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Research Papers

Development of an Inertia-Driven Model of Sideways Fall for Detailed Study of Femur Fracture Mechanics

[+] Author and Article Information
Seth Gilchrist

Department of Mechanical Engineering,
University of British Columbia,
Vancouver, BC V6T-1Z4, Canada
e-mail: seth@mech.ubc.ca

Pierre Guy

Department of Orthopeadics,
University of British Columbia,
Vancouver, BC V5Z-1M9, Canada
e-mail: pierre.guy@ubc.ca

Peter A Cripton

Department of Mechanical Engineering,
University of British Columbia,
Vancouver, BC V6T-1Z4, Canada
e-mail: cripton@mech.ubc.ca

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received April 10, 2013; final manuscript received August 9, 2013; accepted manuscript posted September 12, 2013; published online October 4, 2013. Assoc. Editor: Tammy Haut Donahue.

J Biomech Eng 135(12), 121001 (Oct 04, 2013) (8 pages) Paper No: BIO-13-1183; doi: 10.1115/1.4025390 History: Received April 10, 2013; Revised August 09, 2013; Accepted September 12, 2013

A new method for laboratory testing of human proximal femora in conditions simulating a sideways fall was developed. Additionally, in order to analyze the strain state in future cadaveric tests, digital image correlation (DIC) was validated as a tool for strain field measurement on the bone of the femoral neck. A fall simulator which included models for the body mass, combined lateral femur and pelvis mass, pelvis stiffness, and trochanteric soft tissue was designed. The characteristics of each element were derived and developed based on human data from the literature. The simulator was verified by loading a state-of-the-art surrogate femur and comparing the resulting force-time trace to published, human volunteer experiments. To validate the DIC, 20 human proximal femora were prepared with a strain rosette and speckle paint pattern, and loaded to 50% of their predicted failure load at a low compression rate. Strain rosettes were taken as the gold standard, and minimum principal strains from the DIC and the rosettes were compared using descriptive statistics. The initial slope of the force-time curve obtained in the fall simulator matched published human volunteer data, with local peaks superimposed in the model due to internal vibrations of the spring used to model the pelvis stiffness. Global force magnitude and temporal characteristics were within 2% of published volunteer experiments. The DIC minimum principal strains were found to be accurate to 127±239μɛ. These tools will allow more biofidelic laboratory simulation of falls to the side, and more detailed analysis of proximal femur failure mechanisms using human cadaver specimens.

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Figures

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Fig. 2

In previous tests on osteoligamentous pelvises an instrumented impactor was dropped on the greater trochanter at 4.5 m/s. When the impactor came into contact with the trochanter a force spike was seen before deformation of the pelvis had begun, as indicated by highlighted peak in the inset graph (adapted from Beason et. al [55] with permission). This spike was created by the acceleration of the mass of the lateral pelvis and femur. Illustrations adapted from Gray et al. [67], copyright expired.

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Fig. 1

Schematic of the fall simulator showing the mass and spring structures that influence loading a fall to the side. meff_p+f is the effective mass of the lateral pelvis and femur.

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Fig. 3

A photo of the fall simulator showing each element of the model

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Fig. 4

An example response of the fall simulator plotted with human pelvis drop data [50]. The dashed line indicates the initial loading slope of the scaled volunteer data and the circle indicates the location of the peak forces. The human data was scaled by the ratio of the impact velocities.

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Fig. 5

The averages and standard deviations of the strain errors measured for each specimen. Three specimens, 5, 8, and 10, were subjected to camera vibration, leading to incorrect DIC strain readings.

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Fig. 6

Example data from the DIC analysis. Time versus strain plot (a) for specimen 16 shows the character and magnitude of the random noise, and an example DIC strain contour map (b) shows how the strain varied over the surface of the bone at the maximum applied load. The bone is oriented such that superior is to the left and lateral to the top. The head of the femur is in the lower left and the trochanter occupies the upper portion of the image, with the strain gauge wires visible on the right.

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