How to implement CAD/CAM into your practice

With a wide variety of materials now available to use with the E4D Dentist System, this how-to by Dr. Walter Renne, DMD, offers up examples of restorations and a review of material choices.

Despite the increasing popularity of the current CAD/CAM laboratory systems and their continuing technical advances, some clinicians have remained reluctant to incorporate the very same CAD/CAM techniques into their clinical chairside practices. Two often-repeated misconceptions relate to the perceived lack of strength and lack of esthetics of the ceramics available for use with these systems.

A wide variety of materials are now available to use with the E4D Dentist System and each has a separate set of esthetic and mechanical properties that must be considered. This article will review current materials and show clinical examples of restorations made using the E4D System.

A big plus
One distinct advantage of chairside CAD/CAM is having the ability to make restorations in a single visit from a solid pre-manufactured block that is essentially flawless in construction. A pre-manufactured block is made in ideal conditions, and as a result, has an ideal density with none of the residual porosity found in many layered or pressed porcelains.

Porosities may act as a weak point and lead to the buildup of internal tensile stress in the ceramic and eventually cause a catastrophic failure. Monolithic restorations have several distinct advantages over layered restorations when it comes to mechanical properties. Layered restorations are often veneered with weak feldspathic glasses that can chip or break, especially if not supported properly by the framework. Furthermore, one does not need to worry about delamination and micro-chipping of the veneering porcelain, which has been reported to be as high as 25% for porcelain-fused-to-zirconium restorations.

Empress CAD is a feldspathic glass with approximately 45 percent leucite crystals for dispersion strengthening. The 5 µm leucite crystals improve strength and fracture toughness by acting as “roadblocks” to prevent crack propagation. Empress CAD is a very esthetic material and is available in polychromatic stacked shades that give the restoration a layered appearance. However, it is a weaker ceramic with a flexural strength around 160 MPa and requires meticulous isolation and attention to detail when bonding to ensure long-term success.

Empress has been on the market for approximately 24 years and as a result good clinical research on the longevity of these restorations exists in the literature. A literature review conducted by Brochu and El-Mowafy evaluated and summarized 6 clinical studies that met their inclusion criteria. They concluded the survival rates for IPS-Empress inlays and onlays ranged from 96 percent at 4.5 years to 91% at 7 years. IPS-Empress crowns had a survival rate ranging from 92 percent to 99 percent at 3 to 3.5 years. For both crowns and onlays most failures were due to bulk fracture. In general, Empress has higher failure rates in the posterior than the anterior and higher fracture rates on molars compared to premolars. Therefore, Empress is an excellent material choice in the anterior for esthetically demanding patients. However, better materials exist for posterior use.

The case
A new patient called the office and said his crown “exploded.” He presented to the clinic with the crown missing on tooth No. 9 (Fig. 1). The E4D system was used to take a digital impression of the preparation and the bite registration. Using the intuitive design features in the E4D software, a restoration was designed (Fig. 2). An Empress CAD Multi A1 restoration was milled and custom characterized using IPS Empress Universal Stains. For delivery the crown was prepared by etching with 9.5 percent HFL acid for 60 seconds and silanated for 60 seconds with Monobond-Plus from Ivoclar Vivadent. The tooth was pumiced clean, Optibond XTR (Kerr) was applied and cured for 20 seconds and Nexus 3 resin cement (Kerr) was used to bond the crown (Fig. 3).

Material options
The use of Empress has been declining partly because of the popularity of IPS e.max® CAD (lithium disilicate). e.max® CAD comes in a weaker lithium metasilicate state (blue color) that is not fully crystallized so it is able to be machined.

The milled restoration is then placed in the oven for 19 to 26 minutes to crystallize the glass. During crystallization, the lithium metasilicate crystals are replaced with lithium disilicate crystals and the flexural strength significantly increases from around 160 MPa to 360 MPa. e.max was introduced to the market in 2006 and clinical studies are just now being published. Gehrt and colleagues followed 104 e.max crowns in 44 patients and found the corresponding survival rate for all restorations was 97.4% after five years and 94.8%  after 8 years of clinical service with location not significantly impacting survival rate. These results were for e.max press restorations that were cutback and veneered. It can be hypothesized that monolithic chairside milled e.max would perform better.

In a 10-year study kern et. al. found three-unit fixed partial dentures (FPDs) made from monolithic lithium disilicate ceramic showed five- and 10-year survival and success rates that were similar to those of conventional metal-ceramic FPDs. They concluded that for the monolithic lithium disilicate FPDs the calculated survival rate was 100% after 5 years and dropped to 90.8% (when considering only catastrophic ceramic fractures) and 87.9 % (when considering catastrophic ceramic fractures and biological failures) after 10 years. It is interesting to note that all catastrophic failures occurred in molars.

Single unit monolithic e.max can be expected to preform better than FPDs in this study. Interestingly for both clinical studies mentioned, the restorations that were conventionally cemented preformed just as well as those that were bonded. Therefore, assuming proper retention and resistance form has been achieved, it is acceptable to conventionally cement monolithic e.max restorations. With the incredible flexural strength of e.max some clinicians were concerned that e.max may be aggressive on the opposing dentition. In a clinical study Silva et. al. found e.max to be more gentle on the opposing enamel than feldspathic ceramics with a wear rate on enamel similar to natural teeth.

