Fatigue studies on dental composites and bonding systems

• Main Author: Padipatvuthikul, Pavinee
• Published: University of Liverpool 2007
• Subjects: 617.6
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485845

Introduction: Adhesion has become an important concept in modern restorative dentistry. It offers the ability to bond materials to the tooth without invasive tooth preparation. Numerous in-vitro strength tests have been used to determine the bond strength of adhesive systems. However, because the occlusal forces applied to. a restoration are complex, and made up of a combination of forces, no one test can satisfactorily predict the in-vivo behavior of an adhesive system. The majority of bond strength studies have used monotonic tests to assess the bond strength of materials and between the materials and the tooth. These tests are expedient, but do not simulate the cyclic forces that operate in the mouth. Tests that characterize this type of . stress are called fatigue tests. Fatigue can result in wear and fracture of materials or bonds. .Objectives: To investigate fatigue behavior of modern resin composites and resinbonded joints of both metal to enamel and ceramic to enamel. The main approaches to fatigue assessment, 'Fatigue Limit' and 'Fatigue Life'were compared Materials and Methods: Surface effects of fatigue One hundred and eighty samples of two historical composites1-2 and seven modern composites3 - 9 were subjected to 2000 stress cycles between 0 and 120N or 0 and 400N. Surface damage was measured as the diameter of the fatigue scar and subsurface damage was determined by silver nitrate staining. The hardness of both the surface and subsurface was also determined. Fracture Composite to composite Two hundred and twenty composite disks were fabricated using three materials.7 • 9 After one day, one week, four weeks, and twelve weeks, fifty-five specimens of each material were removed from' water and divided into three groups of fifteen and one group of ten. Each group of samples was treated with one of three bonding systems10- 12 before adding a sec~nd increment. For each material, ten samples were subjected to Shear test in a Universal Testing Machine13 (CHS= 50 mmlmin). The fatigue limit test using fifteen samples per group were used to determine the fatigue limit using the staircase method (Draughn 1979). Metal or Ceramic to Enamel (via resin) Three hundred and forty-two discs of Ni/Cr-alloy14 were cast and treated by either sandblasting with aluminium oxide, or by sandblasting followed by electrolytic-etching in HCI. The disks were bonded to etched enamel with one of three dental bonding systems.1S - 17 One hundred and seventy-one ceramic disks were fabricated by sintering ceramic powder.18 One surface of each disk was etched with porcelain etching-gel19 for fifteen minutes and sandblasted with 50 J.Im A120 3. The prepared disks were then divided into three groups and were bonded to etched enamel using one of three dental bonding systems.1S - 17 Ten specimens of each group were sUbjected to a shear bond test (CHS 50 mm/min) and seventeen specimens of each group to a staircase fatigue test to determine the fatigue limit of the bonds. The remaining specimens from each group were placed in the custom made fatigue testing machine and allowed to cycle to failure between 0-20 kg, 0-10 kg or 0-5 kg (n=10 per load). The number of cycles at failure was analysed by Weibull statistics to determine the fatigue life Results: The surface studies in composites indicated that both surface and subsurface damage increased with increasing load. In general, small-particle composites experienced less damage than the large particle materials. At 12 kg, the surface damage was inversely proportional to the surface hardness, whereas at 40 kg, it was proportional to the subsurface hardness. At both loads, subsurface damage was directly proportion to subsurface hardness. For the composite to composite bonds, the fatigue limit values were approximately 30% of the shear bond strength values and the values were significantly different (p<0.01) for all nine groups. For metal to enamel bonds, the fatigue limit (after 5000 cycles) varied between 10.7 and 16.8 MPa compared to 21.3 and 48 MPa for the shear strength. The values for all groups was significantly different (p<0.001). There was no significant correlation between the shear bond strength and the fatigue limit values (Pearson Correlation P<0.01). For all groups, the threshold stress at which the samples equid withstand over one million cycles (fatigue Life) was 2.5 MPa. For ceramic to enamel bonds, the fatigue limit (after 5000 cycles) varied between 11.41 and 13.74 MPa compared to 21.3 and 48 MPa for the shear strength. The values for all groups were significantly differ~nt (p<0.001). There was no significa~t correlation between the shear bond strength and the fatigue limit values (Pearson Correlation P<0.001). For all groups, the threshold stress at which the samples could withstand over one million cycles (fatigue Life) was 2.5 MPa. Conclusion: Fatigue damage to the surface and subsurface of composite was related to the hardness of the material. The values of the fatigue limit were significantly lower than the shear bond strength values. There was no correlation between fatigue limit and shear bond strength. The long term safety limit for resin bonded joints to enamel is 2.5 MPa. Neither the shear test, nor the fatigue limit test was an accurate predictor of the long-term fatigue behaviour of resin-bonded restorations. A fatigue limit test using 100,000 cycles may be a useful predictor of the fatigue life which, in these studies, was half of the fatigue limit at 100, 000 cycles but the only reliable test is to test to failure. The data presented in this thesis indicated that the shear bond strength is not pred!ctor of long term failure. lClearfil Posterior, Cavex. Holland. 20cclusin. ICI. UK. 3Concise, 3M. USA. 4Admira, VOCO, Germany. 5Grandio. VOCO. Germany. 6Grandio Flow, VOCO, Germany. 7Spectrum, Dentsply, Germany. 8Durafill VS, Heraeus Kulzer, Germany. 9Herculite XRV, Kerr, USA. 10Prime&Bond. Dentsply, Germany. 110ptibond solo plus, Kerr, USA. 12BisGMAffEGDMA. 3M ESPE. USA. 13Nene Instruments Ltd.• UK. 14yerabond II, Aalba Dent Inc., USA. 15Calibra with Prime & Bond Resin, Dentsply, Germany. 16Panavia with ED-Primers. Kuraray, Japan. 17Nexus with Optibond Solo Plus Resin, Kerr, USA. 18Vitadur Alpha, VITA Zahnfabrik. Germany. 19Porcelain Etch-it gels, American Dental Supply. USA.