Exogenous Collagen Crosslinking is Highly Detrimental to Articular Cartilage Lubrication

Healthy articular cartilage is a remarkable bearing material optimized for near-frictionless joint articulation. Because its limited self-repair capacity renders it susceptible to osteoarthritis (OA), approaches to reinforce or rebuild degenerative cartilage are of significant interest. While exogenous collagen crosslinking (CXL) treatments improve cartilage's mechanical properties and increase its resistance to enzymatic degradation, their effects on cartilage lubrication remain less clear. Here, we examined how the collagen crosslinking agents genipin (GP) and glutaraldehyde (GTA) impact cartilage lubrication using the convergent stationary contact area (cSCA) configuration. Unlike classical configurations, the cSCA sustains biofidelic kinetic friction coefficients (μk) via superposition of interstitial and hydrodynamic pressurization (i.e., tribological rehydration). As expected, glutaraldehyde- and genipin-mediated CXL increased cartilage's tensile and compressive moduli. Although net tribological rehydration was retained after CXL, GP or GTA treatment drastically elevated μk. Both healthy and "OA-like" cartilage (generated via enzymatic digestion) sustained remarkably low μk in saline- (≤0.02) and synovial fluid-lubricated contacts (≤0.006). After CXL, μk increased up to 30-fold, reaching values associated with marked chondrocyte death in vitro. These results demonstrate that mechanical properties (i.e., stiffness) are necessary, but not sufficient, metrics of cartilage function. Furthermore, the marked impairment in lubrication suggests that CXL-mediated stiffening is ill-suited to cartilage preservation or joint resurfacing.

Comparative Tribology: Articulation-induced rehydration of cartilage across species

Articular cartilage is a robust tissue that facilitates load distribution and wear-free articulation in diarthrodial joints. These biomechanical capabilities are fundamentally tied to tissue hydration, whereby high interstitial fluid pressures and fluid load support facilitate the maintenance of low tissue strains and frictions. Our recent ex vivo studies of cartilage sliding biomechanics using the convergent stationary contact area (cSCA) configuration, first introduced by Dowson and colleagues, unexpectedly demonstrated that sliding alone can promote recovery of interstitial pressure and lubrication lost to static compression through a mechanism termed 'tribological rehydration.' Although exclusively examined in bovine stifle cartilage to date, we hypothesized that tribological rehydration, i.e., the ability to recover/modulate tissue strains and lubrication through sliding, is a universal behavior of articular cartilage. This study aimed to establish if, and to what extent, sliding-induced tribological rehydration is conserved in articular cartilage across a number of preclinical animal species/models and diarthrodial joints. Using a comparative approach, we found that articular cartilage from equine, bovine, ovine, and caprine stifles, and porcine stifle, hip, and tarsal joints all exhibited remarkably consistent sliding speed-dependent compression/strain recovery and lubrication behaviors under matched contact stresses (0.25 MPa). All cartilage specimens tested supported robust, tribological rehydration during high-speed sliding (>30 mm/s), which as a result of competitive recovery of interstitial lubrication, promoted remarkable decreases in kinetic friction during continuous sliding. The conservation of tribological rehydration across mammalian quadruped articular cartilage suggests that sliding-induced recovery of interstitial hydration represents an important tissue adaptation and largely understudied contributor to the biomechanics of cartilage and joints.