Making a digital impression
When Dr Gordon Christensen called digital impressions part of the ’Current paradigm shift in dentistry’ (Christensen, 2007), as usual he was ahead of the rest of the profession, certainly on this side of the Atlantic. Up until then, there had only been one system of digital impression machines available to dentists: CEREC from Sirona.
The theory of digital impressions in dentistry started with the French dentist François Duret, who first envisioned his theory in 1970 as a dental student, and finally produced a single crown on a willing subject (his wife) for the 1983 Academy of French Dentistry meeting. His original idea was to send the scanned information to a dental laboratory.
At the same time, the Swiss dentist Werner Mormann was inventing what became the CEREC system, producing a chairside crown in 1985. These two fundamental types of workflow persist today: a digital impression then a laboratory produced restoration and a digital impression then a chairside produced restoration.
In 2007, the Itero system came to market, the second generation of intra-oral scanner. This system now has produced over two million restorations in the USA alone. There have been many entrants to the market, but the main players are: CEREC (Sirona), Itero (Align Technology), True Definition (3M) and Trios (3Shape). The majority of these machines do not produce a crown at the chairside – only the CEREC can do that. Even Sirona has realised that a limited number of dentists are interested in the time investment and learning curve involved in milling your own restorations.
To my eye, hand-stacked porcelain by a skilled dental technician has a superior appearance, important in the anterior segment, but that is just my opinion.
These machines, however, do look good in the modern practice environment (Fig 1).
All this is very interesting to anoraks like me, but why change? A recent study found that 44.2 per cent of dental impressions of prepared teeth submitted to three dental labs in the UK were unsuitable for making a dental restoration (Storey, Coward, 2013).
Dental impression materials are subject to distortion, tearing, and inadequate adhesive use, which pulls material away from trays. Trays that are too flexible contribute to inaccuracy. Let’s not even mention moisture control. Polyether impression material is very popular with implant dentists. How many of us can guarantee that our impressions are cast within 24 hours, or are transported to the lab in an environment which has less than 50 per cent humidity, as per manufacturer’s instructions? Or are they in the same bag as a wrapped wet alginate?
This is without beginning to list the things which could possibly go awry on the laboratory side: incorrect stone mixtures, voids in stone, dropped models, incorrect trimming of models, incorrect fitting of analogues to impression copings, etc.
The models produced with scanning show excellent detail and clear margins. There is also ample evidence in implant dentistry that it can be difficult to take implant fixture head impressions, in patients with limited opening for instance, but inaccuracy of conventional impressions has been shown to occur with increased implant depth and implant angle (Linkevicius, et al., 2012, Mpikos, et al., 2012).
This is without any reference to how our patients feel about having an implant fixture head impression taken. Again, studies show that there is a positive patient reaction to digital impressions in comparison to conventional impressions (Wismeijer, et al., 2013).
There is ample evidence already (Ender and Mehl, 2011, and Seelbach, Brueckel and Wöstmann, 2013) that digital impressions are very accurate and have potential to improve on the accuracy of conventional impressions. This is even more pertinent when conventional models would need to be digitalised for production of titanium or zirconia abutments (Almeida, et al., 2013, and Güth, et al., 2013).
Furthermore, Lee and Gallucci (2013) found that operators preferred taking digital implant impressions when in direct comparison with conventional impressions.
So much implant dentistry is now CAD/CAM designed. At the moment our impressions are cast in stone, trimmed and then digitalised for design of copings in titanium or zirconia. It makes perfect sense to avoid the middle step of casting and hand trimming and go straight to digital design. Technicians also trim and design digital models on a large computer screen, which is much easier for identification of margins and soft tissue contours. The printed models handle well and technicians like them. The preparations and margins are very clear.
The results for dentists are that restorations require less adjustment to fit, and there is a reduced number of remakes. From personal experience, I can attest to this. There is an excellent fit (Fig 3).
