Effect of a focal articular defect on cartilage deformation during patello-femoral articulation.

The objective of this study was to determine cartilage strains near, and in apposition to, a focal defect during patello-femoral articulation. Bovine osteochondral blocks from the trochlea (TRO) and patella (PAT) were apposed, compressed 12%, and subjected to sliding under video microscopy. Samples, lubricated with synovial fluid, were tested intact and then with a full-thickness defect in PAT cartilage. Shear (E(xz)), axial (E(zz)), and lateral (E(xx)) strains were determined locally for TRO and PAT cartilage. For articulation with a focal defect, the strain amplitudes of PAT cartilage near the surface were ∼2-8× lower in E(xz) and ∼1.4× higher in -E(zz) than intact PAT cartilage. At 20% depth, E(xz) and E(xx) for PAT cartilage with a focal defect were ∼2× and ∼10-25× higher than intact PAT, respectively. For TRO articulating against a focal defect, E(xz) and -E(zz) near the surface and at 20% depth were ∼2-4× lower than that for articulation against intact cartilage. The results elucidate dramatic region-specific changes in strain due to lateral motion. In these regions, such altered cartilage mechanics during knee movement may cause focal defects to extend by induction of damaging levels of strain to bordering regions of cartilage.