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, the tibial 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.
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:
- displaced bucket handle
- parrot beak
- displaced flap
A complex tear includes more than one pattern.
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.
What are the signs and symptoms of a meniscus injury?
Acute tears are often sports related and usually the result of a twisting injury in the younger, active adult population. Symptoms of an acute tear are usually pain, swelling, and movement irregularities. When the tear gets in the way of normal knee motion, the knee can "catch" or "lock" as it moves.
Degenerative tears are more common in the older population. The patient may experience repeated swelling, but often can't recall any specific injury. The swelling also may be the result of an injury caused by a very minor movement. Mechanical symptoms, such as the knee catching or locking, often exist. Or, the patient may simply experience pain.
How is a meniscus injury diagnosed?
The orthopaedic surgeon will first take a history of the injury to help determine if the signs and symptoms might suggest meniscal damage.
Next the doctor will evaluate the knee for swelling and tenderness in a physical examination. The knee will be tender when pressed on the injured side where the tibia and femur meet. The McMurray's maneuver is a test in which the doctor applies pressure and moves the knee from straight to bent to straight again to see what positions cause pain or catching (indications of a meniscal tear).
The doctor may use imaging to assess the amount of damage. X-rays can show any fractures or arthritic conditions in the knee. A narrow joint space or bone changes indicate bone-on-bone rubbing and arthritis. If the diagnosis is still not clear, an MRI (Magnetic Resonance Image) may be ordered to reveal damage to ligamaments and menisci. This exam is 70 - 95% accurate in revealing meniscal tears, and can also show any ligament damage.
How is a meniscus injury treated?
When determining the treatment for a meniscal tear, the orthopaedic surgeon will consider the following factors:
The patient's activity level
The patient's age
The location of the tear and the type of tear
When the injury happened
Any other associated injuries
After considering these factors the doctor will choose to treat the injury non-operatively or surgically.
Many small meniscal tears will heal without surgical treatment. Also, some tears may have no symptoms and in other tears, symptoms may eventually disappear. Partial tears, degenerative tears, and stable tears may be observed for 2 - 3 months. If symptoms disappear, no surgery is needed. The use of a knee brace and restriction of activities may be recommended to prevent further injury.
Surgical treatment for a meniscal tear may be indicated if:
symptoms are disabling or last for more than 2 - 3 months
a displaced tear causes the joint to lock
the anterior cruciate ligament is also injured - In this case, the knee is highly unstable and excessive motion exists within the joint. The meniscus is unlikely to heal without treatment.
the patient is a high-level athlete
If surgery is recommended, the procedure chosen is usually dependent on the location and type of meniscal tear. All procedures are performed through an arthroscope and usually don't require an overnight hospital stay.
Trephination/ Abrasion Technique
This procedure is used for stable tears located on the periphery near the meniscus and joint capsule junction, where there's a good blood supply. Multiple holes or shavings are made in the torn part of the meniscus to promote bleeding, which enhances the healing process.
This surgical procedure is used for tears located in the inner 2/3 of the meniscus where there is no blood supply. The goal is to stabilize the rim of the meniscus by removing as little of the inner meniscus as possible. Only the torn part of the meniscus is removed. If the meniscus remains mostly intact with only the inner portion removed, the patient usually does well and does not develop early arthritis.
This procedure involves the complete removal of the damaged meniscus. This technique is only performed if absolutely necessary. Removal of the entire meniscus frequently leads to the development of arthritis.
Repairs are performed on tears near the outer 1/3 of the meniscus where a good blood supply exists, or on large tears that would require a near-total resection. The torn portion of the meniscus is repaired by using either sutures or absorbable fixation devices. These devices include arrows, barbs, staples, or tacks that join the torn edges of the meniscus so they can heal.
Experimental attempts to replace damaged meniscus are seen as important recent advances in orthopaedic medicine. The new technology mentioned here has been performed at a few surgical centers across the country on a small number of patients.
Collagen meniscus implant - This is a scaffold of collagen inserted into the patient's knee. Over time, a new meniscus may grow within the joint. This procedure is currently in FDA trials in the United States and has just been approved as an accepted surgical procedure in Europe.
