The main finding of our study is the self-weight loading mainly influences the ultrasound attenuation in calcaneus QUS measurement. And the loading-induced changes on QUS parameters were more on postmenopausal women than those on premenopausal women. The results of this study indicated that calcaneus QUS measurement performed with loading may be a probable way to improve the QUS based osteoporosis diagnosis.
It is well-known that the BMD measured by DXA is the golden standard for osteoporosis diagnosis. However, in this study we did not use the BMD as the reference indicator of osteoporosis. The reason why we did so are two: firstly, we consider the radiation of DXA measurement may do harm to the subjects; secondly, this is a preliminary study in which we just want to address the potential but not to draw the conclusion whether adding a physical loading can improve the QUS based osteoporosis diagnosis. Therefore, considering the high and direct correlation between menopause and osteoporosis, we used the postmenopausal women as approximations of osteoporosis or osteopenia in this study when DXA measurement was not performed. Moreover, we understand that further tests using DXA measurement as diagnosis reference must be performed in proceeding study to conclude whether adding a physical loading can improve the QUS based osteoporosis diagnosis.
Since trabecular bone is an inhomogeneous porous medium, the interaction between physical loading and bone and the interaction between ultrasound and bone are highly complex phenomenon. Modelling ultrasonic propagation through trabecular bone tissue using porous media theories showed that the sound velocity and attenuation in trabecular bone depend on the frequency of the ultrasound, the elastic properties of the constituting materials, porosity, permeability, tortuosity, and effective stress [15
]. Previous studies also suggested that QUS parameters were highly associated with bone microstructure and mechanical properties [15
]. While a loading is exerted on bone, the bone trabecula buckle and bend, and a strain of the bone takes place. The deformation of the bone may change the microstructure and some other properties of the bone (e.g. the porosity and permeability), resulting in the changes of reflection and scattering of the ultrasound signal transmitting through the bone, thus the ultrasound property parameters change accordingly.
Paired sample T-test was used to determine whether there were significant differences between pairs of values measured on the same subject under two different loading conditions. The testing results (shown in Table ) indicated that QUS parameters, mainly nBUA, significantly changed when loading was exerted on calcaneus. The nBUAs obtained with and without loading were different in all subject groups. We presumed that the deformation of the bone microstructure under self-weight loading is the main reason for these differences. As pointed out by Haiat et al [24
], nBUA showed strong correlations with the ratio of bone volume to tissue volume (BV/TV) that 20% change of BV/TV resulted in a change of nBUA by 45
. In our study, the propagation path of the ultrasound beam can be regarded as a cylinder of bone tissue. Since the diameter of the transducers (i.e. the diameter of the cross section of the cylinder) and the width of the calcaneus (i.e. the height of the cylinder) are constants, the volume of the cylinder is a constant (i.e. the tissue volume (TV) is a constant) either with or without loading. The calcaneus trabecula however buckle and bend when loading is applied, and these deformations lead to the strain of bone. The strain induced by loading will reduce the space among trabecula, compress more bone tissue into the ultrasound path, increase the value of BV, and then increase the value of BV/TV. And the ultrasound attenuation increased correspondingly.
It was found in our results that the sensitivities of SOS and STI to loading were lower than that of nBUA. There were only statistically significant differences between SOS measured with and without loading in group 1 and 3. Different behaviours of SOS and nBUA to loading may probably result from the fact that nBUA is mainly influenced by the microstructure of the bone while SOS is mainly determined by bone density [7
]. Self-weight loading causes deformations on the bone (deformation of microstructure); however, it does not alter the density and elasticity of the bone. In addition, as a combination of BUA and SOS, STI measured with loading only differed from that measured without loading in the control group. One possible reason is that STI was calculated from an empirical equation (equation (3)) based on the experimental results measured without loading and this equation may be no longer suitable for calculating STI under loading condition.
Our results showed that some QUS parameters, mainly nBUA, substantially changed if loading was exerted on bone during measurement. Then, it should be validated that whether the changes of QUS parameters induced by loading influence the QUS based osteoporosis diagnosis or not. The experimental results reported by Wright et al [25
] may provide some hints to us. According to their study, under the same loading stress, the strain of healthy bone was much lower than that of the decalcified and deproteinized bones. Since the calcium and protein in the osteoporotic bone are lower than those in the healthy bone, the strain of healthy bone should be lower than that of the osteoporotic bone under the same loading condition. That results in smaller deformations on the microstructure of the healthy bone than that of the osteoporotic bone. The resultant changes of QUS parameters of the healthy bone should be smaller than those of the osteoporotic bone. As a result, the differences on QUS parameters between healthy and osteoporotic bones should be enlarged when the QUS measurement is performed with the same loading exerted on the bone.
The analyses above can be well validated by the data shown in Table . Although the relative changes of QUS parameters in postmenopausal women with respect to healthy women under loading condition were similar to those measured without loading, the relative changes of weight-normalized QUS parameters (i.e. with loading and all subjects subject to approximately the same stress -1kg stress) were much higher than those of the QUS parameters obtained without loading. These results indicated that the differences between pre and postmenopausal women were larger when weight-normalized QUS parameters were used.
In ROC analysis, the AUCs of QUS parameters measured with loading were not higher (even a few lower) than those measured without loading. However, after corresponding QUS parameters being normalized by weight, the AUCs increased significantly (higher than 0.9) from their counterparts obtained from QUS parameters measured without loading. This meant that only the QUS parameters measured under the same amount of physical loading showed stronger discriminatory ability for menopause in this study. On another word, weight-normalized QUS parameters eliminated the variations of weight induced loadings which exerted on different subjects, and revealed the true effect of physical loading on ultrasound measurement. The results of ROC analysis suggested that the discriminatory ability of weight-normalized QUS parameters for menopause was stronger than that of traditional QUS parameters.
To our best knowledge, this is the first study that investigated the effect of loading on the bone QUS measurement and further proposed the weight-normalized QUS parameters. The weight-normalized QUS parameters were proposed by exerting the physiological loading on traditional QUS parameters. In this study, weight-normalized QUS parameters were significantly lower in postmenopausal groups than those in the control group and the relative changes of the weight-normalized QUS parameters after menopause were much higher than those measured without loading. It is worth noting that there is more weight in subjects in postmenopausal groups than those in the premenopausal group in this study. However, this fact cannot be the main reason for the results above, since previous studies [26
] have shown that QUS parameters, including SOS, nBUA and STI, were in positive correlation to weight. It means that more weight companioned with higher values of QUS parameters. Meanwhile, the mean weight of group 3 was the lowest in postmenopausal groups, but the corresponding mean value of weight-normalized nBUA and weight-normalized STI were the lowest and the relative changes of weight-normalized nBUA and weight-normalized STI were the highest in postmenopausal groups. So the lower values and the higher relative changes of weight-normalized QUS parameters in postmenopausal groups cannot be simply ascribed to more weight in subjects in postmenopausal groups.
In this study, the effect of loading was adopted as an initial attempt to bone QUS measurement. Although there are some limitations on these parameters, our results suggested that weigh-normalized QUS parameters could discriminate pre and postmenopausal women more effectively. And by considering the high relationship between menopause and osteoporosis, weight-normalized QUS parameters showed its potential advantages on osteoporosis diagnosis and further investigations about how much stress exerted on the bones is suitable for this method and how to exert the same stress on different subjects are needed.