The added benefit for dentists working with the Trios is the flexibility of the machine. If, for instance, you are concerned about clearance of a preparation, it can give you a digital guide on screen. If more adjustment is required, you can ’erase’ that small area and re-scan it.
You can examine the margins and shape of preparations prior to emailing the impression to the laboratory. The machine has a scanner head with an autoclaveable/ removable tip, which is easy to use (it is remarkable how many intra oral-scanning devices have non-autoclaveable tips!). The scanning process itself is very quick.
Also, a copy will be kept for all your models digitally. A great reduction in the amount of gypsum in your practice.
The results speak for themselves. Fitting times are much reduced in implant and in conventional crown and bridge work.
In this article, I will describe the workflow that occurs when an intra-oral scanner is used in implant dentistry. I have been fortunate enough to have been the first UK dentist to use the 3M LCOS scanner (a second-generation scanner) and also the 3Shape Trios more recently (a third-generation scanner), which I use in practice today. It is used in all situations where conventional impressions for restorative work would be taken.
The difference between the Trios (and the new Cerec Omnicam), compared with machines of a previous generation, is that there is no need for ’powdering’ the teeth before scanning. Previous generations of machine would require a layer of titanium dioxide powder to be sprayed over the teeth prior to scanning, otherwise it was unable to record those surfaces. This can make moisture control more tricky.
After implant placement and suitable time after second- stage surgery, an appointment is required for fixture head impression. A digital lab sheet is prepared on the Trios. Similar to a conventional lab sheet, this contains information such as patient details, delivery time and type of restoration. It also allows for all case information needed, including pre-op
scans if wanted, or the scan of a wax-up in a more cosmetic case. This allows for shade mapping and annotation for individual teeth, if required.
The opposing arch is scanned, this usually takes 1.5 minutes. The operative arch is then scanned after removing the healing cap/caps. This gives a soft tissue impression as the machine is very good at picking up soft tissue contours. A small scan flag is placed in the implant and secured.
A ’scan flag’ is a small plastic computer impression coping (Fig 4). There are currently scan flags for all major brands of dental implants. This gives the software not only the type of implant being restored, but its 3D co-ordinates and angulation. As usual, a check radiograph is taken to ensure full seating of the scan flag. The area around the implant is then rescanned. There is a bite scan which then occludes the digital models. This process usually takes a total of 2-4 minutes depending on the number of implants to be scanned.
There are several post-scanning features that are of use. You can review the scans and the scan information. If you suspect an area is inadequate, then you can erase it and rescan the area. There are also functions which check restorative space and occlusal contacts (Fig 5). After that all you do is press the ’send’ button.
This information is then sent to your dental laboratory. The technicians will receive the information within 10 minutes and be able to review your impression and respond with a message back to you if there are any issues with it.
If you require a monolithic restoration, or a temporary restoration direct to an implant fixture head or on a titanium base, then the laboratory need no further information and can mill the restoration. However, depending on the case and what you wish, they will usually print a model. This can now be printed with soft tissue if required.
Any type of restoration can then be constructed, whether a gold-based crown, either screw or cement retained, implant bar restorations, zirconia or titanium abutments and crowns.
I have included a couple of cases to illustrate the workflow, and results of the Trios.
In the first case, the patient had been wearing a partial acrylic denture for several years. She was very unhappy with the appearance of her teeth. We undertook some preliminary veneer work on anterior teeth, which had some unsightly fillings After implant placement and healing, I scanned the mouth and then fitted milled temporary crowns and bridges to increase the vertical dimension a little, increasing restorative space.
After further tissue maturation, I scanned again to ensure we incorporated the new emergence profiles in the final restorations. Figure 7 shows the scan on the technicians’ screen where you can see the soft tissue shape. Figure 8 shows the same but with the scan flags in place. A printed model with soft tissue was produced (Fig 9), showing the model with soft tissue on one side and soft tissue removed showing anologues on the other. The patient was very happy with the result (Fig 10).
