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Understanding Fracture Toughness by Thomas Hill, PhD

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Header: Understanding Fracture Toughness
by Thomas Hill, PhD


Today, industry advertisers are increasingly referring to the "fracture toughness" values of their materials. It's important for clinicians to understand the relevance of these values, and how to use that information when making clinical practice decisions.

How fracture toughness values can affect layered restorations
The International Organization for Standardization (ISO) has noticed that clinical outcomes appear to be related to the core ceramic's fracture toughness. "Fracture toughness" describes how hard it is to drive a crack through a ceramic. Values range from about 0.8 MPa•m1/2 for veneering porcelains to more than 5.0 for transformation-toughened zirconia. Metals start at about 14 MPa•m1/2 and go into the hundreds, because they can relieve stress concentration at crack tips, with atomic flow leading to crack blunting. No such relief mechanism exists for ceramics, however.

Bilayer restorations can also be governed by a material's fracture toughness, because fractures generally start from the intaglio, or cementation, surface. Stress in the strongest layer is what matters, and the load-bearing strength of the crown increases by the square of its total thickness—even if the crown is made of a weaker material, such as porcelain.

Bilayer prostheses are just as durable as full-thickness prostheses, as shown in studies that tested single load-to-failure and fatigue failure under water.1 So by understanding the clinical indications of specific ceramics, clinicians can be more certain of which ones can be used for particular applications by matching the fracture toughness with an existing and known material.



Fig. 1


Fracture toughness is consistent, but a material's "strength" varies
Strength alone is not a good predictor of a material's clinical indications, because it is determined by both fracture toughness and the size of any flaws. While strength is derived partly from fracture toughness, it's also extremely sensitive to surface condition, and can vary a thousandfold depending on how a specimen has been made. Strength measurement doesn't take into account variables that can be very influential in crown failure, such as the effects of water, cement, a laboratory or clinical surface treatment (e.g., sandblasting, etching, silanization), or the elastic modulus of the cement and build-up material.

Fracture toughness, meanwhile, is more inherent. This makes it more predictive of the durability of single crowns, and it correlates with marginal integrity and attritive wear of resin-based composites.

As data is collected, expect a new required measurement to be added to materials
The dental industry now has at least five—and often up to 10—years of clinical data about existing all-ceramic systems or closely related ones.2 With this information, the ISO standard committee working group has developed a novel classification scheme for the 2008 revision of ISO 6872, based on indications from clinical data. As part of this, researchers also introduced fracture toughness—although still in an annex—which eventually will become a required measurement.

In the meantime, clinicians can help to move the profession forward by asking manufacturer representatives about the fracture toughness of products at shows and during office visits.



Fig. 1 The Ivoclar Vivadent R&D team spend countless hours each and every day testing materials to ensure a high level of care for the dental industry.


Fig. 1 SEVNB depth of cut measurements for analyzing fracture toughness testing


References
  1. Dibner and Kelly, J Prosthet Dent, 2016
  2. Della Bona and Kelly, JADA, 2008


Dr. Thomas Hill Thomas Hill, PhD, graduated from the University of Florida in 2001 and earned a doctorate in material science and engineering with a focus on ceramics. He specialized in dental ceramics at the university while on a National Institutes of Health grant. For 15 years, Hill has been the manager of scientific services at Ivoclar Vivadent, where he oversees the North American Ivoclar-sponsored clinical trials and in?vitro studies. He also mentors university students, and has been published on various topics in dental materials. He is a member of the Academy of Dental Materials and a member of the ADA committees on development of standards for dental ceramics and CAD/CAM in dentistry.




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