Stress distribution pattern in a root of maxillary central incisor having various root morphologies: A finite element study

Objective:To investigate stress distribution in the roots of maxillary central incisors bearing various types of root morphologies with regard to application of different types of orthodontic forces using the finite element model (FEM).Materials and Methods:FEMs of maxillary central incisors with different root morphologies (normal, short, blunt, dilacerated, and pipette) were constructed, and orthodontic forces in various directions (intrusion, extrusion, tipping, and rotational) were applied to the tooth axis at the bracket level.Result:On application of various forces, significantly increased stress was seen at the apex of the root with dilacerated morphology and at the cervical one-third region of the tooth with the short root. Increased stress was observed at the middle one-third region in the tooth with the pipette-shaped root during intrusion and extrusion.Conclusions:In the present study, the stress distribution pattern indicates that the maxillary central incisors with deviated root morphology are at higher risk of root resorption.     

弯根牙的临床综合治疗及正畸早期矫治的可能性

弯根牙是指由于各种原因造成的牙冠与牙根（或部分牙根）形成一定弯曲角度的牙齿。文章总结了临床弯根牙的病因、形成机制、临床错牙合表现、治疗原则、治疗时机选择、治疗的生物学及生物力学考虑;介绍了临床正畸治疗方法与原则,并且着重对牙根正在形成的前牙弯根牙的临床治疗体会及临床治疗对前牙发育的优势进行了阐释,认为早期弯根牙治疗能促进牙根形成、避免弯根严重化及弯曲牙根暴露。同时,文章还强调并提出前牙弯根阻生牙需早期矫治的临床证据,探讨了前牙弯根牙早期治疗的可能性,为弯根牙的临床早期正畸治疗提供了新思路。     

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??Tooth dilaceration is considered as a dental deformity characterized by pronounced angulations of the longitudinal tooth axis. This article reviewed the etiology?? mechanism?? clinical features?? treatment principles?? treatment timing and treatments based on biological and biomechanical thoughts of tooth dilaceration. This article especially illustrated early orthodontic intervention for impacted and dilacerated maxillary central incisors. The early orthodontic intervention showed that the successful tooth alignment could be achieved?? the forms of dilacerated root were improved?? and the chance of root exposure in oral cavity when angular teeth were aligned was reduced. The author suggested the early orthodontic intervention of impacted teeth with dilacerated root was better for root morphologic development and could be a clinical choice for the tooth dilaceration treatment.     

Surgical repositioning of an impacted dilacerated incisor in mixed dentition

BACKGROUND: Treatment options for a dilacerated incisor are either extraction or surgery and orthodontic traction. Because patients with such incisors usually are young, and because of the root angulation of the impacted incisor, treatment usually is lengthy and complicated. Surgical repositioning provides another option for treatment of this unique problem. CASE DESCRIPTION: The author presents the case of a 9-year-old girl with an impacted dilacerated maxillary central incisor to demonstrate the timing, technique and results of the surgical repositioning treatment approach. The advantages of this approach include immediate esthetic improvement, use of a single and simplified surgical procedure, simple and short orthodontic therapy, a normal gingival margin and the possibility of the developing root's adapting to the new position. CLINICAL IMPLICATIONS: Surgical repositioning is a simplified treatment for dilacerated incisors. It is especially valuable in cases of difficult-to-treat impaction. Timing of surgical repositioning depends on the incisor's root development and the space available for the transplant.     

Unusual root development following surgical repositioning of horizontally developing central incisor

Dilaceration is one of the causes of maxillary central incisor eruption failure. If the dilacerated maxillary permanent central incisor is in a horizontal or vertical position and root formation is in early stages, surgical repositioning is frequently the treatment of choice. In this article, the case of a horizontally impacted and dilacerated maxillary central incisor is presented which was treated by surgical repositioning. The tooth developed an unusual form of the root with discontinuity at the middle third region with the coronal and apical parts growing separately. Radiographic and clinical monitoring of the case was regularly performed. The tooth completely erupted in 2 years after the repositioning and even after 6 years of surgery is currently successfully functional in the arch with the malformed root. This article highlights the ability of Hertwig's epithelial root sheath to withstand trauma and its ability to recover.     

