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Continued
First, let's take a look at the question of strength. Most tests evaluate the compressive strengths in the raw state of the product. A better test is the strength and longevity of the product once it is placed in the mouth. Strong products in a raw state can become weak if they are improperly
handled before they reach your patient's mouth.
When it comes to compressive testing, 3M's Lava zirconium frames are stronger than e.max units. Lava scored 1200 mpa's compared to e.max's 400 mpa's. When you add in the factor of water solubility over time in the oral environment, Lava wins again since it degrades to 1000 mpa, versus the unknown degradation of e.max. Generally speaking, we advise our dentist partners that a 900 mpa strength is suitable for the posterior and 600 mpa for anterior.
3M Lava units can be used in anterior and posterior bridges, while e.max bridges are more limited, to three units and anterior only. Lava restorations are stronger in attachment to the tooth when cemented, while e.max is stronger when etched and bonded to enamel. Both products require adequate room for proper thickness to reach maximum strength.
Let's switch to reviewing the weaknesses of the two products. One notable weakness is the veneering of the ceramic porcelains, much as it is with porcelains fused to metal products. The cause of the failure is not typically the metal of the PFM, but the veneering of ceramics. Almost all veneering ceramics are felspathic materials with an average strength of 100 mpa. Even at that strength, it has worked very well for patients because the proper application process renders it strong with the metal substrates. Again, the proper handling of materials is essential to the strength and longevity, both in manufacturing and placement.
A lot of the perception that zirconium products are weak comes from the improper processing of the veneering material, specifically the heating and cooling times in the firing process. Metal heats and cools much more rapidly, thus requiring a slower heating and cooling process. This is a unique aspect of handling zirconium products.
Chipping and fracturing primarily comes from thermal differentiation, where there is a difference in temperature between the veneering porcelain closest to the zirconium core and the outer surface of the veneering porcelain. When there is too great of a difference, it causes internal stress within the porcelain, and under usage, it chips on the surface or gives way to pieces. The solution to this problem is to allow for proper cooling time, which evens the cooling throughout the porcelain, eliminating the internal stresses.
At Perry & Young, our lab uses Noritake porcelain to veneer our 3M Lava frameworks and their instructions detail the proper heating and cooling requirements. That is not the case with every porcelain company. Slow heat and slow cool means that our client's are not experiencing the chipping and fracturing problem with their patients.
e.max disillicate material is not subject to thermal differentiation because it does not have any supportive framework. It heats and cools evenly throughout the firing cycle. It is, however, subject to coefficient thermal expanding issues, as are other veneered products, when it is cut back and layered. Whether milled or pressed as one piece, with no layering, it is colored and glazed, etched and bonded to enamel for its ultimate strength.
e.max states that it can be cemented into place. We believe time and testing will tell if that will be successful. It is recommended in the molar regions, but not as a unit connected to a bridge. It is also recommended for inlays and onlays in the posterior region. e.max, as a veneered materials pressed onto frameworks, gives a great strength to it as a completed unit. This is still in the early states of development.
So, which one is stronger? The answer is all about how it processed at the lab and placed orally in the patients. Both are fine products and have a place in the industry. For one company to say that their product is stronger than the other is weak at best.
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