Layered all-ceramic crowns have become widely used since the introduction of strong alumina and zirconia cores and the availability of computer-aided design/computer-aided manufacturing (CAD/CAM) milling techniques, but problems related to brittle fracture and to the esthetics of core materials remain. Design practices have been based more upon empirical guidelines than upon clinically relevant scientific data. Remarkably little scientific data on optimal design of all-ceramic crowns, or even of metal ceramic crowns, has been published. Ceramic copings are often generically milled to arbitrary thicknesses of 0.4 or 0.6 mm. This may not provide uniform or appropriate thickness for veneering porcelain. Study of failure mechanisms and clinical outcomes may guide clinical practice.

  Fracture appears to be the most common clinical failure mechanism of all-ceramic crowns. Overall crown thickness may be of primary importance in resisting fracture; a minimum overall thickness of 1.5 mm has been recommended. However, relative layer thickness is also important. Relative layer thickness influences strength, stress distribution, and failure mode. It has been suggested that a 1 to 1 ratio of core to veneering porcelain thickness may provide reasonable strength, esthetics, and fabrication tolerance. However, the importance of adequate core thickness may be paramount. The stiffness, or elastic modulus, of the core material is also influential. A stiffer core will better resist flexure under load. This is important because ceramics have low critical strains and are poorly supported by flexible dentin.However, stiffer core materials may also be more vulnerable to radial cracks originating from their internal surfaces Pertinently, zirconia is stronger, tougher, and more flexible than alumina. Thus, zirconia-based crowns might be expected to differ from alumina-based crowns in clinical failure mode and in overall clinical performance, but such comparative data is absent.Optimal design parameters might also differ.

  Fractographic analysis of clinically failed crowns suggests that many failures occur from internal surfaces that are subjected to tensile stress during loading.These data suggest that great technical care should be taken in the fabrication of the internal surfaces of all-ceramic crowns, and that crude adjustment must be avoided. Other failure modes of all-ceramic crowns include bulk porcelain fracture, a focus of the present report. Approximately one third of all fractures in a 3-year clinical trial of 223 glass-infiltrated alumina In-Ceram (VITA Zahnfabrik, Bad Sackingen, Germany) crowns involved the veneering porcelain, without any damage to the core.That study pooled both cohesive and adhesive porcelain failures. The overall 3-year survival rate in that study was 96%, consistent with data on other strong, layered all-ceramic crown systems.

  The authors experienced approximately 30 instances of cohesive porcelain failure with the use of milled zirconia copings machined to arbitrary thicknesses; 2 representative examples are illustrated (Fig 1 + 2 ). The time to failure varied from months to years. Porcelain fracture is one of several common failure modes of all-ceramic crowns. Like most brittle materials, porcelain has high compressive strength and low tensile strength.Therefore, copings for crowns must be designed to minimize tensile loading of veneering porcelain. Appropriate porcelain and core thickness may decrease internal stress, reduce mechanical failure, and optimize esthetics. Lessons learned from the introduction of metal ceramic restorations many decades ago were revisited. The purpose of this article was to describe the customization of milled zirconia copings to provide even and controlled porcelain thickness with the aim of decreasing cohesive porcelain fracture and other failures.

Clinical report

A longstanding patient of record, a 68-year-old man, presented after a tooth fracture. The patient had excellent oral hygiene and a low caries rate. The maxillary left first molar had existing mesio-occlusal and disto-lingual amalgam restorations. The buccal cusps had fractured. The patient requested an esthetic restoration. He was advised of the available metal ceramic and all-ceramic options before selecting a zirconia-based all-ceramic crown.

Missing buccal tooth structure was replaced using a bonded composite resin (Build-It FR and Bond-1; Pentron Clinical Technologies, Wallingford, Conn) restoration. The tooth preparation, impression, die, and scan were made in the customary manner. Full-contour waxing and cut back, as performed for metal ceramic crowns, was used to obtain uniform and adequate occlusal porcelain thickness to support the veneering porcelain, and to maximize framework strength and stiffness, the proximal lingual or palatal coping margins were maintained in zirconia (Procera AllZircon; Nobel Biocare USA, Yorba Linda, Calif). The zirconia extended at least 2 mm above the margins with a thickness of 1 mm or more at the porcelain junction ( Fig. 3 ).This provided sufficient space to allow an even thickness of veneering porcelain throughout the crown form , a butt margin junction between porcelain and zirconia, and a strong, stiff coping. As for all brittle materials, butt joints are preferred to feather edges. The patient had been educated to expect that there would be approximately 2 mm of white ceramic along the gingival margin on the lingual surface. It was explained that this was preferable to the appearance of metal margins. Core ceramic shoulders are also preferable to the “disappearing” margin, when a feather edge of porcelain is applied to a thinned coping margin, providing less than optimal strength and esthetics, and greater technical difficulty in accurately contouring and smoothly finishing the marginal area.

To optimize esthetics, a porcelain labial margin was created on the buccal surface where it may be visible during normal smiling and speech. A distinct butt transition from the porcelain to zirconia margins was created just palatal or lingual to the proximal contacts . Internal line angles on the coping were rounded, as were internal line angles on the preparation itself.

A dual-scan procedure was used to create and merge the datasets from the die and the wax pattern. First, the die was scanned, then the cut-back wax pattern was sealed to the die which was scanned a second time. The scanner (Procera Forte; Nobel Biocare USA) used the submarginal data points to orient the 2 scans and merge the data sets. The merged file was transmitted to a milling facility and the coping was manufactured. The porcelain veneering (Nobel Rondo; Nobel Biocare USA) was completed (Fig.4 ), and the crown cemented (Fuji PLUS; GC America, Alsip, Ill) (Fig. 5 and 6 ). This particular restoration has been in service for over 1 year; no complications have occurred.

Fig. 4

Fig. 5 and 6

Discussion

The advantages of customizing coping design are that: core and porcelain thicknesses can be controlled; marginal areas can be optimized for strength with a high shoulder or for esthetics with a porcelain labial margin; and butt joints between the porcelain and core can be facilitated. Until more is known about clinical failure modes and clinical performance parameters, precise recommendations cannot be made with confidence. The disadvantage of using this customizing technique is primarily the dental laboratory technician time involved in full-contour waxing and cut back, as well as in completing a second scan. This may be offset by greater ease in porcelain application and in a potential, but unknown, improvement in clinical service.

Over the past year, approximately 150 customized milled zirconia crowns have been placed by the authors. In contrast to their prior experience, the authors have not yet encountered any instances of cohesive porcelain fracture or core fracture. However, conclusions cannot be drawn from such a small sample size in so short a time.

Despite the widespread belief that porcelain must be uniformly supported by a metal coping or all-ceramic core, the apparent clinical acceptance of several metal ceramic crown systems lacking this feature demands comment. Several crown systems use thin ductile metal foils, powder metallurgy, or electroforming to produce thimble copings as matrices for porcelain application ^, but rigorous clinical performance data has yet to be published. It is likely that these types of restorations are generally used for anterior teeth, which tend to have superior survival rates for all-ceramic crowns.  The relatively low elastic moduli of these thimble matrices and of veneering porcelain, along with a toughened interior surface, may reduce their vulnerability to tensile radial cracking. However, they may be at increased risk of other failure mechanisms.

Clearly, more research is needed to relate materials properties, crown design geometry, and tooth preparation parameters to the clinical failure mechanisms and clinical performance of all-ceramic crowns.

Summary

A technique for custom designing strong milled ceramic cores for all-ceramic crowns was presented. A full-contour waxing was used in conjunction with a dual-scan technique to ensure optimal coping design and appropriate porcelain thickness.

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