Accurate in vitro identification of fracture onset in bones: failure mechanism of the proximal human femur.

Bone fractures have extensively been investigated, especially for the proximal femur. While failure load can easily be recorded, and the fracture surface is readily accessible, identification of the point of fracture initiation is difficult. Accurate location of fracture initiation is extremely important to understand the multi-scale determinants of bone fracture. In this study, a recently developed technique based on electro-conductive lines was applied to the proximal femoral metaphysis to elucidate the fracture mechanism. Eight cadaveric femurs were prepared with 15-20 electro-conductive lines (crack-grid) covering the proximal region. The crack-grid was connected to a dedicated data-logger that monitored electrical continuity of each line at 700 kHz. High-speed videos (12,000 frames/s, 0.1-0.2 mm pixel size) of the destructive tests were acquired. Most crack-grid-lines failed in a time-span of 0.08-0.50 ms, which was comparable to that identified in the high-speed videos, and consistent with previous video recordings. However, on all specimens 1-3 crack-grid-lines failed significantly earlier (2-200 ms) than the majority of the crack-grid-lines. The first crack-grid-line to fail was always the closest one to the point of fracture initiation identified in the high-speed videos (superior-lateral neck region). Then the crack propagated simultaneously, at comparable velocity on the anterior and posterior sides of the neck. Such a failure pattern has never been observed before, as spatial resolution of the high-speed videos prevented from observing the initial opening of a crack. This mechanism (fracture onset, time-lag, followed by catastrophic failure) can be explained with a transfer of load to the internal trabecular structure caused by the initial fracture of the thin cortical shell. This study proves the suitability of the crack-grid method to investigate bone fractures associated to tensile stress. The crack-grid method enables significantly faster sampling than high-speed cameras. The present findings elucidate some aspects of the failure mechanism of the proximal human femoral metaphysis.

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

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 (R(2)>0.98) up to 99% of the fracture force. When the last 1% of the force-displacement curve was included, linearity slightly decreased (minimum R(2)=0.96). Similarly, all force-strain curves were highly linear (R(2)>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.