Elsevier

Journal of Biomechanics

Volume 101, 5 March 2020, 109599
Journal of Biomechanics

New insights on the proximal femur biomechanics using Digital Image Correlation

https://doi.org/10.1016/j.jbiomech.2020.109599Get rights and content

Abstract

Finite element analyses (FEAs) of human femurs are mostly validated by ex-vivo experimental observations. Such validations were largely performed by comparing local strains at a small subset of points to the gold standard strain gauge (SG) measurements. A comprehensive full field validation of femoral FEAs including both strains and displacements using digital image correlation (DIC) full field measurements, especially at medial and lateral surfaces of the neck that experience the highest strains, provide new insights on femurs’ mechanical behavior.

Five cadaver femurs were loaded in stance position and monitored at the shaft and neck using two DIC systems simultaneously. DIC strains were compared to SG measurements at a limited number of locations so to corroborate DIC measurements by the gold standard technique. These were used to quantitatively assess the validity of FEA strains prediction especially at the neck where fracture usually occurs.

Strains measured by DIC correspond well to the SG observations. An excellent agreement was observed between DIC and FEA predicted strains excluding the superior neck surface: FE=1.02×DIC-17,r2=0.977. At the superior neck however, strains were not well predicted by FEA models: Although the FEA predicted high strains at the ’saddle region’, these were not observed experimentally. On the other hand, strain concentrations were measured by DIC at numerous vessel holes which were not represented by FE models. Since fractures usually initiate at the subcapital region in stance position ex-vivo experiments, where numerus vessel holes exist, these vessel holes may be required to be accounted for in future FE models so to allow a better estimation of the fracture load. Full field measurements are mandatory to allow a better validation of fracture load and location predictions which are of high clinical importance.

Introduction

Finite element (FE) models of femurs based on quantitative computed tomography (QCT) have been used extensively to estimate bone stiffness and strength (Yosibash et al., 2007, Trabelsi et al., 2011, Engelke et al., 2016, de Bakker et al., 2017). These models may be used in clinical practice for fracture risk prediction due to osteoporosis and metastases (Yosibash et al., 2014, Dragomir-Daescu et al., 2011, Keyak et al., 2011, Sternheim et al., 2018) as well as for an optimal implant selection (Katz et al., 2018, Cilla et al., 2017). FE bone models were usually validated by comparing computed surface strains to experimental measurements at same locations. Average strains on a small surface, measured by strain gauges (SGs) are considered to be the gold standard in FE model validation of femurs (Trabelsi et al., 2011, Wille and Rank, 2012, Eberle et al., 2013, Pettersen et al., 2009, Ruess et al., 2012, Schileo et al., 2014). In recent years, digital image correlation (DIC) has emerged as a promising technology to validate FE models accounting for thousands of measured points (Jetté et al., 2017, Grassi et al., 2016, Grassi et al., 2014, Grassi et al., 2013, Väänänen et al., 2013, Dickinson et al., 2011). DIC is a non-contact optical technique that enables a three-dimensional (3D), full field measurement and a complete evaluation of displacements and strains on bone surface.

Here, we apply DIC techniques on cadaveric femurs to monitor their mechanical response at areas where highest principal strains are noticed including the femoral neck which has been mostly neglected in past publications. These measurements are used to further enhance the validation of QCT based FE models, providing new insights on the mechanical behavior and modelling of the femoral neck, which is of high importance for fracture prediction.

Only few validation studies performed 3D DIC measurements on femurs loaded in a stance position, composite femurs were mostly considered (Jetté et al., 2017, Väänänen et al., 2013, Grassi et al., 2013, Dickinson et al., 2011). DIC measurements are easier on composite femurs because of their smooth and dry surface, whereas cadaveric femurs have rough surfaces in some areas and are wet or greasy (which may cause glare). Additionally, composite femurs do not represent well the geometry and material properties of cadaver femurs (Grant et al., 2007, Zdero et al., 2008). To the best of our knowledge there exists only one study that addressed actual femurs (Grassi et al., 2014), however the superior neck region was not reported nor a validation of the DIC to gold standard SGs was performed. Such a comparison between DIC and SG was not yet documented on cadaver femurs (Ghosh et al., 2012, Gilchrist et al., 2013).

Femoral frontal surface is most commonly monitored by DIC (Väänänen et al., 2013, Grassi et al., 2013, Grassi et al., 2014, Grassi et al., 2016, Jetté et al., 2017). This approach has some limitations: (a) Because strains are derivatives of the displacements, DIC technique cannot calculate strains on the edges of the viewed region where the extreme values are found. Also, strains on these edges may be erroneous due to the curvature of the specimen (Jetté et al., 2017, Dickinson et al., 2011). (b) Area of measurement is close to the neutral axis where strains are low and noise may become dominant.

