Dentistry has changed. Patients no longer come to the office for traditional, "drilling, filling, and billing." Now many patients want cosmetic dentistry and are no longer satisfied with amalgam fillings, partial dentures, and metal-based crowns. Progressive clinicians who now restore teeth with composites, porcelains, and CAD/CAM crowns and bridges are addressing the patients' needs and concerns about mercury and other metals that are used in dentistry. In addition to patients' concerns regarding the types of materials used for their fillings and crowns are the concerns about restoration longevity. According to Christensen,1 amalgam will last 10 years. The first composite for use as a class II restorative material was introduced in 1968 and also has an expected 10-year longevity but requires more technical skill to place. PFM aesthetic longevity is reportedly 10 years, and functional longevity is 20 years.2-11
The new zirconia-based CAD/CAM prostheses have high strength characteristics; therefore the longevity looks ‚ "highly promising." CAD/CAM (computer-assisted design and computer-assisted manufacture) is not a new concept. This technology has been used for years in the space industry as well as in computer chip design and manufacture.3 Two types of CAD/CAM applications are currently being used in dentistry: laboratory-based and office-based. Sirona Dental Systems markets the in-office CAD/CAM as CEREC. Three of the laboratory systems are Cercon (DENTSPLY Ceramco), Lava (3M ESPE), and Procera (Nobel Biocare).
Each system has benefits. The Procera system, with its tooth-colored blanks, makes shade matching simpler for the technician. The alumina composition of the Procera blanks allows construction of crowns only. Bridges can be constructed, but the abutments and pontics must be constructed separately and then connected.12 Cercon and Lava are constructed from zirconia (ZrO2), which is a stronger material than alumina or pressed ceramic materials and allows bridge construction as well as single crowns. The span for Cercon bridges can be as wide as 47 mm with predictability and longevity. A bridge replacing 2 missing molars is now feasible with this material.4,7 Partial and even full dentures are being replaced with implant-borne prostheses. Even with implant retainers, metal-free crown and bridge is possible with CAD/CAM.3-5
Patients today are very computer savvy and frequently surf the Internet for healthcare information. Frequently they come across articles and information sheets that implicate mercury and other metals used in dentistry as potential allergens. Although there have been very few documented cases of contact allergies to metals used in dentistry,10,11 both mercury and nickel have been implicated in these rare soft-tissue irritations and add a reason for using nonmetallic dental restorations. Despite the infrequency of such allergic reactions, many patients request the use of alternative materials if available.
With CAD/CAM restorations we can satisfy our patients and deliver high-quality dentistry without the use of metal.
CAD/CAM TECHNIQUES AND MATERIALS
|Figure 1. Diamonds No. 847 KR-016, 839-014, and 379-023.|
Tooth preparation for a CAD/ CAM crown is similar to the preparation for a PFM crown.11,12 The burs of choice for the preparation are as follows: 847 KR 016 (Axis) for basic reduction and application of either a deep chamfer or inclined butt joint margin. Sharp line angles are contraindicated, since they can incorporate stress and potential fracture lines in the crown.13 The 379-023 football-shaped diamond (Brasseler) is good for occlusal reductions and rounding of occlusal and lingual line angles. The 862-014 (Alpen diamond, Coltene/ Whaledent) (safe end) is good for removing the ditching or J margin at the gingival margin, which can cause an incorrect scan for the Procera coping.13 These are the only 3 diamonds needed to complete the preparation of a tooth for a crown constructed by CAD/CAM (Figure 1).
|Figure 2. The 3 diamonds used to accomplish CAD/CAM crown preparation demonstrating the correct preparation design.|
Reduction of the tooth should provide enough bulk to support the undercarriage of the alumina/zirconia/porcelain coping and application of the porcelain for aesthetics; 2-mm bulk reduction of the tooth will allow sufficient space for the coping, which can be as thin as 0.4 mm, and application of porcelain, which can be as thin as 1.5 mm. Occlusal reduction should also be 2 mm.12 Two-plane reduction is important to give the technician room for both the coping and porcelain14 (Figure 2).
