Background
Understanding of the importance of the menisci in the biomechanics of the knee has progressed steadily since 1968, when Jackson wrote, "The exact function of that structure (meniscus) is still a matter of some conjecture."[1] At that time, it was common to remove the entire substance if any doubt existed regarding the integrity of the meniscus. Today, it is known that the menisci are not optional or expendable structures; they have an integral role in normal knee joint mechanics. The physician treating an athlete with a known or suspected meniscal tear needs to understand the structure and function of the meniscus and the factors involved in treating an athlete with nonoperative versus operative treatment. This article presents a program for rehabilitation after meniscal injuries, meniscectomy, and meniscal repair based on current knowledge of knee biomechanics. (See images below.)
Magnetic resonance imaging scan showing a normal meniscus. 
Magnetic resonance imaging scan showing a torn medial meniscus. Pathophysiology
Anatomy
The menisci are C-shaped wedges of fibrocartilage located between the tibial plateau and femoral condyles. The menisci contain 70% type I collagen. The larger semilunar medial meniscus is attached more firmly than the loosely fixed, more circular lateral meniscus. The anterior and posterior horns of both menisci are secured to the tibial plateaus. Anteriorly, the transverse ligament connects the 2 menisci; posteriorly, the meniscofemoral ligament helps stabilize the posterior horn of the lateral meniscus to the femoral condyle. The coronary ligaments connect the peripheral meniscal rim loosely to the tibia. Although the lateral collateral ligament (LCL) passes in close proximity, the lateral meniscus has no attachment to this structure.
The joint capsule attaches to the entire periphery of each meniscus but adheres more firmly to the medial meniscus. An interruption in the attachment of the joint capsule to the lateral meniscus, forming the popliteal hiatus, allows the popliteus tendon to pass through to its femoral attachment site. Contraction by the popliteus during knee flexion pulls the lateral meniscus posteriorly, avoiding entrapment within the joint space. The medial meniscus does not have a direct muscular connection. The medial meniscus may shift a few millimeters, while the less stable lateral meniscus may move at least 1 cm.
In 1978, Shrive et al reported that the collagen fibers of the menisci are oriented in a circumferential pattern.[2] When a compressive force is applied in the knee joint, a tensile force is transmitted to the menisci. The femur attempts to spread the menisci anteroposteriorly in extension and mediolaterally in flexion. Shrive et al further studied the effects of a radial cut in the peripheral rim of the menisci during loading. In joints with intact menisci, the force was applied through the menisci and articular cartilage; however, a lesion in the peripheral rim disrupted the normal mechanics of the menisci and allowed it to spread when a load was applied. The load now was distributed directly to the articular cartilage. In light of these findings, it is essential to preserve the peripheral rim during partial meniscectomy to avoid irreversible disruption of the structure's hoop tension capability.
Blood supply
The blood supply to the menisci is limited to their peripheries. The medial and lateral geniculate arteries anastomose into a parameniscal capillary plexus supplying the synovial and capsular tissues of the knee joint. The vascular penetration through this capsular attachment is limited to 10-25% of the peripheral widths of the medial and lateral meniscal rims. In 1990, Renstrom and Johnson reported a 20% decrease in the vascular supply by age 40 years, which may be attributed to weight bearing over time.[3]
The presence of a vascular supply to the menisci is an essential component in the potential for repair. The blood supply must be able to support the inflammatory response normally seen in wound healing. Arnoczky, in 1982, proposed a classification system that categorizes lesions in relation to the meniscal vascular supply.[4]
- An injury resulting in lesions within the blood-rich periphery is called a red-red tear. Both sides of the tear are in tissue with a functional blood supply, a situation that promotes healing.
- A tear encompassing the peripheral rim and central portion is called a red-white tear. In this situation, one end of the lesion is in tissue with good blood supply, while the opposite end is in the avascular section.
- A white-white tear is a lesion located exclusively in the avascular central portion; the prognosis for healing in such a tear is unfavorable.
Repair of lesions in the red zone has yielded good results, according to Stone. Reports describe techniques for manufacturing a vascular access channel from the peripheral vasculature to improve the chance that tissue in the central region will repair itself.
Biomechanics
The menisci follow the motion of the femoral condyle during knee flexion and extension. Shrive et al presented a model of normal meniscal function.[2] During extension, the femoral condyles exert a compressive force displacing the menisci anteroposteriorly. As the knee moves into flexion, the condyles roll backward onto the tibial plateau. The menisci deform mediolaterally, maintaining joint congruity and maximal contact area. As the knee flexes, the femur externally rotates on the tibia, and the medial meniscus is pulled forward. Studies by Shrive, Fukubayashi, Walker, and Kurosawa state that the menisci directly influence the transmission of forces, distribution of load, amount of contact force, and pressure distribution patterns.
Mechanism of injury
Meniscal injuries, particularly sports-related injuries, usually involve damage due to rotational force. A common mechanism of injury is a varus or valgus force directed to a flexed knee. When the foot is planted and the femur is internally rotated, a valgus force applied to a flexed knee may cause a tear of the medial meniscus. A varus force on a flexed knee with the femur externally rotated may lead to a lateral meniscus lesion. According to Ricklin, the medial meniscus is attached more firmly than the relatively mobile lateral meniscus, and this may result in a greater incidence of medial meniscus injury.[5]
Epidemiology
Frequency
United States
Although the exact incidence and prevalence of meniscal injury are unknown, it is a fairly common sports-related injury among adults. Although less common than in adults, knee meniscal injuries do occur in individuals who are skeletally immature. Meniscal injuries are rare in children younger than 10 years with morphologically normal menisci.[6]
Mortality/Morbidity
Meniscal injuries usually are associated with pain that results in gait deviation and loss of time from work and/or sport.
Race
A correlation of race and meniscal injuries is not known to exist.
Sex
Meniscal injuries are more common in males, which may be a reflection of males being more involved in aggressive sporting and manual activities that predispose to rotational injuries of the knee.
Age
Meniscal injuries are common in young males who are involved in sporting or manual activities. A second peak of incidence is observed in elderly persons older than 55 years; this incidence is secondary to a degenerate meniscus being susceptible to injuries with minor trauma.[7] Meniscal injuries are rare in children younger than 10 years with morphologically normal menisci.[6]
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