Effect of different polishing methods and composite resin thickness on temperature rise of composite restorative materials
Dr. MirzaKoucheki Boroujeni P .,1 Dr. Daneshpour N.,2 Dr. Zare Jahromi M. 3
1 Assistant Professor and Head of Restorative Dentistry Department, School of Dentistry, Islamic Azad University, Khorasgan Branch. Esfahan , Iran . 2 Dentist . 3 Assistant Professor, Department of Endodontic, School of Dentistry, Islamic Azad University, Khorasgan Branch. Esfahan , Iran.
(Received 5 Oct, 2011 Accepted 16 Jul, 2012)
Background and Aim : Along with improvements in aesthetics and longevity of restorations, finishing and polishing, can produce potentially injurious temperature rise within the pulp chamber. The purpose of the current study was to find whether different polishing methods and thickness of composites have any effect on temperature rise of composite restorative materials .
Materials and Methods : Sixty composite resin specimens 9 mm in diameter were prepared and assigned to three experimental groups with three sample thicknesses (2, 3 and 4mm). Each group was divided into four subgroups randomly. Polishing in subgroups 1 and 2 (continuous and intermittent dry polishing) and subgroups 3 and 4 (continuous and intermittent wet polishing) was carried out with a slow speed contra-angle hand piece at a medium speed for 120 seconds in a roll on motion. Immediately after polishing, temperature was measured on the top and bottom surface of each sample using a laser thermometer. One and two way ANOVA, Duncan, and paired T-test was used to analyze the data .
Results: The mean temperature rise after polishing in different methods was significant continuous dry polishing produced the maximum temperature rise. In addition, increasing the thickness of composite resin up to 4mm did not significantly affect thermal transfer from the top surface to the base during polishing .
Conclusion : Copious use of water coolant during finishing and polishing procedures is considered a simple and effective method for pulpal protection. Increasing the thickness of composite resin does not have a significant role in compensating the heat generated during polishing procedure .
Key words : Composite resin - Dental polishing - Thermal transfer
Corresponding Author: Dr. MirzaKoucheki Boroujeni P., Assistant Professor and Head of Restorative Dentistry Department, School of Dentistry, Islamic Azad University, Khorasgan Branch. Esfahan , Iran.
Dentists today have numerous materials from which to select when restoring teeth, including amalgam, composite, glass ionomer cement, gold foil, cast metals, ceramics, and metalceramics. Specific clinical situations, however, dictate a much narrower range of appropriate restoration options.
The clinical decision as to which restorative material to place is complex, involving factors relating to the tooth, the patient, the clinician, and the properties of the restorative materials. Individual restorative materials ideally are applied in a defined set of clinical circumstances, and it is not possible to freely substitute one material for another and expect long-term success.
Because it is anticipated that this report will be read by individuals with dental knowledge ranging from limited to expert, this section begins with a background discussion of some of the factors that must be considered when selecting the appropriate restorative material for a specific clinical situation. These factors include the diagnosis of dental canes, treatment and material options, the properties of dental restorative materials, longevity of materials, and clinical decision-making in determining when a restoration has failed.
Finally, this section provides a brief description of all the currently available posterior restorative materials, including their individual advantages and disadvantages, as well as indications and contraindications for their use.
Diagnosis of Dental Caries (Tooth Decay)
Caries-producing bacteria are continuous residents of the oral cavity for people who have teeth and, thus, the opportunity for caries to manifest itself is always present for these individuals. Individuals may seek care from a dentist when they become aware of caries in their mouth. Pain, discoloration, a bad taste or odor, a sharp tooth or restoration edge felt by the tongue, or a dark spot on a tooth can trigger such a visit. The majority of caries, however, is likely to be asymptomatic (painless) and is found by the dentist during a periodic examination�the traditional "checkup." This process includes a direct visual and tactile examination, often supplemented with radiographic information, which permits assessment of the teeth even in areas that cannot be visualized directly.
When there is questionable, or very early evidence of caries, seen as a "white spot," or slight sticking of the explorer in a pit or fissure of a tooth, application of conservative preventive procedures, such as demineralizing fluoride applications, dental sealants, or preventive resin restorations (to be discussed in detail later) might be employed. These procedures, combined with follow-up observation of the suspected areas at later appointments, are a realistic alternative to preparing and restoring the tooth. At this point oral bacterial screening for strep mutans and lactobacillus may also be appropriate for assessing the caries disease risk for the individual.
