| Residual ridge resorption(RRR) has puzzled prosthetic dentitions for a longtime ,which lead to be difficult for the denture's retention and stabilizationbecause of the loss of bone mass. Many studies had researched the etiologicalcause of RRR, and tried to find out the way of prevention and treatment. Howeverthe conclusions were conflict due to the limit of the animal model and way ofclinic research, difficult to control the condition of the stress. So the physiologicaland pathological mechanism of the RRR was still not clear.The stress condition of the alveolar is changed because of the absence of theteeth. If the chewing force coming from the denture is over the normal stress, theresidual ridge will cause traumatic resorption;if below or there was no denture, itwill cause disuse atrophy. From these we can conclude that a certain stress levelshould be exited for the resorption of residual ridge when the stress above it, whilethe exact mechanism of it is still unclear.Except the stress there are many etiologic factors that can cause RRR, suchas systemic and local anatomic factors, but the local mechanical stress oftengenerated by removable prostheses is the most important and is the main way toprevent through reducing the stress. By deeply researching the physiological andpathological mechanism of RRR, we can design and build the reasonable,scientific, suitable prostheses, on the other hand we can control the pathologicalprocess of the residual ridge from the level of the molecule and gene.In this study we firstly analyze the stress changing of supporting tissue ofcomplete denture under different chewing force by the method of finite elementanalysis, and aim at investigating the influence of the stress distribution under thecomplete denture with cusp inclination, and finding out the suitable design ofcomplete denture that can improve the stability and prevent the resorption ofresidual ridge. The separated jaw bone and complete denture was scanned by CT,and the data was further processed by Photoshop software and transferred intocomputer and then the models were established by software of Solidworks 2001.The different finite element analysis models were established and calculated onthe condition of different cusp inclination and RRR. The results were asfollowing:The side-stress which was on the alveolar bone under the completedenture with same chewing force (cusp inclination) was increased with theaggravation of residual ridge resorption;if the alveolar bone is in the samecondition, when the cusp inclination increased, the factors of unsteadiness willincrease;the stress distribution will be more evenly when the denture was lateralloaded compare with bilateral loading. Under the vertical loading the horizontaldisplacement of the denture with cusp inclination of 0 degree was the smallest;theside stress of the supporting tissue was the biggest under the vertical loading whenthe inclination of cusp was 33 degree;the tress under the complete denture withcusp inclination of 20 degree is between that of 0 degree and 33 degree, thecomplete denture with cusp inclination of 20 degree is fit for the patient whosealveolar bone's resorption isâ… or â…¡ degree.Secondly the traumatic resorption models of the rat residual ridge after toothextraction were used to investigate the functions of RANKL and OPN in RRR,and to show the mechanism of traumatic resorption of RRR, and further toprovide the strong theory proof about the molecule and gene treatment for RRR.The 50 healthy ten weeks male Wistar rats were allotted at random into twogroups, experiment group and control group. The left and right maxilla firstmolars were extracted, after 4 weeks of healing time the denture bases were madeof the self-cured acrylic resin. The height of the denture base of the experimentgroup was 0.8mm over occlusal plane, but equal to occlusal plane for the controlone. 50 rats were perfused and sacrificed at 3 day, 1 week, 2 week,4 week, 8 week.All the buccolingual sections in the first molar region were processed, thenproceeded with HE staining, RANKL immunohistochemical staining, TRAPstaining and OPNmRNA in-situ-hybridization staining. The histopathologicalchange, RANKL expression level, the number of TRAP staining positive cells, thechanging of OPNmRNA expression of the bone tissue in two groups wereinvestigated and the results were analyzed by the Computer Image AnalyzingSystem and treated by statistics test.The result showed that there was no significant histopathological change, noevident inflammation and bone resorption in the mucous of the control group;RANKL expression level was low, lying in the few of osteoblasts and fibroblasts;there was few TRAP staining positive cells in the residul ridge and OPNmRNAexpression level was low, lying in the few of osteoblasts. For the experimentgroup, at 3 day, the RANKL expression level was a bit stronger and theosteoblasts ,matrix cells in the marrow had positive granules, but there was nosignificant different compared with the control group;the number of TRAPstaining positive cells was few and the OPNmRNA expression level had nochanged compared with control group.At 1 week and 2 week , the significanthistopathological change was noted for the mucous of the alveolar .The epithelium,the lamina propria mucosa were severely compressed. Numbers of osteoclasts andHowshiop's lacunae were observed on the bone surface. The RANKL expressionlevel in the osteoclasts was much stronger comparing with the control group;Thenumber of the TRAP staining positive cells and OPNmRNA expression level inthe osteoclasts was much stronger than the control group. After 1 week loading,the number of TRAP staining positive osteoclasts increases greatly, especially onthe bone surface, during this time the OPNmRNA expression level was alsoincreased greatly. At 2 week, in the connection of bone and mucosa plenty ofTRAP staining positive osteoclasts concentrated, and the number came to the top.While the OPNmRNA expression level was in the same as that of 1 week.For 4 week and 8 week, the histopathological changing of the mucosa had noevidence change and the number of osteoclasts on the bone surface was decreasedbut the number of osteoblasts was increased. The RANKL expression level waslower, no evident different comparing with the control group. The number of theTRAP staining positive cells and the OPNmRNA expression level was lower, noevident different comparing with the control group.The above study has showed that the normal masticatory pressure can not ledto pathological changing of residual ridge, while excessive masticatory pressurecan lead to traumatic resorption of the residual ridge;under the long timeexcessive masticatory force the histopathological changing was not evidentbecause the transferred force reduced with the resorption of residual ridge.Theexcessive masticatory pressure can led to the expression of RANKL in osteoclaston the bone surface of residual ridge and the level was associated with the boneresorption. This give us the proof that RANKL has participated the process ofresorption of residual ridge. When residual ridge was under the excessivemasticatory pressure,the number of the TRAP staining positive cells werehigher,so ,excessive masticatory pressure can induce residual ridge resorption.Thechange of the OPNmRNA expression level is a complex of biology reaction, thatis induced by mechanical forces on alveolar bone. Under the modulation of OPN,osteoclast can polify ,which leads to the residual ridge traumatic resorption.From above all we can conclude that change the incination of cusp of thedenture can induce the strength of the masticatory pressure,and the strength of themasticatory pressure is associated with the pressure disposition and pressurepeculiarity of the alveolar bone under the denture. Through the detection ofRANKI and OPNmRNA in alveolar bone,we can conclude that excessivemasticatory pressure can induce the traumatic bone resorption. Therefore, adoptreasonable and scientific design of complete denture in clinical can decrease theresorption of alveolar bone.
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