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Inside Dentistry

September 2009, Volume 5, Issue 8
Published by AEGIS Communications


Clinically Relevant Testing of Dental Porcelains for Fatigue and Durability With an Innovative Mouth Motion Simulator.

Abstract

Objective: To evaluate theeffect of mouth-motion-step-stress-fatigue on reliability and failure modesof monolithic IPS e.max CAD (Ivoclar Vivadent) full crowns. Null Hypothesis: There is no difference in reliability and failure modes of monolithic IPSe.max CAD full crowns and veneered Yttria-stabilized Tetragonal ZirconiaPolycrystal Ceramic (Y-TZP) crowns with hand-layer veneering.

Methods: Following cementation and aging, anatomically correctfull-crown specimens were tested and the results compared to our previousstudy: Group CAD (n = 19) — IPS e.max CAD full crowns. Group Ceram (n = 21) — IPS e.max Ceram on ZirCAD core. All crowns glazed with standard cooling procedure as finaltreatment.

Executive Summary:

  • Porcelain veneered zirconia crowns (Y-TZP) and monolithic lithium disilicate crowns (IPS e.max CAD) were tested
  • Using a mouth-motion-simulator step-stress-fatigue was used to examine reliability and failure modes
  • Failure was considered to be chip-off fractures of veneering ceramic or fracture through the crown
  • Three step-stress profiles were used up to failure or up to 900 N and 180K cycles after which a staircase fatigue method was implemented to a load at which 50% of specimens could be expected to survive 1 million cycles
  • Veneered zirconia crowns resulted in limited reliability — approximately 90% of specimens would fail from veneer chip-off fracture by 100K cycles at 200 N. These results are similar to previous findings for other veneered zirconia systems (LAVA, Cercon, Vita) tested using this methodology (Coelho PG, Silva NR, Bonfante EA, et al. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater. April 2009. [Epub ahead of print])
  • Approximately 90% veneered zirconia specimens failed by 350 N independent of the number of cycles (Appendix 1)
  • None of the e.max CAD lithium disilicate specimens failed below 900 N and 180K cycles independent of loading profile
  • The e.max CAD lithium disilicate specimens survived r ratio fatigue of 1 million cycles at loads of 1,000 N. There appears to be a threshold for damage/fracture for the lithium disilicate in the range of 1,100-1,200 N.

In comparison to veneered zirconia systems (ZirCAD, LAVA, Cercon, Vita zirconia) tested in our laboratory the IPS e.max CAD full crowns would be expected to have excellent clinical performance relative to chipping or fracture based upon our fatigue findings. IPS e.max lithium disilicate (Ivoclar Vivadent) is the most robust all-ceramic system tested to date. The failures reported in this study mimic the failures reported in clinical studies.

Commentary

By Howard E. Strassler, DMD

As one of the primary restorative materials used inrestorative dentistry for indirect restorations for anterior andposterior teeth, ceramics are used to restore the natural dentition or implants using CAD/CAM systems, veneered-overcast metal, zirconia, and alumina substructures,whether pressed or stacked and fired. They are the most esthetic and durable restorative materials we can provide to ourpatients. Typically, in the research and development of dental materials,they are tested for physical properties to meetminimal standards based on defined criteria for strength characteristics.Are these physicalproperties adequate to predict clinical success? Most physical property testing of ceramics includes loading the restorative materialto catastrophic failure. Unfortunately, certain materials perform well in this type ofcatastrophic failure testing but their clinicalperformance has been disappointing. Some of the earlier ceramic systems did well in in vitro testing but when used to restoreteeth they did not withstand the rigors of oralfunction and parafunction.

When laboratory-fabricated ceramic restorations fail, it is usually not a result of a catastrophic single event, unless there is a traumatic accident. Ceramics fail because of the fatigue stress of repetitive function. These repetitive events can include normal and parafunction. We need to test materials, especially ceramics, differently. The ceramic testing group at New York University College of Dentistry, under the guidance of Dr. Van Thompson, has responded to these previous deficiencies in typical physical property testing by developing a “mechanical mouth simulator.” Their mechanical testing, as seen in this extremely well-done research protocol, can stress restorations using clinically relevant directed loads and over many thousands of cycles until fracture occurs. In fact, one unique aspect of this testing model is that the mechanical mouth can be stopped any time during the cycling to evaluate the initial causes of fatigue stress failure. They will know exactly what is happening at every phase of the investigation. With this information, ceramics (and other materials) can be reformulated to address and overcome structural flaws to improve clinical durability. In fact, this is what Ivoclar Vivadent has done with their lithium disilicate ceramic (IPS e.max CAD).

Guess PC, Zavanelli R, Silva NR, Thompson VP. New York University College of Dentistry. Presented at: 39th Annual Session of the American Academy of Fixed Prosthodontics, February 2009, Chicago, IL.

About the Author

Howard E. Strassler, DMD
Professor and Director of Operative Dentistry
Department of Endodontics, Prosthodontics and Operative Dentistry
University of Maryland Dental School
Baltimore, Maryland


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Image Gallery

Figure 1  <em>Left:</em> failure of the veneering ceramic on a zirconia restoration loaded at 200 N after only 60,000 cycles; <em>Center:</em> type of loading used in the mouth motion simulator; <em>Right:</em> the monolithic IPS e.max CAD restoratio

Figure 1