The menisci are two C-shaped pieces of tough, specialized fibrocartilage—the medial (inner) and lateral (outer)—that sit between the thigh bone (femur) and the shin bone (tibia). They primarily function as shock absorbers, distributing the body’s load across the joint surfaces and increasing knee stability. Meniscus extrusion is a specific condition where one of these menisci is displaced radially, or pushed out, from its normal position between the bones. This displacement signifies functional failure, and understanding its cause is important for determining the correct treatment path.
Understanding the Meniscus and Extrusion
The menisci are wedge-shaped, anchored firmly to the top of the tibia by root attachments and surrounding ligaments. This structure allows them to convert the vertical load of the body into “hoop stress,” distributing force evenly across the joint. The medial meniscus is C-shaped and less mobile, while the lateral meniscus is more circular and flexible.
Meniscus extrusion occurs when the meniscal tissue moves outward, extending beyond the margin of the tibial plateau. This displacement is measured using Magnetic Resonance Imaging (MRI). A displacement of \(3\) millimeters or more beyond the edge of the tibia is considered pathological extrusion.
When the meniscus extrudes, it loses the ability to generate hoop stress and function as a shock absorber. The full force of weight-bearing is then concentrated onto the articular cartilage. This increases contact forces on the unprotected joint surfaces, accelerating the breakdown of cartilage. Extrusion is associated with the progression of degenerative joint changes and the early onset of osteoarthritis.
Primary Causes of Meniscus Displacement
Extrusion is generally a consequence of underlying structural failure within the knee joint. The most common mechanism leading to displacement is a meniscal root tear. Root attachments secure the meniscus to the tibial plateau, and when they are torn, the structure becomes untethered.
A torn root prevents the meniscus from resisting outward-pushing forces during weight-bearing, leading to substantial displacement. This tear effectively causes the meniscus to lose its protective function.
Extrusion can also result from chronic degenerative changes, often seen in older individuals due to long-term wear. Over time, the meniscal tissue weakens and softens, making it less able to withstand normal joint forces. Other contributing factors include pre-existing ligamentous instability, such as a chronic deficiency in the anterior cruciate ligament (ACL). Additionally, varus malalignment (bow-legged appearance) can place excessive load on the inner knee compartment, physically pushing the medial meniscus out of position.
Identifying Meniscus Extrusion
Clinical symptoms of meniscus extrusion are often non-specific and overlap with other knee injuries. Patients commonly report pain, which may be localized to the joint line or generalized throughout the knee. Swelling, stiffness, and a popping or clicking sensation are also noted. If the underlying cause is a significant tear, the patient may feel instability or the knee “giving way.”
Definitive diagnosis relies heavily on medical imaging, as the condition is a sign of functional failure. Magnetic Resonance Imaging (MRI) is the gold standard for confirming and quantifying extrusion. MRI allows clear visualization of soft tissues and bony margins, enabling the physician to measure the distance of the meniscal edge from the tibial plateau.
Imaging is essential for identifying the root cause, such as a posterior meniscal root tear or severe degenerative changes. The presence of extrusion on an MRI is a significant finding because it correlates with a higher risk of developing bone marrow lesions and greater loss of articular cartilage volume over time.
Treatment Options and Recovery Paths
Management is determined by the underlying cause and symptom severity. For degenerative extrusion without a major tear, or in patients with mild symptoms, a non-surgical approach is attempted first. This conservative management typically involves the RICE protocol (rest, ice, compression, and elevation) to manage initial pain and swelling.
A structured physical therapy program is central to non-operative treatment, focusing on strengthening the muscles around the knee, particularly the quadriceps and hamstrings. The goal is to improve dynamic joint stability and reduce stress on the meniscal tissue. Nonsteroidal anti-inflammatory drugs (NSAIDs) and bracing may also be used to manage pain and inflammation.
If symptoms persist, or if the extrusion is caused by a significant injury like a meniscal root tear, surgical intervention is recommended. The primary goal of surgery is to restore the meniscus’s normal function to prevent further joint degeneration.
The preferred procedure for a repairable root tear is a meniscal root repair, which uses strong sutures to reattach the torn root to its bony insertion site on the tibia. This repair often includes centralization, a technique pulling the extruded meniscal body back into its normal position within the joint space.
Recovery from a meniscal root repair is lengthy, requiring strict non-weight-bearing for four to six weeks to allow healing. Patients use crutches during this time, with a gradual return to full activity taking six to nine months.
If the meniscal tissue is irreparable, the surgeon may perform a partial or total meniscectomy, trimming or removing the damaged section. Recovery from a meniscectomy is significantly faster, allowing patients to bear weight soon after surgery and return to light activities within a few weeks. However, this procedure carries a higher long-term risk of accelerated osteoarthritis.