About Discoid Meniscus

A discoid meniscus is a rare congenital anomaly of the knee joint where the meniscus is thicker and more circular in shape than normal. It typically affects the lateral meniscus and can be found bilaterally in up to 20% of cases.

The meniscus is a cartilage structure that acts as a shock absorber and secondary stabilizer in the knee joint. In a normal meniscus, it is crescent-shaped and attached to the lining of the knee joint along its periphery. In a discoid meniscus, the meniscal body is thicker and more circular, resembling a disk. This anomaly can lead to tears and instability in the meniscus, especially if it is not properly attached to the tibia.

Discoid menisci are often asymptomatic but can cause symptoms such as pain, swelling, and a snapping or clicking sound in the knee, especially during bending or straightening movements. The condition is more common in Asian populations, with an incidence of 10-15% compared to 3-5% in Western populations.

The Watanabe classification system categorizes discoid menisci into three types:

• Incomplete: The lateral meniscus is thicker and wider than normal but does not cover the entire tibial plateau.
• Complete: The meniscus covers the entire tibial plateau.
• Wrisberg-ligament variant: The meniscus has an abnormal posterior attachment to the posterior cruciate ligament.

Diagnosis is typically made through magnetic resonance imaging (MRI), which can show the characteristic thickened meniscal body and its relationship to the tibial plateau. Treatment usually involves conservative management with physical therapy and bracing, but surgical intervention may be necessary if symptoms persist or if there are significant tears in the meniscus.

Common Symptoms

The most common symptoms of a discoid meniscus or a torn discoid meniscus include:

• Pain in the knee, often on the outer or front side
• Sharp pain with activities like running, jumping, cutting, or deep squatting
• Swelling in the knee
• Tenderness on the outer side of the knee
• Catching, locking, or popping sensations inside the knee while walking or squatting
• Inability to fully extend or straighten the knee
• Loss of knee motion, especially getting “stuck” while trying to fully bend or straighten
• Loss of strength or weakness in the thigh muscle
• Discomfort with daily activities like walking up and down stairs

Individuals with a discoid meniscus can be asymptomatic for years and never experience any problems. However, if there is a tear or instability in the abnormally shaped meniscus, symptoms typically develop. Symptoms often begin during childhood, with older children (8-10 years) more commonly experiencing pain with activity compared to younger children.

The most common tear pattern is a horizontal tear, likely caused by repetitive microtrauma to the abnormal collagen arrangement after mucoid degeneration. Even without a tear, a click or pain can develop due to the abnormal shape and instability of the discoid meniscus.

Cause & Anatomy

While the exact cause is unknown, discoid meniscus is considered a congenital anatomic variant that predisposes the meniscus to tears, especially with athletic activities, due to its abnormal shape and thickness. It is believed to be a congenital disorder that most patients are born with. The most accepted theory is that it develops during fetal development. The condition is more common in Asian populations.

Some key points about the causes of discoid meniscus:

• It is a rare anatomic variant that typically affects the lateral meniscus of the knee.
• Discoid meniscus occurs in less than 1% of patients in North America, but the incidence can be as high as 5% in Asian countries.
• It is often found bilaterally, with up to 20% of patients having discoid menisci in both knees.
• The abnormal shape and thickness of a discoid meniscus makes it more prone to tears compared to a normal meniscus.
• Most patients do not experience symptoms until they participate in sports or other activities that cause the meniscus to tear.

Although most patients present with symptoms in adolescence, discoid menisci can remain asymptomatic or silent for a patient’s entire life or can present in adulthood.

Diagnosis

MRI is the most accurate diagnostic tool, using specific width and continuity criteria to diagnose discoid meniscus. Physical examination findings and arthroscopy can confirm the diagnosis in equivocal cases, especially in children. However, a significant number of discoid menisci remain asymptomatic and undiagnosed.

