Sample selection and grouping
We retrospectively evaluated 1,008 patients who underwent arthroscopic surgery for MMT from June 2014 to May 2020. Patients with isolated MMT were chosen for the study. The exclusion criteria were as follows: MMT combined with ligament injuries, lateral meniscal tears, rheumatoid arthritis, septic arthritis or tumors; a previous history of knee surgery; and unavailability of MR images of sufficient quality. All arthroscopic surgeries were performed within two weeks of the MRI examination. Patients were divided into an MMS group and a control group on the basis of whether MMS was present on MR images.
MRI protocol and measurement
All patients underwent MRI examinations in the supine position. MRI examinations were performed with a 3T MRI unit (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany) using a quadrature extremity coil. The MRI protocol incorporated the following sequences: a T1-weighted turbo spin-echo (TSE) sequence in the sagittal plane (FOV: 160 mm, slice thickness: 4 mm, interslice gap: 0.5 mm, TR: 800 ms, TE: 12 ms, matrix: 256 x 256 mm) and a proton density-weighted TSE sequence with fat saturation in the coronal, sagittal and transversal planes (FOV: 160 mm, slice thickness: 4 mm, interslice gap: 0.5 mm, TR: 3300-3600 ms, TE: 48 ms, matrix: 256 x 256 mm).
The MME distance was measured using the General Electric Healthcare PACS program (GE, Centricity Universal Viewer Zero Footprint, version 5.0 sp7.1). The MME distance was quantified in millimeters (mm) on a coronal MR image obtained at the midpoint of the medial femoral condyle. Two vertical lines were drawn that intersected the outer edge of the medial meniscus and the outer margin of the medial tibial plateau. The distance between the two lines was defined as the MME distance (Fig 1). Osteophytes were excluded when determining the medial margin of the tibial plateau.
Measurement of medial meniscal extrusion: Using a mid-coronal MR image, the first vertical line was drawn to intersect the medial edge of the medial tibial plateau (dotted line a), and the second vertical line was drawn to intersect the medial edge of the medial meniscus (solid line b). The distance between the two lines (arrow line c) was defined as the medial meniscal extrusion distance.
Two trained surgeons who were blinded to the study design, clinical information and radiological reports measured the MME distance. The mean distance was used in the assessment. An MME distance ≥3 mm was considered to indicate MMS. The interobserver reliability of the measurements between the two surgeons was analyzed using the intraclass correlation coefficient (ICC), with an ICC of 0.40 indicating poor reproducibility, an ICC of 0.40–0.75 indicating fair to good reproducibility, and an ICC greater than 0.75 indicating excellent reproducibility [15]. The ICC was 0.887 (range: 0.841–0.921).
Demographic and clinical data
Demographic and clinical data included sex, age, body mass index (BMI), side of the affected knee, duration of symptoms, severity of chondral lesions involving the ipsilateral medial compartment and type of MMT. The duration of symptoms was defined as the amount of time with knee pain before the operation was performed. The severity of chondral lesions involving the ipsilateral medial compartment was described according to the Outerbridge classification: Grade 0 signified normal cartilage; Grade I lesion referred to cartilage with softening and swelling; Grade II lesion described a partial-thickness defect with fissures that did not exceed 0.5 inches in diameter or reach subchondral bone; Grade III lesion was characterized by chondral fissures with a diameter >0.5 inches with an area reaching subchondral bone; and Grade IV indicated erosion of the articular cartilage that exposed subchondral bone [16]. The types of MMT were categorized as horizontal tears, longitudinal tears, bucket-handle tears, radial tears, posterior medial meniscus root tears (PMMRT), and complex tears. These data were collected as variates for multivariate logistic regression analysis.
Statistical analysis
The normality of continuous data distributions was assessed using the Kolmogorov-Smirnov test. Normally distributed data were expressed as the mean (±standard deviation), and the median (interquartile range) was used to express data with a skewed distribution. Categorical data were presented as frequencies and percentages. Differences in numerical data between the MMT group and control group were determined by independent-sample t-test for continuous data with a normal distribution and Mann-Whitney U test for continuous data that were not normally distributed. The chi-squared test was used to compare categorical data. Factors associated with MMS were identified by multivariate logistic regression analysis. The odds ratio (OR) and 95 % confidence interval (CI) were calculated as an approximate index of the relative risks. Statistical analysis was performed with SPSS software (version 25, SPSS, Chicago, IL). A P value less than 0.05 was considered statistically significant.