The knee joint relies on several components to function smoothly. The menisci are two C-shaped pieces of fibrocartilage that sit between the thigh bone and the shin bone, acting as cushions. When the medial meniscus, located on the inner side of the knee, is forced out of its normal position, the condition is known as medial meniscus extrusion. This displacement compromises the biomechanical stability and function of the knee. Addressing this displacement is crucial for long-term joint health.
Understanding the Meniscus and Extrusion
The medial meniscus is a crescent-shaped band of fibrocartilage covering the inner portion of the tibial plateau. Its function involves distributing compressive forces, absorbing shock, and transmitting a significant portion of the weight-bearing load. Structural integrity relies on circumferential collagen fibers, which convert axial compression into “hoop stress” to resist outward pushing.
Unlike the lateral meniscus, the medial meniscus is relatively fixed due to its firm attachment to the joint capsule and the medial collateral ligament, making it less mobile and more susceptible to injury. Medial meniscus extrusion (MME) occurs when the meniscal tissue shifts radially outward, past the bony margin of the tibial plateau. This displacement results in the loss of contact between the meniscus and the femoral condyle, rendering the tissue functionally incompetent. Pathological extrusion is defined on medical imaging as a horizontal displacement of three millimeters or more beyond the edge of the tibia.
Primary Causes and Clinical Recognition
Medial meniscus extrusion arises from two main origins: acute traumatic injury or chronic degenerative changes. Traumatic causes involve sudden, high-energy events that lead to tears of associated structures, most significantly the meniscal root attachments. A tear at the posterior root of the medial meniscus detaches the meniscus from the tibial plateau, disrupting the hoop stress mechanism and causing the tissue to extrude.
Degenerative extrusion is a slower process often seen in older individuals or those with pre-existing conditions like knee malalignment. This type of extrusion results from the gradual breakdown of internal collagen fibers, reducing the meniscus’s ability to withstand normal stresses. The loss of structural integrity causes the tissue to stretch and bulge outwards, even without a complete root tear.
Patients experiencing MME commonly report pain localized to the inner joint line, which may be accompanied by swelling, clicking, or mechanical instability. Clinical examination provides initial clues, but a definitive diagnosis relies on advanced medical imaging. MRI is the standard method for confirming MME, as it accurately visualizes the soft tissues and allows for precise measurement of the meniscal displacement from the tibial margin.
Conservative and Surgical Treatment Options
Management strategies for medial meniscus extrusion depend on the patient’s symptoms, the degree of extrusion, and the presence of associated tears or arthritis. For cases with minimal symptoms or low-grade displacement, initial conservative care is recommended to reduce inflammation and stabilize the joint. This approach includes anti-inflammatory medications, modified activity levels, and physical therapy.
Physical therapy aims to strengthen the muscles surrounding the knee, particularly the quadriceps and hamstrings, to improve joint stability and offload the medial compartment. Non-operative treatment manages symptoms and prevents further damage, but it does not correct the underlying mechanical problem. If conservative measures fail or if the extrusion is significant, surgical intervention becomes necessary to restore function.
Surgical options are tailored to the specific pathology, differentiating between tissue removal and full repair. A partial meniscectomy involves arthroscopically removing torn or degenerative portions, but this may not address the extrusion itself. The preferred surgical solution for root tears causing significant extrusion is a meniscal root repair, typically performed using a transtibial pullout technique.
Root repair involves pulling the torn root back into its anatomical attachment site on the tibia and securing it with sutures passed through a bone tunnel. This procedure re-establishes hoop tension, reducing the degree of extrusion and restoring the meniscus’s load-bearing capacity. Newer techniques like meniscus centralization are also used to directly pull the extruded meniscal body back onto the tibial plateau.
Long-Term Effects of Untreated Extrusion
Ignoring medial meniscus extrusion can lead to severe and progressive deterioration of the knee joint structure. When the meniscus is extruded, it loses its ability to effectively cover the articular cartilage on the tibial plateau, causing a dramatic reduction in the contact area between the bones. This functional loss is biomechanically equivalent to having the entire meniscus removed, placing high, concentrated pressure on the underlying cartilage.
The resulting increase in contact stress accelerates wear and tear on the joint surfaces, directly causing early-onset osteoarthritis. Untreated MME is a strong predictor of degenerative joint disease progression, leading to chronic pain, joint space narrowing, and the formation of osteophytes. This accelerated deterioration increases the likelihood of requiring a total knee replacement surgery at a younger age.