rison of the fracture several all-ceramic mp
Miller, DDS,a Cole, DDSd College
strengths crowns Barbara
of ceramometal DDS,C
Fracture resistance to forces applied to the incisal edges of four types of anterior crowns was tested. Ceramometal crowns fractured at significantly higher values (720 psi) than the all-ceramic crowns (approximately 360 psi). No significant difference was found among the fracture values of the Dicer crowns, the aluminous porcelain jackets, and the crowns fabricated from Dicer veneered with aluminous body and incisal porcelain. (J PROSTHET DENT 1992;68:38-41.)
racture of dental porcelain is a perplexing reality in contemporary restorative dentistry. Many methodshave been used to determine the relative strengths of ceramic and ceramometal crowns. Various authors have investigated ceramic components by using a three-point beam test,lm3a torsional test,4 a Vicker’s indentation test,5 a biaxial flexure test,6 a diametrically compressedcylindric specimentest,7 and many other ingeniousprotocols, Other investigators have made crowns a.ndfractured them in an attempt to simulate a truer prosthetic configuration of the components.8-13 Most clinicians would agree that porcelain-fused-tometal crowns are more resistant to fracture than allceramic units. Yet when individual componentsare tested in the laboratory, feldspathic porcelains generally test weaker than the aluminous porcelains used in jacket crowns. It is difficult to recreate in the laboratory the exact conditions that would causea fracture in the mouth. The halfmoon gingival fracture, which is often seen in the allceramic crown, is consideredto be the end result of chronic stressesand strains related to occlusal forces, thermal shocks,5aqueousdegradation,6 and flaws in the ceramic microstructure.14McLean14related this fatigue phenomenon to the propagation of microcracks, and he developed the bonded alumina crown in an attempt to slowthis propagation in a cost-effective manner. Internal stressesalsocan causefracture in ceramometal crowns, but most patients will relate fractures in these types of restorations either to accidentally biting on some object, occlusal wear, or a singular traumatic event. The half-moon fracture is not seenas often in metal ceramics, although this could changewith the increaseduse of porcelain butt margins. aAssociate bAssistant CAssistant “Assistant IO/l/37281
Professor, Professor, Professor, Professor,
Department Department Department Department
of of of of
Fixed Prosthodontics. Fixed Prosthodontics. Dental Materials. Operative Dentistry.
This study evaluated the relative strengths of ceramometal crowns and several all-ceramic crowns, including aluminous porcelain jackets, Dicer (Dentsply International, York, Pa.), and Dicer veneeredwith aluminouspor-
Fig. 1. Force to incisal edgeof all-ceramic crown on maxillary right central incisor resulted in vertical fracture.
2. Mid incisal loading down long axis of test crown showstypical fracture.
600 P y
n-10 SD=117 E
eelain. All crown specimens (Fig. 1).
of mean fracture
n=7 SD=65 Porcelain
were loaded on the incisal edge
Ten metal-ceramic crowns were made for a maxillary right central incisor using the same die and cast. The preparation was done with a conventional beveled shoulder. The frameworks were waxed and cast using Degu Plus II (Degussa Dental, Long Island City, N.Y.). The porcelainbearing surfaces were thinned to 0.3 mm (+ 0.1 mm). Vita (Vident, Baldwin Park, Calif.) opaque body and incisal porcelain was applied according to manufacturer’s guidelines. An initial bake and correction bake were followed with an autoglaze. After completion, the porcelain thickness was measured at 1.1 mm (rt 0.1 mm). All-ceramic crowns were fabricated for a maxillary right central incisor using the same die and cast. The preparation incorporated a standard porcelain jacket shoulder. Six conventional aluminous jackets were constructed. Platinum foil (0.001 inch) was adapted to the die, and Vita-dur-n (Vident) porcelain was used, according to manufacturer’s recommendations, to fabricate the aluminous jackets. The foil was removed before testing. The completed crowns had a thickness of 1.2 to 1.3 mm. Seven Dicor crowns were made according to manufacturer’s guidelines. Four firings of the surface porcelains were used. The thickness of these crowns was 1.3 to 1.4 mm. Thirteen crowns were made with a Dicer coping veneered with aluminous body and incisal porcelains, often referred to as “Willy Glass.” The Dicer copings were waxed to approximately 0.8 mm thickness, cast, and fired. Next, they were thinned to 0.5 mm, and Vita-dur-n body and incisal porcelains were applied and fired according to manufacturer’s guidelines. After a correction bake and final contouring, the crowns were glazed. The completed crowns had a thickness of 1.2 to 1.3 mm.
Fig. 4. Stylus shape is evident in incisal edge of natural tooth. Enamel has essentially been crushed and crazed. Continued loading of this tooth resulted in catastrophic splits after stylus had penetrated well into dentin.
All test crowns were filled with autopolymerizing orthodontic acrylic resin (L. D. Caulk, Milford, Del.) and positioned in a block of acrylic tray resin (L. D. Caulk) so that the long axis of the crown was perpendicular to the floor and the incisal edge was parallel with the floor. The resin was manipulated so that it did not cover the external axial contours of the specimens. The crowns were then loaded in the center of the incisal edge until a fracture was evident (Fig. 2). An Instron machine model 1125 (Instron Corp., Canton, Mass.), was used at the crosshead speed of 0.05 in/min; the load values were recorded at fracture.
RESULTS The mean fracture strengths were as follows: for the 10 metal-ceramic crowns, 720.5 psi (SD, 117); for the six alu-