ObjectiveWe have developed a fully automatic three-dimensional MRI analysis software that measures the projected cartilage area ratio (PCAR) to allow for the quantification of the cartilage in the knee. Our objectives were to verify our software’s accuracy for segmentation of cartilage and meniscus and to quantify cartilage and meniscus extrusion in our cross-sectional study. We also examined which cartilage quantification was most affected by age and analyzed the relationship between PCAR and meniscus extrusion.MethodsMRI data from 108 subjects were selected for training, and Dice similarity coefficients were determined from 5 other subjects to verify the accuracy. Our cross-sectional study included other 561 subjects between 30–70 years of age. We quantified cartilage thickness, cartilage volume, and PCAR (0.0–1.5 mm) in medial femoral, lateral femoral, medial tibial, and medial lateral regions. Each region was divided into nine subregions, with particular focus on the medial central (mc) subregion. The medial meniscus coverage ratio (MMCR) was investigated as a quantification for meniscus extrusion.ResultsDice similarity coefficients for cartilage and meniscus were both approximately 0.9. Among cartilage quantifications, the highest absolute value of the correlation coefficient with age was mcMT PCAR 1.0 mm in females and mcMT cartilage thickness in males. In females, mcMT PCAR 1.0 mm was correlated with MMCR, although MMCR was not correlated with age. In males, mcMT PCAR 0.0 mm was correlated with MMCR.ConclusionsOur software showed high segmentation accuracy and provided numerous quantifications of cartilage related to age and meniscus extrusion.Trial registration: UMIN, UMIN000032826. Registered 1 September 2018, https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000037299