The human proximal femur behaves linearly elastic up to failure under physiological loading conditions

Abstract

It has not been demonstrated whether the human proximal femur behaves linearly elastic when loaded to failure. In the present study we tested to failure 12 cadaveric femurs. Strain was measured (at 5000Hz) on the bone surface with triaxial strain gages (up to 18 on each femur). High-speed videos (up to 18,000frames/s) were taken during the destructive test. To assess the effect of tissue preservation, both fresh-frozen and formalin-fixed specimens were tested. Tests were carried out at two strain-rates covering the physiological range experienced during daily motor tasks. All specimens were broken in only two pieces, with a single fracture surface. The high-speed videos showed that failure occurred as a single abrupt event in less than 0.25ms. In all specimens, fracture started on the lateral side of the neck (tensile stress). The fractured specimens did not show any sign of permanent deformation. The force–displacement curves were highly linear (R2>0.98) up to 99% of the fracture force. When the last 1% of the force–displacement curve was included, linearity slightly decreased (minimum R2=0.96). Similarly, all force–strain curves were highly linear (R2>0.98 up to 99% of the fracture force). The slope of the first part of the force–displacement curve (up to 70% fracture force) differed from the last part of the curve (from 70% to 100% of the fracture force) by less than 17%. Such a difference was comparable to the fluctuations observed between different parts of the curve. Therefore, it can be concluded that the proximal femur has a linear-elastic behavior up to fracture, for physiological strain-rates.