Attachment of cartilage wear particles to the synovium negatively impacts friction properties

Abstract

Cartilage wear particles are released into the synovial fluid by mechanical and chemical degradation of the articular surfaces during osteoarthritis and attach to the synovial membrane. Accumulation of wear particles could alter key tissue-level mechanical properties of the synovium, hindering its characteristically low-friction interactions with underlying articular surfaces in the synovial joint. The present study employs a custom loading device to further the characterization of native synovium friction properties, while investigating the hypothesis that attachment of cartilage wear particles increases friction coefficient. Juvenile bovine synovium demonstrated characteristically low friction coefficients in sliding contact with glass, in agreement with historical measurements. Friction coefficient increased with higher normal load in saline, while lubrication with native synovial fluid maintained low friction coefficients at higher loads. Cartilage wear particles generated from juvenile bovine cartilage attached directly to synovium explants in static culture, with incorporation onto the tissue denoted by cell migration onto the particle surface. In dilute synovial fluid mimicking the decreased lubricating properties during osteoarthritis, wear particle attachment significantly increased friction coefficient against glass, and native cartilage and synovium. In addition to providing a novel characterization of synovial joint tribology this work highlights a potential mechanism for cartilage wear particles to perpetuate the degradative environment of osteoarthritis by modulating tissue-level properties of the synovium that could impact macroscopic wear as well as mechanical stimuli transmitted to resident cells.

Keywords: Cartilage wear particles; Friction coefficient; Synovium; Tribology.

Figure 1.

Schematic of custom friction tester for the real-time measurement of friction coefficient of native synovial joint tissues in sliding contact motion (adapted with permission from Krishnan, Caligaris et al. 2004) (a). Images of juvenile bovine synovium explants affixed to custom loading platen with example cartilage counterface (b).

Figure 2.

Friction coefficient as a function of time over duration of representative 60 min tests for synovium against a glass counterface at either 25 or 100 kPa contact pressure, in PBS or bovine synovial fluid (SF) (a). Mean equilibrium friction coefficient (average of final 100 cycles) for synovium-on-glass testing at 25 or 100 kPa contact pressure, in PBS or SF. N = 9-10 samples/group. *P < 0.05 vs. indicated group (b).

Figure 3.

Schematic of CWP application to synovium explants in static culture and resulting attachment after 5 days (a). Side-view of confocal z-stack of cartilage wear particle (green) incorporated into surface of the synovium explant by synoviocytes (red) migrated onto particle surface (b).

Figure 4.

Mean equilibrium friction coefficient for synovium under physiologic 25kPa load in ‘worst case’ conditions: large (unfiltered) CWP attachment, no lubricant (PBS bath), in sliding contact with glass, demonstrating large significant increase compared to untreated controls. N = 9-10 samples/group. *P < 0.05 vs. CTL (a). Mean equilibrium friction coefficient for synovium under 25kPa load in OA-mimicking conditions: small (<70μm) CWP attachment, diluted (50% v/v) synovial fluid, in sliding contact with cartilage (b) and other synovium (c), showing significant increase compared to untreated controls (CTL). N = 6 samples/group. *P < 0.05 vs. CTL.