Mechanical stress generated by orthodontic forces on apical root cementum: a finite element model

OBJECTIVES: 1) To determine the mechanical stress generated at the root apex during different types of tooth movement using a finite element model of an ideal, human maxillary central incisor. 2) To determine the relationship of thickness of cementum and the magnitude of mechanical stress at the root apex. DESIGN: Computer simulation. SETTING AND SAMPLE POPULATION: Not applicable, computer simulation. EXPERIMENTAL VARIABLES: Tooth and investing tissue layers (enamel, dentin, cementum, pulp, periodontal ligament, and alveolar bone). OUTCOME MEASURE: Von Mises and maximum principal stresses. RESULTS: Increasing the apical thickness of cementum increases the amount of mechanical stress. CONCLUSION: A finite element model incorporating all layers of a human maxillary central incisor has been developed. This model was used to determine the location and magnitude of mechanical stress generated for all regions of the tooth, PDL, and enclosed alveolar bone, when orthodontic forces are applied to the tooth. Mechanical stresses were found to increase at the root apex with increasing thickness of apical cementum.     

A study of the relationship between incisor intrusion and root shortening

Apical root shortening is one of the most common complications of orthodontic treatment. Force magnitude has been suggested as an important factor. Studies on the occurrence of root resorption show equivocal results. The aim of the present study was to evaluate the relationship between intrusion with low forces (25 gm) using utility arches in the bioprogressive technique and root shortening. Age, sex, facial type, treatment time, extraction versus nonextraction therapy, width of the symphysis, and the angle of the incisors to skeletal reference planes also were studied for their relationship to intrusion and root shortening. Root shortening was found to average 1.84 mm for maxillary incisors and 0.61 mm for mandibular incisors subjected to intrusive force. Intrusion of incisors in a population exhibiting growth was found to be one of "holding against growth" and in the upper arch to a change in angulation of the maxillary incisors. Furthermore, when extraction was a part of the orthodontic treatment, it was related to intrusion of maxillary incisors but not to intrusion of mandibular incisors. No relationship was found between the amount of root shortening and degree of intrusion achieved. However, a long treatment time was significantly correlated to root shortening. None of the other characteristics studied were related to either intrusion or root shortening. In the present study, it was found that intrusion with the utility arch type of technique is not related to amount of root shortening. The degree of root shortening was markedly higher in the maxilla than the mandible. In general, treatment time was the most significant factor for occurrence of root shortening.(ABSTRACT TRUNCATED AT 250 WORDS)     

A finite element model of apical force distribution from orthodontic tooth movement

This study was undertaken to determine the types of orthodontic forces that cause high stress at the root apex. A 3-dimensional finite element model of a maxillary central incisor, its periodontal ligament (PDL), and alveolar bone was constructed on the basis of average anatomic morphology. The maxillary central incisor was chosen for study because it is one of the teeth at greatest risk for apical root resorption. The material properties of enamel, dentin, PDL, and bone and 5 different load systems (tipping, intrusion, extrusion, bodily movement, and rotational force) were tested. The finite element analysis showed that purely intrusive, extrusive, and rotational forces had stresses concentrated at the apex of the root. The principal stress from a tipping force was located at the alveolar crest. For bodily movement, stress was distributed throughout the PDL; however, it was concentrated more at the alveolar crest. We conclude that intrusive, extrusive, and rotational forces produce more stress at the apex. Bodily movement and tipping forces concentrate forces at the alveolar crest, not at the apex.     

Effectiveness of an anterior mini-screw in achieving incisor intrusion and palatal root torque for anterior retraction with clear aligners:: A finite element study

