Contrast-enhanced CT facilitates rapid, non-destructive assessment of cartilage and bone properties of the human metacarpal

Abstract

Objective: The aim of this work is to establish the human metacarpal as a new whole joint surface early-stage osteoarthritis (OA) model that enables comparisons of articular cartilage and subchondral bone through high resolution contrast-enhanced CT (CECT) imaging, mechanical testing, and biochemical analysis.

Design: The fourth metacarpal was obtained from 12 human cadaveric donors and baseline μCT imaging was followed by indentation testing. The samples were then immersed in anionic (Ioxaglate) and cationic (CA4+) iodinated contrast agent solutions followed by CECT. Cartilage GAG content and distribution was measured using the 1,9 dimethylmethylene blue (DMMB) assay and Safranin-O histology staining. Linear regression was performed to compare cartilage and subchondral bone properties.

Results: Strong and significant positive correlations were observed between CA4+ CECT attenuation and both GAG content (R(2) = 0.86) and equilibrium modulus (R(2) = 0.84), while correlations using Ioxaglate were insignificant (R(2) ≤ 0.24, P > 0.05). Subchondral bone plate (SBP) thickness negatively and significantly correlated with SBP mineral density (R(2) = 0.49). Cartilage GAG content significantly correlated with several trabecular bone properties, including positive correlations with bone volume fraction (%BV/TV, R(2) = 0.67), trabecular number (Tb.N, R(2) = 0.60), and trabecular thickness (R(2) = 0.42), and negative relationships with structural model index (SMI, R(2) = 0.78) and trabecular spacing (Tb.Sp, R(2) = 0.56). Similarly, equilibrium modulus correlated positively with %BV/TV (R(2) = 0.50), Tb.N (R(2) = 0.59) and negatively with Tb.Sp (R(2) = 0.55) and SMI (R(2) = 0.60).

Conclusion: This study establishes the human metacarpal as a new early-stage OA model suitable for rapid, high resolution CECT imaging, mechanical testing, and biochemical analysis of the cartilage and subchondral bone, and for examining their inter-relationships.

Keywords: Cartilage; Compressive modulus; Computed tomography; Human; Osteoarthritis; Subchondral bone.

Figure 1

Representative color maps from a central, sagittal, CECT slice for the metacarpal sample with a) the highest GAG content and b) the lowest GAG content, as well as the corresponding Safranin-O stained histological section. Color scale bar indicates corresponding CECT attenuation in Hounsfield Units (HU) for all CECT color maps. The distribution of the CA4+ contrast agent (top row in a & b) more clearly matches the distribution of the Safranin-O stain than does that of Ioxaglate (bottom row in a & b) for both samples. For easier comparison with the corresponding histological sections, the middle column displays a magnified image of region indicated by the red box on the full-size color maps.

Figure 2

Correlations for human metacarpal cartilage samples: CECT Attenuation (HU) vs. a) GAG Content (% of cartilage wet mass) and b) mean Ip value (a measure of red pixel density from Safranin-O stained histological sections that is indicative of GAG). c) GAG content (■) and axial rigidity (□, GAG content multiplied by cartilage thickness) vs. indentation equilibrium modulus (E); and d) CECT attenuation vs. E. For CECT attenuations, unfilled data points indicate the Ioxaglate data, while filled data points represent the CA4+ data. All correlations were strong (R² ≥ 0.70) and significant (p < 0.05), except for the correlations involving Ioxaglate (R² ≤ 0.24, p > 0.05).

Figure 3

(a) A significant positive correlation between subchondral bone plate mineral density (SBP BMD) and SBP thickness. Representative sagittal μCT slices reflecting the SBP BMD vs. SBP relationship for a sample with b) thick and c) thin SBP is shown. The region within the red box has been magnified to highlight this positive correlation.

Figure 4

Correlations between the subchondral trabecular network properties and GAG content: a) bone volume fraction (■, %BV/TV) vs. GAG content and structural model index (□, SMI) vs. GAG Content, b) Trabecular Number (Tb.N) vs. GAG content, c) Trabecular Thickness (Tb.Th) vs. GAG content, and d) Trabecular Spacing (Tb.Sp) vs. GAG content. As GAG content declines, the %BV/TV decreases, SMI increases (transitioning from plate to rod-like trabeculae), Tb.N decreases, Tb.Th decreases and Tb.Sp increases. All correlations were significant (p < 0.05).

Figure 5

Representative CT images for a sample with a) the highest GAG content and b) the lowest GAG content. From a two-dimensional (2D) μCT scan (III, IV), one can construct (I, II) a three-dimensional (3D) CECT color map (CA4+ enhanced CT scan shown), and (V, VI) a 3D reconstruction of the subchondral trabecular network. Unlike histology, which only permits 2D sectioning along one plane and is both time-consuming and destructive, CECT enables rapid, facile, non-destructive (I, II) 3D color maps indicative of cartilage GAG content and compressive stiffness, and (V, VI) the subchondral trabecular network, which can be visually sectioned in real-time in one to three planes simultaneously. Color scale bar indicates corresponding CECT attenuation in Hounsfield Units (HU) for all CECT color maps.