Using Finite Element Techniques to Examine the Changes in Femoral Strength due to Long Duration Bedrest
K. O. Genc,
1, 2
and P.R. Cavanagh
1
1
Department of Orthopaedics and Sports Medicine, University of Washington, Seattle, Washington, USA,
2
Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio, USA
Email: cavanagh@u.washington.edu
INTRODUCTION
We are conducting a bedrest study to examine the effect of individualized loading profiles on bone homeostasis by calculating bone mineral density (BMD) changes from computed tomography (CT) scans [1]. The purpose of the current study is to expand this BMD analysis and examine the use of CT and voxel-based subject-specific finite element (FE) models to quantify the changes in femoral strength.
METHODS
To date, eleven subjects (6 male, 5 female), have been randomized to control (n=6) or treatment (exercise; n=5) groups and confined for 12 weeks to 6-degree head-down bedrest in the Clinical Research Unit at the Cleveland
Clinic. The treatment group underwent individualized daily exercise programs in the Zero Gravity Locomotion
Simulator (ZLS) designed to replace their daily mechanical load stimulus experienced during free-living. QCT scans from the start and end of bedrest were used to assess changes in bone status.
The CT dataset was calibrated using a five-sample calibration phantom (Mindways Software Inc.) placed below the hips during scanning. Simpleware (Simpleware Ltd) and Matlab (Mathworks) was then used to semi-automatically segment the right proximal femur, re-sample it to 3mm
3
voxels, and assign elastic modulus and strength values to each voxel. These model geometric and material properties were then exported to ABAQUS (Dassault Systemes,
Simulia) for FE analysis. A loading condition representing a fall onto the posterolateral aspect of the greater trochanter was modeled (Figure 1), and the failure load of the proximal femur was computed after Keyak et al. [3].
RESULTS AND DISCUSSION
The failure loads and locations in the proximal femur showed a pattern consistent with previous models from the literature [3]. With further sensitivity analysis, this modeling technique shows promise to determine if the exercise countermeasures used were able to prevent losses in bone strength.
Figure 1: (a) Transverse plane QCT image with the segmented (right) femur shown in green and the calibration phantom shown below. (b) Voxel-based FE model of proximal femur with loading conditions representing a fall onto the posterolateral aspect of the greater trochanter. A load is applied to the femoral head (red arrows) and the greater trochanter and base of the femoral shaft are constrained (orange and blue triangles).
REFERENCES
[1] Genc KO et al. (2009) American Society of Biomechanics; August 26-29; University Park, PA. [2] Keyak J.H. et al. (2009) Bone 44, 449-53. [3] Keyak JH, et al. (1998) J Biomech 31, 125-33.
ACKNOWLEDGEMENT: The advice of Dr, Joyce Keyak during this study has been much appreciated.