Treatment and Material Options
For much of this century it was believed that dental caries could be treated away with restorations (Anusavice, 1989). Clearly, this is not the case. The long-term consequences of the insertion of the first restoration in any tooth always must be a consideration in the treatment decision (Lutz et al., 1987). Dental restorations have a limited clinical durability. As restorations need replacement, increasing amounts of tooth structure are lost and the patient may enter into a repetitive restorative cycle with larger restorations, weaker teeth, and more complex therapy (Elderton and Davies, 1984). Indeed, it has been estimated that as many as two-thirds of restorations placed each year are replacements for existing restorations (Maryniuk and Kaplan, 1986). As the cavity size expands, the range of restorative materials to effectively employ becomes limited, and the option of appropriately placing a more economical direct restorative material that conserves tooth structure is lost.
Where active dental caries is evident (some longstanding caries may be arrested or nonactive and not require treatment), the dentist must decide whether or not to restore the tooth and, if restoration is required, which restorative material to employ for the anticipated situation.
Many factors must be considered relative to the placement of a restoration.
- Extent of caries
- Strength of remaining tooth structure
- Specific characteristics of the patient's dentition and periodontal health
- Patient's oral hygiene and dental caries history
- Financial costs of the procedure to the patient
- Risks and benefits of the procedure to the patient
- Ability of the dentist to perform the procedure
- Preferences of the dentist and the prevailing standard of care
- Acceptance by the patient.
These many factors, several of which legitimately could be viewed differently by different patients and dentists, make it desirable to have a variety of options available for consideration. It is neither feasible nor desirable to use a single approach.
The range of acceptable treatment options for the patient who has overt caries includes: 1) The tooth can be restored; 2) the tooth can be extracted; or 3) no treatment can be rendered. A decision to have the tooth extracted or to forego treatment has both short- and long-term consequences, which are usually negative. Traditionally, this decision is one in which patients have participated actively through informed consent.
For teeth that are to be restored, the second decision concerning which procedure and material to use is traditionally one in which patients have been involved less fully. Dentists generally offer patients a "case presentation" outlining overall treatment options. For individual restorations, however, the specific choice of procedures and materials routinely has been made by the dentist.
Although caries is the predominant reason for restoration of teeth, several other clinical conditions, such as tooth fracture, restoration failure, and trauma, also may require restoration. The most common clinical conditions, treatment options, and restorative material options are summarized Table 1.
Table 1. Indications, Treatment, and Restorative Material Options
for the Restoration of Posterior Teeth
The Search for the Ideal Restorative Material
Despite modern dental materials and techniques, the oral cavity presents a demanding environment for restorative materials. Restorative materials break down for a variety of reasons including: dietary factors, masticatory stresses, acid-base shifts, temperature changes, failure of the tooth structure itself, the adhesive nature of plaque, the complex and different structures of cementum, dentin, and enamel, and interaction with other materials. The consequences of breakdown include recurrent caries, surface wear, leakage at the tooth-restoration interface (often referred to as microleakage), marginal fracture, bulk fracture, discoloration, corrosion, lack of biocompatibility, and sensitivity of the pulp to bacteria, chemicals, temperature, and pressure. Indeed, no test system is available that can duplicate readily the combined stresses of the oral cavity over a lifetime. Yet, even though the ideal restorative material does not exist, ideal characteristics can be outlined, as suggested below.
Technical Features for the Provider
Although this list is extensive, undoubtedly there are additional desirable characteristics for a dental restorative material. Given the number and range of characteristics, it is not surprising that no restorative material available today meets all, or even most, of the requirements for each category of ideal properties.
Direct and Indirect Dental Restorative Materials
Dental restorations may be classified as direct or indirect. Direct restorative materials are inserted into cavity preparations in a soft, pliable state and then set hard. For direct restorations, the tooth is prepared and the filling material is placed during the same appointment. Direct restorations usually require less destruction of intact tooth tissues than indirect restorations. Direct fillings are appropriate only when sufficient tooth structure remains to maintain the integrity of the restorative material. The greater the loss of tooth structure, the more likely that an indirect restoration is indicated. Amalgam, resin based composite materials, glass ionomer cements, and compacted gold foil are examples of direct restorative materials.
