Medial Meniscal Extrusion: A Narrative Review of Etiology, Diagnosis, and Therapeutic Strategies

Introduction

The menisci are crescent-shaped fibrocartilaginous structures between the femoral condyles and the tibial plateau, playing crucial roles in load transmission, shock absorption, and joint stability. The medial meniscus covers approximately 50–75% of the medial tibial plateau and attaches firmly to the joint capsule, whereas the lateral meniscus covers about 75–93% of the lateral tibial plateau and exhibits approximately twice the mobility of its medial counterpart.¹ Given its higher clinical prevalence, firmer capsular restraint, and closer association with medial compartment overload and osteoarthritis progression, this review focuses on medial meniscal extrusion. The posterior root of the medial meniscus attaches anteromedially to the posterior cruciate ligament at the medial tibial eminence. Integrity of the meniscal root attachments is essential for maintaining normal knee kinematics and preventing degenerative changes in the joint.²

In cross-section, the meniscus is wedge-shaped. Compressive forces from the femoral condyle generate stress within meniscal tissue, pushing it radially outward. This displacement is a stress-relieving mechanism and can lead to meniscal extrusion.^3,4 Meniscal extrusion is defined as radial displacement of the meniscus beyond the margin of the tibial plateau,⁵ a concept first introduced in 1991.⁶

Extrusion alters load distribution and biomechanical conditions within the knee joint, potentially resulting in bone marrow lesions (BMLs), subchondral cyst formation, cartilage loss, joint space narrowing, and accelerated progression of knee OA. Recent evidence indicates that meniscal extrusion is an independent risk factor for OA.^7,8 Furthermore, meniscal extrusion is strongly associated with spontaneous osteonecrosis, correlating with both lesion staging and volume.^9,10

Meniscal extrusion is commonly quantified as the absolute distance, in millimetres, between the peripheral meniscal edge and the outer margin of the tibial plateau on a coronal slice through the meniscal body, whereas relative approaches express extrusion as a ratio between the extruded width and the total meniscal width or tibial coverage, which can reduce the influence of patient size but is less frequently reported in clinical series. Although a medial extrusion value greater than 3 mm is widely used to denote pathological extrusion, this threshold was derived from imaging conventions and normative distributions rather than a validated biological cut-off, and its clinical meaning depends on concomitant tear morphology, alignment, and osteoarthritic stage.

While prior reviews have often discussed extrusion as an imaging correlate of osteoarthritis or meniscal injury, an integrated synthesis that links etiologic mechanisms, measurement variability under loading, and decision-oriented selection of emerging procedures such as centralization and combined alignment correction remains limited. In particular, previous reviews have not comprehensively connected how structural failure, dynamic loading assessment, and alignment-sensitive treatment selection interact in the clinical management of medial meniscal extrusion.

This review summarizes recent advances in understanding the etiology, imaging diagnosis, and treatment strategies for medial meniscal extrusion, and it frames extrusion as the measurable consequence of failed meniscal restraint under adverse joint loading, which helps translate imaging findings into patient-specific management decisions.

Etiology

The etiology of meniscal extrusion involves the complex interaction of multiple factors. The progression of meniscal extrusion is driven by structural disruption and mechanical imbalance, resulting in a vicious cycle of OA. Postoperative complications and bone micro-injuries may further exacerbate extrusion. Figure 1 summarizes a decision-oriented treatment algorithm for medial meniscal extrusion and illustrates how commonly used interventions address the dominant drivers of extrusion.

Figure 1 Treatment algorithm for medial meniscal extrusion. Patients are first stratified by symptoms, with asymptomatic cases managed conservatively, while symptomatic cases are further classified by the presence of a medial meniscus posterior root tear. For symptomatic MMPRT, posterior root repair is recommended when tissue quality is adequate, with escalation to posterior root repair plus high tibial osteotomy, a combined triple procedure, or posterior root repair with centralization according to flap displacement, associated supporting structure injury, and osteoarthritis stage, whereas meniscal contouring (partial meniscectomy) is considered as a palliative option when tissue quality is poor and osteoarthritis is end stage. For symptomatic extrusion without a posterior root tear, varus alignment is addressed with high tibial osteotomy, and midbody extrusion with or without supporting structure injury is managed with centralization, with combined high tibial osteotomy and centralization when both malalignment and extrusion instability are present. Arrows indicate the direction of clinical decision making, and brackets indicate stratification levels. Abbreviations: MME, medial meniscal extrusion; MMPRT, medial meniscus posterior root tear; OWHTO, open-wedge high tibial osteotomy; OA, osteoarthritis.

