Dental ceramics represents one of the major structural materials in modern fixed prosthodontics. As a material in dental medicine, zirconium-dioxide was introduced in the 1970s when different types of coverage for dental implants were investigated (1). The first use of zirconium-dioxide in fixed prosthodontics was when small amounts of aluminum-oxide (Al2O3), in glass-infiltrated ceramic (In Ceram, Zahnfabrik Vita, Germany), were replaced by zirconium-dioxide crystals (ZrO2). Later on, the development of zirconium-dioxide was followed by the development and improvement of CAD/CAM technology, as the only commercial way for making restorations in fixed prosthodontics from this material. To understand the values of zirconium-ceramics and its excellent mechanical properties, the development of dental ceramics in general must be considered.
Most dental ceramics consist of an amorphous part and crystals. The amount and size of crystals determine the mechanical properties. The amorphous part predominantly consists of SiO2 (glass), which gives ceramics an esthetically pleasant and natural looking appearance (translucency), and insures chemical bond with resin cements. Basic types of ceramics, such as feldspathic ceramic, predominantly consist of glass, and only have small amounts of crystals, which cannot insure good mechanical properties and functional longevity for crowns and bridges in posterior region. Because of this, these ceramics have been veneered on metal framework, which insures mechanical stability while the esthetics of ceramic remains. Demands for better esthetics and natural looking appearance led to development of new ceramics with increased amount of crystals that could withstand greater forces and can be used as a single material without metal framework. These were glass-infiltrated ceramics, that are crystalline-based systems (mostly alumina, Al2O3) with added glass, and glass-ceramics, that are glass-based systems with added crystals. But, due to its limited mechanical properties, they could only be used in up to three-unit bridges in premolar region. Because of this, new polycrystalline ceramics, such as aluminum-oxide and zirconium-oxide ceramics were introduced. These ceramics consist only of crystals and do not have an amorphous part.
Zirconium-dioxide has excellent mechanical properties, with flexural strength of 900-1200 MPa and hardness of 1200 HV (2) (Table 1). These values are almost the same as of metals used for metal-ceramic restorations, and significantly higher than values of all other ceramics used in dentistry. Apart from the absence of an amorphous part, the reason for such good mechanical properties lies in the behavior of the zirconium-dioxide under applied stress. Unlike other ceramics, which develop cracks due to applied loads, leading to material fracture, after the crack formation in zirconium ceramics, those cracks are constricted and stopped and do not progress further.
|Fracture toughness KIC||MPa/m||7-10|
|High termal expansion||K-1||11x10-6|
Structure of zirconium-dioxide
To understand excellent mechanical properties of zirconium-dioxide the basic structure must be understood. Zirconium-dioxide is a polycrystalline material that can be found in nature in mineral that has monoclinic crystalline structure. By heating it up to 1170oC, zirconium-dioxide transforms into tetragonal phase; and by heating it up to 2370oC it transforms into the cubic phase. This transformation between crystalline phases is reversible and cooling causes the return into monoclinic phase. With the phase transformation, the volume of crystalline grains is changed, and exactly this volume change was used for getting excellent mechanical properties. To get the stable material that can be used in dentistry, and medicine in general, the zirconium-dioxide is stabilized by adding 3-5% Y2O3 in tetragonal phase. These small amounts of Y2O3 inhibit tetragonal to monoclinic transformation below 1170o allowing the presence of tetragonal phase at the room temperature. This tetragonal phase is only partially stabilized, and under stress applied to the surface can transform into monocline phase. That is why this material is called yttrium-partially-stabilized zirconium-dioxide (Y-PSZ) (3, 4). The zirconium-dioxide is stabilized in tetragonal phase, because in the tetragonal phase the material is white in color and has excellent mechanical properties. The phase stability and mechanical properties depends on crystalline grain size. Crystals which are too large cause unstable material and spontaneous transformation into monoclinic phase, whereas crystals which are too small inhibit transformation, leading to reduced fracture toughness (5). As mentioned, stress applied to the surface of ceramic causes crack formation leading to the fracture of the material. Unlike other ceramic systems, when cracks initiate in the zirconium-ceramic phase the transformation from tetragonal to monocline which occurs around the crack with the volume increase of crystalline grains of approximately 4.5%. This volume increase causes constriction of crack and hardening of the material (6-8) (Figure 1).
