Assessment of CA4+ Impact on Mechanical Properties of Articular Cartilage

Abstract

X-ray imaging of articular cartilage could be a breakthrough for the early diagnosis of tissue degeneration. This approach relies on radiopaque contrast agents to enhance the visualization of soft tissues. The potential impact of contrast agents on the mechanical response of articular cartilage should be considered in the frame of both clinical and research applications. Attention has been drawn to a solution containing molecules with six iodine atoms and four positive charges (CA4+), which has been shown to improve the X-ray visibility of articular cartilage. This study aimed to determine the effects of a CA4+ solution on tissues' mechanical properties. An experimental pipeline based on indentation tests was applied to paired samples of articular cartilage before and after the immersion in either CA4+ or phosphate-buffered saline solution, maintained at a temperature of 22 ± 2 °C, for 22 h to determine the differences in instantaneous, viscous, and equilibrium responses between the articular cartilage of the two groups. The 22 h immersion of articular cartilage in either CA4+ or phosphate-buffered saline solution had a significant detrimental effect on the overall response, including the instantaneous, viscous, and equilibrium responses, of the articular cartilage. However, this detrimental effect was greater with exposure to the CA4+ solution. Specifically, the articular cartilage was found to be less stiff in both the instantaneous response (approximately -25%) and the equilibrium response (approximately -38%). The softening effect could be attributable to an alteration of the interaction between the proteoglycans of articular cartilage, induced by the positive charges within the CA4+ contrast agent. Further investigations are needed to elucidate whether this hypothesized mechanism is reversible.

Keywords: X-ray imaging; articular cartilage; cartilage degeneration; characterization of materials; contrast agent; contrast-enhanced imaging; early diagnosis; indentation; mechanical properties; soft tissue.

Figure A1

The effect of varying the time constant τ is shown by comparing τ = 6 s (black curve) and τ = 3 s (grey curve), while keeping all other parameters of the stretched exponential function unchanged (S₀ = 5 N; β = 0.5; S_eq = 0 N). Note: The 50% reduction in the time constant was made for illustrative purposes only, to make the effect of the variation visible in the graph. S_eq was set to zero to better highlight how the final part of the curve tends toward a horizontal asymptote.

Figure A2

The effect of varying the stretching parameter β is shown by comparing β = 0.5 (black curve) and β = 0.4 (grey curve), while keeping all other parameters of the stretched exponential function unchanged (S₀ = 5 N; τ = 6 s; S_eq = 0 N). Note: The 20% reduction in the stretching parameter was made for illustrative purposes only, to make the effect of the variation visible in the graph. S_eq was set to zero to better highlight how the final part of the curve tends toward a horizontal asymptote.

Figure 1

The median (markers), 75th percentile (upper edge of error bars), and 25th percentile (lower edge of error bars) of the percentage changes in S₀ (a) and E₀ values (b) before (1st, 2nd, and 3rd indentation test) and after (4th, 5th, and 6th indentation test) 22 h of immersion in solution are shown (grey markers: CE group; white markers: Control group). The values shown in the figure were calculated as the differences between the medians of the 108 values measured in the first three and the 108 values measured in the second set of indentation tests.

Figure 2

The median (markers), 75th percentile (upper edge of error bars), and 25th percentile (lower edge of error bars) of the percentage changes in τ (a) and β values (b) before (1st, 2nd, and 3rd indentation test) and after (4th, 5th, and 6th indentation test) 22 h of immersion in solution are shown (grey markers: CE group; white markers: Control group). The values shown in the figure were calculated as the differences between the medians of the 108 values measured in the first three and the 108 values measured in the second set of indentation tests.

Figure 3

The median (markers), 75th percentile (upper edge of error bars), and 25th percentile (lower edge of error bars) of the percentage changes in Eeq values before (1st, 2nd, and 3rd indentation test) and after (4th, 5th, and 6th indentation test) 22 h of immersion in solution are shown (grey markers: CE group; white markers: Control group). The values shown in the figure were calculated as the differences between the medians of the 36 values measured in the third and the 36 values measured in the fourth set of indentation tests to exclude the effect of the downward trend.

Figure 4

Load trends measured on matched paired OC cores (grey line: OC core immersed in CA4+ solution; white line: OC core immersed in PBS solution) during the 4th indentation are shown as a function of indentation depth, up to 15% of AC thickness (a), and as a function of time (b). The peak load is reached in 1 s. In both graphs, the load values have been normalized to the maximum load measured on the same OC core during the 1st indentation (note: if there were no alterations in the mechanical response of the AC, the peak load should have reached 100%).