Intraoral scanner resolution – does it really matter? And how much?
NewsPosted by: Dental Design 12th November 2024
Digital impressions are growing in popularity across the dental profession, but it is important that we understand their true benefits and limitations. Newer techniques should provide clinical advantages over older ones, to justify their adoption. Intraoral scanners undoubtedly offer benefits over poor quality analogue impressions, and we know that dental laboratories receive many sub-standard impressions.[i],[ii],[iii] But what about the dentists who have truly mastered the art of analogue impression taking? Will the quality of their treatment be maintained when switching to a contemporary digital technique?
How we got here
Many areas of dentistry have evolved substantially in the past couple of decades, but none more so than the digital workflow. Intraoral scanners were initially introduced to dentistry in the 1980s, and there are a growing number of users today, especially within the removable orthodontic aligner field. Though more dentists have or are planning to switch from analogue impression techniques to digital solutions, the majority still utilise traditional impressions and remain on the fence about whether to purchase an intraoral scanner. The global market for these scanners is estimated to grow from an estimated $288.3 million in 2023, to $764.0 million in 2032.[iv]
There are numerous studies that examine the trueness and precision of current dental scanners, but readers should be reminded that our measurement methods are crude and possibly not fully applicable clinically. For example, ISO 20896-1:2019[v] require metrics such as the dimensions of a conical hole (to represent an inlay prep), or the scanned distances between spheres superimposed onto a dental model. This can give artificially good results, where the actual scanned surfaces are inaccurate or distorted locally, but the mean distance measurements recorded remain correct. A prosthodontist might argue that the clinically important measure is the local accuracy of the margin of the (simulated) inlay, and not just the mean distance across the preparation. How does a scanner compare to the resolution of a silicone or polyether impression taken in a rigid tray with a type 4 stone model from a clinical accuracy perspective?
Fgure 1 – Visual comparison of the resolution of various forms of digital impression compared to the original Type 4 stone model. (A) Contemporary intraoral scanner (B) Dental model scanner (C) Research-grade model scanner used at Leeds University (D) Original Type 4 stone model. Note the bur marks and margin sharpness increase progressively from A to D. When striving for a 30 micron quality of fit, only (C) is likely to come close. [Colour removed to enable true 3D shape visualisation]
A comparison
Anyone who has ever seen an intraoral scan will appreciate the beauty of the image, the crisp lines and the texture mapping that modern software can achieve. There is little argument that the scan looks great and can be a very useful tool in treatment planning and patient education. However, when the artificial colour and filters are stripped away, the results are not always what they seem.
Andy Keeling, Professor of Prosthodontics and Digital Dentistry at the University of Leeds, comments:
“Type 4 dental stone has been used very successfully to produce high quality fixed prostheses in many clinical situations for many years. Over 100 years of development in analogue dentistry have proven the need for this fine grit stone – clinicians can tell instantly if a lab has incorrectly used a coarser stone (or plaster) for a prosthodontic case based on the resolution of the model.
“According to ISO 6873,[vi] type 4 dental stone is required to reproduce a 50-µm line as a minimum. In a high-quality dental stone, more than 35% of particles have been found to be smaller than 45 microns in diameter,[vii] with 10% below 2 microns.[viii] This could conceptually be translated to an “analogue triangle size” of approximately 2-3 microns, compared to typically >100 microns for intraoral scans.”
“Triangle edge lengths in scan data from intraoral scanners are typically large, particularly over the relatively smooth cusp tips where the software tends to over-simplify the 3D mesh. I took a scan of my own mouth using the most advanced scanner on the market and extracted a full-arch mesh at the highest available resolution. The mean triangle edge length was 140 microns, with 675,000 triangles in the mesh. Triangle size was smaller in areas of high curvature – fissures, margins, gingivae – due to intelligent mesh resampling, but the smallest triangle sizes were still 50 microns.”
“In contrast, modern Type 4 stone particles are an order of magnitude smaller than these triangles, leading inevitably to greater detail (higher resolution). Combine this with the lack of literature detailing scanner resolution and its clinical effect, is it any wonder that many prosthodontists remain on the fence about moving to digital impressions?”
