Digital Impressions OR Conventional Impressions - What's better?
Each year, humans make great strides by developing innovative technology and constantly working towards making that technology more efficient and convenient. In dentistry, continually evolving and improving technologies offer patients, dentists and dental labs all round better dental experiences by refining both the clinical and technical aspects of these experiences.
In recent years, digital scans/impressions have begun to take over the place held for so long by traditional physical impressions. By using the correct technique, digital impressions can increase productivity and accuracy, and decrease lab turnaround times. CAD/CAM technology is also used by clinicians, chairside, to produce restorations in one single patient appointment.
Further advantages include increased patient tolerance; decreased case cost and there is no risk of lost impressions and impression distortion associated with physical or analog impressions. Missing data can easily be rescanned to the original digital scan file when intraoral scans exhibit distortions. Distorted analog impressions however result in longer chairside times and present a disruption when patients are recalled or their appointments are extended so that dentists can reimpress to achieve an accurate impression.
Additionally, the analog impression method involves a greater number of steps to fabricate the final restoration, increasing the likelihood of errors along the way. While digital scans are sent directly from the scanner to the relevant lab portal eliminating the risk of conversion errors associated with analog impressions and models. Although digital methods are gaining popularity amongst clinicians and dental labs, analog impressions still have a place in modern dentistry. Let’s review each of the two impression techniques and their suitability for crown and bridge, dental implant restorations and removable prosthetics.
1. Crown and Bridge
Over the last decade, the accuracy of digital impressions has surpassed analog impressions for crown and bridge (C&B) restorations. 1–3 Cayster’s data on labs across the country show that for crown and bridge cases, remakes for analog cases are up to 4 times more frequent than digital cases. Remake cases affect the whole dental ecosystem - patients, dentists and dental labs are not only inconvenienced when these occur. But they can have a significant impact on the relationships between dentists and their patients and can erode the trust that dentists have in their labs.
One could assume that this metric alone would see a marked increase in the sale and use of intraoral scanners, but analog impressions remain a standard practice in just under 50% of dental offices around the US. Today most dental labs digitize physical C&B impressions either by scanning the analog impression or final poured model. Once this is completed the case enters the digital design and milling workflow that is common to almost all labs today. The various steps to get an analog impression to this design phase present many opportunities for data to be lost or distorted along the way.
There are however times when physical impressions are indicated for C&B cases:
1. Deep, sub-gingival margins where retraction and/or laser treatment is not an option. If the scanner cannot capture a margin fully and clearly - then it is best to take an accurate analog impression. 4
2. Dentists who are not as proficient at scanning as they are at taking physical impressions. These are usually dentists who have recently introduced intraoral scanners into their practices and are still mastering the learning curve associated with scanning techniques.
Fig 1. Digital impression of #15 with a deep margin | Fig 2. Analog impression of the same #15 with a fully captured margin
Through the Cayster platform, dentists have the opportunity to submit both digital and analog C&B impressions for the same case. This hybrid C&B workflow enables the lab to evaluate the impressions, side-by-side and to proceed with the one that displays a clear preparation with a well delineated margin. This is particularly useful for dentists who are transitioning from taking analog impressions to intraoral scanning.
2. Implant Restorations
For implant restorations, recent studies show that for simple implant restorations, digital impressions offer reliable and accurate restorative outcomes.5,6 A number of studies however suggest that physical impressions are more accurate for cases involving multiple implants, particularly when they are located across the arch.1,7 Although most labs do not charge for poured implant models produced from physical implant impressions, labs will charge for each soft tissue model and lab analog for digital implant cases.
Fig 3. Single implant restoration scan with a scan body.
Both increased accuracy and lower costs are therefore realized when using conventional impressions in large, full arch implant cases. It is important to note that this depends on the technique used by the dentist - ideally an open tray impression taken after the seating of the impression copings have been confirmed radiographically. For digitally scanned implant cases, Cayster recommends that the seating of scan bodies are also verified the same way.
Fig 4. A digitized physical impression of 4 implants.
Photogrammetry is an emerging digital implant impression technology used for capturing multiple implants and is worth mentioning here. This technique establishes the geometrical properties of objects and their spatial arrangement from a series of 2D photographs using an extraoral receiver and specialized scan bodies. There are conflicting reports from recent studies examining the accuracy of photogrammetry and whether it can offer better restorative outcomes than conventional and digital impressions for cross arch implant restorative cases.8,9 More research is however needed to support either claim, as was the case with when digital scanning was introduced over 30 years ago.
Fig 5. The Imetric 4D photogrammetry domino scan posts used for implant orientation.
3. Removable Restorations
The general consensus is that when extensive tissue capturing is required, the gold standard remains traditional physical impressions. However full dentures have been successfully fabricated using the hybrid techniques described below:
- The edentulous arches are scanned, models are printed and a custom tray or base plate is fabricated for a final physical impression after which the traditional stages for denture fabrication are followed.
