The meniscus is a half moon shaped piece of cartilage that lies between the weight bearing joint surfaces of the femur and the tibia. It is triangular in cross section and is attached to the lining of the knee joint along its periphery. There are two menisci in a normal knee; the outside one is called the lateral meniscus, the inner one the medial meniscus.
The menisci play an important role in absorbing about a third of the impact load that the joint cartilage surface sees. It's been shown that complete removal of a meniscus can result in progressive arthritis in the joint within a decade or so in a younger patient, sooner in patients who are older with preexisting "wear-and-tear" osteoarthritis. The menisci also cup the joint surfaces of the femur and therefore provide some degree of stabilization to the knee.
The meniscus itself is for the most part avascular, that is it doesn't bleed if cut and doesn't have blood vessels inside. The exception to this is at the periphery where it joins to the vascular knee lining providing the outermost 20% of the meniscus with a blood supply. As a result of this avascularity a torn meniscus doesn't have the ability to heal itself unless there is just a small tear confined to the peripheral vascular zone. Similarly the nerve supply providing pain and sensation to the meniscus is for the most part limited to the zone where the blood vessels are located.
In terms of descriptive terminology orthopedic surgeons divide the meniscus into thirds with three geographical zones; the front third is referred to as the anterior horn, the back third the posterior horn, and the middle third the body.
There are two different mechanisms for tearing a meniscus.
Traumatic tears result from a sudden load being applied to the meniscal tissue which is severe enough to cause the meniscal cartilage to fail and let go. These usually occur from a twisting injury or a blow to the side of the knee that causes the meniscus to be levered against and compressed. A football clipping injury or a fall backwards onto the heel with rotation of the lower leg are common examples of this injury pattern. In a person under 30 years of age this typically requires a fairly violent injury although any age group can sustain a traumatic tear.
Degenerative meniscal tears are best thought of as a failure of the meniscus over time. There is a natural drying-out of the inner center of the meniscus that can begin in the late 20's and progresses with age. The meniscus becomes less elastic and compliant and as a result may fail with only minimal trauma (such as just getting down into a squat). Sometimes there are no memorable injuries or violent events which can be blamed as the cause of the tear. The association of these tears with aging makes degenerative tears in a teenager almost unheard of.
A meniscus can tear in almost any conceivable geometric pattern and in any location. Tears confined to the anterior horn of the cartilage however are unusual. Typically tears begin in the posterior horn and then can extend forward into the middle body and even anterior horn.
Examples of tear types
Arthroscopic view of a complex posterior horn meniscal tear
A torn meniscus will usually cause pain on the side of the knee that is localized to the meniscus (at the level of the joint line between the femur and tibia). Swelling of the joint may occur although meniscal tears by themselves usually don't cause a large, tensely swollen knee. Typically low-level swelling sets in the next day after the injury and is associated with stiffness and limping.
Sometimes the knee becomes "locked" in a bent position and is quite painful with attempts to straighten it. This scenario is often caused by the mechanical blockage of the joint from a displaced bucket handle type meniscal tear. The torn fragment actually acts like a wedge to prevent the joint surfaces from moving, hence the knee appears locked.
Any twisting, squatting or impacting activities will pinch the meniscus tear or flap and cause pain. Often the pain may improve with rest after the initial injury so that the limping resolves but as soon as aggressive activity is attempted the pain recurs. Meniscal tissue doesn't heal (with the exception noted above) due to its lack of a blood supply so symptoms are persistent until the tear is treated.
The diagnosis of a tear is made based on the history and joint line findings. A physician can often stress and manipulate the knee joint in a way that provokes the meniscal tear to snap or cause pain which makes the diagnosis likely. Other times an MRI scan will be obtained to visualize the meniscus and assess its integrity. A good MRI scanner has a very high accuracy rate in determining if a tear is present.
|About MRI scans
Once a meniscal tear has been diagnosed it should be treated. This doesn't have to be done urgently although patients with a painful locked knee may want surgery as soon as scheduling permits. Arthroscopic surgery is the only way to treat the tear since there are currently no medications, braces, or physical therapy treatments that have been shown to promote healing in the avascular tears. As explained above it's conceivable that a short (<1 cm) stable tear limited to the outer 20% of the meniscus where the blood supply is could heal with a period of immobilization but this a rare circumstance.
