Comparing the fracture limits of the proximal femur under impact and quasi-static conditions in simulation of a sideways fall

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Highlights

  • Eight pairs of proximal femurs were tested in quasi-static and impact conditions.

  • Fracture load was higher in impact (falls) than quasi-static (typical research rate).

  • Fracture locations showed qualitatively good agreement between the two groups.

  • Forces were well correlated and a relationship between the two conditions was defined.

  • Applied to a previous quasi-static study showed a strong prediction of impact failure.

Abstract

Sideways falls onto the hip are responsible for a great number of fractures in older adults. One of the possible ways to prevent these fractures is through early identification of people at greatest risk so that preventive measures can be properly implemented. Many numerical techniques that are designed to predict the femur fracture risk are validated through performing quasi-static (QS) mechanical tests on isolated cadaveric femurs, whereas the real hip fracture is a result of an impact (IM) incident. The goal of this study was to compare the fracture limits of the proximal femur under IM and QS conditions in the simulation of a sideways fall to identify any possible relationship between them.

Eight pairs of fresh frozen cadaveric femurs were divided into two groups of QS and IM (left and right randomized). All femurs were scanned with a Hologic DXA scanner and then cut and potted in a cylindrical tube. To measure the stiffness in two conditions of the single-leg stance (SLS) and sideways fall (SWF), non-destructive tests at a QS displacement rate were performed on the two groups. For the destructive tests, the QS group was tested in SWF configuration with the rate of 0.017 mm/s using a material testing machine, and the IM group was tested in the same configuration inside a pneumatic IM device with the projectile target displacement rate of 3 m/s.

One of the IM specimens was excluded due to multiple strikes. The result of this study showed that there were no significant differences in the SLS and SWF stiffnesses between the two groups (P = 0.15 and P = 0.64, respectively). The destructive test results indicated that there was a significant difference in the fracture loads of the two groups (P < 0.00001) with the impact ones being higher; however, they were moderately correlated (R2 = 0.45). Also, the comparison of the fracture location showed a qualitatively good agreement between the two groups.

Using the relationship developed herein, results from another study were extrapolated with errors of less than 12%, showing that meaningful predictions for the impact scenario can be made based on the quasi-static tests. The result of this study suggests that there is a potential to replace IM tests with QS displacement rate tests, and this will provide important information that can be used for future studies evaluating clinical factors related to fracture risk.

Introduction

Osteoporosis is a generalized skeletal disorder in which a reduction in Bone Mineral Density (BMD) decreases bone strength and can result in an increased risk of fracture. This disease is more common in older adults and is the one of the main reasons for a broken bone among this population. One of the sites commonly affected by osteoporosis is the proximal femur, the fracture of which greatly decreases mobility and function, as well as being responsible for high health care costs for society (Mears and Kates, 2015). Therefore, it is crucial to prevent fractures from happening through early identification of people at a great risk of sustaining a fracture, who may then use protective measures such as hip protectors (Laing and Robinovitch, 2008), energy attenuating floors (Bhan et al., 2014), targeted exercise (Nikander et al., 2010), and pharmacological interventions (L. Yang et al., 2014).

To realistically estimate a person's fracture risk, one needs to understand the fracture mechanism. Many studies have endeavored to create a framework to predict femur fracture risk from a fall. In these studies, the correlation of the fracture load with Finite Element Analysis (FEA) predictions (Dall'Ara et al., 2013; Koivumäki et al., 2012), BMD (Boehm et al., 2008; Leichter et al., 2001), bone's geometry (X. J. Yang et al., 2018), and other factors has been investigated, and typically validated via constant-displacement-rate experiments on cadaveric femur specimens (Gilchrist et al., 2014). However, in reality, the incident that leads to a fracture is rarely quasi-static (QS) or at a constant displacement rate (Gilchrist et al., 2013).

Investigating how a QS constant-displacement-rate experiment might differ from a fall impact (IM) simulation has great importance: if the femur's structural behavior under impact is comparable to a quasi-static test, or at least the relationship between them can be established, experimental impact testing can be replaced with quasi-static ones. It also allows us to extrapolate results from previous studies to conditions more representative of real-life falls.

