Representative assessment of long bone shaft biomechanical properties: an optimized testing method.

Whole bone bending tests are commonly used in mechanical evaluation of long bones. Reliable information about the midshaft can only be obtained if the bending moment is uniformly distributed along the shaft, and if the distribution of the bending stress is not adversely influenced by rigid clamping of the bone ends. A testing device was developed to determine bending stiffness of long bones in 24 directions, perpendicular to the bone axis. For optimal distribution of bending moment and stress, four-point bending was performed, and bone ends were simply supported, not rigidly clamped. The method was validated by repeated testing of a stainless steel rod, and a sheep femur. Left-right ratios were assessed twice in 2 groups of 5 sheep: one control group, and one group in which the left femur was stabilized with a stainless steel interlocking nail for 2.5 yr, after a midshaft osteotomy. Test results obtained with the steel rod reproducibly were close to predicted values. Measurements with the sheep femurs were reproducible and precise for 3 of the 4 parameters of the bending test. Stiffness parameters were significantly higher in the operated sheep than in the control group. We conclude that the method described here provides accurate and reproducible information, which is representative for the long bone shaft.