Why do ceramics have a low fracture toughness?
Resilience of ceramics - causes of fracture losses
In contrast to metallic or polymeric materials, dental ceramic restorations suffer from their extreme brittleness and are therefore very prone to fractures and chipping. With the introduction of zirconium oxide as a high-strength and fracture-resistant framework material, results can be achieved that are equivalent to the clinical success of metal-supported restorations in terms of load-bearing capacity and service life.
Despite the high-strength properties of dental ceramic restorations, fractures are still cited as the most common cause of failure. Chippings are particularly common in veneering ceramics on zirconium oxide frameworks. The article uses clinical examples to explain various causes such as production-related factors and gives recommendations for processing suitable for ceramics. To this day, metal-supported restorations are the method of choice for indications in the posterior region that bear chewing loads. Dental ceramics are used as veneering material. In addition to veneering, their suitability was limited to single crown restorations, inlays and veneers. The silicate, amorphous origin of these ceramics did not allow any more extensive indications, especially under shear or tensile loads. In proven, metal-ceramic systems, the annual rate of veneer fractures between 0 and 4% after 2–7 years has been calculated.1 Attempts to manufacture all-ceramic crowns or even bridges for the posterior region from conventional silicate ceramics mostly failed due to the inadequate load-bearing capacity. Clinically, high survival rates have been reported for dental ceramics, especially in the inlay area. An earlier review article on the quality of CEREC (Sirona, Germany) inlays reports a survival rate of 97.2% after 4.2 years of observation.2 A more recent publication reports a 90% survival rate of CAD / CAM-fabricated inlays and onlays after 10 years.3 Prospective, clinical studies on the named indications showed survival rates between 93.7% after 6 years4, 90.4% after 10 years5, 95% after 11.5 years6 or 86% after 12 years of observation.7 Likewise, in studies with high case numbers of 2,328, 1,588 or 1,010 inlays / onlays, survival rates of 95.5%, 97% and 84.9% were found after 9 years, respectively8, 10 years9 or 11.8 years10 recorded. The restoration fracture emerged as one of the most common causes of clinical failure (in addition to secondary caries).11 Fractures are recorded especially in the early stages or after long periods of wear.7 In order to find causes via the fracture mechanism, an attempt was made, among other things, to correlate clinical long-term experience with a leucite-reinforced glass ceramic (Empress, Ivoclar, Liechtenstein) with experimental life time predictions from the laboratory.12 The degradation (or corrosion) of silicate ceramics under fatigue stress was identified as a decisive criterion for late clinical fractures, while grinding processes are often responsible for early failure.
Metal-free single crown restorations can be made either as a uniform restoration from a monolithic ceramic or in conjunction with a high-strength ceramic framework. For the chewing load-bearing posterior area, the framework-supported systems made of lithium disilicate ceramic, aluminum oxide or zirconium oxide have proven to be more effective. In this way, all-ceramic systems increasingly achieve a fracture resistance comparable to that of metal-borne systems. A systematic review article on controlled prospective and retrospective clinical studies assessed the long-term success of all-ceramic compared to metal single crown restorations and found comparable survival rates of 93.3% and 95.6% after 5 years, respectively.13 Furthermore, the authors made a classification according to the materials used. In particular, the systems made of densely sintered aluminum oxide (Procera, Nobel Biocare, Sweden) and leucite-reinforced glass ceramic (Empress, Ivoclar, Liechtenstein) showed no significant differences to metal ceramics. Lower life expectancies were observed for infiltrated oxide ceramics (InCeram Alumina / Spinell, VITA, Germany) and non-particle-reinforced, glass-ceramic crowns in the posterior region. The most common cause of fracture for all-ceramic crowns was the complete restoration fracture followed by chippings in the veneer. Chippings were observed less frequently on all-ceramic restorations than on their metal-supported counterparts. Zirconium oxide (ZrO2) has experienced a real boom in the last ten years as a completely bio-inert, high-strength ceramic for extensive bridge constructions in the chewing load-bearing area of the posterior teeth due to the massive progress in CAD / CAM technologies. The use of zirconium oxide for single crown restorations has been less researched precisely because of its outstanding properties in terms of strength and toughness (and thus the preferred use in all-ceramic bridge prosthetics). Nevertheless, clinical studies also show an outstanding survival rate of 100% after 214 or 315 Years. Another study, albeit with a small number of cases, showed a survival rate of 93.4% after 2 years for zirconium oxide-supported single crowns16. However, if the comparison between metal-supported and all-ceramic dentures is extended to three or more freely supported bridge units, a higher survival rate and reliability of the metal-supported systems (94.4% after 5 years) compared with all-ceramic dentures (88.6 % after 5 years).17 The significant difference has its origin in the still insufficient strength of the framework ceramics used. A particularly large number of framework fractures