TEMPORARY

CEMENT

EFFE:CT

ON COMPOSITE

CORES

genol-containing temporary cement. When resin cores are coupled with a resin cement for final cementation, there is a potential for unprecedented retention values. The interim use of a eugenol-containing temporary cement is definitely contraindicated, and probably causes great reduction in retention. This observation is peculiar to resin cement only, in that there was no effect on the final outcome with zinc phosphate cement. With the availability of more final cements and several new resin cements, additional research is necessary to establish material compatibility. CONCLUSIONS

REFERENCES 1. Phillips 2.

3. 4. 5.

In vitro comparison of the retention of base metal cylinders placed on composite resin cores showed that resin cements produced retention values that were two to three times greater than retention produced with zinc phosphate cement. Retention was greatly reduced when sam-

A review

ples were pretreated with eugenol-containing temporary cement.

of the strength

properties

RW. Skinner’s science of dental materials. 8th ed. Philadelphia: WB Saunders, 1982498. Grajower R, Hirschfeld Z, Zalkind M. Compatibility of a composite resin with pulp insulating materials. A scanning electron microscope study. J PROSTHET DENT 1974;32:70-7. Grajower R, Hirschfeld Z, Zalkind M. Observations on cavity liners for composite resin restorations. J PROSTHET DENT 1976;36:265-73. Millstein PL, and Nathanson D. Effect of eugenol and eugenol cements on cured composite resin. J PROSTHET DENT 1983;50:211-15. Millstein PL, Nathanson D, Hazan E, Pierce A. Effect of aging on temporary cement retention in vitro. J PROSTHET DENT 1991;65:768-71.

Reprint

requests

to:

DR. PHIUP L. MILLSTEIN 609 ALBANY ST., RM J601 BOSTON, MA 02118

of dental

ceramics

Steven 0. Hondrum, DDS, MS’ The U.S. Army Institute of Dental Research,Washington, D.C. New ceramic materials for restorative dentistry have been developed and introduced in recent years. This article reviews advantages and disadvantages of dental ceramics, concentrating on strength properties. Included are factors affecting the strength of dental ceramic materials and the most common mechanisms for increasing the strength of dental ceramics. The properties of presently available materials such as dispersion-strengthened ceramics, cast ceramics, and foilreinforced materials are discussed. Current research efforts to improve the fracture resistance of ceramic restorative materials are reviewed. A description of methods to evaluate the strength of ceramics is included, as is a caution concerning the interpretation of strength data reported in the literature. (J PROSTHET DENT 1992;67:869-66.)

N

ew ceramic materials for restorative dentistry have been developed and introduced in recent years. Besides esthetics, the most important property of these new materials is strength. The purpose of this paper is to review the advantages and disadvantages of dental ceramics, concentrating on strength properties. Included are mechanisms for increasing the strength of dental ceramics, the strength of products presently available to the dentist,

The views expressedherein are those of the author and do not necessarilyreflect the views of the United States Army or the Department of Defense. %hief, Dental Materials. 10/l/36072

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and present and future research efforts to improve the strength of restorative ceramic materials. BACKGROUND Dental ceramics are used as restorative materials to provide esthetic realism. Dental ceramics allow regular and diffuse t.ransmission, as well as diffuse and specular reflectance of light, and therefore have the potential to reproduce the depth of translucency, depth of color, and texture of natural teeth.‘, 2 In addition, dental ceramics are resistant to degradation in the oral cavity, are biologically compatible, and have a coefficient of thermal expansion that is similar to that of tooth structure. The major disadvantage of dental ceramics is their susceptibility to fracture during placement, mastication, and 859