Meniscal transplant - This procedure involves transplanting a meniscus from a donor into the injured knee. Only a limited number of surgeons perform this procedure on a routine basis. The long-term outcomes are still being evaluated.
The rehabilitation program for non-operative treatment of a meniscus injury may include:
- using crutches for protective weight bearing while walking. Crutch use is usually recommended for the first 2 - 3 days after injury, or until pain and swelling have subsided.
- flexion - extension exercises for motion and strength. An exercise program begins about 2 - 4 weeks after injury. No rotational exercises are permitted until the knee is symptom-free.
- return to activities at about 4 - 6 weeks after injury. If the knee is still symptomatic after 2 - 3 months, further medical evaluation and surgery may be necessary.
The rehabilitation program for a partial resection of the meniscus may include:
- crutch use for the first 2 - 3 days following surgery due to post-operative pain and swelling. After this, the patient may be weight-bearing as tolerated.
- range of motion exercises are emphasized at first.
- strength exercises begin once swelling has subsided.
- return to activities can start at about 4 - 6 weeks following surgery.
Rehabilitation after meniscal repair depends on the size of the tear, stability of the repair, and other injuries. In general, for an isolated meniscal repair:
- Full weight bearing is not permitted for 1 - 6 weeks after surgery, depending on the type of injury and repair. Crutches will be used initially following surgery.
- Many surgeons brace the knee and restrict motion for 6 weeks, to prevent excessive flexion and extension.
- Range of motion exercises begin anywhere from 0 - 6 weeks after surgery, depending on the type of repair.
- Strengthening exercises begin once full range of motion has returned.
- Return to vigorous activities, such as sports, may begin 3 - 4 months after repair.
Persistent pain, mechanical symptoms, or stiffness after meniscal repair may indicate the need for further treatment. If the meniscus does not heal, its revision or removal may be necessary.
Can a meniscus heal?
Small tears in the meniscus that are not dislodged may heal, or may eventually be symptom-free. Larger tears that displace, and tears associated with instability, are less likely to heal. Tears in the outer 1/3 of the meniscus are more likely to heal than tears toward the inside of the meniscus because the blood supply is better in the outer region.
When does a meniscal tear need surgery?
A meniscal tear needs surgery when: 1) the tear causes symptoms such as pain, swelling, catching, or locking, 2) a displaced portion of the meniscus is causing the knee to lock, or 3) the tear is associated with knee instability.
Is an MRI needed to diagnose a meniscal tear?
An MRI is not always required to diagnose a meniscal tear. A meniscal tear can be accurately diagnosed with a doctor's physical examination. However, an MRI can be useful to determine the extent of the injury, the displacement of a tear, and help determine if there are any other associated injuries.
How does the doctor decide between repairing the tear vs. removing the torn piece of meniscus?
The final decision is made during an arthroscopy when the surgeon gets a close look and probes the tear. Tears in the outer third of the meniscus are often repaired. This region has a better blood supply for healing. Also, the outer portion of the meniscus is thicker and resection of these tears will leave little meniscus remaining. Tears in the inner two-thirds of the meniscus often require that the torn portion be removed because the poor blood supply to this region limits healing. Also, the inner portion of the meniscus is the thinnest section so removing a torn piece here requires a minimal loss of tissue.
Why doesn't the surgeon just take out the entire damaged meniscus?
The meniscus has an important function inside the knee as a shock absorber that helps distribute the load of the body. If the entire meniscus is removed, the rest of the joint gets overloaded and the knee is susceptible to arthritis. This is why the surgeon will save as much of the meniscus as possible.
How is a meniscal tear fixed?
There are many techniques and instruments available to repair a torn meniscus. Meniscal tears may be repaired using sutures or devices (such as arrows, tacks, and screws) that the body absorbs after the meniscus has healed.
When can I play sports again after meniscal surgery?
This will depend on the type and location of tear, the size of the tear, and whether the meniscus was surgically repaired or partially removed. In general, rehabilitation is faster after a partial removal than after a repair. Patients can often return to sports three months after meniscal surgery.