The
medial meniscus and lateral meniscus are specialized
structures within the knee. These crescent-shaped shock absorbers between
the tibia and femur have an important role in the function and health
of the knee. Once thought to be of little use, the menisci (plural)
were routinely removed when torn. Now we know that the menisci contribute
to a healthy knee because they play important roles in joint stability,
force transmission, and lubrication. When possible, they are repaired
if injured. There are even experimental attempts to replace a damaged
meniscus, possibly an important advance in orthopaedic medicine.
There are two categories
of meniscal injuries - acute
tears and degenerative tears.
An
acute tear usually occurs when the knee is bent and forcefully
twisted, while the leg is in a weight bearing position. Statistics
show that about 61 of 100,000 people experience an acute tear of the
meniscus.
Degenerative
tears of the meniscus are more common in older people. Sixty percent
of the population over the age of 65 probably has some sort of degenerative
tear of the meniscus. As the meniscus ages, it weakens and becomes
less elastic. Degenerative tears may result from minor events and
there may or may not be any symptoms present.
What are the menisci? The
two menisci of the knee are crescent-shaped wedges that fill the gap between
the tibia and femur. The menisci provide joint stability by creating
a cup for the femur to sit in. The outer edges are fairly thick while
the inner surfaces are thin. If the menisci were missing, the curved femur
would move on the flat tibia.
The medial meniscus, located on the inside of the knee, is more
of an elongated "C"- shape, as the tibial surface is larger on that side.
The medial meniscus is more commonly injured because it is firmly
attached to the medial collateral ligament and joint capsule. The lateral
meniscus, on the outside of the knee, is more circular in shape. The
lateral meniscus is more mobile than the medial meniscus as there
is no attachment to the lateral collateral ligament or joint capsule.
The
outer edges of each meniscus attach to the tibia by the short coronary
ligaments. Other short ligaments attach the ends of the menisci
to the tibial surface. The inner edges are free to move because they are
not attached to the bone. This lets the menisci change shape as the joint
moves. The front portion of the meniscus is referred to as the anterior
horn, the back portion is the posterior horn, and
the middle section is the body.
Under the microscope, the meniscus is fibrocartilage that
has strength and flexibility from collagen fiber.
Its resilience is due to the high water content in the spaces between
the cells. There is not much blood supply to the menisci. Blood flows only to the outer edges from small arteries around
the joint. The poor blood supply to the inner portion of the meniscus
makes it difficult for the meniscus to heal.
What does the meniscus do?
The meniscus acts as a shock absorber for the knee by spreading compression
forces from the femur over a wider area on the tibia.
The medial meniscus bears up to 50%
of the load applied to the medial (inside) compartment of the knee.
The lateral meniscus absorbs up to
80% of the load on the lateral (outside) compartment of the knee.
During the various phases of the walking
cycle, forces shift from one meniscus to the other, and forces on the
knee can increase to 2 - 4 times body weight.
While running, these forces on the
knee increase up to to 6 - 8 times body weight. There are even higher
forces when landing from a jump.
The
important role of the meniscus in force transmission can be seen
when the menisci are removed.
If
the menisci are removed, the forces are no longer distributed over
a wide area of the tibia.
Without
the medial meniscus, thetibial contact area is decreased
50 - 70%. This means the same forces from the femur are concentrated
on a smaller area of the tibia.
When
the lateral meniscus is removed, there is a 45 - 50% decrease in
contact area. This results in a 200 - 300% increase in contact
pressure, which can eventually damage the cartilage on the ends of
the bones. This can lead to degenerative
arthritis.
In
the 1960s and 1970s, it was common to remove a damaged meniscus entirely.
This frequently led to early degenerative arthritis in many patients.
Removing
the entire medial meniscus can lead to a bow-legged deformity
and medial joint arthritis.
Removing
the entire lateral meniscus can cause a knock-kneed deformity
and lateral joint arthritis.
What
is a meniscus injury?
Patients describe meniscal tears in a variety of ways. Knowing where
and how a meniscus was torn helps the doctor determine the best
treatment.
Location
-A tear may be located in the anterior horn, body, or posterior horn.
A posterior horn tear is the most common. The meniscus is broken down
into the outer, middle, and inner thirds. The third in which the tear
is located will determine the ability of the tear to heal, since blood
supply in that area is critical to the healing process. Tears in
the outer 1/3 have the best chance of healing.
Pattern-
Meniscal tears come in many shapes. The pattern of the tear influences
the doctor's decision on treatment. Examples of the various patterns
are:
Completeness
- A tear is classified as being complete or incomplete. A tear is
considered complete
if it goes all the way through the meniscus and a piece of the tissue
is separated from the rest of the meniscus. If the tear is still partly
attached to the body of the meniscus, it is considered incomplete.
Stability
- A tear can be stable or unstable. A stable tear does
not move and may heal on its own. An unstable tear allows the
meniscus to move abnormally and is likely to be a problem if it is
not surgically corrected.
The
medial meniscus and lateral meniscus are specialized
structures within the knee. These crescent-shaped shock absorbers between
the tibia and femur have an important role in the function and health
of the knee. Once thought to be of little use, the menisci (plural)
were routinely removed when torn. Now we know that the menisci contribute
to a healthy knee because they play important roles in joint stability,
force transmission, and lubrication. When possible, they are repaired
if injured. There are even experimental attempts to replace a damaged
meniscus, possibly an important advance in orthopaedic medicine. 
The
two menisci of the knee are crescent-shaped wedges that fill the gap between
the tibia and femur. The menisci provide joint stability by creating
a cup for the femur to sit in. The outer edges are fairly thick while
the inner surfaces are thin. If the menisci were missing, the curved femur
would move on the flat tibia. 
The
outer edges of each meniscus attach to the tibia by the short coronary
ligaments. Other short ligaments attach the ends of the menisci
to the tibial surface. The inner edges are free to move because they are
not attached to the bone. This lets the menisci change shape as the joint
moves. The front portion of the meniscus is referred to as the anterior
horn, the back portion is the posterior horn, and
the middle section is the body. 
Blood flows only to the outer edges from small arteries around
the joint. The poor blood supply to the inner portion of the meniscus
makes it difficult for the meniscus to heal.
