Computed tomography porosity and spherical indentation for determining cortical bone millimetre-scale mechanical properties

Oliver R Boughton,Richard L Abel,James Marrow,Quentin Grimal,Laura Peralta,Liye Yan,Justin P Cobb,Finn Giuliani,Shaocheng Ma,Pascal Laugier,Ulrich Hansen,Xiran Cai

doi:10.1038/s41598-019-43686-6

Abstract

The cortex of the femoral neck is a key structural element of the human body, yet there is not a reliable metric for predicting the mechanical properties of the bone in this critical region. This study explored the use of a range of non-destructive metrics to measure femoral neck cortical bone stiffness at the millimetre length scale. A range of testing methods and imaging techniques were assessed for their ability to measure or predict the mechanical properties of cortical bone samples obtained from the femoral neck of hip replacement patients. Techniques that can potentially be applied in vivo to measure bone stiffness, including computed tomography (CT), bulk wave ultrasound (BWUS) and indentation, were compared against in vitro techniques, including compression testing, density measurements and resonant ultrasound spectroscopy. Porosity, as measured by micro-CT, correlated with femoral neck cortical bone’s elastic modulus and ultimate compressive strength at the millimetre length scale. Large-tip spherical indentation also correlated with bone mechanical properties at this length scale but to a lesser extent. As the elastic mechanical properties of cortical bone correlated with porosity, we would recommend further development of technologies that can safely measure cortical porosity in vivo.

Highlights

The mechanical properties of bone strongly influence the likelihood of a person sustaining a fracture and obtaining a good result from orthopaedic surgery[1]
The spatial resolution of dual x-ray absorptiometry (DXA) is relatively low and most patients suffering from a fracture have a bone mineral density (BMD) that is not within the osteoporotic range (T score less than −2.5) on DXA10
As patients age and the bone quality deteriorates, the trabecular bone contribution to femoral neck strength decreases and in patients with low bone mineral density (BMD) on dual x-ray absorptiometry (DXA) imaging the cortical bone contributes to 3.7 times more of the femoral neck strength than the trabecular bone[19]

Summary

Introduction

The mechanical properties of bone strongly influence the likelihood of a person sustaining a fracture and obtaining a good result from orthopaedic surgery[1]. Patients with lower bone mineral density, as measured by dual x-ray absorptiometry (DXA), have a higher chance of sustaining hip fractures[2], having cementless knee replacements subside[3] and having cemented hip replacements loosen over time[4]. We chose to investigate the mechanical properties of femoral neck cortical bone, assessing the relationship between the bone’s porosity, as measured by x-ray computed tomography (CT), with its millimetre-scale mechanical properties. Reference point indentation (RPI) involves repeated indentations at the same location and the indentation distance increase (IDI) is measured This involves the bone tissue sustaining plastic deformation and is a potential surrogate measure for the fracture toughness of bone[26]. Depth-sensing microindentation can estimate the stiffness of bone from the unloading slope of the load-displacement curve during indentation[33] and a variety of sizes and shapes of indenter tips can be used, which is why this method of indentation was used over RPI in this study

Methods

Results

Discussion

Conclusion