Abstract
[Purpose] Joint effusion is common in knee osteoarthritis (KOA) and contributes to functional limitations and disease progression. Persistence at 1 month is clinically relevant, as it may predispose patients to long-term effusion; however, the associated intra-articular lesions remain unclear. We aimed to identify intra-articular lesions associated with effusion persistence at 1 month using ultrasonography (US) for effusion quantification and magnetic resonance imaging (MRI) for lesion assessment. [Participants and Methods] This retrospective cohort study included 57 patients with medial compartment KOA and joint effusions. Joint effusion was quantified as the suprapatellar pouch distance (SPP-d) by US at baseline and 1 month. Baseline intra-articular lesions were graded using the MRI Osteoarthritis Knee Score. Participants were classified into persistent (SPP-d ≥4 mm) and non-persistent (SPP-d< 4 mm) effusion groups at 1 month. Binary logistic regression identified lesions independently associated with persistence. [Results] The persistent effusion group (n=40) showed a significantly higher medial meniscus extrusion (MME) score than the non-persistent effusion group (n=17) at baseline. In multivariable logistic regression, the MME score was independently associated with effusion persistence (odds ratio, 6.42; 95% confidence interval, 1.95–21.1). [Conclusions] MME is independently associated with joint effusion persistence at 1 month.
Key words: Joint effusion, Knee osteoarthritis, Medial meniscus extrusion
INTRODUCTION
Joint effusion is a common clinical feature of knee osteoarthritis (KOA) and is considered a surrogate marker of intra-articular inflammation1). A randomized controlled trial demonstrated that joint aspiration provides short-term symptomatic relief; however, effusion frequently reaccumulates within 1 week2), indicating a tendency to persist. Persistent effusion is associated with ongoing knee pain, muscle weakness, and structural progression, including cartilage damage and joint deformity3,4,5). In patients with KOA, effusion persistence at 1 month after intra-articular corticosteroid injection predicts prolonged effusion at 1 year6), whereas improvements in synovitis at 1 month are maintained for up to 6 months7). These findings underscore the clinical importance of early effusion resolution to prevent long-term persistence and functional disability.
Longitudinal studies with 24-month follow-up have demonstrated that cartilage damage, bone marrow lesions (BMLs), and osteophytes are associated with joint effusion8). Additionally, medial meniscus extrusion (MME) has been associated with effusion in 48-month studies9). However, whether these intra-articular lesions influence effusion persistence at 1 month remains unclear. This gap may partly reflect the reliance on magnetic resonance imaging (MRI) for effusion assessment in prior studies8, 9), which is unsuitable for repeated short-interval measurement due to cost and logistical constraints. In contrast, ultrasonography (US) shows good validity relative to MRI for detecting joint effusion10) and is non-invasive and readily repeatable, making it well suited for short-term evaluation.
We aimed to identify intra-articular lesions associated with effusion persistence at 1 month in KOA by quantifying effusion with US and grading lesion severity, including BMLs, cartilage damage, osteophytes, and MME using MRI Osteoarthritis Knee Score (MOAKS). We hypothesized that greater severity of BMLs, osteophytes, and MME would be independently associated with persistent joint effusion at 1 month.
PARTICIPANTS AND METHODS
This multicenter retrospective cohort study investigated the association between intra-articular lesions and effusion persistence using clinical data from four orthopedic clinics between June 2024 and August 2025. This study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of Morinomiya University of Medical Sciences (Approval No. 2025-065). Following this ethical approval, permission to conduct the study was obtained from each participating institution. Given the retrospective design using existing clinical and imaging data, the requirement for written informed consent was waived by the Institutional Review Board. Instead, an opt-out procedure was implemented at each participating institution by publicly posting information about the study and providing participants with the opportunity to decline participation.
Patients with medial compartment KOA and joint effusion at baseline were included. For bilateral KOA, the more symptomatic knee was selected. All participants underwent outpatient rehabilitation. Inclusion criteria were: age ≥40 years, femorotibial angle ≥176° on anteroposterior radiographs, and confirmed joint effusion (as defined below). Exclusion criteria included Kellgren–Lawrence (K–L) grade IV, prior knee surgery, comorbid autoimmune or central nervous system diseases, and absence of baseline knee MRI.
The timeline for the baseline phase was defined as follows. At the initial visit, baseline demographic data (age, sex, height, weight, and BMI) were recorded, and anteroposterior radiographs were performed as part of routine clinical flow. Baseline knee MRI and the US examination were subsequently performed within 1 week of the initial visit. No intra-articular procedures (such as joint aspiration or injections) were performed prior to these baseline MRI and US examinations. The date of this baseline US examination was operationally defined as the baseline for the 1 month follow-up evaluation.
