1. Conventional Impression Technique, Digital impression Technique
Since the eighteenth century, conventional impression techniques have been used to fabricate dental models. However, conventional impression techniques might have some errors regarding volumetric changes of impression materials and expansion of dental stone. Also, conventional technique is much influenced by the proficiency of practitioners.

To overcome these difficulties, Digital impression was developed for dental practice with numerous advantages for practitioners. CAD/CAM was first introduced to Dentistry for single unit restorations about thirty years ago, and it has substantially contributed to advancement in technology in the last decade. Nowadays, Intraoral Scanner (IOS) and CAD/CAM provide easier planning of treatment, case acceptance, communication with laboratories, reduced operative time, storage requirements, and reduced treatment times.

For appropriate using of IOS, This review would provide and share the information of IOS in respect to mechanism, limitations and applications.

2. IOS technology
IOS is a medical device composed of a handheld camera, a computer and software. The goal of IOS is to record a precise three dimensional geometry of objects.

  • File Format
    • STL (Standard Tessellation Language)
      1. already used in many industries
      2. Triangulated surface
    • PLY: to record color, transparency or texture of dental tissue.
    • OBJ
  • Mechanism of imaging

Irrespective of the type of imaging technology, all IOS require the light projection then recorded as individual images and compiled by the software. X and Y coordinates of each point are evaluated on the image, and Z coordinate is calculated depending on the distance to object technologies.

  • Light projection and capture
    1. Passive techniques use only ambient lightning to illuminate intraoral tissues and are reliant on a certain level of texture of an object.
    2. Active techniques use white, red or blue structured lights projected from the camera onto the object that is less reliant on the real texture and color of tissues for reconstruction.
      1. Luminous point projection
      2. Light pattern projection (line or mesh) (Fig. 1)

        [Fig. 1]

  • Distance to object technologies
    • Triangulation is based on a principle that the position of a point of a triangle can be calculated knowing the positions and angles of two points of view. These two points of view may be produced by two detectors, a single detector using a prism, or captured at two different points in time. (Fig.2a)
    • Confocal imaging is a technique base in acquisition of focused and defocused images from selected depths. This technology can detect the sharpness area of the image to infer distance to the object that is correlated to the focal length of the lens. A tooth can then be reconstructed by successive images taken at different focuses and aperture values and from different angles around the object. (Fig.2b)
    • Active wave-front sampling (AWS) is a surface imaging technique, requiring a camera and an off-axis aperture module. The module moves on a circular path around the optical axis and produces a rotation of point of interest (POI). Distance and depth information are derived and calculated from the pattern produced by each point. (Fig.2c)
    • Stereophotogrammetry estimates all coordinates (x, y and z) only through an algorithmic analysis of images. As this approach relies on passive light projection and software rather than active projection and hardware, the camera is relatively small, its handling is easier, and its production is cheaper. (Fig.2d)

(a) Triangualtion

(b) Confocal imaging

(c) Active Wave-front Sampling

(c) Active Wave-front Sampling

(d) Stereophotogrammetry

(d) Stereophotogrammetry

[Fig. 2]

  • Reconstruction Technology
    • One of the major challenges of generating a 3D numerical model is the matching of POI taken under different angles.
    • Distance between different pictures may be calculated using an accelerometer integrated in the camera, but a similarity calculation is more often used to determine the point of view of the image.
    • Using algorithms, similarity calculation defines POI coincident on different images.

