Molecular basis for tooth eruption and its clinical implications in orthodontic tooth movement

The process of tooth eruption is a complex sequence of biological eventswhich leads to the timely eruption of teeth in the oral cavity. The factors responsible for the initiation of this process and its timely occurrence are still controversial and subject to intensive research efforts. In this article, the authors review some of the fundamental aspects and principles involved in the eruption process of teeth and discuss the factors involved in delayed eruption. In addition, clinical situations are discussed in which the absence of tooth eruption in a significant portion of the dentition causes great clinical challenges in orthodontic patient management. Also specific clinical situations which may be of help to the clinician are described. From an orthodontic standpoint, the sequenced and timely eruption of teeth in the oral cavity is critical to the timing of treatment initiation as well as the selection of treatment modalities. Treatment length may also be directly associated with the timely eruption of teeth and a better understanding of this process may help managing patients more efficiently.     

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.     

Osteoimmunology in orthodontic tooth movement

The skeletal and immune systems share a multitude of regulatory molecules, including cytokines, receptors, signaling molecules, and signaling transducers, thereby mutually influencing each other. In recent years, several novel insights have been attained that have enhanced our current understanding of the detailed mechanisms of osteoimmunology. In orthodontic tooth movement, immune responses mediated by periodontal tissue under mechanical force induce the generation of inflammatory responses with consequent alveolar bone resorption, and many regulators are involved in this process. In this review, we take a closer look at the cellular/molecular mechanisms and signaling involved in osteoimmunology and at relevant research progress in the context of the field of orthodontic tooth movement.     

Leukotrienes in orthodontic tooth movement

Prostaglandins (PGs) and leukotrienes (LTs) are products of arachidonic acid conversion. PGs have an established role in mediating orthodontic tooth movement. The role of LTs in modulating or mediating orthodontic tooth movement was investigated in this study. One hundred thirty-two Sprague-Dawley rats were used; the animals weighed 300 to 400 gm with equal numbers of male and female rats. They were divided into five main groups of 24 animals each and a sham group of 12 animals. An orthodontic appliance was placed and activated on all the animals except the sham group; in this group the appliances were not active. Each main group was given one of the following treatments daily: distilled water, 5% gum arabic solution, PG synthesis inhibitor indomethacin, LT synthesis inhibitor AA861, and a combination of both drugs. Each group was divided into six subgroups of four animals; the animals were killed at either 1, 3, 5, 7, 10, or 14 days, and tooth movement measured. The three sham subgroups received distilled water and were killed at 1, 7, or 10 days. The first maxillary molar (the moved tooth) and surrounding tissues were removed from all animals in the sham group and the subgroups killed at 1, 7, and 10 days in the gum arabic solution group and the LT synthesis inhibitor group. Prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) were extracted, measured with radioimmunoassay (RIA), and standardized per milligram of protein in the sample. A significant inhibition of tooth movement occurred beginning on day 7 in the indomethacin, AA861, and combination groups; there was no significant difference among these groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prediction of orthodontic tooth movement.

Orthodontics is rapidly advancing from the stage of fortuitous success to one of planned success. When appliance design is based on simple concepts of equilibrium, prediction of desirable and undesirable tooth movement becomes possible. The flow chart that has been presented allows the clinician to systematically treat dental discrepancies more effectively in potentially less treatment time, since the number of desirable and undesirable side effects can be known in advance and managed accordingly. Simple laws of equilibrium, based on a knowledge of the force systems required for specific tooth movement, also permit the design of proper preactivation bends for an appliance. The procedure may be enlarged to include treatment of each arch since, once the teeth are ideally positioned and rigidly held within each segment, each arch can be considered to consist of three "large, multirooted teeth."     

Biological basis of orthodontic tooth movement

The effect of orthodontic therapy is dependent of the biological possibilities and limitations of the dento-alveolar complex. Biomechanical effects determine the first phase of tooth movement. In the second phase hyalinisation occurs in almost all cases. Elimination of the hyalinised tissue is associated with undermining bone resorption. Next, 'real' tooth movement starts. At the pressure side the normal structure of the periodontal ligament is destroyed and so is the tooth attachment. At the tension side deposition of trabecular bone is found and the tooth attachment remains. The regulation of these processes is still not completely understood, but cytokines and growth factors play an important role. The biological system does not react according to a simple dose-response relation and large individual differences in susceptibility of the system exist.     

Intermittent force in orthodontic tooth movement

A single orthodontic activation lasting one hour can initiate tooth movement. The purpose of this study is to examine tooth movement, osteoclasts, and root resorption in rats following several one-hour activations. Rats (n = 144) were randomly assigned to intermittent (multiple activations of 1 hr/day), continuous, and sham appliances. Twelve rats were killed at 3, 5, 7, and 14 days. Tooth movement, osteoclasts, osteoclast %, and root resorption % were quantified. Continuous force moved molars mesially at days 3 and 14 (p < 0.05), but intermittent and sham did not. Intermittent and continuous force increased osteoclast numbers at days 3, 5, and 7 (p < 0.05). Continuous force increased osteoclast surface on days 3 and 14 (p < 0.05). Continuous force increased root resorption at days 5, 7, and 14 (p < 0.05). These results demonstrate that orthodontic force for one hour in 24 stimulates osteoclasts at compression sites but does not stimulate tooth movement or root resorption.     

