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Biomechanical Measurements of Torsion-Tension Coupling in Human Cadaveric Femurs

[+] Author and Article Information
Rad Zdero1

Martin Orthopaedic Biomechanics Laboratory, Shuter Wing (Room 5-066), St. Michael’s Hospital, 30 Bond Street, Toronto, ON, M5B 1W8, Canada; Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, ON, M5B 2K3, Canadazderor@smh.ca

Alison J. McConnell

 Medtronic International Trading Sàrl, Tolochenaz CH-1131, Switzerland

Christopher Peskun

Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, M5S 1A8, Canada

Khalid A. Syed

Division of Orthopaedic Surgery, Toronto Western Hospital, Toronto, ON, M5T 2S8, Canada

Emil H. Schemitsch

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, Toronto, ON, M5B 1W8, Canada; Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, M5S 1A8, Canada

1

Corresponding author.

J Biomech Eng 133(1), 014501 (Dec 22, 2010) (6 pages) doi:10.1115/1.4002937 History: Received September 12, 2010; Revised October 11, 2010; Posted November 02, 2010; Published December 22, 2010; Online December 22, 2010

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 12Nm. 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±283.8mm, R2=0.83±0.20, n=25), internal rotation tests (torsion/tension ratio=495.3±233.1mm, R2=0.87±0.17, n=24), left femurs (torsion/tension ratio=542.2±262.4mm, R2=0.88±0.13, n=24), and right femurs (torsion/tension ratio=506.7±260.0mm, R2=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 R2 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.

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Figures

Grahic Jump Location
Figure 3

Cylinder spring models of the human femur in rotation around the mechanical axis: (a) experimental femur geometry and orientation yielded a measured average torsion/tension ratio=524.1 mm, (b) cylinder model using midshaft femur diameter DS=26.7 mm predicts a torsion/tension ratio=34.3 mm, and (c) cylinder model using full mediolateral femur width DW=79.1 mm predicts a torsion/tension ratio=300.8 mm. Note: DW≈(head diameter+neck length)×cos(neck-to-shaft angle−90 deg)×cos(assumed value ofshaft-to-mechanical axis angle)=(49.9+54.2 mm)×cos(129.5−90 deg)×cos(7 deg)=79.1 mm.

Grahic Jump Location
Figure 2

Typical torsion-versus-tension plot (femur no. 4, internal rotation). Experimental points are shown. Torsion/tension ratio was computed from the slope of the theoretical straight line-of-best-fit. The coefficient of linearity (R2) indicated the agreement between experimental data and the theoretical straight line-of-best-fit. Nominal intercept loads on the axes due to experimental axial preload or femur misalignment are not shown for presentation purposes, since they do not affect the parameters of interest.

Grahic Jump Location
Figure 1

Experimental setup with a human femur mounted for torsional testing. Torsion and tension levels were monitored simultaneously. Screws were used to secure the femoral head, which was inserted into the cup of a steel block simulating the pelvis. A fixation ring was used to position the distal condyles into place.

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