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Predictable Preparation, Staining, and Cementation Procedures for Chairside CAD/CAM Dentistry

In the modern dental era, dental professionals have numerous materials available to restore their patients’ failing dentition. There are increasing social pressures from the patient population for an aesthetic and biological alternative to metal and metal-ceramic materials (Figure 1). At the same time, practitioners have recognized the inability of metal restorations to reinforce remaining tooth structures, whereas bonded restorations have shown the ability to restore cuspal deformation to levels similar to an unprepared, sound tooth and return teeth to a normal strength value using the acid-etch technique.1,2

The metal-free nature and aesthetic advantages of bonded resins have led to their increased use as restorative materials. The primary disadvantage of composite resins is polymerization shrinkage of the resin and the difficulty associated with obtaining an optimal proximal contact.3 Various composite layering techniques have been described in the literature to minimize the effects of the polymerization shrinkage.4-7 These techniques further prevent premature failure of the restoration and allow the clinician to develop more ideal proximal contacts.8

When used appropriately, direct resin procedures provide acceptable clinical results. Nevertheless, as the size of the preparation design increases and a large bulk of composite is used, a greater amount of cuspal deflection and deformation will occur within the tooth. The author recommends direct restorations be used for small or medium carious lesions and that indirect restorations should be used for restoring larger lesions or when cuspal replacement is involved.

There are numerous CAD/CAM protocols available on the market enabling practitioners to provide metal-free, aesthetic, indirect restorations. These CAD systems range from laboratory systems to chairside systems. With laboratory systems, a traditional impression is made and forwarded to the laboratory. The laboratory pours the model and scans dies using the system’s scanner. A coping is then designed on the computer and fabricated using the system’s milling chamber. The laboratory technician then layers the final porcelain on the coping material and returns the restoration to the clinician.

Chairside CAD/CAM systems work in a different manner. The tooth is prepared by the clinician in the traditional manner, but instead of a traditional polyvinyl or hydrocolloid impression, a digital scan is taken of the tooth chairside. The restoration (eg, inlay, onlay, crown, or veneer) is then designed, milled, and cemented on the tooth all in a single visit.

The CAD/CAM Protocol

Regardless of the restorative technique used, the tooth must be prepared to all-ceramic preparation criteria. One critique of all-ceramic restorations in general has been that their preparations are more aggressive than those of traditional crowns due to the bulk of ceramic material required for strength. Since many all-ceramic restorations use the adhesive technique to retain the restoration to the tooth, their preparations, in fact, can be less aggressive than preparations for traditional materials that require a full-coverage crown (Figure 2).9

 

(Continued from page 1 )

 

Preparation Design

Two important factors affecting the longevity of all-ceramic restorations are the preparation design and the strength of the dentin bonding agent.10 For optimal results, the preparations should contain the following characteristics:

  • Smooth preparation contours: For ceramic restorations, sharp internal points lead to over-milling of the porcelain. Over-milling occurs when the CAD/CAM milling chamber, in an attempt to reach the sharp points, removes additional porcelain to ensure the restoration will seat. Soft preparation contours allow for the most intimate adaptation of the porcelain to the tooth structure as well as a reduction of the internal stress in the porcelain which prevents the restoration from premature fracture.
  • Adequate thickness: A minimum reduction of 1.5 mm in the central groove area as well as 2-mm reduction in the cuspal area is recommended for optimal strength.10 Should the tooth need to be reduced axially, the reduction requirements are similar to laboratory-fabricated all-ceramic restorations: generally 1 mm to 1.2 mm (Figure 3).
  • Adequate Design: Traditional preparations required parallel walls to retain the restorations. Since ceramic restorations rely on adhesive bonding, axial walls are not necessary; the preparations should be left entirely supragingival (Figure 4). When possible, the use of an onlay design should be considered over full-coverage designs to preserve tooth structure and provide restoration margins in enamel.
  •  

Once the preparation is complete, an optical image of the tooth is taken. The margin of the restoration is defined by the clinician and the proposed restoration is created digitally. Various tools in the CAD software are available to the clinician to modify and fine-tune the planned restoration. After final design of the restoration, the desired shade of the restoration is selected and placed in the milling chamber, which creates the restoration from the inputted data.

Material Selection and Aesthetic Enhancement

The author prefers to use a milled composite material for inlays and porcelain for onlays and crowns. Porcelain is found to be the closest in characteristics to human enamel and will reinforce and return the stiffness of the tooth that has been significantly altered by tooth preparation.11

The materials used for milling are also available in various translucencies. A high-translucency block is recommended when fabricating an inlay or small onlay so as to allow the porcelain to blend in inconspicuously with the surrounding tooth structure. A low-translucency block is used when the clinician is fabricating large onlays or crowns so that the adjacent teeth can be matched more appropriately.

(Continued from page 2 )

Once the restoration has been milled, the clinician must decide which technique best aesthetically enhances the restoration, based on the expectations of the patient and the position of the tooth in the mouth. For molars, polishing with wheels and brushes using a diamond polishing paste are sufficient. For cases where the aesthetic desires of the patient are higher, or the restorations more visible, the clinician can choose to stain and glaze the restoration.