ObjectivesTo analyze the biomechanical system of anterior retraction with clear aligner therapy (CAT) with and without an anterior mini-screw and elastics.Materials and MethodsModels including a maxillary dentition (without first premolars), maxilla, periodontal ligaments (PDLs), attachments, and aligners were constructed and imported to finite element software. Three model groups were created: (1) control (CAT alone), (2) labial elastics (CAT with elastics between the anterior mini-screw and buttons on central incisors), and (3) linguoincisal elastics (CAT with elastics between the anterior mini-screw and precision cuts on the lingual sides of the aligner). Elastic forces (0–300 g, in 50 g increments) were applied.ResultsCAT alone caused lingual tipping and extrusion of the incisors. Labial elastics caused palatal root torquing and intrusion and mesial tipping of the central incisors, while linguoincisal elastics produced palatal root torquing and intrusion of both central and lateral incisors. Second premolars were intruded in all three groups, with less intrusion in the linguoincisal elastics group. For the control group, stress was concentrated on both labial and lingual root surfaces, alveolar ridge, and cervical and apical PDLs. Stress was more concentrated in the labial elastics group and less concentrated in the linguoincisal elastics group.ConclusionsCAT produced lingual tipping and extrusion of incisors during anterior retraction. Anterior mini-screws and elastics can achieve incisor intrusion and palatal root torquing. Linguoincisal elastics are superior to labial elastics with a lower likelihood of buccal open bite. Root resorption and alveolar defects may occur in CAT, more likely for labial elastics and less likely for linguoincisal elastics.     

青少年期正畸治疗与上中切牙牙根吸收的关系

目的 探讨青少年错He畸形患者在正畸治疗中，牙齿移动与上中切牙根尖吸收之间的关系。方法 随机选择具有完整病例资料的青少年错He畸形患者为研究对象，经与对照组匹配后，确定其中58例为研究样本，分别以治疗前后根尖片测量中切片根吸收的情况，以头颅侧位片测量治疗前后牙根在前后向及垂直向的位移植并作多元回归分析。结果 根吸收与牙根尖的龈向压入具有极显著的负相关关系（P＜0.001)，与牙根尖的He向伸长也表现出有统计学意义的负相关（P＜0.05)，但根吸收与牙根尖在水平方向的位移却未表现出明显的相关。多元回归结果还显示，治疗过程中牙根尖位移为零时，根尖的平均吸收量达到2.29mm，有显著的统计学意义。结论 青少年在生长发育期进行正畸治疗，正畸牙移动量与牙根尖吸收量之间存在正相关的可能性很小。     

埋伏弯根上颌中切牙序列矫治的研究

目的探讨埋伏弯根上颌中切牙有效的正畸牵引治疗方法。方法选择11例患者共12颗埋伏弯根上颌中切牙为研究对象,常规拍摄定位X线片,配合口内检查、模型分析等,确定埋伏牙的位置及其与邻牙的相互关系,采用封闭式导萌、选择性根管治疗及根尖切除术、修复治疗等方法进行矫治。结果12颗埋伏牙通过治疗后顺利进入正常牙列,未见牙根吸收粘连,建立了良好的咬合关系。9例唇侧埋伏弯根上颌中切牙患者的临床牙冠唇倾角度较大,4例根尖位于唇侧黏膜下,进行了根管治疗加根尖切除术,其中1例行烤瓷冠改形修复;其余5例未做根管治疗,暂行观察。2例腭侧埋伏弯根上颌中切牙的疗效稳定。结论埋伏弯根上颌中切牙采用序列矫治可以取得良好的美观和功能效果。     

Severe crowding and a dilacerated maxillary central incisor in an adolescent

This study reports the treatment of an adolescent patient with dilacerated maxillary incisor. She complained of severe crowding with a high-positioned left upper canine. Her left central incisor had been impacted and moved to proper position at the age of eight years, resulting in a severe root dilaceration. To avoid any progression of root dilacerations and resorption in the maxillary incisor, maxillary lateral expansion and molar distalization plus multibracket appliance were selected as the best nonextraction treatment plan. The maxillary expansion and molar distalization should provide adequate space for the correction of the severe crowding, and treatment with a multibracket appliance was initiated. After a 17-month treatment with a multibracket appliance, an acceptable occlusion was achieved with a Class I molar relationship. An acceptable occlusion was maintained without recurrence of the crowding and impairment of the dilacerated root in the maxillary incisor during three years of retention. It is emphasized that careful planning is required to avoid any progression of the root dilaceration and resorption through orthodontic treatment. A shortening of the period of applying orthodontic force on the dilacerated incisor and avoidance of tooth extraction will minimize the risk factors.     