Bone is well understood to be a viscoelastic material (Carter & Hayes, 1977), and some previous studies have investigated how bone material properties such as elastic modulus and compressive strength depend on the strain rate (Linde et al., 1991; Prot et al., 2016). In addition, the behavior of a whole bone at higher strain rates has been shown to be different from small samples due to the entrapped marrow (Askarinejad et al., 2019; Linde et al., 1991). However, the question remains as to how much this will influence the proximal femur during a sideways fall with an average impact velocity of 3 m/s (Feldman and Robinovitch, 2007; Fleps et al., 2018; van den Kroonenberg et al., 1996). The effect of loading rate on the proximal femur fracture load has been investigated in three previous studies by Courtney et al. (1994), Gilchrist et al. (2014), and Askarinejad et al., (2019). The results of the first study showed that both fracture load and structural stiffness were greater by 20% and 200% with a higher quasi-static displacement rate (displacement rate of 100 mm/s vs. 2 mm/s). In the second study (Gilchrist et al., 2014), three rates of 0.5 mm/s, 100 mm/s, and 3 m/s (IM) were tested and the results showed no significant differences between the stiffness and fracture load in the IM group (displacement rate of 3 m/s) and the QS group with a displacement rate of 0.5 mm/s (QS slow). However, the comparison between the IM group and the QS group with a rate of 100 mm/s (QS fast) showed a significant difference. Finally, in the third study (Askarinejad et al., 2019), while most of the fracture loads in the impact condition were higher than QS ones, no statistically significant difference was observed due to the small sample size and lack of paired specimens. Although bones are expected to have higher fracture loads in impact (Enns-Bray et al., 2018), these three experimental studies (Askarinejad et al., 2019; Courtney et al., 1994; Gilchrist et al., 2014), didn't have a clear and strong consensus on how displacement rate influences proximal femurs structural fracture load, since the results may have been influenced either by the different characteristics of each group (BMD, geometry, age, sex), or lack of statistical power.

Therefore, a new study with paired specimens was proposed to properly quantify any possible effect of loading rate (quasi-static vs impact) on the strength of proximal femurs in a sideways fall configuration and investigate if there is a potential relationship between the IM fracture loads and QS ones. With respect to that, the objectives of this research were to (1) determine how femoral fracture load differs between QS loading and IM loading in the simulation of a sideways fall, and quantify any possible relationship between them, and (2) to determine if the fracture patterns differ between these two scenarios.

Section snippets

Specimen preparation

This research was approved by the Hamilton Integrated Research Ethics Board (HiREB). Eight pairs of fresh-frozen human cadaveric femurs (67.4 ± 6.6 years old, four males and four females, Table 1) were obtained and cleaned of all soft tissues. There were no reported musculoskeletal diseases for the specimens. To obtain the BMD, each femur was scanned with a Hologic DXA scanner (Hologic Discovery A, Hologic, Inc., MA, USA). To simulate the soft tissue during the scan a plastic container filled

Results

Comparison of the two groups showed no significant differences in total BMD (P = 0.63), (Table 1). Also, neither the QS stiffness measure differed between groups, with SLS averaging 647 ± 155 N/m and 748 ± 111 N/m for the QS and IM, respectively (P = 0.15), and SWF averaging 570 ± 131 N/m and 598 ± 97 N/m for the QS and IM groups, respectively (P = 0.64).

The average velocity of the projectile in the IM group was 3.24 ± 0.08 m/s, with impact durations of less than 30 ms (Fig. 5). Also, in the QS

Discussion

Sideways falls onto the hip are responsible for a great number of hip fractures among older adults (Kannus et al., 2006; Parkkari et al., 1999; Wei et al., 2001). One of the most effective methods to prevent these fractures is the early identification of people at greatest risk (Johnell et al., 2005). In addition to BMD measurement, numerical methods such as finite element analysis and image processing are useful tools currently receiving a lot of attention (Bessho et al., 2007; Dall'Ara et

CRediT authorship contribution statement

Fatemeh Jazinizadeh: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Hojjat Mohammadi: Investigation, Writing - review & editing. Cheryl E. Quenneville: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing - review & editing.

Declaration of competing interest

There are no financial or personal relationships that could inappropriately influence this work, and there is no conflict of interest.

Acknowledgments

This work was supported by the Labarge Optimal Aging Initiative. Also, the authors would like to thank Chantal Saab for help in DXA scanning of the specimens.

References (42)

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