of the infiltrated oxide ceramics or glass ceramics used were recorded here. In contrast, promising clinical success has been observed with the use of zirconia as a framework material. In more recent studies, survival rates of 97.8%18 or even 100%19-21 to be reported. However, even the use of zirconium oxide is not a panacea for fractures.The prerequisite for the success of all-ceramic restorations is strict adherence to preparation and processing guidelines, some of which differ considerably from the usual handling of metal-ceramic. For example, with optimal production, edge design and compliance with the connector dimensions, there are no fractures of the ZrO2- Scaffolding recorded. Figures 1a – c illustrate improper handling of zirconium oxide, which led to a framework fracture.22 Laboratory tests on the edge strength of veneered zirconium oxide frameworks also show that chipping in the veneer is the problem and less the delamination of the supporting framework. The authors of clinical studies report a frequent occurrence of chipping fractures on zirconia-supported bridge structures.18,20,21 Chipping is a criterion that does not necessarily have to lead to a replacement (and in many cases can be repaired with plastic), but this cannot be the requirement for a high-quality, aesthetic and, moreover, expensive restoration.
Ceramic material properties and fracture mechanisms
Dental ceramics can be divided into three groups based on their composition (see Fig. 2a-c):
1. Silicate ceramics are made up of quartz, feldspar and alumina, with the amount of alumina being extremely low due to the translucency required. Silicate ceramics always consist of an amorphous glass and a crystal phase. Although the translucency of the glass phase is aesthetically advantageous, it is more susceptible to mechanical and chemical loads than the crystal phase. Silicate-based ceramics can be etched, silanized and adhesively processed with hydrofluoric acid.
2. Polycrystalline oxide ceramics made from Al2O3 or ZrO2 have almost no amorphous glass phase, but rather densely packed, crystalline grain structures consisting of single-phase, one-component metal oxides. These materials have enormous strength and high fracture toughness and are therefore preferred as framework ceramics. Since zirconium oxide in particular has a diamond-like hardness, the sintered, “chalk-like” green compacts are shaped in the CAD / CAM process and then densely sintered. Oxide ceramics can neither be etched with hydrofluoric acid nor silanized.
3. Glass-infiltrated oxide ceramics refer to porous oxide ceramic frameworks which are subsequently infiltrated with a special lanthanum glass. After completion, they contain crystalline oxide-ceramic crystals and amorphous glass structures. These materials are available based on spinel, alumina or zirconium oxide. The advantage is the easier CAD / CAM shaping due to the soft, porous framework material and subsequent stabilization with glass. The amorphous glass part can be etched and processed using adhesives.
Each ceramic has different physical and mechanical properties depending on its microstructure and composition. What they all have in common is extremely low flexibility (and thus high brittleness), which can lead to spontaneous fractures if a certain load limit is exceeded (see Fig. 3a-c). Clinically, however, fractures are also recorded in the veneering or framework ceramics, although the average masticatory forces even in the chewing load-bearing area of the posterior teeth are well below the fracture strength of the ceramics used.23 Since ceramics are very resistant under normal occlusal loads, it is very likely that other factors contribute to triggering intraoral fractures anyway. For example, incorrect firing can lead to internal stresses which then exert additional stress on the restoration. The breaking strength of a restoration is also significantly reduced by introducing defects into the surface (grinding, see Fig. 4) or into the microstructure close to the surface (bubbles, pores, impurities).24 A load (e.g. chewing load) below the tolerable (critical) material strength is referred to in the technical batch as "subcritical" load. Such subcritical loads do not lead to spontaneous fractures, but rather weaken a restoration through constant, e.g. cyclical, fatigue loading.25 In a brittle material such as dental ceramics, these forces cause cracks close to the surface to be generated and further, slow propagation of such cracks into the interior of the ceramic, leading to fracture. The ability of a ceramic to withstand such subcritical crack propagation then also determines the frequency of late-occurring fatigue fractures and thus ultimately the service life of a restoration in the patient's mouth.12 Since cracks in the ceramic have spread and increased, the material strength is of course also reduced after fatigue loading and it can then lead to spontaneous breakage of a restoration under normal chewing load. Crack propagation is supported by the corrosive action of water, especially in silicate ceramics such as glass and feldspar ceramics.26 A high proportion of glass or a low, crystalline proportion in the microstructure favors this mechanism in a humid environment. Such ceramics are often used as veneering material, as the high proportion of glass ensures good translucency and aesthetic appearance. These materials are generally less mechanically stable than oxide-ceramic framework materials such as zirconium oxide or aluminum oxide. It is therefore understandable that the use of such materials without a supporting framework leads to a tendency towards greater susceptibility to breakage (Fig. 5a and b).
