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Impressions in Implantology. Literature Review

Neada Hysenaj*, Edit Xhajanka, Anis Thodhorjani, Assoc. Silvana Bara, Ramazan Isufi, Fatmir Lela, Tedi Verçani, Endrit Paparisto, Edlira Mulo

Faculty of Dental Medicine, University of Medicine, Tirana

Abstract: The aim of the impression in implant prosthetic restorations is to register in an accurate way the relation of the analogue or abutment to the oral tissue that surround the implant. Its purpose is to register the position and axis of the implant. A search in PubMed was done, focusing on the full text articles, describing the characteristics of conventional and digital impressions in implantology. The inclusion criteria were articles that described: the open tray technique, closed tray technique, digital impression, impression material, digital accuracy, scan abutment materials, scanning technique and the passive fit. 201 articles were found, 78 articles were selected to conduct this review. There are several factors that influence the success of an impression: the proper components of the implants, the adequate selection of the tray: stock or individual, the selection of the impression material, the choice of the impression technique, of the screw driver: hex, external or internal, the selection of healing caps, impression copings and abutments. The success of the impression depends also on the number of implants, implant angulation, splinting the impression copings, setting expansion of dental stone, polymerization shrinkage of the material. In the multiple implant clinical cases, the splinting or no splinting process seems to be important in the impression accuracy. There are several possible mistakes which can occur during implant impression, such as unseated impression copings, the dimensional change of the material used for the impression, minor movements which might occur by retrieving impression copings and connecting analogues, and the movement of analogues in the cast because of the dimensional changes of plaster. With the high development of the dental industry, digital impression is an important part of implantology. It is important to evaluate the accuracy of the digital impressions, its advantages, disadvantages and its indications. Another important aspect is the comparison of the accuracy between the traditional impression and digital impression.

Keywords: digital impression, conventional impression, accuracy

Received November 06, 2023; Revised December 07, 2023; Accepted December 13, 2023

Citation: Neada Hysenaj, Edit Xhajanka, Anis Thodhorjani, Assoc. Silvana Bara, Ramazan Isufi, Fatmir Lela, Tedi Verçani, Endrit Paparisto, Edlira Mulo, Impressions in Implantology. Literature Review. International Journal of Dental Sciences and Research, 11 (2). 27-32. January 2023. doi:10.12691/ijdsr-11-2-3

1. Introduction

Implant prosthetic restorations generally require two steps: the implant placement and the positioning of the prosthetic part after four to six months of osteointegration. The passive fit of the prosthetic restoration is another key factor for the success of implant prosthetic restorations [1-3]. The passive fit of the fixed restorations constructed from the conventional impression is properly described in the literature, while there is lack of evidence regarding the passive fit of the restorations over implants constructed with digital impression [4,5]. The aim of implant impression is to reproduce in an accurate way the 3-dimensional position of the implant platform [6]. There are several techniques to realize the impression procedure [7]. Although the open tray technique is most widely accepted, especially in the rehabilitation of the edentulous arch [8,9]. This technique is proven to offer acceptable clinical results. In the recent year digital techniques have been developed. They have several advantages: they reduce the time of the clinical and laboratory procedures, several steps are avoided such as pouring, disinfecting, and sending molds to the laboratory, the procedures are more comfortable for the patients, especially those with gag reflex, it makes easier the communication among doctor, lab technician and patient. The intraoral scanner offers the possibility to the dentist to have a 3D dimension of the implant and realize an accurate picture of the oral tissue situation of the patients. The traditional technique might cause to the patient the discomfort, especially in some clinical cases such as difficulty in breathing, tooth sensitivity, difficulty in mouth opening. They are also difficult to achieve accuracy especially in long span implants [10-12].

2. Measurements of Impression Accuracy

There are contradictory results in the literature regarding the accuracy of implant impression. It might occur because the different techniques which are used to evaluate the impression accuracy. The accuracy can be measured based on absolute or relative values. The technique which uses absolute distortion values includes an external reference point for each scan [13]. While in the technique where relative distortion concepts are used, there are included the position of the implant/abutment and is focused on the distance between implants and their angulation related to the master cast [14]. In literature the impression accuracy is defined by trueness and precision. Trueness is the ability of the measurement to have the real value being measured, while precision is the ability of the measurement to coincide the same value several times. The purpose is to evaluate the impression accuracy through 3 D technique through scanning. Trueness can be determined by comparing the STL files obtained from the scanning of the model with an intra-oral scanner and the scanning of the same model with an equipment with very high accuracy such as a coordinating measuring machine, or an industrial optical scanner. The inter-implant distance and inter-implant angle can be measured and compared. Precision can be evaluated by superimposing different scan made from the same model and from the same intra oral scanner (or device). Different techniques have been used to evaluate the impression accuracy. This might be also a reason why there are no consensual results and conclusions.