Structural severity was assessed using K–L grade and femorotibial angle from anteroposterior radiograph. The number of intra-articular corticosteroid injections, hyaluronic acid injections, and joint aspirations from baseline to 1 month was extracted from medical records. Aspiration was performed when effusion was identified clinically and deemed necessary by an orthopedic surgeon; injections were administered as clinically indicated.
Joint effusion was quantified as the suprapatellar pouch distance (SPP-d) using US. Four US devices with commercially available linear transducers were used: Aplio me (Canon Medical Systems Corp., Tochigi, Japan), SONIMAGE MX1 and SONIMAGE HS1 (Konica Minolta Inc., Tokyo, Japan), and LOGIQ P10 (GE Healthcare, Chicago, IL, USA). A designated examiner at each institution independently measured the SPP-d; all four examiners were healthcare professionals specializing in musculoskeletal rehabilitation (physical therapists and ju-do therapy practitioners) and were well-trained with 2, 5, 6, and 15 years of US experience, respectively. A single measurement was recorded per assessment.
Participants were positioned supine with the knee in slight flexion. The transducer was aligned in the long axis, with the base of the patella as the lowest reference point. The SPP-d, the maximum diameter of the hypoechoic area between the quadriceps tendon and prefemoral fat pad (Fig. 1), was considered indicative of effusion when ≥4 mm.11) This method has demonstrated high intra-rater and inter-rater reliability12). To ensure cross-institutional consistency, a standardized imaging protocol was established before study initiation, and all examiners underwent comprehensive training before data collection.
Fig. 1.
Assessment of joint effusion using ultrasonography.
(a) The participant is positioned supine with the knee in slight flexion. The transducer is aligned along the long axis, with its lower end positioned at the level of the base of the patella. (b) Ultrasound image showing the measurement of the maximum diameter of the hypoechoic area between the quadriceps tendon and the prefemoral fat pad using the caliper tool. SPP-d: suprapatellar pouch distance; Ant: Anterior.
Measurements were obtained at baseline and at 1 month, both prior to joint aspiration. Participants with an SPP-d ≥4 mm at 1 month were classified into the persistent effusion group; those with SPP-d<4 mm into the non-persistent effusion group11).
Knee MRI was performed at four institutions using the following systems: 0.3-T MRI (AIRIS Vento, Fujifilm Medical Corp., Tokyo, Japan), 3.0-T MRI (Discovery MR750w, GE Healthcare Japan Inc., Tokyo, Japan), and two 1.5-T MRI systems (ECHELON OVAL and ECHELON Smart Plus, Fujifilm Medical Corp.). All examinations were conducted in the supine position without contrast enhancement, covering the entire knee joint. Imaging protocols included sagittal T1-weighted, T2-weighted, and T2 fat-suppressed sequences; coronal T1-weighted, T2-weighted, and T2 fat-suppressed sequences; and axial T2 fat-suppressed sequences. Detailed scanner parameters are provided in Supplementary Table 1.
Intra-articular lesion severity was assessed using the MOAKS, a validated semi-quantitative scoring system13). Given the focus on medial compartment KOA, BMLs, cartilage damage, and osteophytes were evaluated in the medial femoral condyle (MFC) and medial tibial condyle (MTC). Effusion-synovitis and MME were also scored.
To minimize variability related to MRI field strength across facilities, all MOAKS evaluations were performed by a single physical therapist with 5 years of experience blinded to SPP-d values. Intra-rater and inter-rater reliability were assessed using intraclass correlation coefficients (ICC [1,1] and ICC [2,1]) in 10 cases not included in the main cohort, independently scored by the primary reader and a board-certified orthopedic surgeon with >20 years of musculoskeletal imaging experience and prior MOAKS training. ICC values were interpreted as: <0.50, poor; 0.50–0.75, moderate; 0.75–0.90, good; and >0.90, excellent14). Subchondral BMLs are scored semi-quantitatively based on the percentage of subregional volume affected (including cysts) on a 4-point scale: 0 (none), 1 (<33%), 2 (33–66%), and 3 (>66%). In this study, six subregions were evaluated: the anterior, central, and posterior areas of the MFC and MTC. Total BML scores for each of the three subregions within both the MFC and MTC (each 0–9 points) were used in statistical analysis.
