In this blog post, we’ll explore the characteristics of zirconia crowns used for dental implants and the principles behind enhancing their strength, while also examining how modern dental materials have evolved.
What Kind of Material Is Zirconia Used in Dental Implants?
When you think of actor Lee Byung-hun, the first thing that comes to mind is his smile, which reveals his white, healthy teeth. In the movie ‘The Man Who Became King’, Lee Byung-hun sticks a piece of seaweed between his front teeth and pretends to be a fool to make the Queen Consort, who has lost her smile, laugh. Han Hyo-joo, who plays the Queen Consort, eventually bursts into laughter. However, if Lee Byung-hun had actually been missing two front teeth, Han Hyo-joo might have recommended dental implant surgery rather than finding it funny.
The dental implant procedure generally involves reinforcing insufficient alveolar bone with bone graft material as needed, then placing a titanium implant fixture into the jawbone, attaching an abutment to it, and finally fitting a crown. Here, the crown refers to the outer part of the tooth that we actually use for chewing. Crowns are widely used not only for dental implants but also for restoring damaged teeth. So, what exactly is zirconia, and how has it become a material with a form and properties so similar to those of natural teeth?
How has the strength of zirconia increased?
Zirconia used for dental implant crowns broadly falls under the category of ceramic materials. In materials engineering, materials are classified into metals, ceramics, and polymers. Ceramics refer to oxides, carbides, and nitrides formed when elements such as silicon (Si), aluminum (Al), and zirconium (Zr) combine with oxygen, carbon, nitrogen, and other elements. Ceramic products are created by shaping these ceramic materials into the desired form and then sintering them at high temperatures to ensure sufficient strength.
When zirconia was first discovered, it had an ivory color similar to that of teeth and a density comparable to teeth, suggesting its potential as a dental material. However, its application was severely limited due to its very low strength, which made it prone to breaking. Since it was a material that could be used in various fields if only its strength could be improved, materials engineers began research to increase the strength of zirconia.
The strength of a material depends on the arrangement of its atoms and molecules. Even for the same substance, the crystal structure can vary depending on conditions such as temperature, pressure, and additives during the production process. By appropriately controlling these external conditions, the strength of the material can be significantly improved. In materials engineering, the phenomenon of altering a material’s properties by inducing such changes in its crystal structure is called martensitic transformation. Applying this principle to zirconia led to a significant increase in its strength, and subsequently, it began to be used not only in dental crowns but also in various industrial fields.
How does martensitic transformation work?
What is the mechanism by which martensitic transformation enhances the strength of ceramics?
When zirconia and magnesium oxide (MgO) are used together, a much wider variety of crystal structures can form than when used as a single material. When zirconia mixed with a specific proportion of magnesium oxide is heated to a high temperature, a crystalline structure different from the original forms. Subsequent rapid cooling (quenching) creates a metastable state in which this crystalline structure—which is unstable at room temperature—is maintained.
Simply put, quenching refers to a rapid cooling process. It can be understood as similar to the process used by blacksmiths in the past, who would hammer red-hot iron and then immediately immerse it in cold water to harden it.
Undergoing this heat treatment process alters the crystal structure, significantly improving the material’s mechanical properties. Ultimately, when the arrangement of molecules changes, various physical properties—including strength, melting point, and thermal expansion characteristics—change as well. However, since the conditions required for these changes in crystal structure vary for each material, extensive research and experimentation are necessary. If this research continues to advance, even materials that had little practical value in the past could be reborn as high-performance materials with new functions.
In the past, when teeth were damaged, they were often capped with gold or silver, or dentures were used. Today, however, materials that are virtually indistinguishable from natural teeth in appearance are preferred, and dental implants are widely used for their functional benefits. People want a healthy smile that is almost indistinguishable from their natural teeth, rather than teeth with visible gold or silver. Advances in materials engineering have made these demands possible, and they are expected to continue driving the development of dental materials that are even more natural-looking and durable.