Indirect restorations, such as inlays, onlays, and crowns, are fabricated in a dental laboratory on models made from impressions of the tooth prepared by the dentist. These restorations generally require multiple visits and placement of temporary restorations in the prepared teeth between appointments. In contrast to direct restorative materials, all indirect restorations are cemented as one-piece restorations and so require the removal of all undercuts, undermined tooth structure, and, often, significant amounts of healthy tooth tissues in order to produce parallel walls of the cavity preparation to allow insertion of the restoration and to provide adequate bulk of restorative material for strength. The two-step procedure and laboratory costs make indirect restorations significantly more expensive for the patient.
Recently, techniques have been developed where a composite inlay is prepared in the mouth, hardened outside the mouth, and cemented into the tooth during the same visit. A relatively new and not widely available technique, CADCAM (Computer-Aided Design and Computer-Aided Manufacture), uses a computer to record the prepared tooth optically and to direct the grinding of a ceramic (porcelain) block to produce an inlay, onlay, or crown for cementation at the same visit (Mörmann et al., 1990). These techniques eliminate the need to make an impression or temporary restoration, but are also significantly more expensive than a direct restorative material and are not generally available in dental offices.
Longevity and the Diagnosis of Failure in Restorative Materials
The longevity of a restoration depends upon many factors varying according to tooth type, location, condition, type of restoration, age of the patient, materials used, clinician capability, and the proper diagnosis of restoration failure. One of the major reasons that clear-cut longitudinal longevity data are deficient is the lack of objective measures for determining when a restoration has failed.
The dentist's decision to replace or repair a restoration involves numerous factors, including breakdown in marginal integrity, presence of recurrent or new caries, unacceptable esthetics, excessive wear, and pain symptoms. Criteria for quantifying the clinical failure of restorations have not been well defined, and diagnostic techniques used to determine the quality or functional status of restorations are grossly inadequate. Such criteria are necessary for definitively determining such factors as the clinical significance of leakage at the restoration tooth interface, bacterial colonization, presence of caries under restorations, and breakdown of marginal integrity at interproximal and subgingival margins (Anusavice, 1989). Currently, the decision to classify any of the above clinical conditions as a failure requiring replacement draws on the individual dentist's clinical judgment, which has been shown to be highly variable and not defined clearly (Maryniuk, 1984). Merrett and Elderton (1984) found that great variation exists among dentists in their decision to replace a restoration and that a third of these decisions would not be agreed upon by a randomly selected second dentist.
Surveys by Mjör (1979, 1980) of 85 dentists in private practice detailed the reasons for replacing amalgam and composite restorations. The primary reason for replacing amalgam restorations was recurrent caries (58 percent). Marginal degradation (9 percent), isthmus (bulk) fracture (13 percent), and tooth fracture (12 percent) also were commonly cited. For composites, poor marginal adaptation and anatomic form (40 percent), recurrent caries (20 percent), and discoloration (19 percent) were the most commonly cited reasons for failure. In these studies, the resin-based restorations were predominantly of a Class III type, with a few Class V restorations (see glossary for definitions).
Clear-cut data on longevity also are lacking because of the difficulty in designing studies that include the many pertinent variables, such as quality of the restoration, patient hygiene and dietary habits, materials used, operator proficiency, and conditions under which the restorations were placed. Maryniuk (1984) reviewed longevity data from 21 published studies of various restorative materials and, on the basis of study design, validity of data, and failure criteria, concluded that because of methodological flaws in these studies and discrepancies in the determination of failure, no generalizable information is available to describe and predict the lifespan of restorations.
Nevertheless, available longevity data for the various restorative materials suggest that indirect restorative materials, cast metal, and metal-ceramic crowns likely will have the greatest clinical longevity of the available posterior restorative materials, with a median survival rate of 12 to 18 years (Schwartz et al., 1970; Kerschbaum and Voss, 1977; Coornaert et al., 1984; Leempoel et al., 1985). Of the direct restorative materials, amalgam is estimated to last 8 to 12 years and composite 6 to 8 years (Osborne et al., 1980; Crabb, 1981; Patterson, 1984; Klausner and Charbeneau, 1985; Maryniuk and Kaplan, 1986; Qvist et al., 1986a and b; Mj
Dentists have more than a century of experience using amalgam as a direct filling material. Amalgam is strong and durable enough to withstand the pressures of chewing; it is relatively inexpensive and easy to place; and it has properties that may help prevent recurrent caries (Phillips, 1984; 0rstavik, 1985). Dental amalgam is widely considered to be unaesthetic, however, and questions regarding its safety have been raised virtually from the time of its first use.