3. Influencing factors of IOS technologies

    • scattering
      • To prevent scattering due to reflective surface.
        1. Changing the orientation of the camera to increase diffuse light.
        2. Use cameras with polarizing filter.
      • Some other scanners, a 20~40 microns powder coating is required.
        1. Powder could be relatively uncomfortable for patients and additional scanning time is needed.
        2. Powder could be contaminated with saliva during impression.
        3. Concerning full arch scans, IOS using powder free technologies appears to be recommended due to the difficulty to maintain powder coating on all the teeth for the during the scan.
      • An inappropriate opacization technique may result in layers of different thicknesses at various points of the teeth, with the risk of errors that reduce the overall quality of the scan
    • Scanning strategies
        1. IOS must be used according to a specific movement to increase accuracy of scanning.
        2. Keep fluid movement and proper distance between object and scanner head.
        3. Making an “S” sweep for vestibular, occlusal and lingual faces of anterior region. (Fig.3a, 3b)
        4. A linear movement on all occlusal-palatal surfaces followed by buccal surface seems to limit spatial distortion by finishing capture at the initial point.
        5. During impression, tracking could be lost which may destabilize the software when distance to the object or scan path is not respected.
        6. A scan strategy must be followed beginning with easy parts (occlusal faces of posterior region) so that the software has enough information if tracking is lost.
        7. Repeated captures of same area might induce the distortion of images.
    • Scanning Speed
      1. Scanning speed is certainly a matter of great importance for an IOS
      2. IOS have different scanning speeds, and the latest-generation devices are generally faster than the oldest ones.
      3. The scanning speed does not depend only on the device, but largely on the experience of the clinician.
    • Size of the tip
      1. The size of the tip plays a role as well, especially in the case of the posterior regions.
      2. A scanner with a tip of limited dimensions would be preferable for the patient’s comfort during the scan.
      3. Even scanners with more voluminous tips allow excellent scanning in posterior areas
    • Mesh quality
      1. A high mesh density for the whole tooth is not relevant due to long computing time involved.
      2. Some files incorporate a routine mesh on flat zones and a more dense mesh for high curvatures. (Fig.4)

        [Fig. 4]

      3. A large number of triangles are sufficient to follow precisely the emergence profile whereas a low number could lead to smoothing of margins.
      4. The presence of blood, saliva or gingival fluid can also falsify the picture acquired. It might lead to an error especially around margin area.
      5. The latest IOS also provide color and texture. Nevertheless, the rendering of file in the graphical user interface might be misleading on the accuracy of a scan due to the algorithms of software.

4. Accuracy of IOS technologies

The main feature of IOS should have is accuracy. A scanner should be able to detect an accurate impression. Accuracy is the sum of trueness and precision.

  • Definition of IOS accuracy: The accuracy is described by Trueness and precision
    • Trueness: The average of the difference in values between the scan data and the reference data
    • Precision: The average of the difference in values between each scan data from the same scanner
  • Precision and trueness of IOS Files
    • Many literatures have reported clinically acceptable trueness and precision of current IOS in vitro and in vivo.
    • Accuracy and precision of 3 intraoral scanners and accuracy of conventional impressions: A novel in vivo analysis method (Nedelcu et al)
      1. IOS has an equally or higher accuracy and precision than conventional impressions
      2. No statistically difference was found for IOS precision.
      3. Intraoral scanners can be used as a replacement for conventional impressions when restoring up to ten units without extended edentulous spans.
    • Investigation of accuracy and reproducibility of abutment position by Intraoral scanners (Fukazawa et al)
      1. The distance precision of intraoral scanners was within the same error range as that of a laboratory scanner, demonstrating that some IOS can precisely reproduce abutment positional relationships.
      2. IOS can eliminate the errors associated with materials. Therefore, IOS might qualify the clinical application of the digital impression method for implant treatment of multiple teeth.
    • Clinical marginal fit of zirconia crowns and patients’ preferences for impression techniques using intraoral digital scanner versus polyvinyl siloxane material (Sakornwimon et al)
      1. No differences were found in the clinical marginal fit of Zirconia crowns fabricated from either digital impressions compared with PVS impressions.
      2. Patients’ satisfaction with digital impressions was significantly higher than with conventional impressions.
    • Influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions (Mejia et al)
      1. Conventional dental impressions alone or those further digitized with an extraoral digital scanner cannot reliably reproduce abutment tooth preparations when the total occlusal convergence angle is close to 0 degrees.
      2. In contrast, digital impression made with intraoral scanning can accurately record abutment tooth preparations independently of their geometry.
  • The trueness and precision obtained with the digital impressions for these types of short-span restorations are comparable to those obtained with conventional impressions. However, digital impressions do not appear to have the same accuracy as conventional impressions in the case of long span restorations such as partial fixed prostheses with more than 10 units or full-arch prostheses on natural teeth or implants.
  • The error generated during intraoral scanning of the entire dental arch does not appear compatible with the fabrication of long-span restorations, for which conventional impressions are still indicated.

