[Mechanical properties of chondrocytes isolated from normal articular cartilage: experiment with rabbit knees]

To measure the mechanical properties of chondrocytes of articular cartilage of knee in order to provide relevant technical parameters into the development of cartilage tissue engineering.

Eight NZW rabbits were killed, and their bilateral knee joints were taken out. The articular cartilage of the left knees were used for histological study, and articular cartilage of the right knees was digested with 0.4% pronase and 0.025% collagenase type II so as to make isolation of chondrocytes. The viability rate of the isolated chondrocytes was detected by trypan blue staining. The diameters of the chondrocytes in the histological sections and single cell suspension were measured respectively. The mechanical properties of the chondrocytes were determined using micropipette aspiration technique coupled with half-space model.

In the normal articular cartilage 4 structural zones were differentiated: tangential, transitional, radial, and calcified zones. The viability rate of chondrocytes was 98.2% on average after isolation. The mean diameter of the chondrocytes in the histological sections was (14.2+/-2.9) microm, not significantly different from that in the single cell suspension [(14.9+/-2.2) microm, t=1.31, P=0.19]. In response to a prescribed pressure, the chondrocytes exhibited viscoelastic solid creep behavior characterized initially by a jump in displacement followed by a monotonically decreasing rate of deformation that generally reached an equilibrium displacement. The cells were observed to deform to a length of as much as three times the radius of the micropipette without completely entering the micropipette. The cells were then extruded from the micropipette and completely recovered. The Young's modulus of the normal chondrocytes was (0.57+/-0.43) kPa, and the k1, k2, and micro of the viscoelastic parameters were (0.37+/-0.07) kPa, (0.29+/-0.04) kPa, and (6.36+/-1.12) kPa-s respectively. The k1 is positively correlated to the cell diameter (r=0.4, P=0.031).

Chondrocytes from normal articular cartilage behave as a viscoelastic solid. Micropipette aspiration technique is an efficient method for the study of chondrocyte biomechanics.