Cartilage damage in each subregion was evaluated based on the affected surface area, graded from 0 to 3 (0: none; 1: <10%; 2: 10–75%; 3: >75%). Six subregions were assessed: the anterior, central, and posterior areas of the MFC and MTC. Total cartilage damage scores for the MFC and MTC were calculated separately by summing the surface area scores of the three corresponding subregions, yielding a total score ranging from 0 to 9 points for each condyle.
Osteophyte size at each location is graded on a 4-point scale: 0 (none), 1 (small), 2 (medium), and 3 (large). Four subregions were analyzed: the anterior, central, and posterior areas of the MFC, and one MTC region. The MFC total score (0–9 points) was derived by summing the three subregion scores; the MTC score was analyzed independently.
Effusion-synovitis was evaluated on non-contrast-enhanced MRI as a measure of joint capsular distension. Distension attributable to intra-articular fluid and synovial thickening was graded on a 4-point scale: 0 (physiologic fluid), 1 (mild), 2 (moderate), and 3 (severe distension).
MME was defined as the perpendicular distance from the medial tibial margin to the peripheral edge of the medial meniscus and was graded as follows: grade 0 (<2 mm), grade 1 (2.0–2.9 mm), grade 2 (3.0–4.9 mm), and grade 3 (≥5.0 mm).
Baseline characteristics were compared between groups using Student’s t-test or the Mann–Whitney U test, according to data distribution, and the χ2 test for categorical variables. Between-group differences in MOAKS parameters were examined using Student’s t-test or the Mann–Whitney U test with Bonferroni correction applied for multiple comparisons.
Binary logistic regression was performed to identify intra-articular lesions associated with effusion persistence at 1 month. To reduce the risk of overfitting, each MOAKS parameter was evaluated in a separate adjusted model. Based on prior evidence, BMI15), K–L grade16, 17), and baseline SPP-d6) were included as covariates in each model with K–L grade entered as a continuous variable. As a sensitivity analysis, the multivariable logistic regression analysis was repeated with institution included as a categorical variable represented by three dummy variables, with one institution designated as the reference category. Statistical analyses were performed using SPSS (version 27.0; IBM Corp., Armonk, NY, USA), with significance set at p<0.05.
RESULTS
Of 130 initially screened participants across four participating facilities (Facility 1: n=71; Facility 2: n=24; Facility 3: n=20; Facility 4: n=15), 59 met eligibility criteria after exclusions (Facility 1: 35, Facility 2: 10, Facility 3: 9, and Facility 4: 5). Two were lost to follow-up, yielding a final analytic sample of 57 participants: 40 in the persistent and 17 in the non-persistent effusion group (Fig. 2).
Fig. 2.
Flowchart of participant selection.
SPP-d: suprapatellar pouch distance.
Compared with the non-persistent effusion group, the persistent effusion group showed significantly higher body weight (median [interquartile range {IQR}]: 62.1 [15.0] kg vs. 54.3 [10.5] kg, p=0.020) and BMI (mean ± standard deviation [SD]: 25.0 ± 3.4 kg/m2 vs. 22.2 ± 3.0 kg/m2, p=0.006). Furthermore, baseline SPP-d was significantly higher in the persistent effusion group (median [IQR]: 7.1 [2.6] mm vs. 5.6 [1.7] mm, p=0.007). The demographic and clinical characteristics of both groups are shown in Table 1.
Table 1. Participant characteristics.
| Characteristic | Persistent effusion group (n=40) | Non-persistent effusion group (n=17) |
| Age (years) | 73.5 (13.8) | 66.0 (9.0) |
| Female, n (%) | 25 (62.5) | 11 (64.7) |
| Height (m) | 1.6 (0.1) | 1.6 (0.1) |
| Weight (kg)* | 62.1 (15.0) | 54.3 (10.5) |
| Body mass index (kg/m2)** | 25.0 (3.4) | 22.2 (3.0) |
| Kellgren–Lawrence grade (I/II/III) | 5/22/13 | 4/9/4 |
| Femorotibial angle (°) | 180.0 (3.0) | 180.0 (1.0) |
| SPP-d at baseline (mm)** | 7.1 (2.6) | 5.6 (1.7) |
| SPP-d at 1 month (mm)*** | 6.7 (1.9) | 2.4 (1.0) |
| ΔSPP-d (mm)*** | −0.7 (2.5) | −3.4 (1.4) |
| IACS (times) | 0.3 (0.8) | 0.6 (0.6) |
| HA (times) | 1.2 (1.5) | 1.2 (1.5) |
| Joint aspiration (times) | 1.7 (1.6) | 0.9 (1.1) |
Values are presented as mean (standard deviation), median (interquartile range), or number (percentage), as appropriate. Continuous variables were compared between groups using Student’s t-test or the Mann–Whitney U test, depending on data distribution, while categorical variables were analyzed using the χ2 test. *p<0.05, **p<0.01, ***p<0.001. SPP-d: suprapatellar pouch distance; IACS: intra-articular corticosteroid injection; HA: hyaluronic acid; K–L grade: Kellgren–Lawrence grade.