Meniscal Tears

Knees with structural damage to the meniscus, particularly root tears, usually present with significantly increased meniscal extrusion.¹¹ Medial meniscus posterior root tear (MMPRT), defined as a radial tear within 10 mm from the tibial attachment of the posterior root, leads to a marked progression of meniscal extrusion and degeneration of articular cartilage in the medial compartment.¹² Larger gaps in MMPRT are associated with increased absolute and relative extrusion.¹³ In a biomechanical standpoint, MMPRT disrupts circumferential hoop stress, diminishing the meniscus’s load-distributing capacity and leading to abnormal increases in joint contact pressures. Consequently, this triggers cartilage damage and accelerates OA progression.⁴

Meniscotibial Ligament Dysfunction

In addition to intrinsic meniscal injury, dysfunction of the ligamentous structures critically affects meniscal stability. Due to its firm capsular attachments, the medial meniscus has limited mobility and is thus more prone to tears.⁴ In knees with intact menisci and mild knee pathology yet showing extrusion ≥3 mm, abnormalities of the meniscotibial ligament (MTL) are common.¹⁴ A biomechanical study of human cadaveric knees demonstrated that the MTL is pivotal in stabilizing the meniscus. MTL injury induces extrusion, while MTL repair significantly reduces extrusion and may preserve meniscal function, potentially delaying OA progression.¹⁵ The study by Krych et al revealed a temporal relationship among MTL injury, meniscal extrusion, and MMPRT, proposing a sequential pathological model of “MTL injury–extrusion–root tear”.¹⁶ Analysis of 63 MRI scans from 27 patients showed that MTL injuries and meniscal extrusion consistently preceded the diagnosis of MMPRT. The study indicates that MTL dysfunction compromises meniscal stability, increasing extrusion and exposing the posterior root to abnormal stress, eventually leading to MMPRT and accelerated cartilage degeneration. This finding challenges the traditional view that MMPRT directly causes extrusion, explaining the clinical observation that isolated posterior root repairs often fail to correct meniscal extrusion. However, these conclusions require further validation through prospective studies, as the small sample size and retrospective nature may limit their generalizability.

Biomechanics and Loading

Varus malalignment appears to influence medial meniscal extrusion across a continuum rather than at a single universally accepted cut-off. Nevertheless, the available literature suggests that even mild-to-moderate varus may be clinically relevant. In a clinical root-repair cohort, patients with preoperative varus alignment of less than 2.5° showed significantly better extrusion correction and Lysholm scores than those with varus greater than 2.5°, supporting this value as a pragmatic threshold for risk stratification in surgical decision-making.¹⁷ This clinical observation is consistent with biomechanical data showing that medial meniscal extrusion increased progressively with varus loading, reaching 2.36 ± 0.13 mm at 2° varus and 3.16 ± 0.24 mm at 4° varus, the latter exceeding the commonly used 3-mm pathological threshold.¹⁸ In addition, upright weight-bearing imaging demonstrated that a more varus hip-knee-ankle angle was moderately associated with a greater increase in extrusion under load, while dynamic gait analysis in mild-to-moderate osteoarthritic knees further linked abnormal varus-related loading, reflected by the knee adduction moment, to increased extrusion during walking.^19,20 Taken together, these findings suggest that clinically relevant varus should not be viewed as an all-or-none phenomenon, but values exceeding approximately 2.5° may already warrant closer attention, particularly when extrusion is progressive or when isolated meniscal repair is being considered.

Systemic factors, such as obesity, are also closely associated with meniscal extrusion. Achtnich et al²¹ in a large-scale ultrasound study, demonstrated a significant correlation between body mass index (BMI) and weight-bearing meniscal extrusion. These results partly align with Ding’s findings, who suggested that increased BMI and bone size indirectly drive meniscal extrusion via mechanical stress or osteophyte formation.²² However, Crema et al²³ reported no independent association between BMI and meniscal extrusion in the Multicenter Osteoarthritis Study (MOST), a cohort of adults aged 50–79 years who either had knee osteoarthritis or were at high risk of developing it; therefore, this finding should be interpreted in the context of an OA/at-risk population rather than healthy individuals without OA. In contrast, in symptomatic and established OA populations, Raynauld et al²⁴ further revealed a strong relationship between elevated BMI, severe meniscal extrusion, and accelerated cartilage loss, indicating that in the pathological context of OA, obesity may amplify meniscal dysfunction through abnormal load distribution.

