MRI-based normative reference values for medial and lateral meniscal extrusion in asymptomatic Korean knees

Abstract

Purpose

To establish MRI-based normative reference values for medial and lateral meniscal extrusion in asymptomatic Korean adults and to assess compartmental differences using absolute and proportional indices.

Methods

A retrospective study of 38 independent knees from asymptomatic adults (mean age, 30.6 ± 4.5 years; 13 male, 25 female) was performed using a single-knee-per-subject protocol. Knees with trauma, ligament injury, significant effusion, meniscal tears, meniscotibial abnormalities, cartilage defects, or bone marrow lesions were excluded. Mid-coronal 1.5-T fat-suppressed proton density MRI was used to measure absolute extrusion and calculate the Meniscal Extrusion Index (MEI). Upper reference values were determined using mean + 2 SD and the 95th percentile. Compartments were compared using paired t-tests.

Results

Mean medial meniscal extrusion (MME) was 1.86 ± 0.64 mm (95% CI, 1.65–2.07), with a mean + 2 SD of 3.13 mm and a 95th percentile of 2.84 mm. Mean lateral meniscal extrusion (LME) was 1.40 ± 0.59 mm (95% CI, 1.20–1.59), with a mean + 2 SD of 2.58 mm and a 95th percentile of 2.40 mm. Medial extrusion was greater than lateral extrusion (p = 0.0029; Cohen’s d = 0.52). Mean medial and lateral MEI values were 22.5% ± 8.9% and 15.4% ± 9.2%, respectively.

Conclusions

Mild meniscal extrusion is present in structurally intact knees. Medial values approaching 3 mm and lateral values approaching 2.6 mm may represent normal anatomical variation. Interpretation should incorporate compartment-specific reference limits and proportional indices such as the MEI.

Keywords

meniscal extrusion magnetic resonance imaging reference values meniscal extrusion index Korea

Introduction

The meniscus plays a critical role in load distribution, shock absorption, and joint stability by converting axial compressive forces into circumferential hoop stresses.¹ Displacement of the meniscal body beyond the tibial articular margin, commonly referred to as meniscal extrusion, is a recognized imaging biomarker for structural failure. Severe medial meniscal extrusion is strongly associated with posterior root tears, cartilage degeneration, and the progression of knee osteoarthritis.^2,3 Historically, an absolute threshold of 3.0 mm has been widely adopted to define pathological meniscal displacement on magnetic resonance imaging (MRI).²

However, this 3.0-mm cutoff was not originally derived from strictly asymptomatic populations and may fail to account for normal physiological variation. Recent investigations have questioned whether rigid millimetric thresholds adequately reflect normal anatomical variation across different populations, noting that mild extrusion is frequently observed in asymptomatic individuals.^4,5 This mild, asymptomatic displacement has been described as a “paraphysiological” variant rather than an acute structural injury.⁶

Anatomical differences between compartments further complicate interpretation. The medial meniscus is firmly anchored to the joint capsule and coronary ligaments, whereas the lateral meniscus demonstrates greater translational mobility due to the popliteus hiatus.^1,7,8 These differences in attachment and mobility influence how each meniscus responds to axial loading and may contribute to compartment-specific patterns of extrusion. Therefore, compartment-specific reference values may be more appropriate than a single universal cutoff.

Skeletal morphometry and meniscal dimensions vary significantly among ethnic groups and patient demographics.⁹ Proportional assessments such as the Meniscal Extrusion Index (MEI) have been introduced to account for variation in meniscal width and patient size.^10,11 Despite these advances, comprehensive compartment-specific normative data for physiological meniscal extrusion in asymptomatic Asian populations, particularly Korean adults, remain limited.

Therefore, the purpose of this study was to establish MRI-based normative reference values for medial and lateral meniscal extrusion in asymptomatic Korean adults using both absolute millimetric measurements and proportional indices. We hypothesized that measurable meniscal extrusion exists as a physiological variant in structurally intact knees and that medial extrusion would be greater than lateral extrusion.

Materials and methods

Study population

This retrospective observational study was approved by the Institutional Ethic Committee (IEC/2026/005).

Informed consent was obtained from all individual participants included in the study.