3. Conventional Intra-oral Implant Impressions

Current protocols regarding implant impressions are based on conventional prosthodontic techniques. The cast required should not be subject to distortion. As previously stated, the goal of implant impressions is to capture the spatial relationship between the implant platform and the surrounding oral structures. Traditional implant impression approaches include replacing the implant healing abutment with a machined impression coping. Different articles have discussed the ideal impression technique. Variables which may influence implant impression accuracy include the type of tray, impression technique, implant angulation and design and material of the abutments [15].

3.1. Impression Material

An ideal impression material should have a good dimensional stability and should be highly accurate [16]. This statement stands for both tooth and implant impressions. The choice of material vary on the undercuts present, the clinician’s preference, and timing based on dimensional stability. Clinically, the two most commonly used materials are polyether and polyvinyl siloxane. Elastomeric impression materials have a high accuracy, an adequate dimensional stability and proper resistance. Polyether is more hydrophilic than Poly vinyl siloxane. It is also more rigid so has the potential for less distortion under the mass of the die. However, it has lower recovery from strain than PVS, so may not be recommended for use in situations with significant undercuts [17]. Some studies have compared the accuracy of these two materials for implant impressions and have found no significant difference between them [18]. Lee performed a systematic review of the accuracy of implant impressions and found no difference associated with the choice of impression materials. Based on the current evidence, in the clinical cases of parallel implants, the use of either polyether or polyvinyl siloxane is recommended. In cases of angulated implants, polyvinyl siloxane showed higher accuracy.

3.2. Closed Tray Technique vs Open Tray Technique

There is no sufficient evidence related to the comparison between the stock trays and individual trays. An ideal impression tray creates a rigid substructure for the impression and allows for even thickness of the material. There is some evidence that custom trays may produce more accurate impressions in certain clinical situations, with one study finding a statistically significant difference when comparing stock versus custom trays (p=0.01); however the clinical significance of the average difference, 10 μm, is debatable (Burns et al. 2003). The two conventional implant impression techniques include the “closed-tray technique” and the “open-tray technique”. The first one includes tapered impression, the second one can be obtained by using impression copings positioned upon the fixtures. There is not a definite consensus which technique to use. 14 studies make a comparison between the accuracy of open vs closed tray technique. Based on the review no conclusions could be conducted. The advantages of the open-tray technique are: easier access, better control over impression material flow, and low possibility of distortion during the manipulation procedures [19]. The advantage of the closed-tray technique is thought to be in its simplicity. It is suggested when there is not enough space for the screwed copings. The open tray technique is suggested to have a higher accuracy in cases with multiple implants in the same arch. 9 studies evaluated the accuracy between the open tray and closed tray technique. 5 studies concluded that the open tray technique had a higher accuracy (Carr 1991; Barrett et al. 1993; Assunção et al. 2004; Del’Acqua et al. 2008), 3 concluded no difference (Herbst et al. 2000; Naconecy et al. 2004; Wenz & Hertrampf 2008) and only 1 found a greater accuracy with the closed-tray technique (Humphries et al. 1990). Different materials and techniques have been used in splinting the impression copings. 17 studies compared the splinted with the non-splinted technique. It can be concluded that the splinted impression technique had a higher accuracy compared to the non-splinted technique in multiple implants. Regarding short restorations, there was not sufficient scientific evidence, in order to draw conclusions which technique was most accurate. Several factors might influence impression accuracy such as the implant angulation, the type of implant connections. There is no ideal impression technique, it is unavoidable some displacement of the analogues in the model.

