Summary

The four characteristic ultrasonographic findings for the detection of medial meniscus posterior root tear are joint effusion, increased medial meniscus extrusion (MME) in B-mode, blood flow signals into the femur and tibia in Doppler mode, and a smaller MME difference between the extension and flexion positions.

Abstract

Purpose

Medial meniscus posterior root tear (MMPRT) disrupts the meniscal hoop, resulting in the progression of knee osteoarthritis. The early diagnosis and treatment of MMPRT is crucial. While MRI is essential for diagnosing MMPRT, its availability is limited, and it is expensive. Therefore, unnecessary MRIs should be avoided whenever possible. Currently, objective indicators for using MRI are not available. Consequently, the decision has traditionally relied on the subjective judgment of physicians. We hypothesized that ultrasonography (US), which can be performed easily in an outpatient setting, could provide objective indicators. The purpose of this study is to investigate US findings that are characteristic of MMPRT.

Methods

This study is a multicenter prospective study. The study included 100 patients (101 knees; average age 58.3 ± 11.2 years; 58 males and 42 females) who visited hospitals related to Kanazawa University from 2018 to 2023 with knee pain and demonstrated Kellgren-Lawrence grade 1 or less on plain X-ray. Using B-mode US, we investigated the presence of joint effusion and synovial proliferation in the suprapatellar pouch. Findings were considered positive if either joint effusion or synovial proliferation was 4mm or more. We also evaluated the presence of horizontal tears in the medial meniscus and medial meniscus extrusion (MME). Horizontal tears were deemed positive if a hypoechoic area was observed within the meniscus. MME was measured at the position where the medial collateral ligament (MCL) was visualized by US. In Doppler mode, we evaluated inflow signals of the suprapatellar bursa, MCL bursa, and infrapatellar fat pad. Additionally, we assessed the blood flow signals into the femur and tibia. MME was measured in three positions: supine extension, 90-degree flexion, and standing extension. The differences between standing and supine, as well as supine and 90-degree flexion positions, were calculated as ΔMME. We compared the MMPRT group (20 knees, diagnosed with MMPRT by MRI) with the non-MMPRT group (81 knees). Statistical analysis was performed using Student's t-test, chi-square test, Fisher's exact test, and Mann-Whitney U test. The significance level was set at p-value <0.05.

Results

In B-mode US, joint effusion was significantly more frequent in the MMPRT group (p=0.028), while horizontal tears in the medial meniscus were more frequent in the non-MMPRT group (p<0.001). In Doppler mode, blood flow signals into the femur (p=0.008) and tibia (p=0.013) were significantly more frequent in the MMPRT group. MME (supine, standing, and 90-degree flexion) was significantly greater in the MMPRT group (3.04mm, 3.72mm, 3.08mm) compared to the non-MMPRT group (2.21mm, 2.65mm, 1.34mm) in all three positions. The difference in MME between standing and supine positions was not significant between the groups (p=0.162). Conversely, the difference between supine and 90-degree flexion positions was significantly smaller in the MMPRT group (p=0.003).

Conclusion

The four ultrasound findings associated with MMPRT were 1) joint effusion, 2) increased MME in B-mode, 3) blood flow signals into the femur and tibia in Doppler mode, and 4) a smaller MME difference between extension and flexion positions in dynamic evaluation.