1

Introduction

The survival, and to a certain extent, the success of clinical prostheses with a ceramic component are reported as probabilities. Only in very rare circumstances will restorations exhibit 0% successes and 100% failures over a period of 5 years or more. In general, restoration success is defined as the demonstrated ability of a restoration (including a prosthesis) to perform as expected. Restoration failure may be defined as any condition that leads to replacement of a prosthesis. Conditions that constitute restoration failure include secondary caries, irreversible pulpitis, excessive wear of opposing tooth surfaces, excessive erosion and roughening of the ceramic surface, ditching of the cement margin, unacceptable esthetics, cracking, chipping fracture, and bulk fracture. For the present paper, only fracture-related phenomena are considered.

A recent study of the failure of ceramic-based FDPs confirmed a previous estimate that less than 85% or more of fixed dental prostheses were still functional after 10 years in service. However, there was considerable variability in the number of parameters that were reported as well as the extent of details on the failures that had occurred. Even for studies in which a standardized evaluation system such as the USPHS criteria or modified Ryge criteria was used, there was great uncertainty in the determination of whether or not repairable fractures were failures or merely “technical complications.” In addition there was uncertainty on whether the identified causes were directly or indirectly associated with the treatment procedures, for example, during replacement of previous prostheses . Based on this previous review, it was concluded that there is an urgent need to develop a comprehensive classification system for identifying clinical prosthesis failures, technical complications, and biologic complications. The authors also recommended that procedures for making impressions that capture the fracture surface details be clearly described to facilitate fractographic analysis and identification of the most likely cause of fracture.

2

Limitations of clinical studies of ceramic-based prostheses

There are many ceramic materials that can be used for single-unit or multiple-unit restorations. The so-called “gold standard” for performance of ceramic restorations is the metal–ceramic or PFM restoration. Survival estimates for PFM fixed dental prostheses are typically about 97% over seven years or more. Thus, an excellent measure of the performance of all-ceramic prostheses is a clinical trial in which a well-established PFM system is used for control restorations. Unfortunately, very few studies of this type have been reported. Nevertheless, it is important to evaluate the potential significance of the outcome variables that have been reported.

Because of the multifactorial nature ceramic–ceramic and metal–ceramic restorations, researchers may conclude that it is futile to seek correlations between in vitro fracture data with clinical performance data. However, a significant reason why these correlations are not fruitful is that details on the ceramic fractures that occurred in clinical trials are inadequately described. Essential information on cracks, chipping fractures, and bulk fractures (size, location, patient characteristics, and clinicians’ criteria for judging success or failure) are generally lacking in these publications. To address this situation there is a major need to establish reporting guidelines for these clinical trials by starting with a comprehensive list of the details of clinical fractures, including the product description, prosthesis design and dimensions, supporting substructure (dentin, implant abutment, etc.), type of fracture, location, and appearance, and the occlusion conditions under which the failures occurred. Once this information becomes available, designs for well-focused in vitro studies can be established that attempt to replicate the types of ceramic failure that have been observed .

How should we define clinical success and why is it important to define clinical success from a dental materials perspective? Success may be defined as the intact survival of a prosthesis with acceptable surface quality, anatomic contour, and function, and where applicable, with acceptable esthetics. Since the probabilities of success over time are rarely 100% for established prosthodontic treatments, an important question to address is which of the many variables that affect outcomes should be included in classifying the performance of ceramic–ceramic and metal–ceramic restorations?

Obviously, the answer to this question cannot be addressed completely in this paper without soliciting feedback from the research community on proposed guidelines for describing fracture characteristics more comprehensively. Nevertheless, this article will lead us to acquire a more detailed description of fractures that involve zirconia-based FDPs and PFM FDPs that will allow us to answer many pressing questions. Does the ceramic veneer for zirconia frameworks fracture more frequently than PFM veneers? Does a press-on veneer fracture less frequently than manually layered ceramic veneer? How does the severity of chipping fracture affect survival outcomes? Are we overestimating the survival probabilities of zirconia/veneer restorations because of potential investigator bias in industry-supported studies and clinical research that is being performed in the ideal environments of academic institutions? There is some evidence of underreporting of the type, frequency, and severity of adverse effects, such as chipping fracture, bulk fracture, and surface degradation of ceramics, which have been reported in studies that have been supported by corporate sponsors .