- Or, the tissue is recorded by doing an extra-oral 360° digital scan of an existing denture - either as is, or one that has been adapted intraorally with a wash impression. Here the bite is scanned with the dentures in place so that a try-in or final denture can be delivered at the next appointment.10
Fig 6. Scan of an edentulous arch - for custom tray/bite rim fabrication.
Fig 7. Scan of a complete denture - taken outside the mouth.
The biggest challenge with digitally impressing for complete dentures, is that the surrounding vestibular tissue is dynamic. This moveable soft tissue is still most accurately recorded by physical impressions and traditional border molding techniques. While complete dentures sometimes present a challenge for digital scanning, we are seeing more dentists sending digital impressions for removable cases which include:
- Bite splints/occlusal guards.
- Removable partial dentures.
- Clear aligners and retainers.
- Whitening trays.
- Custom tray fabrication for complete dentures.
Fig 8. A digital impression for a flexible partial denture.
Summary:
Cayster supports both analog and digital impressions for all the product categories discussed. Cayster further digitizes analog impressions and/or models for case analysis and easy retrieval in case of a remake; and has successfully integrated physical impressions into the digital workflow.
Ultimately, it's up to the dentist to choose an impression technique for a specific patient and clinical scenario. The information above offers a guideline, which along with the dentist’s comfort and skill level serves to facilitate this decision. As time goes on, digital impressions and their related technology will undoubtedly continue to improve in all areas of clinical dentistry and could one day be the standard of care for all fixed and removable dental prosthetic restorations.
FAQs
1. For C&B, when is it better to do a physical impression rather than a digital scan?
Only when there are subgingival margins that cannot be captured on a scan.
2. For cross arch implant cases, is it possible to proceed exclusively digitally?
No. Large, cross arch cases must be verified chairside, which is an analog process.
3. Is it more cost effective to digitally scan all implant cases?
For cases involving multiple implants, the printed model, analog and soft tissue model costs can make these more expensive.
4. Do digitally scanned implant cases have a quicker turnaround time?
Not necessarily. These can be delayed by 3rd party milling centers who manufacture the abutments for these cases.
5. Can I avoid analog impressions for Complete Denture cases?
Yes - but only if the lab is duplicating an already existing, well fitting denture that is scanned extra-orally.
BIBLIOGRAPHY
1. Ahlholm P, Sipilä K, Vallittu P, Jakonen M, Kotiranta U. Digital versus conventional impressions in fixed prosthodontics: a review. J Prosthodont. 2018;27(1):35–41.
2. Chochlidakis K, Papaspyridakos P, Geminiani A, Chen C, Feng I, Ercoli C. Digital versus conventional impressions for fixed prosthodontics: A systematic review and meta-analysis. J Prosthet Dent. 2016;116(2):184–90.
3. Tsirogiannis P, Reissmann D, Heydecke G. Evaluation of the marginal fit of single-unit, complete-coverage ceramic restorations fabricated after digital and conventional impressions: A systematic review and meta-analysis. J Prosthet Dent. 2016;116(3):328–35.
4. Pavlatos J. Technique and troubleshooting guide [Internet]. GC America Inc. 2010.
5. Lee SJ, Jamjoom FZ, Le T, Radics A, Gallucci GO. A clinical study comparing digital scanning and conventional impression making for implant-supported prostheses: A crossover clinical trial. J Prosthet Dent [Internet]. Editorial Council for the Journal of Prosthetic Dentistry; 2021;1–7. Available from: https://doi.org/10.1016/j.prosdent.2020.12.043
6. Mangano F, Veronesi G. Digital versus Analog Procedures for the Prosthetic Restoration of Single Implants: A Randomized Controlled Trial with 1 Year of Follow-Up. Biomed Res Int. 2018;July:5325032.
7. Kim K, Seo K, Kim S. Conventional open-tray impression versus intraoral digital scan for implant-level complete-arch impression. J Prosthet Dent. 2019;122(6):543–9.
8. Tohme H, Lawand G, Chmielewska M, Makhzoume J. Comparison between stereophotogrammetric, digital, and conventional impression techniques in implant-supported fixed complete arch prostheses: An in vitro study. J Prosthet Dent [Internet]. Editorial Council for the Journal of Prosthetic Dentistry; 2021;1–9. Available from: https://doi.org/10.1016/j.prosdent.2021.05.006
9. Revilla-León M, Att W, Özcan M, Rubenstein J. Comparison of conventional, photogrammetry, and intraoral scanning accuracy of complete-arch implant impression procedures evaluated with a coordinate measuring machine. J Prosthet Dent [Internet]. Editorial Council for the Journal of Prosthetic Dentistry; 2021;125(3):470–8. Available from: https://doi.org/10.1016/j.prosdent.2020.03.005
10. Harder C. Using 3-D printing to improve removable denture workflow. [Internet]. Dental Economics. 2019 [cited 2022 May 31]. Available from: https://www.dentaleconomics.com/science-tech/dentures/article/14039664/using-3d-printing-to-improve-removable-denture-workflow
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