Failure behavior of glass ionomer cement under Hertzian indentation

Objective To investigate the load-bearing capacity and failure mode of various types of glass ionomer cement (GIC) under Hertzian indentation, exploring the relationship between the failure behavior and formulation, and examining claims of filler-reinforcement of GIC. Methods Discs 2 mm thick, 10 mm diameter, 8–18 replicates, were fabricated for two filler-reinforced GICs, four unmodified and unreinforced GICs, and four resin-modified GICs, with a dental silver amalgam and a filled-resin restorative material for comparison. Testing was at 23 °C, wet, after 7 d storage at 37 °C in artificial saliva at pH 6, using a 20 mm diameter hard steel ball and filled-nylon substrate (E: 10 GPa). First failure was detected acoustically; mode was determined visually. At least 1/3 of specimens in each case were examined under scanning electronic microscope for corroboration. Results Reinforced and unmodified-unreinforced GICs were indistinguishable by failure load (one-way analysis of variance, P = 0.425, overall 260 ± 70 N) and mode. Failure loads for resin-modified GICs were 360–1150 N, amalgam not, vert, similar680 N, and filled resin not, vert, similar1200 N. Resin-modified GICs tended to be tougher (incomplete fracture), all others gave complete fracture (radial cracking). The stronger materials (two resin-modified GICs and filled resin) showed some cone cracking. Significance While resin-modified GICs showed various extents of increase of failure load over that of the plain GICs, consistent with the hybrid chemistry, filler-reinforcement was not evident for the two claimed products, consistent with structural and theoretical expectations.