Femur fracture biomechanics and morphology associated with torsional and bending loading conditions in an in vitro immature porcine model

Purpose The objectives of this study were to describe fracture morphology resulting from common loading mechanisms such as bending and torsion in immature bone and to identify differences in the energy required to produce various fracture types under these two loading mechanisms using an immature porcine animal model. Methods Twenty-six in vitro immature porcine femora were mechanically tested in 3-point-bending and torsion. Femur specimens were tested with and without soft tissue and at both quasi-static and dynamic loading rates. Bone geometry and density measures were determined for each femur using dual-energy x-ray absorptiometry and plain film x-rays. Failure load, stiffness, and energy to failure were determined for each specimen from the load-displacement history from mechanical tests. Results 3-point bending tests resulted in 10 transverse fractures and 2 oblique fractures. Torsion tests resulted in spiral fractures. Mean energy required to produce transverse fractures (3.32 Nm) was double that associated with spiral fractures (1.66 Nm). In bending, specimens with soft tissue intact required significantly greater energy to fracture (4.40 Nm) than specimens with soft tissue removed (2.92 Nm). Torsional loading rate did not significantly affect energy to fracture. Conclusions Fracture morphology is dependent upon loading conditions. Energy to failure allows for comparison across various loading conditions, and thus offers an effective means of characterizing fracture thresholds for a wide range of scenarios. Consideration should be given to whether or not soft tissue is left intact when conducting experiments using whole bone specimens given its influence on energy to failure.