Glass or feldspar ceramics as well as densely sintered aluminum or zirconium oxide ceramics consist either partially of crystallites, embedded in a vitreous, amorphous matrix (Fig. 2a and b), or completely of crystal grains (Fig. 2c). Although crystallites have a similar chemical composition to the surrounding matrix, they differ in their physical properties due to their crystal structure. This effect is used specifically to optimize strength and fracture toughness. Particularly toughness-increasing measures (e.g. crack deflection, Fig. 6) are of great value for such brittle ceramics.27 The mechanical properties can be set in a targeted manner via factors such as the crystallite shape, size, concentration, spatial distribution or also through different thermal expansion.28 Measures to increase toughness are particularly effective in the fully crystalline materials aluminum or zirconium oxide and achieve higher mechanical properties compared to silicate materials.29 Materials with little or no glass content are therefore also preferably used as framework materials. Attempts to increase the strength of the glass-infiltrated oxide ceramic InCeram by replacing the aluminum oxide grains (InCeram Alumina, VITA) with zirconium oxide crystallites (InCeram Zirconia, VITA) were not very successful, as the infiltrated glass phase continued to exist and was the weakest part of the material in which it was located Cracks could still spread.29 In densely sintered zirconium oxide, the crystallites are not distributed in a glass matrix, but reinforce each other. For dental applications, zirconium oxide is alloyed with small amounts (2–5 mol%) of cerium oxide or yttrium oxide in order to be able to adjust the size and type of crystal at room temperature (yttria stabilized tetragonal zirconia polycrystal [Y-TZP]). The preferred crystalline, tetragonal structure (t) of zirconium oxide can be seen in Fig. 1c, and is stabilized by the yttrium oxide at temperatures above 1,140 ° C and remains metastable on cooling (at room temperature zirconium oxide would normally be in monoclinic crystal form [m] ). This leads to tensions in the structure, which can be released when cracks spread. Then the spontaneous transformation of the tetragonal to monoclinic crystallites takes place on the crack surface. This phase transformation is associated with an increase in volume of 4–5%. This effect is used specifically to increase toughness and is unique in zirconium oxide. In principle, energy is released at the tip of a spreading crack, which initiates the spontaneous transformation and thus the increase in volume at the crack flanks, which then counteract further crack propagation through increased shear and compressive stresses in the vicinity of the crack tip.30 Furthermore, the expansion of the grains produces small micro-cracks at the grain boundaries, which also consume energy. The property of spontaneous phase transformation makes zirconium oxide, as a highly fracture-resistant ceramic (KIc = 10MPam0.5), very reliable, which suggests its use as a load-bearing material under extensive bridges and has meanwhile been confirmed by clinical studies.
