PROPERTIES OF COMPOSITE SPECIMENS OF OLD AND NEW BONE CEMENT

Abstract

Vast amount of literature is available on mechanical properties of PMMA, but not about the composite specimens of old and new cement. This is important, as in cement revision has become established technique with good clinical results. Originally Greenwald and later Li described properties of such specimens. However in these studies the old samples were only few days old, unlike clinical situation, where the old cement is a few years old.

We therefore decided to test short-term mechanical properties of composite specimens and compare these with new uniform specimens. We choose specimens of cement 3–17 years old (median 11.8) for the manufacturing of the composite specimens.

Material and Methods: Uniform and composite specimens were fabricated and were tested for bending, tensile and shear strength. Beam shaped specimens were fabricated for bending and tensile tests, cylindrical for shear. Seventeen beams and eight cylindrical specimens fabricated earlier (1988–2002) using the same moulds were available to form composite specimens. Old specimens were placed into the moulds and new cement was injected next to these. Specimens were allowed to polymerize at room temperature for 30 minutes and stored in saline at 37 °C for 6 weeks before testing. Specimens were tested in Lloyds EZ 20 machine with customized jig so that the junction was subjected to bending, tensile or shear force.

Results: Bending tests: The load and bending stress for new specimen was 80N and 47MPa as compared with 72N and 38MPa for composite specimens. 4 composite specimens failed though old cement, 3 through the junction and 1 through the new cement. There was no statistical difference in maximum load between uniform and composite specimens (p=.29). However there was a difference in the stress between uniform and composite specimens.

Tensile tests: The load and tensile stress for new specimen was 916N and 29MPa as compared with 795N and 24MPa for composite specimens. 7 composites failed through old cement, 1 through new cement and 1 at junction. There was difference in the load and stress of uniform specimens as compared with composite specimens.

Shear tests: The load and shear stress for new specimen was 2718N and 35MPa as compared with 2055N and 26MPa for composite specimens. There was significant difference in load as well as stress in uniform specimens as compared with composite specimens.

Discussion: This study demonstrates that composite specimens fail at 89.6% of bending load, 77.2% of tensile and 74.6% of shear load as compared with uniform new cement specimens. They have 81.4% of bending stress, 74.9% of tensile stress and 73.3% of shear stress at failure as compared with uniform specimens. Of more importance is the fact that only four of these composite specimens (23.5%) failed at the junction and the rest thirteen failed either through old cement (64.7%) or through new cement (11.8%).