HONDRUM

trauma. Clinicians and scientists have questioned the use of all-ceramic crowns because of their lack of strength. Their counterparts, the metal-ceramic crowns, have been used successfully; the majority of all full-coverage restorations and fixed prostheses are fabricated from metalceramic systems3 that have a failure rate of only 1% to 3 % over 5 years.4s5 Metal-ceramic systems (MCS) have come under scrutiny, however, because of (1) potential alloy corrosion leading to toxicity and allergy concerns; (2) esthetic problems such as lack of translucency, discoloration of some ceramics from silver in the alloy, and excessive value in the cervical third; (3) the amount of tooth reduction necessary, and the associated tenden.cy to overcontour the restoration; and (4) incompatibility between metal and ceramic, and the difficulty in establishing standard tests for bond strength and thermal compatibility. Research has emphasized the development of stronger all-ceramic restorations, largely because of increasing esthetic demands. The most significant dilemma in dental ceramic research has been to increase strength and/or toughness without sacrificing esthetics. FACTORS AFFECTING OF DENTAL CERAMICS

THE

STRENGTH

Dental ceramics are inherently fragile in tension. While the theoretical tensile strength of porcelain is dependent upon the silicon-oxygen bond, the practical strength is 10 to 1000 times less than thie nominal strength.6 Susceptibility to fracture is the result of several factors. A volume and surface distribution of stress-raising microcracks is present in manufactured ceramics. Microcracks are caused by the condensation, melting, and sintering process; by the high contact angle of ceramics on metal; by differences in ,the coefficient of thermal expansion between alloy or core and veneers; by grinding and abrasion; and by tensile stresses during manufacture, function, and trauma. Strength is most dependent upon the number and severity of these flaws.7 The flaws of greatest importance are located in surface areas, and are in the range of 100 pm in diameter.s g The largely covalent 01: ionic bonded structure of ceramics confers resistance to chemical degradation in the oral environment; however, it also imparts brittleness. Ductile materials, such as metallic alloys, can dissipate stress by slip and plastic deformation. Brittle materials, such as dental ceramics, have a limited capacity for distributing localized stress at nominal temperatures.lO The critical strain of dental ceramics is low; the material can withstand a deformation of approximately 0.1% before fracture.‘l Repetitive loading, resulting in fluctuating stresses and strains, may be the most common mechanism of failure of dental ceramics. l2 Failure (fracture) of a ceramic crown intraorally generally occurs by a combination of bending and torsional forces, such as those produced by incisal leverage. These forces involve tensile stresses occurring as a result of comparatively light, but repeated, occlusal loading on the

inner surfaces of the crown,g particularly at the cervical third of anterior crowns.13,l4 These low-energy flexural forces place surface flaws under tension.15 Slow crack growth of subcritical flaws occurs as local residual stresses are relieved by the growth of existing cracks until critical dimensions are reached at the time of failure.16 The longer stresses are applied, the greater the chance of failure over time. The environment of the oral cavity aggravates the low tensile strength of dental ceramics.17 The silicon-oxygen bond is weaker in the presence of moisture. This is called static fatigue or delayed failure. While moisture can abet failure in many ways, failure primarily results from a stress-dependent chemical reaction between water vapor and microcracks on the surface of the ceramic. Absorbed moisture lowers the energy required for propagation at the crack tip. ‘s There is a 20 % to 30 % reduction in strength in a moist environment.lg STRENGTHENING

MECHANISMS

Strengthening or toughening mechanisms are designed to resist the initiation and propagation of microcracks. In dentistry, the most popular methods of accomplishing this are by the use of metallic substructures and by crystalline dispersions within the glass matrix. Enameling

of metals

Ceramic fracture is minimized if a strong bond is effected between a ceramic veneer and a cast metal substructure. The metal at the internal surface distributes stresses and provides rigid support, therefore inhibiting propagation of cracks from small faults at the interface.20 A small coefficient of thermal expansion mismatch (metal higher than ceramic) may assist the bond and resist the initiation and propagation of microcracks by creating tangential compressive stresses in the ceramic at the interface.21 Dispersion-strengthened

ceramics

Since the glass is the weak component, dispersion of a crystalline phase helps to manage crack growth. During firing, the glass melts and flows around the crystals, forming an ionic bond between the matrix and the crystals. Fracture lines will then pass through both phases; the high rigidity of the crystals results in the crystalline phase bearing a higher portion of the load. This results in a restricted flaw size and an increase in the toughness of the system, a function of the volume fraction of the dispersed phase.22T24 If the coefficient of thermal expansion of the dispersed phase is slightly higher than that of the matrix, the glass will be placed in compression upon cooling, thereby increasing the potential strength.25 DISPERSION MATERIALS

