Key words: Accuracy, edentulous, scan bodies, digital impression

Introduction:

Tooth loss results in impairment of masticatory function, speech, aesthetics, and also affects the psychology of the patient. Implant-supported prosthesis, to replace naturally missing teeth, is one of the most common treatment modalities.¹ An accurate implant impression is an essential pre requisite for implant restorations, as inaccurate transfer of the implant position can lead to an ill fitting prosthesis, which may induce unnecessary strain on several prosthetic components and may result in various complications. Moreover, there is no intervening periodontal ligament at the implant-bone interface to compensate for any inaccuracies. The different factors that influence the implant impression accuracy include the impression techniques, materials used, and the number of implants present.²

There are two main conventional implant supported impression techniques: the direct/ pick-up technique that uses an open-tray; and the indirect/ transfer technique that uses a closed tray. The open tray technique is chiefly indicated when the implants are not oriented parallel to each other, and can further be subdivided into splinted and non-splinted techniques. The closed tray technique is mainly indicated in case of restricted mouth opening, limited access areas (posterior region) or in patients with strong gag reflex.³

Nowadays, digital impressions are widespread and have revolutionized the field of implantology. Compared to traditional implant impression techniques, digital impressions eliminate several procedures such as dispensing and setting of impression materials, disinfection, and stone cast pouring. Also, the simplified workflows not only improve time efficiency, but also reduces the possibilities of deformation.⁴

Digital impressions can be achieved with the help of scan bodies. In the field of implantology, standardized scan bodies that are inserted onto the implant instead of impression copings, have been developed and well established. This enables computer-aided determination of the actual implant position using data from the digital scanner.⁵ Implant Scan bodies are precision attachments that are screwed onto the coronal portion of the implant and reproduces its position in the digital model. They also assist on the digital transfer of 3-dimensional position of dental implants from the patient’s mouth to computer-aided design (CAD) softwares.⁶

Implant scan body (ISB) characteristics such as connection type, geometry, dimensions, material and reusability can play a significant role in the overall accuracy of the intraoral digital impression.⁷ Currently, there are different types of scan bodies of respective implant system that are available in the market. These scan bodies are manufactured as monolithic components or by a combination of different materials, as titanium alloy, polyetheretherketone (PEEK), aluminum alloy, and various resins. Scan bodies from different manufacturers also differ in their characteristic geometries as well, such as Flag shaped, Cylindrical, Tapered, Straight with/ without bevel.⁷

However, there is limited literature on the effects of ISB material and geometry on the accuracy of digital impressions made in fully edentulous arches. Hence, there is a need of more detailed investigations on these parameters of scan body which would be helpful for more accurate reproduction of the full mouth implant supported prosthesis. Therefore, the purpose of this systematic review is to evaluate the accuracy of digital impression by using scan bodies of different geometry and material for full arch implant prosthesis.

Material and Methods:

This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines8 with prior registration in PROSPERO (Registration number CRD42023433845). The focused question was “Will different geometry and material of scan bodies have an effect on the accuracy of digital impression for fully edentulous arches for full arch implant prostheses?” The PICO i.e., the Population, Intervention, Comparison, and Outcome format was used (Table 1). The inclusion criteria were studies that evaluated the effect of Scan bodies in fully edentulous arches for full arch prosthesis, studies on accuracy of digital implant impression by using scan bodies and articles appearing in the English dental literature, published after year 2011 till 31st May 2023. The exclusion criteria were studies wherein the use of scan bodies was limited to partially edentulous arches. Review articles, case series and case reports were also excluded.



Electronic search of PubMed (including MEDLINE), Cochrane Central, EBSCO host databases and Google Scholar search engine for articles published from 1st January 2011 to 31st May 2023 was conducted. The controlled vocabulary terms (i.e., MeSH terms) and free text terms were obtained by searching key concepts in the MeSH database and a thorough evaluation of related articles, thesaurus, dictionaries, and entry terms. The terms such as edentulous jaws, edentulous mouth, edentulous patients, fully edentulous arches, completely edentulous arches, full mouth implant impressions, digital impressions, dental digital impressions, complete arch impressions, virtual impressions, dental scan bodies, digital scan bodies, dimensional measurement accuracy, data accuracies were combined using suitable Boolean operators (AND, OR, NOT) (Table 2).



