Elsevier

Clinical Biomechanics

Volume 25, Issue 1, January 2010, Pages 63-69
Clinical Biomechanics

Effect of hip protectors, falling angle and body mass index on pressure distribution over the hip during simulated falls

https://doi.org/10.1016/j.clinbiomech.2009.08.009Get rights and content

Abstract

Background

We examined how a soft shell hip protector affects the magnitude and distribution of force to the hip during simulated falls, and how the protective effect depends on the fall direction and the amount of soft tissue padding over the hip.

Methods

Fourteen young women with either high or low body mass index participated in a “pelvis release experiment” that simulated falls resulting in either lateral, anterolateral or posterolateral impact to the pelvis with/without a soft shell hip protector. Outcome variables were the magnitude and location of peak pressure (d, theta) with respect to the greater trochanter, total impact force, and percent force applied to four defined hip regions.

Findings

The soft shell hip protector reduced peak pressure by 70%. The effect was two times greater in low than high body mass index individuals. The protector shunted the peak pressure distally along the shaft of the femur (d = 52 mm (SD 22), theta = −21° (SD 49) in the unpadded trials versus d = 81 mm (SD 23), theta = −10° (SD 35) in the padded trials). Peak force averaged 12% greater in posterolateral and 17% lower in anterolateral than lateral falls.

Interpretation

Our results indicate that the hip protector we tested had a much stronger protective benefit for low than high body mass index individuals. Next generation protectors might be developed for improved shunting of pressure away from the femur, improved protection during posterolateral falls, and greater force attenuation for low body mass index individuals.

Introduction

Hip fractures are an enormous public health problem for the elderly. Ninety percent of hip fractures are caused by falls. An estimated 1.3 million hip fractures occurred worldwide in 1990 (Johnell and Kanis, 2004). Approximately 20% of older adults hospitalized for hip fracture die within a year, and about 50% suffer a major decline in independence (Empana et al., 2004, Wolinsky et al., 1997). Fracture risk increases exponentially with age, and given the aging of the population, the global incidence of hip fracture is projected to increase 4-fold to 6 million annual cases by 2050 (Gullberg et al., 1997). Health care costs for hip fractures are estimated at $12.1 billion in 2005, and projected to grow incurring $25.3 billion by 2025 (Burge et al., 2007).

Hip protectors represent a promising strategy for preventing hip fractures. They are intended to reduce impact force at the greater trochanter (GT) by shunting the force onto the surrounding soft tissues, or by absorbing energy. Robinovitch et al. (1995a) reported that total force at the femoral neck was attenuated 68% by an energy-shunting hip pad. In a simulated fall experiment, Wiener et al. (2002) asked standing participants to fall sideways on a hard surface while wearing a hip protector. A piezoeletric film sensor was placed between the hip protector and the skin over the hip. They found that only 5% or less impact force was transmitted to the skin sensor. However, the sensor did not cover the whole surface of the protector and therefore did not measure distribution of force or pressure over the entire contact area. Recently, Laing and Robinovitch (2008a) tested soft shell protectors with human subjects, and found that the mean pressure over the GT was reduced 76% by a 14 mm thick horseshoe-shaped protector and 73% by a 16 mm thick continuous protector. However, their measurements provided only the average pressure over circular areas centered at the GT, and not the exact location of peak pressure. In the current study, we obtained high speed, high resolution maps using a two-dimensional pressure distribution device (RSscan) to gain new insight on the pressure distribution profile and the benefit of hip protectors.

It is known that people with high body mass index (BMI, weight/height2) have lower risk for hip fracture in a fall than people with low BMI (La Vecchia et al., 1991). One possible reason is that individuals with high BMI are likely to have a thicker layer of fat tissue over the greater trochanter, which provides mechanical shock absorption during a fall (Robinovitch et al., 1995b, Lauritzen et al., 1993). However, no studies have investigated the effect of BMI on pressure distribution during falls with or without a hip protector.

The effect of impact direction on hip fracture risk was examined by Keyak et al. (2001) and Pinilla et al. (1996), who reported that the failure load of the cadaveric femur decreased by 24% as the loading angle changed from lateral to 30° posterolateral, indicating a greater danger for hip fracture posed by posterolateral falls. Nankaku et al. (2005) measured impact force and velocity of the greater trochanter during simulated falls in the posterior, posterolateral, and lateral directions, and found that impact force was highest in posterolateral falls. These results collectively suggest that posterolateral falls create high risk for hip fracture. However, most hip protectors are designed to protect primarily against sideways falls. An important question is whether they also reduce impact force and redistribute pressure in posterolateral falls.

Against this background, we studied human subjects during falling experiments onto pressure profile sensors and examined how a soft shell hip protector affects the distribution of pressure, and how the protective effect depends on BMI and falling impact configuration.

Section snippets

Subjects

Fourteen young women between the ages of 18 and 35 participated. We included only women because hip fractures are approximately 3-fold more common in older women than men (Chevalley et al., 2007, Bjorgul and Reikeras, 2007, Lonnroos et al., 2006). We excluded individuals with musculoskeletal problems such as arthritis, thoracic outlet syndrome, or recent rotator cuff tears, contracture, sprain, and strain. We measured individuals’ weight, height, and hip girth. Height ranged from 160 to 172 cm.

Results

Peak pressure was associated with presence or absence of the hip protector, impact angle configuration, and BMI. Across all padded and unpadded conditions, peak pressure decreased 70% by use of the protector (F = 13.7, P = 0.003), peak pressure was on average 38% lower in anterolateral than lateral falls (F = 4.9, P = 0.03), and 266% higher in low BMI than high BMI participants (F = 6.7, P = 0.024) (Fig. 4). Furthermore, there was a significant interaction between hip protector condition and BMI (F = 7.8, P = 

Discussion

The soft shell hip protector we tested substantially reduced peak pressure on the GT, and redirected the location of peak pressure distally along the diaphysis. However, it had no effect on total applied force. This suggests that the tested protector had little influence on total stiffness, or consequently total impact force, while causing a substantial change in local stiffness and pressure distribution. The same mechanical analysis explains our observation that BMI was associated with peak

Acknowledgements

This research experiment was funded in part by NSERC operating grant (Grant # RGPIN239735). S. Robinovitch is a consultant to Tytex A/S, manufacturer of the Safehip line of hip protectors (not included in the present study).

References (26)

  • J.P. Empana et al.

    Effect of hip fracture on mortality in elderly women: the EPIDOS prospective study

    J. Am. Geriatr. Soc.

    (2004)
  • B. Gullberg et al.

    World-wide projections for hip fracture

    Osteoporos Int.

    (1997)
  • O. Johnell et al.

    An estimate of the worldwide prevalence, mortality and disability associated with hip fracture

    Osteoporos Int.

    (2004)
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