SUMMARY: ISO testing of Crystal Ultra resulted in approximately 155 MPa to 205 MPa in Flexural strength depending on the testing methodology employed. Our published number of 175 MPa is based upon the 3-point bending testing using the most universally accepted international standards. All testing methods far exceed both FDA and European standards for dental ceramics. For a more useful strength comparison, Crystal Ultra recommends analyzing Compressive Strength, which better simulates the chewing stresses found in the mouth.
SCIENCE: DLMS has not been able to confirm the flexural strength results of our competitors products, and so no apples-to-apples comparison of flexural stength has been conducted of the comparable ceramic dental materials. However, all dental material manufacturers must exceed 100 MPa in flexural strength in order to be registered with the FDA, and they must exceed 50 MPa under European standards, and we believe that all materials meet this basic standard.
Three challenges exist in doing apples-to-apples comparison of flexural strength of competiting materials.
The first challenge is that only Crystal Ultra, among all of the esthetic ceramics, is the only material manufactured in a large disc format, and is therefore one of the only materials which can be used off-the-shelf to make sample sizes as specified by most of the relevant ISO standards. All others must make special batches of material for testing or use smaller samples from chairside milling blocks, which some argue alters the results. Using the break-a-stick-over-your-knee example, you can imagine that it might be more difficult to break an 8 inch stick over your knee than it would to break a 24 inch stick.
The second challenge is that since the goal of testing is to demonstrate a 50 MPa or 100 MPa minimum requirement, regulatory authorities allow manufacturers to use one of many different generally accepted methods for measuring flexural strength, including standards found in ISO 10477, and ISO 6872, or ISO 4049, as published and changed over the years, and as altered based upon available sample sizes, and within those standards, manufacturers can elect to do any of the following tests:
- three-point bending test
- four-point bending test
- biaxial flexure test (piston-on-three-ball)
The third challenge is the basic question about whether flexural strength is the best possible measure of how "strong" a dental material is, especially for side-by-side comparisons. Flexural strength is measured by creating a span of unsupported material across an open space and applying downward force till it breaks. And while flexural strength may be an ideal test for contruction materials, by contrast, dental restorations are usually supported by underlying teeth or implants, and so Compressive Strength is a far better measure of what actually happens in the mount.