Bidirectional Relationship Between OA and Meniscal Extrusion

Meniscal extrusion and OA exhibit a bidirectional pathogenic relationship. Kim et al²⁵ evaluated MRI and radiographic findings in 99 patients, noting that osteophyte size and Kellgren-Lawrence (KL) grading positively correlate with the degree of meniscal extrusion. This suggests that specific pathological changes in OA, such as osteophyte formation, may be important driving factors for extrusion processes. Conversely, meniscal extrusion itself substantially increases joint contact pressures, comparable to a total meniscectomy, thereby accelerating cartilage degeneration, osteophyte formation, and ultimately OA progression.²⁶ Age-related collagen matrix degeneration of the meniscus also increases extrusion prevalence,²¹ partially explaining its higher incidence in OA contexts. Thus, meniscal extrusion is both a structural contributor to OA and an aggravating factor through abnormal load distribution, perpetuating a vicious cycle.

Postoperative Factors

Regarding anterior cruciate ligament (ACL) reconstruction, Narazaki et al²⁷ analyzed MRI scans from 28 ACL reconstruction patients, reporting significantly increased postoperative extrusion compared with healthy controls. Therefore, they hypothesized postoperative external tibial rotation or graft tension could displace the meniscus. They concluded that although ACL reconstruction alone does not directly cause meniscal extrusion, isolated ACL reconstruction in patients with concomitant medial meniscus injury fails to halt extrusion progression.

Bone Micro-Injury

Kishiro et al²⁸ examined the association between subchondral insufficiency fracture of the medial tibial plateau and medial meniscal extrusion in patients presenting with knee pain, and they observed greater extrusion and more frequent medial meniscal injury in the fracture group than in those without fracture. These findings support a close coupling between subchondral microstructural failure and meniscal restraint failure. Yet, the direction of causality is not settled, because extrusion can plausibly elevate focal contact pressure and precipitate subchondral fracture. At the same time, acute or subacute subchondral collapse can also alter joint congruence and secondarily displace the meniscus. Clinically, this bidirectional plausibility argues for parallel assessment of meniscal integrity, alignment, and subchondral status when extrusion coexists with sudden pain escalation and marrow signal changes.

Diagnosis

Physiological meniscal extrusion can be observed even in structurally intact knees, and the medial meniscus typically shows slightly greater displacement than the lateral meniscus. Across studies of asymptomatic or non-osteoarthritic knees, physiological extrusion varies by modality and loading condition. For example, ultrasound measured deep to the MCL shows higher values in standing than supine, highlighting that normal ranges are position-dependent.^11,29 In most MRI-based studies, extrusion is measured on a coronal slice through the meniscal body at the level where the tibial spine and meniscal body are well visualized, using the outer osteochondral margin of the tibial plateau as the reference line while excluding marginal osteophytes when present.^4,5 Because small changes in slice selection and landmark definition can shift millimetre-scale measurements, standardizing the measurement plane and reporting the anatomical reference used are prerequisites for meaningful comparison across cohorts.

MRI Diagnosis

MRI is widely used for evaluating meniscal morphology and its association with osteoarticular pathology due to its superior soft-tissue resolution and advantages in static imaging.³⁰ However, MRI has notable limitations, such as being non-weight-bearing, static, and relatively costly. Stehling et al demonstrated via weight-bearing MRI that meniscal extrusion significantly increased under load-bearing conditions, averaging 2.63±2.36 mm compared to 1.75±1.67 mm in non-weight-bearing conditions.³¹

