Sunday, February 20, 2011
The menisci serve important roles in maintaining proper joint staility, health and function. The anatomy of the the medial and lateral menisci helps explain functional biomechanics. Viewed from the above the medial meniscus appears C shaped and the lateral meniscus appears O shaped.
Each meniscus is thick and convex in periphery (horns) but becomes thin and concave at its center. This serves to provide a larger area for the rounded femoral condyles and the relatively flat tibia. As well, menisci do not move in isolation. They are connected to each other anteriorly and to te anterior cruciate ligament, the patella, the femur, and the tibia by ligaments.
The medial meniscus is less mobile than the lateral meniscus.This is due to its firm connections to the knee joint capsule and the medial collateral ligaments. This decreased mobility , in conjuction with the fact that the medial meniscus is wider posteriorly, is cited as the usual reason for the highr incidence of tears within the medial meniscus than within the lateral one. The semimenbranous muscle (through attachments from the joint capsule) helps retract the medial meniscus posteriorly, serving to avoid entrapment and injury to the medial meniscus as the knee is flexed.
The lateral meniscus is not as adherent to the joint capsule. Unlike the medial meniscus, the lateral meniscus does not attach to its respective collateral ligament. The posterolateral aspect of the lateral meniscus is separated from the capsule by the popliteus tendon. Therefore, the lateral meniscus is more mobile than the medial meniscus. The attachementof the popliteus tendon to the posterolateral meniscus ensures dynamic retraction of the laterar meniscus when the knee internally rotates to return out of the scre-home mechanism. Therefore both the medial and lateral menisci, by having attachements to muscle structures, share a common mechanism that helps avoid injury.
The architecture of the vascular supply to the meniscus has important implications for healing. Capillaries penetrate the menisci from the periphery to provide nourisment. After 18 months of age, as weight bearing increases, the blood supply to the central part of the menisci recedes. Researches have showned that eventually only peripheral 10% to 30% of the menisci or red zone receives this capillary network.
Therefore, the central and internal portion or white zone of these fibrocartilaginous structures becomes avascular with age, relying on nutrition received through diffusion from the synovial fluid. Because of this vascular arrangement, the peripheral meniscus is more likely to heal than the central and posterolateral part.
The primary but not sole function of the menisci is to distribute forces across the knee jointand to enhance stability. Multiple studies shown that the ability of the joint to transmit loads is significantly reduced if the meniscus is partially or wholly removed. Fairbank published a seminal article in 1948 suggesting that the menisci are vital in protecting the articular surfaces. He reported that indiviuals who had undergone total menisectomies demonstrated premature osteoarthritis.
Meniscal tears are classified by their complexity, plane of rupture, direction, location, and overall shape. Tears are commonly defined as vertical, horizontal, longitudinal, or oblique in relation to the tibial surface. Most meniscal tears in young patients will be vertical longitudinal, whereareas horizontal cleavage tears are more commonly found in older patients. The bucket-handle tear is the most common type of vertical (or longitudinal) tear. Tears are also described as complete, full thicknes or partial tears. Complete, full-thickness tears are so named as they extend from the tibial to femoral surfaces. In addition, medial meniscus tears outnumber lateral meniscus tear from 2:1 to 5:1.
Meniscal injuries may result from an acute injury or from gradual degeneration with aging. Vertical tears (e.g. bucket handle tears) tend to occur acutely in individuals 20 to 30 years of age and are usually located in the posterior two thirds of the meniscus. Sports commonly associated with meniscal injuries are Soccer, footbal, basketball, baseball, wrestling, skiing, rugby,and lacrosse. Injury commonly occurs when an axial load is transmitted through a flexed or extended knee that is transmitted rotating. Degenerative tears in contrast are usually horizontal and are seen in older people with concomitant degenerative joint changes.
On the basis of the arthroscopic examiantion, the majority of acute peripheral meniscal injuries are associated with some degree of occult anterior cruciate ligament laxity.
