The normal meniscus of the knee is semilunar in shape. In discoid meniscus, the meniscus of the knee is discoid rater than semilunar in shape. This is a common cause of popping or snapping knee.
The lateral meniscus is most frequently affected, though, on occasion, the condition may occur in the medial meniscus.
Involvement is often bilateral. There is no difference in occurrence in either sex. The condition is diagnosed comparatively rarely, and it may often go unrecognized. Familial incidence of the discoid lateral meniscus is known.
The affected meniscus is made of fibrocartilage, oval or roughly circular in shape. The corresponding surface of the tibial plateau will be almost entirely covered by the discoid meniscus. Variations may occur where the anterior horn and body will be a solid mass, whereas the posterior horn will be normal.
Cystic degeneration with centrally located cavities can occur in the discoid meniscus.
Histology shows mucoid degeneration due to increased wear. Meniscal tears are common.
Classification of Discoid Meniscus
There are three types of discoid lateral meniscus
The primitive type
This type of meniscus is a complete disc. This gets damaged due to opposing movement between the superior and inferior surfaces which lead to tears, ridges, and separation from surfaces.
The movement of the femoral condyle over the ridges that produces the snapping sensation and sound. 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
The intermediate type is also complete but the thickness is lesser than primitive and size is smaller.
The infantile type
The infantile type is discoid in shape but the size is normal. Tears occur here as in normal meniscus.
Another classification of the discoid meniscus is based on arthroscopic visualization. It describes three types
Wrisberg-ligament type in which the lateral meniscus has not attached to the tibial plateau posteriorly except for meniscofemoral ligament (ligament of Wrisberg). This ligament is taut and does not accommodate the normal flexion and extension of the knee.
The hypermobility of the posterior part of the lateral meniscus results in its secondary hypertrophic thickening. Usually, this variety is symptomatic.
The menisci are disc-shaped, covering the entire tibial plateau. The attachments of the lateral meniscus are intact and there is no hyperlaxity or abnormal motion. Complete type of discoid lateral meniscus is usually an incidental finding at arthroscopy
This from the complete type only in size.
The mere presence of a disc-shaped cartilage does not cause any problem. Rather, the condition is often asymptomatic in infancy and early childhood.
By the sixth or eighth year of life the child may complain of snapping, click, giving way or catching. Symptoms may be precipitated by a recent injury, especially in the adolescent.
On examination, fullness may be detected in the lateral parapatellar area at the joint line. A loud clunk is audible during the last 15 to 20 degrees of extension of the flexed knee. The clunk is produced by the lateromedial movement of the semilunar cartilage.
On extension, of the knee joint, the lateral meniscus does not remain in place under the lateral femoral condyle because it is not fixed posteriorly to the tibia, but is dislocated medially onto the intercondylar space by the pull of the short meniscofemoral ligament.
During flexion, the ligament relaxes, and the lateral meniscus is replaced in its usual position by the contracting popliteus and coronary ligaments.
Atrophy of the thigh, joint effusion, and synovial thickening are significantly absent and there is no functional disability. Forced hyperextension of the knee may elicit pain on the lateral aspect of the joint.
Other causes of snapping should be ruled out. These mainly are meniscal cyst, congenital subluxation of the tibiofemoral joint, abnormal movement of the popliteus tendon, snapping of the tendons about the knee and subluxation or dislocation of the proximal tibiofibular joint or of the patellofemoral joint.
Increase in lateral joint space may be found on plain x-ray if the discoid meniscus is thick. Flattening of the lateral femoral condyle and cupping of the lateral aspect of the tibial plateau are other features
Magnetic resonance imaging
It will clearly depict the configuration of the menisci and is the investigation of choice.
Treatment of Discoid Meniscus
The menisci in the knee joint are required for
- Compensation of incongruity between the femur and tibia
- In the distribution of joint pressure
- Shock absorber, for stabilization of the knee, in the provision of rotation, in spreading of synovial fluid, and in the nutrition of articular cartilage.
An intact meniscus transmits 70 to 90 percent of the total load across the knee joint. Therefore, it is desirable to preserve the meniscus whenever possible.
A conservative nonoperative method of management is recommended In the treatment of discoid meniscus if pain and functional disability are minimal.
Silent discoid menisci require no treatment. However, they should be kept under observation.
- Immobilization of the knee
- Restriction of physical activity
- Progressive exercises for the quadriceps.
If the knee locking persists, there is functional disability or pain partial or complete excision of the discoid meniscus is indicated.
Diagnostic arthroscopy is carried out to know the pathologic changes and the type of discoid meniscus.
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.