A Comparative Evaluation of Trueness of Two Intraoral Scanners for a Partially Edentulous Arch - An In Vivo Study

Introduction

Dentistry has always been considered as a digital process as it requires the hands and fingers of humans to skillfully treat patients. With the evolution of digitization and computer-aided design/computer-aided manufacture (CAD/CAM) systems, breathtaking and outstanding opportunities are created for improving the efficiency of restorative dentistry. Digital systems now offer the opportunity to avoid conventional, traditional impression materials, thereby handling limitations associated with them. Intraoral scanners (IOS) have the potential to offer excellent accuracy and precision with a more comfortable experience for the patient as well as dentist and more efficient workflow for the office.¹

The basic technique in dental practice that is used to generate an imprint of the oral situation is intraoral impression making.² The gold standard impression technique today in the field of dentistry is the physical impression with elastomeric impression material and stock trays.³ These conventional approaches are considered unwieldy, bearing in mind the obstacles and challenges for both patient and dentist, including discomfort, forceful removal of highly retentive impressions with a risk for potential damage to the oral and contiguous structures, nausea, unsatisfactory taste, time consumption and remakes in case of air bubble inclusion.⁴ These problems were addressed to an extent since the advent of intraoral scanners (IOS). IOS is a medical device used to record the three-dimensional geometry of an object with precision which is composed of a handheld camera, a computer, and a software. The most widely used digital format is the open STL (Standard Tessellation Language). The various imaging technologies on which an IOS functions are confocal microscopy, triangulation, active wavefront sampling, stereophotogrammetry etc.⁵ 

Several in vitro studies have reported excellent dimensional accuracy and precision of digital impressions when compared with conventional impressions in a laboratory setup.^6,7 It is of utmost importance to evaluate precision and trueness in a clinical environment in the presence of the patient, the operator and related factors that might affect accuracy such as blood and saliva in the mouth, jaw opening, movement of the patient operator movement, obstructions by cheek mucosa and tongue, reflection of light by intraoral structures, metallic restorations and restrictions of space inside the patient’s mouth. Accuracy can only be evaluated in comparison, preferably with a gold standard, which is not easy to establish in the oral cavity. Accuracy consists of precision and trueness. In previous studies, the measurement for accuracy of conventional impressions were most frequently linear distance measurements. This method is restricted and there is a need to analyze the threedimensional changes in a dental model such as torsions and axis deviations. Specialized softwares are available which are used to generate STL files corresponding to the preparation, restoration, soft or hard tissues, which are superimposed over one another, calculating all possible orientations, and selecting the one with the best objectto-object penetration. This type of procedure or function is known as the best-fit-method. The present study has been designed to compare two different intraoral scanners which work on different principles, that is, confocal microscopy (3SHAPE TRIOS® IOS) and active speed 3D video (CARESTREAM CS 3600® IOS) and also to do a comparative evaluation with conventional elastomeric monophase polyvinyl siloxane impression technique in a partially edentulous patient. The study also compared the trueness deviation between maxillary and mandibular arches when digital impressions are performed. The superimposition software used in this in-vivo study was Geomagic (Geomagic; 3Dsystems) which computes the value by superimposing the point cloud data.

Materials and Methods

Participants with partially edentulous dentition desiring to get missing teeth replaced and who fulfilled the inclusion and exclusion criteria were recruited from the patients who reported to the Department of Prosthodontics, Krishnadevaraya College of Dental Sciences. Patients were informed about the study in brief and written informed consent was obtained from all the study participants.

Inclusion criteria:

1. Partially edentulous patients

2. Patients willing to participate in the study

3. Good oral hygiene

Exclusion criteria:

1. Generalized periodontitis

2. Microstomia

3. Reduced mouth opening

4. Generalized attrition

5. Xerostomia

Sample size estimation and method of sampling Statistical Software:

SPSS (Statistical Package for Social Sciences) Version 20.1 (IBM Corporation, Chicago, USA)

Sample Size Calculation Software: N Master, V.2.0, CMC Vellore, India

Sample Size Calculation: A minimum sample size of 16 in each of the three groups was calculated for the study. The sample size calculations are as follows: Sample Size Calculation (Supplementary Figure)

Study design

The impressions were grouped under two types according to the method followed, that is, conventional impression and digital impression. Digital impression was again subdivided into two groups according to the intraoral scanner used, that is, 3SHAPE TRIOS® and CARESTREAM CS 3600® intraoral scanners. The study design is schematically represented in the flow chart (Figure 1 and Figure 2).

