The Basics of CAD/CAM Dental Milling

CAD/CAM technology has helped to transform the manner in which restorative treatment can be provided. In particular, the ability to provide patients with single-visit indirect restorations is appreciated by clinicians and patients. Chairside milling produces restorations that are at least as accurate as traditionally fabricated indirect restorations. Chairside milling also allows clinicians to optimize the process flow, reducing chairside time, increasing efficiency, and saving patients additional visits. As with other procedures, each step must be accurately performed. Once the learning curve has been mastered, chairside milling provides an accurate method to provide patients with efficient and aesthetic restorative care.

Introduction to CAD/CAM Dentistry

CAD/CAM has become mainstream and has changed dentistry, particularly in the fields of restorative and implant dentistry. Single-unit crowns are the most frequently provided indirect restoration, and together with inlays and onlays were the first to be provided using CAD/CAM technology. Since then, bridges, veneers, post-and-cores, and implant abutments and restorations have all been created using CAD/CAM dentistry.

Fabrication of a CAD/CAM restoration may occur in a laboratory after sending the digital files through a secure internet portal, either in a central location designated by the CAD/CAM system’s manufacturer or in laboratories that are authorized by the manufacturer. Such systems are “closed” systems. Increasingly, systems are “open,” meaning that any laboratory with the capability to use the software can fabricate the restoration using virtual models and CAD/CAM milling, or by milling a model and then using a traditional technique to fabricate the restoration.

With digital impressions, a scan is taken of the clinical site, adjacent teeth, and opposing arch, after which the scans are transformed into images for immediate viewing on the screen. This allows the clinician to check the preparation for any deficiencies on a large screen and to immediately correct any deficient areas. From there, the restoration can be designed and fabricated. In contrast, with traditional impressions and a traditional technique, it may be more difficult to spot marginal defects in the impression, and models must be poured from these. Inherently, there are more opportunities for errors with a traditional technique, from defects in impressions or models to manual fabrication of the final restoration. Scanning of traditional impressions is a third option—this does digitize the information, allowing for CAD/CAM fabrication of the restoration; however, it will also scan any impression errors. A CAD/CAM milling unit may also be incorporated into the dental office, making it possible to mill restorations chairside while the patient is in the office.

Chairside Milling

In addition to the obvious advantage of being able to provide single-visit indirect restorations that are esthetic and accurate,1 chairside milling reduces the time required for restorative care, does not require delivery of a provisional restoration, and gives the dentist complete control over the procedure and the process workflow. A further advantage is the ease with which digital scanning and chairside milling can be performed in challenging situations, such as treating elderly patients with reduced cognition, or patients with limited mobility or ability to tolerate long treatments.2 Conversely, the cost of chairside-milling technology is a capital investment not required if laboratory milling is performed, and there is a learning curve associated with its adoption by the dental team and optimization of process workflow including delegation of tasks.1

Block Options

Regardless of the type of restoration, CAD/CAM milling begins with a CAD/CAM block and should end with an esthetic result even for challenging cases. Options include zirconia-based, leucite-based, and feldspar-based ceramics; lithium disilicate; and composite blocks. All-ceramic CAD/CAM blocks offer strength and esthetics, including a chameleon effect.3 The preparation design must take into account the type of block being used, and provide the correct preparation form and clearance for that material. CAD/CAM materials blend well, allowing margins to be placed supragingivally or perigingivally.

Accuracy and Longevity of CAD/CAM Restorations

Several studies have determined that CAD/CAM restorations in general have a marginal fit at least equivalent to those fabricated using traditional impressions and laboratory techniques.4,5 In vivo and in vitro studies have also assessed overall accuracy and fit of single-unit crowns milled using all-ceramic, zirconia or lithium disilicate blocks and found these to be acceptable and comparable to the use of a traditional technique.6,7 Similarly, single-unit implant crowns and abutments were clinically accurate in other studies.8,9 In one recent review, the long-term survival rate for single-tooth indirect restorations was similar following digital or traditional fabrication.10

A significant number of studies have been conducted on chairside-milled restorations using ceramic and composite block materials. The marginal and overall fit of chairside-milled restorations were found to be at least as accurate as those fabricated using traditional methods; the materials were also found to offer sufficient strength and fracture resistance and clinically acceptable outcomes.11-16 In addition, the 5- and 10-year survival rates for chairside milled restorations were found to be 97% and 90%, respectively, in one review.17

The process of CAD/CAM milling

The process flow when providing patients with single-visit CAD/CAM restorations can be simple. After scanning and verifying the margins, interocclusal clearance, and proposed restoration design, it’s simply a question of waiting for the restoration to be milled, any necessary staining and glazing or sintering, and trying in the restoration and seating it. In cases where the shape of the teeth must be duplicated, scans of the pre-existing teeth shapes can be made and used as a virtual template for the restorations, provided the intent is to have restorations with the same shape. This reduces the likelihood of adjustments being needed or patient dissatisfaction.

Chairside milling also allows you to use a process flow that optimizes the treatment schedule by performing other procedures in the same visit as well as delivering the final restoration without an intermediate provisional phase. During the time of onset of local anesthesia, scans of the opposing dentition can be made as well as of the existing teeth at the site(s) if the same shape is desired, and the shade can be selected. Later in the appointment, while waiting for the restoration(s) to be chairside milled, and finished, other treatment needs can be addressed on the same visit. Treatment such as a direct composite restoration or endodontic therapy can be completed, if appropriate, while the restoration is being milled. This saves chairside time, maximizes efficiency, and saves the patient another visit.

Efficiency (Speed)

CAD/CAM imaging and chairside milling together save valuable time by streamlining treatment and permitting delegation of significant portions of the process. In one study, digital scanning alone saved dentists 55% of the time previously required to take a traditional impression.18 Digital images are immediately available for examination, such that any imperfect areas of a preparation can be adjusted and rescanned if necessary, without the patient having to return. Studies have shown that significant time savings can be achieved using CAD/CAM software and milling machines. In addition, CAD/CAM software now has automated marking and evaluation of margins, and uses digital articulating paper to rapidly determine the occlusion and interdigitation of teeth in opposing arches. Full color images have also made it easier for us to view margins and other aspects of the preparation. The case below shows use of CAD/CAM and chairside milling for a single-unit esthetic anterior restoration.

Conclusion

Digital impressions can be taken and restorations created that rival traditional techniques for form and esthetics. Of course, care must be taken to ensure that the digital scans are accurate and that preparation and milling procedures are properly executed. Once the learning curve has been mastered, chairside milling provides an easy and efficient method to provide restorative care. The process workflow with chairside milling can be streamlined and efficient dentistry provided. Chairside time is optimized, accurate and esthetic restorations can be provided, and patient satisfaction is excellent.