Apical root displacement is a critical risk factor for apical root resorption after orthodontic treatment

Objectives:To identify risk factors for apical root resorption (ARR) of maxillary and mandibular incisors using mathematical quantification of apical root displacement (ARD) and multiple linear mixed-effects modeling.Materials and Methods:Periapical radiographs of maxillary and mandibular incisors and lateral cephalograms of 135 adults were taken before and after orthodontic treatment. ARR was measured on the periapical radiographs, and movement of central incisors was evaluated on the superimposed pre- and posttreatment lateral cephalograms. ARD was mathematically calculated from pretreatment tooth length, inclination change, and movement of the incisal edge. Linear mixed-effects model analysis was performed to identify risk factors for ARR, and standardized coefficients (SCs) were calculated to investigate the relative contribution of the risk factors to ARR.Results:Vertical ARD showed the highest SCs for both maxillary and mandibular incisors. Horizontal ARD showed the second highest SC for mandibular incisors but was not significantly correlated with the ARR of maxillary incisors. When horizontal and vertical ARDs were included in the mixed-effects model, the use of self-ligating brackets was significantly correlated with increased ARR of mandibular incisors.Conclusions:ARD is a critical factor for ARR after orthodontic treatment. Careful monitoring of ARR is recommended for patients requiring significant ARD of incisors.     

牙周炎患者前牙区压低后骨缺损形态的初步研究

目的本研究通过正畸压低排齐牙周炎患者伸长、移位的前牙,探讨正畸压低后前牙区的骨缺损形态。方法选取11例牙周炎患者的20颗伸长、移位的患牙,治疗前牙槽骨形态均为水平型骨吸收,正畸压低排齐患牙,采用Morita CT对压低排齐后的骨缺损形态进行评价。结果2名患者的近远中侧均出现了角型吸收,其余患者个别位点出现了角型吸收。结论压低单颗前牙在近、远中侧比较容易出现角型吸收;同时压低2颗或以上的牙齿,压低后一般仍为水平型吸收。     

上颌阻生尖牙牵引治疗后邻近切牙牙根吸收情况的研究

目的 分析上颌阻生尖牙正畸牵引治疗后邻近切牙的牙根吸收情况及可能的相关因素。方法 选取上颌尖牙阻生且符合标准的43例患者,年龄10～23岁,共60颗上颌埋伏阻生尖牙。上颌阻生尖牙正畸牵引治疗前后分别拍摄锥形束CT,分析阻生尖牙邻近切牙的牙根吸收程度,并对可能存在的相关因素进行分析。结果 上颌阻生尖牙正畸牵引治疗后,上颌中切牙发生轻、中、重度牙根吸收的概率分别为71.7%、15.0%、13.3%,上颌侧切牙发生轻、中、重度牙根吸收的概率分别为48.3%、36.7%、15.0%,上颌中切牙与侧切牙牙根吸收严重程度存在差异(P<0.05)。位于腭侧及颌骨内、低位且靠近面中线的阻生尖牙,在正畸牵引治疗后,侧切牙牙根吸收程度较重(P<0.05)。低位阻生尖牙牵引治疗后,中切牙牙根吸收较重(P<0.05)。牵引时间较长时,中切牙牙根吸收程度较重(P<0.05)。治疗前已发生牙根吸收的切牙,在阻生尖牙牵引治疗后发生的牙根吸收程度较重(P<0.05)。结论 上颌侧切牙在阻生尖牙正畸牵引治疗后,发生的牙根吸收程度较重。上颌阻生尖牙正畸牵引治疗后,邻近切牙牙根吸收情况与阻生尖...     

Effects of root morphology on stress distribution at the root apex

It is thought that the stress concentration at the root apex caused by orthodontic force induces root resorption. The purpose of this study was to investigate stress distribution at the root in cases of deviated root shapes using finite element models (FEMs). To clarify this, five three-dimensional FEMs divided by deviated root shape (normal, short, blunt, bent root apex, pipette shape) were constructed and, experimental orthodontic forces, applied in a vertical (intrusive) and horizontal (lingual) direction to the tooth axis. In the short-root model, significant stress was concentrated at the middle of the root. The blunt-shaped root model showed no significant stress concentration at the root. In the models with a bent or pipette-shaped root, significant stress was concentrated at the root apex. During orthodontic force application, stress concentration was observed in the root of the models with short, bent, and pipette-shaped roots, indicating that attention must be paid to root shape during orthodontic treatment.     

