Fabrication Defects

Fabrication defects are created during processing and consist of voids or inclusions generated during sintering. Condensation of a ceramic slurry by hand before sintering may introduce porosity. Sintering under vacuum reduces the porosity in dental ceramics from 5.6 to 0.56 volume percent.⁹ Porosity on the internal side of clinically failed glass-ceramic restorations has been shown to constitute a fracture initiation site.¹⁰ Also, microcracks develop within the ceramic upon cooling in leucite-containing ceramics and are caused by thermal contraction mismatch between the crystals and the glassy matrix.^11–13

Surface Cracks

Surface cracks are induced by machining or grinding. The average natural flaw size varies from 20 to 50 μm.¹⁴ Usually, fracture of the ceramic material takes place from the most severe flaw, which effectively determines the fracture resistance of the restoration. Ceramic engineers analyze failure with a statistical approach, looking at flaw size and spatial distribution.¹⁵

Crystalline Reinforcement

Strengthening by crystalline reinforcement involves the introduction of a high proportion of crystalline phase into the ceramic material to improve the resistance to crack propagation. The crystals can deflect the advancing crack front to increase the fracture resistance of two-phase materials. Microstructural features that typically lead to crack deflection include (1) weakened interfaces between grains in single-phase materials that may be caused by incomplete sintering and (2) residual strains in two-phase materials.¹⁶ The latter constitutes a major issue in dental ceramics.

A crystalline phase with greater thermal expansion coefficient than the matrix produces tangential compressive stress (and radial tension) near the crystal-matrix interface. Such tangential stresses tend to divert the crack around the particle. Leucite particles have a greater thermal expansion coefficient than does the surrounding glassy matrix. Upon cooling, compressive stresses develop at the leucite crystal–matrix interface.¹²

Chemical Strengthening

Chemical strengthening is another method used to increase the strength of glasses and ceramics. Chemical strengthening relies on the exchange of small alkali ions for larger ions below the strain point of the ceramic material. Because stress relaxation is not possible in this temperature range, the exchange leads to the creation of a compressive layer at the surface of the ceramic.¹⁷ Finally, any applied load must first overcome this built-in compression layer before the surface can be placed into tension; this results in an increase in fracture resistance. This technique involves the use of alkali salts with a melting point lower than the glass transition temperature of the ceramic material. Ion exchange strengthening has been reported to increase the flexural strength of feldspathic dental porcelain up to 80%, depending on the ionic species involved and the composition of the porcelain.^18,19 The depth of the ion-exchanged layer could be as high as 50 μm.²⁰ However, this technique is diffusion driven, and its kinetics are limited by time, temperature, and ionic radius of the exchanged ions.

The glass industry also uses thermal tempering (fast cooling) as a strengthening method.²¹

Stress-Induced Transformation

In some ceramic materials such as polycrystalline zirconia, strengthening can be obtained through a stress-induced transformation. Zirconia is monoclinic at room temperature and tetragonal between about 1170° C (2140° F) and 2370° C (4300° F). The transformation between tetragonal and monoclinic zirconia is accompanied by an increase in volume. The tetragonal form can be retained at room temperature by addition of various oxides such as yttrium oxide. Stress can trigger the transformation from tetragonal to monoclinic zirconia, thereby leading to strengthening as a result of an increase in grain volume in the vicinity of the crack tip.²²

Glazing

The addition of a surface glaze can also be used to strengthen ceramics. The principle is the formation of a low-expansion surface layer formed at a high temperature. Upon cooling, the low-expansion glaze places the surface of the ceramic in compression and reduces the depth and width of surface flaws.²³

With contemporary dental ceramics, self-glazing is the standard technique. This consists of an additional firing in air after the original firing, without application of a low-expansion glaze. However, self-glazing does not significantly improve the flexure strength of feldspathic dental porcelain.^24,25

Prevention of Stress Corrosion

The strength of ceramics is reduced in moist environments. This weakening is caused by a chemical reaction between water and the ceramic at the tip of the strength-controlling crack, resulting in an increase in the crack size—a phenomenon called stress corrosion or static fatigue.²⁶ According to Michalske and Freiman,²⁷ the reaction steps involve the following:

