The Clinical Aesthetic Study Of Three PFM

PFM was published since the 50's in last century. It has become one of the most popular clinical restorative procedures after being developed continuously. In China, a point was started to carry on a research at the early part of 80's in 20 centuries, then a series of the non- precious metal porcelain crown such as CWPA, YKH-1, CAI group were developed and extensively applied in clinic. All these have aggressive function to the universality of porcelain crown. The combination of Ni-Cr alloy and porcelain is very well. Although The porcelain is very fragile, the ability of the incisal edge of PFM to resist fracture is better than that of natural tooth .The experiment of Feng Sun and others had proved that, the ability of Ni-Cr alloy to resist fracture is as good as high aurum alloy .The Ni-Cr alloy's physical performance is better, and its price is cheap,so in recent years it is extensively used as oral fixed denture. The PFM technology becomes perfect gradually. But dyeing of Ni-Cr alloy's cervical margin and precision of casting are still the question not to be answered.The study and research on the composition of non- precious metal alloy has proved that, when the non-precious metal alloy was polished and porcelain was fused to surface of the non-precious metal alloy, the ability of Ni-Cr alloy become degrade in corrosion resistance. The metal alloy including Ni is instableand easy to corrode in oral acid environment. Separation of the dangerous Ni ions have tremendous hazard to the human body, in the oral local part, it can also result in gingival discoloration, gingivitis and nigralinea appeared in cervical margin of PFM. Under the body fluid environment, even under the protection of porcelain layer, Ni-Cr alloy will also release Ni ion and Cr ion. Among them, the Ni ion deposite down in the gum after swimming out, making the gum present dark or gray. Ni is dissolved into ion form in the electrolyte, has a latent toxicity. And after Ni ion depositing in the gum edge, the dark and gray metals side appeared clearly, it influence esthetics result .Jun Xu and others think that ,it is possible that the dyeing of gingival cellula epithelialis is relevant to sheding off of the corrosion output that was produced in oxidize preparing of Ni-Cr alloy inner crown.Ni-Cr alloy is applied extensively in our country. But because its safety in organism is worried about, it has already used less in the flourishing nation. More precious metal alloys (aurum alloy, palladium alloy) and titanium metal are adopted. The precious metals, particularly high aurum alloy, are ideal material of PFM because of their tolerance of corrosion, excellent nature of counteracting oxidize, high foundry accuracy, good biocompatibility. They has been applied in the western countries widely. Because of its high price,currently the high aurum alloy only is used in the big city with better economic condition in our country .So the relevant clinical application report was few.The application of electroplating technique to the Ni-Cr alloy solves this puzzle successfully. The technique of gilding is a method which transfer gold(anode) through electrolytic solution to the casting (cathode) that was made up by casting metal alloy according to electroplating technique, and then, make the surface of casting enclose with certain thickness of gilded layer. The technique of gilding is used to the inner surface of the non-precious metal inner crown. Speak from the theories, it can make use of the good biocompatibility of the precious metal, in the meantime it has an advantage that its price is cheap, so it don't lose to a kind of ideal repair means.Along with the popularization of technology gilt and gilded PFM's reasonable price, the number of cases is growing in clinical use of gilded PFM. But the evaluation on gilded PFM's result is divergent. Some doctors have observed that in some cases the gilded layer lost, and gingival inflammation has emerged even with intact gilded layer.Under such circumstances, this study collected 35 cases of the past gilded PFM , observed gilded PFM effect, and analyze the reasons of some failedgilded PFM. Methods were put forward to deal with the problem, and then the improved gilded PFM's effect was observed.The standards of choosing case are :⑴Patient` s gender is unconditional, his body is healthy, his dental hygiene is well before reparation, and have no gingivitis, peridentitis,and history of therapia of periodontal disease during the period of surveying.⑵Upper anterior teeth with dental defect or discolored tooth having done with flawless root canal therapy , color and shape of isonym natural tooth in opposite side is not unusual (such as tetracycline pigmentation teeth or cone-shaped tooth)⑶These condition are necessary in dental prosthetic restoration: the length of dental prosthetic restoration's gum margin is not exceed to 1.0mm in gingival groove;its margin must be tightly close;its shape must be recovered correctly;its surface must be smooth.The standards of case exclusion are:⑴the patient that his gingivitis or peridentitis is not yet controlled completely.