When introduced in medicine, zirconium-ceramic, with its excellent biocompatibility and white color was considered an ideal material for clinical practice (hip replacement). However, after the promising start, problems arose and hips started to fracture unexplainably (9, 10). It was discovered that sterilization causes “aging“ of material and spontaneous transformation from tetragonal to monoclinic phase, which weakens the material and causes fracture under very small loads. Concentration and distribution of yttrium-oxide, grain size, the amount of cracks present before clinical use, manufacturing process and finishing procedures all affect aging of zirconium-dioxide (7, 11).
The final production in dental laboratory or dental office, and the quality of material contributes the most to the functional longevity of zirconium-ceramic in the mouth. Due to absence of translucency, zirconium-dioxide cannot be used as a single material for dental restorations and needs veneering with feldspathic or glass-ceramic. For the use in fixed prosthodontics, zirconium-ceramic is manufactured individually or comes as industrially made final products such as posts or implant abutments. If custom made, zirconium-dioxide is used for crowns, bridges and supra-structures on teeth and implants, and manufactured using CAD/CAM technology. Today, most zirconium-ceramics are milled in pre-sintered phase, because the material is very soft, of chalk consistency, which simplifies the production, allowing easy milling without excessive wear of the burs and possible damage of the material. Apart from this, if sintered, zirconium-dioxide is produced using CAD/CAM technology, the mechanical properties of final restorations are poorer than the declared ones (12-14). This phenomenon occurs due to surface microcracks and fractures that appear after such treatment (15). The pre-sintered restorations are approximately 30% bigger, because sinterization causes volume shrinkage, and achieves equal tetragonal structure without cracks. Caution is required if the final sintered restoration needs to be adjusted to avoid crack formation before placing the restoration into the mouth, and transformation into monoclinic phase which could decrease its mechanical properties and cause fracture. This is important, because despite the fact that this phase transformation around the crack causes strengthening of the material, this phenomenon can occur only once. If it occurs before placing the restoration in the mouth, the zirconium-dioxide does not have ability for new transformation, and if new crack appears under occlusal loads the fracture could not be stopped any more and would lead to the fracture of the material (Figure 2). Consequently, zirconium-dioxide is treated differently than metal, using water-cooling and special burs. Sandblasting with Al2O3 under high pressure and using particles which are too large should be avoided, because it can also induce surface phase transformation (16).
The main problem of zirconium-dioxide restorations in clinical practice has become fracture of veneering ceramic or chipping. In general, chipping can be classified as minor and major chipping. Minor chipping does not compromise esthetics and function of the restoration, and in most cases requires only polishing or composite repair, while major chipping implies bigger fractures with zirconium framework exposed. The residual stresses in the veneer ceramic can be considered the most important factor in the chipping phenomenon, but the exact origin of these stresses is still unknown. Residual stresses can be compressive and tensile. Compressive stresses improve mechanical properties of material, while tensile stresses cause material fracture (17, 18). Thermal characteristics of zirconium framework and veneering ceramic and zirconium/veneer interface characteristics are considered the most important factors that influence generating of residual stresses in the veneer ceramic.