Sherrill and O’Brien²⁸ reported a reduction in fracture strength of about 30% when dental porcelains were fractured in water, and other authors have concluded that stress corrosion is important in the performance of dental ceramic restorations.^29,30

Ceramic systems such as Captek^* that are baked on a metal foil may reduce fracture incidence by reducing moisture exposure to the internal surface of the ceramic material, from where the fracture is thought to initiate.¹⁰ In industry, coatings are used to reduce stress corrosion of glass and ceramics, such as optical fibers. Similar coatings have been tried experimentally for their effect on dental ceramics.³¹

Aluminous Core Ceramics

The high-strength ceramic core was first introduced to dentistry by McLean and Hughes³² in 1965. They advocated using aluminous porcelain, which is composed of aluminum oxide (alumina) crystals dispersed in a glassy matrix. Their recommendation was based on the use of alumina-reinforced porcelain in the electrical industry³³ and the fact that alumina has a high fracture toughness and hardness.³⁴

The technique devised by McLean³⁵ involved the use of an opaque inner core containing 50% by weight alumina for high strength. This core was veneered by a combination of esthetic body and enamel porcelains with 15% and 5% crystalline alumina, respectively³⁶ (Fig. 25-3) and matched thermal expansion. The resulting restorations were approximately 40% stronger than those with traditional feldspathic porcelain.²⁶

Fig. 25-3 The strength of an aluminous jacket crown is derived from its high-alumina content core, onto which esthetic body and incisal porcelains are fired. This is analogous to the metal-ceramic crown, whose strength is derived from a metal substructure.

Fabrication procedure

Although declining in popularity with the introduction of innovative all-ceramic products, a technician skilled in the fabrication of the aluminous core porcelain jacket crown produces an exceptionally esthetic restoration. The procedure is outlined in Figures 25-4 to 25-6.

Fig. 25-4 Platinum matrix fabrication. A, A diamond-shaped foil is adapted to the facial surface (a cutting guide is provided with the foil). B, Two cuts are made, one to each incisal corner, and a triangle of foil is removed by cutting at 45 degrees toward the corners. C, The foil is folded onto the lingual surface and burnished. D and E, It is then gathered on the lingual surface with tweezers and adapted with finger pressure. F, The foil is trimmed to follow the lingual contour evenly. The two ends are separated, and one is trimmed to exactly half the width of the other. G and H, The long end is folded over the short end, and relieving cuts are made (see Fig. 25-5). Then the three-thickness joint is folded toward the short end. I, The foil is adapted with a wooden point, always starting from the incisal edge and working toward the margin. J, A beaver-tail burnisher is used to adapt the margin, working the foil toward the internal angle to prevent a perforation. Better adaptation can be achieved by swaging at this stage. The matrix is removed with sticky wax (K) and annealed in a Bunsen flame (L) to relieve work hardening. M, The completed platinum foil matrix.

Fig. 25-5 The tinner’s joint. A, Foil is trimmed so that one end is exactly twice as long as the other. B, The long end is carefully folded over the short end. C, Margin discrepancy at the joint can be reduced by removing a triangular section of foil.

Fig. 25-6 Aluminous core technique. A to C, The platinum matrix is heated to drive off dissolved gases, and the core porcelain is built up. D, A thin blade is used to form a cervical ditch, which will prevent the matrix from becoming distorted during the first firing. E, There must be no porcelain particles on the inner aspect. F, The fired core should be checked with a thickness gauge. Often additional core material is needed to obtain the recommended dimensions. G, The foil is readapted to the margin, and the ditch is filled with additional porcelain. This lingual view shows where the core should be thickest. For esthetic reasons, the core is much thinner on the facial surface. H, The core is seated on the definitive cast before the application of body and incisal porcelains.