It is very difficult to measure resolution, which is why literature is lacking. However, visual comparisons show stark differences, as illustrated in Figure 1. Are we losing clinically significant detail by changing from analogue to digital impressions? Or does this mean that we have been working with greater detail than we needed to for many decades? Or is there still work to do before digital workflows match the standard of existing techniques? Indeed, recent research[ix] has found conventional impressions to still provide significantly better retention than digital scanners for both milled and 3D printed denture bases. Drancourt, Noemie, et al[x] evaluated the accuracy of conventional and digital impressions for full-arch implant-supported prostheses and found conventional impressions to be more accurate than digital impressions.
Does it matter?
Not all dentistry is the same, meaning what is required from an impression may vary between the disciplines. Orthodontic needs seem to be in the order of 100s of microns while prosthodontic needs are potentially in the order of 10s – which has driven the development of light-bodied silicone impressions and Type 4 dental stone. Whilst no consensus has been reached regarding acceptable marginal gap for fixed prosthseses, there is universal agreement that this gap should be as small as possible to prevent secondary caries. A recent study showed secondary caries formation at gaps of 30 microns or more,[xi] so it would seem prudent to strive for this standard in modern dentistry rather than accepting lower standards such as 120microns, which were only loosely agreed upon in the twentieth century.
However, we should remind ourselves that intraoral scanners reduce technique sensitivity and may provide a better option than badly taken analogue impressions, which is why they may already have a place in dentistry where throughput is valued above absolute quality. However, in certain clinical situations such as full-arch dentures, and high-end fixed prosthodontics (aiming for the aforementioned minimal cement gap) analogue impressions will likely still deliver more accurate results.
Professor Keeling summarises:
“There is little doubt that an expertly taken traditional impression produces the highest resolution and accuracy. But questions remain over whether this level of detail is clinically relevant. Maybe one day, if clinical parity is unequivocally shown between intraoral scanners and analogue impressions, then conventional dentists will be able to reduce their material costs by no longer requiring light-bodied washes or expensive die stones, but I suspect this is unlikely.”
Author: Krishan Davda. Dental Health Care, Hertford
[i] The quality of impressions for crowns and bridges received at commercial dental laboratories. Winstanley RB, Carrotte PV, Johnson A. Br Dent J (1997) 183(6): 209–213
[ii] The quality of dental casts used in crown and bridgework. Alhouri N, McCord JF, Smith P. Br Dent J (2004) 197(5): 261–264
[iv] Fortune Business Insights. Intraoral Scanners Market Size, Share & Industry Analysis, Accessed October 2024. https://www.fortunebusinessinsights.com/intraoral-scanners-market-109836
[v] British Standards Institution (2019) BS EN ISO 20896-1:2019 Dentistry. Digital impression devices. Methods for assessing accuracy. Available at: https://bsol.bsigroup.com/ (Accessed: 7 October 2024).
[vi] British Standards Institution (2013) BS EN ISO 6873:2013 Dentistry. Gypsum products. Available at: https://bsol.bsigroup.com/ (Accessed: 7 October 2024).
[vii] Rejab LT, Al-Jubori SH. Examination of the particle’s shape, size and distribution. Al-Rafidain Dental Journal. November 2019; 3(2). 10.33899/rden.2003.165764.
[ix] Chebib N, Imanura Y, El Osta N, Srinvasan M, Müller F, Maniewicz S. Fit and retention of complete denture bases: Part II – conventional impressions versus digital scans: A clinical controlled crossover study. The Journal of Prosthetic Dentistry. 2022 April; 30:S0022-3913(22)00464-4. doi: 10.1016/j.prosdent.2022.07.004. Epub ahead of print. PMID: 36055812.
[x] Drancourt, Noemie, et al. “Accuracy of conventional and digital impressions for full-arch implant-supported prostheses: an in vitro study.” Journal of Personalized Medicine 13.5 (2023): 832.
[xi] Maske TT, Hollanders ACC, Kuper NK, Bronkhorst EM, Cenci MS, Huysmans MCDNJM. A threshold gap size for in situ secondary caries lesion development. J Dent. 2019;80:36–40. https://doi. org/10.1016/j.jdent.2018.10.014