Currently, no consensus exists regarding the optimal MRI measurement plane or standardized quantification threshold for pathological meniscal extrusion, and variations in the selection of anatomical reference points on coronal images significantly affect comparability across studies. Farivar et al³² systematically reviewed 45 studies and identified the most frequently employed anatomical landmarks as the medial collateral ligament (MCL, 38%), the midpoint of the medial meniscus length (23%), and the midpoint of the medial femoral condyle (19%). The extrusion values corresponding to these landmarks were 3.5±0.7 mm (MCL), 3.9±0.8 mm (meniscal midpoint), and 4.5±2.1 mm (femoral condyle midpoint), respectively. This review noted difficulties in accurately identifying some landmarks, such as those used in dynamic imaging methods like ultrasound. Consequently, the posterior border of the MCL has been recommended due to its anatomical reproducibility. To date, there is insufficient evidence regarding differences among these measurement planes, nor is there a consensus on which plane most accurately reflects actual meniscal extrusion. Furthermore, single-plane measurements, despite their simplicity, fail to account for the three-dimensional anatomy of the meniscus, and thus cannot fully represent the overall extrusion magnitude.

Costa et al⁵ measured medial meniscal extrusion at the midpoint of the medial femoral condyle on coronal MRI, proposing a threshold of 3 mm. Although the >3 mm threshold has become widely adopted as indicative of pathological medial meniscal extrusion, this criterion largely derives from expert consensus rather than multicenter validation studies.³³ Svensson et al³⁴ measured extrusion at the widest coronal section of the medial tibial eminence and proposed 4 mm, rather than 3 mm, as the optimal threshold to maximize sensitivity and specificity for OA, BMLs, and cartilage damage. Conversely, some researchers have suggested that a threshold of 2.5 mm at the largest cross-sectional area of the medial tibial eminence is optimal for predicting knee pain and cartilage damage progression over four years.³⁵ Compagnoni et al³⁶ introduced the Meniscal Extrusion Index (MEI), a novel classification system based on the ratio of extruded meniscus width to the total meniscal width, which reduces the impact of anatomical variations and challenges the traditional absolute-value thresholds. Their research indicated that the correlation between MEI and traditional absolute thresholds was weak; for instance, a 3 mm extrusion could correspond to either “mild” or “severe” extrusion according to MEI. Importantly, an MEI >40% consistently identified functionally significant injuries such as root tears, showing superior predictive ability compared to the 3 mm threshold.

Ultrasound Diagnosis

Ultrasound provides a practical approach to quantify extrusion under load-bearing conditions, which is a major limitation of conventional supine MRI. In symptomatic osteoarthritic knees, Tortorella et al³⁷ reported that standing ultrasound yields larger extrusion measurements than both supine ultrasound and standard MRI, with medial extrusion increasing from approximately 4.2 mm on MRI to 5.2 mm under standing ultrasound, and lateral extrusion increasing from about 3.3 mm on MRI to 4.3 mm under standing ultrasound, supporting the concept that loading unmasks clinically relevant displacement.

Beyond static standing, dynamic ultrasound protocols have been combined with motion analysis to map extrusion during functional tasks such as walking and stair climbing, which can directly link time-varying joint moments to meniscal displacement and may help phenotype knees in which extrusion is predominantly load-dependent.³⁸ Additionally, ultrasound has shown promise in postoperative evaluations, such as evaluating meniscal repositioning following surgical repair.³⁹

Diagnostic performance has also been quantified against MRI. In a validation study using MRI as the reference standard, Nogueira-Barbosa et al reported ultrasound sensitivity of 95–96% and specificity of 70–82% for detecting medial meniscal extrusion, together with substantial agreement for absolute measurements.⁴⁰ These data support ultrasound as a reliable modality for quantification, while also emphasizing that agreement is not perfect and depends on standardized probe positioning and landmark selection.⁴¹

At present, there is no universally accepted ultrasound cut-off for pathological extrusion across clinical contexts. Barreira’s synthesis of normative values suggests that thresholds in the range of 2–3 mm may separate typical from atypical displacement in some cohorts, whereas Boksh argues that thresholds should be interpreted in relation to tear morphology, osteophytes, and loading state rather than as a single number applied across all knees.^11,39

Several limitations constrain broader adoption. Measurement is operator dependent, and small deviations in probe orientation can change millimetre-scale readings. In addition, physiological extrusion overlaps with early degenerative states, so imaging results must be interpreted alongside symptoms and structural findings, and multicentre studies are still needed to define context-specific thresholds and test whether ultrasound-guided decision-making improves outcomes.^39,42