Slip-Cast Ceramics

High-strength core frameworks for all-ceramic restorations can be produced with a slip-casting procedure³⁷ such as the In-Ceram (see Fig. 25-9A).^*

Fig. 25-9 Fabrication of a slip-cast alumina restoration. A, The In-Ceram system relies on a high-strength core veneered with an esthetic feldspathic porcelain. B, The definitive cast is duplicated with a special elastomer. C, The special plaster die. D, The alumina slip is ultrasonically mixed and applied to the plaster die (E). F, The VITA Inceramat special porcelain furnace. G, A special colored infiltration glass is painted on the porous sintered alumina and fired. H, Excess glass is carefully removed by grinding and air abrasion. I, Finished substructures on the definitive cast. J, Body and incisal porcelains are applied and fired in the conventional manner.

(A, Courtesy of Morehead Dental Laboratories, Morehead, Kentucky. B to J, Courtesy of Vident, Brea, California.)

Slip-casting is a traditional technique in the ceramic industry and is used to make sanitary ware. The starting medium in slip-casting is a slip that is an aqueous suspension of fine ceramic particles in water with dispersing agents. The slip is applied onto a porous refractory die, which absorbs the water from the slip and leads to the condensation of the slip on the die. The piece is then fired at a high temperature (1150° C [2100°F]). The refractory die shrinks more than the condensed slip, which allows easy separation after firing. The fired porous core is later glass infiltrated, a unique process in which molten glass is drawn into the pores by capillary action at a high temperature.³⁸ Materials processed by slip-casting tend to exhibit lower porosity and fewer processing defects than do traditionally sintered ceramic materials. The strength of In-Ceram is about three to four times greater than that of earlier alumina core materials,^39,40 a finding that has prompted its use in high-stress situations such as fixed dental prostheses (FDPs) (Fig. 25-7). Two modified porcelain compositions for the In-Ceram technique have been introduced: In-Ceram Spinell^* contains a magnesium spinel (MgAl₂O₄) as the major crystalline phase, which improves the translucency of the final restoration (Fig. 25-8). In-Ceram Zirconia contains zirconium oxide (ZrO₂) and is said to provide the highest strength.^41,42 Marginal fit of In-Ceram has been reported as very good⁴³ or good⁴⁴ but also poor,⁴⁵ which emphasizes the technique sensitivity of the process and the need to select a skilled dental laboratory.

Fig. 25-7 A to C, All-ceramic fixed dental prosthesis (FDP) replacing the maxillary left central incisor, through the use of the In-Ceram system. D to F, All-ceramic FDP replacing the mandibular left first premolar, through the use of the Empress 2 system.

(D to F, Courtesy of Ivoclar Vivadent, Inc., Amherst, New York.)

Fig. 25-8 A, Defective maxillary metal-ceramic crowns. Esthetic problems included high value and opacity. B, Crowns removed. The preparations are not discolored and thus allow a translucent all-ceramic crown system. C, Maxillary all-ceramic crowns with a translucent slip-cast spinel core material.

(Courtesy of Dr. R. B. Miller.)

Fabrication procedure

1. Duplicate the working die with an elastomeric impression material (Fig. 25-9B) and pour it with the special refractory die material. Any undercuts must be blocked out first, and two coats of die-spacer must be applied. When the die material has fully set (2 hours), remove the die, mark the margins, and apply the wetting agent (see Fig. 25-9C).

2. Mix the appropriate shade of alumina slip with ultrasonic agitation (see Fig. 25-9D), place the mixture under a vacuum, brush apply it to the plaster die (see Fig. 25-9E), and shape it with a blade, trimming back to the margins carefully.

3. The slip is fired in a special furnace (see Fig. 25-9F), initially through a prolonged drying cycle to 120° C (248° F) that dries the die material, which shrinks away from the core. Then the alumina is fired at 1120° C (2048° F). The resulting core is porous and weak at this stage but can be carefully transferred to the master die after the die spacer is removed. The relatively low sintering shrinkage (about 0.3%) is compensated for by an expansion of the refractory material.

4. Paint a thick coat of the appropriate shade of glass mixture onto the surface of the core (see Fig. 25-9G) and fire at 1100° C (2012° F). As the glass melts, it is drawn into the interstices of the alumina by capillary action, producing a dense composite structure with excellent strength properties.