Although amalgam has a range of defined optimal uses, its low cost to patients, ease of manipulation, and durability allow it to be used in areas where a stronger or more esthetic material ideally would be placed. For example, large amalgam fillings are often used even when a casting would be stronger. Lost cusps are replaced with amalgam when a cast onlay would be more durable and long-lasting. Incipient caries are restored with amalgam when a preventive resin and sealant would conserve tooth structure and function.
- Marginal breakdown
- Some destruction of sound tooth structure
- Not esthetic
- Long-term, corrosion at margins may cause breakdown; however, newer formulations have greater resistance to corrosion
- Potential for galvanic reaction
- Local allergenic potential
- Public perception of mercury toxicity.
Composites have excellent esthetic properties and are applied most frequently in anterior tooth cavities. In the 1980's, the mechanical and physical properties of composite resins, fillers, coupling agents, and bonding agents were improved, and a number of brands have been approved by the American Dental Association for posterior restorations in nonstress-bearing situations. When used in large restorations, including virtually all posterior situations, an incremental filling technique must be utilized to ensure complete polymerization and to minimize the effects of shrinkage of the resin on the final size of the restoration. Compared with amalgam restorations, the longer time necessary to properly complete this procedure has implications relative to moisture contamination and financial cost to the patient.
Exacting techniques are necessary for the successful placement of a composite resin. Composite restorations rely upon mechanical and chemical adhesion of the material to the tooth surface to seal margin areas and, thus, are sensitive to moisture contamination during placement. The difficulties presented in controlling saliva and the moisture normally present on tissues of the tooth create an unfavorable surface for adhesion. This is a major consideration in clinical decision making, because moisture control is difficult in many patients and in the most posterior areas of the dentition. Marginal leakage and the formation of recurrent caries are likely consequences of moisture contamination.
Problems with excessive wear under stress and high technique sensitivity still limit composite use in posterior situations; however, they are popular with individuals who strongly value esthetics. Additionally, composites have been advocated as an alternative for persons concerned about the mercury content of amalgam. This situation may result in the inappropriate use of composite in stress-bearing situations.
Increasingly, individuals desire attractive, as well as functionally satisfactory, teeth. Composite resin currently has limited, but important, applications as a posterior restorative material. Its use in treating incipient lesions in conjunction with sealants is an important step in the long-term conservation of tooth structure. Unfortunately, as a recent worldwide survey has shown, the teaching of placement techniques for posterior composites is limited. Professional dental education rarely includes significant opportunities for students to gain clinical experience in the use of composite resins as posterior restorative materials (Wilson and Setcos, 1989). There are indications, however, that these opportunities are increasing and the use of composite for specific posterior restorative situations, such as preventive resins for lesions in minimal-stress-bearing areas, likely will become a more integral part of the dental curriculum as further research data become available.
Pit and Fissure Sealants and Preventive Resin Restorations
A contemporary report on dental restorative materials must include a discussion of sealants and preventive resins. Although technically a preventive measure, sealants increasingly play an important role in a conservative restorative treatment strategy, in which the goal is to preserve healthy tooth structure.
Some of the pits and fissures of teeth largely are fused during tooth development, while others may remain microscopically open and impossible to clean. The latter fissures are potential sites for the colonization of caries forming bacteria, despite the best oral hygiene efforts. Sealants are resin materials that flow easily and, when applied to the acid-etched surfaces of pits and fissures of posterior teeth, bond to the enamel and seal the pits and/or fissures from bacterial invasion and debris.
The decline in caries rates experienced over the past 30 yeas in the United States has resulted largely from the addition of fluoride the drinkng water and to dentrifices (PHS, DHHS, 1991). Fluoride, however, has its greatest effect on the smooth surfaces of the teeth and lesser benefit protecting pits and fissures. Graves and Burt (1975) found that more than 91 percent of the callous surfaces in permanent first moles of children up to grade 6 were in pits and fissures. The National Children's Oral Health Survey of 1979-80 reported that 84 percent of the cases experience of 5- to 17-year-old children occurred in pit and fissure surfaces (NIDR, 1981).
Many studies have demonstrated the efficacy of pit and fissure sealants in reducing caries. Horowitz et al. (1977) reported a 37 percent reduction in occlusal caries after 5 years. Meurman et al. (1978) reported a 59.8 percent reduction in cases after 5 years and Simonsen (1987) reported a 47 percent reduction in cases after 10 years.