Management of root dilaceration of an impacted maxillary central incisor following orthodontic treatment: an unusual therapeutic outcome

Abstract – There are rare cases of impacted permanent central incisors with dilaceration, a dental deformity characterized by pronounced angulation of the longitudinal tooth axis. This paper presents the orthodontic alignment of a permanent maxillary left central incisor in a 7.5‐year‐old boy, which appeared impacted radiologically and displayed root dilaceration. Following surgical exposure with the closed‐eruption technique and appropriate orthodontic traction, the tooth was successfully aligned into the dental arch and the root was radiologically shown to be straightened and relatively well developed.     

The effect of orthodontic extrusion on traumatically intruded teeth

The management of traumatically intruded permanent incisors is controversial. Some authors suggest a decreased incidence of ankylosis in cases treated with orthodontic extrusion. The purpose of this study was to examine two common management techniques for traumatic intrusion, orthodontic extrusion, and observation for re-eruption. The four first premolars of three shepherd dogs were traumatically intruded with a mallet while a holding device was used to prevent tooth fracture. Five to 7 days following the injury, orthodontic force was applied unilaterally while the contralateral tooth served as the untreated control. To facilitate serial periapical radiography, x-ray jigs were fabricated for each animal and tantalum implants were placed in the bone distal to the permanent canine and first and second premolars. Observations included radiographic measurement of tooth movement, clinical estimates of tooth mobility, and radiographic and histologic assessment of root resorption, ankylosis, and periapical pathosis. The amount of traumatic intrusion varied from less than 0.5 to 4.1 mm. Following 11 to 13 weeks of force activation, 10 of 12 traumatized teeth showed clinical, radiographic, and histologic evidence of ankylosis irrespective of orthodontic treatment. Whereas the ankylosed teeth did not move with orthodontic forces, the teeth used for force application were orthodontically intruded 1.7 to 6.5 mm. When the injury to the tooth was severe, orthodontic extrusion had little effect on repositioning of the injured tooth but resulted in undesirable movement of the anchorage teeth. When the injury was less severe, orthodontic forces facilitated repositioning of the affected tooth.     

Apical root resorption of maxillary first molars after intrusion with zygomatic skeletal anchorage

The aim of this study was to evaluate radiographically the apical root resorption of maxillary first molars after their intrusion was done using zygomatic miniplates as skeletal anchorage in open-bite cases. The study group comprised 16 consecutively treated open-bite cases who had received special titanium miniplates in their zygomatic bones for use as anchorage to apply orthodontic intrusive forces to the maxillary posterior region. The control group consisted of 16 patients, who were matched regarding age, sex, and treatment duration but who had undergone fixed orthodontic treatment without intrusion mechanics for molars. Tooth lengths were measured on pretreatment, and posttreatment panoramic radiographs of all patients and mesiobuccal and distobuccal roots of left and right maxillary first molars were measured on-screen using a software program. The difference between the pre- and posttreatment tooth lengths was defined as apical root resorption. Comparison of the differences in root resorption of the two groups using the t-test for independent samples showed a statistically significant difference (P = .004) only for mesial roots on the right side. But because the mean difference in apical root resorption was only 0.5 mm, it was concluded that the apical root resorption of maxillary first molars after intrusion was done using zygomatic skeletal anchorage was not clinically significant.     

应用CBCT研究不同矫治器正畸拔牙矫治对前牙区牙根及牙槽骨的影响

目的 应用CBCT研究无托槽隐形矫治及传统固定矫治器正畸拔牙对前牙区牙根及牙槽骨的变化情况.方法 选择成人拔牙矫正的患者60例,随机分为无托槽隐形矫治器组及传统固定矫治器组(N=30).正畸治疗前后拍摄CBCT,测量两组正畸患者上下颌中切牙、侧切牙、尖牙的牙根长度及牙槽骨的厚度变化情况.结果 固定矫治器组较无托槽隐形矫...