Standardization: Missing maxillary and mandibular posterior teeth where the number of missing teeth were restricted to less than three were selected.

Conventional impression

Standard perforated metal stock trays were used to generate alginate (DPI Algitex) preliminary impressions. These impressions were poured with type III dental stone (KALSTONE) and the cast was obtained on which a light cure acrylic (VOCO, INDIA) custom tray was fabricated with 2 mm thick thermoplastic resin sheet spacer. The impressions were made for all patients according to the same criteria to standardize the procedure and to avoid errors. After the removal of spacer, relief holes for the impression material were made in the custom tray using a round bur. Tray adhesive (3M ESPE) was applied and vinyl polysiloxane impression material (DENTSPLY Aquasil Ultra Monophase – medium body) was loaded onto the tray and also injected around dried teeth surface using auto-mixing gun so that equal amount of base and catalyst are taken and mixed evenly. The impression was made and was removed from the mouth after 4 min which is the setting time provided by the manufacturer and was poured with type IV dental stone (KALROCK, Mumbai India). The cast was retrieved from the impression after 60 minutes based on the product manual. The casts obtained were scanned with a laboratory scanner -White light scanner (STEINBICHLER) and the scan data was exported in STL data format.

Digital impression

Digital impressions were recorded using 3 SHAPE TRIOS® IOS and CARESTREAM CS 3600® IOS. Before scanning, the teeth were dried lightly using compressed air. Saliva ejector and cotton rolls were also used for isolation. Lips, cheeks and tongue were kept out of the scanner’s view with the help of a finger or mouth mirror. The best scanning method is to start with a molar since it has greater details for easier identification. The scanning angle was changed to 35-55 degrees during scanning to allow the surfaces to overlap. The recommended scanning path consists of three sweeps: occlusal, lingual and buccal to ensure good data coverage of all surfaces (Figure 3 and Figure 4). For centrals, it is important to cover both lingual and labial sides of the teeth; to ensure that, the scanner tip was slowly wiggled between the labial and lingual sides. The unwanted areas were trimmed off before saving the STL file. The scans for the patients were obtained by following the same protocol for both the intraoral scanners. The data sets from each scan were automatically saved by the respective software as STL files, which were later used for three-dimensional analysis.

Three-dimensional analysis

All the STL files were exported to 3D analysis software (GEOMAGIC VERIFY; 3D Systems). To measure the accuracy of the intraoral scanners, the datasets were superimposed via best fit alignment method utilizing this software. The trueness of the scanners was evaluated by superimposing the scan data of both intraoral scanners with the STL data of the model obtained by conventional impression respectively. A value obtained by calculating the root mean square (RMS) of the amount of deviation at each measurement point was taken to represent a deviation. The RMS is a general method to assess the mean value of errors, by directly comparing two data groups with an identical coordinate system. A higher calculated RMS value indicated a large error, that is the difference in the attributes between reference and measurement data. A color map representing visual deviation was set with 15 color segments (Figure 5). For the reference model, the range of maximum and minimum nominal values was set at ±0.0500, and the range of maximum and minimum critical values was set at ±0.2000.

Statistical analysis

The normality of the data was checked using the Shapiro wilk test, the result showed significant ‘p’ value thereby accepting the assumption of normality. The student ‘t’ test was used to determine whether there was a statistical difference between study groups in the parameters measured. Data analysis was carried out using the Statistical Package for Social Science (SPSS) Version 20.1 (IBM Corporation, Chicago, USA).

Ethics

The work was approved by the appropriate ethical committees related to the institution in which it was performed and the subjects provided informed consent for their participation in the work. Results The readings obtained were tabulated. The results for 17 Prathibha P et al., RJDS 2022;14(1):13-19 each parameter (numbers and percentages) for discrete data and averaged (mean + standard deviation) for continuous data are presented in Table 1 & 2 and Figure 6. In this test, “p” value of less than 0.05 was accepted as indicating statistical significance.