Treatment Strategies and Prognosis

Treatment strategies for meniscal extrusion include both non-operative and operative interventions. Non-operative management comprises non-steroidal anti-inflammatory drugs (NSAIDs), weight reduction, and lateral wedge insoles (LWI), aiming to decrease joint load, reduce inflammation, and improve knee function, thereby potentially delaying disease progression. Operative interventions encompass medial meniscal posterior root repair, meniscal centralization, and open-wedge high tibial osteotomy (OWHTO) (Table 1). Because extrusion reflects both tissue failure and the loading environment, treatment selection is best approached by first defining the dominant driver in an individual knee, namely whether extrusion is primarily tear-driven, load-driven, or the combined expression of both. In practice, this means establishing tear morphology and reparability on MRI, grading the osteoarthritic stage, and quantifying coronal plane alignment and medial compartment overload, because the same extrusion magnitude may be treatable with meniscal stabilization in a well-aligned early-stage knee, yet may require concomitant alignment correction when varus torque is the principal amplifier. Although no single alignment threshold has been universally adopted, available clinical data suggest that preoperative varus exceeding approximately 2.5° may reduce the likelihood of effective extrusion correction after isolated root repair, making alignment correction more relevant in selected patients.¹⁷

Table 1 Comparison of Surgical Techniques for Meniscal Extrusion

Non-Operative Treatment

Krych et al⁷³ reported the long-term natural history of degenerative medial meniscus posterior root tears managed without repair, a scenario in which extrusion commonly progresses because hoop tension is not restored. Across follow-up beyond five years, nonoperative regimens such as physiotherapy, injections, unloading braces, and assistive devices can provide transient symptom relief, yet overall clinical failure approached 87%, and approximately 31% of patients underwent total knee arthroplasty, with worse outcomes in women and in those with more advanced baseline osteoarthritis. In these series, failure largely reflected persistent or recurrent symptoms prompting subsequent operative treatment and structural progression culminating in arthroplasty, which underscores that symptomatic management alone rarely addresses the mechanical driver of extrusion in degenerative root tears.

However, recent studies by Ishii et al offer a novel perspective on conservative management. The use of LWI, a conservative treatment for medial compartment knee OA, reduces medial compartment load, thereby decreasing meniscal extrusion and delaying disease progression. Ishii et al first explored the short-term impact of LWI on meniscal extrusion through a cross-sectional study, demonstrating that patients with early-stage OA (KL grade 2) exhibited significant reductions in weight-bearing extrusion and difference between non-weight-bearing and weight-bearing extrusion (ΔMME) after three months of LWI use, whereas patients with advanced OA (KL >2) showed no significant improvement. This may be because structural degeneration in advanced OA reduces meniscal plasticity, resulting in diminished ΔMME and subsequently impairing the biomechanical corrective effect of LWI. This finding provides important evidence for selecting appropriate candidates, highlighting the greater efficacy of early intervention.⁷⁴ Building on this, Ishii et al⁷⁵ subsequently investigated the immediate effects of LWI during walking using dynamic ultrasound, demonstrating that LWI significantly reduced dynamic extrusion and ΔMME, with pain relief closely correlated to ΔMME reduction. These findings emphasize the importance of dynamic evaluation for individualized treatment planning and provide direct evidence that LWI attenuates meniscal extrusion through dynamic load reduction.

To evaluate the effects of LWI over one year, Ishii et al⁷⁶ conducted a 1-year cohort study demonstrating that the duration of daily LWI usage critically influenced clinical outcomes. Patients wearing insoles for over 5 hours per day showed sustained reductions in meniscal extrusion and lower OA progression rates, whereas those using insoles less than 5 hours per day experienced no improvement in meniscal extrusion, and OA progression or varus alignment deterioration occurred in 3 out of 10 cases. Furthermore, the long-duration group demonstrated significant improvements in KOOS pain and daily activity scores, while the short-duration group showed only minor pain improvement. These results indicate that regular, prolonged wear of LWI is essential to maintain biomechanical corrective effects and delay OA progression. However, these findings should be interpreted cautiously given the small sample size and the observational design of the cohort.

