23. THE EFFECT OF A FOCAL ARTICULAR DEFECT ON CARTILAGE DEFORMATION DURING ARTICULATION

Department: Bioengineering
Faculty Advisor(s): Robert Sah

Primary Student
Name: Benjamin L. Wong
Email: blwong@ucsd.edu
Phone: 858-822-4788
Grad Year: 2009

Abstract
INTRODUCTION: Focal defects in articular cartilage are common in symptomatic knees, particularly in the patella, and prone to enlarge if untreated. Under compressive load, focal defects markedly elevate cartilage strains. However, during knee movement, focal defects are subjected to both axial compression and sliding, and effects of lateral motion on intra-tissue deformation remain to be elucidated. Therefore, the objective of this study was to determine the effect of a focal defect on cartilage strains during the physiological stimuli of axial compression and sliding motion.

METHODS: Samples. Macroscopically normal cartilage was harvested from the patella and trochlea of knees from 4 adult bovine animals as osteochondral blocks. Donor matched patella-trochlea samples (n=4) were fluorescently stained and submerged in normal bovine synovial fluid prior to mechanical testing. Experimental Design. Samples were microscopically tested in shear as intact and then retested with a 3mm wide full thickness defect in the center of the patellar cartilage. Microscopic Shear Testing. Each apposed sample pair was compressed 15% and stress-relaxed for 1h in a bi-axial loading chamber. Lateral displacements were applied, and deformation was captured with an epi-fluorescence microscope and used to determine Lagrangian strains in the patellar cartilage. Strains were averaged and interpolated depth-wise in regions at (EDGE), ~0.4mm (MID) and ~0.8mm (FAR) away from the defect edge.

RESULTS: A focal defect resulted in marked changes in intra-tissue strains. The effects of a defect on shear, axial, and lateral strains each varied markedly depending on depth from the articular surface (p<0.01). Shear Strain (Exz). Exz at the surface tended to be decreased by a defect FAR from the defect edge (p=0.08). In contrast, Exz at 25% depth was increased markedly FAR from the defect edge (p<0.05). Axial Strain (Ezz). Ezz at 5% depth was decreased markedly with a defect (p<0.05), being significantly lower at the EDGE up to the FAR regions than in intact. Ezz at 20% depth was not affected by a defect (p=0.5). Lateral Strain (Exx). Exx at 20% depth was markedly increased at EDGE (p<0.05) and MID (p<0.01) region than in intact. Exx at the surface remained near 0 for both groups (p=0.5).

DISCUSSION: These results provide the first maps of altered cartilage strain distributions due to the combination of focal defects and cartilage-on-cartilage lateral articulation. Such altered cartilage mechanics during knee movement may trigger chondrocyte responses, such as cell death and matrix damage, and cause focal defects to expand and progress.

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