⑵the patient that he could not brush the teeth correctly and keep oral health well.Four PFM's distinctions are introduced to the patients, and by patients'svoluntary choice all PFMs are divided into four groups: experimental group : ordinary gilded PFM in 40 cases, highly polished gilded PFM in 40 cases; Control group : nickel-chromium alloy PFM in 40 cases, aurum alloy PFM in 40 cases.By the case selection and the unification of manufacture,the impact on checking result from other factor is avoidable. After clinical try-in, gingival discoloration and gingivitis are evaluated. And they will be rechecked again after a year.Based on the observation of the previous collected cases, we find some gold layer lost, and analyze it maybe relate with the dissimilar metal. Dissimilar metal can promote the corrosion occurred in the mouth. To deal with the problem, this study is rigorously screened to exclude the effect of the dissimilar metals on gilded PFM before restoration.In another seven cases, PFM's adaptation is poor, PFM's neck is overhang and not seal. Under such circumstances, any treatment can not improve the gingival situation. So in this study, HANTIAN agar impression material with better hydrophilic properties is used to obtain a clear cervical margin model in clinic. In any way, the shoulder of cervical margin should not be impaired when technician repaired the model. After bonding, detected with probe, noobvious groove or overhang exists between Crown and teeth. These methods guarantee the adaptation of cervical margin, and then reduce bad effect on gilded PFM. In dental preparation, labial groove-wide shoulder is used to avoid gingival discoloration caused by thin edge.Observed under microscope, black spots and cracks can be seen in most of the inner surface of the crown. Black spots or cracks are not good polished, so can not be gilded. Electrochemical corrosion easily occur in pit, then undermine the existing gold layer .In this study, Cervical margin of the inner surface of crown is highly polished to eliminate minor pits and marks arising from coarse grinding.By the experimental observation, discoloration only occurs at the cervical margin of nickel-chromium alloy PFM in first day. Light pass through the dental cervix into abutment, but after reflection it can not pass through metallic inner crown. On the contrary, the color of metallic inner crown will be reflected by abutment, and pass through the gum, then gum show the color of inner crown, and make itself discolored.After a year, six cases of gingival discoloration and 9 cases of gingivitis emerge in Ni-Cr alloy group. These may be caused by two factors, metal oxides electrochemistry corrosion or nickel ion precipitation. Gingivalcrevicular fluid is an excellent electrolyte. Metal edge of PFM inevitably expose in gingival sulcus, the alloy can produce electrochemical corrosion in gingival crevicular fluid. Alloy corrosion and decomposition lead to gingival discoloration and gingivitis.Three cases of gingival discoloration and 8 cases of gingivitis emerge in general gilded PFM. According to analysis, the reason still is electrochemical corrosion. After bonding , microscopic crevice exist between metal inner crown and abutment's cervical margin and saliva or gingival crevicular fluid permeate into the crevice. Black spots or cracks exist on the surface of gold layer. In the electrolyte environment, residual oxides in cracks generate electrochemical corrosion.No gingival discoloration and one case of gingivitis exist in highly polished gilded PFM group. Gingivitis occurs not in free gingiva, but in gingival papilla. Excessive oppression leads to occurrence of gingivitis.Through this experiment, we found that gilded PFM can reduce the occurrence of gingival discoloration and gingivitis. The reason is that gold layer, like a protective barrier, can protect Ni-Cr alloy from the electrochemical corrosion, then prevent metal ions into the gingival tissues, avoiding the occurrence of gingival discoloration. Pure gold, with inertia and good biocomp-atibility, can not educe metal ions into gingival tissue. With good opaque role, gold layer can avoid the gingival discoloration caused by the reversible optical phenomena.In the same time, it is found if not with highly polishing and ideal results in gilding, general gilded PFM can not play a very good role in the protection of the gingival tissues, only reduce the occurrence of gingival discoloration and gingivitis limitedly. But highly polished gilded PFM play an effective role in the protection of gingival tissue by avoiding pit corrosion. Therefore, in clinical observation gilded PFM achieve a good cosmetic result.