5. Applications

  • Clinical Indications and Contraindications of IOS
    1. IOS are of great utility and are applied in various fields of dentistry, for diagnosis and for fabricating restorations or custom devices in prostheses, surgery and orthodontics.
    2. IOS are used for acquiring 3D models for diagnostic purposes and communicating with the patient.
    3. In prostheses, IOS are used to make impressions of preparations of natural teeth for fabricating a wide range of prosthetic restorations: resin inlays/onlays, zirconia copings, single crowns in lithium disilicate, zirconia, metal-ceramic and all-ceramic as well as frameworks and fixed partial dentures.
    4. In prosthodontic, IOS can be successfully used to capture the 3D position of dental implants and to fabricate implant-supported restorations.
    5. At present, implant-supported single crowns, bridges and bars can be successfully fabricated from digital impressions.
    6. IOS can be successfully used for digital smile design applications, post and core fabrication.
    7. Superimposed model with IOS and CBCT is used for planning the positioning of the implants and to draw one or more surgical stents useful for placing the fixtures in a guided manner.
    8. IOS represent a very useful tool in orthodontics for diagnosis and treatment planning. Digital impressions can be used as a starting point for the realization of a whole series of customized orthodontic devices.
    9. The most common clinical indications and contraindications on the use of IOS are summarized below. (Table. 1)
  • Advantages and disadvantages of Intraoral scanner
    • Advantages

The advantages and disadvantages of digital impressions with respect to conventional impressions are presented below and summarized in [Table. 2]

  1. Less patient discomfort The ability to directly capture all dental arch information of the patient, and consequently their 3D models, without using conventional physical impressions, is one of the advantages of digital impressions. The conventional physical impressions can cause momentary discomfort for the patient due to the inconvenience and hardship stemming from the materials positioned on impression trays.
  2. Time efficient Digital impressions are time-efficient, as they enable reduction of the working times when compared to conventional impressions. With digital impressions, there is no need to pour stone casts and obtain physical plaster models.
  3. Simplified clinical procedures when the learning curve has been completed, the use of IOS may confer further clinical advantages, simplifying impression-making in complex cases. Moreover, if the clinician is not satisfied with some of the details of the recorded digital impression, they may delete them and recapture the impression without having to repeat the entire procedure.
  4. Better Communication with dental technician With IOS, the clinician and the dental technician can assess the quality of the impression in real-time. Immediately after the scan has been performed, the dentist can e-mail it to the laboratory, and the technician can check it accurately. If the dental technician is not convinced of the quality of the received digital impression, the technician can immediately request that the clinician make another one without any loss of time and without having to call the patient for a second appointment.
  5. Better Communication with patients with digital impressions, patients feel more involved in their treatment and it is possible to establish more effective communication with them.
  • Disadvantages
    1. Learning Curve There is a learning curve for adopting IOS in the dental clinic, and this aspect must be considered with attention. Older clinicians with less experience and passion for technological innovations could find using the devices and related software more complex for. It is still unclear whether one scanning strategy is better than the other, as manufacturers provide little information about their scanning strategies. This is an aspect that will certainly be researched in-depth in the coming years, as it is possible that different machines, using different scanning strategies, would produce different results.
    2. Difficulty detecting deep margin lines of prepared teeth One of the most frequent problems encountered with IOS and with digital impressions is difficulty in detecting deep marginal lines on prepared teeth or in the case of bleeding. Light cannot physically detach the gum and therefore cannot register ‘non-visible’ areas. When bleeding, blood may obscure the prosthetic margins.
    3. Purchasing and managing costs Depending on the model, the cost of purchasing an IOS may be about 20,000 to 40,000 dollars. Regardless, the purchase cost of a high-end, last-generation IOS should be cushioned over the year by integrating the device into the clinical workflow across the various dental disciplines.