In preparing the crown, care should be given to taper the preparation occlusally (or incisally) to replicate the emergence profile of the natural tooth being restored. Technicians comment on the difficulty they have in maintaining aesthetics when an area of the preparation doesn't provide enough room for the coping and overlying porcelain.13
Preparations for crowns, bridges, and veneers can be supragingival, and the diamond need not disturb tissue during preparation.
Impression technique should follow the same guidelines as for traditional crown and bridge impressions. Rigid trays (or custom trays) should be used for the final impressions along with a rigid tray impression of the opposing arch. Bite registration should be taken with a vinyl polysiloxane (VPS) material such as Regisil (DENTSPLY Caulk) or Jet Bite (Coltene/Whaledent), and triple-tray impressions should be avoided. There are 2 reasons why triple trays should not be used for any final impression/bite registration:
(1) Torquing of the tray may occur when the patient is asked to bite down.
When the triple-tray impression is removed from the mouth, elastic memory will cause a distortion of the impression and therefore a nonfitting coping (and crown or bridge).
(2) The patient may bite on the rim of the tray and give an incorrect bite registration, requiring a great deal of time in occlusal adjustment.
The author's impression material of choice for metal-free dentistry is vinyl polysiloxane. Many VPS materials are on the market, and they are all good. The author has used the following materials: Aqua-Sil Ultra (DENTSPLY Caulk), Affinis (Coltene/Whaledent), and Flexi-Time (Heraeus Kulzer). Each has its own characteristic benefits. Aqua-Sil Ultra has a very hydrophilic nature. Affinis handles very well and has a new light-body dispensing system, which is very economical. Flexi-Time has a set time based on body temperature, and once placed in the mouth sets fairly rapidly with good impression results.
Many clinicians like to use the polyether-type impression material. However, the main benefit of hydrophylicity is less important in the case of CAD/CAM restorations, since impressions are supragingival. Therefore, the additional cost of the material is not justified, in the author's opinion.
After the laboratory receives the impression, it is mounted, and a wax-up of the preparation is made. In the Cercon process, the wax-up is converted into a coping.
The coping is returned to the office, the framework is checked for fit, and then it is returned to the lab with a shade prescription.
Crowns and bridges can be cemented traditionally with self-cure resin cements such as Principle (DENTSPLY Caulk), or they may be cemented with glass or resin ionomer cements such as Infinity (Den-Mat).14
Following is a case study involving CAD/CAM restorations that met the patient's demands for improved aesthetics and function.
CASE STUDY: CONGENITALLY MISSING LATERAL INCISORS
|Figure 3. Orthodontically repositioned cuspids in the position of the lateral incisors.|
The patient presented with the chief complaint of small teeth and congenitally missing lateral incisors. As is often the case with missing laterals, orthodontic movement of anterior teeth had previously been accomplished. At times the cuspids are moved into the position of the lateral incisors and a fixed or removable partial denture is constructed to replace the missing teeth. At other times the cuspids are moved into the anatomically correct position and fixed; removable partial dentures or implants are the treatment of choice. In this case the cuspids had been moved into the position of the lateral incisors, which made construction of an aesthetic anterior restoration very challenging (Figure 3).
Initially, a removable partial denture was made and worn to correct the aesthetic defect. This proved very difficult for the patient, who was concerned about 3 problems associated with the removable appliance. Her first concern was the inability to speak comfortably, since the partial did not have proper retention. Second, she could not eat well for the same reason and felt that the additional acrylic in her mouth affected her sense of taste. Third, the cosmetic effects of both the shape of the cuspids and the size of her teeth were not pleasing to her.
|Figure 4. Two porcelain-fused-to-gold bridges and 2 porcelain veneers on central incisors.|
A combination of 2 PFM fixed partial dentures and 2 porcelain veneers were constructed to allow better speech and eating functions and improve overall aesthetics. The results were satisfactory for a time, but the patient wanted a more functional and aesthetic restoration (Figure 4).
