The most important finding in this study is that we demonstrated that the HU value of PCL could reflect the grade of degeneration. Also, we revealed that the PCL degeneration in knee OA was severer at proximal portion.
In our study, the HU value in the PCL with mild degeneration was 84.5 HU which is significantly more than 70.6 HU and 72.1 HU in severe and moderate degeneration, respectively. As long as we know, this is the first literature demonstrated the HU value of PCL in knee OA. Woodard et al. and Willner et al. revealed the typical HU values for various tissues in the literatures: tendon; 101.6 HU, muscle; 39.4/40.4 HU, adipose; -30.6/-66.6 HU, for instance . However, they did not mention about the HU of ligamentous tissue. There are some reports described the utility of CT HU for the evaluation of soft tissues. Byun et al. found a direct correlation between the inflammatory activity and the density of extraocular fat and lacrimal gland by using HUs in patients with thyroid-associated orbitopathy, and they concluded that the density could predict active inflammation . Another report by Chikui et al. revealed Lipoma had a specific range of CT HU values and displaced the surrounding soft tissue . They also suggested that it is expected that myxoid and pleomorphic sarcomas could easily be differentiated from lipomas because the CT values would be higher than those of adipose tissue. In another literature, Spruit et al. showed HU value of bone graft in intervertebral metal-cage contents increased in the postoperative period, which demonstrated biological activity and change of bone mineral content of the tissue . Barber et al. evaluated the long-term in vivo degradation of biocomposite interference screw with HU value and confirmed its osteoconductivity . He also analyzed the density of the synthetic multiphase implant for the donor site autologous osteochondral transplantation in another literature. The density declined over time to that of fibrous scar, which revealed no evidence of bone ingrowth, osteoconductivity and ossification of the implant .According to these previous reports, the HU value using CT scans could provide us objective and quantitative evaluation for soft tissues and help us predict properties of those tissues.
Another finding in this study was that PCL in knee OA showed severer degenerative changes in proximal portion with lower HU value compared with that in distal portion. Levy et al. demonstrated that the first changes observed in the PCL were fiber disorganization, mucoid and chondroid metaplasia . The study by Kumagai et al. indicated that chondroid metaplasia is associated with the progression of degeneration in human ligaments . The lower HU value in proximal portion of the PCL could reflect those histological changes. They evaluated PCL by scoring inflammation, mucoid degeneration, chondroid metaplasia, cystic changes other than orientation of collagen fiber. On the other hand, we evaluated the degeneration only in terms of collagen fiber impairment. Viidik demonstrated that changes in collagen fibrils affect the biomechanical properties of the ligament . Therefore, we believe that the impairment of collagen fibers is the most affective factor from the aspect of ligamentous function and evaluated it microscopically.
Severer degenerative change was more likely to be seen in proximal portion of PCL in our study. Although Levy et al. and Kumagai et al. described PCL degeneration with histological evaluation, they analyzed the tissue resected from middle portion and proximal one third of the PCL, respectively . There is no study demonstrated the detailed emergence of degeneration in PCL. It is reported that the degeneration in ACL occurs from proximal portion , however the mechanism and pathogenesis of PCL degeneration is still unknown and should be elucidated in the future.
Aggarwal et al. showed the Knee Society Score, anteroposterior instability, ACL appearance and erosion in the lateral tibiofemoral compartment are predictors of PCL degeneration, however they are not enough information to assess the property of PCL quantitatively . On the other hand, the HU value using CT scans is shown as a number objectively which reflects its property, therefore it could be a new scale to predict the degeneration of PCL.
The most important finding in this study is that we revealed the potential to predict the property of PCL using CT HU value. Even if macroscopic appearance of PCL is normal, degenerative change is observed microscopically . Therefore, it is profitable to know the grade of degeneration in PCL with the CT scan. Considering the fact that MRI is not useful enough to analyze the quality within the ligaments which have a short echo time , the measurement of CT HU value could be a valid method to provide us more details about the ligament property. Although further studies are required, the evaluation using the CT HU value could be applied not only for PCL but also for other soft tissues.
There are some limitations in this study. Firstly, the sample size was small and we need large population for both groups. Secondly, the gap of the number of patients with different gender in both OA and control groups. More women were included in OA group and more men in control group. The influence of gender was not considered in this study. Thirdly, one sagittal image from CT scans might not be enough to correspond to the histological result of the obtained sample. Lastly, bone or soft tissue tumor might have affected the property of PCL. However, they were the best control group that we could obtain, because it is ethically difficult to have healthy people undergo CT scans considering the exposure to radiation.