Discussion

The purpose of this in vivo study was to investigate the accuracy of different IOS’s, that is, 3 SHAPE TRIOS IOS® and CARESTREAM CS3600® IOS and to comparatively evaluate the accuracy between conventional impression and digital impression. This study also investigated the deviations between maxillary and mandibular arches when digital impressions are made. A basic demand in many fields of dentistry is accuracy. Fabrication of an exact fitting prosthesis in replacing dental hard tissue is imperative to ensure proper function and prevent further destruction of the remaining tooth structure. Fixed partial dentures and implant prosthesis should have exact fit on the prepared teeth and implants. In-vivo studies can be done in two ways, i.e., indirect comparison and direct comparison. In indirect comparison studies, a prosthesis is fabricated and the fit of the same is evaluated in terms of marginal accuracy or patient’s comfort in both digital and conventional impressions. Direct comparison studies are performed by comparing the STL files obtained by both digital and conventional impression techniques by a superimposition software. Indirect studies have reported clinically acceptable fit of restorations fabricated using a digital impression technique and when compared to restorations fabricated using conventional impressions.8 It should be kept in mind that these indirect studies take the entire manufacturing process of a prosthesis into consideration while evaluating the accuracy and do not evaluate the impression procedure exclusively. Therefore, a method to compare only the impression process is required. 

Therefore, the aim of this study was to compare the conventional and digital impression techniques using direct technique. However, accuracy, which constitutes trueness, is strenuous to evaluate in-vivo because the original dimensions of the test participant, the gold standard, cannot be easily measured in the oral cavity of patients. In this in vivo study, the deviations in superimposed impressions were evaluated by a superimposition software and could not distinguish superiority of digital impressions or conventional impressions as the latter was taken as reference model. It was not possible to measure the absolute accuracy of each impression technique due to the absence of a reference model as a standard. In this in vivo study, as there was no reference model to compare the trueness, conventional elastomeric impression was considered as the reference as it is the current gold standard. According to Wee, the use of either polyether or addition silicone is recommended for direct implant impressions.9 However, Thongthammachat et al., (2002) related that addition silicone was clearly superior to polyether although some clinicians have suggested that polyether presents superior elastic properties and accuracy for implant reconstruction.10 Monophase technique, whether polyether or vinyl polysiloxane, generally produced better details under either wet or dry conditions compared to the dual-viscosity technique. Possible explanations for this disparity may be differences in the composition and amount of the filler, inadequate polymerization, and poor percent recovery or creep behavior.11 Hence in this study, monophase vinyl polysiloxane impression material was used for making conventional impressions for the partially edentulous cases. Digitization of dental impressions of abutment teeth using a white light scanner was assessed to be a highly accurate method and provided values in a clinically acceptable range.12 Hence in this study, a white light lab scanner was used to scan the model acquired by conventional impression.

Statistical interpretation

This study proved that there was no statistically significant difference between two of the major IOS’s available in the market despite being different technologies. This can be attributed to the fact that both the scanners follow superior intraoral scanning technology, that is, principle of confocal microscopy and principle of active speed 3D video. Further there was no major difference in the accuracy of digital impression between maxillary and mandibular arches which could be due to the superior wand design of the studied IOS’s.

Conclusion

Within the limitations of this study, the following conclusions were made:

1. There were no statistically significant differences between the two digital intraoral impression systems compared, that is, 3SHAPE TRIOS® IOS and CARESTREAM® CS3600 IOS.

2. It was shown that the 3SHAPE TRIOS IOS system showed lesser deviations when compared to CARESTREAM CS3600 IOS system.

3. There was no statistically significant difference between maxillary and mandibular arches for each intraoral scanners compared.

4. Although some minor deviations were observed among different intraoral scanners and between the maxillary and mandibular arches, the clinical impact of these differences was negligible.

Mention of unanswered questions

1. Which is better among the intraoral scanner and conventional impression?

2. How is the accuracy of IOS’s different in hard and soft tissue region?

Mention of new questions raised

1. How can you compare the accuracy between digital and conventional impression in an in vivo study?

2. Can digital impressions be used for all types of prosthetic procedures?