CAD/CAM’s role
Chairside CAD/CAM allows the clinician to predictably provide more conservative restorations such as e.max inlays and onlays that have longevity similar to full coverage crowns. The advantage to onlays over crowns is the conservation of healthy tooth structure and subsequent prolonging of the tooth’s life cycle. Chairside milled onlays are an ideal restoration compared to direct resins. Despite their popularity large posterior resin based composite (RBC) restorations only last 6-7 years. RBC restorations have poor clinical longevity, higher recurrent caries and greater need for replacement compared to the alternative, high copper amalgam. Amalgam and cast gold are not a popular option for many patients because of esthetic concerns, and an E4D onlay restoration is the ideal treatment for many patients that refuse these alternative treatments. Milled inlays and onlays have been shown to be very successful. One study found a success rate of 90.4 percent after 10 years with older feldspathic ceramics as well as older milling and design technology.

In this case the patient was not happy with the esthetics of the amalgam restorations and she had recurrent caries on the bond-Plus (Ivoclar Vivadent). The tooth was pumiced clean, Optibond XTR (Kerr) was applied and cured for 20 seconds and Nexus 3 resin cement (Kerr) was used (Figs. 4-6). Despite the benefits of onlays single unit crowns are still the more preferred restoration for the general dentist and the E4D system fabricates excellent restorations with a short learning curve. With the strength of e.max predictable restoration of second molars using E4D is possible (Figs. 7–9).

Once the learning curve of single unit restorations is mastered it will not be long before the benefits of the E4D system become apparent for more complicated cases. For this next case a 37-year-old male presented for a consult for dentures. He had been to several dentists and an immediate denture was the treatment plan he had selected. He presented with severe acid erosion and abrasion from a combination of gastroesophageal reflux disease (GERD) and bruxism (Fig. 10).

Occlusal examination revealed lack of anterior guidance and posterior support. The lateral pterygoids were sensitive to palpation and upon visual examination it was noted that he had hypertrophic masseters. Lip commissures were folded and he appeared to have a collapsed vertical dimension of occlusion (VDO). He did not close in a repeatable position and had a severe anterior deviation from centric relation. When evaluating the location of the gingival margins it was determined that compensatory eruption had taken place. However, based on the closest speaking space during the production of sibilant sounds the patient had excess freeway space.

Findings, treatment plans
It was determined that the patient lost vertical dimension of occlusion and therefore compensatory eruption did not keep up with the rate of erosion. Two centric relation (CR) records were made using bimanual manipulation, a custom triad jig and a rigid bite material. The case was mounted on a semi-adjustable articular in centric relation and the mounting was verified with the second CR record. It was decided (based on freeway space, esthetics and phonetics) that to recapture the lost VDO the patient needed to be opened 2.5 mm in the anterior; this correlates with around 1 mm in the posterior. A diagnostic wax-up was made.

The teeth were prepared and temporized based on the diagnostic wax-up. The patient was kept in temporaries for 6 weeks to verify tolerance of the new vertical dimension, phonetics (particularly “F” and “S” sounds) and CR. In the temporaries anterior guidance was established with no balancing interferences during lateral excursive movements. CR was stable and at the end of the 6-week trial period the patient was pain free upon palpation of his lateral pterygoid muscles and the temporaries did not show signs of malocclusion such as fracture of accelerated wear.

His central incisors were hitting just inside the wet-dry line of the lower lip during “F” sounds. During “S” sounds, the closest speaking space, the patient’s maxillary and mandibular anterior teeth did not touch. Once verified a vinyl polysiloxane (VPS) impression of the temporaries was made along with a bite registration. At this point centric relation was equal to maximum intercuspal position (MIP). The E4D system has a clone feature that will copy the occlusion and anatomy of the temporaries exactly.

The software allows the clinician to superimpose the temporary “clone” model over the restoration design to determine accuracy (Fig. 11). One of the most powerful features of the software is the ability to turn the clone model clear and analyze how accurately the software has copied the anatomy and occlusion. The accuracy of this is within microns with an intuitive color map displaying the discrepancy that exists between the temporaries and the final crown design. The restorations were milled out of B1 e.max CAD LT. For delivery the restorations were prepared by etching with 9.5 percent HFL acid for 20 seconds and silanating for 60 seconds with Monobond-Plus (Ivoclar Vivadent). The tooth was pumiced clean, Optibond XTR was applied and cured for 20 seconds and Nexus 3 resin cement (Kerr) was used (Fig. 12). The occlusion was identical to the temporaries and thus no adjustments were needed on the day of delivery. With the option now to use e.max HT this case could have a better esthetic result as the LT block appear slightly monochromatic and opaque.

Summary
Dental patients typically want tooth-colored indirect restorations and with newer ceramics that are available for chairside milling the same high quality ceramic restorations labs are promesial of No. 13. The E4D was used to make a digital model and the design software proposed well contoured, anatomical restorations that were milled out of e.max CAD HT A2 blocks. For delivery the restorations were prepared by etching with 9.5% HFL acid for 20 seconds and silanating for 60 seconds with Monoducing can be fabricated in a single appointment. With a chairside CAD/CAM system large technique sensitive and inferior direct resins are treatment planned less with milled e.max onlays taking their place.

Chairside CAD/CAM dentistry is not the only way to provide patients with high quality restorations but it certainly is the most exciting from both a clinician and patient viewpoint. Multiple visit single units restorations, single unit temporaries, difficult resins, expensive monthly lab bills and bonding restorations after weeks of contamination with temporary cement and saliva is routine for most dentists that have not invested in CAD/CAM technology. The old adage “what you don’t know you don’t miss” holds true.

About the author Dr. Walter Renne is a 2003 graduate of the College of Charleston and a 2008 graduate of the Medical University of South Carolina College of Dental Medicine. He is active in undergraduate dental education and holds a full time faculty position in the Department of Oral Rehabilitation at MUSC. He is the course director for CAD/CAM technologies and ceramics and runs the E4D CAD/CAM clinic at MUSC. Dr. Renne maintains an active general dentistry practice utilizing both the CEREC AC and E4D systems.