The human knee is a complex joint, serving as a connection between the thigh and lower leg bones. Within this joint, crescent-shaped cartilage, known as menisci, act as natural shock absorbers, distributing weight and stabilizing the knee. An anatomical variation, a discoid meniscus, occurs when one of these menisci develops with an atypical shape.
Understanding the Discoid Meniscus
A discoid meniscus is thicker and can appear more like a disc or oval, sometimes even completely covering the shin bone (tibia), differing from the typical C-shaped meniscus. This congenital variation means individuals are born with it, and it does not result from injury or trauma. Its abnormal shape often makes it more prone to injury.
There are generally three classifications: the incomplete type, which is slightly thicker and wider than normal; the complete type, which fully covers the lateral tibial plateau; and the Wrisberg ligament type, where the meniscus lacks its normal posterior attachment, leading to instability.
Unpacking Its Rarity
Discoid meniscus is an uncommon anatomical variant. Its prevalence in the United States and Europe is estimated between 1.5% and 5%. However, the actual incidence can be challenging to determine precisely because many individuals with a discoid meniscus never experience symptoms.
It is more common in the lateral meniscus (outside of the knee) than the medial meniscus (inside). While uncommon in Western populations, prevalence is higher in Asian populations, ranging from 5% to 16.6% in countries like Japan and Korea. Approximately 20% of affected individuals may have a discoid meniscus in both knees. Variations in reported numbers stem from differences in diagnostic methods, studied populations, and asymptomatic cases.
When a Discoid Meniscus Becomes an Issue
Many individuals with a discoid meniscus remain symptom-free. When symptoms arise, they often appear during childhood or adolescence, but can also manifest later.
Common symptoms include pain, particularly on the outer side of the knee, which may worsen with activity. Other symptoms include stiffness, swelling, or a sensation of catching, popping, or locking. Some report the knee “giving way” or an inability to fully straighten the leg. The less organized collagen fibers in a discoid meniscus also contribute to its increased susceptibility to tears.
Identifying and Addressing the Condition
Diagnosis typically begins with a physical examination, where a doctor might observe tenderness, swelling, or a popping sensation during knee movement. The McMurray test, involving twisting the knee, can elicit a clunk or pop, suggesting a meniscal issue. While X-rays may show indirect signs like a widened joint space, magnetic resonance imaging (MRI) is the most effective imaging test. MRI provides detailed soft tissue images and can reveal tears, confirming the presence and characteristics of a discoid meniscus.
For asymptomatic individuals without pain or discomfort, no specific treatment is necessary. If symptoms develop, conservative management is often the first step, involving rest, physical therapy, and anti-inflammatory medications. Physical therapy aims to restore range of motion and manage discomfort.
If conservative treatments do not alleviate symptoms, surgical intervention may be considered. The primary surgical approach is arthroscopic meniscectomy, specifically saucerization. This minimally invasive technique reshapes the discoid meniscus into a more typical crescent form, preserving as much healthy tissue as possible to maintain knee function. In cases where a tear is present, the surgeon may trim away damaged portions or, less commonly, repair the tear with stitches, especially if the tear is in an area with good blood supply. Surgery aims to reduce symptoms and prevent further joint damage.