Introduction

The pathogenesis of Mucoid degeneration (MD) is not well understood but several associated factors with MD-anterior cruciate ligament (ACL) were also suggested including posterior tibial slope (PTS) angle or intercondylar notch geometry. The purpose of this study was to determine the anatomical risk factors for the MD-ACL. It is hypothesized that a MD-ACL would be associated with small intercondylar notch geometry, increased PTS and higher degree of K-L grade.

Methods

This was a retrospective cohort study on patients who had undergone knee arthroscopy between June 2011 and Oct 2020. The inclusion criteria for MD- ACL were as follows: (1) patients with intact ACL with no evidence of tears, (2) an increased diameter of the ACL, (3) partial or absent synovial coverage of the ACL, (4) Yellow substance found by probing, and (5) arthroscopy due to symptoms (limitation of range of motion (ROM) or/and pain). The exclusion criteria were as follows: any evidence of ACL injuries, previous surgery history, other combined ligament injuries. Finally, 52 patients were enrolled as MD-ACL group (Group 1) and 52 patients as control group (Group 2) for this study after sex, age matching. Radiologic evaluation included KL grade, mechanical hip-knee-ankle (HKA) angle, posterior tibial slope (PTS) angle, and Insall-Salvati ratio. The notch width index, and transverse notch angle were measured on MRI, and the grade of trochlear dysplasia was also classified.
The data were analyzed using Mann-Whitney U-test, independent t-test, paired t-test, chi-squared test or Wilcoxon signed-rank test according to the results of normality. Logistic regression analysis was performed to evaluate the risk factors for the MD-ACL by backward elimination methods. The associated factors were age, BMI, sex, HKA angle, PTS angle, Insall-Salvati ratio, K-L grade, presence of trochlear dysplasia, notch width index, transverse notch angle and sagittal notch angle. Furthermore, the diagnostic values of risk factors to the presence of the MD-ACL were evaluated by receiver-operating-characteristic (ROC) curves and its area (AUC).

Results

The ROM was found significantly decreased in group 1, whereas the PTS was significantly larger in group 1. In MRI measurements, the ACL diameter and amount of amount of anterior translation (ATT) were increased significantly in group 1, but the ACL angle was smaller in group 1 than that of group 2. Combined ganglion cysts of the ACL were found in 42/52 cases (80.7%) in group 1. In logistic regression analysis, the risk of MD-ACL increased with a steeper PTS angle, increased Insal-salvati ratio, male and higher K-L grade, as well as decreased transverse notch angle and notch width index. The cutoff values in ROC analysis were found as =28.27% for notch width index (AUC, 0.849; p<0.001), >12.2° for PTS angle (AUC, 0.765; p<0.001) and =47.4° for transverse notch angle (AUC, 0.711;p<0.001).

Conclusion

The steeper PTS angle, decreased notch width index, transverse notch angle, male and over grade 2 K-L grade were significantly associated with the presence of MD-ACL. Those factors should be considered during diagnosis or determining the treatment option for the symptomatic MD-ACL, such as notchplasty.