Arthroscopic surgery is performed on an outpatient basis. The surgeon initially inspects the whole knee joint to see if there are any other problems (e.g. arthritic areas, ligament tears or tears of the other meniscus) and then evaluates the meniscal tear by inspecting it with a blunt probe. Based on the location and geometry of the tear the decision is made to either remove or repair the tear(s). Removing the tear (called a meniscectomy) is accomplished by using a variety of small instruments that cut and suck out only the the torn portions of the meniscus. The remaining meniscal rim is then balanced and contoured to provide a gradually tapered transition into the area of the resection. The surgeon tries to leave as much normal meniscal cartilage as possible since this is an important shock-absorbing structure.
|Arthroscopic probing of a posterior horn complex meniscal tear with multiple flaps||After arthroscopic removal of flap tears and contouring of remaining meniscal rim|
Most meniscal tears need to be removed because they involve areas of the tissue that do not have the ability to heal (even if sewn together). On the other hand, if there is a vertical tear at the peripheral rim of the meniscus which is confined to the zone of meniscal blood supply, it is possible and desirable to repair the meniscus. A repair allows the entire meniscus to be saved and retained whereas removing a peripheral tear would require resection of a very large portion of the meniscus. The key to a successful repair is that the meniscus must be able to heal itself; the repair serves only as a means of securely holding the tissue together long enough for this biologic process to occur.
There are a variety of surgical techniques available for repairing a torn meniscus. Initially surgeons used to do an open repair (outside-in) sewing the meniscus back together through an incision, however in recent years the technology has progressed so that these repairs are now routinely done arthroscopically.
One arthroscopic technique is an inside-out method that uses curved guide tubes called cannulas to direct a pair of long needles into the meniscus and out through a small incision in the back of the knee. The suture thread ends connected to the needles are then tied together on the outside of the knee capsule layer to firmly bring the meniscal tear together.
This technique works well but note that it does require a 11/2 - 2" incision to access the area where the knots need to be tied down.
There are now a variety of methods available to the arthroscopic surgeon that permit a true inside-in repair and avoid having to make an incision at all. Some of these include bioresorbable T-arrows and dissolving meniscal "staples" which can each be applied from within the joint arthroscopically. At Orthopedic Associates of Portland we often prefer to use the T-Fix® system for meniscal repair since in our hands it seems to give a strong and tight re-approximation of the meniscus to its rim. The T-Fix® sutures have an anchor that acts like a wall anchor and is deployed after placing the suture through the meniscus, the tear and the peripheral rim . Multiple sutures pairs are placed through long hollow needles and are tied together from inside the joint using a knot pusher instrument that securely snugs the meniscus down and provides an excellent repair.
Peripheral meniscal tear in the zone of blood supply
Knots are pushed down after placing suture anchors
Close up of knot pusher
Arthroscopic view of a meniscus repaired with the T-Fix® system
Appropriate protection of the repaired meniscus is required since even the most secure repair will tear through the meniscal tissue if it is loaded too soon. The role of the repair is only to hold the meniscus together long enough that it can heal firmly. For this reason we do not allow weight bearing on the joint for at least 3 weeks after surgery. On the other hand motion is encouraged and some simple light-load toning muscle exercises are usually permitted.
Once weight bearing is begun the knee is gradually and progressively conditioned with a supervised physical therapy program . Maximal weight training is not allowed for 2-3 months. Return to running and agility sports is permitted after 3-4 months if strength and motion have returned and there is no pain in the joint.
Recovery from removal of a meniscal tear is much quicker and requires the use of crutches for longer walks only until the patient can walk without limping (typically 5-7 days). With a proper rehabilitation program one can usually expect to be back in sports within 4-6 weeks after the meniscectomy (Meniscectomy Rehabilitation Protocol).