Measurement of cancellous bone strain during mechanical tests using a new extensometer device

A device for measuring mid-substance strain in bone during uniaxial compression was developed with special attention to subsequent FE modelling. It is based on dual instrumented cantilever arms that measure the difference in tip-to-tip deflection. The so-called extensometer device was compared to two standard methods of measuring strain based on platen measurements. The extensometer device output was highly linear with tip deflection (r(2)>0.99), and contact of the devices with the specimen was optimized by using an axial distribution of three devices fixed in a free-floating jig. Deflection of the extensometer arms was accurate to 4.8 microm, and precision was between 2-5 microm. Tests included measuring rubber test specimens and cylindrical cancellous bone cores extracted from canine femoral condyles. A trend of decreasing apparent modulus with decreasing strain rate was evident with the extensometer technique. Correlation between the extensometer method and the other two methods was r(2)=0.55. The measure of mid-substance strain avoids non-linearities in the compression tests caused by early failure at the specimen ends, and the uniaxial testing conditions result in boundary conditions that are well suited for subsequent finite element analysis.