MOAKS scoring demonstrated good to excellent intra- and inter-rater reliability, with ICC (1,1) values ranging from 0.89 to 1.00 and ICC (2,1) values ranging from 0.84 to 1.00 (Table 2).
Table 2. Intra- and inter-rater reliability of MOAKS.
| Parameter | ICC (1,1) | 95% CI | ICC (2,1) | 95% CI |
| Total BMLs in MFC | 0.99 | 0.96–0.99 | 0.97 | 0.89–0.99 |
| Total BMLs in MTC | 0.95 | 0.81–0.99 | 0.84 | 0.50–0.96 |
| Cartilage damage in MFC | 0.94 | 0.80–0.98 | 0.91 | 0.70–0.97 |
| Cartilage damage in MTC | 0.89 | 0.65–0.97 | 0.92 | 0.73–0.98 |
| Osteophytes in MFC | 0.99 | 0.97–0.99 | 0.97 | 0.94–0.99 |
| Osteophytes in MTC | 0.90 | 0.67–0.97 | 0.89 | 0.66–0.97 |
| Effusion-synovitis | 0.97 | 0.91–0.99 | 0.96 | 0.88–0.98 |
| MME | 1.00 | 1.00–1.00 | 1.00 | 1.00–1.00 |
Intra-rater reliability was assessed using intraclass correlation coefficient (ICC) (1,1), and inter-rater reliability was assessed using ICC (2,1). Values closer to 1 indicate higher reliability. Confidence intervals (95% CI) are presented to reflect the precision of the estimates. MOAKS: MRI Osteoarthritis Knee Score; ICC: intraclass correlation coefficient; CI: confidence interval; BMLs: bone marrow lesions; MFC: medial femoral condyle; MTC: medial tibial condyle; MME: medial meniscus extrusion.
The persistent effusion group showed significantly higher MME scores (median [IQR]: 2.0 [0.0] vs. 1.0 [1.0], effect size: 0.66, p<0.001) than the non-persistent group. No significant between-group differences were observed for BMLs, cartilage damage, osteophytes, and effusion-synovitis (Table 3).
Table 3. Comparison of MOAKS scores between the two groups.
| Parameter | Persistent (n=40) | Non-persistent (n=17) | Effect size |
| BMLs in MFC | 0.0 (1.0) | 0.0 (1.0) | 0.04 |
| BMLs in MTC | 0.0 (2.3) | 0.0 (1.0) | 0.00 |
| Cartilage damage in MFC | 1.0 (3.0) | 1.0 (1.0) | 0.06 |
| Cartilage damage in MTC | 1.0 (2.0) | 0.0 (1.0) | 0.26 |
| Osteophytes in MFC | 3.0 (2.0) | 2.0 (1.0) | 0.30 |
| Osteophytes in MTC | 1.5 (1.0) | 1.0 (0.0) | 0.24 |
| Effusion-synovitis | 3.0 (1.3) | 2.0 (1.0) | 0.09 |
| MME*** | 2.0 (0.0) | 1.0 (1.0) | 0.66 |
Values are presented as median (interquartile range). Group comparisons were performed using the Mann–Whitney U test. P-values were adjusted for multiple comparisons using the Bonferroni correction, and a value of<0.006 was considered statistically significant. Effect size values are provided to indicate the magnitude of differences between groups. ***p<0.001. MOAKS: MRI Osteoarthritis Knee Score; SPP-d: suprapatellar pouch distance; BMLs: bone marrow lesions; MFC: medial femoral condyle; MTC: medial tibial condyle; MME: medial meniscus extrusion.
Univariable analysis identified MME score as significantly associated with effusion persistence (OR, 8.21; 95% confidence interval [CI], 2.50–27.0, p<0.001). After adjustment for BMI, K–L grade, and baseline SPP-d, the MME score remained independently associated with effusion persistence (OR, 6.42; 95% CI, 1.95–21.1; p=0.002). No other MOAKS parameter was significantly associated with effusion persistence in either analysis (Table 4). In the sensitivity analysis additionally adjusted for institution, the association between MME and effusion persistence remained consistent with the primary analysis (OR, 4.82; 95% CI, 1.44–16.1; p=0.011).
