
Stress in periodontal ligament is the initiating factor of tissue remodeling. The dynamic three-dimensional finite element analysis method adopted by Wang Yuwei and others discussed the stress buffering effect of periodontal ligament. A comparative study was conducted on periodontal ligament attachment and bone attachment. Except for the different thickness of periodontal ligament (0.2mm for periodontal ligament attachment and 0mm for bone attachment), the other aspects are exactly the same. The test results are well comparable. The results showed that CD periodontal ligament made the stress distribution more uniform, while bone attachment made the stress distribution uneven and concentrated Periodontal ligament has obvious stress buffering effect in the neck. The more it moves to the root tip, the weaker the buffering effect is. This result can be explained as follows: when there is periodontal ligament, the tooth will produce large displacement after being subjected to impact load. When the load reaches the peak, the displacement also reaches the peak. At this time, the periodontal ligament produces large deformation and converts the impulse into energy storage. After unloading, the periodontal ligament gradually returned to its original shape and gradually released the stored energy site to the surrounding alveolar bone. Therefore, the stress of alveolar bone is small and uniform in the whole loading process. When there is no periodontal ligament, there is basically no displacement or only slight displacement after the tooth is stressed, which makes the tooth neck bear most of the stress and can not be well transmitted to the root tip. Therefore, the stress on the root tip is very small or even zero, which makes the stress on the bone attached root tip smaller than that of the periodontal ligament adherent.
Fang Bing and others also used the three-dimensional finite element method to explore the stress distribution and displacement trend in the periodontal ligament of maxillary canines and first permanent molars under clinical conditions: when CD intermaxillary and intermaxillary traction forces act on the cuspidium at the same time, the stress distribution in the periodontal ligament at the neck edge is very small, there is no obvious stress concentration at the root tip, and the stress is mainly concentrated in the middle of the cuspidium. The movement trend of fangs is close to the overall movement of thousands to the far middle and to the reserved side. In the results of this study, the stress distribution in the periodontal ligament of the cervical margin of canine is very small, and the maximum stress is concentrated in the middle of the root, which is different from the previous research results of the maximum stress concentration and the cervical margin. This may be that the models used in previous studies are single teeth without the constraint of simulating the main arch wire When the intermaxillary and intermaxillary traction and retroflexion moment were applied to the thousand maxillary first molars at the same time, the stress distribution in the cervical margin and apical periodontal ligament was small, and the stress was mainly concentrated in the middle of the thousand root. The movement trend of tooth body is that it is possible to rotate in the direction of square and near center. The stress distribution in the periodontal ligament of maxillary first molars is much smaller than that of canines, which shows that after multiple teeth bear external forces, they can disperse the forces to thousands of larger periodontal membranes and alveolar septum, which can bear larger external forces. In this study, there is no obvious stress concentration in the cervical periodontal ligament, and the maximum stress is distributed in the middle of the whole root in the thousands of partial crown square, which is different from the previous results of stress concentration in the neck and root tip of thousands of teeth. This suggests that when the teeth are in the whole dentition, there is normal adjacent surface contact, and the dentition is connected as a whole with the main arch wire, the teeth have a strong resistance to the external force.
Nature and location of tooth resistance Center
Resistance center is a controversial issue in the field of orthodontics. It is generally believed that the nature of the resistance center is: when the external force line passes through the resistance center of the tooth, the tooth will move horizontally; When the external force line does not pass through the resistance center of the tooth, the tooth will have a compound movement composed of translation and rotation. Therefore, the relationship between the position of tooth resistance center and the external force line directly affects the movement trend of tooth after stress. Therefore, the definition, nature and location of resistance center is one of the key problems in orthodontic treatment.