The new treatment plan included gingival recontouring with radiosurgery to improve the high lip line and the replacement of the existing veneers with 2 Ceron Crowns; then the replacement of 2 PFM fixed partial dentures with two 3-unit Cercon fixed partial dentures.
|Figure 5. Biological width.|
To facilitate a functional and aesthetic temporary restoration, a preliminary VPS impression was taken with a triple-tray and Affinis VPS impression material (Coltene/Whaledent). The partial denture was left in place to allow for a pontic in the temporary bridge. This impression was then set aside, and the patient was anesthetized. Periodontal probing and radio-graphs demonstrated that the tissue would allow a recontouring of 1 to 2 mm to improve the length-to-width ratio of the teeth and maintain ‚ "golden proportions" to minimize the effect of the high lip line while preserving biological width (Figure 5). Radiosurgery using the Ellman Dento-Surg (set at level 7: Cut and Cauterize) sculpted the gingival levels.16
|Figure 6. Preparations for CAD/CAM bridges and crowns.|
The 2 bridges and the 2 veneers were removed, and the preparations of the teeth were modified to allow for construction of Cercon copings. The preparations should follow general guidelines for all-ceramic restorations (see preparation guidelines and bur selection discussed previously and seen in Figures 1 and 2), which include modified shoulder or chamfer margins. The J margin is removed to preserve preparation integrity for CAD/CAM scanning. Preparation margins may be kept at the crest of the gingiva or supragingival if desired. Subgingival margins are not recommended because there is no metal in the restoration to impart a ‚ "black line" in the event of gingival recession (Figure 6).
Impressions taken with VPS and a rigid tray (stock or custom) should be full arch, and opposing impressions should also be full arch for best occlusal results. Bite registration can be taken with Regisil (DENTSPLY Caulk) or Jet Bite (Coltene/ Whaledent), and the case is sent to the laboratory for Cercon coping construction.
|Figure 7. Temporary crowns and bridges.|
Prevision temporization material (Heraeus Kulzer) was dispensed into the previously taken triple-tray impression and placed in the mouth. In minutes, the material began initial set and was then removed from the mouth for trimming and polishing. The temporary bridge/splint was cemented in the mouth with Prevision temporary cement (Heraeus Kulzer, Figure 7).
|Figure 8. Retracted view of final restorations: 2 Cercon crowns on teeth Nos. 8 and 9; two 3-unit Cercon bridges.||Figure 9. Portrait of final restorations.|
The try-in visit is important to verify marginal fit and adaptation of the copings prior to application of the porcelain. Shade matching is next, and any characterizations, surface anatomy, and/ or translucency formats must be given to the lab for the desired aesthetic results. The final restorations (2 Cercon crowns on teeth Nos. 8 and 9, and two 3-unit Cercon bridges) were returned from the laboratory, and the restorations were cemented using Principle auto-cure resin cement (DENTSPLY Caulk). The final result is seen in Figures 8 and 9.
Dentistry has changed. Our patients now want both cosmetic and functional dentistry, and with advances in CAD/CAM prosthetics, dentists now have the tools with which to achieve both goals.
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The author thanks his laboratory technician Peter Lee, owner of Puredent Dental Laboratory, for laboratory support.
Dr. Abel is an accredited member of the American Academy of Cosmetic Dentistry, a member of the American Society for Dental Aesthetics, a fellow of the Academy of General Dentistry, and a fellow of the International Society for Dental and Facial Aesthetics. He is published nationally and internationally and lectures on the techniques necessary for using composite resins to restore both anterior and posterior teeth predictably. Dr. Abel maintains a cosmetic dental practice in Rockville, Md. He can be reached at (301) 770-1447.