Performance of dental ceramics: challenges for improvements

Digitization and its futuristic approach in prosthodontics

Digitization has become part and parcel of the contemporary prosthodontics with the probability of most of the procedures being based on the digital techniques in near future. Let us think of X-rays or photographs, making impressions, recording jaw movements or fabricating prosthesis, educating and training new dentists or patient motivation for practice build up, all has become digital. CAD-CAM has revolutionized not just the ceramic technology but has also been used for the CAD-CAM implant surgeries, maxillofacial prosthesis and diagnostic splints. Today a practicing dentist needs to be abreast with the latest but with the technology changing so fast, this poses a great challenge. There is endless scope of digitisation and technology in prosthodontics- let it be in the clinical and lab procedures like use of CAD-CAM technology, stereolithography, rapid prototyping, use of virtual articulators and digital face bows, digital radiographs, or in the field of training, education and research by the use of virtual patient programs, dental softwares, optoelectronic recording of jaw motion, digital instron machine, retention testing device, audiovisual aids,… the list will remain endless. The article reviews those various aspects of prosthodontics where digitization has modified the conventional procedures. For discussion they have been considered under the educational aspect, diagnostics, treatment procedures, prosthesis fabrication and lastly the research and futuristic development. The day is not far when remote sensing robotic devices would be performing the restorations under the command and surveillance of the master-the dentist without his immediate presence.

How and when does fabrication damage adversely affect the clinical performance of ceramic restorations?

OBJECTIVES

As compared to factory-processed ceramic parts, one unique trait of all-ceramic dental restorations is that they are custom-fabricated, which implies a greater susceptibility to fabrication defects. A variety of processing techniques is now available for the custom fabrication of all-ceramic single and multi-unit restorations, these include sintering, heat-pressing, slip-casting, hard machining and soft machining, all in combination with a final staining or veneering step. All these fabrication techniques, from shaping to firing, are associated with the production of flaws of various shapes and sizes, in conjunction with thermal residual stresses, all of which are capable of inducing failure.

METHODS

This review will examine the various types of fabrication damage inherent to each technique and attempt to establish a relationship between fabrication defects and clinical performance of all-ceramic dental restorations with particular attention to their longevity in vivo.

RESULTS

Failure mechanisms in dental ceramics can be very complex and often involve the combination of physical factors, to which are added patient and clinician-related variables such as restoration design and in vivo conditions.

SIGNIFICANCE

Tremendous progress has been made in understanding the failure mechanisms of all-ceramic dental restorations over the past thirty years. It remains that there is still a need for laboratory tests that usefully simulate clinical conditions.

Standardizing failure, success, and survival decisions in clinical studies of ceramic and metal-ceramic fixed dental prostheses

The recent increase in reports from clinical studies of ceramic chipping has raised the question of which criteria should constitute success or failure of total-ceramic prostheses. Terminologies such as minor chipping [1], partial chipping, technical complications [2,3], and biological complications have crept into the dental terminology and they have complicated our classification of success and failure of these crown and bridge restorations. Some journals have permitted the reporting of fractures as "complications" and they are not necessarily classified as failures in the study. One study has attempted to classify chipping fractures according to their severity and subsequent treatment [4]. This is a promising approach to resolve the challenges to the classification of chipping fracture. The term 'chipping fracture' is more descriptive than 'chipping' since the latter term tends to imply an event of minor consequence. Two types of statistics are reported routinely in these studies, i.e., percent success, which is a measure of restorations that survive without any adverse effects, and percent survival, which is a measure of all restorations that survive even though they may have exhibited chipping fracture or they may have been repaired. Why has this scenario occurred? One possible explanation is that many of these types of fractures are very small and do not affect function or esthetics. Another reason is that corporate sponsors prefer to use the term chipping since it does not connote failure in the sense that the term fracture does. In any event, we need to be more precise in our scientific observations of fracture and classifications of the various types of fracture including details on the location of fracture and the prosthesis design configuration. Because of the lack of standardized methods for describing chipping fractures, materials scientists are unable to properly analyze the effect of material properties and design factors on the time-dependent survival probability of ceramic fixed dental prostheses (FDPs). Based on the review of clinical trials and systematic reviews of these trials, the present study was designed to develop guidelines for classifying the functional performance, success, survival, and susceptibility to chipping fracture, and subsequent treatment of ceramic and metal-ceramic restorations.

OBJECTIVE

To develop comprehensive descriptive guidelines and a clinical reporting form to assist dental scientists in their analyses of chipping fracture behavior of metal-ceramic and all-ceramic prostheses with particular emphasis on veneered-zirconia restorations. These guidelines are required to optimize the recording of fracture features that can be used to differentiate ceramic chipping fracture from bulk fracture and to assist dentists in identifying subsequent treatment that may minimize the need to replace affected restorations. A recording form for clinical fracture observations must be sufficiently clear and complete so that dental health professionals can translate the most relevant information in a context that allows their patients to fully understand the potential risks and benefits of treatment with ceramic restorations. It should clearly allow a clinician to determine whether or not a ceramic fracture constitutes a failure, which requires replacement of the prosthesis, or whether the fracture surface is relatively small or located in a nonfunctional area, i.e., one that is not contribute to occlusion, esthetics, proximal contacts, or food impaction. To accomplish this task, a review of the relevant publications of clinical trials was necessary to identify the variability in reporting of fracture events. The reviews were focused on clinical research studies of zirconia-based FDPs and PFM FDPs, which had been monitored through recall exams for three years or more. These reports and systematic reviews of all relevant publications were published in English dental journals between 2004 and 2010.The primary focus in this review was on the susceptibility to chipping fracture or bulk fracture of veneered zirconia-based fixed dental prostheses (FDPs) and metal-ceramic FDPs, which are also referred to in this paper as porcelain-fused-to-metal (PFM) FDPs.

Reliability of reduced-thickness and thinly veneered lithium disilicate crowns

The present investigation hypothesized that the reliability of reduced-thickness monolithic lithium disilicate crowns is high relative to that of veneered zirconia (Y-TZP) and comparable with that of metal ceramic (MCR) systems. CAD/CAM first mandibular molar full-crown preparations were produced with uniform thicknesses of either 1.0-mm or 2.0-mm occlusal and axial reduction, then replicated in composite for standard crown dies. Monolithic 1.0-mm (MON) and 2.0-mm CAD/CAM lithium disilicate crowns, the latter with a buccal thin veneer (BTV) of 0.5 mm, were fabricated and then sliding-contact-fatigued (step-stress method) until failure or suspension (n = 18/group). Crack evolution was followed, and fractography of post mortem specimens was performed and compared with that of clinical specimens. Use level probability Weibull calculation (use load = 1,200 N) showed interval overlaps between MON and BTV. There was no significant difference between the Weibull characteristic failure loads of MON and BTV (1,535 N [90% CI 1,354-1,740] and 1,609 N [90% CI 1,512-1,712], respectively), which were significantly higher than that of Y-TZP (370 N [90% CI 322-427]) and comparable with that of MCR (1,304 N [90% CI 1,203-1,414]), validating the study hypothesis.