4. Digital Intra-oral Implant Impressions

The digital impression is a pretty simple procedure in the clinic, but behind it there is a rather complex mechanism ongoing. A light beam or laser is reflected, the light is deformed, and this optical effect is captured from the one or more cameras situated in the tip of the intra oral scanner. The registration is done through the program by creating points and meshes, which gathered together create the three-dimensional virtual model. The digital impression facilitates the connection between the clinician and lab technician. Dental industry has developed very fast. There is enough evidence regarding the accuracy of the digital impression for the construction of the fixed restorations over natural teeth. While there is a lack of scientific evidence regarding the accuracy of digital impressions over implants. Digital intra-oral scanning was first developed in the prosthodontic and then was used in orthodontic, implantology [20]. Digital impressions were developed with the aim to overcome the disadvantages of the traditional impressions. The requirements for realizing a digital impression are: the scanner, the software that constructs the computer aided design and a machine that transforms the designed restoration in the desired product. The digital impression seems a very simple procedure, but in itself is a rather complex mechanism. When choosing an intra oral scanner, a clinician should take into consideration several factors such as: its accuracy, the way of manipulation, the speed of image registration, the size of the tip of the intraoral scanner.

4.1. Intra Oral Scan Bodies

Scan bodies are devices which help to register the position of implants, they are specific to brand implants and they come in different shapes, designs, surfaces and connections. They are constructed from the body, base and scan region. Only four articles have evaluated the effect of the scan bodies in the digital impression accuracy. Based on the current evidence, the accuracy of digital impression depends also on the geometry and design of the scan bodies. The most common shape of the scan bodies is the cylindrical shape. Arcuri concluded that the scan bodies made of PEEK was most accurate, followed by titanium and at least hybrid scan abutments: PEEK and titanium. To come to this conclusion, the linear and angular deviations were evaluated.

4.2. Scanning Technique

Only six articles evaluated the effect of the scanning technique on the digital impression accuracy. Based on these articles, the scanning protocol might influence the performance on the intra oral scanner, as a result it influences the accuracy of the impression. According to Giménez-González, the distance and angular deviations increase throughout the arch. As a result: the first scanned quadrant has better accuracy compared to the second quadrant. Motel21 compared the one step with the two steps scanning technique and concluded the one step technique was more accurate. Motel [21] made a comparison of three different designs of scan bodies: ELOS A/S, nt-trading and TeamZiereis. The ELOS A/S geometry of scan body demonstrated higher accuracy compared to the other two designs of the scan bodies examined. Mizumoto [22] made a comparison between five different scan bodies designs: Atlantis I-Flo (AF), Core3D (C3D), Nt-Trading (NT), Dess-USA (DE), Zimmer Biomet (ZI). The intra-oral scan bodies were positioned on an edentulous maxillary model with four analogs. Five scans of the model were realized with an intraoral scanner, which were superimposed with the scanned of the reference model. The distance deviation and angular deviation of the Intra oral scan bodies was calculated. It was concluded that Zimmer Biomet scan bodies demonstrated higher accuracy compared with the other designs.

5. Accuracy of Digital Impression Compared to the Traditional Impression

The methods used to analyze the accuracy of the impressions in the reviewed studies were varied and included the following: 1. Prosthetic fit analysis, examination of marginal spaces with an optical microscope. 2. Analysis of deviations in the distance between implants with digital calipers. 3. Linear, angular and/or three-dimensional deflection analysis by overlaying models as surface tessellation language (STL) files using software such as Geomagic Control X. Conventional models are transformed into STL files by scanning them with an extraoral laboratory scanner. The matching algorithm is one of the most common methodologies used to investigate accuracy through STL overlay. 4. Analysis of linear, angular and/or three-dimensional deviations with a coordinate measuring machine and the corresponding software. Most studies used the last two methods listed above.

5.1. In Vitro Studies that Compare the Accuracy of Digital and Traditional Impression

15 in vitro studies have compared the accuracy of digital impressions toward conventional impression in total edentulism. Six studies showed that digital impression had higher accuracy, four reported no difference and five studies concluded that conventional impression had higher accuracy. In a study of Papaspyridakos [23], where a case of edentulous master cast with 5 implants were evaluated. 10 digital casts were obtained. 4 groups were created with 10 impressions each: 1. Splinted implant level, 2. non-splinted implant level, 3. Splinted abutment level, 4. Non-splinted abutment level. The master cast was itself scanned and all the STL files obtained were compared. It was concluded that digital impression had a comparable accuracy with the traditional impression. The splinting technique showed higher accuracy. At abutment level there was no difference. The inclination of the implants with 15 degrees had no effect in the accuracy. A similar study was conducted by Amin [24], who found that digital impression had a higher accuracy versus traditional impression. The accuracy of the impression taken with True Definition intra oral scanner was higher compared to the impression taken with Omnicam. The different results based of the two studies might be the different intra oral scanners used and the different level of experience of the clinician. Nine in vitro studies compared the accuracy of the digital impression and the traditional impression in partial edentulism casts. All the studies used 2 implants, except of one study with 3 implants. 7 studies concluded that conventional impression had higher accuracy, 2 studies reported higher accuracy for the digital impression. Regarding the in vitro studies with a single implant, the most recent study was by Yilmaz, who compared the accuracy of conventional impression, with a normal scan abutment and scan-peg healing scan abutment [25]. They concluded that digital impression had higher accuracy compared to the open-tray implant-level traditional impression. The impression from the conventional scan body showed higher trueness compared to scanpeg system.

