MENISCAL INJURIES

DEFINITION


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.


SYMPTOMS


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

 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.


FUNCTIONAL LIMITATIONS


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.


DIFFERENCIAL DIAGNOSIS


ACL or PCL tears
Medial Collateral ligament tear
Osteoarthritis
Plica Syndromes
Popliteal tendinitis
Osteochondritic lesions
Loose bodies
Patellofemoral pain
Fat pad impingement sydrome
Inflammatory arthritis
Physeal fracture
Tumors


DIAGNOSTIC STUDIES
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.

A 3D illustration of a bucket handle tear demonstrates that these tears actually are longitudinal in nature (arrows), coursing parallel to the c-shaped fibers of the meniscus. Displacement of the inner rim of the tear (arrowheads) results in the classic "bucket-handle" configuration



The parrot beak shape of an oblique tear (arrow) is readily apparent on (G) a proton density-weighted axial image of the menisci.




TREATMENT
Initial
The truly locked knee resulting from meniscal tear should be reduced within 24hours oh injury. Otherwise, acute tears of the meniscus may initially be treated with rest, ice, and compression, with weight bearing as tolerated. Patients may be need to use crutches acutely. A knee splint may be applied for comfort of the patient, particularly in ustable knees with underlying ligamentous injury. 
Analgetics such as acetaminpphen or opioids can be used for pain and inflammation.
Arthrocentesis can be performed (ideally in the first 24 to 48 hours) for both diagnostic and treatment purposes when there is a significant effusion.


REHABILITATION


Not all meniscal injuries necessitate surgical interention or resection. In fact, some meniscal lesions have gradual resolution of symptoms during a 6-week period and may normal function by 3 months. Types of tears that may be treated with nonsurgical measures include partial -thickness longitudinal tears, small (<5mm) full thickness peripheral tears, and minor inner rim or degenerative tears. Healing potential is greatest for tears within the red zone. In general. only symptomatical meniscal injuries should be reffered for surgical intervention.


Both nonsurgical and partial meniscectomy patients should undergo similar rehabilitation protocols. Crutches may be used to off-load the affected limb. These can usually discontinued when patients are ambulating without a limp. The goal during the 1st week is to decrease pain and swelling while increasing range of motion and muscle strength and endurance. 
Aerobic conditioning can begin  only if  the patient can tolerate bicycle training or aqua-jogging. As time progresses, a combination of open and closed kinetic chain exercises in all three planes (saggital,coronal and transverse) can be performed in combination with stretching of the lower limb. Gradually and in time, more functional activities are introduced. More challenging proprioceptive and balance activities also can be started as deemed appropriate. Finally plyometric training is started, and the individual is gradually introduced back into sport-specific activities.


Many rehabilitation protocols for the surgically repaired meniscus have been described. Rehabilitation programs ideally need to be individualized to the specific type of repair performed. In addition, there has been considerable controversy among physicians about the patient's weight-bearing and immobilization status soon after the surgical repair. In general, however, initial exercises are nonaggressive, avoiding dynamic shear forces that may occur fro joint active range of motion. Therefore, exercises are initally static, targeting hip abductors, adductors and extensors. Static quadriceps exercises are performed with care to avoid terminal knee extension. While superior and medial patella mobilization is begun, stretching of the lower limp musculature in multiple planes is emphasized. After 2-3 weeks, goals are to increase range of motion and to advance weight-bearing status while a resistance exercise program is to introduced. With the absence of effusion and significant pain, improved knee range of motion from 5 to 110 degrees should be achieved.
More aggressive active exercises could be applied if the surgical repair was in the peripheral vascular zone of the meniscus, since there the healing rate is much higher. More proprioceptive, neuromuscular facilitation activities (PNF)  can be implemented, ensuring that the patient is rehabilitated in all three planes.
By 6-8 weeks, low--impact funtional activities that entail components of the patient's sport or activity are introduced. Brace protection, if it was initially employed may be removed, particularly when the patient demonstrates success with proprioceptive testing. Running, cutting and rotational activitiesare avoided. Athletes may be able to return to their individual activities at about 16 weeks for those with repairs in the vascular zone and 24 weeks for those with repairs in the nonvascular zone.


POTENTIAL DISEASE COMPLICATIONS


Once a meniscal tear occurs, the joint inherently becomes less stable. This instability may promote further extension of the initial tear, turning a nonsurgical lesion into one in which with arthroscopic repair may be necessary. Chronically, the resultantan increased abnormal motion that occurs secondary to the meniscal injury may also lead to damage of the articular surface and predispose to premature osteoarthritis.


POTENTIAL TREATMENT COMPLICATIONS


Analgetics, such as acetaminophen and NAIDS have well-known side effects that may affect the gastric, hepatic and renal systems. Ife the clinician is unfamiliar with appropriate rehabilitation strategies, an overly aggressive regimen may lead to extension of the tear or failure of the meniscus to heal.
A rehabilitative programe that is too conservative, in contrast may also lead to a significant loss of strength with muscle atrophy and decreased range of motion. If the surgical approach resulted in a significant amount of cartilage removed, the knee may be predisposed to development of osteoarthritis as originally described by Fairbank back in 1948. Saphenous nerve injuries as well as infections are also common complications after meniscal repair surgery and arthroscopy.






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