Dental Research Of Lithium Disilicate Glass Ceramic

With the improvement of people’s aesthetic requirement,the purpose of dental restorative therapy is not only to restore masticatory function,but also to obtain natural and realistic aesthetic appearance.Due to their superior aesthetics,excellent biocompatibility,stability in oral enviroment and no adverse effect on Magnetic Resonance Imaging examination,ceramics have become the first choice for aesthetic prosthodontics.In recent years,the properties of dental ceramics have been continuously improved,and the processing technology has gradually matured.At present,ceramics are more and more widely used in dental prosthodontics.All-ceramic restorations can not only bear the occlusal force,but also simulate the hierarchy sense and vitality of natural teeth.With high clinical success rate,they are favored both by dentists and patients,and have become the best for aesthetic prosthodontics.With high flexure strength and superior aesthetics,IPS e.max Press is a lately developed lithium disilicate glass ceramic for hot-pressing technique.It is suitable for all-ceramic veneers,all-ceramic crowns and bridges,and the superstructure of implants.IPS e.max Press all-ceramic system,which is the most promising dental restorative material,has become perfect day by day and has been popularized in clinical therapy.However,several problems have appeared in the process of clinical diagnosis and treatment.So it is of great significance to study IPS e.max Press all-ceramic system comprehensively and deeply,which is helpful to guide clinical practice and ensure clinical efficacy.In this study,a series of basic properties and clinical performance of IPS e.max Press lithium disilicate glass ceramic(LS2)all-ceramic system were studied,aiming at improving the processing technology.The results can provide guidelines for clinical practice.The basic research involves mechanical properties and aesthetic properties,and discusses the effects of various factors on the flexural strength,color and translucency of LS2.The clinical studies include the evaluation of clinical efficacy of LS2 all-ceramic crowns for restoring cracked teeth,ultra-thin veneers for restoring anterior teeth and restorations for sequence treatment of worn dentition.Part 1:The basic research on mechanical and aesthetic properties of IPS e.max Press lithium disilicate glass ceramic(LS2)1.In the present study,we report the impact of thermal shock cycles on flexure strength and microstructure of LS2 relevant so as to improve the processing technology.The water quench experiment and three point bending test were used to analyze the impact of thermal shock on flexure strength.The crystal,morphological and atomic structural characteristics before and after the thermal shock were detected by X-ray diffraction,infrared spectroscopy and field-emission scanning electron microscopy.Furthermore,theoretical calculations based on density functional theory were performed on models for the distorted and undistorted lithium disilicate crystal lattices,to illustrate the mechanism of thermal shock effecting the strength and microstructure of LS2.2.To evaluate the effect of repeated heat treatment on the color and translucency of LS2 to optimize the processing technology.Disc specimens(10 mm in diameter,1.00 mm in thickness)were fabricated using IPS e.max Press HT A2 ingots,and then randomly divided into six groups(S0,S1,S2,S3,S4 and S5)(n=50/group,300 in total).Each group was sintered 0,1,2,3,4 and 5 times individually according to the glazing procedure.After polishing,the color parameters L*,a*and b*values and transmittance τ were respectively measured using ShadeEye NCC colorimeter and spectrophotometer.Subsequently,C*ab andΔE were calculated.The results were statistically analyzed using SPSS 20.0 software for ANOVA and LSD’s multiple comparisons(p=0.05).3.Effect of core/veneer thickness ratio on color and translucency of LS2 all-ceramic restorations were evaluated.Disc specimens were fabricated of HT A2,MT A2,LT A2,MO1 and HO1 ingots from IPS e.max Press system in different thicknesses(0.60,0.80 and 1.00mm)(n=40/group,600 in total).Then A2/TI1 IPS e.max Ceram was layered on one surface of each core specimen and sintered to veneer.Since groups of core specimens were different in thickness,the thickness of veneer varied to achieve the same total thickness of 1.20mm.All the LS2 specimens were polished,and color parameters L*,a*,b*and transmittance τ were measured before and after veneering,using ShadeEye NCC colorimeter and spectrophotometer.Color parameters L*,a*,b*of standard A2 shade tabs were also measured.Color saturation C*ab and color difference ΔE were calculated in the following formulae:C*ab=[(a*)2+(b*)2]1/2,ΔE=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2.One-way analysis of variance ANOVA and Turkey’s multiple comparisons test(p=0.05)were used to analyze the data by SPSS20.0 statistic software.Part 2:The study on clinical efficacy of IPS e.max Press lithium disilicate glass ceramic(LS2)all-ceramic restorations.1.This study was to retrospectively evaluate the clinic performance of LS2 all-ceramic crowns made with modified processing technology for the cracked teeth during a 3-year period.26 posterior teeth of 22 patients having a crack,biting pain and sensitivity to cold and hot,were selected.The LS2 all-ceramic crowns were fabricated with high translucent HT ingots by hot pressing and staining technique,then bonded on the minimally invasively prepared teeth using lately resin cement.Patients were interviewed and clinically examined at 1 week,1 month,6 months,1 year,2 years and 3 years respectively.The remission of pain symptom was checked,as well as X-ray examination of peri-apical lesions and secondary caries.The clinical evaluation of all-ceramic prosthesis was carried out with reference to the criteria of United States Public Health Service.2.This study is to evaluate the clinical aesthetic effect of LS2 ultra-thin ceramic