Mechanical behavior of human morselized cancellous bone in triaxial compression testing

Despite its widespread use as graft material in orthopaedic surgical procedures, morselized cancellous bone has not yet been well characterized from the standpoint of its mechanical properties. To accommodate the noncohesive nature of this loose particulate form of bone, a triaxial compression test apparatus commonly used in engineering soil mechanics was adapted for the testing of fresh-frozen human morselized cancellous bone specimens. Triaxial compression tests were run to 30% axial strain at five different levels of confining pressure ranging from 0.276 to 8.552 MPa. The measured axial stress versus axial strain behavior was bimodal, characterized initially by relatively stiff linear behavior, then by a rapid transition to a much more compliant (but, again, approximately linear) domain until test cessation. The apparent axial moduli of both response regions were found to be nearly linear functions of the transverse confining pressure. As typically prepared surgically, the distribution of particle size was found to have approximately 80% of the bone graft, by weight, encompassed in particles 0.43-3.2 mm in size. Triaxial tests of samples segregated by size showed that particle size had no appreciable effect on apparent material properties. The nominal Young's modulus and Poisson's ratio of morselized cancellous bone were 100 MPa and 0.2, respectively.