A total of 62 asymptomatic knees that underwent MRI for non-knee-related complaints or health screening purposes were initially identified. To preserve statistical independence and eliminate intra-subject correlation bias, a single-knee-per-subject protocol was implemented. Among the 10 patients who underwent bilateral imaging, the right knee was systematically excluded and only the left knee was included in the final analysis. The use of a single healthy knee as a representative anatomical template is supported by bilateral symmetry studies confirming highly reproducible meniscal dimensions between contralateral limbs.^12,13

Patients with a history of recent knee trauma, acute ligament injury, significant joint effusion,¹⁴ or mechanical symptoms, as documented in clinical records and MRI reports, were strictly excluded. Furthermore, knees demonstrating discoid meniscal variants, meniscal tears (including radial, complex, or root tears), meniscotibial ligament abnormalities, cartilage defects, or bone marrow lesions were excluded to isolate structurally intact anatomy. Structural pathology, particularly posterior meniscal root disruption, is known to significantly increase extrusion magnitude.³ Strict screening was applied to avoid the inclusion of incidental abnormalities frequently observed in asymptomatic populations.¹⁵

After application of these criteria, 38 independent asymptomatic knees constituted the final study cohort. The cohort included 13 male and 25 female participants. Sex was determined based on medical record documentation.

MRI acquisition and measurement technique

All magnetic resonance imaging (MRI) examinations were performed using a clinical 1.5-Tesla MRI scanner (Siemens Healthineers). To achieve high-resolution structural evaluation and optimize the signal-to-noise ratio, a dedicated knee coil was utilized for all knee acquisitions, ensuring precise diagnostic assessment of the articular and periarticular structures. Fat-suppressed proton density sequences were acquired with a slice thickness of 3 mm and no interslice gap. All measurements were performed by a single orthopaedic surgeon to ensure methodological consistency.

Morphometric analysis was conducted on the mid-coronal slice identified at the level where the medial tibial spine was most prominent. The medial tibial spine is a validated and reproducible anatomical landmark for meniscal extrusion quantification.^5,16 To ensure anatomical accuracy, slice selection was confirmed across multiple planes. Sagittal scout images verified alignment through the mid-portion of the medial femoral condyle, and axial images confirmed correspondence with the mid-meniscal body.

All measurements were performed parallel to the tibial plateau surface in accordance with validated protocols and contemporary consensus recommendations.^5,17 The tibial osteochondral junction was used as the reference margin. Osteophytes, when present, were excluded from the reference line to prevent artificial overestimation of extrusion.^2,5

Tibial plateau width was measured separately for medial and lateral compartments, from the respective tibial spine apex to the osteochondral margin.¹² Meniscal width was defined as the horizontal distance from the inner free edge of the meniscal body to its outer capsular margin at the mid-body level.^5,12

Absolute meniscal extrusion was defined as the horizontal distance from the outer margin of the tibial plateau to the peripheral border of the meniscus² (Figure 1).

Figure 1.

MRI-based morphometric measurement technique on a mid-coronal fat-suppressed proton density sequence of an asymptomatic right knee at the level of the medial tibial spine. (a) A horizontal dashed line is drawn across the tibial plateau to establish the reference plane, ensuring all subsequent measurements are parallel to the joint surface. (b) Total medial meniscal width is measured via a horizontal dashed line between two solid vertical lines marking the inner free edge and the peripheral capsular border. (c) Absolute medial meniscal extrusion is measured via a horizontal dashed line between two solid vertical lines marking the tibial osteochondral junction and the peripheral capsular border.

Relative extrusion was calculated using the Meniscal Extrusion Index (MEI), defined as (extrusion width/total meniscal width) × 100. This proportional method has been validated in extrusion ratio studies and incorporated into modern classification frameworks.^10,11

Statistical analysis

Normality of continuous variables was assessed using the Shapiro–Wilk test and inspection of Q–Q plots. Continuous variables are presented as mean ± standard deviation (SD), 95% confidence intervals (CI), and observed ranges.

To estimate the upper reference values, both variance-based and distribution-based approaches were used. Mean + 2 SD was calculated as a conventional variance-based estimate of the upper distribution boundary under the assumption of normality, and the 95th percentile was reported as a non-parametric distribution-based estimate.

