The effects of force application on the compressive properties of femoral spongious bone

End artefacts play a major role in uniaxial compression tests with cancellous bone specimens. They lead to misinterpretation of mechanical parameters of bones due to uncontrolled introduction of bending moments into the free ends of trabeculae. This work aims to simplify current methods preventing end-artefacts and furthermore to investigate the influence of end artefacts on plateau stress.

176 cylindrical cancellous bone specimens were taken from human femoral condyles and tested in uniaxial compression. The specimens were divided into 2 groups (direct, end-cap) and compressive modulus, maximum stress, plateau stress, energy absorbtion as well as apparent density were evaluated. Density values are from separate specimens which are immediately adjacent to the mechanical specimen.

All mechanical parameters were significantly higher in the end-cap specimens than in the direct ones by about 30 - 40 %, thus reaching similar differences as the previous studies. Greatest differences between groups were determined for compressive modulus (45 %) and plateau stress (35 %). Energy absorbtion can be explained with great accuracy by plateau stress (P < 0.001; R² = 0.95). Among all parameters plateau stress can be best explained by apparent density using an exponential function (P < 0.001; R² = 0.38).

The end-cap method used here to prevent end artefacts showed variations consistent with the literature when compared to the direct method. Additionally it was shown that the way in which the force is applied to the specimen has a major influence on the failure progression behavior, which was characterized using the plateau stress.