INQUIRY Dental Materials Ceramic restorations Background.—Over the past 25 years many ceramic systems have been introduced for dental uses, but most have demonstrated poor survival rates in clinical situations. The gold standard for ceramic restorations is porcelainfused-to-metal (PFM). These restorations have a 95% 10to 15-year survival rate, with porcelain chipping seen at an incidence of 4% to 10%. Good to excellent esthetics are achieved when porcelain facial margins and proper tooth preparations are employed. The major drawback for PFM restorations is their cost, estimated to be an average of $273, which reflects the dramatic increase in the basic price of gold and other noble metals. Because of the public’s demand for esthetic restorations, ceramic materials have been developed to meet the need. The four groups of ceramic materials with sufficient clinical testing or anecdotal evidence to consider their use are leucite-reinforced glass-ceramics, lithium disilicate glass ceramics, layered zirconia, and monolithic zirconia (Table 1). Leucite-Reinforced Glass-Ceramics.—The first system to use pressing technology to make monolithic and layered restorations was IPS Empress. Layered IPS Empress anterior crowns are probably the most esthetic ceramic restorations now available and have a 5-year survival of 95% for anterior teeth, but just 80% on posterior teeth. IPS Empress crowns must be internally etched and bonded in place. Bonding should use immediate dentin sealing technique and a dual-cure or self-adhesive dual-
Table 1.—Type of Crown and Average Cost (Based on Survey of Five Commercial Laboratories) PFM (high noble) PFM (noble) Cast Gold IPS Empress (layered) IPS e.max (layered) IPS e.max (monolithic) Zirconia (layered) Zirconia (monolithic) (Courtesy of Ahmed SN, Donovan TE: Evaluation of contemporary ceramic materials. J Esthet Restor Dent 27:59-62, 2015.)
$273 $248 $314 $219 $212 $196 $242 $171
cure resin cement. This material is most appropriate for single anterior crowns in patients desiring a highly esthetic restoration. The average price is $219. Lithium Disilicate Glass Ceramics.—Short-term survival rates for lithium disilicate glass ceramic restorations are better than 95% on both anterior and posterior teeth. Advantages of this material include high flexural strength, ability to be used in both monolithic and layered form, and esthetic appearance. Monolithic e.max crowns are used for posterior teeth and layered crowns for anterior teeth. Although the layered crowns are more esthetic, they also fracture twice as often as monolithic crowns. Three-unit fixed partial dentures made of lithium disilicate glass ceramic have an unacceptably high failure rate. Recommended procedures with these crowns are etching and bonding with a self-adhesive dual-cure resin cement. Cost is about $196 to $212. Layered Zirconia.—Layered zirconia crowns have been used for over 10 years and have a very low core fracture rate because of transformation toughening, which intrinsically stops crack propagation. These crowns have satisfactory esthetics and acceptable marginal fit. However, nearly all clinical trials document cohesive chipping of the veneering crowns, which occurs five times more often than with PFM restorations. Although the crown does not always require replacement, the problem persists. Problems have been noted with this material and new formulations are being developed. The low thermal conductivity of the core material has been addressed by using slower cooling cycles. Lack of support of the veneering ceramic by the core has been resolved by improving software programs to ensure the core has optimal support. These new protocols have yet to be clinically tested. Therefore, layered zirconia crowns should be used with caution. In addition, the average cost for these restorations is $242. Monolithic Zirconia.—No long-term clinical trials have tested monolithic zirconia crowns, but few zirconia cores
have fractured in clinical trials and monolithic or fullcontour zirconia crowns are essentially an unveneered core. The material offers very high flexural strength and has been tested in large multi-unit restorations. The material allows more conservative tooth preparations than the other available ceramics or PFM. Also, when wellpolished, they are gentle on opposing tooth structures and demonstrate less wear on enamel than occurs with other ceramics. The monolithic zirconia restorations tend to be opaque and have reduced esthetics compared to layered restorations. They are usually used for posterior teeth, where esthetic concerns are less important, especially for second molars. They can be fabricated with significantly less tooth preparation, so they are appropriate for mandibular anterior teeth. They are inexpensive, with an average cost of $171. Monolithic zirconia cannot be etched but can be subjected to airborne particle abrasion bonding with MDP primers and resin cements. However, the bonds formed are relatively weak and deteriorate with aging. Particle abrasion may also transform the crown or core. The internal surface of these crowns is usually contaminated with saliva and blood during try-in, and salivary proteins are not readily removed. Crowns can therefore be prematurely dislodged. Sodium hydroxide applied for 20 seconds, then a water rinse helps to cleanse these crowns and increase the bond strength.
Discussion.—The various ceramic systems now available have strengths and weaknesses that need to be recognized and tailored to the clinical situation. Layered leucite-reinforced crowns provide excellent esthetics when used on maxillary anterior teeth and etched and bonded appropriately. Monolithic lithium disilicate crowns work best on premolars and first molars. Layered lithium disilicate crowns are appropriate for anterior teeth and premolars. Monolithic zirconia crowns perform well on molars and mandibular anterior teeth.
Clinical Significance.—PFM remains the gold standard for esthetic crowns but the cost can be a serious drawback for patients. Ceramic materials have been developed to address the issue and achieve good to excellent results with adequate longevity. Clinicians should carefully evaluate the requirements of each situation and choose an appropriate material to meet the need.
Ahmed SN, Donovan TE: Evaluation of contemporary ceramic materials. J Esthet Restor Dent 27:59-62, 2015 Reprints available from TE Donovan, Dept of Operative Dentistry, UNC School of Dentistry at Chapel Hill, Chapel Hill, NC; fax: 919537-3990; e-mail: [email protected]
Endodontics Weakening teeth during root canal treatment Background.—The structural changes in dentin that make a devitalized tooth more prone to fracture remain to be determined. Removing tooth tissue during root canal treatment can affect devitalized teeth, altering their mechanical behavior even after they are restored. The relative contributions of cavity preparation steps to overall tooth weakening were evaluated. Methods.—The decrease in tooth strength was analyzed using the finite element method (FEM) to elicit the separate and combined influences of a two-surface Class II preparation and root canal treatment. Root canal treatment was broken into access cavity preparation and root canal enlargement to determine the influence of each of these steps and of the combination of the two. After each phase, the crown was restored with composite resin and FEM was done on the restored tooth. Four models
were developed: model 1 was the intact tooth, model 2 included MO Class II cavity preparation restored with composite resin, model 3 had a MO Class II restoration and access cavity to the pulp chamber but no root canal enlargement, and model 4 was based on scans of the treated tooth and included MO Class II cavity, an access cavity preparation, and root canal enlargement. A failure index based on maximum principal stress criterion (MPCS) was applied. Compressive and tensile stresses were also analyzed, with corresponding failure indices applied as appropriate. Results.—The compression test identified the critical breaking force for the intact tooth as 1025 N at a compressive strain of 0.9 mm. The critical breaking force for the treated tooth was 710 N at a compressive strain of 1.02