An electronic search was conducted independently by two reviewers (S.K., A.P.) A total of 1166 articles were obtained via electronic search. The articles thus obtained were evaluated for duplicates. A detailed summary of data selection has been put forth in the PRISMA 2009 Flow Diagram⁸ (Figure 1). The study characteristics of each systematic review were extracted including study details, search details, analysis and results/findings by two independent reviewers (S.K., A.P.) A third reviewer (N.P.S.) was called in for a final decision if any disagreement persisted between the two calibrated reviewers.

Results:

The 1166 articles that were obtained through the electronic searches were compared meticulously with respect to the author’s name, year of publication, title, abstract as well as the journal name, issue and volume number. The articles thus obtained after the electronic and manual searches, were evaluated for duplicates using the Mendeley Desktop software (v1.19.6). The 2 articles obtained through the manual search were added manually using the ‘add entry manually’ feature of Mendeley Desktop software (v1.19.6). Duplicates were identified and removed using the software’s “Check for Duplicates” feature. 716 duplicate articles were identified and subsequently eliminated leaving behind 450 articles. Two calibrated reviewers (S.K., A.P.) independently screened the relevant titles of the studies found through the electronic search. Out of 450 articles, 213 articles were excluded after screening of the title. The articles thus eliminated were either literature reviews, scoping reviews, case reports, case series, or articles on utilization of scan bodies on partially edentulous arches. Thus, 237 articles were selected after title screening.

Two calibrated reviewers (S.K., A.P.) now independently screened the abstracts of the studies found relevant during the screening of the titles and a total of 188 articles were further excluded after abstract screening. The articles eliminated through abstract screening were mainly involving different impression materials and comparing different brands of scan bodies. 49 articles were included after abstract screening. Hence, 8 articles were selected after abstract screening and thus were included in this systematic review. All the 8 included articles were in-vitro studies (Supplementary Table 1 and 2).





A third reviewer (N.P.S.) was called in for a final decision, if any disagreement over article selection persisted between the two calibrated reviewers. Inter-reviewer reliability was checked via Cohen’s kappa coefficient.⁹ The Cohen’s kappa coefficient values obtained for title, abstract and full text screening was 0.65, 0.72 and 0.68 respectively, indicating moderate inter reviewer agreement for title, abstract and full text screening. The data was subsequently extracted from the 8 included studies and recorded in 2 excel data extraction sheets as mentioned in the summary table (Table 2)

1. DIFFERENT SCAN BODY MATERIALS

• Titanium
• PEEK
• PEEK with Titanium base

2. DIFFERENT SCAN BODY GEOMETRIES

• Cylindrical
• Cylindrical with bevel
• Flag shaped
• One/ Two-piece SB with milled/pyramidal side
• Tapered

The data extracted was entered under the following headings: Author and Year of publication, Study design, Study model, Number of ISB used, Number of scans, Material/ Geometry of Scan body, Type of digital scanner used, Measurement variables, Test of Analysis, Outcome and Additional Outcomes Risk of bias assessment of the included studies was done using the QUIN tool scale10 by two independent reviewers (S.K., A.P.) This scale is primarily used to assess the risk of bias of in-vitro studies. Since all the 8 included studies were in vitro studies, this scale was considered apt for the risk of bias evaluation in this systematic review. The changes made to the scale were validated by the third reviewer (N.P.S.) In this scale, the items are scored 0 if not specified, 1 if inadequately specified or 2 if adequately specified. The results were then summed to obtain an overall score for a given in vitro study. The scores thus obtained were used to grade the in vitro study as high, medium, or low risk (>70%=low risk of bias, 50% to 70%=medium risk of bias, and <50%= high risk of bias) by using the following formula :



The risk of bias of all the 8 included studies ranged from 79% to 91%, which falls under the category of low risk of bias.

