Elsevier

Bone

Volume 39, Issue 2, August 2006, Pages 401-407
Bone

Biomechanical comparison of hard and soft hip protectors, and the influence of soft tissue

https://doi.org/10.1016/j.bone.2006.01.156Get rights and content

Abstract

Introduction:

Hip protectors appear to be promising in preventing hip fractures. Currently, many different hip protectors exist, and it is not clear which hip protector has the best biomechanical properties. Therefore, the objective of this study was to compare the force attenuation capacity of 10 different hip protectors. Both hard hip protectors, which primarily shunt away energy, and soft hip protectors, which primarily absorb energy, were included.

Methods:

Using a drop weight impact testing system and a surrogate femur, a weight of 25 kg was dropped from a height of 8 cm causing a force of almost 7806 N on the bare femur, which simulates a severe fall. After this calibration test, soft tissue and the different hip protectors in combination with the soft tissue were tested. Each test was repeated six times. To simulate normal-weight elderly people, a 1/2-inch-thick layer of foam was chosen, reducing the force by 18%. To examine the influence of soft tissue thickness, soft tissue was also simulated by a 1-inch-thick layer of foam, reducing the force by 49%.

Results:

In the 1-inch soft tissue test, all hip protectors were capable in reducing the impact to below the average fracture threshold of elderly people (3100 N), although the hard types performed significantly better than the soft ones (P < 0.001). In the 1/2-inch soft tissue test, only the hard hip protectors were capable of attenuating the peak force to below the average fracture threshold of 3100 N (hard vs. soft hip protectors: P < 0.001).

Conclusions:

This study showed that the hard, energy-shunting hip protectors were superior to the soft, energy-absorbing ones, especially in a simulation of normal-weight elderly people. With increased soft tissue thickness, soft hip protectors were also capable in reducing the impact to below the average fracture threshold of 3100 N.

Introduction

More than 90% of all hip fractures are the consequence of a fall [1]. However, only 1–2% of all falls result in a hip fracture [2], [3]. For a fall to result in a hip fracture, the force applied to the proximal femur must exceed its strength [2]. Three conditions influencing this outcome are: (a) the faller must land on or near the hip; (b) protective responses must fail; and (c) local soft tissues must absorb less energy than necessary to prevent fracture [2]. Because gait speed decreases with increasing age [4], frail elderly people are more likely to land on the hip. Furthermore, reaction time slows with age and therefore, protective responses may be delayed. Absorption of energy may be decreased due to weakness or atrophy of the muscles and reduced fat around the hip and buttocks. In addition, bone strength decreases with aging.

A preventive measure to reduce the impact of a fall on the hip is the hip protector [5]. Basically, two types of hip protectors exist: (1) hard, shell-shaped protectors, which primarily shunt away energy towards the surrounding tissues, including femoral shaft, iliac crest and soft tissues; and (2) soft protectors, which primarily absorb energy. Their effectiveness in practice depends on two issues: (1) the force attenuation capacity, which is in our study defined as the “the capability of a hip protector to decrease the peak force”; and (2) compliance, which is influenced by wearing comfort [6]. In the literature, two biomechanical studies suggest that the force attenuating capacity of the hard, energy-shunting hip protectors is superior to the soft, energy-absorbing ones [7], [8]. However, compliance may be higher with soft hip protectors [9], [10].

In our study, we examined the force attenuation capacity of hip protectors that are currently commercially available. After the above two biomechanical studies were carried out, several new hip protectors have been developed and the biomechanical properties of existing hip protectors have been improved. Therefore, the aim of this study was to compare the force attenuation capacity of all hip protectors that were commercially available at the start of our study. In addition, based on an earlier study that reported a high correlation between increased soft tissue thickness and decreased peak force, the influence of soft tissue thickness on the peak force of the different hip protectors was examined [11].

Our hypotheses were: (1) The force attenuation capacity of hard hip protectors, which primarily shunt away energy, will be higher than those of soft hip protectors, which primarily absorb energy. (2) The force exerted on the hip will be lower for hip protectors combined with thicker soft tissue than for hip protectors combined with thinner soft tissue, because part of the energy will be absorbed by the soft tissue.

Section snippets

Hip protectors

All hip protectors were selected that could be identified by the literature or the Internet and were commercially available at the start of our study. Of the 11 manufacturers selected, nine different manufacturers were willing to participate. One manufacturer refused to participate, and one hip protector used in a previous patient study was no longer commercially available. Of each type, six underpants, including 12 protectors, were ordered. Of the Safehip hip protector, also the old model was

Results

In Table 2, the coefficients of variation for the different experiments are presented. In general, the coefficients of variation were very low (0.01–0.08), with two experiments having somewhat higher coefficients (0.18–0.19). In Fig. 3, two time-versus-force graphs are presented. The first graph represents the time-versus-force curves of the hard and soft hip protector with the lowest average peak force in the 1-inch soft tissue test. In the second graph, the results of the 1/2-inch soft tissue

Discussion

In this study, a biomechanical comparison of 10 different hip protectors was made. It was shown that, in combination with a thicker soft tissue layer, all hip protectors were able to reduce the peak force below the average fracture threshold of 3100 N. However, in combination with thinner soft tissue, only the hard hip protectors were able to reduce the peak force of a severe fall below the average fracture threshold. In both experiments, the hard hip protectors reduced the impact on the

Acknowledgment

We would like to thank the manufacturers for providing the hip protectors.

References (21)

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