Typically in laboratories, a two-step glaze cycle is used where the appropriate colored stains are placed on the restoration and the restoration is run through the firing cycle. Because the ceramic reconstructions procedure aims to complete the clinical visit in a minimal amount of time, the stain and glaze cycles are conducted in a single step. The glaze is applied first, and then the appropriate stains are placed. The restoration is fired in a single step, which not only fixes the stains, but also fuses the glaze to the surface of the restoration. The entire stain process takes approximately eight to 12 minutes.

Adhesive Bonding

After the restoration is aesthetically enhanced, it should be tried in for fit before bonding (Figure 5) to ensure the least amount of contamination and the highest bond strengths.12 The proper protocol for preparing porcelain is as follows: The porcelain is etched with a hydrofluoric acid treatment and then thoroughly rinsed with copious amounts of water for a full minute to create the optimum amount of surface irregularities in the porcelain.13 Next, silane is applied to the internal surface of the porcelain for a period of three minutes and then dried with warm air to improve bond strength.14 One coat of resin is applied to the silanated porcelain, and the restoration is set aside, ready for adhesive luting.

While the assistant is preparing the porcelain, the clinician can prepare the tooth for adhesive bonding. The first step, if a powder is used in the imaging process, is to remove all powder residue from the tooth by scrubbing it with a cotton pellet soaked in alcohol or chlorhexidine. The tooth is then etched with a 37 percent phosphoric acid solution (Figure 6) to condition the enamel and dentin surface. After etching, the dentin bonding agent is applied to the tooth then sufficiently thinned and pre-cured (Figure 7). By pre-curing the bonding agent, one prevents a collapse of the collagen network from the pressure of seating the restoration and ensures the highest bond strengths.15

Various cements are available for cementation of the different materials but all-ceramic restorations show the highest success rate when using resin cements that bond the porcelain restoration to the tooth structure. At this time, the use of any cement that is not resin-based cannot be recommended for use with all-ceramic restorations.

The cement is placed in the tooth, and the restoration is seated (Figure 8). The cement is lightly tacked and allowed to come to a gel stage at which time initial clean up can begin. The interproximal aspects are flossed, and the excess is removed (Figure 9). Once the majority of the cement is cleaned, final light curing can be accomplished. The occlusion is verified and the patient dismissed (Figure 10).

Conclusion

Ceramic reconstruction has proven to be a viable and substantiated clinical technique with acceptable clinical margins as well as enhanced aesthetics when the restorations are stained and glazed by the practitioner. By placing the final restoration on the tooth, the benefits of sealing the tooth immediately can be realized for decreased postoperative sensitivity and increased comfort for the patient.

*Director of CAD/CAM, Scottsdale Center for Dentistry, Scottsdale, Arizona; private practice, Tarzana, California

References

  1. Spreafico R. Direct vs. semidirect vs. indirect restorations: Establishing criteria for clinical decision making [interview].  In: Sadan A, ed. Quintessence of Dental Technology 2003. Hanover Prk, Il:Quintessence Publishing; 2003:82-89.
  2. Morin D, Delong R, Douglas WH. Cusp reinforcement by the acid-etch technique. J Dent Res 1984;63(8):1075-1078.
  3. Liebenberg WH. Controlling contacts and contours using preformed ceramic inserts. Compend Contin Educ Dent 2000;21(2):147-150.
  4. Choi KK, Ryu GJ, Choi SM, et al. Effects of cavity configuration on composite restoration.  Oper Dent 2004;29(4):462-466.
  5. Nikolaenko SA, Lohbauer U, Roggendorf M, et al. Influence of c-factor and layering technique on microtensile bond strength to dentin. Dent Mater 2004;20(6):579-585.
  6. Kuijs RH, Fennis WM, Kreulen CM, et al. Does layering minimize shrinkage stresses in composite restorations? J Dent Res 2003;82(12):967-971.
  7. Versluis A, Douglas WH, Cross M, Sakaguchi RL. Does an incremental filling technique reduce polymerization shrinkage stresses? J Dent Res 1996;75(3):871-878.
  8. El-Badrawy WA, Leung BW, El-Mowafy O, et al. Evaluation of proximal contacts of posterior composite restorations with 4 placement techniques. J Can Dent Assoc 2003;69(3):162-167.
  9. Broderson SP. Complete crown and partial coverage tooth preparation designs for bonded cast ceramic restorations. Quint Int 1994;25(8):535-539.
  10. Dietschi D, Spreafico R. Adhesive Metal Free Restorations: Current Concepts for Esthetic Treatment of Posterior Teeth. Carol Stream, IL: Quintessence Publishing; 1997:79-99.
  11. Magne P, Belser UC. Porcelain versus composite inlays/onlays: Effects of mechanical loads on stress distribution adhesion and crown flexure. Int J Periodont Rest Dent 2003;23(6):543-555.
  12. Swift B, Walls A, McCabe JF. Porcelain veneers: The effects of contaminants and cleaning regimens on the bond strength of porcelain to composite. Brit Dent J 1995;179(6):203-208.
  13. Canay S, Hersek N, Ertan A. Effect of different acid treatments on a porcelain surface. J Oral Rehabil 2001;28(1):95-101.
  14. Barghi N, Berry T, Chung K. Effects, timing and heat treatment of silanated porcelain on the bond strength. J Oral Rehabil 2000;27(5):407-412.
  15. Magne P. Immediate dentin sealing: A fundamental procedure for indirect bonded restorations. J Esthet Rest Dent 2005;17(3):144-155.
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