Surgical Treatment

Partial Meniscectomy

Partial meniscectomy is a common arthroscopic procedure involving the removal of damaged meniscal tissue to alleviate pain and mechanical symptoms, frequently used to treat meniscal tears. However, its efficacy is significantly limited when meniscal extrusion is also present. Chung et al⁴³ reported in their study on MMPRT that although partial meniscectomy initially relieved symptoms, 35% of patients required TKA within a 5-year follow-up period. Radiological assessments revealed substantial narrowing of joint space (mean decrease of 2.3 mm) and worsening KL grades (≥2 grades progression in 80% of patients). Furthermore, Wang et al⁴⁴ found that OA patients with significant preoperative meniscal extrusion (≥3 mm), despite experiencing pain relief, showed greater progression in joint space narrowing and KL grading after 4 years, as well as a higher proportion progressing to TKA.

Partial meniscectomy fails to restore the structural integrity of the meniscus, thereby leaving abnormal joint stresses unresolved—similar to biomechanical loading abnormalities observed with meniscal extrusion. Partial meniscectomy inherently cannot restore meniscal hoop tension or normal joint mechanics, and therefore it remains biomechanically insufficient for treating extrusion-driven overload. When varus malalignment coexists, partial meniscectomy also fails to correct this additional source of medial compartment overloading, which can further compromise outcomes.

Meniscal Root Repair

Recent advances have significantly improved surgical techniques, postoperative biomechanical understanding, and clinical outcomes associated with MMPRT. Nevertheless, substantial controversies and challenges remain. Current mainstream surgical approaches for MMPRT repair include transtibial pullout repair (TTPR) and all-inside repair techniques, both aiming to restore anatomical attachment and circumferential hoop tension of the meniscus.

TTPR technique involves drilling a guide pin below the posteromedial articular surface of the tibial plateau to create a tibial tunnel. Sutures are subsequently passed through this tunnel and secured at the anteromedial tibial cortex, maintaining the contact pressure between the meniscal root and its bone bed.⁴⁶ While this method effectively reduces extrusion in knee flexion and delays OA progression, its limitations include tunnel positioning inaccuracies and insufficient control of meniscal extrusion in knee extension.^43,47 Several studies emphasize that tibial tunnel positioning critically influences postoperative medial joint space narrowing and meniscal extrusion progression.^46–49 Anatomically positioned tunnels, particularly oriented posterolaterally, significantly reduce meniscal extrusion in flexion but do not entirely prevent extrusion progression in extension over the long term.^47,48,50 Quantitatively, a clinical MRI study reported that posterior extrusion at 90° knee flexion decreased from 4.42±1.38 mm preoperatively to 3.09±1.06 mm at 3 months after transtibial pullout repair, whereas near extension (10°) showed no significant change, underscoring the position-dependent nature of “extrusion correction” after repair.⁷⁷ Although meniscal extrusion and cartilage degeneration continue to worsen within three years postoperatively, clinical outcomes (eg, IKDC scores) progressively improve, indicating a dissociation between structural degeneration and functional recovery—likely attributable to restored meniscal dynamic stability and pain relief.^51,78–80

The all-inside repair technique does not require the creation of a tibial tunnel; instead, it directly anchors the meniscal root at its anatomical footprint arthroscopically using suture anchors. This approach avoids complications associated with tibial tunnels and provides superior radiographic repositioning; however, it demands higher technical expertise, and long-term outcome data are currently lacking.⁸¹

Patient-specific factors critically influence surgical prognosis. Elevated BMI (≥30 kg/m²) significantly correlates with decreased root signal intensity post-repair, whereas advanced age (≥50 years) and preoperative varus alignment (>2.5°) may diminish the effectiveness of extrusion correction.^17,78,82 Notably, absolute quadriceps strength (rather than strength improvement alone) appears important for delaying extrusion progression, underscoring the role of structured rehabilitation.⁸³ From a technical perspective, outcomes remain sensitive to tibial tunnel positioning and anatomic reduction, particularly in transtibial pullout repair, and long-term durability in higher-risk subgroups such as those with severe varus deformity or advanced cartilage degeneration still requires further validation.^43,45

OWHTO

Concurrent varus alignment and meniscal extrusion significantly increase the risk of OA progression.⁵² OWHTO is a crucial surgical procedure to correct varus deformity and realign the mechanical axis of the lower extremity. It plays a pivotal role in managing OA or MMPRT accompanied by meniscal extrusion. The primary mechanism involves correcting varus alignment to reduce medial compartment loading, thereby alleviating meniscal extrusion and delaying OA progression.

