Cortical and trabecular load sharing in the human femoral neckJournal of Biomechanics

About

Authors
Shashank Nawathe, Bich Phuong Nguyen, Nasim Barzanian, Hosna Akhlaghpour, Mary L. Bouxsein, Tony M. Keaveny
Year
2015
DOI
10.1016/j.jbiomech.2014.12.022
Subject
Orthopedics and Sports Medicine / Rehabilitation / Biophysics / Biomedical Engineering

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Text

Cortical and trabecular load sharing in the human femoral neck$

Shashank Nawathe a, Bich Phuong Nguyen b, Nasim Barzanian c, Hosna Akhlaghpour c,

Mary L. Bouxsein d, Tony M. Keaveny a,c,n a Department of Mechanical Engineering, University of California, Berkeley, CA, USA b Department of Chemical Engineering, University of California, Berkeley, CA, USA c Department of Bioengineering, University of California, Berkeley, CA, USA d Beth Israel Deaconess Medical Center, Boston, MA, USA a r t i c l e i n f o

Article history:

Accepted 26 November 2014

Keywords:

Femoral neck

Load sharing

Euler beam theory

Cortical vs trabecular a b s t r a c t

The relative role of the cortical vs trabecular bone in the load-carrying capacity of the proximal femur—a fundamental issue in both basic-science and clinical biomechanics—remains unclear. To gain insight into this issue, we performed micro-CT-based, linear elastic finite element analysis (61.5-micron-sized elements; 280 million elements per model) on 18 proximal femurs (5M, 13F, ages 61–93 years) to quantify the fraction of frontal-plane bending moment shared by the cortical vs trabecular bone in the femoral neck, as well as the associated spatial distributions of stress. Analyses were performed separately for a sideways fall and stance loading. For both loading modes and across all 18 bones, we found consistent patterns of load-sharing in the neck: most proximally, the trabecular bone took most of the load; moving distally, the cortical bone took increasingly more of the load; and more distally, there was a region of uniform load-sharing, the cortical bone taking the majority of the load. This distal region of uniform load-sharing extended more for fall than stance loading (7778% vs 5176% of the neck length for fall vs. stance; mean7SD) but the fraction of total load taken by the cortical bone in that region was greater for stance loading (8875% vs. 6479% for stance vs. fall). Locally, maximum stress levels occurred in the cortical bone distally, but in the trabecular bone proximally. Although the distal cortex showed qualitative stress distributions consistent with the behavior of an Euler-type beam, quantitatively beam theory did not apply. We conclude that consistent and well-delineated regions of uniform load-sharing and load-transfer between the cortical and trabecular bone exist within the femoral neck, the details of which depend on the external loading conditions. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction

Starting out in the early 1990s with my PhD, I looked up to a number of the “big guys” in the field, one of whom was Rik

Huiskes. He was an editor of Journal of Biomechanics at the time, and had written some of the best papers in what had been my dissertation topic, finite element analysis of total hip replacements. In my mind, this guy was a superstar! I remember seeing

Rik while he was boarding a plane (he was with his family on the way to ORS). I recognized him—we had been introduced very briefly once before—but I was too shy to say anything to him, and he, not recognizing me and presumably pre-occupied with his family, looked right through me. So much for that. But shortly later I actually met him. And he was so nice and friendly, and so genuinely interested in my work. All the young crowd wanted to hear what

Rik had to say about their work, and he always generously engaged in what was always an insightful discussion. And then there were the beers and stories and fun times. Rik was an incredible role model to all us young crowd back then. And he still is. The topic of this paper is right out of his playbook. - TMK

A fundamental but poorly understood structure–function issue in both basic-science and clinical biomechanics is the relative role of the cortical and trabecular bone in the load-carrying capacity of the proximal femur, particularly in the femoral neck. Cadaver studies have reported that both cortical bone geometry (Bousson et al., 2006) and trabecular bone density (Lang et al., 1997), as measured with quantitative computed tomography (CT), are strongly associated with femoral strength. However, views diverge as to the relative contributions of the cortical and trabecular compartments to overall femoral strength (Bouxsein and Fajardo, 2005; Mayhew et al., 2005; Turner, 2005). Whereas, some have reported that the contribution of the trabecular bone to the strength of the femoral neck is only marginal (Holzer et al., 2009; Koivumaki et al., 2012), others have suggested that both cortical and trabecular bone work in synergy (Manske et al., 2009; Nawathe et al., 2014; Riggs et al., 2004). Using finite element

Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com

Journal of Biomechanics http://dx.doi.org/10.1016/j.jbiomech.2014.12.022 0021-9290/& 2014 Elsevier Ltd. All rights reserved. ☆Funding source: National Science Foundation via XSEDE TG-MCA00N019. n Corresponding author at: 6175 Etcheverry Hall, University of California, Berkeley, CA 94720-1740, USA. Tel.: þ1 510 642 8017; fax: þ1 510 642 6163.

E-mail address: tmk@me.berkeley.edu (T.M. Keaveny).

Journal of Biomechanics 48 (2015) 816–822 analysis, Lotz et al. (1995) estimated that the cortical bone supports approximately 50% of the load at the mid-neck and 96% of the load at the proximal end of the neck, during both normal gait and a sideways fall on the greater trochanter. However, that analysis included only one bone, and its limited spatial resolution prevented a realistic description of the true cortical thickness. Addressing the need for higher resolution, more recent investigations developed more mechanistically correct models (Nawathe et al., 2014; Verhulp et al., 2008); however, these investigations have not yet focused on quantifying load-sharing between the cortical and trabecular bone.

Thus, the overall goal of this study was to elucidate how load is shared between the cortical and trabecular bone in the human femoral neck, accounting for real bone microstructure, multiple bones, and different loading conditions. 2. Materials and methods 2.1. Specimen preparation and imaging