Discoid Meniscus: Understanding, Symptoms, Diagnosis, and Treatment

What is a Discoid Meniscus?

A discoid meniscus is an uncommon, abnormally shaped meniscus in the knee, typically thicker, wider, and more disc-like than the usual crescent-shaped meniscus, often leading to symptoms like clicking, popping, and pain.

Understanding the Meniscus

To comprehend a discoid meniscus, it's essential to first understand the normal anatomy and function of the menisci. The knee joint contains two menisci: the medial (inner) and lateral (outer) menisci. These are C-shaped (crescent-shaped) wedges of fibrocartilage that sit between the femur (thigh bone) and the tibia (shin bone). Their primary functions include:

• Shock Absorption: Distributing forces across the knee joint during movement and impact.
• Joint Stability: Enhancing congruence between the rounded femoral condyles and the relatively flat tibial plateau.
• Lubrication and Nutrition: Assisting in the spread of synovial fluid, which lubricates the joint and nourishes the articular cartilage.

The lateral meniscus is slightly more circular, while the medial meniscus is more C-shaped. Both are crucial for the long-term health and function of the knee.

What Makes a Meniscus "Discoid"?

A discoid meniscus is a congenital anomaly, meaning an individual is born with it. Instead of the typical crescent or C-shape, the affected meniscus, most commonly the lateral meniscus, retains a more primitive disc-like or oval shape. This abnormal morphology results in a meniscus that is often:

• Thicker: It has greater depth from top to bottom.
• Wider: It covers a larger area of the tibial plateau.
• Less Stable: It may have abnormal or absent posterior attachments, leading to increased mobility within the joint.

This unusual shape and potential instability can predispose it to tearing or symptomatic presentation.

There are three main types of discoid menisci, classified by Watanabe:

• Complete Discoid: The entire tibial plateau is covered by the meniscus, which is thick and disc-shaped.
• Incomplete Discoid: The meniscus is thicker and wider than normal but still retains some resemblance to a crescent shape, not fully covering the tibial plateau.
• Wrisberg-Ligament Type: This is the rarest type. The meniscus is relatively normal in shape but lacks its usual posterior attachments to the tibia, instead attaching only to the femur via the Wrisberg ligament. This makes it highly unstable and prone to snapping or shifting.

The lateral discoid meniscus is significantly more common than the medial discoid meniscus, with an estimated prevalence ranging from 1% to 5% of the population.

Causes and Risk Factors

A discoid meniscus is a developmental anomaly, meaning it occurs during fetal development. It is not caused by injury or activity. While the exact genetic mechanisms are not fully understood, it is generally considered a congenital condition. There are no known modifiable risk factors, as individuals are simply born with this anatomical variation.

Symptoms of a Discoid Meniscus

Many individuals with a discoid meniscus remain asymptomatic throughout their lives, never experiencing any problems. Symptoms typically arise when the abnormal meniscus is subjected to stress or injury, leading to a tear or mechanical irritation. When symptoms do occur, they commonly appear in childhood or adolescence and may include:

• Clicking or Popping Sensation: Often described as a loud "clunk" or "thud" with knee movement, particularly during flexion and extension. This is sometimes referred to as a "snapping knee" syndrome.
• Pain: Localized to the affected side of the knee (lateral pain for a lateral discoid meniscus), especially during activity, squatting, or twisting movements.
• Swelling: May occur acutely after an injury or chronically with ongoing irritation.
• Knee Locking or Catching: The knee may momentarily get stuck or "give way" due to the abnormal tissue interfering with joint mechanics.
• Feeling of Instability: A sensation that the knee is "giving out."
• Limited Range of Motion: Difficulty fully extending or flexing the knee.

Symptoms are often exacerbated by physical activity and may be intermittent.

Diagnosis

Diagnosing a discoid meniscus involves a combination of clinical evaluation and imaging studies.

• Physical Examination: A healthcare professional will assess the knee for tenderness, swelling, range of motion, and stability. Specific tests, such as McMurray's test, may elicit the characteristic clicking or popping sound.
• X-rays: While X-rays do not show the meniscus itself, they can sometimes reveal indirect signs, such as widening of the joint space on the affected side or flattening of the femoral condyle, which can be suggestive of a discoid meniscus. They also rule out other bony abnormalities.
• Magnetic Resonance Imaging (MRI): MRI is the most definitive non-invasive diagnostic tool. It provides detailed images of soft tissues, allowing the radiologist to visualize the abnormal shape, size, and signal characteristics of the discoid meniscus, as well as identify any associated tears.
• Arthroscopy: In some cases, particularly when diagnosis is uncertain or surgical intervention is being considered, diagnostic arthroscopy may be performed. This minimally invasive procedure involves inserting a small camera into the knee joint to directly visualize the meniscus and confirm the diagnosis.

Treatment Approaches

Treatment for a discoid meniscus depends largely on whether the individual is experiencing symptoms and if there is an associated tear.

Conservative Management

For asymptomatic individuals or those with mild, intermittent symptoms without a significant tear, conservative management is typically the first line of treatment. This includes:

• Rest and Activity Modification: Avoiding activities that exacerbate symptoms.
• Ice and Compression: To reduce pain and swelling.
• Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): To manage pain and inflammation.
• Physical Therapy: A structured rehabilitation program can help strengthen the muscles around the knee (quadriceps, hamstrings, glutes), improve proprioception, and restore range of motion, thereby enhancing knee stability and reducing stress on the meniscus.

Surgical Intervention

Surgical intervention is considered for individuals with persistent, debilitating symptoms, mechanical locking, or a significant tear that is not responsive to conservative measures. The primary goal of surgery is to relieve symptoms while preserving as much functional meniscal tissue as possible. Surgical procedures are typically performed arthroscopically (minimally invasive).

• Saucerization (Partial Meniscectomy): This is the most common procedure. The surgeon reshapes the discoid meniscus, trimming the excess tissue to create a more normal, crescent-like shape. The goal is to remove the unstable, symptomatic portion while leaving a stable, functional rim. This is often preferred for complete and incomplete types.
• Meniscal Repair: If the discoid meniscus has a repairable tear, particularly in a well-vascularized zone, the surgeon may attempt to repair it. This is more common in children and adolescents.
• Total Meniscectomy: In rare cases of extensively damaged or irreparable discoid menisci, a complete removal may be necessary, though this is avoided if possible due to the increased risk of early osteoarthritis.

Recovery and Rehabilitation

Following surgical intervention, a structured rehabilitation program is crucial for optimal recovery and return to activity.

• Immediate Post-Op: Focus on pain and swelling management, gentle range of motion exercises, and early weight-bearing as tolerated, depending on the specific procedure.
• Progressive Strengthening: Gradually increasing exercises to strengthen the quadriceps, hamstrings, calves, and hip muscles.
• Proprioception and Balance Training: Exercises to improve the knee's ability to sense its position in space, which is vital for stability.
• Functional Training: Incorporating activities that mimic daily movements and sport-specific actions.
• Return to Activity: A gradual return to sports and high-impact activities is guided by the surgeon and physical therapist, typically taking several weeks to months, depending on the extent of surgery and individual progress.

Living with a Discoid Meniscus

The long-term outlook for a discoid meniscus, particularly after successful surgical saucerization, is generally good. Most individuals experience significant relief from symptoms and are able to return to their desired activity levels. However, like any meniscal surgery, there is an increased risk of developing osteoarthritis in the affected knee later in life, especially if a large portion of the meniscus was removed or if there were pre-existing cartilage issues. Adherence to rehabilitation protocols and maintaining good knee health through appropriate exercise and activity modification are key to a positive long-term outcome.