Before we look at meniscus injury, we first need to understand its anatomy. At the knee, the rounded ends of the femur, or thighbone, meet with the flat ends of the tibia (one of the shin bones). Two rings of cartilage, called the medial and lateral menisci, conform to the shape of each end of bone and form a tight fit between them; much the same way a washer does between a nut and a bolt (see figure 1).
The menisci are composed of three collagen fibre layers that give them a tough yet flexible structure. The deepest layer of collagen is woven into bundles and runs parallel to the outer edge of the meniscus. In the middle layer, the fibres are arranged around the edge to resist the hoop stress of weight bearing. This arrangement prevents the meniscus from being squeezed out from the joint. The outer layer of fibres runs radially to tie the layers together and resist the shear forces that occur at the knee.
Both menisci are wedged on the outer edge. The medial is crescent shaped and the lateral is more circular. Each meniscus is attached to the surface of the tibia via ‘anterior and posterior horns’. A coronary ligament runs around the edge of each meniscus attaching it to the tibia, while the medial borders move freely within the joint. The medial meniscus is also attached to the medial collateral ligament (see figure 1).
The blood supply characteristics of the menisci exert a major influence on the healing process and the potential for return to function after injury. Only the outer parts of the menisci actually receive a direct blood supply. Termed the red zone, this area comprises the outside 10-30% of each meniscus(1). The remaining portion, known as the white zone, is nourished via diffusion from the synovial fluid within the joint.
The menisci were long thought to be left over from a leg muscle no longer needed. Research in the mid to late twentieth century brought to light their importance, which is to distribute weight-bearing forces across the knee joint by increasing the joint contact surface area. In knees where a meniscus is removed, joint contact pressures increase from 50%-200%(2).
The menisci also provide shock absorption, lubrication to the joint, joint stability, and facilitate the gliding of the bones as the knee flexes and extends. When the knee flexes, the menisci are displaced towards the front (anteriorly). When the knee extends, the menisci are displaced towards the rear (posteriorly). The lateral meniscus is more ‘mobile’ than the medial, due to the medial meniscus’ attachment to the medial collateral ligament and its firm posterior horn attachment.
Mechanism of injury
There are two types of meniscus injuries– degenerative and traumatic. Degenerative injury is a result of ageing. Years of weight bearing result in the breakdown of the cartilage leaving it thin and vulnerable to tearing. This is most common in people over 40 years of age.Traumatic meniscus injuries primarily occur in athletes. A common traumatic mechanism of injury is hyperextension of the knee. When the knee is rapidly hyperextended, the femur may impinge a portion of the meniscus that has not yet moved posteriorly out of the way. The resulting stress from the taut pull of the medial collateral ligament and the femoral impingement can result in a longitudinal or ‘bucket handle’ tear in the outer portion of the meniscus (see figure 1).
Lateral knee trauma can also cause tearing of the meniscus. When the knee is forced into a valgus position with rotation, as in a lateral tackle or a plant and cut manoeuvre, a stress is placed on the medial collateral ligament. Again, as the ligament is strained, the medial meniscus may pull away from its attachment there. The ligament itself may also tear. Adding a coinciding tear in the anterior cruciate ligament forms an unhappy triad of O’Donoghue(3).
A hyperflexion injury of the knee causes the back of the lateral meniscus to become impinged, resulting in an oblique tear. This type of tear starts at the interior of the meniscus and extends toward the outer edge, leaving a tongue of free tissue. A horizontal tear that begins at the outer edge has the most chance of healing due to its location in the red vascular zone. A radial tear extending from the interior outwards is the most disabling, as it disrupts the fibres around the outside and hinders the ability of the meniscus to bear hoop stress.
If the meniscus is torn, an athlete may experience a sudden pop or giving way of his knee. While he may be able to continue play initially, pain and swelling will soon result. The knee may catch or lock with a limited range of motion, especially if the torn piece of meniscus drifts into the joint space.
Investigators in Greece examined 378 injured knees in 364 athletes with isolated meniscal tears to determine frequency of meniscus injuries among types of sports(4). They found that most of the tears (73%) occurred in athletes who were soccer players, basketball players, or skiers. The medial meniscus was torn more frequently than the lateral (70% of the tears in the study were medial), and most of the tears (74.8%) were located in the posterior horn of both menisci.
Treatment
Conservative treatment is recommended initially with rest, ice, compressive wraps and elevation (RICE). Over-the-counter Ibuprofen, taken as directed, may help to alleviate the pain and swelling. Clinical tests to determine the integrity of the meniscus can be done by an athletic trainer or physiotherapist; however an examination by an orthopaedic physician is recommended and an orthopaedic doctor will likely order an MRI scan to evaluate the knee.If the meniscus has a small tear in the red, vascular zone, conservative treatment is initiated. Recommendations include rest from sports activity, immobilisation of the knee, partial or non-weight bearing using crutches for up to two weeks, and use of over-the-counter, non-steroidal medication to manage pain and inflammation. If the tear is significant, surgery is recommended. Most meniscus surgery is done via arthroscopy. Now that the function of the meniscus is well understood, every attempt is made to salvage and repair it, especially in the young athletic population. According to a study at Queensland University in Australia, meniscal repair in athletes is quite successful(5). Among 42 elite athletes who underwent meniscal repair via arthroscopy, 81% returned to sports at or near their previous level of competition. Those who did not return to sport cited fear of re-injury as their reason, not dissatisfaction with the outcome of their surgery. The mean return to sport time for those in the study with isolated meniscal repair was 5.6 months.
