Wen | Mo Qingyan
Editor|Mo Qingyan
preface
Computer-aided design and computer-aided manufacturing technology have been very hot in recent years, and it is precisely because of its continuous system development, advanced quality control, and the introduction of new materials that it occupies most of the country in medical prosthetics.
Computer-aided design and fabrication is widely recognized in implant rehabilitation, and its accuracy is similar to that of traditional fixed repair techniques, using digital workflows to increase efficiency, reduce production time, and also omit some laboratory steps.
There are two ways to acquire digital impressions, the first is to directly use an intraoral scanner for image acquisition, and then generate a 3D virtual model, the advantage of this method is that it is no longer necessary to use traditional impressions.
The second method is to indirectly scan the stone model using an extraoral scanner, and then create a 3D virtual model, which corresponds to the 3D data set of the model, and the standard subdivision language file is sent and stored electronically, which can greatly improve efficiency.
In 1980, because the intraoral scanner was too slow and had the disadvantage of limited accuracy, the extraoral scanner came into being.
Originally, extraoral scanners were developed to scan plaster, but now that there are more advanced systems that can also be used to scan dental impressions and digitize impressions and models indirectly, how does an intraoral scanner work?
Intraoral scan
The first step in intraoral scanning is the impression, but many factors can affect the accuracy of the full-bow implant impression, such as the characteristics of the impression material, the impression technique, the splint of the impression cover, the depth and angle of the implant, or the connection of the implant.
In 2018, a systematic review was published abroad, which recommended specific techniques and materials to achieve the highest accuracy of multi-implant impressions.
And in this evaluation, it is mentioned that if the accurate transfer of implant position and angle can be achieved, then people can obtain a satisfactory, precisely fitting prosthesis.
However, based on the next few studies, and taking into account accuracy factors, scientists do not recommend the use of intraoral impressions with optical scanners in multiple implant cases.
Despite the increasing use of intraoral scanners, this technology still requires a long learning curve and a large initial investment, and has been shown in many clinical situations that intraoral scanners are not as accurate as extraoral scans.
And scanning impressions benefit from eliminating the mold-making process, but the process takes a lot of time and can cause some errors, so scientists believe that scanning conventional impressions may be a good choice in the oral rehabilitation process of implants.
The scientists say the purpose of the in vitro study is to evaluate digital implant impressions in the rehabilitation of full arch implants by using two silicones, corresponding plaster casts and two laboratory scanners.
The idea is proposed, how exactly should it be implemented, and what materials will be used in this process?
Materials and methods
The scientists placed six Straumannian bone horizontal implants in an artificial mandible with artificial gums, each with an internal hexagonal connection and a corresponding screw-fixing abutment.
To simulate the clinical situation, the scientists also scanned the master model using a 120,000-pixel reference scanner and stored the scanned data in a standard template library file, which was later converted as a digital master.
Based on the master mold from this scan, the scientists used two silicone and splint covers with open trays to imprint using dual hybrid technology.
The scientists photographed 10 impressions using two types of silicones, and the trays were made using 3D printing, where the operator pre-applied silicone adhesive to the impressions and tightened the top cover into a random reservoir.
The operator then uses a methyl methacrylate splint, and after 24 hours of polymerization, they slice the splint and add a small amount of other resin, wait until the resin polymerizes, unscrew the top cover, and remove the indentation from the parent casting.
The operator then carefully trims any excess impression material and buccal surface of the impression with a scalpel, which allows the impression to be exposed to light, and then stores the impression at a temperature of 8°C for 23 hours.
For digitizing the main casting mold using an out-of-mouth scanner, titanium dioxide sprays particles according to the specification to obtain the minimum thickness of the coating on the impression.
To digitize the impressions, the operator uses two laboratory scanners, a blue LED scanner and a structured light optical scanner, and the scanned images are stored in a standard template library file
Before pouring the plaster, the operator removes the scanning simulation and screws the multi-unit analogue into the transfer unit, which is followed by cleaning and drying the impression and placing artificial gums on the impression.
Plaster made of elite rock is used to cast castings, the impression is poured under constant vibration, and after 2 hours, the operator separates the impression from the casting by unscrewing the transfer plate.
The operator digitized the impression using an extraoral scanner and placed a scan of the implant in it, after which the operator sprayed titanium dioxide particles on the stone model.
The operator then used two scanners to scan the stone model and save the scans in a standard template library file, all in a controlled environment, and finally the research team obtained the data results.
