Bone growth increases the knee flexion contracture angle: A study using rats

Objectives: To assess the impact of bone growth on the flexion contracture angle at the knee, to measure the bone growth pattern in rats, and to asses s the impact of immobility on bone growth. Design: Experimental, controlled study. Setting: Bone and joint laboratory. Animals: Sixty Sprague-Dawley rats. Interventions: Knee joints of 40 rats w ere immobilized unilaterally in flexion. Sham-operated animals (n = 20) wer e controls. Main Outcome Measures: The contracture angle and the femur and tibia length s on radiographs. Results: The angle of flexion increased over time and was largely explained by bone growth (r =.725, p <.01). Femur and tibia grew in rats until they were 11 months old. Immobility enhanced growth in bone length, especially o f the femur, after 16 and 32 weeks of immobility (p <.05), Conclusions: Knee flexion contracture angle increased as a consequence of n ormal bone growth, a situation that is also encountered in skeletally immat ure children. The continued growth in length of bones in children may influ ence the progression of contractures and add to the therapeutic challenge. Ongoing bone growth should be considered when interpreting reports that use animal models for bone and joint diseases.