The Effect of Aging on Biaxial Flexural Strength of Lithium Disilicate and Strength Gradient Zirconia

In the world of modern dentistry, the materials used to replace missing or damaged teeth have evolved from simple fillings to sophisticated ceramics that mimic the look and feel of natural enamel. Among these, two types of ceramic stand out for their balance of beauty and durability: lithium disilicate, a glass-based material known for its lifelike appearance, and zirconia, a tough, crystalline ceramic that offers exceptional strength. For a dental restoration to succeed, it must withstand the relentless forces of chewing and the constant shifts in temperature caused by hot coffee and cold ice cream. Over time, these repeated stresses can cause microscopic cracks to form, potentially leading to the material breaking. Scientists and clinicians need to know if these materials can survive years of such wear and tear without losing their structural integrity, or if the daily cycle of hot and cold, combined with the pressure of biting, will eventually weaken them.

A team of researchers at the University of Jordan set out to answer this question by putting two specific dental ceramics to the test. They focused on a popular lithium disilicate product and a newer, strength-gradient zirconia material, which is designed to vary in composition from the inside out to balance strength and aesthetics. To simulate the harsh conditions of a human mouth, the researchers created forty small, flat discs from these materials, each two millimeters thick and ten millimeters wide. They split these discs into four groups to see how the materials held up under different conditions. Two groups served as a control, representing the materials in their fresh state. The other two groups were subjected to a rigorous artificial aging process designed to mimic five years of clinical use. This process involved two distinct challenges: first, the discs were repeatedly dipped into hot water at 55 degrees Celsius and then cold water at 5 degrees Celsius for 6,000 cycles, simulating the thermal shock of eating and drinking. Immediately following this, the aged discs were subjected to over a million cycles of mechanical pressure, where a force of 50 Newtons was applied to them 1.5 times every second, replicating the repetitive action of chewing.

Once the aging simulation was complete, the researchers tested the strength of every single disc. They placed each one on a testing machine and applied a steady, increasing force to the center until the disc shattered. This method allowed them to measure the exact amount of pressure the material could withstand before failing. The results revealed a clear and significant difference between the two materials. The zirconia discs proved to be far stronger, with an average breaking point of roughly 1,185 megapascals, compared to the lithium disilicate discs, which broke at an average of about 473 megapascals. This confirmed that the zirconia material possesses a much higher capacity to resist breaking under stress. However, the most surprising finding concerned the aging process itself. Despite the intense heat cycling and the million-plus chewing simulations, the strength of neither material changed in any statistically meaningful way. The zirconia that had been aged held just as much strength as the fresh zirconia, and the same was true for the lithium disilicate.

The study also looked deeper into the reliability of the materials by analyzing how consistent their strength was across the different samples. While the zirconia was stronger overall, its strength varied more from one disc to another, whereas the lithium disilicate showed a more uniform performance across the board, even though it was weaker. This suggests that while zirconia can handle heavier loads, its failure point is slightly less predictable than that of the glass-based ceramic. Ultimately, the research indicates that for these specific materials, the combination of thermal shock and repetitive chewing forces, even when accelerated to simulate five years of use, does not degrade their ability to withstand breaking. The findings suggest that both materials maintain their mechanical integrity under these conditions, offering reassurance that they are robust enough for long-term use in the mouth, provided they are selected for the right clinical situations based on their inherent strength differences.