The second case is a simple single tooth replacement. The patient attended with a failed post crown in tooth 24. Unfortunately, it had been left with a root fracture for a considerable time. The tooth was removed and, following healing, an implant was placed (Fig 11). A temporary Rochette bridge was constructed. After second stage surgery, a scan flag was inserted (Fig 12) and scanned to construct a temporary, milled from Telio CAD (Ivoclar Vivadent). The scan appears very clearly and it is easy to manipulate on the screen of the Trios (Fig 13).
The scan was sent in seconds to the dental laboratory. The technician imports the scan into the 3Shape software and designs the temporary crown. Figure 14 shows the manipulation of emergence angle. The temporary can be checked for excursive contacts on one of 5 ’virtual’ articulators (Fig 14).
The occlusal carving of the temporary can be finessed on screen (Fig 15).
The temporary was fitted and further tissue maturation was allowed. If required, additional material can be added to the Telio CAD temporary to build further tissue support. Another scan was taken to record the new emergence profile, and a final restoration was constructed: a screw retained zirconia crown.
If the final restoration is a monolithic one, for example full contour zirconia or Lava Ultimate restoration, then no model will need to be made. For a gold/porcelain or, as in this case zirconia porcelain, then a model will be printed and used in the construction of the final restoration (Fig 16). The patient was delighted with the try in of the restoration (Fig 17). However we decided improvements were needed and the final restoration had improved colour and length (Fig 18).
Digital dental impressions are a clinically available technology that will revolutionise dental practice, both in patient acceptance and clinical efficiency. Improved communication with thedental laboratory will mean better restorations from both the patient’s and the clinician’s perspective. It is time to change. It will be a gradual change but this is happening all over Europe and around the world. The UK is slowly changing as practitioners begin to see the benefits. As patients begin to hear about these technologies being available, they will also begin to demand their use.
I would like to thank Craig Smith of DTS for his excellent porcelain work.
About the author
Dr Duncan Black BDS, MJDF RCS (Eng) is the principal dentist at Somerset Place Consulting. He is a member of the Association of Dental Implantology and the Academy of Osseointegration.
Almeida E Silva, JS, Erdelt, K, Edelhoff, D, Araújo, E, Stimmelmayr, M, Vieira, LC, Güth, JF, 2013. Marginal and internal fit of four-unit zirconia fixed dental prostheses based on digital and conventional impression techniques. Clinical Oral Investigations, May 29. [Epub ahead of print]
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Lee, SJ, Gallucci, GO, 2013. Digital vs. conventional implant impressions: efficiency outcomes. Clinical Oral Implants Research, May;17(4): 1201-8.
Linkevicius, T, Svediene, O, Vindasiute, E, Puisys, A, Linkeviciene, L, 2012. The influence of implant placement depth and impression material on the stability of an open tray impression coping. Journal of Prosthetic Dentistry, Oct;108(4):238-43.
Mpikos, P, Tortopidis, D, Galanis, C, Kaisarlis, G, Koidis, P, 2012. The effect of impression technique and impression angulation on the accuracy of external- and internal-connection implants. International Journal of Oral and Maxillofacial Implants, Nov-Dec;27(6):1422-8.
Seelbach, P, Brueckel, C, Wöstmann, B, 2013. Accuracy of digital and conventional impression techniques and workflow. Clinical Oral Investigations, Sep; 17(7): 1759-64.
Storey, D, Coward, TJ, 2013. The quality of impressions for crowns and bridges: an assessment of the work received at three commercial dental laboratories, assessing the quality of impressions of prepared teeth. European Journal of Prosthodontics and Restorative Dentistry, Jun;21(2):53-7.
Wismeijer, D, Mans, R, van Genuchten, M, Reijers, HA, 2013. Patients’ preferences when comparing analogue implant impressions using a polyether impression material versus digital impressions (Intraoral Scan) of dental implants. Clinical Oral Implants Research, Aug 14 doi:10. 1111/clr. 12234. [Epub ahead of print]