Cationic agent contrast-enhanced computed tomography imaging of cartilage correlates with the compressive modulus and coefficient of friction

Abstract

Objective: The aim of this study is to evaluate whether contrast-enhanced computed tomography (CECT) attenuation, using a cationic contrast agent (CA4+), correlates with the equilibrium compressive modulus (E) and coefficient of friction (μ) of ex vivo bovine articular cartilage.

Methods: Correlations between CECT attenuation and E (Group 1, n = 12) and μ (Group 2, n = 10) were determined using 7 mm diameter bovine osteochondral plugs from the stifle joints of six freshly slaughtered, skeletally mature cows. The equilibrium compressive modulus was measured using a four-step, unconfined, compressive stress-relaxation test, and the coefficients of friction were determined from a torsional friction test. Following mechanical testing, samples were immersed in CA4+, imaged using μCT, rinsed, and analyzed for glycosaminoglycan (GAG) content using the 1,9-dimethylmethylene blue (DMMB) assay.

Results: The CECT attenuation was positively correlated with the GAG content of bovine cartilage (R(2) = 0.87, P < 0.0001 for Group 1 and R(2) = 0.74, P = 0.001 for Group 2). Strong and significant positive correlations were observed between E and GAG content (R(2) = 0.90, P < 0.0001) as well as CECT attenuation and E (R(2) = 0.90, P < 0.0001). The CECT attenuation was negatively correlated with the three coefficients of friction: CECT vs μ(static) (R(2) = 0.71, P = 0.002), CECT vs μ(static_equilibrium) (R(2) = 0.79, P < 0.001), and CECT vs μ(kinetic) (R(2) = 0.69, P = 0.003).

Conclusions: CECT with CA4+ is a useful tool for determining the mechanical properties of ex vivo cartilage tissue as the attenuation significantly correlates with the compressive modulus and coefficient of friction.

Figure 1

(A) Photos showing locations where osteochondral plugs were harvested from bovine patellae, femoral grooves, and femoral condyles. Plugs were randomly selected after freezing. (B) Schematic of mechanical testing setup. A- Frame of machine, B- Plug fixture with set screws to anchor plug by its subchondral bone, C- 7mm diameter osteochondral plug, D- physiologic saline, E- aluminum platen, F- Torque Cell, G- Load Cell, H- Actuator. Each osteochondral plug from both groups was subjected to a 4- step compression against the aluminum platen while immersed in saline. Plugs from Group 2 were also subjected to a 720° rotation following the 45-min relaxation after the final compressive step.

Figure 2

Correlations between CECT Attenuation (HU) and GAG content (mg/mg) of cartilage samples for (A) CECT vs. E samples (Group 1, unfilled data points indicate degraded samples) and (B) CECT vs. μ samples (Group 2). Both correlations were strong (coefficients of variation greater than or equal to 0.74) and statistically significant (p≤0.001). Color maps of representative, non-degraded samples with (C) low (2.86%) and (D) high (4.88%) GAG contents.

Figure 3

Correlations between (A) Equilibrium Compressive Modulus (E) (MPa) and GAG content and (B) CECT attenuation (HU) and Equilibrium Compressive Modulus (E) (MPa) for Group 1(unfilled data points indicate degraded samples). Both correlations were strong (coefficients of variation equal to 0.90) and statistically significant (p<0.0001).

Figure 4

Correlations between CECT attenuation (HU) and three different experimentally-determined coefficients of friction (μ): (A) μ_static, (B) μ_{static_eq}, (C) μ_kinetic for Group 2.

Figure 5

Correlations between GAG content of each sample and three different experimentally-determined coefficients of friction (μ): (A) μ_static, (B) μ_{static_eq}, (C) μ_kinetic for Group 2.

Figure 6

Correlations between CECT attenuation (HU) excluding the STZ of each sample and three different experimentally-determined coefficients of friction (μ): (A) μ_static, (B) μ_{static_eq}, (C) μ_kinetic for Group 2.