PROJECTED CARTILAGE AREA RATIO, EVALUATED USING THREE-DIMENSIONAL MRI ANALYSIS SOFTWARE, IS A USEFUL INDEX FOR ASSESSING CARTILAGE IN THE MEDIAL COMPARTMENT OF THE KNEE JOINT, COMPARABLE TO CARTILAGE THICKNESS MEASUREMENTS

INTRODUCTION

The projected cartilage area ratio, evaluated using three-dimensional MRI, is defined as the proportion of the region of interest (ROI) occupied by sufficiently thick cartilage and serves as a quantitative index for cartilage assessment (1). However, the relationships between the projected cartilage area ratio and factors such as cartilage thickness, lower limb alignment, patient characteristics, and the medial meniscus coverage ratio have not been fully clarified.

OBJECTIVE

The aim of this retrospective study was to investigate the correlations between the projected cartilage area ratio and cartilage thickness, lower limb alignment, patient characteristics, and the medial meniscus coverage ratio.

METHODS

A total of 53 patients who underwent medial meniscus repair or high tibial osteotomy for the treatment of medial knee OA were included. MRI was performed using a 3.0-T system (Achieva 3.0TX, Philips, Netherlands). Sagittal images of the knee joint were obtained using both fat-suppressed spoiled gradient echo and proton-weighted sequences. DICOM data were processed using SYNAPSE VINCENT 3D software (FUJIFILM Corp., Tokyo, Japan). Tibial cartilage was projected vertically onto a plane aligned with the bone’s long axis, while femoral cartilage was projected radially around the intercondylar axis, defined as the line connecting the centers of the medial and lateral femoral condyles. These centers were identified by approximating each condyle to an ellipse on lateral views. The software automatically delineated the ROI using bone contours and divided the medial femoral condyle (MFC) into nine subregions based on anatomical morphology. The projected cartilage area ratio was calculated as the ratio of the projected cartilage area exceeding a defined thickness threshold to the total ROI area in each region and subregion. Average cartilage thickness and the medial meniscus coverage ratio were also automatically computed. The medial meniscus coverage ratio was defined as the ratio of the area covered by the meniscus within the medial tibial cartilage area to the total medial tibial cartilage area. Correlations between the projected cartilage area ratio or average cartilage thickness and patient demographics, lower limb alignment, Kellgren–Lawrence (KL) grade, and the medial meniscus coverage ratio were assessed using Spearman’s rank correlation coefficient.

RESULTS

A strong positive correlation was observed between the projected cartilage area ratio and average cartilage thickness in both the MFC (r = 0.96, p < 0.001) and the medial tibial plateau (MTP) (r = 0.96, p < 0.001) (Figure 1). Body weight was not correlated with the projected cartilage area ratio or cartilage thickness; however, BMI showed significant negative correlations with the projected cartilage area ratio (MFC: r = -0.45, p < 0.001; MTP: r = -0.33, p = 0.02) and cartilage thickness (MFC: r = -0.41, p = 0.002; MTP: r = -0.35, p = 0.01) (Figure 2). Height correlated positively with the projected cartilage area ratio in the MTP (r = 0.34, p = 0.01) and with cartilage thickness in both the MFC (r = 0.29, p = 0.03) and MTP (r = 0.41, p = 0.003), but not with the projected cartilage area ratio in the MFC (Figure 1). KL grade showed significant negative correlations with both PCAR (MFC: r = -0.54, p < 0.001; MTP: r = -0.57, p < 0.001) and cartilage thickness (MFC: r = -0.53, p < 0.001; MTP: r = -0.52, p < 0.001). Regarding lower limb alignment, both the projected cartilage area ratio and cartilage thickness demonstrated moderate positive correlations with the weight-bearing line ratio and moderate negative correlations with the joint line convergence angle. Significant positive correlations were also observed between the projected cartilage area ratio and the medial meniscus coverage ratio in both the MFC (r = 0.50, p < 0.001) and MTP (r = 0.44, p < 0.001).

CONCLUSION

The projected cartilage area ratio appears to be less influenced by body physique compared with cartilage thickness and may serve as a reliable index for assessing cartilage status in the medial compartment of the knee joint.