5. Remove excess glass from the core by grinding (see Fig. 25-9H) and airborne particle abrasion. Body and incisal porcelain is applied to the core in a manner similar to that for metal-ceramic crowns, as shown in Figure 25-10. Powder distribution is governed by a detailed prescription of the patient’s shade (see Chapter 23). With experience, the technician will be able to mix different powders to match almost any shade. If necessary, test firings can be used to help select the correct blend in difficult situations.

6. After moistening the core, mix the powder with modeling liquid and apply in increments with a brush (see Fig. 25-10A to C).

7. Remove moisture with a paper tissue held against the lingual surface. The capillary action condenses the porcelain particles. Slight vibration brings further moisture to the surface before the next increment is added. To prevent voids from forming between increments, always add to a moist surface.

8. When the crown has the correct shape, cut it back to allow room for incisal porcelain (see Fig. 25-10D).

9. Apply incisal porcelain, overbuilding the incisal edge by 1 or 1.5 mm to allow for firing shrinkage (see Fig. 25-10E).

10. Lightly condense the buildup with a large whipping brush. Absorb excess moisture with a tissue.

11. Remove the crown from the definitive cast and add material interproximally to allow for shrinkage (see Fig. 25-10F).

12. Dry the crown and fire it (see Fig. 25-10G and H).

Fig. 25-10 Body and incisal porcelain application. A, Moistening the high-strength core. B and C, Applying gingival and body porcelains. D, Cut-back for the incisal porcelain. E, The incisal porcelain. F, Adding to the interproximal areas. G, The completed buildup. H, The fired crown is seated on the definitive cast.

Heat-Pressed Ceramics

Leucite based

Heat-pressed ceramics have been popular in restorative dentistry since the early 1990s. The restorations are waxed, invested, and pressed in a manner somewhat similar to that for gold casting. Marginal adaptation seems to be better with heat pressing than with the high-strength alumina core materials,⁴⁵ although the results from individual dental laboratories may not support the research findings. Most heat-pressed materials contain leucite as a major reinforcing crystalline phase, dispersed in a glassy matrix. The crystal size varies from 3 to 10 μm, and the leucite content varies from about 35% to about 50% by volume, depending on the material. Research has shown that residual tangential stresses remain around the leucite crystals after cooling.¹² Ceramic ingots are pressed at a high temperature (about 1165° C [2130° F]) into a refractory mold made by the lost-wax technique. The ceramic ingots are available in different shades. Two finishing techniques can be used: a characterization technique (surface stain only) and a layering technique, involving the application of a veneering porcelain (Fig. 25-11G and H). The two techniques lead to comparable mean flexural strength values for the resulting porcelain composite.⁴⁶ The thermal expansion coefficient of the core material for the veneering technique is usually lower than that of the material for the staining technique to be compatible with the thermal expansion coefficient of the veneering porcelain. Among the currently available leucite-containing materials for heat-pressing are IPS Empress,^* Optimal Pressable Ceramic,^† and two lower fusing materials, Cerpress^‡ and Finesse.^§

Fig. 25-11 Heat-pressed ceramic technique. A, Ceramic inlay restoration for a maxillary molar. A wax pattern is made in a manner similar to that for conventional gold castings. B, After the pattern is invested, it is burned out, and a ceramic ingot and alumina plunger are placed in the heated mold. C and D, The pressing is done under vacuum at 1165° C. E, Sprue removal. F, The pressed restoration seated on the die. G and H, For esthetic anterior restorations, only the dentin-colored ceramic is pressed. The incisal porcelain is brush applied in the conventional manner. I, Three-unit fixed dental prosthesis and veneer waxed to anatomic contour. J, The technician ensures that the connector size is adequate (4 × 4 mm). K, A silicone putty matrix is used to aid in cut-back of the wax pattern. L, Framework is sprued and invested (M), and the lithium-silicate ceramic is pressed into the mold. N, The pressed restoration. O, Framework on definitive cast. P, Applying veneering porcelain. Q, Completed restorations.

(Courtesy of Ivoclar Vivadent, Inc., Amherst, New York.)