Despite numerous published studies on the safety and effectiveness of sealants, the dental profession has been slow to adopt their use. The 1985-86 National Children's Oral Health Survey found that less than 7 percent of children 5 to 17 years old had received sealants (NIDR, 1989). Dentists have cited a number of reasons for their reluctance to place sealants, including concern about sealing in cases. Several studies, however, have demonstrated that sealants can be applied over incipient active cases, resulting in a rapid drop in viable bacteria count and elimination of the nutrition source, rendering the bacteria nonviable and stopping further progression of the disease (Jeronimus et al., 1975; Handelman et al., 1976; Going et al., 1978; Mertz-Fairhurst et al., 1987). Studies also have cited a perceived lack of cost-effectiveness (Stiles et al., 1976; Messer and Nustad, 1979; Lennon et al., 1980; Simonsen, 1982), and lack of third-party coverage as reasons why sealants have not been accepted more widely (ADA,1981). Other cost-effectiveness studies, however, indicate decreased long-term expense for sealed teeth, as compared to unsealed teeth (Stiles et al., 1976; Simonsen, 1989). The more cases-prone the population, the more effective is this treatment modality.
The 1983 National Institutes of Health Consensus Development Conference on Dental Sealants in the Prevention of Tooth Decay concluded that pit and fissure sealants were a safe and effective means for preventing pit and fissure cases. "Expanding the use of sealants would substantially reduce the occurrence of dental cases ... and improve the health of the public and reduce expenditures for the treatment of dental disease" (NIH, 1984).
Investigators are examining other uses for sealants, such as sealing over the surface of amalgam restorations to reduce or eliminate the release of mercury vapor from the surface. Promising results also have been reported in improving wear rate and marginal integrity and in reducing bacterial leakage for both posterior composites and amalgams by applying sealants over the surface (Mertz-Faithurst and Ergle, 1991; Dickinson et al., 1990).
Preventive Resin Restorations(PRR) utilize a combination of composite and sealant to treat early caries in pits and fissures. Despite the name of preventive resin, this technique is employed after caries has formed and the caries is judged to be deeper into dentin than appropriate for management by fissure sealant alone (Anusavice, 1989). In the interest of conserving tooth structure, PRR involves only removing the affected tooth structure, acid-etching the enamel, placing composite in the prepared cavity, and using sealant in the remaining pits and fissures. These conservative restorations are minimal in size and are used in nonstress-bearing situations. The PRR can be considered an alternative and, in most situations, preferable to the placement of conservative Class I amalgams (Anusavice, 1989; Simonsen, 1990; Mjor, 1991). A treatment pattern starting with early identification of caries, fissure sealants, and preventive resins will conserve tooth structure and help to forestall, or significantly defer, the need for major restorative care later.
Few data are available on the long-term clinical evaluation of preventive resins. In one study, Houpt et al. (1986) demonstrated a 72 percent survival rate for PRR after 5 years. Simonsen and Landy (1987) have also reported favorable results. These studies, however, are small, and comparisons of preventive resins with restorations for which they are generally substituted, (i.e., Class I amalgam) are needed.
Glass ionomers were introduced commercially about 10 years after dental composites and enamel-bonding materials came to the market. Composites proved to have a competitive edge over glass ionomers as restorative materials because of their higher strength.
The original glass ionomers had a number of clinical drawbacks that limited their acceptance. Clinical failings were related to manipulation, setting sequence, early moisture sensitivity, esthetics, and surface texture. Consequently, glass ionomer, as restorative materials, did not gain the acceptance of dentists to the same extent as composites.
For a few important reasons, glass ionomers recently have gained wider acceptance as a restorative material for defined situations. They bond chemically to tooth structure and release fluoride. Patient response to glass ionomers is usually excellent because the placement technique can be extremely conservative and requires little, if any, drilling (Hunt 1990); the procedure is usually quick and painless and often does not require local anesthesia; and the resulting restoration is fairly esthetic.
Developments in the formulation of glass ionomers have made them useful as a cavity-lining material and for cementation and preventive applications, as well as for their original intended use as a direct filling material. As a filling material, glass ionomers are perhaps best used in restoring deciduous teeth and in Class V restorations involving gingival erosion and abrasion defects in adults. The use of glass ionomer may play an increasingly important role in the growing geriatric population which is retaining their teeth longer, but facing a concomitant increased risk of root caries.
While glass ionomer appear to be satisfactory in many anterior applications and primary teeth, their use continues to be limited in permanent posterior teeth, particularly with stress-bearing restorations. Limitations include low tensile strength, low impact and fracture resistance (brittleness), and degradation.