Meta Analysis

Eight studies evaluating the accuracy of digital impression with different geometry and material of scan bodies for fully edentulous arches were included in the systematic review. Five studies which evaluated the effect of different geometry of scan bodies on accuracy (Mizumoto et al.,2019; Motel et al., 2019; Alvarez et al., 2022; Lawand et al.,2022; Gomez-Polo et al., 2022)^1,13,7,16,17 were excluded from meta-analysis due to lack of uniformity in the comparison groups. Three studies which evaluated the effect of PEEK vs Titanium material scan body on the accuracy of digital impression (Arcuri et al., 2019; Kim et al., 2020; Karthhik et al., 2022)^12,14,15 were included for meta-analysis.

The Review Manager software (Version 5.4.1) was used to perform meta-analysis. Mean values and standard deviations for linear discrepancies were included for the analysis. The linear discrepancies were measured along three axis- X, Y and Z axis. The primary outcome measures the accuracy of digital impression, and was evaluated by measuring the linear deviations along X, Y and Z axis. More the linear deviations present, lesser is the accuracy. The data was tabulated under the headings of study name, group, and effect size. The effect size was calculated on the continuous raw data entered for mean, standard deviation and sample size. 95% confidence interval for each effect size was also computed. The heterogeneity of effects was assessed by the Higgin’s I² test.¹¹

A statistically significant difference was observed between the two materials of scan body along X axis (P < 0.05, pooled mean difference = 0.00 CI = 95%), along Y axis (P < 0.05, pooled mean difference = 0.07 CI = 95%), and along Z axis (P < 0.05, pooled mean difference = 0.03 CI = 95%) as stated in the forest plot. The results of the meta-analysis for linear marginal discrepancy showed minimum discrepancy in PEEK and maximum discrepancy in Titanium scan body along X, Y and Z axis (Figures 2, 3, 4 respectively)

Discussion:

This systematic review analyzed the effect of the various characteristics of ISBs on the accuracy of implant impression, and is chiefly based on in vitro studies. Among the parameters assessed in this study were the scan body material and geometry.

The ISB material is a crucial factor that has to be scrutinized, as it can have a significant impact on the biocompatibility and scanning accuracy which depends on surface reflections and in turn can influence the number of stitching points in order to attain the desired results.¹² Different materials which are used include polymers such as polyetheretherketone (PEEK), titanium alloys, aluminum alloys, and resin materials. PEEK and Titanium are the most commonly used for ISB fabrication. PEEK is high-performance thermoplastic polymer with excellent physical and mechanical properties, chemical stability and low weight. It is often used in ISBs because it can easily be scanned compared to other materials and it does not cause any surface reflections which could cause hinderance in intraoral scanning.¹²

However, the selection of polymeric materials has recently come into question, as repeated sterilization, clamping forces, and even chewing forces can deform and abrade the polymeric materials. This is the reason why PEEK ISBs are suggested only for single usage. Ti-alloy components are also commonly selected for the fabrication of ISB. It represents excellent biocompatibility and is resistant to deformation on repeated sterilization.¹³ Hashemi et al evaluated the effect of repeated use of PEEK and Titanium Scan Bodies on the transfer accuracy of implant position and demonstrated that the inter-implant distance variations were more in titanium as compared to PEEK scan bodies. The results further indicated that titanium scan bodies had lesser dimensional changes as compared to PEEK scan bodies after repeated use.¹⁹

Nevertheless, there were inconsistencies between the different studies about which is the preferable material for ISB. In a study given by Arcuri et al, PEEK ISBs showed optimal results on both linear and angular measurements, which was followed by Titanium. PEEK with titanium base showed least accuracy because of its bi component configuration.¹² In a study given by Karthhik et al, PEEK SB material showed more scanning accuracy as it reduced the problem of light reflectance that can occur in the metal alloy.15 PEEK showed optimal results on both the studies however this was in disagreement with another study by Kim et al, which presented better trueness and stability of the Ti-ISB compared to PEEK ISBs.¹⁴

