Subjects
In the Osteoarthritis Initiative (OAI) participants (https://nda.nih.gov/oai), MOAKS scores were measured at different time points according to the projects. The number of participants who were receiving baseline, 12 months or 24 months was larger than other time points, and therefore the periods from baseline to 12 months and from baseline to 24 months were analyzed in this study. As shown in the flowchart (figure 1),records with insufficiency measurement or duplicate records were screened out. Participants with BMI >30 or Kellgren-Lawrence (KL) grade > 1 were excluded because overweight [17] [18] or existing radiologic OA might lead to deterioration of meniscus. Basic demographics, including sex, race, side, age, height, weight, KL grade, Knee Injury And Osteoarthritis Outcome Score Quality Of Life Score (KOOSQOL), joint space narrowing (OARSI grades 0-3) medial compartment (XRJSM), joint space narrowing (OARSI grades 0-3) lateral compartment (XRJSL), were exported from the OAI database.
Muscle strength measurement
The isometric knee extensor strength (N) was measured using the “Good Strength Chair” (Metito Oy, Jyvaskyla, Finland) at 60° knee flexion [19, 20]. Maximum extensor force was measured from the isometric strength test. In this study, extensor muscle strength was described by the maximum extensor force/weight. The high extensor strength group and the low extensor strength group were divided referring to the median extensor muscle strength/weight at the baseline.
Semi-Quantitative measurements of menisci
MOAKS was applied in this study, which described what the abnormality was and where this occurred in the menisci. MOAKS scaling could be divided into extrusion part and morphology part. Extrusion in four positions (medial meniscus medial extrusion, medial meniscus anterior extrusion, lateral meniscus lateral extrusion, lateral meniscus anterior extrusion) was graded as Grade 0:<2mm; Grade 1: 2 to 2.9mm, Grade 2: 3-4.9mm; Grade 3: >5mm. Signal abnormality, vertical tear, horizontal and radial tear, complex tear, root rear, partial maceration, progressive partial maceration, complete maceration, cyst, and hypertrophy were assessed as presence or absence in the anterior horn, body, and posterior horn of each meniscus. Root tear was only assessed in the posterior horn. In order to conduct the longitudinal evaluation, definitions for progression of MOAKS features proposed by Runhaar et al.[21] were applied in this study (supplementary 1). The overall meniscus progression was seen as occurring when at least one classified progression was observed.
Quantitative measurements of menisci
The volume of menisci was measured based on the knee MRI scans to assess the OA progression. The analyses on these images were performed by biomediq [22, 23]. The framework demonstrated precision and accuracy comparable to manual segmentations. The framework combined multi-atlas rigid registration with voxel classification and was trained on manual segmentations with varying configurations of bones, cartilages, and menisci. [22].
Statistical analyses
Independent t-tests (for continuous variables) and Pearson chi-squared tests (for categorical variables) were used to investigate the differences between the high extensor strength group and the low extensor strength group in demographics at baseline, the volume change of meniscus, and meniscus progression. Considering the distinction in muscle strength between males and females, the primary outcomes of this study in each sex were separately analyzed. Logistic regression models adjusted for baseline age, height, weight, side, KL grade were used to assess the relationship between extensor muscle strength and meniscal deterioration. SPSS version 25 was used for statistical analysis, and the difference was considered as significant when P-value < 0.05.