Abstract
Background
Although knee osteoarthritis (OA) has first been thought to be a wear-and-tear disease of the articular cartilage, it is now established that it is a disease of the entire joint. Unfortunately, its complex physiopathology is still incompletely understood, limiting the design of efficient therapeutic options. Recently, it has been suggested that OA could be related to the breakdown of the relationships among knee tissues rather than to isolated alterations of one or more tissues. However, there is a paucity of data regarding relationships among knee tissues. Better understanding the bone and cartilage relationships is thus of particular interest as both tissues contribute to the transfer of the mechanical loads through the joint and altered loading is known to be a main factor of OA onset and progression. Specifically, there is an interest to determine if cartilage thickness (CTh) and subchondral bone mineral density (sBMD), two properties related to loading, are adapted in healthy femurs.
Method
CT-arthrography of the knee was performed unilaterally on 16 healthy subjects (8 males; 61.8± 2.8 years old). Bone and cartilage boundaries were segmented on the CT images in order to calculate 3D CTh and sBMD maps. sBMD was calculated as the average intensity of the CT-arthrography in the superficial 3mm of bone. The 3D CTh and sBMD maps were transformed into 2D anatomically-standardized maps to allow comparison among individuals. According to literature, relationships between CTh and sBMD were assessed both in terms of magnitude and spatial distribution. The magnitude of CTh and sBMD was quantified using the ratio between the (CTh and sBMD) values in the load-bearing region of the medial and lateral condyles. The spatial distribution was quantified using the location of thickest cartilage or densest bone in the load-bearing region of the medial condyle. Pearson correlations were performed between CTh and sBMD metrics, using an alpha-level at 5%.
Results
The medial-to-lateral ratios of sBMD and CTh were positively correlated (r = 0.66, p = 0.005). The locations of thickest cartilage and densest bone were positively correlated along the anterior-posterior direction (r = 0.66, p = 0.005), but the medial-lateral locations were not significantly correlated (p = 0.26).
Conclusion
The correlations identified in this study between CTh and sBMD metrics provide important new elements to support the physiopathology model of knee OA based on relationship among tissues. Specifically, the positive correlations suggest that the magnitude and spatial distribution of CTh and sBMD are adapted to one another in healthy knees, likely due to shared loading environments. Further studies are needed to determine if similar relationships exist in OA knees. In the future, having a better understanding the healthy relationships and their alterations with knee OA could indicate targets for therapeutic interventions.
