Zirconia, particularly tetragonal yttria-stabilized zirconia (Y-TZP), has become a preferred material in dentistry due to its superior mechanical properties, including high flexural strength, fracture toughness, and biocompatibility. In crown restorations, the use of monolithic zirconia crowns (without veneering porcelain) has gained increasing popularity, as it eliminates the common problem of veneer chipping.
However, for zirconia restorations to function successfully over long periods in the oral environment, the material must withstand occlusal loading as well as long-term degradation caused by moisture, temperature fluctuations, and thermal cycling. One of the most critical aspects in this context is the fracture resistance of zirconia and how factors such as structural thickness and artificial ageing influence its performance.
A study conducted by a Faculty of Dentistry UGM student, Mega Cicilia, under the supervision of drg. Heriyanti Amalia K., S.U., Sp.Pros(K), and drg. Murti Indrastuti, M.Kes., Sp.Pros(K), entitled “Pengaruh Ketebalan Oklusal dan Lama Ageing Mahkota Zirkonia Monolitik terhadap Ketahanan Fraktur”, concluded that occlusal thickness has a significant effect on fracture resistance, whereas the duration of ageing did not demonstrate a significant influence.
This article reviews the characteristics of zirconia as a restorative material, factors affecting fracture resistance (including thickness and ageing), and their clinical implications.
Characteristics of Zirconia as a Dental Material
Zirconia, particularly Y-TZP, is widely used in dentistry due to the following properties:
- Resistance to crack propagation, achieved through a tetragonal-to-monoclinic phase transformation around crack tips that absorbs energy.
- High flexural strength compared to many other dental ceramics.
- Biocompatibility and relative inertness in the oral environment.
- Improved translucency in newer generations, making it suitable for aesthetic restorations, although it remains more opaque than glass ceramics.
Despite these advantages, zirconia is not immune to external factors such as cyclic loading, structural degradation (ageing), and fluctuations in temperature and humidity. Consequently, restoration design (thickness and geometry), tooth preparation, surface finishing, and artificial ageing protocols are critical in ensuring long-term fracture resistance.
Factors Affecting Fracture Resistance of Zirconia
Several key factors influence the fracture resistance of monolithic zirconia crowns:
- Structural Thickness (particularly occlusal thickness)
UGM research demonstrated that greater occlusal thickness (1.5 mm) produced the highest mean fracture resistance (47.33 ± 0.93 N/mm²), while thinner restorations (0.5 mm) showed the lowest values (21.52 ± 1.33 N/mm²).
Adequate thickness allows for more even stress distribution and reduces local stress concentrations that may initiate cracks. - Artificial Ageing / Thermal Degradation
Ageing processes (e.g., autoclaving or thermal cycling) may induce tetragonal-to-monoclinic transformation at the zirconia surface, potentially weakening mechanical properties. However, in this study, ageing duration (3 hours vs. 5 hours) did not significantly affect fracture resistance, possibly because the tested ageing range remained within the material’s tolerance limits.
This could be due to the fact that the tested aging range is still within the material tolerance limits, or that the surface transformation has not been sufficient to significantly weaken the structure. - Surface Condition and Finishing
Polishing, grinding, and surface roughness can generate residual stresses or microcracks that act as crack initiation sites. Other studies have shown that surface condition and residual stress significantly influence susceptibility to ageing. - Material Quality and Densification
Porosity, inclusions, or microstructural defects within zirconia increase the likelihood of crack initiation. - Restoration Design and Occlusal Conditions
Cusp contours, internal angles, and load distribution determine areas of maximum stress concentration.
Clinical Implications and Recommendations
Based on material theory and research findings, the following clinical recommendations are proposed:
- Ensure adequate occlusal thickness.
An occlusal thickness of approximately 1.5 mm provides a greater safety margin compared with extremely thin restorations, particularly in posterior teeth subjected to high occlusal loads. - Pay close attention to surface finishing.
Proper polishing and elimination of microcracks are essential to prevent residual stresses from becoming fracture initiation points. - Avoid excessive external ageing conditions.
Although ageing duration was not significant in the cited study, long-term clinical exposure to moisture, temperature cycling, and repeated loading may still contribute to degradation. - Control occlusion and anatomical design.
Balanced occlusion and well-designed cusp anatomy help prevent excessive stress concentration. - Long-term monitoring and evaluation.
Periodic follow-up is necessary to detect early microcracks or failures, allowing timely intervention.
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Zirconia materials, particularly in monolithic crown form, offer excellent mechanical potential for dental restorations. Fracture resistance is strongly influenced by occlusal thickness, with greater thickness providing higher fracture strength. Although variations in ageing duration did not significantly affect fracture resistance in the cited study, long-term clinical ageing effects must still be considered, especially when combined with cyclic loading and real oral conditions. With appropriate restoration design, surface finishing, controlled occlusion, and routine clinical monitoring, zirconia crowns can provide durable and reliable restorative solutions.
References
MEGA CICILIA, drg. Heriyanti Amalia K., S.U., Sp.Pros(K); drg. Murti Indrastuti, M.Kes., Sp.Pros(K), PENGARUH KETEBALAN OKLUSAL DAN LAMA AGEING MAHKOTA ZIRKONIA MONOLITIK TERHADAP KETAHANAN FRAKTUR, https://etd.repository.ugm.ac.id/penelitian/detail/112465
Author: Rizky B. Hendrawan | Photo: Freepik
