Correlation between the surface area of synovial membrane and the surface area of articular cartilage in synovial joints of the mouse and human

Background Articular cartilage is avascular. Evidence suggests that in mature animals synovial fluid is the dominant source of nutrition for articular cartilage. If so, there might be a direct correlation between the surface area of synovial membrane and the surface area of articular cartilage in synovial joints. Methods The hindlimb joints of young adult Swiss Webster mice were prepared for histology and sagitally sectioned at regular intervals. The surface area of synovial membrane and the surface area and volume of articular cartilage were calculated from serial photomicrographs of knee, ankle, and first metatarsophalangeal (MTP) joints using ImageJ software and Cavalieri's method. Sagittal E12 slices from numerous synovial joints in a single human cadaver were similarly analysed. Results There was a strong statistically significant positive linear correlation between the surface area of synovial membrane and articular cartilage in the hindlimb joints of the mouse (r = 0.96, 95% CI 0.90-0.98, P < 0.0001). A similarly strong highly statistically significant corelation was observed between the surface area of synovial membrane and volume of articular cartilage. This relationship was also observed across a wider range of synovial joints in the human (r = 0.83, 95% CI 0.48-0.95, P = 0.0009). All analyses remained highly statistically significant after adjusting the standard errors and consequent P values for the linear models based on multiple observations in the same subject. Conclusions This study demonstrates for the first time that there is a direct positive linear correlation between the surface area of synovial membrane and the surface area of articular cartilage in synovial joints in the mouse and human. These novel findings support the concept that the nutrition of mature articular cartilage is dependent on synovial fluid and may also explain why some joints communicate with surrounding bursae. Perhaps more consideration should be given to synovial membrane when studying the pathology of articular cartilage.