pQCT study on diffusion and equilibrium distribution of iodinated anionic contrast agent in human articular cartilage - associations to matrix composition and integrity

Objective: X-ray imaging of articular cartilage using anionic contrast agents has been introduced for quantification of tissue glycosaminoglycan (GAG) concentration. In this in vitro study we investigated diffusion and equilibrium distribution of an anionic contrast agent in human articular cartilage and related the results to tissue composition and integrity. Methods: Osteochondral cylinders (d = 4.0 mm, n = 24) were prepared from femoral medial condyles (FMCs, cartilage thickness 2.13 ± 0.54 mm, mean ± standard deviation [SD]), and tibial medial plateaus ([TMPs]1.99 ± 0.38 mm) of human cadaver knees. Samples were immersed for 24 h at room temperature in 21 mM concentration of anionic contrast agent Hexabrix™. The X-ray absorption maps and profiles were measured before immersion, and after every 2 h of immersion using clinical peripheral quantitative computed tomography (pQCT). Results: An increase in X-ray attenuation along cartilage depth, indicating a characteristic density profile increasing from superficial to deep tissue, could be seen in pQCT images acquired without contrast agent. The complete diffusion of the contrast agent into cartilage took more than 12 h. However, the uronic acid concentration correlated with the contrast agent concentration in femoral cartilage (r = -0.76, n = 12, P = 0.004) as early as after 2 h of immersion, and the linear correlation was virtually unchanged during the remaining 22 h. Similarly, the histological tissue integrity (Mankin score) correlated positively with the contrast agent concentration in tibial cartilage (r = +0.75, P = 0.005) after 2 h of immersion. The X-ray absorption profiles before immersion, i.e., without the contrast agent, and after 24 h of immersion were significantly correlated (r = -0.76 ± 0.34, mean ± SD). Conclusions: Although the complete contrast agent diffusion into human articular cartilage in vitro took more than 12 h, significant apparent correlations were revealed between the spatial proteoglycan (PG) and contrast agent distributions already after 2 h of immersion. At the stage of incomplete penetration, however, the spatial contrast agent concentration distribution cannot directly reflect the true PG distribution as the Donnan equilibrium has not been reached. However, in degenerated cartilage the diffusion rate increases. Obviously, this can lead to the reported correlation between the bulk PG content and the bulk contrast agent concentration already at the early stages of diffusion