Shear deformation kinematics during cartilage articulation: effect of lubrication, degeneration, and stress relaxation

Abstract

During joint articulation, the biomechanical behavior of cartilage not only facilitates load-bearing and low-friction, but also provides regulatory cues to chondrocytes. Elucidation of cartilage kinematics under combined compression and shearing conditions clarifies these cues in health and disease. The objectives of this study were to elucidate the effects of lubricant, tissue degeneration, and stress relaxation duration on cartilage shear kinematics during articulation. Human osteochondral cores with normal and mildly degenerate surface structures were isolated. Paired blocks from each core were apposed, compressed, allowed to stress relax for 5 or 60 min, and shear tested with a micro-scale video microscopy system using phosphate-buffered saline (PBS) or synovial fluid as lubricant. During applied lateral motion, local and overall shear strain (Exz) of articular cartilage were determined. The applied lateral displacement at which Exz reached 50% of the peak (Deltax(1/2)) was also determined. Quantitatively, surface Exz increased at the onset of lateral motion and peaked just as surfaces detached and slid. With continued lateral motion, surface Exz was maintained. After short stress relaxation, effects of lubrication on Exz and Deltax(1/2) were not apparent. With prolonged stress relaxation, Exz and Deltax(1/2) near the articular surface increased markedly when PBS was used as lubricant. Similar patterns were observed for overall Exz and Deltax(1/2). With degeneration, surface Exz was consistently higher for all cases after the onset of lateral motion. Thus, cartilage shear kinematics is markedly affected by lubricant, cartilage degeneration, and loading duration. Changes in these factors may be involved in the pathogenesis of osteoarthritis.

Figure 1

Schematics of (A) sample and testing configuration, (B) micro-shear test setup, and (C) loading protocol.

Figure 2

Micrographs taken during shear loading of apposing (A,B) normal and (C,D) degenerate samples lubricated with PBS (A,C) or SF (B,D) after 60 minutes of stress relaxation and (I) 0, (II) 0.2, (III) 0.4, and (IV) 0.6 mm of applied lateral displacement (Δx). Cell nuclei tracking method was used to determine (I–IV) maps of shear strain (color maps) with (i–iv) magnified views of the surface above. Bars = 150 mm.

Figure 3

Lateral surface displacement (u_s) versus applied lateral displacement (Δx) for normal (NL: ●, ○) and degenerate (DGN: ■, □) cartilage after (A) 5 and (B) 60 minutes of stress relaxation time (t_sr). Samples were tested with phosphate buffered saline (PBS: ○, □) and synovial fluid (SF: ●, ■).

Figure 4

Local shear strain, E_xz, versus normalized tissue depth for normal (NL: ●, ○) and degenerate (DGN: ■, □) cartilage after 5 (A–D) and 60 (E–H) minutes of stress relaxation and (A,E) 0, (B,F) 0.2, (C,G) 0.4, and (D,H) 0.6 mm of applied lateral displacement (Δx). Samples were tested with phoshate buffered saline (PBS: ○, □) and synovial fluid (SF: ●, ■).

Figure 5

(A,B) Surface and (C,D) overall shear strain, E_xz, versus applied lateral displacement (Δx) for normal (NL: ●, ○) and degenerate (DGN: ■, □) cartilage after (A,C) 5 and (B,D) 60 minutes of stress relaxation time (t_sr). Samples were tested with phoshate buffered saline (PBS: ○, □) and synovial fluid (SF: ●, ■).

Figure 6

Effect of lubricant (synovial fluid and PBS), degeneration (normal and degenerate), and stress relaxation time (5 and 60 minutes) on (A) surface and (B) overall Δx_1/2 at 50% peak E_xz(Δx_1/2).

Figure 7

Four sequential events, (I) adherence, (II) adherence and shear deformation, (III) detachment as shear deformation peaks, and (IV) sliding with maintenance of shear deformation, that occurs during cartilage-on-cartilage articulation. (A) Schematic and (B) where these events occur in a representative shear strain (E_xz) versus applied lateral displacement (Δx) diagram.