The role of lubricin in the mechanical behavior of synovial fluid

Abstract

Synovial fluid is a semidilute hyaluronate (HA) polymer solution, the rheology of which depends on HA-protein interactions, and lubricin is a HA-binding protein found in synovial fluid and at cartilage surfaces, where it contributes to boundary lubrication under load. Individuals with genetic deficiency of lubricin develop precocious joint failure. The role of lubricin in synovial fluid rheology is not known. We used a multiple-particle-tracking microrheology technique to study the molecular interactions between lubricin and HA in synovial fluid. Particles (200 nm mean diameter) embedded in normal and lubricin-deficient synovial fluid samples were tracked separately by using multiple-particle-tracking microrheology. The time-dependent ensemble-averaged mean-squared displacements of all of the particles were measured over a range of physiologically relevant frequencies. The mean-squared displacement correlation with time lag had slopes with values of unity for simple HA solutions and for synovial fluid from an individual who genetically lacked lubricin, in contrast to slopes with values less than unity (alpha approximately 0.6) for normal synovial fluid. These data correlated with bulk rheology studies of the same samples. We found that the subdiffusive and elastic behavior of synovial fluid, at physiological shear rates, was absent in fluid from a patient who lacks lubricin. We conclude that lubricin provides synovial fluid with an ability to dissipate strain energy induced by mammalian locomotion, which is a chondroprotective feature that is distinct from boundary lubrication.

Fig. 1.

Random walk exhibited by a 200-nm-diameter particle embedded in BSF from start (a) to end (b) after tracking its position over 12 s. Each pixel represents 64.5 × 64.5-nm resolution.

Fig. 2.

Time-dependent MSD of 200-nm beads in BSF, 80% glycerol solution, ET-BSF, CACP, and BSF diluted with 80% glycerol. The power law with a slope near unity is exhibited at low and high lag times for all samples except BSF, which exhibits a slope less than unity (α ≈ 0.6) for short lag times (τ ≤ 300 ms).

Fig. 3.

Normalized displacement PDFs of particle tracers embedded in the synovial fluid samples measured after 60 (red), 300 (blue), and 600 (green) ms. The RMSD of the tracers in both BSF (a) and ET-BSF (b) solutions exhibit a scaling with the cubic root of lag times <300 ms and with the square root of lag times >300 ms (Insets). (c) Similar to a purely viscous fluid, CACP-hSF scales to the square root of all lag times.

Fig. 4.

Shear-rate-dependent viscosity of the CACP-hSF, BSF, ET-BSF, UHA, and UHA with added lubricin measured in the rheometer. After addition of lubricin, the zero-shear viscosity of UHA decreased considerably. An increase in zero-shear viscosity was also noted in BSF after digestion with trypsin. The onset of power-law behavior (between arrows) at greater shear rates in the CACP-hSF sample may be attributable to a wider range in the molecular weight distributions of HA.

Fig. 5.

Comparison of MPTM technique and rheometry in measurement of shear storage modulus (G′) and shear loss modulus (G″) for BSF. The MPTM technique recapitulates the observations made by rheometry for G″ and more closely for G′.