5.2. In Vivo Studies that Compare the Accuracy of Digital and Traditional Impression

There is also lack of sufficient research regarding the comparison of the accuracy of digital and conventional impression in clinical studies. In a study at Harvard University, it was evaluated the efficiency of the traditional and digital impressions in implantology. The digital impression had a higher efficiency and had less difficulties in the operating technique, compared to the traditional impression. Andressien et al. realized a clinical study where the accuracy of digital impression was compared to the accuracy of traditional impression. The study was conducted in 25 patients. The iTero intraoral scanner was used. The inter implant and angular deviations were measured. This study concluded that digital impression was not sufficiently accurate to be used in the prosthetic restoration construction over two implants in the mandibular jaw. A possible reason might be the absence of the reference points in the total edentulous clinical cases. This was the oldest study related to this topic. The latest study was by Schmidt, who involved 39 patients. Individual reference keys were constructed on implants. Both digital and traditional impression were taken. The implant distances were registered through the coordinate measuring machine, and the superimposition of the files were obtained, to assess the deviations and to compare the accuracy of the two techniques. Based on the study, the accuracy of the digital impression was comparable to the accuracy of the traditional impression for the total and partially edentulous cases. Chochlidakis [33] included 16 patients in the study, who were treated with total fixed dentures over implants. The casts were scanned with an extraoral scanner with high accuracy, respective STL files were obtained. The intra oral scan was also done, the STL file obtained was superimposed with the STL files from the extraoral scanner. The conclusion was that the digital impression has an acceptable accuracy in total reconstruction of fully edentulous patients. Regarding the clinical studies that compare the accuracy of the two technique, further research is necessary, especially in the cases with full edentulism.

6. Comparison of Conventional vs. Digital Impression Techniques

A recent study that compared the digital impressions with traditional impressions of the fixed restorations, concluded that digital impressions are clinically acceptable [26]. Syrek et al. determined that digital impression of single crown constructed from zirconia, had a higher accuracy than the traditional impressions [27]. Anyway, there is a lack of agreement regarding the accuracy of the digital impressions over implants, especially in the rehabilitation of the edentulous patients. Vandeweghe et al. concluded that one of the four scanners he analyzed in his study was not adequate for the use in completely edentulous patients with multiple implants. While, there are several in vitro researches who support the accuracy of digital impressions in edentulous patients [28-31]. Anyway, most of these studies were in vitro studies with their limitations. A latest study was conducted by Eliasson & Ortorp, who made a comparison of the accuracy of implant analogue positions in casts using digital impressions of coded healing abutments (Encode) versus conventional implant-level impressions. The study was conducted over an acrylic master model, with 2 groups of implant analogues. On the experimental side encode healing abutments were positioned, in the other side traditional impression copings were positioned. Impressions were made with polyvinylsiloxane. The measurement was made by a laser machine, in the center point of the implant analogues in 3 dimensions. They concluded that both methods had slight inaccuracies of implant position (35.0-47.3μm in the digital technique and 13.9-18.5μm in the conventional technique). Despite the difference was statistically important, authors could not make a conclusion on the clinical significance. Both techniques had reliable results for the majority of the clinical situations. Howell et al [32] also concluded that the digital impression technique with encode abutments had a lower accuracy compared to conventional close-tray or open-tray impressions. There is a lack of research regarding the Straumann Scanbody system. Different in vitro studies have concluded that the iTero intraoral scanner can take accurate digital impressions for the construction of fixed restorations over teeth, but there is no sufficient scientific evidence related to the partial implant supported restorations impressions.

