Prediction of bone mechanical properties using QUS and pQCT: Study of the human distal radius
Introduction
In recent years, research on fracture risk prediction has been directed towards the clinical assessment of bone strength factors. Quantitative ultrasound (QUS) techniques have emerged, and showed their ability to discriminate osteoporotic patients from healthy patients [1], [2], [3]. In addition to their non-ionising, portable, and inexpensive characteristics, the elastic nature of ultrasound waves implies that QUS measurements are potentially able to provide information on bone biomechanical competence. By comparison, techniques based on X-ray absorptiometry only provide an indirect assessment via the well-accepted general relationship that exists between bone mineral content and mechanical properties.
Axial transmission is a QUS modality specifically developed for cortical bone assessment, that can be applied to various skeletal sites, such as the tibia, radius, metatarsus and finger phalanges [4], [5], [6], [7]. Axial transmission is a generic term that designates a technique in which emitters and receivers are placed on the same side of the skeletal site. The measured parameter is the velocity of a signal propagated along the bone axis. Various axial transmission approaches have been reported. Some of them measure the velocity of the first arriving signal (FAS). Among these devices exploiting the FAS, the first commercially produced axial transmission device measured cortical velocity using a 250 kHz pulse transmitted along the cortical layer of the mid-tibia [8]. A commercial system, the Sunlight Omnisense™, operates at 1.25 MHz at the distal one-third radius site, and potentially other skeletal sites including the ulna, phalanges, metacarpal or metatarsus [9], [10]. A prototype device has been developed by Bossy et al. [11], operating at 1 MHz and based on a bi-directional axial transmission in which an ultrasonic pulse is transmitted along the bone surface in two opposite directions, for an automatic correction for soft tissue thickness. In vitro studies have shown a strong association between the FAS velocity and bone properties such as porosity, mineralization [4] and tissue elasticity [12]. In contrast, the FAS is observed as relatively insensitive to cortical thickness [13].
Besides the devices exploiting the FAS, others use lower frequencies, in order to excite guided wave modes propagating in the bone. One such device uses 200 kHz broadband transducers [7] and another uses 110 kHz transducers [14]. In particular, the guided waves theory in plates or hollow tubes [7] describes well one slow-guided mode, associated to a signal arriving later than the FAS. This mode shows a higher sensitivity to cortical thickness than the FAS [15]. Thus, with the axial transmission technique, several propagation modes can be excited, which velocity is governed by a specific sensitivity to cortical thickness and material or microstructural properties.
Consequently, the axial transmission parameters may reveal different bone properties, depending on the modality. The direct relation of cortical ultrasound velocity to structural bone mechanical properties were previously investigated. Measurements were performed at the radius [13] with a high frequency (1.25 MHz) device. Site-matched measurements of cortical velocity (FAS) and failure load were correlated. However, these studies revealed the inability of the FAS velocity to improve the failure load prediction over bone mineral density (BMD) and geometrical features assessed with dual X-ray absorptiometry (DXA) or peripheral computed tomography (pQCT). To the best of our knowledge, the relationships between velocity of low frequency guided waves and biomechanical competence have never been reported.
The aim of the study was to understand the relationships between axial transmission parameters and mechanical bone properties, and to compare the prediction of bone properties provided by axial transmission to that provided by pQCT measurements, widely used for clinical applications.
The novelty of this study relies on two points:
Three axial transmission devices have been used to provide different velocities. Two of them were high frequency devices (Sunlight Omnisense and a bi-directional axial transmission prototype), providing the ultrasound FAS velocity. The third one was a low frequency prototype, providing a slower signal velocity, corresponding to an A0-mode velocity (first antisymetric Lamb mode, guided by a plate). Therefore, different ultrasound velocities were explored for mechanical parameters prediction.
In addition to failure load, the Young's modulus prediction by different QUS parameters has also been assessed. The studies cited above did not include Young's modulus prediction, probably because Young's modulus measurement usually requires simple geometry samples, difficult to obtain experimentally. The combination of experimental mechanical testing measurements, and finite elements modelling (FEM) allowed us to evaluate the Young's modulus in samples with random geometry (diaphyseal shafts) [16].
This study was conducted on a collection of human radius, in which three axial transmission ultrasound velocities and pQCT parameters were assessed. These ultrasound and pQCT parameters were compared to mechanical properties such as load, stress and strain at failure, as well as Young's modulus.
Section snippets
Specimens
Thirty-two fresh human radii (14 female and18 male, mean age 73 years, S.D. = 12 years, range 49–90 years) were assessed. Soft tissue was removed and specimens were kept frozen at –20 °C between measurement sessions, and measured at room temperature. All measurements were performed at a region of interest corresponding to 45% of the radius length from the distal end, close to the “1/3 distal radius” region used clinically (Fig. 1). As part of a previous study [4] the 30% distal part of the radius
Results
Three types of parameters were measured on the same collection of specimens: ultrasound velocity, pQCT parameters (cortTh and density), and mechanical parameters (Young's modulus, stress, strain and failure load). The ability of the different ultrasound velocities to predict mechanical properties was compared to that of the pQCT parameters.
In Table 1 are summarized the values of the correlation coefficients obtained between ultrasound velocities and pQCT parameters on one hand, and mechanical
Correlation to mechanical properties
All parameters were correlated to the Young's modulus of the specimens. Correlation coefficients were generally weak to moderate. Among the ultrasound velocities, the best predictor of the Young's modulus was the bi-directional velocity VBidir, descibring 30% of the Young's modulus variability. It is worth to notice that only trends can be observed, given that the different correlation coefficients are rather close to each other. Assuming bone to be isotropic, the velocity of the FAS, in a
Conclusion
In this study, we compared the respective abilities of three ultrasound axial transmission modalities in terms of mechanical parameters prediction at the radius. Cortical ultrasound velocities were compared to site-matched pQCT estimation of BMD and cortTh. pQCT provided better information on failure load, whereas QUS allowed a better estimation of Young's modulus. In addition, the low frequency ultrasound velocity gave a good cortTh description. Our results illustrate that multiple ultrasound
Conflict of interest
All authors disclose any financial and personal relationships with other people or organizations that could inappropriately influence their work.
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