STRENGTHENED

While dispersion-strengthened cores and crowns have not been popular in the United States, new materials and technique innovations may soon increase their use. .lllNE

,992

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Alumina

CERAMICS

core

1. The alumina-reinforced ceramic core crown, using a platinum foil technique, has been the traditional allceramic crown for the last 25 years, and is the original dental application of dispersion strengthening. The core is a composite of low to medium fusing ceramics and alumina in amounts of 40% to !jO% by volume. The alumina particles in the glass resist the deepening of microcracks by increasing the elastic modulus and therefore the toughness of the system by approximately 50% .26v27 The flexure strength is approximately 125 MPa.2s 2. Hi-Ceram (Vita Zahnfabrik, Bad Sackingen, Germany) is an example of a dispersion strengthened core crown utilizing a refractory die. It is similar in chemistry to the traditional alumina core but has higher alumina content. The crystalline phase has been increased by means of a specific size distribution that allows an increase in the volume percent without markedly worsening handling characteristics and opa’city. Flexural strength is approximately 155 MPa.2g, 30* 3. In-Ceram (Vita Zahnfabrik) is the most recent advancement in this line ofresearch. The alumina content has been increased to more than 90%) with a particle size distribution averaging 3.8 /Am.Fabrication requires a severalhour sintering procedure on a refractory die in a special high temperature oven, followed by infusion of a low-fusing glass (slipcasting). Flexural strength is three times higher than that of most. current materials.31 This material may be strong enough for routine use in posterior teeth and for three-unit bridges.

Magnesia

core

The magnesia ceramic: core is strengthened because 40% to 60% of its weight consists of a fine dispersion of crystalline magnesia in a glass matrix. Flexure strength is similar to that of the alumina core material but may be doubled by the application of a glaze.32 The glaze probably works by filling surface porosities, reacting with the core material to further crystallization, and placing the ceramic surface in compression (the coefficient of expansion of the glaze is lower than that of the core).

Injection

molded

core

An injection molded core material, formerly marketed as Cerestore, has been reintroduced as Alceram (Innotek Dental Corp., Lakewood, Colo.). This technique uses a shrink-free, crystallized magnesium aluminum oxide spine1 as a dispersion-strengthened core. The flexure strength of the original material is similar to that of the traditional alumina core; the strength of the new material is about 70 % to 90% higher.30t

not a core material. The technique is similar to the porcelain jacket crowns popular before 1970, but utilizes a refractory die instead of a platinum foil. Strength probably arises from nucleation and growth of a fine dispersion of a high volume fraction of leucite crystals. Despite the increase in crystallization, the material retains its translucency, apparently because of the closeness of the refractive index of leucite with that of the glass matrix. The flexure strength is approximately 140 MPa.2g* The manufacturer is promoting this material for use in three-unit bridges.

CAST Dicer

CERAMICS

Dicer (Dentsply, York, Pa.) is a fluorine-containing tetrasilicic mica that is supplied as ingots in the vitreous state, cast in full form by the lost wax process, and converted to a partially crystalline state by a controlled heat treatment. Nucleation, followed by growth of interlocking mica crystals, occurs within the body of the casting. The intersection of crystals causes crack deflection, branching, and blunting, which account for its strength. Strength is also dependent upon crystal diameter and crystal-glass expansion mismatches.33 The flexure strength of Dicer is approximately 135 MPa.2g, 30,34 The poor strength of Dicer reported in some laboratory tests is thought to be caused by porosity, especially in the “ceram layer.“35 Some clinicians claim that this material is strong enough for routine use in posterior teeth. A recent innovation is laminated feldspathic ceramic over a Dicer substructure.36 The purported advantage is the ability to stain internally. The fracture strength of these crowns is similar to that of the traditional Dicer crowns.37t