veneers made with modified processing technology for anterior teeth.126 anterior teeth of 54 patients were chosen for aesthetic restore.Ultra-thin ceramic veneers were made of IPS e.max Press HT ingots with high translucency.Patients were interviewed and examined at 1 month,6 months,1 year,2 years,3 years,4 years and 5 years respectively.The clinical evaluation of ultra-thin ceramic veneers was carried out with reference to the criteria of United States Public Health Service.3.The clinical performance of IPS e.max Press LS2 restorations in occlusal reconstruction of worn dentition was studied.A patient with typical worn dentition was restored with all-ceramic veneers、laptops、crowns and fixed bridges made of IPS e.max Press LS2.All the restorations were adhered to the worn teeth with resin cement to reconstruct the whole dentition for mastication function and aesthetic outcome.The patient was followed up and interview to evaluate the clinical efficacy of all the prosthesis.The results and conclusions are as follows:1.In this present work,the results suggest that more than 5 thermal shock cycles cause a significant decrease of the flexure strength of LS2 samples.IR spectroscopy and FE-SEM imaging show that the surface of LS2 is significantly affected by the thermal shocks and that a change in surface morphology is connected to a decrease in mechanical property.We suggest further that the morphological and mechanical change of LS2 can be ascribed to the distortion of the Si-O-Si structural unit which is supported by DFT calculations.The above results provide a prospective guideline for clinicians to avoid thermal damage during the producing process and to elongate the life time of LS2 all-ceramic restorations.The heat treatment in processing procedure should be no more than 4 times.2.Repeated heat treatment can affect the color parameters(lightness,hue,color saturation)and translucency τ of LS2.After sintering 1-3 times,the L*,a*,b*,C*ab values and τ value significantly increased.The color parameters and transmittance of group S3 were higher other groups.After sintering 4-5 times,the L*,a*,b*,C*ab values and τ value significantly decreased.Compared with the group not repeatedly sintered,the color differences ΔE were 0.89 minimally and 2.01 maximally after different sintering times,which can be clinically accepted.It is recommended that the heat treatment should be carried out only 2-3 times including glazing after clinical grinding for occlusal modification.3.As core material,the translucency of all groups of LS2 specimens decreased when the thickness increased.After veneering,the translucency and lightness of core ceramic material decreased,the color became redder and yellower,the color saturation increased and the color difference could be distinguished by eyes.The all-ceramic restorations made from high transparent HT A2 ingot were the most translucent.When core/veneer thickness ratio became higher,LS2 all-ceramic restorations were redder and yellower,less transparent and more lightness.Compared with standard A2 shade tab,the color difference of HT A2 all-ceramic restoration was minimum,and couldn’t be distinguished by eyes.The color difference of MT A2 and LT A2 all-ceramic restoration could be distinguished but still could be clinically accepted.The color of MO land HO1 all-ceramic restoration was quite different from A2 shade tab,and the color difference was too big to be clinically accepted.4.The cracked teeth were restored with IPS e.max Press LS2 all-ceramic crowns made with modified processing technology.At 1 week,2 teeth(7.7%)of 2 patients still had sensitivity to cold and biting;3 teeth(11.5%)of 2 patients still had sensitivity to biting;1 tooth(3.8%)of 1 patient still had sensitivity to cold.At 1 month,only 2 teeth(7.7%)of 2 patients had sensitivity to cold.1 tooth needed endodontic treatment after 6 months.All the teeth and patients were followed up during the observation period of 3 years.24(92.3%)crowns were maintained well with excellent color,no fracture,no food impaction,no gingivitis,and satisfactory clinical performance.Normal root and peri-apical tissues and no secondary caries were detected with X-ray examination.It can be concluded that LS2 all-ceramic crowns made with modified processing technology have high strength and are very effective and successful in relieving the pain symptom,preventing cracked teeth from fracture,and preserving the teeth.5.The anterior teeth were restored with ultra-thin ceramic veneers made of IPS e.max Press LS2 with modified processing technology.At 1 month,2 veneers(1.6%)were B class in color match,and the other indicators were A class.At 6 months,1 year and 2 years,all veneers achieved A class in each evaluating indicator.At 3 years,gap as detected on the margin of 2 veneers(1.6%),which were B class in marginal suitability.At 4 years,2 veneers(1.6%)were C class in integrity with 1mm incisal fracture.At 5 years,1 veneer(0.8%)was lost;1 veneer(0.8%)was C class in integrity with disto-incisal fracture;slight discoloration was observed in 5 veneers(4.0%).All the veneers were clinically effective with no sensitivity symptoms,no secondary caries,no gingivitis and acceptable marginal suitability,excellent translucency and natural appearance.In a word,ultra-thin ceramic veneers made of IPS e.max Press LS2 with modified processing technology have been an ideal choice for anterior aesthetic restoration.6.After one year of clinical observation,all IPS e.max Press LS2 restorations have good retention,marginal suitability,complete and aesthetic appearance,good occlusion function,healthy gingival,and the patient is quite satisfied with the effect of treatment.The long-term efficacy is under observation.Occlusal reconstruction of worn dentition is a complicated sequence therapy process.In order to achieve ideal restoration effect,it is of great significance to make a reasonable treatment plan,select appropriate dental materials and attach importance to each step in treatment.