Duplicating the Functional Path With Conservative Restorations

INTRODUCTION
Resin and ceramic restorations have been used successfully as aesthetic alternatives to metallic restorations for several decades. However, duplicating preexisting wear patterns and contours of teeth with direct or indirect techniques has proven to be technically daunting. Recent technological advances using CAD/CAM systems have made it possible to duplicate the original preoperative forms of teeth, including functional paths, using a function called correlation.
Bonding to enamel is generally considered to be preferable to dentin bonding due to substantially more favorable bond strengths. In some situations, it is desirable to preserve enamel for aesthetic as well as functional reasons. However, when anterior teeth are compromised with large Class III restorations, the most appropriate treatment is usually considered to be a full-coverage restoration with a prosthetic crown. When the entire facial wall and incisal edge of such teeth are intact, full-coverage therapy requires reduction of the facial and incisal aspects of the tooth to create space for aesthetic restorative materials. Replicating natural enamel with artificial dental materials aesthetically is challenging, especially when adjacent teeth are not planned for restoration. Therefore, there is an indication for a lingual three quarter crown restoration so that the facial wall of enamel can be preserved and utilized for the maximum aesthetic outcome and for optimal bonding of the restoration.
This clinical case report describes a technique for the fabrication of a chairside indirect three quarter crown on the lingual aspect of a maxillary left canine utilizing CAD/CAM technology.

CASE REPORT
Diagnosis and Treatment Planning
A 57-year-old male presented with large restorations on the mesial-lingual and distal-lingual aspects of the maxillary left canine. A horizontal fracture was present that extended from one restoration to the other (Figure 1a). The facial enamel, including the incisal edge and facial line angles, was pristine (Figure 1b). The tooth, which tested vital to thermal stimuli and electric pulp testing, was asymptomatic. The patient was in a disease-control mode of dental rehabilitation, and restoration of the adjacent teeth was not in the treatment plan for the immediate future due to financial and disease-control reasons. Occlusal equilibration had been previously completed, which had provided canine guidance into anterior crossover with stable and balanced posterior centric occlusion.

Conventional restorative modalities were considered and discussed with the patient. Replacement of the existing restorations with a single large bonded direct composite resin restoration was ruled out due to difficulty in reproducing the optimal functional path of eccentric guidance. Core buildup and full-coverage therapy with a ceramic or metal-ceramic crown was ruled out due to financial considerations, due to the difficulty involved in reproducing the functional pathway of guidance, and due to aesthetic concerns because the shade of the natural adjacent teeth fell outside of normal restorative shade selections. Furthermore, preservation of the facial enamel was desirable for maximum bonding of the restoration and for aesthetic benefit. Therefore, the decision was made to restore the tooth with an aesthetic lingual three quarter crown using a CEREC AC (Sirona Dental Systems) chairside CAD/ CAM system. Milled porcelain, lithium disilicate, or resin would have all been appropriate materials to use. In this case, a Paradigm MZ100 (3M ESPE) composite resin block was chosen to minimize the potential for wear on the opposing natural enamel. This choice would also facilitate preparation of this restoration as a core buildup for full-crown therapy during full-mouth reconstruction in the distant future. Furthermore, this type of restoration had been tested by similar restoration of the right maxillary canine tooth 3 years previously on this same patient (Figures 2a and 2b).

Operative and Technical Protocol
After anesthesia was administered, the tooth to be restored (No. 11) and the adjacent teeth were powdered according to standard technique. The CEREC AC unit was set for the fabrication of a crown in master mode because the current software (V1.10) did not allow for designing a veneer on the lingual aspect of an anterior tooth. The correlation function was selected, which allows for duplication of existing tooth form. An image was captured and placed into the occlusion window. Placing the image into the antagonist window would use biogeneric programming to design a restoration within the bounds of the existing tooth form, but it would not have duplicated the contours that comprise the functional path of occlusion.

Figure 4. After capturing the image of the preparation, the margin was outlined. Note the placement of the incisal margin midway through the incisal edge of the tooth.	Figure 5. The area of desired duplication was highlighted (green) on the occlusion image of the original tooth.
Figure 6. Since the software did not enable design of a lingual veneer, the computer interpreted the natural enamel incisal-facial line angle as the margin. This misinterpretation of data was reflected in the computer-generated proposal.	Figure 7. The “flash” on the incisal edge was removed using the graphic manipulation tools.

