Evaluating acrylic and glass-ionomer cement strength using the biaxial flexure test.

Frequently, bone cement strengths are evaluated from uniaxial tests, such as three- or four-point flexure. Measurement of material strength in this manner may not provide an accurate characterisation of a bone cements true load-bearing capacity. In most orthopaedic applications, there exists a state of biaxial stress and so biaxial strength information is most useful. To address this issue, the biaxial flexure strength of two polymethylmethacrylate orthopaedic (PMMA) bone cements and two glass-ionomer dental cements have been determined. The biaxial strength of orthopaedic bone cements have been compared to the three-point bending strength. Furthermore, the calculated theoretical biaxial strength was compared with a value of biaxial strength utilising the finite element method. For all materials tested the calculated biaxial strength is significantly greater than the three-point bending strength. The biaxial test offers several advantages over three-point bending because it critically explores surface flaws--as it does not matter in which orientation the crack lies. However, it does minimise the volume or surface area investigated and also the edge effect. The difference in strength calculated for the two testing methods can be explained quantitatively in terms of the volume of material under stress. This work has demonstrated that the biaxial flexure test can be used for the testing of orthopaedic bone cements.