Glass ionomers are not recommended for restorations where toughness and resistance to wear are major considerations (Sulong and Aziz, 1990). It has been recognized, generally, that the wear resistance of glass ionomer is inadequate in areas of occlusal contact. Clinical studies have shown that a gradual loss of contour can be expected because of chemical degradation and surface wear (McLean, 1980). One study of a glass ionomer product, using a commercial composite resin as a control, reported that the glass ionomer abraded about three times more rapidly, by volume, than the composite (Smales and Joyce, 1978).
In the early to mid-1980's, it was found that the introduction of metal fibers or powder in the glass ionomer system (glass-cermet cements) significantly improved abrasion resistance (McLean, 1984). The addition of silver alloy powder to glass ionomer, in particular, resulted in a number of improvements in its physical properties (Simmons, 1990). The silver cermet material has a light gray color, which is no more unaesthetic than silver amalgam, but it has a major disadvantage in that it has a low fracture toughness, making it of limited value in regions subjected to the stresses of mastication (Croll, 1990; McLean and Gasser, 1985).
Glass ionomes, including cements, are technique sensitive (Knibb and Plant, 1989; Mount, 1990b; Smales et al., 1990; Smales and Gerke, 1990; Watson, 1990). The setting reaction and maturation of glass ionomer restorations are relatively slow. Even with the most skillful placement technique, however, the success of a glass ionomer restoration may hinge on the composition of commercial glass ionomer materials, which may vary widely from manufacturer to manufacturer (Smith, 1990).
Although glass ionomers exhibit significantly less polymerization shrinkage than composites, some curing contraction generally occurs, leading to the formation of marginal gaps (Feilzer et al., 1988; Saunders et al., 1990). Marginal leakage associated with glass ionomer can be reduced still further if the restoration is covered with a thin layer of posterior composite resin (Guelmann et al., 1989).
For centuries, gold foil has been applied to various surfaces for ornamentation or utility. Early use of foil also included adaptation to teeth where defects existed. With time, as new instruments became available and better skills were developed, more and more uses were found for this material in dentistry. Newer forms of the gold appeared and made easier the meticulous task of condensation, first with powdered gold (Baum, 1965), then with other forms of electrolytic-formed gold (mat gold).
Properly placed, direct-filling gold restorations are excellent replacements and can be expected to last for 20 years or more. Their clinical indications, however, are limited. Most frequently, they are placed into small cavities in nonstress-bearing situations, or to repair defective margins of cast gold inlays, onlays, and crowns. Large restorations of foil are difficult to place. In addition, pure gold is too soft and ductile to withstand the forces that are exerted on most posterior restorations. Furthermore, larger restorations in the anterior of the mouth are not esthetic.
The major difficulties with direct gold restorations are the technique sensitivity of placement, the skill and meticulous attention required of the dentist, the potential damage to the pulp and/or periodontal tissues because of trauma during placement, and the overall cost to the patient in time and money.
Although many dentists still believe that this material should continue to be placed and that the technique should be taught, the use of gold foil is limited and diminishing. Its use is declining primarily because of the high cost associated with this technique, the limited number of applications for its use, and the availability of acceptable alternative materials, primarily composite, glass ionomer, or amalgam.
Cast Metal and Metal-Ceramic Restorations
Cast metal restorations such as inlays, onlays, and crowns are indirect restorations generally requiring two or more appointments. The successful fabrication and placement of these restorations depend on close attention by the dentist and laboratory technician to minute details in a multiprocedural, step-by-step process. Each restoration is designed carefully to restore anatomy, function, appearance, and comfort.
The decision to restore with inlays, onlays, crowns, and/or bridges depends on many factors, including the degree of tooth destruction, esthetic needs, missing teeth, oral hygiene, and the financial capability and desires of the patient. There is over a six-fold increase in price for cast restorations in teeth that could be restored with amalgam.
Cast metal posterior inlays only cover a portion of the occlusal surface. It is believed that these inlays weaken the tooth and may lead to cuspal fracture (Norman, 1991). Therefore, onlays or crowns that cover and protect the cusps are the recommended restoration for highstress-bearing situations where there is inadequate natural tooth remaining to support a direct restorative material and where one or more cusps need replacement.
Since tooth preparations for full crowns are easier for the dentist to prepare and are less likely to involve the pulp than tooth preparations for an onlay, they are becoming the cast restoration of choice when cuspal coverage is indicated.