6. Limitation

    • Current complete arch scanning is not sufficiently accurate for fabricating long span bridge.
    • Fabricating partially and completely removable prostheses still presents some issues due to the absence of reference points and the impossibility of registering soft tissue dynamics.
    • Distance and angulations errors were too large to fabricate well-fitting frameworks on implants in edentulous mandibles. The main reason for the unreliable scans seemed to be the lack of anatomic landmarks for scanning, irrespective of the technology employed.

7. Conclusion

An understanding of the IOS technology is necessary for any practitioner to have a successful clinical strategy during the scanning of prepared teeth. Both in vitro and in vivo studies have concluded that the accuracy of IOS is acceptable for the clinical purposes.

Digital impressions have several advantages over conventional impressions. The most important is the reduction of patient stress and discomfort. Moreover, digital impressions, moreover, are time-efficient and can simplify clinical procedures for the dentist, especially for complex impressions.

In addition, digital impressions eliminate plaster models, saving time and space, and allow for better communication with the dental technicians and patients. Therefore, IOS is a powerful marketing tool for the modern dental clinic. Regarding accuracy as compared to conventional impressions, digital impressions are equally accurate. The current clinical applications of IOS are extremely wide, as these devices can not only be used in fixed prosthodontic to obtain the virtual models needed to manufacture a whole range of prosthetic restorations on natural teeth and implants, but also in implantology for guided surgery and in orthodontics

Conversely, the disadvantages of using digital impressions are the difficulty in detecting deep margin lines in prepared teeth and/or in the case of bleeding, the learning curve, and the purchasing and managing costs. Conventional impressions still appear to be the best solution currently for long-span restorations, such as fixed full arches on natural teeth and implants, edentulous arch.

However, the latest-generation scanners are characterized by very low errors in full arch impressions. Preparing and publishing a scientific article generally takes time, whereas manufacturers release new powerful software for mesh construction very frequently.

References

  1. Mangano et al, Intraoral scanners in dentistry: a review of the current literature. BMC Oral Health 2017
  2. Park, K. Son, K. Lee, Feasibility of using an intraoral scanner for a complete-arch digital scan. Journal of Prosthetic Dentistry. Vol.121, Issue 5, May 2019. Pages 803-810
  3. Brian, A. Wennerberg, Truness and precision of 5 intraoral scanners for scanning edentulous and dentate complete-arch mandibular casts: A comparative in vitro study. Journal of Prosthetic Dentistry. March 2019
  4. Fukazawa, C Odaira, H. Kondo, Investigation of accuracy and reproducibility of abutment position by intraoral scanners, Journal of Prosthodontic research, 2017, pages 450-459
  5. Nedelcu, P. Olsson, I. Nystrom, J. Ryden, A. Thor, Accuracy and precision of 3 intraoral scanners and accuracy of conventional impressions: A novel in vivo analysis method, Vol. 69, Feb 2018, pages 110-118
  6. Andriessen, D. Rijkens, W. Meer,D. Wismeijer, Applicability and accuracy of an intraoral scanner for scanning multiple implants in edendtulous mandibles: A pilot study, The Journal of Prosthetic Dentistry, Vol. 111, Issue 3, Mar 2014, Pages 186-194
  7. Mejia, K. Wakabayashi, T. Nakamura, H. Yatani, Influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions, Vol. 118, Issue 3, Sep 2017, pages 392-399
  8. Kim et al, Understanding and application of digital impression in dentistry, Journal of dental materials, Vol. 41, Dec 2014, pages 253-261
  9. Richert et al, Intraoral scanner technologies: A review to make a successful impression, Journal of healthcare engineering Vol. 2017

Appendix