Another characteristic feature that was assessed in the included studies was the ISB geometry. A relatively flat, more simply constructed scan body resulted in significantly smaller deviations within the digital impression. In two studies ISB with overall cylindrical geometry with a partially beveled upper part showed better scanning trueness in distance deviation.^1,6 The current study found out that worst results were seen in scan bodies which had a very complex anatomy with irregular surface. This finding agrees with the study published by Kurz et al, which shows that more intricate the scan body surface, as in sharp edges, more are the errors registered.²⁰ The ISB geometry bevel also has a significant effect on the scanning accuracy and the lingual placement of the bevels is proposed for best results.¹⁷ An interesting finding is that the scanning trueness of ISBs was improved by subtractive modifications in design, whereas it was reduced by additive alterations.18 Surfaces which are more challenging to scan include steep, sharp, deep undercuts, angled or overcrowded surfaces.

The position and location of implant and ISB has impact on the scanning trueness in distance deviations. If the most distal implant was tilted mesially, there would be better trueness in scanning accuracy. The extent of edentulism is another important factor; Free-end partial edentulism (Kennedy I, II) results in higher deviations compared to Kennedy III, IV. This is can be possibly attributed to inter-implant space which is limited in Kennedy I and II cases.²¹ In this systematic review, majority of studies had linear and angular discrepancies as measurement variables. In terms of material, PEEK showed least discrepancies, followed by Titanium followed by PEEK with Titanium base. Considering the geometrical aspect of ISB, Scan bodies with cylindrical and simple configuration showed least discrepancies.

The QUIN Tool has been used here based on the in-vitro study design of the included studies to identify the risk of bias of the individual studies.¹⁰ Three out of 8 included studies seemed to be relatively homogenous in their study design and outcome variables. Hence, a quantitative analysis by means of a meta-analysis was planned. Meta-analysis is a systematic procedure that is used for assessing and combining statistical information based on results of available independent studies regarding the same topic. The results of the quantitative analysis have been provided in the form of forest plots for easy visualization. The heterogeneity of the primary studies has been evaluated using the Higgins’s I2 test.¹¹ Heterogeneity refers to differences in results between primary studies that are greater than expected by chance alone. The results of the meta-analysis for linear marginal discrepancy showed minimum discrepancy in PEEK and maximum discrepancy in Titanium scan body along all three planes.

After evaluating different studies in the current systematic review, PEEK biomaterial scan bodies do show a promising outcome for better accuracy for full arch implant supported prosthesis as compared to that of Titanium scan bodies. Also, when it comes to the geometry of scan bodies, it can be said that scan bodies with least complex geometry will show minimal errors during scanning, especially in full arch cases. Hence, clinicians will have a better understanding in selection of scan bodies when it comes to scanning of fully edentulous arches.

Limitations of this systematic review were that all the included articles were in vitro in their study design, as there were nearly no clinical studies in the available literature. The search for this study was also limited to articles published in the English language only. Other ISB parameters like location of ISB, wear, inclination of dental implants and ISB dimensions were not included in this systematic review as it would contribute more to the heterogeneity of the current systematic review. Hence, the results of this systematic review should be applied with caution to the clinical scenario and more in vivo randomized controlled trials should be carried out to support the current evidence.

CONCLUSION

Although intraoral scan bodies and their characteristics vary widely, they significantly influence implant impression accuracy. The majority of studies agree that, among the various characteristics, the material, and the geometrical design affect the impression accuracy significantly.

1. In terms of materials, PEEK scan bodies reported least discrepancies, thereby deeming to be more accurate than Titanium scan bodies.
2. There is an overall increase in dimensional accuracy of digital impression recorded by scan bodies of cylindrical and simpler geometry.

These conclusions enable the clinician in proper decision making to choose the PEEK scan bodies with simple geometry whenever possible for digital impressions of their full arch implant cases. However, more clinical studies are necessary for safer conclusions, since the available scientific evidence is not yet conclusive about the optimal Intraoral Scan Body.

REFERENCES