Biomechanical Modeling of Repair Articular Cartilage: Effects of Passive Motion on Osteochondral Defects in Monkey Knee Joints

Abstract

A new biomechanical methodology was developed, based on the linear biphasic finite element formulation and nonlinear optimization techniques under conditions of creep indentation, to determine the in situ intrinsic material properties of healing neocartilage in osteochondral defects. This biomechanical methodology was then used in conjunction with morphological and histological analyses to quantify neochondrogenesis in large (3.2 mm in diameter) osteochondral defects in the femoral condyle and patella of eight cynomolgus monkeys. Postoperatively, the animals were managed with either intermittent passive motion or cast immobilization, followed by 6 weeks of ad libitum cage activity. Cast-immobilization patellar neocartilage had a higher aggregate modulus than intermittent passive motion tissue (p < 0.05) and intermittent passive motion repair tissue in the medial condyle appeared to be slightly more permeable than cast-immobilization neocartilage (p < 0.10). Normal articular cartilage was four times stiffer (p < 0.0001) and had a larger Poisson's ratio (p < 0.01) than repair (intermittent passive motion and cast-immobilization combined) neocartilage. Histologic studies did not demonstrate differences in the volume or type of repair tissue formed in osteochondral defects treated with intermittent passive motion or cast immobilization. Our biomechanical methods, based on the linear biphasic theory for articular cartilage, allow for the first-time simultaneous determination of all three intrinsic material properties of neocartilage in situ on the joint. We conclude that for large, full-thickness defects at 8 weeks postoperatively, cast immobilization induced stiffer and less permeable cartilage than intermittent passive motion, but the cast-immobilization cartilage was less stiff and more permeable than normal cartilage. Our results suggest that for large, ungrafted defects, intermittent passive motion may not be an effective postoperative management modality for cartilage healing enhancement.