CerePearl CerePearl (Kyocera, San Diego, Calif.) is a castable apatite ceramic that, when reheated, becomes crystalline oxyapatite and, when exposed to moisture, becomes crystalline hydroxylapatite, similar to tooth enamel but with an irregular arrangement. Strength is dependent upon these crystals and the bond between the crystals and the noncrystalline inorganic matrix. 38Flexure strength is similar to that of Dicor.3g

Castable

calcium

phosphate

glass40

Castable calcium phosphate glass material is a combination of calcium phosphate and phosphorous pentoxide plus trace elements. The material is somewhat weaker than other castable ceramics and appears to be too opaque for use in anterior teeth.30

Optec-HSP

FOIL-REINFORCED

While Optec (Jeneric/Pentron Inc., Wallingford, Corm.) is an additional example of dispersion strengthening, it is

In a literal sense, foil crowns are a modification of the MCS. They are an attempt to avoid the lost wax process

*Vita Zahnfabrik. tInnotek Dental

*Jeneric/Pentron iSather DA.

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Personal communication, 1989. Corp. Personal communication, 1990. OF PROSTHETIC

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Inc. Personal

CERAMIC

Personal communication, communication, 1989.

CROWNS

1989.

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and still get an MCS crown. The advantages are a reduction in the thickness of the metal (less tooth reduction is required), less cost, and less dependence on the properties of the alloy during firing of the ceramic.

Ion exchange is an established method of improving the strength and hardness of glass-like materials by creating compressive stresses within the body of the ceramic, thereby reducing the effect of tensile stresses on microcracks. The process involves the substitution of larger alkali

ions (potassium in the medium) for smaller ions (sodium in the ceramic) at the surface. 53 There exists a high temperature, several-hour immersion method54 that has not gained popularity, and a lower temperature method (Ceramicoat/Tufcoat, GC International, Tokyo, Japan) in which the medium is applied and the crown is placed in an oven at 450° C for 30 minutes.55T56 The manufacturer claims an increase of 40% to 98% in tensile strength, with no significant increase in opacity.* Thermal tempering of ceramics involves rapid cooling of a ceramic by forced convective cooling in air or in a liquid medium. This process results in a prestress on the glass surface; residual compressive stresses are induced, inhibiting the initiation and the growth of cracks. Research indicates a threefold increase in the flexural strength of feldspathic porcelain.56-58 Zirconium dioxide dispersion-strengthened cores and crowns and zirconia-based enamels for metal substrates may be developed to take advantage of the high fracture strength of zirconia. Transformation toughening is possible with zirconia. This involves phase transformations that create compressive stressesacross the plane of propagating cracks (crack shielding), thereby reducing the tensile stresses acting at the crack tip.s* 5g,6o Fiber or whisker reinforcement of dental ceramics may be possible. Ceramic whiskers of alumina may bridge the flaws, resulting in whisker-toughened intergranular bridges.8 Dispersion strengthening with leucite takes advantage of controlled crystallization altering the volume fraction, particle size, and crystal size distribution. Transformation toughening may also be possible with leucite.61 New castable ceramics may have superior strength properties and be colored internally through temperature control during crystallization and by the addition of metals and metal oxides.15,33,57 Adhesive luting agents are increasingly being used to improve the bond between ceramic restorations and dentin/enamel. Dental composite resin cements are used in conjunction with an acid etch/silane treatment on the intaglio of the crown or laminate, and a dentin bonding/ cleansing agent on dentin and an acid etch on enamel. The goal is to transfer functional stresses to the supporting structure of the tooth, thereby strengthening the system without necessarily improving the strength of the materials themselves.62 There are new test and evaluation methods available to the research scientist. Determination of fracture toughness is a means of measuring strain-absorbing ability, the level of tensile stress that must be obtained in the vicinity of the crack tip before catastrophic fracture. The advantage of this method is that specimen preparation and loading are not as critical as with routine strength tests.63-65Frac-

