INJURY
Tears are the most common form of meniscal injury, and are generally classified by appearance into four categories: longitudal tears (also referred to as bucket handle tears), radial tears, horizontal tears, and oblique tears.33 (See Figure 3.) Research indicates that radial or horizontal tears are more likely to occur in the elderly population while younger patients have a higher incidence of longitudal tears.34-36 Each can be further described as partial thickness tears or complete thickness tears, depending on the vertical depth of the tear. (See Figure 4.) Meniscal damage can be caused by either trauma or gradual degeneration. Traumatic injury is most often a result of a twisting motion in the knee or the motion of rising from a squatting position, both of which place particular strain and pressure on the meniscus. More often than not, traumatic injuries occur during athletic activity. The ratio of degenerative to traumatic tears increases from equal incidence in those under 20 years of age to a ratio of 7:8 in the 30 to 39 age group, to nearly 4:1 in individuals over the age of 40.20 This pattern of increased degenerative breakdown is to be expected with age, as joint wear will result from years of mechanical stress. Unlike the anatomy of younger and more active patients, however, the fibers in older patients are less capable of healing themselves, due to decreased diffusion of synovial fluid with lessened motion.37
A basic ability to identify meniscal tear symptoms is essential for diagnosis and treatment of injury. (See Figure 5.) The first symptom typically indicative of a meniscal tear is pain. In the case of a traumatic tear, pain may present immediately at the time of injury and is often accompanied by an audible pop. In a degenerative tear, the onset of pain may be more gradual, with no definite moment of injury. In both cases, pain may be accompanied by swelling and subsequent limitation in range of motion. Another hallmark of meniscal tears is clicking, popping, or locking in the knee joint. These symptoms are most likely a result of a torn flap of meniscal tissue which catches in the joint during movement. Instability and weakness are also both common symptoms because a damaged meniscus, as well as damaged ligaments and tendons, inhibits normal mechanical function.
The severity of initiating trauma, as well as the nature and characteristics of the tear, plays an important role in the meniscus’ ability to heal. (See Figure 6.) Tears that are shorter, partial thickness, and located in the vascular red zone have a much better chance of healing than extensive, complete thickness tears located in the white zone.38, 39 When other cartilages and ligaments are injured in the knee, this can also have a detrimental effect on the meniscus’ ability to heal on its own. Because of the interdependence of each of the knee’s mechanisms, meniscal injuries often occur in conjunction with other internal ligament damage; the most common example of this is O’Donoghue’s “unhappy triad,” the correlated injury of the meniscus (debatably either medial or lateral), tibial collateral ligament, and ACL.40-42 The severity of meniscal lesions has been found to increase in direct proportion to ACL injury and/or laxity, and create less favorable conditions for repair.43 Furthermore, previous injury to either the meniscus or any other ligament inside the knee can increase the risk of future injury to the meniscus, even if the injury has healed or been surgically repaired.
Another condition which can be both a cause and complication of meniscal tears is a discoid meniscus. (See Figure 7.) A discoid meniscus occurs when the lateral meniscus takes on the shape of a disc, rather than a crescent, and is most often manifested in adolescence.44 Although the cause has never been officially determined, the repercussions of a discoid meniscus have been widely documented. Often referred to as “snapping knee syndrome,” this condition is identified with its only symptom, snapping on extension. The “snap” is caused when the femur and the meniscus are not able to move in sync with each other and the femur either slips over a ridge in the meniscus or off of the meniscus altogether.45 Unlike the normal meniscus, which is shaped to fit the condyle of the femur, a discoid meniscus lacks the configuration to serve as a stable surface for motion. This abnormal tracking adds stress on the meniscus, increasing the probability of lateral meniscus tears.46 Unfortunately, discoid menisci often remain undetected when no symptoms present prior to injury, and the only other way to identify a discoid meniscus is by magnetic resonance imaging (MRI).
| Figure 3. Common types of meniscal tears. |
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| Figure 4. Depths of tears in the meniscus. |
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| Figure 5. Symptoms of meniscus tears. |
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| Figure 6. Factors affecting the healing of a torn meniscus. |
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| Figure 7. Discoid meniscus of right knee. |
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