Spatial Investigation of Facet Joint Cartilage Properties Reveals Specific Structure-Function Relationships

Facet joints are highly susceptible to osteoarthritis, which can manifest in distinct patterns of cartilage degeneration across the facet joint surfaces. Understanding how cartilage structure translates into functional properties of the tissue may help to identify regions that are, from a biomechanical perspective, at higher risk for degenerative changes. Therefore, this in vitro study aimed to determine structure-function relationships based on spatially assessed surface properties of intact facet joint cartilage.

Biomechanical properties were assessed by spatial stress-relaxation tests, which were performed at 12 measurement points on the cartilage surfaces of six inferior and six superior ovine facets, respectively. Cartilage thickness was determined at corresponding locations by the needle penetration method. Spatial histological analyses were performed to assess collagen and glycosaminoglycan contents, quantified from Picrosirius-red- and Safranin-O-stained sections, respectively. Differences between inferior and superior facets were analyzed using linear-mixed models; structure-function relationships were analyzed using Spearman's correlation.

Cartilage thickness and glycosaminoglycan coverage were significantly higher at superior facets (p < 0.05). Biomechanical properties did not significantly differ between inferior and superior facets (p ≥ 0.05). The relaxation behavior indicated a significant (p < 0.05) positive correlation with cartilage thickness at inferior facets (ρ = 0.606) and with glycosaminoglycan coverage at superior facets (ρ = 0.597).

The biomechanical behavior of intact ovine facet cartilage was associated with its structure and the assessed properties indicated spatial variation across the facet joint surface. Overall, this study highlights the importance of topographic characterization of human facet cartilage to better understand facet osteoarthritis.