Orthodontic root absorption in rat molars is a useful model for investigating the mechanism of dental root resorption. In this research, we used SD models to investigate whether MMP-2 is involved in orthodontically induced inflammatory root resorption (OIIRR). We demonstrated the expression of MMP-2 in rat root resorption and the localization of this enzyme in osteoblasts, osteoclasts and fibroblasts. Based on immunohistochemical analysis, a high level of MMP-2 expression was revealed in the rat root resorbing tissue, indicating that MMP-2 should take part in root resorption during orthodontic tooth movement.
Root resorption is an unwelcome effect of orthodontic treatment. Although bone resorption craters have been shown to be cured by cementation, the apical area is separated from the rest of the root, and the root will be permanently shortened. The process of root resorption is closely related to injury and necrosis of the PDL. When a heavy orthodontic force is used for a sustained period of time (weeks or months), necrosis (transparency) of the compressed PDL may develop rapidly . According to previous studies, the establishment of orthodontic teeth movement of Model in rats, 50 g or so of nickel-titanium helical pull force allows teeth to move effectively, along with force increasing, the speed at which the teeth move is reduced, and the probability of root absorption occurs increases, and can cause excessive damage to periodontal tissue. As the force increases, the area where absorption occurs increases. To reduce the occurrence of root absorption, they advocate the use of light force in the clinic. In a study of the movement of common rat teeth, the magnitude of the correction force set in the model was 20 g-50 g. Therefore, the force value of the nickel-titanium tension spring set in this experiment was 60 g to ensure that the orthodontic force-induced animal model could be successfully established. Histological appearance of root resorption in HE staining show that when the force 7d, the root tip of the tooth and the root fork are visible in the root absorption phenomenon, and tired of the essence of the tooth, absorption of a large number of multicore bone-breaking cells visible in the trap, and can be seen that the potential bone absorption is taking place, inflammatory cells immersion is obvious, indicating the successful establishment of orthodontarosic mobility root absorption model.
Brudvik and Rygh studied mice and rats and confirmed that OIIRR is part of the transparent strip elimination process[19–24]. As noted above, MMPs are the major enzymes involved in the degradation of ECM. Under normal conditions, their activity is low, but their activity is elevated during the repair or remodeling of diseased or inflammatory tissues . Tissue inhibitors of metalloproteinases (TIMPs), a major endogenous inhibitor of metalloproteinases, reversibly inhibit MMPs in a 1:1 stoichiometry . MMP-2 is collagens (IV, V, VII, X), gelatin, elastin. Our results showed that orthodontic forces affect both MMP-2 protein levels on the compression and tension sides, although to different extents, MMP-2 protein levels change in a time-dependent fashion (table 1). In the control group, the expression of MMP-2 was at baseline levels(Fig. 3.A),which was similar to that at 1 day. Then, the expression of MMP-2 was induced by compression and increased significantly in a time-dependent fashion, reaching a peak after 7 days of force application and then gradually returning to weakly positive at 21 days. Studies have demonstrated that before the decomposition of the matrix, there is an enzyme formation and storage phase, followed by protease decomposition and matrix destruction. Therefore, we conclude that the 1d and 3d of the intensification in this experiment are the formation stages of the enzyme, so the content is slightly increased. After 5 days of exertion, obvious root resorption occurred, and the enzyme content increased significantly. After 7 days of intensification, the expressive content reached the maximum(Fig. 3.E). The disordered arrangement of periodontal fibers hinders the cushioning effect of the periodontal ligament on the orthodontic force, resulting in a relative increase in local pressure, which leads to an increase in the level of relevant inflammatory mediators, which accelerates the activation of osteoclasts and ultimately aggravates inflammatory root resorption. This is part of the possible reasons. Then, it gradually declines until 21 d after the force, and the content reaches a dynamic stable level, with no obvious fluctuations(Fig. 3.F).Previous investigations proved that decomposition of the matrix is the result of the interaction of TIMPs and MMPs[27, 28]. As the expression of MMPs increases, TIMPs also increase, inhibiting the excessive degradation of MMPs to tissues. Both MMPs and TIMPs are obtained by adult periodontal ligament fibroblasts. The degradation of periodontal ligament tissue may be related to the increase in local MMP and the decrease in TIMPs. This may be explained by the inhibition of MMP-2 expression at the beginning of TIMPs, so the expression of MMP-2 is decreased in the late stage of posteenergization. In addition, it may be accompanied by the repair of the root tissue during the root resorption process, the root inflammation is relieved, and the released necrosis is reduced so that MMP-2 is not excessively produced. At 21 days, the expression of MMP-2 in periodontal tissues was significantly reduced(Fig. 3.F), but it was still more than the physiological state, suggesting that periodontal tissues are still actively remodeling and reconstructed until the internal environment state is stable.
The statistical results showed that there was a significant difference in expression levels between the control group and each experimental group, indicating that MMP-2 is sensitive to root resorption, and we conclude that it may be involved in root resorption.
In conclusion, the results confirmed the hypothesis that MMP-2 plays a significant role in OIIRR. However, it is unclear how MMP-2 works and how it can inhibit root resorption. This is also the focus of the next group.