Ultrasound assessment of boundary effect on osmosis-induced shrinkage and swelling of articular cartilage in vitro

This study aims to use ultrasound to investigate the boundary effect of the cut edge on the osmosis-induced shrinkage and swelling of articular cartilage while the distance from the scanning site to the edge decreases with the reduction of specimen size. Sixteen cartilage-bone specimens (of diameter 6.35 mm) were prepared from normal bovine patellae. The cartilage width was gradually reduced to 4.35 mm and to 2.35 mm. Shrinkage and swelling were induced by changing the concentration of the saline solution and monitored using nominal 50 MHz focused ultrasound. The parameters including shrinkage and swelling peak strains (εP1 and εP2, respectively), shrinkage and swelling equilibrium strains (εE1 and εE2, respectively), and shrinkage and swelling slopes (k1 and k2, respectively) were extracted. The εP1, εE1, εE2, k1, and k2 of the 2.35 mm specimens were significantly different (p < 0.05) from those of the 6.35 mm specimens. For the 4.35 mm specimens, εE1 and εP1 were, respectively, significantly different (p < 0.05) from εE1 of the 6.35 mm specimens and εP1 of the 2.35 mm specimens. The percentage of coefficient of variation (18.5% for shrinkage and 16.3% for swelling) of the 2.35 mm specimens was much higher than that (<8.5%) of the 6.35 mm specimens. The relative root mean square difference (rRMSD%, 12.0% for shrinkage and 10.6% for swelling) of the 2.35 mm specimens was also much higher than that (<5.5%) of the 6.35 mm specimens. The results indicated that the boundary effect of the cut edge on the osmosis-induced shrinkage and swelling of articular cartilage increases with the reduction of specimen size.