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Model-free crowns with CAD/CAM dentistry

Categories: CAD-CAM

Author(s): G. Shull

Date: 09-20-2017 12:31:09 pm

CAD/CAM has provided dentists and dental laboratories with the opportunity for collaboration on the fabrication and delivery of crowns, without the use of either traditional impressions or models. Inherently, this is a game changer that can be viewed from several perspectives.

Traditionally, impressions were taken and a model was either poured and trimmed in the lab area at the back of the dental office or sent to the laboratory for pouring. Impression materials must be of high quality, the actual impression taken requires expertise, and the impression must be properly treated/stored/shipped. Flaws found in impressions include voids due to the incorporation of bubbles or contamination, tears associated with undercuts, shrinkage due to drying or high temperatures, and margins missing due to incorrect isolation or lack of flow of the material around the margins of the prep. These pitfalls can be avoided with careful technique, use of a quality impression material, and proper handling. Potential flaws also include those associated with the pouring of the model – such as air voids and lack of detail due to poor reach of the stone into the model prior to setting (for instance, if a vibrator is not used to encourage flow of the stone material into all detailed areas of the impression).

Further considerations include the need for dies when fabricating indirect restorations from models as well as the potential for abrasion or damage to models, which may result in oversized or poor contacts, occlusal and intaglio dimensional inaccuracies. Infection control is also an essential component in the handling of traditional impressions. The impression must first be rinsed to remove saliva, blood and any other debris present, and then disinfected in accordance with the impression material manufacturer’s instructions before being dried, then shipped or poured. With CAD/CAM scanning, the scanner itself must be treated appropriately as recommended by the manufacturer.

There is no risk of material-related flaws with digital impressions. Provided tissue retraction (if required) and isolation are used, and the scan is properly executed and captured, the impression will be accurate. Model-free crowns remove the need for a poured or milled model, and speed up fabrication. CAD/CAM still permits milling of resin-based models if traditional fabrication of a crown is preferred or if the laboratory and dental office would like to check the fit of a milled crown on a model. In either case, the potential for error associated with stone models is avoided.

Increasingly, CAD/CAM model-free crowns are being fabricated, i.e., without the use of a milled model even for checking the contact and occlusions. Virtual articulators are used while designing the crown, and they result in an accurate and suitable crown occlusal form.

When introducing fabrication of model-free crowns in the laboratory, there are several important considerations for versatility and accuracy. The first is the accuracy of the CAD/CAM scanner and whether the system is open or closed. With an open system, the digital scans can be used with any system and design software. This clearly improves versatility, as does using a high-quality milling machine that is compatible with scans from any system. This also means that the office and laboratory are not locked in with one CAD/CAM solution provider. As such, an open system and versatile work flow are attractive options using a scanner and software system available from the same company, thereby simplfying manufacturer support.

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For both the dental office and dental laboratory, model-free crowns represent a marked change in procedure that can initially seem daunting, as there is no model for the dental technician or dentist to check the contours, margins and contact points. However, model-free milled crowns have proven to be reliable. It is always possible to mill models initially and to ascertain the fit of the model-free crown using the model until you have built up confidence that the results are accurate. After that, periodically using a milled model for spot checks is also an option, if preferred. As long as the scan was accurate in the first place, the model-free crown will be accurate. For crowns requiring esthetic customization, such as in the anterior zone, the crown can be milled and then custom-stained and glazed. By working with a local laboratory, if complex shade matching and customization are required it is also possible to have the patient visit the laboratory or for the laboratory technician to visit the dental office while the patient is being treated.

An additional aspect of CAD/CAM indirect restorations is to ensure that the luting agent used is compatible with the CAD/CAM block material. If in doubt, the CAD/CAM block manufacturer can be consulted as well as the luting agent manufacturer. Crowns arriving from the laboratory include information on the type of block that was specified and used, and commonly contain information on suitable luting agents.

Model-free crowns are faster to produce, accurate and less expensive than fabrication using models – mainly due to improved workflow and the automated design and fabrication, which reduces labor costs. In summary, model-free crowns can benefit the laboratory, the dental office and the patient.

References

1. Beuer F, Schweiger J, Edelhoff D. Digital dentistry: an overview of recent developments for CAD/CAM generated restorations. Br Dent J. 2008 May 10;204(9):505-11.

2. Ceranic D. Trends in implant dentistry: model-less restorations. Inclusive Implant Magazine. 2013;4(2). www.glidewelldental.com/inclusivemagazine/ volume4-2/implant-dentistry-trends.aspx. Accessed May 9, 2014.

3. Joda T, Brägger U. Digital vs. conventional implant prosthetic workflows: a cost/time analysis. Clin Oral Implants Res. 2014 Sep 2.

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