Synovial adhesions are more important than pannus proliferation in the pathogenesis of knee joint contracture after immobilization: an experimental investigation in the rat.

Objective: To measure intraarticular pannus proliferation after early and prolonged joint immobility using an animal model.

Methods: Forty rats underwent unilateral immobilization of a knee joint with an internal fixator for periods of 2, 4, 8, 16, and 32 weeks. Twenty rats received sham surgery. The knee joints were harvested and processed for histological examination. The synovial intima length and the subintimal area were measured on standardized sagittal sections with image analysis software. The measurements were recorded with regard to their location (anterior or posterior; superior or inferior).

Results: Intra and interrater reliabilities for all measurements were > 87.9%. The synovial intima length was smaller in immobilized knees than in controls at all time points. At 4 and 32 weeks, the difference was statistically significant (p < 0.05). The differences were marked in the posterior synovium, where the intima length of immobilized knees was significantly smaller than in controls after 4, 8, 16, and 32 weeks of immobilization (p < 0.05). The subintimal area was comparable in immobilized and control knees at all time points.

Conclusion: We standardized the quantification of intraarticular pannus in a joint contracture model after immobility of up to 32 weeks' duration. This study revealed a significant decrease in synovial intima length but no change in the subintimal area of immobilized knees compared with controls. The decrease in synovial intima length with immobility suggests that adhesions of synovium villi rather than pannus proliferation are the major pathophysiological changes leading to contracture after immobility.