The menisci play a critical protective role for the knee joint through shock absorption and load distribution. We hypothesized that cartilage degeneration will be abruptly progressed if meniscal subluxation exceeds a critical point.
Of 56 cases that showed medial meniscal subluxation without cartilage degeneration of ipsilateral medial femoral condyle (MFC) on initial MRI, from January 2005 to June 2007, meniscal subluxation index (MSI), the ratio of meniscal overhang to meniscal width in mid-coronal image of initial MRI, was measured. After 2 years, 40 cases were evaluated for cartilage degeneration of ipsilateral MFC on follow-up MRI. The relationship between medial MSI on initial MRI and cartilage degeneration of MFC on follow-up MRI was analyzed. Logistic regression analysis was conducted to find a critical point of meniscal subluxation related to cartilage degeneration.
Abrupt progression of cartilage degeneration was observed from which MSI was 0.38. Logistic regression showed that if MSI was at the critical point, which was 0.38 in our study, then the probability of cartilage degeneration to grade 3 or 4 after 2 years was 44 %. If MSI was 0.4, then the probability was 50 %. If MSI was 0.6, then the probability was 99 %.
The results suggest the existence of critical point from which the protective function of the meniscus appears to be significantly altered, and the degree of cartilage degeneration of ipsilateral femoral condyle corresponding to the amount of medial meniscal subluxation may be predictable.