Imaging

• MRI is the most accurate imaging modality for diagnosing discoid meniscus.
• On MRI, a discoid meniscus is diagnosed when the ratio of the minimal meniscal width to the maximal tibial width is more than 20% on the coronal image.
• The ratio of the sum of the width of both lateral horns to the maximal meniscal diameter on the sagittal image should be more than 75%.
• Continuity of the meniscus between the anterior and posterior horns on three or more 5-mm thick consecutive sagittal slices also indicates a discoid meniscus.
• An abnormal, thickened, bow-tie appearance of the meniscus on MRI is suggestive of discoid meniscus.

Physical Examination

• Patients may show joint line tenderness, effusion, joint line bulging, and even a positive McMurray test.
• Twisting the knee in flexion and extension can elicit a popping or clunking sensation, especially with the Wrisberg ligament variant.
• There may be limitation of motion during knee flexion and extension, and an anterolateral bulge at full flexion.

Arthroscopy

• Diagnostic arthroscopy may be needed to confirm the diagnosis of discoid meniscus in a symptomatic patient with instability or with the Wrisberg variant, showing a normal shape and MRI appearance.

Limitations

• Some authors have reported low sensitivity in detecting lateral discoid meniscus in children with MRI.
• MRI may not provide enhanced diagnostic utility over clinical examination alone in children, as checking the stability of a discoid meniscus on MRI is difficult.
• Some incomplete discoid menisci often look normal on MRI.

Prevention

• A discoid meniscus is a congenital condition present at birth, so it cannot be prevented.
• Maintaining appropriate lower-extremity mobility and muscular strength are the best methods for preventing any type of knee injury in those with a discoid meniscus.
• Being aware of any knee pain, particularly with squatting, running, or turning, as these are signs of a potential knee injury. Addressing injuries early can improve treatment outcomes.

Treatment

While a discoid meniscus cannot be prevented as it is a congenital condition, maintaining lower extremity strength and flexibility can help prevent injuries. If symptoms develop, conservative treatment with physical therapy is often effective, but arthroscopic partial meniscectomy may be needed for persistent symptomatic discoid menisci. The goal is to preserve as much meniscal tissue as possible to maintain joint health.

Many people with a discoid meniscus never experience symptoms and do not require treatment. If symptoms develop, such as pain, swelling, catching, locking, or popping sensations in the knee, treatment may be necessary. Conservative treatment with physical therapy is often the first line of management.

Physical therapy aims to:

• Restore normal range of motion
• Strengthen the muscles around the knee
• Provide manual therapy techniques
• Manage pain and swelling

If conservative treatment fails, arthroscopic surgery may be indicated for symptomatic discoid menisci. The current preferred surgical treatment is meniscal reshaping (partial meniscectomy) to remove the abnormal central portion while preserving a rim of meniscus. Total or subtotal meniscectomy is avoided if possible, as it can lead to early osteoarthritis.

After surgery, a course of physical therapy rehabilitation is usually recommended to restore function.

Surgery

Surgical Indications

• Discoid meniscus is often asymptomatic and does not require surgery if not causing symptoms.
• Arthroscopic surgery is indicated for a discoid meniscus causing pain, popping, locking, or other symptoms.

Surgical Techniques

• Arthroscopic saucerization is the most common procedure, where the abnormal central portion of the discoid meniscus is removed to reshape it into a crescent.
• For complete or incomplete discoid menisci with tears, the torn portion is trimmed away after saucerization.
• Some tears can be repaired with sutures instead of being removed.
• For the hypermobile Wrisberg variant, saucerization is combined with sutures to stabilize the meniscus to the joint lining.
• In rare cases of a very thick discoid meniscus with a tear at the vascular periphery, repair of the tear may be attempted.

Rehabilitation

• Physical therapy is essential after surgery to restore range of motion, strength, and function.
• Patients are advised to avoid deep squatting, sitting cross-legged, and heavy lifting for at least 4 months to allow healing.
• Return to sports is guided by the surgeon and physical therapist based on the individual patient’s progress.

FAQ’s

How is a symptomatic discoid meniscus treated?
Treatment depends on symptoms:

• Asymptomatic discoid menisci do not require treatment
• Conservative treatment with physical therapy is first line for mild symptoms
• Arthroscopic surgery is indicated for significant pain, locking, or instability
• Surgery involves reshaping the meniscus (saucerization) and repairing tears if possible