Temporal changes in synovial fluid composition and elastoviscous lubrication in the equine carpal fracture model

Abstract

The objective of this study was to examine temporal variations in synovial fluid composition and lubrication following articular fracture. Post-traumatic osteoarthritis (PTOA) was induced by creating an osteochondral fracture in the middle carpal joint of four horses while the contralateral limb served as a sham-operated control. Horses were exercised on a high-speed treadmill, and synovial fluid was collected pre-operatively and at serial timepoints until 75 days post-operatively. Lubricin and hyaluronic acid (HA) concentrations were measured using sandwich ELISAs, and the molecular weight distribution of HA was analyzed via gel electrophoresis. Synovial fluid viscosity and cartilage friction coefficients across all modes of lubrication were measured on days 0, 19, 33, and 61 using a commercial rheometer and a custom tribometer, respectively. HA concentrations were significantly decreased post-operatively, and high molecular weight HA (>6.1MDa) did not recover to pre-operative values by the study termination at day 75. Lubricin concentrations increased after surgery to a greater extent in the OA as compared to sham-operated limbs. Viscosity was significantly reduced after surgery. While boundary and elastoviscous mode friction coefficients did not vary, the transition number, representing the shift between these modes, was lower. Although more pronounced in the OA limbs, similar derangements in HA, HA molecular weight distribution, viscosity, and transition number were observed in the sham-operated limbs, which may be explained by synovial fluid washout during arthroscopy. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Keywords: articular fracture; hyaluronic acid (HA); lubricin/PRG4; post-traumatic osteoarthritis; viscosity.

Figure 1.

Synovial concentration of (a) lubricin increased after injury with levels peaking at 2–3 weeks after injury and not returning toward baseline until day 61 after injury. In contrast, the total concentration of (b) HA decreased after injury, especially the (c) high molecular weight fraction (>6.1 MDa). For the >6.1 MDa fraction, synovial concentrations of HA did not recover to baseline values over the course of the study while the 3.1–6.1 MDa fraction did not recover until 61 days after surgery. Note that † indicates that the combined sham and OA data are significantly different than day 0 at the specified timepoints and *indicates a significant difference between treatment groups across timepoints (p < 0.05, n = 4).

Figure 2.

Viscosity decreased after injury and did not recover to baseline values. Note that † indicates that the combined sham and OA data are significantly different than day 0 at the specified timepoints (p < 0.05, n = 4).

Figure 3.

(A) Stribeck curves were created by plotting friction coefficients as a function of the Sommerfeld number, which factors in the viscosity of synovial fluid and tribological test parameters including sliding speed, contact width, and normal load. Stribeck curves were used to analyze how lubrication changed over time with respect to (B) boundary lubrication and (D) elastoviscous lubrication and the (C) transition point between these modes. Note that † indicates that the combined sham and OA data are significantly different than day 0 at the specified timepoints (p < 0.05, n = 4).