In the 1950s, it was once thought that the rotation center of the tooth directly affected the movement trend of the tooth after being stressed, because the inclined movement of the tooth always rotates around the rotation center. Muhlemn (1951 ~ 1960) studied the inclined movement of the tooth mostly in the series of articles on tooth movement. It was considered that the tooth rotates around the rotation center, and the position of the rotation center is the key to determine the movement trend of the tooth, However, there is little discussion on the fundamental properties of the center of rotation. The research literature from the 1960s to 1970s gradually found that the rotation center is not fixed, and it is difficult to intuitively find out the relationship between the rotation center and the applied force. For example, Burstone (1969) pointed out that the tooth movement has a logarithmic function relationship with the applied force, and the rotation center determines the coupling moment / force ratio of the thousand action on the tooth, rather than the magnitude of the thousand force alone. From 1980s to 1990s, the research literature gradually appeared
Now the resistance center is the key to determine the movement trend of teeth. Resistance center and rotation center are two different concepts, and the nature and definition of resistance center are discussed. According to the principles of physics, we know that any closed Erhe figure in the two-dimensional plane always has a certain centroid. When the external force line acts through this centroid, the figure will translate along the action direction of the external force, and the resistance center of the Erhe figure is its centroid; If the object is a free body in mass space, when the external force line acts through the mass center of the free body, the free body can also be translated along the direction of the external force, and the resistance center of the free body is its mass center; If the free body is in the thousand gravity field, its resistance center is its center of gravity. At this time, the binding force on the object is only gravity; The tooth is located in the thousand alveolar fossa and is constrained by periodontal supporting tissues such as periodontal ligament and alveolar bone. The resistance center of the tooth is the simplified center of the binding force of periodontal supporting tissues. Of course, the tooth itself is also constrained by gravity, but the weight of the tooth is almost negligible compared with the strong binding force of the periodontal ligament. Therefore, the tooth resistance center is related to the shape and length of the root and the distribution of periodontal ligament on the root surface. If the anatomical shape of periodontal supporting tissue remains unchanged, the tooth resistance center is also certain.
Although many mathematicians believe that there is a resistance center in the tooth, there is a debate about the location of the resistance center. Burstone (1965) abstracted the root shape of a single tooth into a parabola (two-dimensional), and believed that the resistance (resistance) center of the root, or its geometric centroid, is located at 40% of the total length from the top of the tooth to the root tip. Davidian (1971) established a two-dimensional calculation model based on the basic principle of static equilibrium force system, and calculated the resistance center of the tooth from the root tip to 56% ~ 61% of the alveolar crest. Based on the two-dimensional model, Burstone (1980) established a three-dimensional model. The tooth root is a rotating parabola (aluminum, IO magnification central incisor). The model is placed in the simulated alveolar fossa with silicone rubber as the periodontal membrane and artificial stone instead of alveolar bone. Measured by laser holographic interference method, it is found that the position of the resistance center is basically consistent with the centroid of the three-dimensional model (1 (3 places of the neck on the long axis). It is quite different from the centroid of the two-dimensional model (2 / 5 of the root on the long axis). Dermaut (1986) studied the maxillary first molar of a cadaveric bone by speckle interferometry, and simulated the periodontal ligament with artificial material (araldif208). The experimental results are as follows: the resistance center of the first molar is about 1000, and the bifurcation is slightly close to the reserved square. Burstone (1988) used a three-dimensional finite element model to calculate the resistance center of the upper central incisor. The model has two improvements: @ the root shape adopts the real shape rather than the rotating throwing object; @ Periodontal ligament press
The measured value of Coolidge is uneven in thickness. The results show that the resistance center of maxillary central incisor is 24% from the crest of thousand alveolar to the root tip. In the early study of two-dimensional model, the position of resistance center was 40% (i.e. 2 / 5 of the root on the long axis) and 52% (i.e. 56% ~ 61% from the root tip to the alveolar crest). In the study of three-dimensional model, the position of resistance center was 33% (i.e. 1 / 3 of the neck on the long axis). With the improvement of model similarity, the resistance center was closer to the reserved side. Liu Fuxiang (1989) studied the resistance center of single tooth on cadaveric skull by laser holographic interferometry. It is also considered that the resistance center of single tooth on the upper jaw is 1 / 3 of the neck of thousands of teeth.