Biomechanical measurements of torsion-tension coupling in human cadaveric femurs

The mechanical behavior of human femurs has been described in the literature with regard to torsion and tension but only as independent measurements. However, in this study, human femurs were subjected to torsion to determine if a simultaneous axial tensile load was generated. Fresh frozen human femurs (n=25) were harvested and stripped of soft tissue. Each femur was mounted rigidly in a specially designed test jig and remained at a fixed axial length during all experiments. Femurs were subjected to external and internal rotation applied at a constant angulation rate of 0.1 deg/s to a maximum torque of 12 N m. Applied torque and generated axial tension were monitored simultaneously. Outcome measurements were extracted from torsion-versus-tension graphs. There was a strong relationship between applied torsion and the resulting tension for external rotation tests (torsion/ tension ratio = 551.7 ± 283.8 mm, R 2 = 0.83 ± 0.20, n=25), internal rotation tests (torsion/tension ratio =495.3 ± 233.1 mm, R 2 = 0.87 ± 0.17, n = 24), left femurs (torsion/tension ratio =542.2 ±262.4 mm, R 2 =0.88 ± 0.13, n =24), and right femurs (torsion/tension ratio = 506.7 ± 260.0 mm, R 2 = 0.82 ± 0.22, n = 25). No statistically significant differences were found for external versus internal rotation groups or for left versus right femurs when comparing torsion/tension ratios (p = 0.85) or R 2 values (p =0.54). A strongly coupled linear relationship between torsion and tension for human femurs was exhibited. This suggests an interplay between these two factors during activities of daily living and injury processes.