Lateral Cartilage Meniscus Abnormality
The Discoid Lateral Meniscus
Menisci are small fibrocartilagenous pads present between the articulating bone ends in the knee (and a few other joints). There are two of these, the one on the inside is called the medial meniscus while that on the outside is called the lateral meniscus. Both menisci are semi lunar in shape, concave from above and flat from below. The articulating surface of the tibia (lower leg bone) is almost flat, while the articulating end of the thigh bone (femur) has two rounded heads (condyles). These menisci form a cavity to hold the condyles of the femur, thus providing functional stability to the joint.
The menisci serve to:
- Reduce friction
- Stabilise the joint
- Absorb about one third of the impact load and disperse this load uniformly, when the condyles of the femur and tibia make point contact during movement.
Discoid Lateral Meniscus:
Discoid meniscus is a morphological anomaly of the lateral meniscus of the knee, in which the normal semi-lunar shaped meniscus is replaced by a discoid meniscus. Though the medial meniscus may also show the deformation, this is very rare. Even lateral discoid meniscus is a rare finding, though studies suggest a comparatively higher incidence in Asians (Japanese and Koreans).
The discoid shape prevents normal contact between the articulating ends of the femur and tibia, which increases the risk of mechanical distortion.
A discoid lateral meniscus often does not produce any symptoms and, for this reason, is rarely diagnosed. Sometimes, it may give rise to a clicking or snapping sound heard on moving the knee or it may become painful after a traumatic incident.
There are three variants of discoid meniscus according to the Watanabe Classification:
- The complete type
- The incomplete type
- Wrisberg-ligament variant (the loose type)
A. Incomplete Lateral Discoid Meniscus (middle, in right side diagram)
B. Complete Lateral Discoid Meniscus (last, in right side diagram)
C. Wrisberg-ligament variant
A complete discoid meniscus almost completely covers the lateral half of the articulating surface of the tibia, while an incomplete type covers a smaller surface.
The third type may have a normal shape, however it is unstable and hyper mobile. The reason is the lack of tibial attachment at the posterior aspect, which leads to hyper mobility and instability. Instead, the meniscus is only attached to the Wrisberg ligament, which inserts into the condyle of the femur.
A fourth type, a ring-shaped meniscus has also been added to the discoid meniscus category, based on diagnostic and operative findings.
There are two theories about the formation of a discoid lateral meniscus. According to one, the problem is congenital; during development the resorption process remains incomplete, leading to a discoid shape of the meniscus. Others claim that the discoid shape is a result of increase in the size of the normal meniscus (hypertrophy) due to abnormal motion of the meniscus that imposes undue stresses on its structure.
Discoid lateral menisci may remain completely trouble free or may give rise to a variety of symptoms. Complete and incomplete discoid menisci produce symptoms different from the Wrisberg type variant. The former remain symptom free unless there is a meniscal tear which leads to pain and swelling. Symptoms are therefore those of a meniscal tear. Pain is felt at the side or in the centre of the knee. There is swelling and stiffness, restricting the range of motion. There may also be locking or catching of the joint.
The Wrisberg type discoid meniscus is mostly associated with clicking or snapping sounds, locking or catching of the knee or a feeling as if the knee is going to collapse. There may also be pain or joint effusion (water on the knee).
Although discoid menisci commonly occur bilaterally, symptoms can occur on one side only.
Physical examination may reveal an inability to fully extend the knee (extension block).
Imaging techniques help to reach a definitive diagnosis. X-rays may provide useful hints such as a wide joint space on the lateral side, however, MRI or arthrography are required for confirmation.
A problem-free discoid meniscus discovered incidentally should be left alone. Surgery is only indicated if the symptoms are troublesome or if there is tearing of the meniscus.
Arthroscopic partial menisectomy:
A part of the meniscal tissue is removed arthroscopically, preserving the peripheral rim (contouring the disc shape to a more normal semi-lunar shape). The results are usually satisfactory.
If the meniscus is severely damaged it may have to be removed completely.
The surgical treatment of the Wrisberg variant comprises a near-total or total removal, as it has shown better results. Recently, there have been attempts to tie it to the tibia to improve its stability.