Estrous-cycle-dependent variation in orthodontic tooth movement

Sex hormones, including estradiol, play important physiological roles in bone metabolism. The purpose of this study was to investigate whether there is estrous-cycle-dependent variation in orthodontic tooth movement, and, if so, to determine the mechanism. Ten-week-old female Wistar rats were used. They received repeated orthodontic force during specific phases in the estrous cycle. Tooth movement in animals that received force principally in estrus was about 33% greater than that in animals that received such force principally in pro-estrus (p < 0.05). Serum estradiol levels also varied according to the estrous cycle, with a peak during pro-estrus and a nadir during estrus, and were inversely related to tooth movement. Furthermore, there were negative correlations between estradiol and both serum TRAP activity and pyridinoline (r = -0.42, p < 0.05; r = -0.59, p < 0.001). These results suggest that cyclic changes in the estradiol level may be associated with the estrous-cycle-dependent variation in tooth movement through its effects on bone resorption.     

加速正畸牙齿移动的生物学方法

正畸治疗的平均疗程在两年以上,较长的正畸治疗周期可能增加牙周、牙体并发症发生的风险。因此科学有效地加速牙齿移动,缩短正畸周期具有重要的临床意义。目前加速牙齿移动的方法主要有两大类：一类是外科手术法,包括截骨术、皮质切开术、压电皮质切开术、微骨手术等;另一类是非手术理化刺激法,包括低能激光辐照疗法、低强度脉冲超声、共振震动、脉冲电磁场、低频脉冲疗法、化学物质局部注射等。本文对以上方法的研究进展作一综述,以期为临床医生选择加速牙齿移动的方法提供参考。     

加速正畸牙齿移动的生物学方法

正畸治疗的平均疗程在两年以上,较长的正畸治疗周期可能增加牙周、牙体并发症发生的风险。因此科学有效地加速牙齿移动,缩短正畸周期具有重要的临床意义。目前加速牙齿移动的方法主要有两大类：一类是外科手术法,包括截骨术、皮质切开术、压电皮质切开术、微骨手术等;另一类是非手术理化刺激法,包括低能激光辐照疗法、低强度脉冲超声、共振震动、脉冲电磁场、低频脉冲疗法、化学物质局部注射等。本文对以上方法的研究进展作一综述,以期为临床医生选择加速牙齿移动的方法提供参考。     

Clear aligners generations and orthodontic tooth movement

Clear aligner technology has evolved over the last 15 years, with these appliances continually being modified to increase the range of tooth movements that they can achieve. However, there is very little clinical research available to show how these appliances achieve their results. This article describes the different generations of clear aligners that are available and highlights their use. However, until more clinical research becomes available, aligners cannot be routinely prescribed as an effective alternative to fixed labial appliances.     

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目的    探讨正畸力作用下蛋白激酶B（Akt）在兔牙周组织改建过程中的作用。方法    2009年1—4月在中国医科大学中心实验室选取日本大耳白兔24只,建立正畸牙移动动物模型,将实验动物上颌右侧戴矫治器,作为实验侧;左侧未戴矫治器,作为对照侧。分别在戴矫治器后3、5、7、14 d各处死6只实验动物。用实时荧光定量聚合酶链反应（RQ-PCR）方法对牙周组织中Akt表达进行检测。结果    RQ-PCR检测结果显示,加力3 d后牙周组织中Akt mRNA表达增强（P < 0.05）,7 d后牙周组织中Akt mRNA明显增强（P < 0.01）,随后缓慢下降。与对照侧相比,实验侧牙周组织中Akt mRNA表达明显增强（P < 0.05）。结论    Akt参与牙周组织改建,并在牙周组织改建中起重要作用。     

A model for evaluating friction during orthodontic tooth movement

Orthodontic forces for sliding tooth movement during space closure are applied at a distance from the centre of resistance of the teeth. For this reason, the teeth will tip until contacts are established between the archwire and diagonally opposite corners of the bracket wings. They will also rotate until the wire contacts opposite corners of the ligature tie or the buccal shield with self-ligating brackets, and the base of the slot. Frictional forces measured with models that do not enable such movements may therefore not be representative of the clinical condition. To test this hypothesis, a dentoalveolar model that allowed accurate reproduction of the width of a material of similar elastic properties as the periodontal ligament (PDL) was fabricated. In addition, a device was designed that allowed accurate adjustment of the bracket slot in all three planes of space during mounting of the model in an Instron machine. Frictional forces during sliding of ceramic brackets with 0.022 x 0.028-inch bracket slots along 0.019 x 0.025-inch stainless steel wires were tested using models with simulated PDL widths of 0.00, 0.33, 0.67, and 1.00 mm. ANOVA detected a significant effect of PDL width on mean frictional force (P < 0.001). Pairwise comparisons at 0.05 significance level indicated no differences between the models without PDL and those with a width of 0.33 mm, and between models with PDL widths of 0.67 and 1.00 mm. However, the two models with smaller widths produced significantly lower frictional forces.