*Schaffer nication,

*GC

Twin-foil

and associated

techniques13

The twin-foil technique involved firing core porcelain onto a tin-plated and oxidized foil that remained as a permanent part of the crown. Microcrack propagation was inhibited by the bond between the core and the foil. These techniques were not popular in the United States, possibly because of the laboratory effort required, the popularity of the MCS, and contradictory research reports regarding the fracture strength of these crowns.41p42Studies have shown that simply leaving a foil matrix in the crown or test specimen increases strength to a degree.43-47This finding has given rise to the present foil systems.

New

foil systems

New foil systems include RenaissanceKeplatec (Williams Gold Refining Co., Inc., Buffalo, N.Y.), Sunrise (Tanaka Dental, Skokie, Ill.), Flexobond (Elephant Edelmetaal, Hoorn, The Netherlands), and Plati-deck (Schone Edelmetaal, Degussa, Amsterdam, The Netherlands). All these techniques are modifications of the original foil crowns or simply a porcelain jacket crown with the foil remaining in the crown. Layered noble metal foils are adapted, swagged, and brazed on a die; feldspathic porcelains are then veneered over the foils. Strength values are controversial; various test methods reveal a strength of about 30 % to 80% of the MCS.4s, 4g* Some clinicians propose use of these crowns for posterior teeth.

CLINICAL

STUDIES

Because of the cost in time, money, and labor, only a few long-term clinical studies of the failure rate of all-ceramic crowns have been accomplished. While all-ceramic crowns are generally successful in anterior teeth, research seemsto show that caution is indicated when placing all-ceramic restorations in posterior regions of the oral cavity. Twinfoil crowns experienced a 15% failure rate in molars over a period of 7 years.50 Cerestore crowns exhibited a failure rate of 18.5% in posteri0.r teeth over 4 years.51 The Dicer crown failure rate for molars has been reported to be as high as 35% over 3 years.52

EFFORTS TO IMPROVE OF DENTAL CERA.MICS

SS, Williams 1985.

Gold

Refining

THE

Co., Inc.

STRENGTH

Personal

commuInternational.

Personal

communication,

JUNE

1992

1989.

VOLUME

67

NUMBER

6

STRENGTH

OF DENTAL

CERAMICS

tography involves an analysis of the location, direction, and force of fracture of hrittle materials. Analysis of fractured ceramic crowns reveals that the origin of fracture is generally on the surface of the crown, the fracture energy is low, and that fracture is most commonly flexuraLg, l5 Fractography may also reveal the role of microstructural components and the chemical composition of fracture sites.@

MEASUREMENT QF THE DENTAL CERAMICS

STRENGTH

OF

There is considerab1.e controversy surrounding the strengths of various ceramic materials, and contradictory research will undoubtedly continue to appear in the literature. Reliable laboratory data concerning the strengths of brittle materials are difficult to obtain and coefficients of variation are high. Values are affected by a variety of factors such as geometry, temperature, loading rates, technique variations, and fabricational and thermally-induced imperfections. When testing crowns mounted on laboratory materials, inherent material strength may not be measured as much as the characteristics of the load-the base for loading, the load-bearing applicator, and the rate of load application.15s 67 All these factors partially account for variations in reported flexure strengths (Table I). Flexure strength is used as a measure of crack propagation from surface microcracks, traditionall:y on the undersurface of a rectangular bar.2gs67-70More recently, biaxial flexure tests using disk specimens have become popular to avoid the effects of edge flaws common with rectangular bars. Biaxial tests are also relatively insensitive to specimen geometry and are independent of flaw direction.71-73 While these and similar laboratory tests may be appropriate for comparison of the inherent ultimate tensile strength of dental ceramics, results do not necessarily extrapolate to more complex test specimens74 nor to the situation of the oral cavity. The laboratory cannot accommodate intraoral variables such as the periodontal ligament, the physical properties of the cement, the fit, and occlusion. When crowns are cemented intraorally, factors other than and likely more important than the inherent mechanical strength of the materials come into play. For example, a poorly fitting crown may be weaker in a practical sense than a tightly fitting crown, regardless of the materials used; a linear relationship exists between fit and breaking strength.75 Rank order strength may be affected as well as magnitude. 15,47In addition, the strength of core materials, because they are used in such thin cross-sections, may contribute only moderately to the strength of the final restoration, and then only when maximum tensile stressesoccur at the internal surface of the restoration.76 All these factors emphasize the need for a wide margin of safety when placing brittle materials in stressful environments. The strength of dental ceramics may be of less consequence than clinical factors such as case selection,