The selection of casting alloys depends on the location of the tooth in the mouth, the presence and type of adjacent restorations and opposing teeth, the need for esthetics, and the patient's financial capability.
Casting alloys for metal-cesmic restorations are divided into three categories: high noble, with at least 60 percent noble metal content and at least 40 percent gold; noble metals, with at least 25 percent noble metal; and predominantly base metal, which has less than 25 percent noble metal. The noble metals in casting alloys are primarily gold, platinum, and palladium (ADA, 1984).
Base metal alloys, which can include nickel, beryllium, cobalt, and chromium have gained widespread use, especially in the United States, because of their low cost and superior physical properties. These properties include: high mechanical strength, resistance to sag when fired with porcelain at high temperatures, porcelain bond strength, thermal compatibility between porcelain and metal, and resistance to corrosion. A survey of dentists in Minnesota by Olin et al. (1989) revealed that 62 percent of dentist prescriptions written in that year were for base metal alloys.
Fabricating fixed prosthetics like crowns, inlays, and onlays is extremely technique-sensitive, and the skill and attention to detail by both the dentist and technician play a major role in the longevity of these devices.
Metal-ceramic restorations (porcelain fused to metal, PFM) combine the strength of cast metal with the esthetics of porcelain. In these restorations, porcelain is baked onto a thin coping (cast metal substructure) prepared from an impression of the tooth. Metal-ceramic restorations have been successfully employed for single crowns and multiunit bridges for the past 30 years. These restorations are used for more than 60 percent of the crown and bridge restorations performed (Anusavice, 1991).
One of the main disadvantages of metal-ceramic crowns is the high abrasive potential of ceramics relative to opposing natural teeth or other dental materials. Mahalick et al. (1971) reported a high wear rate of enamel-porcelain surface interactions, as compared to gold alloy against enamel. DeLong et al. (1986) reported a high coefficient of friction between enamel and dental porcelain and concluded that the wear of porcelain appears to be one order of magnitude (10X) greater then that of dental amalgam. When the surface of the porcelain is roughened through occlusal adjustment, care must be taken to restore a highly polished surface or severe wear of the opposing tooth structure may result.
The longevity of noble metal inlays compared with amalgam was reported in two studies. Jahn et al. (1989) found no significant difference between gold inlays and amalgam after 2 years. Mjor et al. (1990) reviewed a number of clinical trials and reported longevities of cast metal restorations that ranged from slightly less than to 90 percent greater than that of amalgam restorations. Schwartz d al. (1970) reported a mean lifetime of 10.3 years for full metal crowns. Recurrent caries was the primary cause of failure for 58 percent of the crowns. Kerchbaum and Voss (1977) estimated that only 3 percent of PFM restorations failed over a 10-year period. When properly fabricated, however, it is likely that a cast metal or metal-ceramic restoration will be in service for many years longer than large, direct restorations.
The failure rates reported for PFM restorations appear to be relatively low (Kerchbaum and Voss, 1977; Coomaert et al., 1984; Glantz et al., 1984; Leempoel etal.,l985;Christiansen,1986). The reasons for failures of PFM crowns and bridges fall into five major categories: (1) clinical deficiencies, (2) laboratory deficiencies, (3) inadequate dentist-technician communication, (4) technique sensitivity of materials, and (5) patient factors. The principal cause of failure varies considerably among dentists and among laboratory technicians.
Although 70 percent of the dentists indicated that PFM crowns with porcelain occlusion on maxillary first molars were highly successful, only 26 percent indicated that they would have used PFM crowns with porcelain occlusal surfaces for their own personal treatment. Most of these dentists preferred, for their own maxillary first molars, a three-quarter gold crown (53 percent), compared with a PFM crown with metal occlusion (7 percent), a seven-eighths gold crown (11 percent), or a full-gold crown (1 percent) Christensen, 1986). This preference is likely because of the potential for increased wear if the porcelain surface loses its glaze or polish.
The success of any cemented restoration will depend on the strength and lack of solubility of the luting agent (cement), as well as the ability to achieve an extremely close fit between the tooth and restoration. A tight junction must be established between the restoration and the finish line of the preparation on the tooth. A space of only 50 microns between the restoration and tooth will result in a visible cement line. This cement line eventually will result in a defective seal that will permit progressive dissolution of the cement from beneath the restoration. When the cement dissolves, food particles, oral fluids, and bacteria can enter the defect and may cause caries in the supporting tooth (Zander, 1957).