Despite the excellent mechanical properties of zirconium oxide, more and more reports of chipping fractures in the veneers of a zirconium oxide ceramic are reported (Fig. 7a – f). In laboratory experiments it could be shown that the main cause of failure when using zirconium oxide is to be found in chipping fractures, while e.g. lithium disilicate ceramics do not show this.31 If a crack spreads from the surface of a veneering ceramic (low modulus of elasticity, low fracture toughness) into the interior and hits the boundary with the ceramic framework (high modulus of elasticity, high fracture toughness), it is either stopped or deflected at the interface.32,33 The complete fracture of the zirconium oxide framework is extremely rare, because it requires extremely high forces that are well above the normal occlusal forces.31 The chipping phenomenon in zirconium oxide ceramics can also be explained by the development of thermal stresses in the veneering ceramic. Different thermal properties (thermal expansion, thermal conductivity) can lead to the build-up of these tensions during the firing process. This can be very significant, especially if the coefficients of thermal expansion (CTE) of the framework and the veneer differ significantly.34 In general, the compressive stresses in the veneering ceramic are beneficial for the prevention of fractures and are caused by different CTEs. A coefficient of expansion is selected for the veneer that is slightly reduced compared to the ceramic framework. As the sintering temperature rises, the framework and veneer are heated equally until the glass melts.In the event of a positive thermal difference (CTE (veneer)
The effect of the increase in strength is mainly based on the internal thermal stresses introduced, especially the compressive stresses close to the surface. If a crack should propagate through the compressive stress zone, it can spread much more easily inside the tensile ceramic veneer, which inevitably leads to a fracture. The crack can develop towards the interface with the zirconium oxide or it can be deflected and spread through the veneer, which then manifests itself as chipping. For a zirconium oxide veneering ceramic composite, it was possible to show that the maximum stresses develop when the temperature is rapidly cooled from 20 ° C above the glass transition temperature.36 This makes it clear that by controlling the cooling process in particular during the sintering process, the susceptibility to fractures and thus the clinical lifespan can be influenced. The very thin surface zone, which is under compressive stress, thus serves as protection for the areas underneath, which are under tensile stress and therefore weaker. The removal of the surface layer or exposure of the underlying areas can be brought about in two different ways: by contact abrasion in the mouth (Figs. 7a – f and Figs. 8a and b) or by adjusting the occlusion intraorally (Fig. 9a–) c). While the former occurs over years in the patient's mouth due to the natural chewing process (or accelerated in the case of non-physiological bruxism stress), in the latter the protective layer is removed in minutes and the restoration is weakened. Both rotating processing and natural abrasion not only reduce the resilience of a restoration, but also create new cracks due to the increasing roughness, which can spread and cause fractures (Fig. 8a and b).24 In fact, it could also be shown clinically in a prospective study over twelve years that the causes of fractures are to be found in the initial grinding or in the abrasion process.7,12
Strategies for the prevention and avoidance of ceramic fractures
Due to the described characteristics of brittle ceramics and zirconium oxide and due to the clinical experience gained to date in handling the material, recommendations for handling ceramics-friendly could be defined (further information is also available at www.ag-keramik.eu). Ceramic fractures or chippings in the veneer can be minimized by considering the following criteria throughout the entire production chain of a restoration (manufacturer - laboratory - practice):
- Pay attention to the contraindications for all-ceramics: bruxism, parafunction, missing anterior canine guidance, cover / deep bite, temporomandibular joint problems, loosened teeth, inadequate oral hygiene, etc. (practice).
- Choice of flawless starting materials from certified manufacturers, both as framework and veneering materials (manufacturer / laboratory).
- For extensive restorations in the chewing-loaded posterior region, only the materials indicated for this need to be selected (preferably high-strength, high-toughness materials; laboratory / practice).
- Coordination of framework and veneering materials with regard to adapted thermal expansion in order to avoid tension in the manufacturing process (recommendation: stay in the system; laboratory).
- Relaxation cooling (slow cooling after sintering the veneering ceramic), especially when using zirconium oxide to avoid internal tension in the veneering ceramic (risk of chipping; laboratory).
- Ceramic-compatible preparation with regard to minimum layer thicknesses, transition angles (inner angles and coronary stump edges are to be rounded, design as right angles as possible) and connector dimensions. The anatomical shape of a crown cap or a bridge framework is recommended in order to achieve an even veneer layer (practice).
- Preparation of steps and chamfers, no flat chamfers, tangential preparations and bevels (practice).
- Avoid extensive grinding of the framework and the inner lumen without water cooling (especially with coarse-grain diamond grinders) or blasting of the ceramic surfaces with too high a blasting pressure or too coarse blasting media (laboratory / practice).
- If machining has to be carried out, only fine diamond tools with water cooling or sandblasting with fine grain sizes (35 µm) and gentle pressure (<1.5 bar) should be used (laboratory / practice).
- Try-in is recommended before veneering or before glaze firing (practice).
- If possible, a cohesive, adhesive cementation is preferable to conventional cementation (practice).
- Observance of functional conditions combined with repeated follow-up checks of the occlusion after integration (practice).
- Final coating by polishing or by additional glaze firing to increase the service life of a restoration in the mouth. Glaze firing is preferable to a final polish (practice).
- The service life of a ceramic restoration can be increased by regular follow-up checks for signs of abrasion and subsequent polishing (practice).
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