7. The Passive Fit

The clinical evaluation of the passive fit between framework and implants is difficult process. The passive fit of the prosthetic restorations depend on several factors including the accuracy of the impressions. The connection between the implant and abutment should be accurate in order to have a long term successful restoration. The misfit can cause biological and mechanical problems. Regarding the acceptable misfit, there are several opinions: An acceptable misfit value is considered 150 µm, Several authors: Branemark 10 µm, Jemt 150 µm, Di Fiore 30–50 µm, Klineberg and Murray 30 µm. Generally, in literature a misfit of 150 µm is acceptable. The Sheffield test, radiographs, visual inspection, and tactile sensation are the most used techniques to realize the evaluation. The misfit of the prosthetic restorations based on implants, constructed from the conventional impression is well documented, while there is no sufficient data related to the misfit of the prosthetic restorations over implants constructed from digital impression.

8. Discussion

The impression in implant prosthodontics is a key factor in the success of the restorations. It influences in the accuracy of the cast and as a result in the restorative construction. There are several factors that influence the accuracy of impressions such as the material, the technique of the impression, the construction of the working cast and the ability of the doctor. The ideal impression should be simple, reliable, accurate, comfortable for the patient and require minimal clinical time [34-38]. Traditional impressions can be technique sensitive and can cause patient discomfort, while digital impressions require clinicians to master a new treatment modality. As digital impression technology is relatively new in its application to implant dentistry, there are a low number of published studies. Only a small number of studies have directly compared traditional impression procedures and digital impression approaches [39-44]. Further modifications are suggested in the techniques and in the development of the intra oral scanners in order to improve the efficiency and accuracy of digital impressions. The advantages of implant digital impressions: it avoids the need for use of traditional impression materials, avoids distortion of the impression materials, avoid possible inaccuracies because of impression material strain, displacement of impression components, and plaster expansion, it is more comfortable for the patients: no gagging or unpleasant taste, less time consuming, potentially reduces production time and error from analogue techniques, the evaluation of the emergence profile, specifically in the frontal area, the 3-D models (in lab) can be examined for clearance, contacts, positioning of the implants and lab analogs, the prosthetic space can be assessed virtually, digital data can be stored in the computer, the possibility of re-scanning, it permits for day-of-surgery impressions and indexing for constructing custom abutments to position the final restoration earlier in the healing period after the osteo-integration of the implant26. Digital implant impression, also allow for immediate impression correction and interface with digital platforms for computer-aided design and manufacture techniques. The disadvantages are: 1. Multiple implants can cause difficulty in identifying the correct positions of the abutments. 2. Inability to scan the surrounding soft tissue when implants are close to each other. Not all implant manufacturers currently have a system designed for digital intra-oral implant impressions; currently only Biomet 3i (Encode System, Biomet 3i), Camlog (Scanbody, Camlog) and Straumann (Straumann CARES Mono Scanbody, Straumann) have developed specific intra-oral scanning abutments for their own dental implant systems. Several laboratories are promoting the construction of custom intra-oral scanning abutments; however it should be under careful control regarding the quality. It is not possible yet to scan through peri-implant soft tissue to implant platform at bone level. As such, capture of implant position has to be via a transmucosal component or “scanning abutment” which has inherent potential transfer error similar to traditional impression copings. Furthermore, as digital scanning requires multiple hard tissue landmarks in order to assemble or “stitch” together the intra-oral images taken to produce a complete digital impression, there is some limitation in full arch scenarios where most of the impression area is covered by moveable mucosa.

9. Conclusions

Within the limitation of this study, it can be concluded that:

Based on the current evidence digital impression is an accurate procedure in implantology. In edentulous cases, there is no sufficient scientific data to achieve firm conclusions. In partial and single implant cases the traditional impression seems to be more accurate, anyway due to the short distance, the digital impression can also be used. There are several factors that influence the digital impression accuracy, such as the scan abutment design and material, the scanning technique, the clinician experience, the type of intra oral scanner, the characteristics of the environment. The implant angulation seems to have no effect on the accuracy of the impressions.

Acknowledgements and Funding

We would like to express our sincere gratitude to the Ministry of Education and Sport of Albania, which, based on Order No. 468, dated 02.08.2022, for "Financing/Co-financing of Joint Projects in the Framework of Internationalization of Study Programs", financed the study and the project entitled "Evaluation of fixed metal-ceramics and all-ceramics implant supported restorations".

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