The argument that OP and OA may exhibit an inverse relationship has been proposed 40 years ago. However, the real relationship between these two age-related diseases is still unclear. With the improvement of the understanding of OP, the role of trabecular bone microstructure is widely acknowledged. It is believed that 60% of the mechanical properties of bone depend on bone density, and the remaining may be associated with a number of factors related to bone quality [27, 44–47].
In this study, the structural parameters of the two disease groups of women had significant differences in apparent bone volume fraction, trabecular thickness and mean roundness. Our findings also found that trabecular bone number had no statistical significance between OP and OA group. It was reported that trabecular number did not change with age in elderly . Patel et al. discovered that knee joint of normal and OA patients does not show any difference in the trabecular number. But, Chappard et al. found that late-period OA patient’s trabecular bone number and OP group have significant differences. At the same time, they also believed that it was the loss of OP’s trabecular bone that caused this difference. Stauber and Muller  also agreed that the OP patient’s trabecular bone number had dropped. In the ovariectomized animal models, trabecular bone number was usually lower compared to blank comparison group .
The thickness of trabecular bone tends to decrease when the trabecular bone space increases. It is generally accepted that trabecular bone thickness reduces along with age, but trabecular bone space gets increasingly wider. At the same time, OP makes this tendency to be increasingly serious regardless of human patients or animal models. Most of the authors think the change in OA patients is just the opposite, namely the trabecular bone thickness increases, which was the same with our results. But interestingly, although the trabecular bone thickness in OA group was higher than in OP group significantly, the difference between their trabecular bone spaces was not statistically significant.
Ding et al. found that patients with early OA gained bone volume, while it reduced in normal and OP groups. Although this kind of change may increase the bone volume in OA patients, Dequeker et al. thought that low mineralization of the subchondral bone made mechanical characteristics still lower than the normal population. Buckland-Wright et al. found that the medial tibial subchondral bone becomes thick in patients with anterior cruciate ligament rupture, while the lateral side had no significant changes. However, in OA patients whether the change of cancellous bone was earlier than that of cartilage was still unclear. Unlike other imaging methods, MRI with multiplanar imaging may intuitively demonstrate all structures within the joints at the same time, and provide help to better understand the relationship between bone and cartilage. Some scholars found that trabecular bone could accurately reflect the degree of bone loss in pigs with arthritis and the trabecular bone underneath the cruciate ligament become thin . Chappard et al. found that there was no significant difference in male patients with early OA and OP; on the contrary, there was significant difference between patients with later period OA and OP. They adopted advanced high-resolution Micro-CT system, but patient’s age and gender will possibly influence the comparison result and it could only be studied in vitro. It was also found that bone volume fraction is associated with the Young’s modulus . In the current study, we found that apparent trabecular bone volume fraction in OP group was significantly lower than the OA group. Although the mean trabecular bone area in OA is higher than in OP, there was no statistical significance between them.
Roundness is an indicator reflecting the morphology, and roundness of the object is related to value closer to 1. The more this value approaches to 1, the more the structure tends to blunt. As for a network, larger value of roundness implies good connectivity (low perimeter). In the current study, the mean roundness of trabecular bone in women with OP was significantly lower than OA group (p < 0.05), which suggested that the trabecular bone of women with OP was smaller or with less connectivity.
Our research also found that Euler number in the OP group was significantly higher than in OA group, which implied that the connectivity of trabecular bone network structure in OP patients was poorer and bone microstructure was severely damaged. Euler number is the common index reflecting bone connectivity . It may reflect the connection degree of the complex trabecular bone network microstructure. Better the connection of trabecular bone network, lower the value. It could even be negative in a highly connective trabecular bone network . It was also found that the Euler number and other parameters reflecting bone quality and bone structure had remarkable relevance, and they both correlated with the strength of bone . Euler number has nothing to do with bone quantity, but is related with the Young’s module, and may help evaluate cavities and the number of opened or closed loops in bone marrow. They also confirmed that spatial distribution of the cavities and loops in trabecular bone is a major factor of bone strength. In our study, this index was related to mean trabecular bone area in the OP group, but there was no relation with structural parameters in OA group. The multivariate linear regression analysis demonstrated that Euler number was the only parameter, which has linear correlation with BMD in OA group. Contrast and inverse difference moment have linear correlation with BMD in OP group; this suggested that the mechanism of trabecular bone connectivity to bone microstructure and bone quantity under different disease conditions can be very complex and require further study.
We must admit that this research has some limitations. First of all, the sample was still not big enough as a clinical study, and it was insufficient to completely evaluate the real relationship between OP and OA. The heterogeneity of the small sample size is also a drawback. However, we established strict criteria for case selection. Secondly, this series has not been able to integrate ‘completely normal’ control subjects. We also desired to gather age-matched normal people as control group to enhance the effectiveness of comparative studies. But in the same age scope, it was difficult to find postmenopausal women with no disease. The original experimental design intended to determine the erosion index of high-resolution images, which can reflect the degree of destruction of trabecular bone. But, the calculation of erosion index requires special software to carry out the three-dimensional reconstruction of the image. As the software was not currently available domestically, developing software from the beginning costs high and takes a longer time. The significance of Euler number overlaps with erosion index partly. However the index was not included in this study. We hope that with the development of appropriate computer hardware and software, this could be further improved. Lack of data on BMI is also a limitation of the study.
The reason why we chose proximal tibia area to evaluate the trabecular bone microstructure is that this area is more suitable for the coil which is used for HR MRI examination, and the fact that it had been chosen in earlier studies as well. We also tried to evaluate the hip area, but it needs bigger coil, which had poorer quality of HR images. We think it was also feasible to conduct the study had we chosen other area’s BMD, like lumber. However, in our hospital, we examine the hip region as routine.