The success of arthroscopic meniscal repair was echoed in a Tufts University study(6). This study focused on the young athlete, enrolling 26 patients 17 years old or younger, with 85% of them participating in varsity athletics at the time of injury. The average follow-up time between surgery and the study was five years. Patients were evaluated via a follow-up survey and a clinical examination.
All but two of the patients in the study returned to their previous level of competition. At follow up, all patients reported participating in strenuous to moderate activities such as skiing and tennis. This study reports 100% success in the surgical repair of the meniscus. Interestingly, 28 of the 29 repairs were in the highly vascular red zone, a location known for improved rates of healing. All tears were in the posterior horn. The same surgeon performed all procedures. However, the study’s conclusion that aggressive surgical repair in youth is warranted based on these outcomes may be biased due to the singular type of tears studied.
In fact, ten years later, a larger study conducted at the Mayo Clinic on children with an average age of 16 years, reported slightly less glowing results(7). This study followed 44 patients with isolated meniscal repair via arthroscopy, for two or more years. Seven different surgeons performed the repairs, which were varied in location and degree.
The repair success rate overall was 62%; however, a great discrepancy exists between the success of simple tears (80%) and complex tears (13%). Of the 17 repairs that failed, 12 occurred while performing the same sports activity that caused the initial tear. Age at injury, return to sport time and gender did not have a significant impact on success rate. What did impact success was the distance from the red-white junction to the central margin of the tear. Those tears greater than three millimetres from the red-white junction were more likely to fail. Complex tears were also more susceptible to failure.
Is repair always the right choice?
When the tear is complex or outside of the red zone, surgeons may opt to perform a partial or complete removal of the meniscus via arthroscopy. Swedish surgeons performed 20 meniscectomies (nine total and 11 partial) on 19 elite athletes(8). Seventy-four percent of them reported excellent results (full activity, no symptoms) and 21% reported good results (full activity with minimal symptoms). The advantage of removal rather than repair of the meniscus is an aggressive rehabilitation schedule that allowed 68% of the patients to return to training and competition within two weeks after surgery.The risk with meniscectomy, however, is the development of osteoarthritis (OA). It is well accepted that injury to the meniscus is associated with the development of OA. In a collaborative study, researchers in Sweden and Massachusetts evaluated the long-term OA risk of meniscus injuries and whether or not repairing the meniscus decreased that risk(9). An evaluation of the research showed that those who underwent a meniscectomy were ten times more likely to develop OA 10 to 20 years post-procedure, than those of the same age distribution without a known knee injury. However, they did not find evidence that repairing the meniscus, rather than removing it, protected one from such risk.
Another literature review explored this issue in relation to athletes. The author concluded that the alteration of the biomechanics of the knee after meniscectomy could not be the sole catalyst for degenerative OA changes in the knee(10). He developed a risk assessment scale to help guide clinicians and athletes in their decision making after meniscectomy (see table 1).
Prevention
Athletes at particularly high risk for meniscal injuries are those whose sports demand running, cutting and jumping. A Yale University study theorised that because these athletic manoeuvres require the body to recover from unstable positions, a decrease in neuromuscular control of the trunk would decrease stability and control at the knee(11). They also hypothesised that decreased proprioception in the core would predict knee injury in women, but not men.The researchers then evaluated the active proprioceptive repositioning of 277 Yale varsity athletes (140 women, 137 men) and followed the athletes for three years. During that period, 25 subjects sustained knee injuries (11 female, 14 male). Among the females, there was a significant difference in the baseline active proprioceptive repositioning score between those injured and uninjured.
This study found that decreased core proprioception is significantly associated with increased incidence of knee injuries in women. These results warrant core function screening in pre-season training. Those at risk should be assigned a core strengthening protocol as a preventative measure to maintain joint health.
Seeking a way to decrease the incidence of knee and ankle injuries in young athletes, researchers in Norway did just that(12). They implemented a core strengthening protocol into the early season training warm up of 61 youth handball clubs. Fifty-nine clubs acted as matched controls and continued with their regular training.
The protocol consisted of running drills, plyometrics, balance training, and quad and hamstring strengthening. The athletes spent 15-20 minutes performing the assigned activities prior to each training session for 15 consecutive sessions, then once per week for the remainder of the season. Significantly fewer knee ligament and meniscus injuries occurred in the intervention group than in the control group. In fact, the intervention group experienced significantly fewer injuries overall.
As children, many of us learned that the knee bone connected to the thighbone, and the thighbone connected to the backbone. These studies bear out the wisdom of that well known song. The best way to preserve the menisci is to respect the connectedness of the body and train the core, not just the knees, for the demands of sports.
Alicia Filley, PT, MS, PCS, lives in Houston, Texas and is vice president of Eubiotics: The Science of Healthy Living, which provides counselling for those seeking to improve their health, fitness or athletic performance through exercise and nutrition
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