Medial and lateral extrusion values within the same knee were compared using paired Student’s t-tests. Effect size was calculated using Cohen’s d for paired samples. Statistical significance was defined as p < 0.05. Statistical analyses were performed using IBM SPSS Statistics version 25.0.

Results

Thirty-eight independent asymptomatic knees from 13 male and 25 female participants (mean age, 30.6 ± 4.5 years) were included in the final analysis.

The mean medial meniscal width was 8.83 ± 2.50 mm (range, 5.3–17.9 mm), and the mean lateral meniscal width was 9.78 ± 2.25 mm (range, 6.0–15.6 mm) (Table 1).

Table 1.

MRI-based morphometric analysis of meniscal extrusion.

Measurement	Medial compartment	Lateral compartment	p-value
Total meniscal width (mm)	8.83 ± 2.50	9.78 ± 2.25	—
Absolute extrusion (mm)	1.86 ± 0.64	1.40 ± 0.59	0.0029^a
95% confidence interval (mm)	1.65–2.07	1.20–1.59	—
Observed range (mm)	0.5–3.2	0.2–2.5	—
95th percentile (mm)	2.84	2.40	—
Mean + 2 SD (mm)	3.13	2.58	—
Meniscal extrusion index (%)	22.5% ± 8.9%	15.4% ± 9.2%	—

^aPaired t-test comparison between medial and lateral extrusion.

The mean medial meniscal extrusion (MME) was 1.86 ± 0.64 mm (95% CI, 1.65–2.07 mm; range, 0.5–3.2 mm). The variance-based upper reference value (Mean + 2 SD) was 3.13 mm, and the 95th percentile was 2.84 mm.

The mean lateral meniscal extrusion (LME) was 1.40 ± 0.59 mm (95% CI, 1.20–1.59 mm; range, 0.2–2.5 mm). The Mean + 2 SD for LME was 2.58 mm, and the 95th percentile was 2.40 mm.

Medial extrusion was significantly greater than lateral extrusion (t = 3.19, p = 0.0029), with a moderate effect size (Cohen’s d = 0.52) (Figure 2).

Figure 2.

Box-and-whisker plot demonstrating the physiological distribution of absolute meniscal extrusion in the medial and lateral compartments of asymptomatic knees. The medial compartment exhibits significantly greater extrusion (p = 0.0029). The ‘X’ markers denote mean values (1.86 mm medial vs 1.40 mm lateral), while the horizontal lines within the boxes represent the medians. Whiskers indicate the full observed range from minimum to maximum values.

The mean medial MEI was 22.5% ± 8.9%, whereas the mean lateral MEI was 15.4% ± 9.2%.

Discussion

The primary finding of this study is that measurable meniscal extrusion exists in healthy, asymptomatic Korean adults. The 95th percentile for medial extrusion was 2.84 mm, closely approximating the commonly cited 3.0 mm boundary. These findings suggest that mild extrusion may represent a normal anatomical variant rather than structural failure, consistent with the concept of paraphysiological displacement.⁶

Our findings align with population-based analyses demonstrating that mild medial extrusion is a common physiological feature in asymptomatic knees rather than a strictly pathological condition.⁵ According to the MOAKS semi-quantitative scoring system, meniscal extrusion of 2.0–2.9 mm is categorized as Grade 1.¹⁸ In the present asymptomatic cohort, the 95th percentile value of 2.84 mm falls within this Grade 1 range. This finding suggests that low-grade extrusion defined within osteoarthritis scoring systems may overlap with the upper distribution of physiologic extrusion in structurally intact knees.

The widely used 3-mm threshold for pathological medial extrusion² was not derived from strictly asymptomatic cohorts. Population-based analyses have suggested that raising the threshold to 4 mm improves specificity for structural osteoarthritis⁵ However, the present study was designed to define the physiological distribution in structurally intact knees rather than to establish a diagnostic threshold for osteoarthritis. In our cohort, extrusion values closely approximating 3 mm were observed in anatomically intact knees, indicating that rigid millimetric cutoffs may oversimplify interpretation. Given the frequency of incidental findings in asymptomatic individuals,^4,15 extrusion near 3 mm should be interpreted within clinical and structural context.