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Table I. Approximate flexure strengths of ceramic crown materials*t strength

Flexure

(MW

Feldspathic porcelain Dispersion-strengthenedmaterials Alumina core Hi-Ceram core In-Ceram core Magnesia core (glazed) Cerestorecore Alceram core Optec-HSP Castable ceramics Dicer CerePearl CCPG Foil-reinforced (twin-foil) Metal-ceramic (estimated by McLean7s)

-

45-90 70-150 140-180 420-520 270 go-130 160-240 105-170 115-150 150 115 140-170 450-550

-

*See references 19, 28 through 31, 34, 39,40,46, and 77 through 79. tVita Zahnfabrik, Bad Sackingen, Germany; Inn&k Dental Corp., Lakewood, Cola.; and Jeneric/Pentron Inc., Wallingford, Conn. Personal communications.

tooth preparation, supporting structure, and the skill of the dentist and the technician. Success remains dependent upon the skill of the dentist and his or her knowledge of the basic behavior and indications of restorative dental materials. REFERENCES 1. Clark FJJ. Measurement of color of human teeth. In: McLean JW, ed. Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:441-89. 2. O’Brien WJ, Johnston, WM, Fanian F. Double layer effects in porcelain systems. J Dent Res 1985;64:940-3. 3. Christensen GJ. The use of porcelain-fused-to-metal restorations in current dental practice. J PROSTHET DENT 1986;56:1-3. 4. Morris HF. Clinical performance of metal-ceramic restorations: a fiveyear progress report. In: Anusavice KJ, ed. Quality evaluation of dental restorations: Criteria for placement and replacement. Chicago: Quintessence Publishing Co, 1989:325-41. 5. Strub JR, Stiffler S, Scharer P. Causes of failure following oral rehabilitation: biologicalversus technical factors. QuintessenceInt 1988;19:21522. 6. Jones DW.

7.

8. 9. 10. 11.

The strength and strengthening mechanisms of dental ceramics. In: McLean JW, ed. Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:83-141. Jones DW. Ceramics and glass as restorative materials. In: Smith DC, Williams DF, eds. Biocompatibility of dental materials. vol IV. Biocompatibility of prosthodontic materials. Boca Raton, Fla: CRC Press, 198279-122. Marshall DB, Ritter JE. Reliability of advanced structural ceramics and ceramic matrix composites-a review. Ceram Bull 1987;66:309-17. Kelly JR, Giordano R, Pober R, Cima, MJ. Fracture surface analysis of dental ceramics. Int J Prosthodont 1990;3:430-40. Flinn RA, Trojan PK. Engineering materials and their applications. 2nd ed. Boston: Houghton Mifflin, 1981:313. Jones DW. The strength and strengthening mechanisms of dental ceramics. In: McLean JW, ed. Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:83-141.