It seems prudent to develop a more detailed reporting system for describing fractures of ceramic-based restorations by reducing the variability in reporting of survival outcomes in this field of study. The development of a standardized reporting format for these clinical trials with a refinement of descriptors and terminology that are currently used in peer-reviewed publications should greatly improve our ability to analyze the outcomes of these studies more critically.

2

Limitations of clinical studies of ceramic-based prostheses

There are many ceramic materials that can be used for single-unit or multiple-unit restorations. The so-called “gold standard” for performance of ceramic restorations is the metal–ceramic or PFM restoration. Survival estimates for PFM fixed dental prostheses are typically about 97% over seven years or more. Thus, an excellent measure of the performance of all-ceramic prostheses is a clinical trial in which a well-established PFM system is used for control restorations. Unfortunately, very few studies of this type have been reported. Nevertheless, it is important to evaluate the potential significance of the outcome variables that have been reported.

Because of the multifactorial nature ceramic–ceramic and metal–ceramic restorations, researchers may conclude that it is futile to seek correlations between in vitro fracture data with clinical performance data. However, a significant reason why these correlations are not fruitful is that details on the ceramic fractures that occurred in clinical trials are inadequately described. Essential information on cracks, chipping fractures, and bulk fractures (size, location, patient characteristics, and clinicians’ criteria for judging success or failure) are generally lacking in these publications. To address this situation there is a major need to establish reporting guidelines for these clinical trials by starting with a comprehensive list of the details of clinical fractures, including the product description, prosthesis design and dimensions, supporting substructure (dentin, implant abutment, etc.), type of fracture, location, and appearance, and the occlusion conditions under which the failures occurred. Once this information becomes available, designs for well-focused in vitro studies can be established that attempt to replicate the types of ceramic failure that have been observed .

How should we define clinical success and why is it important to define clinical success from a dental materials perspective? Success may be defined as the intact survival of a prosthesis with acceptable surface quality, anatomic contour, and function, and where applicable, with acceptable esthetics. Since the probabilities of success over time are rarely 100% for established prosthodontic treatments, an important question to address is which of the many variables that affect outcomes should be included in classifying the performance of ceramic–ceramic and metal–ceramic restorations?

Obviously, the answer to this question cannot be addressed completely in this paper without soliciting feedback from the research community on proposed guidelines for describing fracture characteristics more comprehensively. Nevertheless, this article will lead us to acquire a more detailed description of fractures that involve zirconia-based FDPs and PFM FDPs that will allow us to answer many pressing questions. Does the ceramic veneer for zirconia frameworks fracture more frequently than PFM veneers? Does a press-on veneer fracture less frequently than manually layered ceramic veneer? How does the severity of chipping fracture affect survival outcomes? Are we overestimating the survival probabilities of zirconia/veneer restorations because of potential investigator bias in industry-supported studies and clinical research that is being performed in the ideal environments of academic institutions? There is some evidence of underreporting of the type, frequency, and severity of adverse effects, such as chipping fracture, bulk fracture, and surface degradation of ceramics, which have been reported in studies that have been supported by corporate sponsors .

It seems prudent to develop a more detailed reporting system for describing fractures of ceramic-based restorations by reducing the variability in reporting of survival outcomes in this field of study. The development of a standardized reporting format for these clinical trials with a refinement of descriptors and terminology that are currently used in peer-reviewed publications should greatly improve our ability to analyze the outcomes of these studies more critically.

