Shearing as a mechanism for lamina splendens formation

Abstract

Event Abstract Back to Event Shearing as a mechanism for lamina splendens formation Sierra Cook1, Cory N. Brown1, Noah J. Pacifici1 and Delphine Gourdon1 1 Cornell University, Materials Science and Engineering, United States Introduction: The amorphous surface layer of the superficial zone of cartilage is the lamina splendens (LS). The LS functions as the load-bearing layer when articular joints are sheared and marks the interface between the cartilage surface and the synovial fluid (SF). The LS comprises higher concentrations of the same components as SF and it was recently proposed that it is formed by shear-induced aggregation of SF components [1]. Here we use a Surface Forces Apparatus (SFA) to investigate the effects of applied load, shearing speed, and loading pattern on LS formation. Furthermore, we discriminate between the roles of surface-bound SF and SF from the synovial sac (present in the medium) in LS formation. Materials and Methods: The SFA can simultaneously measure the separation distance and normal forces between opposing surfaces with 1 nm and 1 µN resolution, respectively. When the surfaces are sheared, the SFA measures the resulting friction forces with 100 µN resolution. In our experiments, SFA mica surfaces were coated with fibronectin, a protein present at the cartilage surface and reported to tether SF components to mica [2]. The surfaces were then coated with SF and sheared either in PBS or in SF. Besides friction forces, film thickness and refractive index were also monitored to correlate lubrication with structural changes in the film. Additionally, normal forces were collected before and after shearing to detect any permanent thickening/aggregation of SF components attributed to gel formation. Results and Discussion: Our results indicate the formation of a shear-induced ‘gel’ layer which depended on shearing speed, initial loading, and shearing medium. While permanent film thickening was measured during low-load, low-velocity shearing in both PBS and SF media, transient film thickening was observed under all other conditions (see Figure 1). This transient film thickening did not result in wear or increased friction, suggesting that once the ‘gel’ initially formed, its thickness did not further impact friction. Associated local increase of refractive index in the sheared film indicated aggregation of SF constituents. At high shearing velocities, although no wear was observed when surfaces were sheared first at low load then at high load, damage eventually occurred when sheared directly at high load. This implies that low velocity and/or incremental loading are necessary to form a sustainable lubricating gel. Lastly, the presence of SF in the medium resulted in overall thicker films but did not improve lubrication or wear protection suggesting that cartilage-bound SF (rather than SF from synovial sac) plays a predominant role in LS formation. Conclusions: Our findings suggest that shear is likely responsible for the formation of a LS-like gel in joints. This result is exciting because it reveals that the LS is highly adaptive, forming in joints to absorb shock only when needed. Figure 1. Representative film thickness and friction force measured over time when SF coated mica surfaces were sheared across PBS. Data were recorded under low-load and high-velocity shearing conditions, exhibiting transient film thickening accompanied by decreasing friction. We would like to thank the Fortier Laboratory at Cornell University for providing the synovial fluid used in these experiments.