Several studies have demonstrated indirect improvements in meniscal extrusion following mechanical axis correction through OWHTO. Ishii reported a significant correlation between postoperative improvement in meniscal extrusion and changes in the HKAA as well as the %MA.⁵³ Furthermore, clinical outcomes after OWHTO are superior in patients whose postoperative meniscal extrusion values were below 3 mm compared to those exceeding 3 mm.⁵⁴ Armin Mucha and Yavuz Selim Karatekin further suggested that isolated OWHTO, even without MMPRT repair, could prevent the progression of meniscal extrusion and OA by redistributing joint loading, demonstrating the inherent protective effects of lower limb mechanical axis correction alone.^55,56 In contrast, Horita reported that isolated OWHTO was insufficient to significantly reduce extrusion in the presence of MMPRT; instead, a combined procedure involving medial meniscal repair achieved a greater decrease in extrusion and a lower postoperative MME than OWHTO alone. This discrepancy may originate from differences in study populations: Horita’s research was based on cadaveric specimens focusing on MMPRT, whereas Karatekin enrolled patients with combined varus deformity and MMPRT. Across the currently available literature, no universally accepted varus threshold has been established to determine when osteotomy should be favored over isolated meniscal procedures. However, several studies provide clinically informative reference ranges. In cohorts undergoing root repair, preoperative varus exceeding approximately 2.5° has been associated with less favorable correction of meniscal extrusion, indicating that even mild-to-moderate coronal malalignment may diminish the efficacy of isolated meniscal stabilization.¹⁷ At the other end of the spectrum, a recent international Delphi consensus recommended consideration of concomitant realignment osteotomy when the weight-bearing axis passes medial to the medial tibial eminence on hip-knee-ankle radiographs or when varus alignment exceeds 5°, particularly in active patients undergoing medial meniscus root repair.⁸⁴ Furthermore, studies of OWHTO have demonstrated that reduction in meniscal extrusion is associated with correction of the hip-knee-ankle angle and percentage mechanical axis, supporting an alignment-sensitive treatment strategy in which the severity of coronal malalignment is interpreted together with tear morphology, extrusion severity, and osteoarthritic stage, rather than according to a single universal angular cut-off.⁵⁷

Regarding the debate over combining OWHTO with meniscal repair, some studies highlight synergistic benefits. Dong Won Suh observed that OWHTO combined with all-inside meniscal repair maintained joint space width (JSW) more effectively, while Okamura reported that OWHTO combined with root repair significantly improved outcomes in varus-aligned knees with MMPRT. However, residual meniscal extrusion and incomplete healing persisted in some patients, suggesting either limitations in patient selection or inherent constraints of the combined surgical approach that warrant further optimization.^65,66 Conversely, Lee and Nha argued that adding root repair to OWHTO yielded no significant differences in postoperative imaging findings or clinical outcomes, and meniscal healing rates did not correlate with symptomatic improvement. They proposed that the therapeutic effects of OWHTO might primarily stem from mechanical axis correction rather than structural meniscal repair itself.^67–69

Meniscal Centralization

Centralization involves pulling the meniscus centrally toward the joint and securing it to the posteromedial apex of the tibial plateau, typically utilizing either suture anchors or a transtibial tunnel fixation method.^58–60 This technique anatomically reconnects the meniscal midbody-capsular complex centrally at the peripheral edge of the tibial plateau.⁶¹ The primary objective of meniscal centralization is to anatomically reduce and stabilize the meniscus, restoring its biomechanical function and thereby delaying the progression of OA.

Arthroscopic all-inside techniques using knotless or double-loaded anchors simplify the procedure by introducing an additional medial portal, thus avoiding the complexities associated with transtibial tunnels.^59,85 Alternatively, peripheral stabilization sutures passed through a tibial tunnel can further enhance mechanical stability following meniscal repositioning.⁶⁰ However, biomechanical advantages between these different surgical approaches remain unclear. Notably, the presence of osteophytes may impede effective meniscal reduction.⁷⁰ Therefore, it is recommended to assess the severity of joint degeneration preoperatively and, if feasible, remove osteophytes intraoperatively to enhance the medial meniscus mobility towards the joint center.

