The normal meniscus of the knee is semilunar in shape. In discoid meniscus, the meniscus of the knee is discoid rather than semilunar in shape. This is a common cause of popping or snapping knees. This condition is also called popping knee syndrome.
Discoid menisci are also incidentally found in 3-5% of knee MRIs.
The lateral meniscus is most frequently affected.
Often, the condition affects both the knees. There is no difference in occurrence in either sex. The condition may often go unrecognized. Familial occurrences of the discoid lateral meniscus are known.
There are two menisci in a knee – present one each between the lateral and medial condylar articulation of tibia and femur.
They are crescent-shaped normally.
Menisci are, fibrocartilaginous and appear smooth glossy-white. These attach to the intercondylar area and periphery of the tibial plateau. Lateral meniscus display greater variety in size, shape, thickness, and mobility than medial.
The medial meniscus is nearly semicircular but the lateral meniscus is nearly circular.
The meniscofemoral ligaments are also known as the Humphrey and Wrisberg ligaments. They connect the
Menisci are stabilized by
- Medial collateral ligament
- Transverse ligament
- Meniscofemoral ligaments
- Attachments at the anterior and posterior horns
The menisci act to
- Compensate of incongruity between the femur and tibia
- In the distribution of joint load
- Absorb shock
- Facilitate nutrition of articular cartilage.
Pathology of Discoid Meniscus
The discoid shape of the meniscus is believed to be due to failure of apoptosis [Programmed] in utero. Therefore the disc retains extra tissue.
The affected meniscus is made of fibrocartilage, oval, or roughly circular in shape. This leads to the covering of tibial plateau almost entirely.
Many variations can occur leading to different degrees of plateau cover.
Cystic degeneration with centrally located cavities can occur in the discoid meniscus.
Classification of Discoid Meniscus
There are three types of the discoid lateral meniscus
This type of meniscus is a complete disc. This gets damaged due to opposing movement between the superior and inferior surfaces which leads to tears, ridges, and separation from surfaces.
The movement of the femoral condyle over the ridges produces the snapping sensation and sound [popping knee syndrome]. Because of hypermobility of the posterior part of the lateral meniscus, it frequently gets displaced medially, gets caught between the femoral condyle and tibial plateau leading to loud click.
The intermediate type is also complete but the thickness is lesser than primitive and size is smaller.
The infantile type is discoid in shape but the size is normal. Tears occur here as in normal meniscus.
Arthroscopic classification of the discoid meniscus is based on arthroscopic visualization and describes three types
- Wrisberg-ligament type
- Lateral meniscus not attached to the tibial plateau posteriorly except for meniscofemoral ligament (ligament of Wrisberg)
- Complete type
- Menisci are disc-shaped, covering the entire tibial plateau.
- Incomplete type
- Differs from the complete type only in size.
Symptoms and Signs of Discoid Meniscus
The discoid meniscus may be asymptomatic in a number of cases.
Symptoms may be noted in adolescence. There patient may complain of
- Knee giving way
- Catching of knee
The symptoms often get pre precipitated by a recent injury.
On examination, fullness may be detected in the lateral parapatellar area at the joint line.
The last 15-20 degrees of extension of the flexed knee may result in a click or clunking sound due to the lateromedial movement of the discoid meniscus.
[This happens because the lateral meniscus is not fixed posteriorly to the tibia, and on extension is pulled medially onto the intercondylar space by a short meniscofemoral ligament. As the ligament relaxes during flexion, and the lateral meniscus is replaced in its usual by the contracting popliteus and coronary ligaments.]
The knee and thigh appear normal for other examinations.
- Meniscal cyst
- Congenital subluxation of the tibiofemoral joint
- Abnormal pull of popliteus tendon
- Snapping of the tendons about the knee
- Subluxation or dislocation of the proximal tibiofibular joint
- Subluxation of the patellofemoral joint.
Knee x-rays may suggest
- Increae in lateral joint space [esp. if the discoid meniscus is thick]
- Flattening of the lateral femoral condyle
- Cupping of the lateral aspect of the tibial plateau
It will clearly depict the configuration of the menisci and is the investigation of choice.
Treatment of Discoid Meniscus
An intact meniscus transmits 70-90% of the total load across the knee joint. Therefore, it is desirable to preserve the meniscus whenever possible.
Asymptomatic discoid meniscus requires no treatment. But these need to be observed and followed.
Conservative measures are indicated in people with minimal symptoms. These are
- Immobilization of the knee
- Restriction of physical activity
- Progressive exercises for the quadriceps.
Operative Measures are indicated if
- Persistent knee locking
- Functional disability
- Persistent pain
A diagnostic arthroscopy is carried before the procedure out to ascertain the pathologic changes and the type of discoid meniscus.
Partial or complete excision of the discoid meniscus is indicated.
Partial resection of the discoid meniscus is preferred when it is of the complete or incomplete type with minimal tearing and slight degeneration
Excision of the entire meniscus is performed when it is of the Wrisberg type or when it is torn and there is marked degeneration.
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