There are limits to the use of PFM and cast metal restorations. For the most part, they are used only on permanent teeth in adults because the necessary removal of tooth structure for proper fabrication would threaten pulp vitality in children and even many young adults. Also, the restorations are costly, amounting to more than eight times the cost of amalgam.
Approximately 30 years ago, the term glass ceramic was given to certain formulations of porcelain which, by the controlled nucleation and growth of crystals at elevated temperature, fanned a polycrystalline material. Compared with feldspathic porcelain, the resultant material exhibited greater strength and toughness, a variable coefficient of thermal expansion, greater ease of fabrication, lower processing shrinkage, better translucency control, good thermal shock resistance, and excellent chemical durability. Its use in dentistry has expanded rapidly.
Dental porcelain and newer glass ceramics have a multitude of applications. Different types are employed in the construction of artificial denture teeth, full crowns, inlays, onlays, and laminate veneers, and as the esthetic veneer over a metal substructure for crowns and bridges. Although some porcelains and glass-ceramics have been considered for bridges, the failure rates to date have been unacceptably high.
Strength tests on the newer glass ceramics encouraged manufacturers to develop all-ceramic crowns for posterior teeth. However, the strength of all ceramic crowns is significantly less than that of porcelain-fused-to-metal (PFM) crowns. Thus, all-ceramic crowns should be restricted to lower-stress situations, such as the anterior teeth and in patients with smaller biting force and no history or evidence of bruxing. For posterior teeth, all-ceramic crowns should be considered only for low-stress conditions in which PFM and metal crowns are unacceptable.
Dental ceramics generally are used to restore extensively damaged, diseased, or fractured teeth because of their excellent esthetics, wear resistance, chemical inertness, and low thermal conductivity. In addition, they match the characteristics of tooth structure fairly well. Ceramic restorations represent one of the few esthetic choices for treatment of small-to-large defects in posterior teeth. Compared with glass ionomers, dental ceramics are more durable, less technique sensitive, and more predictable from an esthetic viewpoint, but they are more costly by a factor of more than six. Compared with office-produced composites (direct) and lab-processed composites (indirect), ceramics are more color-stable, higher in flexural strength, more resistant to abrasion, potentially more abrasive to opposing enamel, and again, over six times more costly. Compared with all-metal or ceramic-metal crowns, ceramic restorations are more esthetic, but generally have a shorter life expectancy.
Ceramic inlays, onlays, full crowns, and veneers have become popular alternatives over the past 5 years because of improvements in physical properties, cementation techniques, and an increased public demand for esthetic materials.
One of the problems encountered with the use of ceramic materials in dentistry has been their inherent brittleness and low tensile strength. A small flaw can enhance tensile stress that can initiate a crack and cause fracture of the restoration.
Although newer formulations are significantly stronger than earlier types of porcelain, data on clinical success are not available.
CAD/CAM�One of the potential uses of glass ceramics is in the production of machined inlays, onlays, and crowns by means of CAD/CAM systems, as described earlier. The precision of defining and machining the marginal area of CAD/CAM prostheses has been reported to be in the range of 0 to 250 m (Mörmann et al., 1987). Recent improvements in hardware and software have considerably improved the overall precision of this method; however, the technique is very demanding (Roulet and Herder, 1990). Rekow et al. (1991) stated that crowns produced using a CAD/CAM system can fit at least as well as those produced with ideal casting conditions.
Herder (1988) reported that postoperative pain occurred in 31 percent of the cases after inlays were cemented in place, even though marginal openings could be detected in only 0.3 percent of the cases overall. This pain disappeared in all cases within a period of 4 to 12 weeks. Of greater concern is the observation by Herder (1988) float, after 6 months, submargination (loss of material at the restoration-tooth junction) occurred in 19 percent of the interproximal sites and 50 percent of the occlusal sites, which was explained by the excessive wear of the resin cementation material (Roulet, 1987). In summary, inadequate long-term clinical data are available from controlled studies, and no data are available to indicate the performance of these inlays under routine private-practice conditions. CAD/CAM is not widely available and CAD/CAM restorations will likely be similar in price or slightly higher than metal-ceramic restorations.
Because the failure rates of all-ceramic restorations are relatively high, the esthetic demands for posterior restorations are not sufficient to recommend their general use in preference to metallic restorations, especially for molar sites. Metal ceramic restorations, which are indicated for moderate-to-high stress conditions, can be recommended when esthetics are of concern.
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