 (CD14), OX42 (CD11b), OX6 (anti-class II MHC molecules), ED2 (anti-tissue-resident macrophages), 8A2 (CD11c) and WT.1 (CD11a) varied in the mesial, distal and periapical regions of the distal root and the furcal region. This finding suggests that there are several subpopulations of ED1-positive cells which express various combinations of these markers. Immunoelectron microscopy revealed that a small, but distinct, subpopulation of ED1- and OX6-positive cells did have a DC-like ultrastructure, although the majority of these cells were identified as macrophages. The DC-like cells were characterized by poorly developed lysosomal structures and an absence of phagocytic vesicles. It was concluded that the normal rat PDL is equipped with heterogeneous populations of macrophages with regional variations in density. The DC-like cells may function as antigen-presenting cells.

Quantitative evaluation of newly formed bone in the alveolar wall surrounding the root during the initial stage of experimental tooth movement in the rat

By using a chronological lead-labelling technique and computer image analysis the volume of this newly formed bone was evaluated. Rat maxillary first molars were moved mesially by a fixed, closed coil-spring appliance for 6 days using three different magnitudes of initial tensile force (27, 60 and 136 g). Sham-treated rats wearing an inactivated appliance were used for the control study. All animals were injected twice intraperitoneally with lead-disodium EDTA, 3 hr before the beginning and 3 hr before the end of treatment. The unit volumes of newly formed bone (mm3/mm2) were assessed with reference to lead-labelling lines in the alveolar walls of the root socket by computer image analysis. In the control group, two distinct lead-labelling lines indicated continuous bone formation on the mesial side of the root sockets, but only a jagged line was found on the distal side. After experimental mesial tooth movement, only a single lead line could be found on the mesial/pressure side of the root sockets; on the distal/tension side, a wide layer could be detected between the two lead lines. The volume of newly formed bone on the distal/tension side in the experimental groups was significantly greater than that in the control group. However, there was no significant difference in the volumes of newly formed bone among the three experimental groups. The study demonstrates that the volume of newly formed bone in the alveolar walls during the initial stage of tooth movement can be quantified and that the magnitude of the tensile force of tooth movement may not influence directly the volume of newly formed bone in the alveolar wall on the tension side.

Biometric analysis of tooth migration after approximal contact removal in the rat

Nineteen 8-week-old female Sprague-Dawley rats were used to observe macroscopically the direction of tooth movements following removal of approximal contacts. In 10 rats, under anaesthesia, approximal contact between the second and third right maxillary molars (M2-M3) was removed by grinding. These animals and a control group of nine rats were housed with normal diet and water ad libitum for 7 weeks. After killing, the skulls of all animals were removed, dried, and fixed in a standardized position on a Horsley-Clarke type stereotaxic frame. Using a micromanipulator graduated to 1/100 mm and mobile in three orthogonal directions, distances between the teeth (M1 or M3) and some bone structures that served as landmarks were measured. These measurements showed the direction of tooth movements in the experimental approximal space. After statistical analysis of the data, the results showed that the approximal space between M2 and M3 was closed by both a distal drift of M1-M2 and a mesial migration of M3. It was concluded that, in the rat, a mesial drift exists that can be induced by the loss of approximal contact. This movement is distinct from the distal physiological dental migration classically described in histological studies.