Additionally, some authors have proposed that meniscal centralization might also address abnormalities of the MTL, potentially reducing meniscal extrusion.¹⁴ However, this hypothesis is based on studies of highly selected patients with isolated meniscal extrusion and minimal concomitant knee pathology, which limits its external validity. Specifically, these cohorts excluded meniscal tears, moderate-to-severe chondromalacia, and advanced osteoarthritic change, thereby removing many of the structural and degenerative conditions that commonly coexist with symptomatic medial meniscal extrusion in routine clinical practice. As a result, the observed association between MTL abnormality and extrusion may better characterize a relatively uncommon, low-degenerative subgroup rather than the broader extrusion population. Accordingly, caution is warranted when extrapolating these findings to patients with extrusion associated with meniscal root tears, complex meniscal pathology, or more advanced cartilage degeneration, in whom the mechanisms driving extrusion and the potential response to centralization may differ substantially.

Combined application of centralization with other surgical procedures, such as meniscal root repair and OWHTO, demonstrates synergistic benefits. Multiple studies have reported that meniscal centralization in conjunction with root repair effectively reduces extrusion, enhances clinical function, and maintains favorable short- to mid-term outcomes (1–2 years) in patients with severe extrusion or MMPRT.^61–63,70 This conclusion is further supported by biomechanical evidence provided by Deichsel et al, demonstrating that centralization reduces stresses experienced during posterior root repair under varus loading conditions.⁶⁴ Nakamura introduced an arthroscopic meniscal centralization procedure combined with OWHTO and root repair for MMPRT. The use of OWHTO decreases medial compartment stress, permitting earlier weight-bearing, and its associated medial collateral ligament release provides better visualization for root repairs.⁷² Meniscal centralization may help reduce the posterior root toward its native attachment, and the centralization anchor functions as a load-sharing point that distributes hoop tension at the root repair site, which supports root repair stability and meniscal containment. Katagiri reported that combined OWHTO and centralization significantly improved JSW compared to isolated OWHTO, although short-term clinical outcomes were similar. This suggests that combined mechanical correction and structural repair optimize the biomechanical environment, but long-term effects on outcomes, such as conversion rates to TKA, require further investigation.⁷¹

Although preliminary clinical experience with meniscal centralization has been encouraging, the published studies have used different patient populations, surgical approaches, and reported outcomes. At present, these reports support the potential role of centralization in selected cases, but they do not yet allow firm conclusions regarding its indications, durability, or comparative benefit.

Conclusion

Meniscal extrusion should be interpreted as a mechano-structural failure state in which disrupted meniscal restraint and adverse joint loading amplify each other and accelerate medial compartment degeneration. Across the evidence reviewed, posterior root tears and degenerative complex tears represent the most frequent structural triggers, while varus malalignment, increased body mass index, osteophytes, and subchondral insufficiency fracture modulate the magnitude and clinical consequences of extrusion, which explains why identical millimetre values can carry different prognostic meaning across patients. From a diagnostic standpoint, MRI remains indispensable for defining tear morphology and concomitant cartilage and subchondral pathology, whereas ultrasound and weight-bearing protocols add value when the clinical question is load-dependent displacement or when serial monitoring is required.

As a narrative review without a protocol-driven systematic search or formal risk-of-bias assessment, this synthesis reflects the available and selected literature and should be interpreted as a clinically oriented framework rather than definitive evidence-based guidance. In terms of management, the available data support an individualized pathway that links treatment choice to tear reparability, extrusion severity, alignment, and osteoarthritic stage, because addressing only one component of the failure state often leaves the mechanical driver intact. Structured nonoperative care can be appropriate for low-demand patients and early degenerative change, but long-term observational series show high rates of clinical failure after untreated degenerative root tears, which justifies a lower threshold for surgical stabilization when symptoms persist and joint degeneration is not end stage. When surgical treatment is indicated, restoring hoop tension through anatomic root repair, augmenting peripheral restraint with centralization when appropriate, and correcting coronal plane overload with high tibial osteotomy in varus knees form a coherent strategy aimed at reducing extrusion and slowing compartmental deterioration, although the supporting studies remain heterogeneous in techniques, outcome definitions, and follow-up duration. Future work should therefore prioritize standardized measurement planes and reporting conventions, explicit definitions of radiographic and clinical failure, and prospective comparative studies that clarify which combinations of repair, centralization, and alignment correction provide durable benefit across patient subgroups.