All caries were removed from tooth No. 11, and it was then prepared for a lingual three quarter bonded crown with the intention of maximizing the amount of enamel left for bonding. Glass ionomer (Fuji IX [GC America]) was used to base a small undercut on the mesial pulpal wall (Figures 3a and 3b). The preparation was powdered according to standard protocol, and a series of digital images was captured and placed into the preparation window. A lingual three quarter crown restoration was designed and milled using the CEREC CAD software and milling unit. (Figures 4 to 8b).
The sprue was then removed with an abrasive rubber disc (Kenda 244 Blue [Kenda AG Dental Manufacturing) on a straight mandrel. The restoration was tried-in (Figure 9) and polished with a rubber disc (Burlew 5/8″ Knife Edge [Pacific Abrasives]) and chamois wheel with acrylic polishing compound (Kenda 244 Blue). Due to the delicate nature of the restoration, occlusion was not evaluated at this time.
The intaglio surface of the restoration was steam cleaned, silanated (Silane [Ultradent Products]) for 60 seconds, and then air-dried. Afer proper retraction and isolation (Isolite System [Isolite Systems]), the tooth was prepared for bonding using a total-etch technique. Optibond FL (Kerr) primer was applied according to the manufacturer’s instructions, and Optibond FL adhesive was placed on the surfaces of the preparation and thinned but not cured at this time. The silanated restoration was loaded with a dual-cured resin cement (Resiment Ready-Mix Dual-Cure Plus Resin Cement [J.L. Blosser]) and placed on the preparation. Under light pressure, the restoration was exposed for 5 seconds to a standard curing light. Excess cement was gently removed, and the restoration was cured for an additional 40 seconds (Figure 10).

Figure 9. The restoration was tried-in, and optimal margin adaptation was confirmed.	Figure 10. The margins of the bonded restoration are difficult to discern visually or tactilely.

Figure 11. A combination of a separate facial cervical restoration and the lingual three quarter resin crown yielded an aesthetically pleasing final product that showcases the aesthetics of the natural facial enamel of tooth No. 11.

A separate Class V direct composite resin (Esthet•X Flow [DENTSPLY Caulk] bonded with Optibond FL) restoration was placed facially where a noncarious cervical lesion was present (Figure 11).
Occlusion was verified in both maximum intercuspal position (Figure 12a) and in lateral excursions (Figure 12b). The restoration was polished using diamond paste (Diashine [VHTechnologies LTD]) with a rotary brush.

DISCUSSION
When anterior teeth have been heavily restored previously but require reconstruction for improved structural integrity, full-coverage restorations are generally considered to be the treatment of choice. However, there are occasions when the needs of the patient call for the preservation of precious healthy enamel. In the case presented, it was desirable to preserve the facial enamel for optimal aesthetics and optimal bonding strength. CAD/CAM technology makes it possible for exact duplication of existing tooth structure while minimizing critical tooth structure.
Although a variety of materials could have been used to fabricate the restoration in this case, composite resin was chosen for several reasons. The treating dentist anticipated the future fabrication of a full crown, and the possibility of future need for endodontic therapy existed due to previous extensive restoration and dentinal fracture. Therefore, it seemed logical to use a restorative material (Paradigm MZ100) that could be more easily prepared with rotary instrumentation than various conventional or high strength porcelains. This patient’s dental history suggested that this type of restoration would fare well for several years, as evidenced by a previously placed three quarter resin crown on tooth No. 6.
Approximation of the functional path of occlusion and appropriate restoration of the lingual contours of tooth No. 11 could have been completed conventionally through the use of full-arch maxillary and mandibular impressions, the making of meticulous bite records, the taking of a face-bow record, and appropriate utilization of a semi-adjustable articulator. However, this would have required provisionalization of the preparation. The thin enamel wall would have been in danger of fracture during the time necessary for laboratory processing of a similar restoration.

CONCLUSION
This clinical case report outlined the steps in the chairside fabrication of a lingual three quarter crown using CAD/CAM technology while demonstrating the implementation of recent technological advances using CAD/ CAM systems that allow the clinician to duplicate the original preoperative forms of teeth, including functional paths. Using a single-appointment approach to the restoration of the tooth in this case also allowed the clinician to avoid the need for extensive record taking and ensured the preservation of the thin facial enamel wall of the preparation.

Dr. Huff is a private general dentist in Dover, Ohio. Among his teaching commitments, he is a clinical instructor at the Case School of Dental Medicine in Cleveland, Ohio; is the coordinator of oral mucosal screening for the Mercy Medical Center General Dental Practice Residency Program in Canton, Ohio, and he is visiting faculty at Spear Education in Scottsdale, Arizona. He can be reached at (303) 364-2011 or dr@doctorhuff.net.

Disclosure: Dr. Huff reports no disclosures.

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