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12. Leckie F, quoted in: Anuaavice KJ, deRijk WG. Performance of dental biomateriala: conference report. Dent Mater 1990$X9-72. 13. McLean JW, Seed IR. The bonded alumina crown. 1. The bonding of platinum to aluminous dental porcelain using tin oxide coatings. Aust Dent J 1976;21:119-27. 14. Farah JW, Craig RC. Distribution of stresses in porcelain-fused-tometal and porcelain jacket crowns. J Dent Res 1975;54:255-61. 15. Hoekstra KE. Dicer research facts. In: Pamiejer CH, ed. Proceedings of the International Symposium on alternatives to the use of traditional porcelain. Framingham, Mass. Restorative techniques. 1986:49-76. 16. Wiederhorn SM. Subcritical crack growth in ceramics. In: Bradt DC, Hasselman DPH, Lange FF, eds. Fracture mechanics of ceramics. ~012. New York: Plenum Press, 19743613-46. 17. Morena R, Beaudreau GM, Lockwook PE, Evans AL, Fairhurst CW. Fatigue of dental ceramics in a simulated oral environment. J Dent Res 1986;65:993-7. 18. Wiederhorn SM. Moisture assisted crack growth in ceramics. Int J Fracture Mecban 1968;4:171-7. 19. Sherrill CA, O’Brien WJ. Transverse strength of aluminous and feldspathic porcelain. J Dent Res 1974;53:683-90. 20. Yamamoto M. Metal-ceramics. Principles and methods of Makoto Yamamoto. Chicago: Quintessence Publishing Co, 198517-22. 21. Mackert JR. Effects of thenmally induced changes on porcelain-metal compatibility. In: Preston JD, ed. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago: Quintessence Publishing Co’, 1988:53-64. 22. Hasselman DPH, Fulath RM. Proposed fracture theory of a dispersionstrengthened glass matix. J Am Ceram Sot 1966;49:68-72. 23. Binns DB. Some physical properties of two-phase crystal glass solids. In: Stewart GH, ed. Science of ceramics. ~011. London: Academic Press, 1962:315-34. 24. Dinsdale A, Camm J, Wilkerson WT. Mechanical strength of ceramic tableware. Trans Br Ceram Sot 1967;66:367-404. 25. Jones DW. Ceramics and glass as restorative materials. In: Smith DC, Williams DF, eds. Biocompatibility of dental materials. vol IV. Biocompatibility of prostbodontic materials. Boca Raton, Fla: CRC Press, 1982:79-122. 26. McLean JW, Hughes TH. The reinforcement of dental porcelain with ceramic oxides. Br Dent J 1965;119251-67. 27. Jones DW. The strength and strengthening mechanisms of dental ceramics. Im McLean JW, ed. Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:83-141. 28. McLean JW, Kedge, MI. High strength ceramics. Quintessence Int 1987;18:97-106. 29. Se&i RR, Dalwr T, Caputo A. ReIative flexurd strengths of dental restorative &.a~.‘Deut‘Er 1990;6:181-4.. 30. Woblwend A, Stmb JR, Sohaer P. Metal ceramic and all-porcelain restorations: clinical considerations. Int J Prosthodont 1989;2:13-26. 31. Segbi RR, Sorenson JA, Engleman MJ, Roumanas TJ, Tones TJ. Flexural strength of new ceramic materials. J Dent Res 1990;69:299. 32. O’Brien WJ. Magnesia ceramic jacket crowns. Dent Clin North Am 1985;29:719-24. 33. Grossman DG. The science of castable glass ceramics. In: Preston JD, ed. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1988117-33. 34. American Dental Association. Council on Dental Materials, Instru ments and Equipment. O’Brien WJ. Recent advancements in materials and processes for ceramic crowns. Am Dent Assoc 1985;110:548-9. 35. Campbell SD, Kelly JR. The influence of surface preparation on the strength and surface microstructure of a cast dental ceramic. Int J Prosthodont 1989;2:459-66. 36. Geller W, Kwiatkowski SJ. The Willi’s glass crown: a new solution in the dark and shadowed zones of esthetic porcelain restorations. Quintessence Dent Tech 1987;11:!133-42. 37. Bales DJ, Duffin JL, Johnson GH. Evaluation of the fracture resistance of two ceramic crown techniques [Abstract]. J Dent Res 1990;69:176. 38. Hobo S. Castable hydroxyapatite ceramic restorations. In: Preston JD, ed. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1988:135-52. 39. Hobo S, Iwata T. Castable apatite ceramics as a new biocompatible restorative material, I. Theoretical considerations. Quintessence Int 1985; 2:135-41.

