Meniscus extrusion on standing weightbearing ultrasound is associated with limb alignment and posterior tibial slope in healthy knees

Abstract Purpose The purpose of this study was to report meniscus extrusion in asymptomatic knees using ultrasound sonography (US) and evaluate the association between the extrusion and joint space width, coronal alignment, and posterior tibial slope (PTS). Methods Patients ≥18 years with meniscus allograft transplantation with a contralateral knee that was asymptomatic, Kellgren‐Lawrence grade ≤1, without a history of pathology were included. Knee pain or acute injury requiring orthopaedic evaluation, prior knee surgery, and inflammatory arthropathies were excluded. Meniscus position was captured via US in supine, bipodal, and unipodal stances. Participants underwent radiographic imaging, and joint space width, alignment, and PTS were measured. Statistical analyses included t ‐tests, Mann–Whitney U tests, Pearson correlation, Friedman and repeated ANOVA for further comparisons. Statistical significance was set at p < 0.05. Results Twenty‐eight patients (mean age: 39 ± 11 years, 36% females) were included. No significant differences were found between medial and lateral extrusion across stances (all p > 0.05; medial 2.7 mm vs. lateral 2.7 mm in supine). Extrusion CSA of the medial meniscus was greatest in the bipodal (22.4mm 2 ), followed by unipodal (21.3mm 2 ), then supine (16.0mm 2 ) stances ( p < 0.01). Extrusion distance of both menisci was greatest in the unipodal (medial 3.2 mm, lateral 3.1 mm), followed by bipodal (medial 3.1 mm, lateral 3.0 mm), then supine stances (medial 2.7 mm, lateral 2.7 mm, p < 0.01). Age, body mass index (BMI), and joint space width (medial 7.4 ± 1.5 mm, lateral 7.3 ± 1.4 mm) were not correlated with extrusion (all p > 0.05). For the lateral meniscus, greater varus alignment correlated with less change in extrusion from supine to unipodal ( r = –0.70; p < 0.01) and bipodal ( r = 0.52; p = 0.02) stances. For the medial meniscus, PTS had a positive correlation with extrusion from supine to unipodal ( r = 0.50; p = 0.02) and bipodal ( r = 0.53; p = 0.01) stances. Conclusions Meniscus extrusion varied with loading conditions and bony morphology. Varus alignment was associated with less lateral meniscus extrusion, and increased PTS with greater medial meniscus extrusion. These results establish baseline values for dynamic meniscus extrusion in healthy knees, to guide US‐based monitoring after surgery. Level of Evidence Level IV, case series.