Table 4. Association between persistent joint effusion and intra-articular lesions.
| Parameter | Univariable OR (95% CI) | Multivariable OR (95% CI) |
| BMLs in MFC | 0.91 (0.68–1.23) | 0.86 (0.57–1.29) |
| BMLs in MTC | 0.95 (0.73–1.24) | 0.80 (0.57–1.13) |
| Cartilage damage in MFC | 1.24 (0.83–1.86) | 1.45 (0.87–2.40) |
| Cartilage damage in MTC | 1.59 (0.92–2.76) | 1.53 (0.82–2.84) |
| Osteophytes in MFC | 1.28 (0.94–1.75) | 1.24 (0.84–1.83) |
| Osteophytes in MTC | 1.61 (0.77–3.35) | 1.52 (0.53–4.39) |
| Effusion-synovitis | 1.14 (0.57–2.27) | 1.07 (0.46–2.51) |
| MME | 8.21 (2.50–27.0) *** | 6.42 (1.95–21.1) *** |
Univariable and multivariable logistic regression analyses were performed to evaluate the association between intra-articular lesions and persistent joint effusion. Results are presented as odds ratios (ORs) with 95% confidence intervals (CIs). The multivariable model was adjusted for body mass index (BMI), Kellgren–Lawrence (K–L) grade, and baseline SPP-d. ***p<0.001. OR: odds ratio; CI: confidence interval; BMLs: bone marrow lesions; MFC: medial femoral condyle; MTC: medial tibial condyle; MME, medial meniscus extrusion; SPP-d: suprapatellar pouch distance; BMI: body mass index.
DISCUSSION
Among all intra-articular lesions evaluated using MOAKS, MME was the only factor independently associated with effusion persistence at 1 month in patients with KOA after adjustment for relevant covariates. This finding suggests that MME may be a structural feature associated with short-term effusion persistence. Contrary to our hypothesis, BMLs and osteophytes were not associated with effusion persistence.
Consistent with a previous prospective cohort study6), the persistent effusion group in our study showed higher baseline SPP-d, body weight, and BMI, supporting the relevance of these anthropometric and effusion-related variables to effusion persistence.
A 24-month longitudinal study reported that BML severity, cartilage damage, and osteophytes were associated with subsequent changes in effusion volume8, 18). In contrast, MME was the only structural lesion independently associated with 1-month effusion persistence in our study. Because MME may precede bone abnormalities19), this early morphological change may be particularly relevant to short-term effusion persistence. Previous biomechanical studies have shown that MME reduces medial compartment load-bearing capacity and increases abnormal mechanical stress20, 21), which may be cause irritation to adjacent synovial tissue22). In addition, a 4 year longitudinal study demonstrated that MME contributes to osteoarthritis progression-mediated synovitis9). These previously proposed mechanical and inflammatory mechanisms may partly explain the observed association between MME and persistent effusion; however, these mechanisms were not directly examined in the present study.
Early assessment of MME may help identify patients at increased risk of persistent effusion. However, whether interventions targeting MME can reduce effusion persistence or delay structural progression requires investigation in future prospective studies.
This study has several limitations. First, the retrospective cohort design precludes causal inference between MME and effusion persistence. Second, US is inherently operator-dependent; residual inter-center and inter-examiner variability cannot be excluded despite the standardized protocol. Similarly, differences in MRI field strengths across institutions may have affected image quality and lesion detectability, potentially introducing measurement heterogeneity. Third, the timing and frequency of interventions during follow-up, including joint aspiration, corticosteroid injection, hyaluronic acid injection, and rehabilitation, were not standardized or included in the adjusted models and may have influenced effusion status at 1 month. Although the numbers of aspirations and intra-articular injections did not differ significantly between the groups, residual confounding cannot be excluded. Fourth, the sample size was small, particularly in the non-persistent group. Consequently, the adjusted estimates may have been unstable or imprecise despite limiting the number of covariates and analyzing each MOAKS parameter in a separate logistic regression model. The multivariable results should therefore be interpreted cautiously and require confirmation in larger cohorts. Finally, restriction to participants with baseline MRI may have introduced selection bias toward more symptomatic patients, limiting generalizability to the broader KOA population.
In conclusion, MME was independently associated with effusion persistence at 1 month in patients with KOA. These findings suggest that MME may be a clinically relevant imaging feature associated with short-term effusion persistence.
Funding
This research received no external funding.
Conflict of interest
The authors declare that they have no competing interests.
Supplementary
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