Meniscus Extrusion: Vital Details for Knee Stability

The meniscus plays a crucial role in knee stability by distributing load and absorbing shock. When it shifts beyond the tibial edge—a condition known as meniscus extrusion—joint mechanics are disrupted. This displacement is linked to cartilage degeneration and osteoarthritis progression, making early detection essential.

Definition And Biomechanics

Meniscus extrusion refers to the abnormal displacement of meniscal tissue beyond the tibial margin, typically exceeding 3 mm. This disrupts normal load distribution, increasing stress on the articular cartilage and subchondral bone. The meniscus, primarily composed of fibrocartilage, functions as a shock absorber and stabilizer, but when misaligned, its ability to cushion compressive forces diminishes. This accelerates degenerative changes, particularly in the medial compartment.

The meniscus is stabilized by its anchoring attachments, including the meniscotibial (coronary) ligaments, meniscofemoral ligaments, and posterior root. When these structures weaken due to degeneration, trauma, or surgery, the meniscus becomes more prone to extrusion. The posterior horn of the medial meniscus is especially vulnerable, as it bears significant axial loads and helps resist anterior tibial translation. Loss of this stabilizing function contributes to joint instability and cartilage wear.

Biomechanically, meniscus extrusion alters contact force distribution in the knee. A properly positioned meniscus transmits about 50% of the load in extension and up to 85% in flexion. When extruded, it loses this ability, shifting the load onto the articular cartilage. This increased focal pressure accelerates cartilage breakdown, a process well-documented in osteoarthritis. Finite element modeling studies show that even minor extrusion significantly raises peak contact stresses, particularly in the medial compartment.

Imaging Techniques

Assessing meniscus extrusion requires imaging that captures both structural and functional aspects of the knee. MRI is the preferred method due to its high-resolution soft tissue contrast, allowing precise visualization of meniscal position. T2-weighted and proton density sequences are particularly useful, with coronal plane imaging being the most reliable for measuring extrusion. An extrusion of 3 mm or more is considered clinically significant, as it correlates with cartilage degeneration and osteoarthritis progression.

While MRI is the gold standard, ultrasonography has emerged as a valuable tool for dynamic assessment. Unlike MRI, which provides a static image, ultrasound enables real-time evaluation of meniscus mobility under load-bearing conditions. High-frequency linear transducers enhance resolution, allowing for precise measurement of medial meniscus displacement. Studies show strong correlation between ultrasound and MRI findings, making ultrasound useful for longitudinal monitoring.

CT arthrography offers an alternative for patients with MRI contraindications, such as metallic implants or severe claustrophobia. By injecting contrast material into the joint, CT arthrography enhances visualization of meniscal tears and displacement. Though it lacks MRI’s soft tissue detail, it provides superior spatial resolution of bony structures, which is beneficial when osteophytes or subchondral bone changes contribute to meniscal instability. It is particularly useful in postoperative assessments where MRI interpretation is complicated by surgical artifacts.

Potential Etiological Factors

Meniscus extrusion develops due to a combination of degenerative, mechanical, and structural factors that compromise meniscotibial attachments. Age-related degeneration plays a significant role, as collagen disorganization and proteoglycan loss weaken the fibrocartilaginous matrix. Aging menisci exhibit reduced cellularity and increased matrix fibrillation, diminishing their ability to resist compressive and shear forces. This is especially pronounced in the medial meniscus, where chronic weight-bearing stress accelerates structural failure.

Mechanical overload worsens this process, particularly in individuals with malalignment. Varus knee deformity, commonly associated with medial compartment osteoarthritis, increases compressive forces on the inner knee, promoting meniscal displacement. Gait studies show that individuals with varus alignment experience higher peak knee adduction moments, correlating with greater medial meniscus extrusion. Lateral extrusion, though less common, is often linked to valgus malalignment. These biomechanical imbalances disrupt normal load-sharing, driving progressive extrusion and cartilage deterioration.

Structural insufficiencies, whether congenital or acquired, also contribute to meniscus instability. Posterior root tears are a major precursor to extrusion, as they render the meniscus nonfunctional by eliminating its ability to anchor under load. Cadaveric studies show that complete posterior root detachment increases extrusion by up to 5 mm, mimicking the effects of total meniscectomy. Similarly, prior meniscal surgeries, such as partial meniscectomy, can predispose patients to extrusion by disrupting the hoop stress mechanism that maintains normal meniscal positioning.

Comparison Of Medial And Lateral Extrusion

Meniscus extrusion is more common in the medial compartment due to the biomechanical demands placed on the inner knee during weight-bearing activities. The medial meniscus is more rigidly anchored to the tibial plateau by the deep medial collateral ligament and meniscotibial ligaments. This limits its mobility but makes it more susceptible to degenerative changes. As these stabilizing structures weaken, the medial meniscus is prone to displacement, particularly in individuals with varus alignment, where excessive compressive forces exacerbate extrusion. This contributes to the rapid progression of medial compartment osteoarthritis.

Lateral meniscus extrusion is less frequent because the lateral meniscus has greater mobility due to looser attachments. The popliteus tendon and meniscofemoral ligaments allow for dynamic adaptation to tibiofemoral motion, reducing the likelihood of significant displacement. When lateral extrusion occurs, it is often associated with acute trauma, such as ACL injuries, which disrupt stabilizing structures and lead to secondary meniscal displacement. Unlike medial extrusion, which develops gradually due to degeneration, lateral extrusion is more often linked to acute structural disruptions.

Quantifying And Measuring Meniscus Extrusion

Accurately assessing meniscus extrusion is essential for understanding its impact on knee biomechanics and disease progression. MRI is the most widely used technique for quantification, with extrusion measured as the perpendicular distance from the outer meniscal border to the tibial plateau edge on coronal images. A displacement exceeding 3 mm is considered pathological, as it strongly correlates with cartilage degeneration and osteoarthritis severity. This threshold helps distinguish between physiological variations and clinically significant extrusion.

Dynamic assessments provide additional insights into meniscus displacement under load. Weight-bearing MRI and ultrasound imaging reveal that extrusion can vary depending on joint loading conditions, with some cases showing increased displacement during weight-bearing. This highlights the importance of considering functional knee mechanics when interpreting imaging findings. Studies suggest that progressive extrusion over time may predict worsening osteoarthritis, reinforcing the need for longitudinal monitoring. Standardized measurement protocols and consistent imaging techniques are critical for reliable assessments, particularly in clinical and research settings where extrusion severity influences treatment decisions.