Root deformation and root resorption

描述：正畸牙根吸收是一个主要的临床和医学法律问题。大约有一半的美国人接受了正畸治疗，据估计，90%的患者出现了可检测到的牙根脱落。5%到58%的人经历了严重的（50毫米）损失。其机制尚不清楚，但人们相信破骨细胞对这种基因调节的损失负有责任。最终，吸收是对牙冠负荷的一种反应。当正畸力叠加在改变的咬合上时，“自然”的牙根变形在大小、方向、持续时间和频率上都发生了变化。提出这些变形通过两种方式引起根部损伤。首先，当根弯曲时，凸弯曲侧被拉伸，而另一侧（凹）侧被压缩。这种张力/压迫会损害牙骨质和/或牙本质。其次，牙根弯曲也会在牙髓-牙本质界面内产生剪切应力，导致可能的分层。进一步假设正常的生理吸收部位作为应力集中器，加剧局部根损伤。反过来，受损的根更容易被吸收。这些现象将使用机械测试、组织学、微型ct的新应用和有限元（FE）计算机模型进行检查。

DESCRIPTION: Orthodontic root resorption is a major clinical and medico-legal concern. Approximately half the US population is treated orthodontically, and according to some estimates, 90 percent of patients experience detectable root-loss. Five to 58 percent experience severe (>5mm) loss. The mechanisms remain obscure, but it is believed that osteoclasts are responsible for this genetics-modulated loss. Ultimately, resorption is a response to loads on the crowns of teeth. When orthodontic forces are superimposed on altered occlusion, the "natural" root deformations are changed in magnitude, direction, duration, and frequency. It is proposed that these deformations cause root damage in two ways. First, as the root flexes, the convex bending side is stretched, while the other (concave) side is compressed. This tension- /compression can damage the cementum and/or dentin. Second, root bending also produces shear stresses within the cementum-dentin interface, leading to possible delamination. It is further hypothesized that normal physiological resorption sites act as stress concentrators that intensify local root damage. In turn, the damaged root is more vulnerable to resorption. These phenomena will be examined using mechanical testing, histology, a novel application of microCT, and finite element (FE) computer models. The Specific Aims of this study are to (1) quantify root fatigue damage, (2) develop/validate microCT damage assessment, (3) correlate stress gradients with damage, (4) relate root-shape to stresses, and (5) assess damage in the adjacent contralateral root to determine its suitability for use as control. Cyclic loading will be applied to the crowns of fresh cadaveric canine (dog) mandibular central incisors. Blocks with a tooth and its surrounding alveolar bone will be microCT imaged and then examined for damage using the traditional basic fuchsin histological method. The two damage assessments will be compared and the locations of damage will be correlated with the results of FE calculated stress gradients. This multidisciplinary project will contribute in two disparate areas - understanding the mechanism of orthodontics-associated root resorption, and demonstrating the potential of 3D microCT imaging to become the new "gold standard" of calcified tissue damage assessment.