3

Data and data analysis: complexities of multifactorial clinical trials

In medical trials, Taylor et al. challenged the methods used to define the clinical success of surgical revascularization of the lower extremities that has been traditionally limited to graft patency or limb salvage, but the definition failed to consider other intuitive measures of importance. Clinical success was redefined as achieving all of the following criteria: (1) graft patency to the point of wound healing; (2) limb salvage for one year; (3) maintenance of ambulatory status for one year; and (4) survival for six months. Despite achieving acceptable graft patency for 72.7% of the subjects and limb salvage for 73.3% of the subjects at 36 months, the clinical success was only 44.4% when outcome data for all four defined parameters were considered. Thus, in spite of achieving “acceptable graft patency and limb salvage”, fewer than half of the patients achieved success when using a definition that included multiple parameters. In a similar manner, we can define the success of dental restorations on the basis of multiple variables rather than survival percentages alone.

Although the adverse effects of ceramic-based restorations are relatively minor compared to the “quality of life” issues described above, we can draw a parallel when reporting the overall success versus survival of our restorative treatments. However, we need a quantitative rating scale for clinical fracture resistance such as the following one:

• (1)

  Superior performance: Survival of all FDPs (100%) for at least five years and a success rate of 95–100% (surviving FDPs minus altered FDPs based on two of the three grades scale of chipping fractures ).

• (2)

  Excellent performance: Survival of 95–100% all FDPs for at least five years and a success rate of 90–95%.

• (3)

  Good performance: Survival of 90–95% of restorations for at least five years and a success rate of 90–95%.

• (4)

  Poor performance: Survival of less than 90% of restorations or a success rate of less than 90%.

For evaluations in clinical trials, investigators have used the traditional four-point scales of the California Dental Association or that used by the U.S. Public Health Service that generally list two categories (Alpha and Bravo and or Romeo and Sierra) for acceptable performance, and two categories for unacceptable performance (Charlie and Delta or Sierra and Tango, respectively). However, to assist biomaterials scientists and dental practitioners with decision-making that is relevant for prosthetic treatments, much more specific information is needed to ensure that our probabilities of success or failure are reasonably accurate and reproducible from one population to another.

A more “sensitive” list of criteria for evaluating the performance of restorative materials has been proposed as compared with those developed by the California Dental Association (CDA) in 1973 and published in 1977 , and those later described by Ryge . The CDA system was set up to evaluate restoration quality, but it also covered 14 other components of dental care including history and clinical examination, radiographic examination, diagnosis, treatment planning, and all other aspects of clinical dentistry. Hickel et al. seem to have ignored or overlooked these broad-ranging objectives of the CDA system that covered many more variables than restoration quality.

Hickel et al. criticized the insensitivity and the lack of rigor of previous approaches for evaluating the performance of restorative materials in clinical studies. Instead, these investigators proposed that future clinical studies on new materials and procedures be conducted as randomized, controlled trials with a clear hypothesis and protocol description to allow meta-analyses to be performed. In addition, they proposed three categories (esthetic, functional, and biological) to simplify clinical evaluation procedures and to encourage a more detailed analysis of failures. Of greatest significance in this report is the final recommendation that journal editors allow sufficient space for authors of RCTs to report all of the relevant data as outlined in this proposal. As stated earlier, the lack of detailed information on technical complications such as chipping fractures, refinishing, repairs, and bulk fractures represents a major deficiency of previous reports on performance and survival of ceramic and metal–ceramic restorations. Thus, this recommendation should receive the greatest attention in the future. However, the specific allocation criteria for esthetic, functional, and biological properties outlined in Table 2 of their report fall far short of the types of details that are needed to more clearly identify the causes of chipping fracture or bulk fracture of ceramic and metal–ceramic prostheses. Although one of the categories of criteria for clinical observation is focused on “fractures and retention,” there are insufficient descriptive details to ensure that information such as prosthesis dimensions, evidence of bruxing, occlusion, grinding damage, specific location and size of each crack, chip, or fracture will be captured. On the positive side, their criteria included a category called “patient’s view”, for which a rating of a “clinically poor” restoration could lead to a replacement simply solely because the patient may be completely dissatisfied with the restoration.