In addition, true anterior cruciate ligament tears are associated with lesions of the posterior horns of the menisci. Lateral meniscal tears appear to occur with more frequency with acute ACL injuries, whereareas medial meniscal tears have higher incidence with chronic ACL injuries. With chronic cruciate ligament injuries, the medial meniscus may be more frequently damaged because its posterior horn serves as an important secondary stabilizer of anterior-posterior instability.
The history will help diagnose a meniscal injury 75% of the time. Young patients who experience mensical tears will recall the mechanism of injury 80% to 90% of the time and may report a ''pop'' or a ''snap'' at the time of injury. Deep knee bending activities are often painfull, and mechanical locking may be in 30% of patients. Bucket-handle tears should be suspected in cases of mechanical locking with loss of full extension. If locking is reported approximately 1 day after the injury, this may be due to ''pseudoocking'', which results from harmstring contracture. Knee hemarthrosis may also occur acutely, especially if the vascularized peripheral portion of the meniscus is involved. In fact 20% of all acute traumatic knee heamrthroses are caused by isolated meniscal injury. More typicall, however, knee swelling occurs approximately 1 day later as the meniscal tear causes mechanical irritation within the intrarticular space, creating a reactive effusion. Typically, this effusion is secondary to a lesion more in the central portion of the meniscus.
In contrast, degenerative meniscal tears are not classically associated with a history of trauma. In fact, the mechanism of injury, which may not be reported by the patient, can be simple dayly activities, such as rising from a chair and pivoting on a planted foot. Patients with degenerative tears often also report recurrent knee swelling, particularly after activity.
Physical examination aids in diagnosis of a meniscal injury accuretely in 70% of patients. Gait evaluation may reveal an antalgic gait with decreased stance phase and knee extension on the symptomatic side. A knee effusion is observed in about half of meniscal tear cases. Quadriceps atrophy may be noted a few weeks after injury. Palpation of the joint line frequently results in tenderness. Posteromedial or lateral tenderness is most suggestive of a meniscal tear. The result of a ''bounce home'' test may be positive. This test result is positive when pain or mechanical blocking is appreciated as the patients knee is passively forced into full extension. Classically, the result of the McMurray test is positive 58% of the time in the presence of a tear but is alaso reported to be positive in 5% of normal individuals. The Apley compression test is an insensitive indicator of meniscal injury. With this test, the prone knee is flexed to 90 degrees and an axial load is applied. A painful responce is considered a confirmatory test result with a reported sensitivity of 45%. No singular meniscal test has been showen to be predicitve of meniscal injury compared with findings of arthroscopy. Physical examination findings are less reliable in patients with concomitant ACL deficiencies.
Patients with meniscal injuries may have difficulty with deep knee bending activities, such as traversing stairs, squating, or toileting. In addition, jogging, running and even walking may become problematic, particularly if any rotational componetn is involved. Laborers who repetively squat may report mechanical locking with loss of full knee extension on rising.
ACL or PCL tears
Medial Collateral ligament tear
Fat pad impingement sydrome
Standing plain radiographs are often normal in isolated meniscal injuries. Presence osteoarthritis, as with degenerative meniscal tears can be detected with weight-bearing anteroposterior and lateral knee films.
With nondegenerative tears, MRI has largely replaced plain radiographic examination in tracing injuries.
Saggital views demonstrate the anterior and posterior horns of the menisci, coronal images can be vital in diagnosis of bucket handle and parrot-beak tears.
There are three grades of meniscal injury as detrmined by the location of T2 signal intenity within the black cartilage. By definition, only grade 3 tears qualify as true meniscal tears; however, a few grade 2 lesions seen on MRI will be found to be true tears on arhtroscopy.With use of arthroscopy as the ''gold standard'', the sensitivity of 83% to 93%. MRI appears to have a false-positive rate of 10% A 5% false-negative rate is also reported and may be due to the incidence of missed tears at the meniscosynovial junction.
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Buckup Klaus, M.DClinical Tests for the Musculoskeletal System Examinations—Signs—Phenomena © 2004 Thieme,ISBN 1-58890-241-2
Kapandji I.A, Churchill Livingstone. The physiology of joints vol.2 Lower Limb. Paris: Librairie, Maloine,Paris, 1987.0443036187