Bone replaces unloaded articular cartilage during knee immobilization. A longitudinal study in the rat

Abstract

Joint immobility results in deleterious changes such as capsule shortening, bone loss and articular cartilage damage. Immobilization of rat knees in flexion for 32weeks resulted in the distinctive feature of well-established replacement of articular cartilage by bone. Determining the time of onset of bone replacement is critical for the prevention of this likely irreversible complication of joint immobilization. To determine the onset and progression of bone replacement in the anterior tibial articular cartilage following knee immobilization in flexion. One hundred forty-nine adult male Sprague-Dawley rats were used. The experimental groups had one knee immobilized at 135°of flexion for durations of 2, 4, 8, 16 or 32weeks and were compared to age-matched controls. The knees were evaluated histologically for the presence and cross-sectional area of bone within the articular cartilage of the tibia. Distance between the anterior aspect of the tibia and intact articular cartilage and cross-sectional bone area of the tibial epiphysis were also measured. Bone replacement in the articular cartilage was observed in 14%, 75%, 95%, 100% and 100% of knees after 2, 4, 8, 16 and 32weeks of immobilization, respectively. No bone replacement was seen in the control knees. The mean area of bone replacement increased from 0.004±0.007mm2 after 2weeks to 0.041±0.036mm2; 0.085±0.077mm2; 0.092±0.056mm2 and 0.107±0.051mm2 after 4, 8, 16 and 32weeks of immobilization, respectively, (p<0.001) largely restricted to the anterior tibial articular cartilage. Mean distance to intact articular cartilage increased from 0.89±0.69mm at 2weeks to 1.10±0.35mm; 1.65±0.77mm; 1.48±0.63mm; and 1.78±0.58mm after 4, 8, 16 and 32weeks of immobilization, respectively (p=0.001). Epiphyseal bone cross-sectional area was significantly reduced following 4, 8, and 16weeks of immobilization compare to controls (all 3 p<0.05). Knee immobilization in flexion resulted in bone replacement in the anterior tibial articular cartilage that began after 2weeks and was prevalent after 4weeks of immobilization. The bone replacement progressed in an anterior-to-posterior direction and stopped at the area of contact between tibia and femur. These findings stress the importance of mobility to maintain joint health.