The data obtained are described by the scientists as median and interquartile values for different groups, the Shapiro-Wilke test is used to determine the normality of the distribution, and the equality of variance is determined by the homogeneity of variance test.
The scientists then compared the differences between groups by using a nonparametric Mann-Whitney test, and statistically significant values used when using specialized software for p<0.05.
discuss
In fact, this study did not find differences between the extraoral scanners used in the experiment, between the impression materials, or between the Canter silicone and the corresponding plaster model, so the previous two null hypotheses were accepted, but the third null hypothesis was partially rejected.
The accuracy of an experiment is defined by realism and precision, as accuracy corresponds to reproducibility between repeated measurements, while realism describes the proximity of real-world dimensions.
To assess accuracy between the master model and the virtual model, the scientists used digital software with a best-fit overlay algorithm, which has been recommended for accuracy analysis.
Although some scholars have reported that this method cancels out the results due to the positive and negative deviations between the reference model and the dummy model, on the other hand, the global best-fit overlay does not allow people to determine the direction of the difference, so it does not fully show the true divergence.
Another technique used is local best-fit overlays, where certain overlap regions are defined, but the global best-fit algorithm is still more repeatable, and the manual selection used by the local best-fit algorithm is more error-prone.
In the experiment, the scientists chose to use rigid custom trays, which is not a key factor affecting the accuracy of implant impressions, although the elastic material has a high degree of dimensional stability, but in the scientists' research, they found that the accuracy than rigid custom trays will be higher than that of non-rigid trays.
However, the splint technique used in the study could not be used for the immediate implant loading protocol for full arches, and a second appointment would be required if the splint was cut off 24 hours after polymerization and more resin was subsequently added.
For impression materials, the scientists chose polyether and polyethylene siloxane, both of which are excellent impression materials for implant rehabilitation, and all impressions in this study were made of polyethylene siloxane.
Experimental results show that optical impressions are reliable for small span cases, but when considering large span cases with more than 22 implants, the accuracy of intraoral scanning impressions is still not comparable to the accuracy of traditional impressions.
The use of laboratory scanners for impression digitization in these cases is an effective alternative, as scientists have shown that for one of the elastomers studied, Golden Mark silica gel, it has shown better accuracy than stone castings in research.
The scientists found that the differences between the Canter and Jinmac materials may be related to differences in optical or mechanical properties between silicones, but this information is only speculation, and there is no exact literature to point out.
The digital acquisition method of implant impressions allows people to skip some of the steps that need to be passed in the laboratory, thereby reducing the chance of errors.
When using an intraoral scanner, if the scanned area is larger, the more overlapping images will be made, which will accumulate to create regional distortion.
However, this does not occur with laboratory scanners, as most of the impressions are captured at the same time, minimizing the impact of errors.
In addition, the narrow and deep shapes hidden in the impression make it difficult for the operator to scan accurately, yet there is no deep surface in the impression of completely edentulous patients, which undoubtedly increases the potential for scanning accuracy.
In addition to accuracy, the complete digital workflow for full arch implant rehabilitation has some limitations, mainly due to the two challenges of digital face arch and virtual joints made of plaster.
One of the limitations of the study is the use of an anti-reflective spray, which can make a big difference between the scan inside the mouth and the scanning spectrometer.
Another limitation of the study was the use of a reference model designed by parallel multi-unit designs of the same brand, as the results may vary if some or all of the targets were unconventional gums.
However, we should note that in the full-arch impression, the appropriate selection of abutments can often correct those most unfavorable conditions, and further in vivo studies are needed if more information is to be obtained.
The use of a rotating scanner in this survey does not allow people to extrapolate results based on the accuracy of a single rehabilitation, as it is subject to many influences, such as the brand of implant used, which does not provide a non-rotating digital library for the multiunit used.
In addition, because repeated tightening will cause wear on the scanning body or the scanning simulant, which may affect the accuracy of the scan, in this study, the scientists replaced these components every five experiments.
The purpose of this last in vitro study was to evaluate the accuracy of impressions using two different silicones in the rehabilitation of total arch implants with corresponding plaster casts, using two laboratory scanners.
The scientists created a master model of six dental implants, scanned with a scanner and used as a digital master model, used the opening technique through two silicone impression systems, and obtained 600 implant impressions after pouring plaster.
The scientists used two extraoral scanning systems to scan impressions and stone molds, and best-fit a superposition between the main model and the obtained virtual model, resulting in a result of p < 05.82.
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