Medial extrusion was significantly greater than lateral extrusion, reflecting established biomechanical differences between compartments.^1,5 The medial meniscus is firmly anchored to the joint capsule, coronary ligaments, and deep medial collateral ligament, resulting in limited anteroposterior translational mobility. Under axial compressive loading, these constraints restrict dynamic repositioning of the meniscal body, which may contribute to greater radial displacement beyond the tibial margin when containment forces are exceeded. In contrast, the lateral meniscus demonstrates greater translational mobility due to the popliteus hiatus, while its radial excursion is dynamically restricted by the menisco-tibio-popliteus-fibular complex (MTPFC).^7,8 This combined mobility and lateral tethering permits more effective redistribution of load through anteroposterior translation, thereby limiting radial extrusion.⁷ These anatomical and biomechanical distinctions explain the higher physiological extrusion values observed in the medial compartment and support the use of compartment-specific reference limits rather than a universal cutoff.

Proportional analysis demonstrated that a medial MEI of approximately 22% can be observed in healthy adults. Recent qualitative frameworks have proposed distinguishing paraphysiological extrusion from true pathology,⁶ while proportional assessments have suggested that an MEI < 20% is generally paraphysiological and 20% to 40% represents a diagnostic “grey zone”.¹¹ In the present study, a baseline medial MEI of 22.5% was observed in asymptomatic Korean adults, suggesting that values within the lower grey-zone range may represent the upper limit of physiological variation in this cohort. Relative assessment may improve interpretation because absolute millimeter measurements do not account for natural variation in meniscal width across individuals.^9,10 Park et al.¹⁰ specifically emphasized that uniform length criteria may be insufficient due to interindividual variation in transverse meniscal diameter. Proportional indices have therefore been proposed and validated as more individualized indicators of meniscal position.^10,11

Clinically, extrusion must be interpreted alongside structural integrity, particularly posterior root and meniscotibial ligament status.¹⁹ Longitudinal studies suggest a chronological cascade of meniscal failure in which subtle meniscotibial ligament laxity precedes root tearing and permits minor radial drift.²⁰ In our asymptomatic cohort, mild displacement up to 2.8 mm may reflect baseline capsuloligamentous laxity without representing acute biomechanical failure. Although marked extrusion remains strongly associated with posterior root tears,³ mild extrusion may occur in structurally intact knees.

Limitations

The sample size was modest (N = 38), which may influence percentile-based estimates of physiological extrusion. However, the strict exclusion of bilateral data and structural abnormalities was designed to isolate anatomically intact knees. MRI was performed under non-weight-bearing conditions. Dynamic and weight-bearing imaging studies have demonstrated greater extrusion; therefore, the present values should be interpreted within the context of standard supine MRI protocols.^21–23 Intraobserver reliability was not formally assessed. However, measurements were performed using standardized and previously validated anatomical landmarks to minimize variability.^16,17 Longitudinal follow-up was not available to determine future structural progression in this asymptomatic cohort. Finally, the findings reflect a Korean population and may not be generalizable to other ethnic groups.

Conclusions

Mild meniscal extrusion is observed in asymptomatic Korean knees. Medial extrusion approaching 3 mm and lateral extrusion approaching 2.6 mm may fall within normal anatomical variation rather than indicating definite structural pathology. Extrusion should be interpreted using compartment-specific reference values and proportional indices such as the MEI, with careful assessment of meniscal root integrity and capsuloligamentous structures.

Footnotes

ORCID iD

Kaushik Are

Ethical considerations

This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Institutional Ethic Committee (MURUP/IEC/2026/005).

Consent to participate

Informed consent was obtained from all individual participants included in the study.

Consent for publication

The authors affirm that human research participants provided informed consent for publication of the data.

Author contributions

The study was conceptualized by Dr. Woon Hwa Jung. All authors contributed to the design. Material preparation, data collection and analysis were performed by Dr. Kaushik Are and Dr. Harshit Khare. The first draft of the manuscript was written by Dr. Kaushik Are and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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