Although the superior neck is of special interest since neck fractures usually start there in stance position experiments, no DIC measurements are reported in past publications for that location. Displacements measured by DIC are also rarely addressed. The single study to our knowledge accounting for displacements is by Jetté et al. (2017), where composite femurs were used.

Here five fresh frozen femurs were used to validate QCT based high order FE models (one of the femurs has sever osteoarthritis as may be common in the elderly population). DIC strains were compared to strain gauge measurements and thereafter compared to FE computed strains for validation. Displacements were also addressed for validation. Lateral and medial surfaces of shaft and neck were examined (similarly to past studies performed with SGs). These surfaces experience highest strains and thus are of interest. DIC measurements on the neck’s superior surface allow new insights on its mechanical behaviour and modelling.

Section snippets

Methods

Five fresh frozen femurs were defrosted, thoroughly cleaned of soft tissue using scalpels and tweezers, cut at the shaft 240 mm distally of the lesser trochanter and imbedded inside a steel cylinder using PMMA (Fig. 1). Femurs were CT scanned with K2HPO4 calibration phantoms (Mindways, 2002, Katz et al., 2019). Donors’ and CT details are given in Table 1.

Femurs were painted white and speckled black using spray paint (white matte aerosol spray by Tambour LTD and black 2X Ultra Cover Flat Spray).

Comparing DIC to SG

A comparison between SG strains to DIC strains is presented in Fig. 3. Locations of the SGs inside the analyzed strain field are given in the supplementary material (Fig. A.1).

RMSEs between DIC and SG were below 50 μs in most cases. A poor agreement was obtained for SG2 at FFI1L neck (RMSE  = 407 μs). This SG was located near the edge of the DIC analyzed field which lead to high noise in the DIC strains. Because of the noise, principal strain is not zero at zero load after time smoothening.

Discussion

To the best of our knowledge, this is the first study that considered full field measurements using two DIC systems (4 cameras) simultaneously to monitor the mechanical response of fresh frozen cadaver femurs, monitoring medial and lateral bone surfaces which experience the highest strains. In previous studies (Jetté et al., 2017, Grassi et al., 2013, Grassi et al., 2016, Väänänen et al., 2013) the femoral frontal surface (including neutral axis) was monitored. In these studies, the superior

Limitations and future work

The observations reported in the current study are based on five femurs, only in four of which the superior neck was monitored (three different donors). Further specimens should be investigated in the future to further increase the sample size and enhance validity of the conclusions.

The misrepresentation of the femoral neck mechanical response demand further investigation. The high strains at the neck ’saddle region’ may be due to poor cortex identification in the CT scan (Prevrhal et al., 2003

Declaration of Competing Interest

YK has no conflicts of interest. ZY has a financial interest in PerSimiO.

Acknowledgements

The authors thank Dr. Nimrod Snir from Sourasky Medical Center, Tel Aviv, Israel for his help in cleaning the bones from soft tissue and helping with the CT-scans and Ms. Gal Dahan from TAU for her help with the experiments.

References (40)

Cited by (26)

  • Digital image correlation based on convolutional neural networks

    2023, Optics and Lasers in Engineering
    Citation Excerpt :

    Over the past three decades, digital image correlation (DIC) technique has already become a classical non-destructive photomechanical method to characterize spatio-temporal deformation fields of specimen surfaces [1–6]. Owing to its flexibility, reliability, robustness and ease of use [7–9], DIC has been increasingly applied to a large number of engineering kingdoms concerning non-contact full-field measurements, such as experimental mechanics [10–14], biomechanics/cell mechanics [15–17], structural health monitoring [18,19], fracture mechanics [20–22] and composite mechanics [23–26]. The core of the traditional DIC algorithms lies in implementing numerical comparisons between undeformed speckle images (i.e., reference images) and the corresponding deformed ones (i.e., target images) and eventually acquiring full-field displacement and strain knowledges [27–31].

  • The influence of foramina on femoral neck fractures and strains predicted with finite element analysis

    2022, Journal of the Mechanical Behavior of Biomedical Materials
    Citation Excerpt :

    The larger region where high strains are measured, compared to the predicted strains, suggests that other factors, such as remaining soft tissue, could play a role. Similar results have been observed under loading in single-leg-stance, where high strains measured with DIC were observed outside of the foramina (Katz and Yosibash, 2020). These strains were also not captured by FE models based on clinical CT scans.

View all citing articles on Scopus
View full text