Correlation between 3D microstructural and 2D histomorphometric properties of subchondral bone with healthy and degenerative cartilage of the knee joint

Abstract

Cartilage degeneration of the knee joint is considered to be a largely mechanically driven process. We conducted a microstructural and histomorphometric analysis of subchondral bone samples of intact cartilage and in samples with early and higher- grade arthritic degeneration to compare the different states and correlate the findings with the condition of hyaline cartilage. These findings will enable us to evaluate changes in biomechanical properties of subchondral bone during the evolution of arthritic degeneration, for which bone density alone is an insufficient parameter. From a continuous series of 80 patients undergoing implantation of total knee endoprosthesis 30 osteochondral samples with lesions macroscopically classified as ICRS grade 1b (group A) and 30 samples with ICRS grade 3a or 3b lesions (group B) were taken. The bone samples were assessed by 2D histomorphometry (semiautomatic image analysis system) and 3D microstructural analysis (high-resolution microCT system). The cartilage was examined using the semiquantitative real-time PCR gene expression of collagen type I and II and aggrecan. Both histomorphometry and microstructural and biomechanical analysis of subchondral bone in groups A and B consistently revealed progressive changes of both bone and cartilage compared with healthy controls. The severity of cartilage degeneration as assessed by RT PCR was significantly correlated with BV/TV (Bone Volume Fraction), Tb.Th (Trabecular Thickness) showed a slight increase. Tb.N (Trabecular Number), Tb.Sp (Trabecular separation) SMI (Structure Model Index), Conn.D (Connectivity Density) and DA (Degree of Anisotropy) were inversely correlated. We saw sclerotic transformation and phagocytic reticulum cells. Bone volume fraction decreased with an increasing distance from the cartilage with the differences compared with healthy controls becoming greater in more advanced cartilage damage. The density of subchondral bone alone is considered an unreliable parameter for classifying changes evolving over time. The progressive damage of subchondral bone seen in the present study correlates well with cartilage changes. Trabecular orientation is also impaired, which explains the changes in biomechanical parameters and the inadequate load transfer and excessive loading of cartilage. Besides subchondral bone density, which in turn correlates with cartilage thickness, other parameters such as structure model index and grade of anisotropy best reflect mechanical properties such as Young modulus, compressive strength, tensile stress, and failure energy. However, it remains unclear whether the mechanical interaction of the mineralized subchondral tissues with articular cartilage works vice versa. The possibility of a biochemical signalling from the degenerating cartilage via the synovial fluid and bone- cartilage crosstalks via subchondral pores may indeed explain a certain depth dependency of subchondral bone changes.