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Time dependent mechanical properties of bone cement. An in vitro study over one year.

Changes in mechanical properties of bone cements over time are of clinical importance, but not well documented. Specifications for testing do not address the time factor. This study recorded changes in compressive properties and microstructure of one bone cement stored under simulated physiological conditions (water at 37 degrees C) from 20 min up to 1 year and in dry air at 37 degrees C for comparison. Compressive strength increased within the first week (p < 0.001), decreased at 1 month (p < 0.001), and remained at that level at 1 year. Elastic modulus showed a similar development. Maximum strain values, indicating plastic deformability, increased continuously over 1 year. Microscopy revealed microcracks between the pre-polymer beads and the matrix in specimens tested after 20 min, whereas there were less cracks in 1 year specimens. Increase in strength during the first week is due to polymerization and formation of interpenetrating molecular networks. The subsequent decrease could be due to the plasticizing effect of water uptake, as supported by higher values for dry specimens. It can be speculated that microcracks which could be initiated while reducing an arthroplasty at 15 min, acting as initiators for fatigue fractures in the cement mantle, contribute to cement failure. It is recommended that testing of bone cements should be performed after extended ageing at simulated physiological conditions, for the present cement at least 5 weeks. Results obtained at less than one week could be influenced by ongoing polymerization, as well as microcracks and lower coherence between the prepolymer beads and the matrix.