Abstract
Summary
Macroscopic grading, histologic grading, morphometry, mineral analysis, and mechanical testing were performed to better understand the changes that occur in the cartilage, calcified cartilage, and subchondral bone in early osteoarthritis.
Introduction
The earliest changes in osteoarthritis (OA) remain poorly understood due to the difficulty in detecting OA before patients feel pain. We have published details of the mature bovine patella model showing the pre-OA state where no gross macroscopic changes are visible yet microstructural changes indicate very early degeneration. In this new study, we proceed to investigate this model further by more comprehensively quantifying the changes in articular cartilage (AC), zone of calcified cartilage (ZCC), and subchondral bone (SB) in pre and early OA.
Methods
Patellae from mature cow were studied. Gross examination with India ink was used to classify macroscopic cartilage degeneration. Two groups were selected in this study: one with no visible surface degeneration (pre-OA) and the other with mild to moderate macroscopically visible surface degeneration (early OA). Histologic staining with Safranin O and Fast Green was analysed with two osteoarthritic scoring systems: Mankin and OOCHAS. Differential Interference Contrast (DIC) microscopy was used to quantify morphometric changes. Degree of mineralisation was analysed with energy dispersive X-ray spectroscopy (EDS) to quantify the calcium and phosphorus content of the mineralised tissues. Material properties of calcified cartilage and subchondral bone were tested macroscopically using 3 point bending.
Results
In the early OA group, cartilage was fissured and showed matrix loss. In its hydrated state, average cartilage thickness was significantly greater (p<0.05) in the early OA group by 24% compared to pre-OA group. The early OA group showed an 88% increase in ZCC thickness. Early OA tissue was graded significantly higher in OOCHAS grading and structure scores, cellularity, and staining scores of Mankin grading but not in the tidemark integrity score. Pre-OA and early OA tissues showed no significant differences in ZCC or SB mineralisation although all samples showed an increase in the degree of mineralisation going from the upper to the deeper ZCC and SB. Macroscopic mechanical testing showed no significant differences in mechanical properties between pre-OA and early OA groups. However within groups, the ZCC was an order of magnitude less stiff than the SB. Micromechanical testing showed that deeper ZCC and SB were stiffer than their regions closer to the joint surface.
Conclusions
Early osteoarthritic changes in the joint tissues produce macro-level cartilage degeneration as well as microstructural changes. The combination of mineralisation and mechanical data show that though calcified cartilage and subchondral bone have similar mineralisation profiles, their material properties are drastically different, suggesting that stiffness is not purely the result of the mineral phase.