Zirconium-dioxide and ceramics used for veneering have different coefficient of thermal expansion (CTE). This mismatch in CTE is similar to metal-ceramic restorations, and was designed on purpose in order to achieve the similar mechanical properties. Despite that, laboratory results showed the presence of tensile stresses in the deep layers of veneer ceramic, unlike metal-ceramic restorations where only compressive stresses were recorded (19-22). Apart from this, zirconium-dioxide conducts heat slower than veneering ceramic (23, 24). As a result, heating and cooling processes during veneer firing can additionally influence thermal characteristic differences between these two materials and increase residual stresses within veneer ceramic. In vitro tests showed that when slow heating and cooling during firing of the veneer ceramic were applied, the restorations achieved better fracture resistance (24, 25). On the other hand, latest observations of stress distribution within veneer ceramic using focus ion beam nanotomography (FIB-nt), showed the presence of tensile stress in the deep layers of veneer ceramic submitted to slow cooling (18). Knowing the fact that tensile stresses lead to material fracture, these findings are contrary to previous in vitro results (24, 25). Apart from thermal characteristics, the interface between framework and veneering ceramic can also induce the presence of residual stresses. As mentioned, different surface treatments on zirconium framework can cause the change of phase and size of surface crystals. Apart from decreasing of mechanical properties, this transformation causes generation of stresses within framework surface that could secondly cause development of residual stresses in the veneer ceramic (26). Apart from this, monoclinic zirconium-dioxide has much lower CTE, which further influences the generating of stresses in these areas (27). In addition to surface changes, a good bond between framework and veneer insures homogenous structure and stress distribution from framework to the bond layer and finally to the veneer. If this bond is affected by different surface changes the stress is distributed directly to the veneer, causing increased stress, which can be up to 12 times bigger than in homogenous interface (28).
Repeated occlusal loads on the veneer during function, with generated residual tensile stresses can lead to crack formation and veneer fracture. To try to overcome this problem, new veneering ceramics and techniques have been proposed. Instead of veneering only with weak feldspathic ceramic, zirconium framework is veneered with glass-ceramic. Apart from this, the over-pressing technique has been introduced, where the whole veneer is over-pressed to the zirconium framework at once. This ceramics is industrially prepared, has always the same quality and the possible procedure mistakes during layering that can lead to potential flaws and artefacts which could then lead to veneer fracture are excluded (29). Apart from veneering ceramics and application technique, anatomically designed framework and veneer/framework thickness ratio, with lower ratio, also contribute to better clinical performance of zirconium-ceramic restorations and decreased chipping. Modification of zirconium framework design to anatomical shape leads to uneven thickness of strong framework and equal veneer thickness. Since the zirconium framework has excellent mechanical characteristics, they would not be greatly influenced by uneven thickness, but would insure uniform support for weak veneer ceramic and better clinical performance (30).
All these factors must be taken into account to minimize the failures of these restorations. Moreover, the newest techniques include CAD/CAM production of whole veneer from lithium-disilicate ceramics, which is then joined with zirconium framework. These restorations show much better mechanical properties compared to layered or over-pressed zirconium restorations (31, 32). The latest development in production of zirconium-dioxide is high-translucency (HT) zirconium-dioxide, which allows production of full-contoured monolithic restorations without need for veneer and risk of chipping (Figure 3). These restorations are stained to get better esthetics, but the development of HT zirconium still has not progressed sufficiently to achieve excellent esthetics, and therefore its indications are still in the posterior region, whereas for the anterior teeth the veneered zirconium-ceramic is used.
Clinical studies regarding reliability and longevity of zirconium-ceramic restorations are in the correlation with the in vitro studies (33-48). Fracture of the veneer ceramic is the main complication, but the incidence of major and minor chipping is hard to strictly determine since there is no standardized fracture categorization. On the other hand, zirconium framework fracture occurs very rare. Anatomical design of zirconium framework decreases the possibility of chipping and leads to better clinical performance (36, 38-40). Different veneering techniques also affect the reliability of zirconium restorations, with the greatest improvements when veneer ceramic was over-pressed to the anatomically designed zirconium framework (38). Nevertheless, the survival rate of zirconium-ceramic restorations is very high, because most of the reported veneer fractures are minor ones, and are easy to repair in the mouth without compromising the esthetics and function.
After great expectations from zirconium-ceramics, the clinical use has shown imperfections of this material. There is still no standardized technical procedure in manufacturing of fixed restorations, which leads to great discrepancies in reliability of zirconium-ceramics. From its introduction, it has undergone many improvements, but further enhancements are needed for it to become the gold standard in dental medicine.