The inferomedial femoral neck is compromised by age but not disease: Fracture toughness and the multifactorial mechanisms comprising reference point microindentation

Abstract

The influence of ageing on the fracture mechanics of cortical bone tissue is well documented, though little is known about if and how related material properties are further affected in two of the most prominent musculoskeletal diseases, osteoporosis and osteoarthritis (OA). The femoral neck, in close proximity to the most pertinent osteoporotic fracture site and near the hip joint affected by osteoarthritis, is a site of particular interest for investigation. We have recently shown that Reference Point micro-Indentation (RPI) detects differences between cortical bone from the femoral neck of healthy, osteoporotic fractured and osteoarthritic hip replacement patients. RPI is a new technique with potential for in vivo bone quality assessment. However, interpretation of RPI results is limited because the specific changes in bone properties with pathology are not well understood and, further, because it is not conclusive what properties are being assessed by RPI. Here, we investigate whether the differences previously detected between healthy and diseased cortical bone from the femoral neck might reflect changes in fracture toughness. Together with this, we investigate which additional properties are reflected in RPI measures. RPI (using the Biodent device) and fracture toughness tests were conducted on samples from the inferomedial neck of bone resected from donors with: OA (41 samples from 15 donors), osteoporosis (48 samples from 14 donors) and non age-matched cadaveric controls (37 samples from 10 donoros) with no history of bone disease. Further, a subset of indented samples were imaged using micro-computed tomography (3 osteoporotic and 4 control samples each from different donors) as well as fluorescence microscopy in combination with serial sectioning after basic fuchsin staining (7 osteoporotic and 5 control samples from 5 osteoporotic and 5 control donors). In this study, the bulk indentation and fracture resistance properties of the inferomedial femoral neck in osteoporotic fracture, severe OA and control bone were comparable (p > 0.05 for fracture properties and <10% difference for indentation) but fracture toughness reduced with advancing age (7.0% per decade, r = -0.36, p = 0.029). Further, RPI properties (in particular, the indentation distance increase, IDI) showed partial correlation with fracture toughness (r = -0.40, p = 0.023) or derived elastic modulus (r = -0.40, p = 0.023). Multimodal indent imaging revealed evidence of toughening mechanisms (i.e. crack deflection, bridging and microcracking), elastoplastic response (in terms of the non-conical imprint shape and presence of pile-up) and correlation of RPI with damage extent (up to r = 0.79, p = 0.034) and indent size (up to r = 0.82, p < 0.001). Therefore, crack resistance, deformation resistance and, additionally, micro-structure (porosity: r = 0.93, p = 0.002 as well as pore proximity: r = -0.55, p = 0.027 for correlation with IDI) are all contributory to RPI. Consequently, it becomes clear that RPI measures represent a multitude of properties, various aspects of bone quality, but are not necessarily strongly correlated to a single mechanical property. In addition, osteoporosis or osteoarthritis do not seem to further influence fracture toughness of the inferomedial femoral neck beyond natural ageing. Since bone is highly heterogeneous, whether this finding can be extended to the whole femoral neck or whether it also holds true for other femoral neck quadrants or other material properties remains to be shown.

Keywords: Bone quality; Femoral Neck; Fracture Toughness; Osteoarthritis; Osteoporosis; Reference point indentation.

Fig. 1

Sample preparation for material testing and imaging. a) Femoral head and neck sample removed from femora through surgical cut, hack saw cut and fracture as appropriate, b) Samples machined from the femoral neck using hack saw, low-speed saw and polishing for: c) Indent imaging via micro-computed tomography (μCT), Fluorescence Light Microscopy (FLM), Atomic Force Microscopy (AFM) and Polarised Light Microscopy (PLM) and d) Material testing via fracture toughness followed by indentation. Indicating the number of osteporotic fracture (OP), osteoarthritic hip replacement (OA) and control (C) donors used for each part of the testing and imaging.

Fig. 2

The whitening front tracking method showing example curves for the three cohorts (osteoporotic fracture, osteoarthritic hip replacement and control) alongside gamma corrected sample videography and whitening front tracking of the subtraction image for a control sample (63 year old male).

Fig. 3

Indent imaging segmentation from a 65 year old male control donor. Unsegmented images are shown for a) micro computed tomography (μCT) and b) fluorescence microscopy (FLM). Segmented images showing the indent (blue), microdamage (red) and closest pore (green) are shown for c) μCT and d) FLM (overlayed on the red-green subtraction image). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Sampled FLM image stack for an 88 year old osteoporotic female donor showing the segmented indent (blue), microdamage (red) and closest pore (green) overlayed on the subtracted red-green image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

a) A three dimension visualisation of the μCT segmented images (control 65 year old male) and the diagram b) indicates how the damage extent is approximated through the μCT mean crack length or through assuming the FLM damage area is approximately semi-circular or hemispherical and the measurement of the proximity to the closest pore.

Fig. 6

Comparison between donors in the osteoporotic fracture (OP), osteoarthritic THR (OA) and control groups in terms of a) indentation distance (TID), b) fracture toughness (K_max) and c) maximum whitening area (W_Area) with p values displayed where significant (p < 0.05) or close to significance (p < 0.1). The young osteoarthritic donor is considered anomalous and has been excluded from the analysis.

Fig. 7

Indent imaging through a) fluorescence light microscopy, b) micro computed tomography, c) polarised light microscopy and d) atomic force microscopy showing examples of the indent imprint (I) and pile up elastic response (P), haversian canal (H) and crack extension resistance mechanisms (B – Bridging, D – Deflection and M – Microdamage).

Fig. 8

Correlation beween RPI measured indentation parameters (IDI and TID) and indent imaging measurement: a) Indent Depth, b) Pore Proximity and c) Damage/Crack Extent across the imaging modalities of micro computed tomography and fluorescence microscopy of the serial sectioned stack or the central slice of the indent. Each circle indicates one indent (which may have been imaged across multiple imaging modalities) – preferentially selecting measurements based on, or most similar to, micro-CT.

Fig. 9

Effect of an individual measurement (a) compared to the median of multiple repeat measurement on the same sample (b). c) Indicates a hyperbolic relationship indicating that a single measurement in close proximity to a pore is higher than the median measurement (the circled measurement, in very close proximity to a pore, is excluded from the analysis).

Fig. 10

Correlation of fracture toughness measurements with: a) age, b) RPI indentation parameters (IDI) and c) whitening area. A young osteoarthritic donor (circled) has an anomalously high toughness and has been excluded from the analysis.