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EPIDEMIOLOGY OF MENISCAL INJURIES

Knee injuries are a common concern resulting in over 1 million surgeries performed to the knee in the United States every year.^1-3 According to the National Athletic Trainers’ Association, knee injuries account for 10% to 19% of high school sports injuries and 60.3% of all high school athletic-related surgeries.⁴ Similar studies of collegiate sports have shown that knee injuries make up 7% to 54% of athletic injuries, varying by the nature of the sport.^5-9 The leading injuries to the knee, in both adults and children alike, are primarily patellofemoral derangements or ligament strains and tears.^10-12 Secondary to these injuries are meniscal tears, which have generated particular interest in both the young and elderly population as studies over the past several decades have revealed a rise in both degenerative and traumatic meniscal injuries. Meniscal tears occur as early as childhood, where they serve as the leading cause of pediatric arthroscopy, and increase with age and activity.^{13, 14} An estimated one sixth of knee surgeries are performed for lesions of the meniscus, and it is likely that many more remain untreated every year.^{15, 16} In one study of cadaver knees, untreated meniscal lesions were found in 34% of the autopsied subjects.¹⁷

A significant percentage of meniscal injuries result from athletic injury. On a professional level, meniscal tears accounted for 0.7% of all injuries sustained in the National Basketball Association, totaling 3,819 days missed by NBA athletes over a 10 year span.¹⁸ In college sports, studies conducted over a 16 year span by the National Collegiate Athletic Association Injury Surveillance System found internal knee derangement was second only to ankle sprains in both men’s and women’s college basketball and men’s and women’s soccer.^5-8 An independent study of college football had equally devastating statistics, reporting injuries to the meniscus in roughly one in five elite college football athletes.9 With participation in college sports on the rise, the number of meniscal injuries and subsequent surgeries are consequently rising at an alarming rate.¹⁹ Although athletes appear to have the highest instance of injury, meniscus injuries can happen anywhere, regardless of a person’s level of activity. A research study conducted in Greece showed that meniscal tears developed equally from traumatic and non-traumatic causes with 72% of all meniscal tears occurring during normal activities of daily living.²⁰

ANATOMY & FUNCTION

The menisci (plural of meniscus) are a pair of C-shaped fibrocartilages which lie between the femur and tibia in each knee, extending peripherally along each medial and lateral aspect of the knee. (See Figure 1.) The anatomy of both menisci is essentially the same, with the only exception being that the medial meniscus is slightly more circular than its hemispherical lateral counterpart. Each meniscus has a flat underside to match the smooth top of the tibial surface, and a concave superior shape to provide congruency with the convex femoral condyle. Anterior and posterior horns from each meniscus then attach to the tibia to hold them in place. The meniscus is comprised of approximately 70% water and 30% organic matter. This organic matter is primarily a fibrous collagen matrix consisting of type I collagen, fibrochondrocytes, proteoglycans, and a small amount of dry noncollagenous matter.^{21, 27} There has been a great deal of speculation and research dedicated to what exact function the meniscus serves, but today there is general consensus that the menisci provide stability in the joint, nutrition and lubrication to articular cartilage, and shock absorption during movement.^21-25 The menisci provide stability to the knee joint by both restricting motion and providing a contour surface for tibiofemoral bone tracking. The function of stability is shared with several ligaments which work together to prevent overextension of any motion. The transverse ligament connects the two menisci in the front of each knee and prevents them from being pushed outside of the joint at any point. Hypermobility is avoided through the connection of the medial collateral ligament (MCL) to the medial tibial condyle, femoral condyle, and medial meniscus, and the connection of the lateral collateral ligament (LCL) to the lateral femoral epicondyle and the head of the fibula; these ligaments provide tension and limit motion during full flexion and extension, respectively. The anterior and posterior meniscofemoral ligaments form an attachment between the lateral meniscus and the femur and remain taut during complete flexion. Lastly, the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are responsible for preventing too much backward or forward motion of the tibia.^{23, 24}

The menisci also provide shock absorption and stability by equally distributing weight across the joint. It is estimated that 45% to 70% of the weight-bearing load is transmitted through the menisci in a completely intact joint.²¹ By channeling the majority of this weight evenly, the meniscus is able to avoid placing too much direct stress at any one point of the knee. In turn, proper weight transmission in the knee reduces stress on any other joints in the body affected by load bearing.²⁵

One of the most vital roles of the meniscus is to provide lubrication to the knee, which it accomplishes through diffusing synovial fluid across the joint. Synovial fluid provides nutrition and acts as a protective measure for articular cartilages in the knee.²⁶ The femoral condyle in the knee is covered in a thin layer of articular cartilage, which serves to reduce motional friction and to withstand weight bearing. This cartilage is very susceptible to injury both because of its lack of proximity to blood supply and the high level of stress placed on it by excessive motion.^{27, 28} The meniscus, therefore, is able to provide a much-needed source of nutrition to the femoral and tibial articular cartilage by spreading fluid to that avascular area.

By acting as a spacer between the femur and tibia, the meniscus eliminates any direct contact between the bones, preventing any contact wear.²⁹ To see what effect the presence of the meniscus has on degeneration within the knee, researchers from the UK at the Institute of Medical and Biological Engineering conducted an in vitro study by mounting dissected bovine knee joints in a pendulum friction simulator and monitoring wear on knee cartilage both with and without a meniscus. Their results showed no change in surface integrity or loss of cartilage with an intact meniscus, but removal of the meniscus resulted in immediate surface wear and cartilage deterioration.³⁰

The ability to preserve the meniscus, unfortunately, is somewhat hampered by the fact that only a very small percentage (10% to 25% peripherally) of the meniscus receives direct blood supply.³¹ This area is often referred to as the red zone, and the inner portion of the meniscus which does not receive blood supply is referred to as the white zone. (See Figure 2.) While the red zone has a moderate chance of healing from injury, the white zone is almost completely incapable of healing itself in the event of injury.³²

Figure 1. Anterior aspect of the right knee.

Figure 2. Superior aspect of right knee showing red and white zones.