The most important finding of the current study is that a greater MMEUS was associated with a higher prevalence of MMPRTMRI based on a positive finding of the ghost meniscus and creft/truncation signs, which corresponds to the rupture of the posterior root of the medial meniscus on sagittal and coronal T2*-weighted MRI. Notably, the current finding was consistently significant in patients with medial knee pain both with and without definitive radiographic OA changes defined by the KLG. Moreover, the cut-off value of MMEUS differed according to the stage of radiographic knee OA.
The prevalence of MMPRT in the current study was 31.3% (25/80 knees) and 29.1% (16/55 knees) in the non-ROA and ROA groups. This prevalence of MMPRT was higher than the previous epidemiological data, ranged from 10.1 to 27.8%.30–32 Interestingly, Bin et al.30 and Hwang et al.32 from Korea reported relatively similar prevalence of MMPRT with the current study. Asian people are more likely to experience MMPRT due to their lifestyle of frequent squatting and sitting on the floor with the legs deeply folded.5 A greater MMEUS was consistently associated with a higher prevalence of MMPRTMRI, thereby verifying our study hypothesis. In accordance with the significant loss of meniscal function resulting from MMPRT, some previous cohort studies reported that patients with medial meniscus posterior root injury on MRI33,34 demonstrate a greater MME on the corresponding MRI. In line with these previous cohort study findings, our study findings showed that the use of US is compatible with that of MRI in terms of measuring MME for MMPRT prevalence determination. Furthermore, US is likely to be clinically relevant for the easily validation of the risk of MMPRT in patients with medial knee joint pain in the outpatient clinic.
However, there is insufficient evidence to connect the relationship between MMEUS and the prevalence of MMPRT at this point. A cadaveric biomechanical study reported that the resection of the posterior root of the lateral meniscus affected the degree of MMEUS.25 For knees with a total resection of the posterior root, a greater MMEUS was observed in comparison to that of knees with a partial resection. Specifically, when an axial load was applied, MMEUS was significantly greater than when a non-axial load was applied; the latter was simulated as non-weight bearing condition.25 Karpinski et al. conducted a study with a similar design to that of the current study to elucidate the relationship between the prevalence of MMPRTMRI and the values of MME measured by both US and MRI.26 They observed a greater MME value in participants who had MMPRT in the knee OA population with a relatively early stage of KLG 0–2. This study26 also evaluated the alterations between the weight and non-weight bearing conditions of MMEUS. Interestingly, the results of Karpinski et al. conflicted with those of Rowland et al.25 wherein weight bearing condition did not change the value of MMEUS.26 Based on these limited US data, there remains a controversy with regard to the relationship between the value of MMEUS and the prevalence of MMPRT. The biomechanical effect of MMPRT on MMEUS would change in accordance with the severity of cartilage degeneration,35,36 lateral or medial meniscus involvement, and weight or non-weight bearing condition. Moreover, to improve inter-rater reliability, future studies should be conducted to determine a consistent method of MMEUS measurements.
The most important limitation of previous US studies25,26 is the small sample size; the reliability of MMEUS was not high enough to determine the prevalence of MMRPT. Thus, the lack of evidence makes it difficult for musculoskeletal healthcare providers to consider the values of MMEUS that validate the prevalence of MMPRT at various stages of knee OA. Based on our study data, with a relatively large sample size, we can progress in the further discussion of applying the optimal cut-off of MMEUS to determine the prevalence of MMPRT. In the current non-radiographic OA population with KLG 0–1, a 5-mm MMEUS is the optimal cut-off to detect MMPRT on T2*-weighted sagittal MRI. In contrast, a 7-mm MMEUS cut-off is optimal for detecting MMPRT in the definitive radiographic OA population with KLG ≥ 2. Compared to the findings of Karpinski et al., the current cut-off values are greater; notably, the upper error bar of the supine position MMEUS was between 5 mm and 6 mm in the study by Karpinski et al.26 In other words, further discussion would be needed to conclude which MMEUS value is best to detect the prevalence of MMPRT, and future large-sample studies will shed light on the detailed mechanism of the effect of meniscus root rupture on the deterioration of meniscal function.
Our study had several limitations. First, we could not validate whether the current study participants had grossly visible MMPRT using arthroscopy. Second, the US evaluation was performed only in the supine position with a non-weight bearing condition. Therefore, we did not discuss how the weight bearing condition affects MMEUS based on the current data. Third, the sample size of the advanced knee OA stage (KLG 3–4, N = 20 knees) was relatively small. Finally, the design of the current study was cross-sectional, and therefore we could not establish a cause-effect relationship between MMEUS value and MMPRT development or aggravation. Despite these limitations, the current study emphasizes the clinical relevance of MMEUS in detecting the prevalence of MMPRT by healthcare providers. The use of US can potentially aid in the evaluation of posterior root meniscus rupture in an outpatient room.