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1992

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67

NUMBER

6

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tion Number69for DentalCeramic.AccreditedStandardsCommittee MD 156.Chicago:AmericanDentalAssociation. 71. Ban S, Anusavice KJ. Influence of test method on failure stress of brittle materials. J Dent Res 1990;69:1791-9. 72. Grossman DG. Biaxial Aexure strength of CAD/CAM materials. J Dent Res 1991;70:433. 73. Fairhurst CW, Lockwood P, Ringle R, Thompson WO. The effect of glaze on porcelain strength [Abstract]. J Dent Res 1991;70:433. 74. Hondrum SO, O’Brien WJ. The strength of alumina and magnesia core crowns. Int J Prosthodont 1988;1:67-72. 75. Brukl CE, Philp GK. The fit of Cerestore, Twin foil and conventional ceramic crowns [Abstract]. J Dent Res 1985;64:362. 76. Southan DE. Laminate strength of dental porcelain. Quintessance Int 1987;18:357-9. 77. McLean JW. Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 198225.

Long-term Pamela

outcomes J. Guenzel,

PhD,a

78. McLeanJW.Dentalceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:2&X 79. Starling LB. Transfer molding all ceramic crowns: the Cerestore system. In: O’Brien WJ, Craig RG, eds. Ceramic Engineering and Science Proceedings. Proceedings of the Conference on Recent Developments in Dental Ceramics, October lo-12,1983. Columbus, Ohio: The American Ceramic Society, 1985:41-56. Reprint

Knight,

to:

USAIDR,c/o KACH SGRD-UDR-DMICOL HONDRUM FT. GEORGE G. MEADE, MD

for remedial

and G. William

requests

COMMANDER

20755-5305

students DDS,

MSb

The University of Michigan, Schoolof Dentistry, Ann Arbor, Mich. A recent survey of dental schools concluded that current efforts toward remediation are inadequate. A remedial waxing course providing recognition training before production attempts, emphasis on formative selfand peer-evaluation of projects, and application of a highly structured format for ensuring relevant practice had been developed and favorably evaluated previously. The current report follows the progress of two differently trained remedial groups and the remainder of the class in two courses following remediation. On the five subsequent practical examinations analyzed, the experimental group continued to perform at the class mean. On one practical examination, the experimental group significantly outperformed the traditional group (p < 0.02). For three of the five examinations, the traditional group was significantly outperformed by the class. One of the six students in the experimental group required additional remediation. Of the seven in the traditionally remediated group, one left school and four required additional remediation. An apparent changing remediation pattern in the preclinical training period is described and possible reasons for the change are explored. (J PROSTHET DENT 1992;67:865-9.)

P

sychomotor skill acquisition is central to success in dental school. When a student is unable to demonstrate adequate technique in a regular course, remediation becomes a necessary, frustrating, and expensive proposition for both student and faculty. At The University of Michigan 13% of first semester dental students have been required to participate in a remedial program focusing on waxing skills after a dental anatomy course given during the first 8 weeks of school. These same students are more likely to continue to require remediation than the rest of the class. For example, in the 2 years preceding an innovative remedial program recently aLecturer bAssistant tistry.

in Dentistry, Professor,

Office of Educational Department of Cariology

10/l/36753 THE

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OF PROSTHETIC

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described,l of the 26 students required to participate in the remedial waxing course, seven (27 % ) were also required to take the summer amalgam restorations remedial course, compared with only 4% of the rest of the class. The approach to remediation had been to do more of the same tasks as in the regular course but with closer faculty supervision. Essentially students work independently on projects, requesting demonstrations or additional information as they require it. While early identification of these “at-risk” students is possible, traditional remediation strategies have been ineffective in assuring successin subsequent courses. The current report compares the performance of the two differently remediated groups and the rest of the class subsequent to that remediation. It describes an apparent changing remediation pattern in the preclinical training period, and explores possible reasons for the change. 865