| Background and objectiveDental implants should be considered as one of the highly sophisticated medical devices. As they provide useful value to our patients and have the ability to improve their life quality. Screw- and cement-retained implant restorations are two major accepted treatment options for the replacement of missing teeth. Compared with the screw-retained implant restorations, the upper structure of the cement-retained restorations was simple and can be used for the cheap materials, and the doctors and technicians came the ability to restore the implant in a manner similar to how we deal with the natural dentition. It can save the clinical time and reduce the cost. There is no screw access to ensure the integrity of the surface and the strength of the crow restorations. It was beneficial to the uniform distribution of stress, improved the vertical direction of the load force. Even if the restorations and abutments were not quite close, it can make the restorations and abuments as a whole, not to form a gap, reach more passive fit of the casting by cement grouting effect. Although there are still some disputes about the advantages of the screw-and cement-retained restorations, a non negligible fact is that the cement-retained restorations have been widely studied and applied, and achieved good results.For the cement-retained implant restorations, clinicians are most concerned with the problem of retention and marginal seal. Factors that influence the retention of the cement-retained implant restorations have many such as type of cement, choice of dental impalnt system, abument taper, height, size, length, roughness, and treatment of crown internal surface. Dental cements have become essential mediums for retention of implant restorations. Originally, dental cements have been manufactured for use with the natural dentition. As a result, the properties of cements are designed to have an interaction with the natural tooth. For example, many release fluoride as an anticaries agent, some cements etch dentine, some chelate to the calcium ion found in tooth tissue, etc. None of these properties are required with dental implants. In fact, some are detrimental with certain implant materials. Currently there is no ideal cement manufactured for the cementation of implant-supported restorations. For the past few years, stomatologist are tent to use definitive resin-based cements. Because marginal seal between the restorations and the abuments is one of the important factors for the effectiveness. Microleakage can lead to loose, fall off of the implant restorations. A large number of plaque will be attached to the edge of the crown, which will lead to inflammation of the surrounding tissue of the implant, and even implant failure. At the same time, luting cements from the cement-retained implant restorations were difficult to control. If residual cement was got into the implant periodontal tissue, which easily caused implant lesions. If luting cements can meet the retention, as little as possible excess cement discharge. The residual cements were also very concerned about a problem.In the first part of the experiment, the bonding properties were studied on crown adhesion between luting cements and implant straight abutment. The tensile strengths, adhesion fracture modes were evaluated between luting cement and implant straight abutment using the temporary cement, phosphoric acid zinc cement, glass ionomer cement, resin modified glass ionomer cement and resin cement. The tensile strengths and titanium abutment surface microstructure about short abutemnts were observed by resin modified glass ionomer cement and resin cement, while the abutmens were respectively treated by nonsandblasting, sandblasting, sandblasting and metal primer. By evaluating and comparing the retention values of restoration using different abutment modifications, open, closed, internal vent abutment, and internal cone, a comparison bond strength and the weigh of residual excess cements was produced.In the second part of the experiment, marginal seals were studied between the crown restorations and implant abuments. Marginal seals and residue cement control were observed by two kinds of bonding method including direct method and indirect method. The difference of marginal seals were evaluated by thermal cycle on four different cement bonding planting. Finally, the effect of the cement-screw-retained restortions were observed in region of posterior missing teeth.Research methodsIn the first part of the experiment, fifty prefabricated abutments were placed on titanium screw implants torqued to 35 Ncm. Metal copings were cast with Co-Cr alloy using burnt-out plastic copings. Cements used were temporary zinc oxide cement, zinc phosphate (ZPC), glass ionome rcement (GIC), resin-modified glass ionomer cement (RMGIC), and composite resin (CR). All specimens were immersed in water at 37? for 24 hours and then tested for retention using a universal testing machine. The adhesive failure mode was observed. Furthermore, Copings were cemented on 2 mm,3mm and 4mm height abutment, using two types of cements:resin-modified glass ionomer cement (RMGIC), and composite resin (CR). Copings were removed using a universal testing machine and bond strengths were recorded.2 mm,3mm short abutment were sandblasted, metal primer, or sandblasted and metal primer. 4mm abutment was control group, which was given any treatment. The tensile strength and interfacial fracture mode were tested.Coupling microstructure of abutment surface were observed by scanning electron microscope (SEM). Finally, the retention values of restoration using different abutment modifications, open, closed, internal vent abutment, internal cone and the cement retained were evaluated and comparied in 4 groups each contain 10 specimens.In the second part of the experiment, first of all, the experiment was divided into two groups including the direct method and indirect method group. Each group of 32 specimens were respectively bonded on the implant abutment using RMGIC. All specimens were immersed in drying air environment for 24 hours. Marginal seals were evaluated according to the modified United States Public Health service (USPHS) criteria. Furthermore,40 casting crowns were randomly divided into four groups each have ten specimens. Four group were respectively cemented using ZOE, ZPC, RMGIC, CR. All specimens were immersed in deionized water 37? for 20 days. Then, the samples were submitted to thermal cycles (10,000 cycles,5? ± 2° to 55? ± 2°) for thirty seconds in each bath. Next, the samples were submitted to marginal seals observation before and after thermal cycling. Finally, a total of 114 dental implants were placed in the posterior regions of 80 partially edentulous patients (The experiment was approved by ethics committee of Wuhan General Hospital, PLA).64 crowns were cement-retained, while 50 crowns were cement-screw-retained. After 12 months of restoration, peri-implant marginal bone resorption, modifled plaque index (mPLI), modified sulcus bleeding index (mSBI) and patient satisfaction were recorded.Research resultsAccording to the results, the following conclusions could be achieved.1. ZPC, CR and RMGIC can obtain clinically acceptable retentive strength and marginal adaption. Bonding strength of temporary cements and GIC were poor, which were used in order to obtain retrievability.2. Surface modification can significantly improve the bonding strength for short abutments, sandblasting and metal primer can improve the tensile strength. SEM observation results proved that roughness of abutment surface can significantly improve by surface modification including sandblasting treatment. By reasonable choose of surface modification treatment can expand the application of the short abutment.3. Internal vent abutment and abutment insert can obtain good tensile strength compared to closed abutment, open abutment. In addition, they can reduce the excess cement from the crown margin. Internal vent abutment have many advantages, such as simple manufacture, reducing the excess cement from the crown edge and better biological complications of dental implants.4. The CCA approach can better marginal adaption and limits excess cement to an absolute minimum. Different treatment of abutment surface reduced marginal adaption of restorations, but it was acceptable in clinical. The indirect method can considered to use in clinical.5. Cement-screw-retained restortions is an inexpensive, simple bonding technique. The short-term clinical effect of mixing retained are satisfactory, which can be used in implant restorations. |
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