Mini Short-Rod Fracture Toughness Testing of Bone Cements Exhibitng Elastic-Plastic Behaviour

Fracture of bone cement is considered to be a major factor contributing to total joint replacement failures because of implant loosening. While plane-strain fracture toughness (Klc) provides a parameter for assessing fracture resistance, the standard methods for testing (ASTM E399) require (1) the use of specimens greater than a certain minimum size (for bone cement this minimum size is greater than that found in clinical applications) and (2) fatigue precracking of specimens prior to testing. The short-rod fracture toughness test proposed by Barker offers a method of testing not requiring fatigue precracking and furthermore, it appears that a valid fracture toughness value can be determined using a modified miniature test specimen that we have proposed. Our mini short-rod specimen size approaches the actual dimensions of bone cement as used clinically. In this study mini short-rod fracture toughness test specimens were used to assess fracture toughness for two commercial brands of bone cement (Simplex-P and Zimmer LVC). An elastic-plastic fracture mechanics (EPFM) analysis was used to obtain experimental results with a plasticity correction factor being introduced. The cements were assessed as a function of method of preparation (hand mixed versus centrifuged) and aging conditions (3 days in air versus 7 days in distilled water at 37° C). The results indicated that (1) true fracture toughness values could be obtained only using an EPFM analysis for these specimens, (2) centrifugation did not significantly affect fracture toughness and (3) a significant difference in the plasticity correction factor due to water aging at 37° C was indicated for only the hand-mixed Zimmer LVC cement. There was, however, no significant change in fracture toughness due to water aging.