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Research Papers

The Force Attenuation Provided by Hip Protectors Depends on Impact Velocity, Pelvic Size, and Soft Tissue Stiffness

[+] Author and Article Information
Andrew C. Laing1

Faculty of Applied Sciences, Injury Prevention and Mobility Laboratory, School of Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6, Canadaalaing@sfu.ca

Stephen N. Robinovitch

Faculty of Applied Sciences, Injury Prevention and Mobility Laboratory, School of Kinesiology, and School of Engineering Science, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6, Canada

1

Corresponding author.

J Biomech Eng 130(6), 061005 (Oct 09, 2008) (9 pages) doi:10.1115/1.2979867 History: Received January 29, 2008; Revised April 19, 2008; Published October 09, 2008

Abstract

Wearable hip protectors represent a promising strategy for preventing hip fractures. However, there is lack of agreement on biomechanical testing standards and subsequent uncertainty about the ability of hip protectors to attenuate impact force during a fall. To address this issue, we designed a fall impact simulator that incorporated a “biofidelic” surrogate pelvis, which matched the surface geometry and soft tissue stiffness measured in elderly women $(n=15)$. We then used this system to measure the attenuation in peak femoral neck force provided by two commercially available soft shell protectors (Safehip Soft and Hipsaver) and one rigid shell protector (Safehip Classic). Finally, we examined how the force attenuation provided by each protector was influenced by systematic changes in fall severity (impact velocity), body size (pelvis size), and soft tissue stiffness. With the biofidelic pelvis, the force attenuation averaged over all impact velocities was 27% for Safehip Soft, 17% for Safehip Classic, and 19% for Hipsaver. However, the rank order of hip protectors (and especially the performance of Safehip Classic) varied with the test conditions. Safehip Classic attenuated force by 33% during a low velocity $(1m∕s)$ fall, but only by 8% for a high velocity $(4m∕s)$ fall. In the latter condition, improved attenuation was provided by the soft shell hip protectors (19% by Safehip Soft and 21% by Hipsaver). As soft tissue stiffness increased from softest to most rigid, the attenuation provided by Safehip Classic increased 2.9-fold (from 26% to 76%), while Safehip Soft increased 1.7-fold (from 36% to 60%) and Hipsaver increased 1.1-fold (from 36% to 38%). As pelvis size decreased from largest to smallest, the attenuation provided by Safehip Classic increased 8-fold, but for a high velocity fall and moderate tissue stiffness, never exceeded that provided by Safehip Soft and Hipsaver. Our results indicate that, under biofidelic testing conditions, the soft shell hip protectors we examined generally provided greater force attenuation (averaging up to 27%) than the hard shell protector. Measured values of force attenuation were highly sensitive to variations in impact velocity, pelvic size, and pelvic soft tissue stiffness. This indicates the need to develop international testing standards to guide market approval, the selection of protectors for clinical trials, and the design of improved hip protectors.

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Figures

Figure 1

Demonstration of the steps used in determining pelvic surface geometry from the elderly participants. (a) Photograph of the participant during the experiment, showing the 10×10 grid of markers recorded by the motion capture system. (b) 3D representation of the markers in the motion capture software. (c) We aligned the frontal and sagittal midlines from each subject and defined a polar (r,θ) coordinate system having its original at the common body midpoint. For each subject we plotted best-fit splines representing the marker positions at each transverse plane. We then determined the average (r) distance from the origin to the spline surface at ten angles: reference (body midpoint to left GT), 10deg, 20deg, 30deg, 40deg, and 50deg posterior to the reference angle and 10deg, 20deg, 30deg, and 40deg anterior to the reference angle.

Figure 2

Methods used to determine soft tissue stiffness. (a) Schematic of the indentation device. (b) Sample force-deflection curve; stiffness was determined as the slope of the curve between 90N and 110N. (c) Average soft tissue stiffness for elderly women (n=15) and the surrogate pelvis. Error bars show one SD.

Figure 3

Elements of the hip impact simulator and associated test methods. (a) Simon Fraser University hip impact simulator. (b) Schematic of the surrogate pelvis. (c) Photographs of the three hip protectors tested in the current study (from top: Safehip Classic, Safehip Soft, and Hipsaver). (d) Schematic of the small, medium, and large pelvis size conditions.

Figure 4

Sample force versus time traces for unpadded and padded trials with the biofidelic surrogate pelvis at an impact velocity of 3.0m∕s: (a) total force and (b) femoral neck force

Figure 5

Ftotal and Fneck in the unpadded (baseline) and hip protector conditions as a function of (a) impact velocity with the biofidelic surrogate pelvis, (b) pelvic surface geometry, and (c) soft tissue stiffness

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