Meniscus injuries affect the young and physically active population. Although meniscus injuries are common in many sports, football, soccer, basketball, and wrestling are associated with the greatest risk. In an occupational setting, jobs requiring kneeling, squatting, and increased physical activity level have the greatest risk. Meniscus injury can be isolated to the meniscus or associated with other concomitant injuries, including anterior cruciate ligament tears and tibial plateau fractures. The frequency of meniscal repair is increasing because of a better understanding of meniscal pathophysiology, technological advancements, and a focus on meniscal preservation following injury to mitigate long-term consequences such as osteoarthritis.
Key points
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The incidence of meniscus injury ranges from 0.61 to 0.70 per 1000 person-years in the United States, and is common in the young and physically active population.
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The medial meniscus is 2 to 3 times more likely to be injured than the lateral meniscus.
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Radial and vertical tears are most prevalent with acute traumatic knee joint injuries, whereas horizontal and complex tears of the meniscus are most commonly associated with degenerative joint disease and osteoarthritis.
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Modifiable risk factors include activity level and body mass index, whereas nonmodifiable risk factors include age, gender, and anatomic factors, to include posterior tibial slope, medial meniscal slope, a biconcave medial tibial plateau, and knee malalignment.
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Football, soccer, basketball, and wrestling have the greatest incidence of meniscus injuries. Concomitant knee joint injuries, such as anterior cruciate ligament tears and tibial plateau fractures, are associated with high rates of meniscus injury.
Introduction
Historical Perspective
The first description of meniscus injury came in the late eighteenth century. Between 1782 and 1803, British surgeon William Hey published several articles describing “internal derangement” of the knee as being caused by “semilunar displacement.” The first detailed anatomic description of the meniscus came from the brothers Weber and Weber in 1836, in their book Mechanik der menschlichen Gehwerkzeuge . From the original description, the menisci were referred to as internal and external semilunar cartilage. Their anatomy and function were further examined by John Goodsir, Professor of Anatomy at the University of Edinburgh, in his anatomic memoirs in 1868. Goodsir and Lonsdale described the menisci as a “curved, yielding, but elastic railway” that moves with flexion and extension of the knee.
During the nineteenth century, the semilunar cartilage was a well-known cause of internal derangements of the knee. In 1889, British surgeon Herbert Allingham described the internal and external semilunar cartilages as the “most generally recognized causes of internal derangements of the knee-joint.” He stated that the knee may be “thrown out of gear, and from a cartilage moving too quickly or too slowly, or a condyle moving too slowly or too quickly, great damage may arise.” He also stated that meniscus injuries are “virtually neglected, and have failed to receive from most of the profession that consideration which they really deserve.” Up until 1885, meniscus injuries were treated by closed means, as originally described by William Hey, through closed reduction with flexion followed by extension of the knee and casting. Meniscal injuries are now frequently treated surgically.
Incidence and Prevalence of Meniscal Tears
The incidence of meniscus injury ranges from 0.61 to 0.70 per 1000 person-years in the general population in the United States to as high as 8.27 per 1000 person-years in young and physically active populations. Meniscal disorder ranges from the discoid meniscus in the pediatric population to traumatic tears in athletes. Asymptomatic, degenerative tears are common in older adults. In high school athletes, for whom most of the epidemiologic data exist, meniscus tears occur at a rate of 0.51 per 10,000 athlete exposures (AEs), accounting for approximately 10% to 20% of all knee injuries. Meniscus injuries occur in a variety of sports but are most frequent in football, soccer, basketball, and wrestling. Pathologic conditions, such as anterior cruciate ligament (ACL) tears and tibial plateau fractures, are associated with meniscus injury.
Not all tears are symptomatic and require treatment. With the advancement of MRI, meniscus tears are becoming easier to identify. In a study of patients aged 18 to 39 years, the prevalence of asymptomatic meniscus tears was 5.6%. In asymptomatic individuals, there is an increased prevalence of meniscal abnormalities on MRI with increasing age. In the elderly population, aged 50 to 90 years, the prevalence of meniscus tears is 31%. These tears tend to be horizontal and complex.
Epidemiology of Tear Types
Location (medial vs lateral)
When discussing the incidence of medial versus lateral meniscus injury, several factors come into play, to include mechanism, acuity of injury, and age. Despite these factors, the medial meniscus is consistently 2 to 3 times more likely to be injured compared with the lateral meniscus. In 1889, Allingham had 19 medial meniscus injuries, with only 6 lateral meniscus injuries, giving a ratio of 3:1. In 1948, Fairbank’s original study included 80 total medial meniscectomies and 27 lateral meniscectomies, producing a ratio of 3:1 medial to lateral meniscus injury. Over the years, this proportion has been documented in athletes, military service members, asymptomatic adults (aged 18–39 years), and symptomatic adults with knee osteoarthritis.
A few exceptions to the 3:1 rule exist. In acute and chronic ACL tears, the proportion of lateral meniscus injury is higher, producing a ratio closer to 1:1, medial to lateral meniscus tears. Tibial plateau fractures also have a higher proportion of lateral meniscus injuries. In contrast, in occupations requiring kneeling, a higher proportion of medial meniscus tears are present, up to 20:1 in a study of miners.
Type
Meniscus tears are categorized by type and location. Horizontal, complex, radial (and flap tears), vertical (longitudinal and bucket-handle tears), root tears, and ramp lesions are frequently described. Horizontal and complex tears are most common and typically exist on the osteoarthritis spectrum. Radial and vertical tears are common in acute injuries, whereas root tears and ramp lesions are typically higher-energy injuries associated with ACL tears. Unlike horizontal and complex tears, radial and vertical tears are typically seen in a younger population. These tear types are discussed here in their order of prevalence.
Horizontal tears run parallel to the tibial plateau. They are the most common tear type, encompassing 24% to 40% of all tears. Although most common, they exist on the spectrum of degenerative disease. Complex tears also exist on this spectrum. In a study of patients aged 50 to 90 years (Framingham study), the prevalence of meniscus tear was 31%. Most tears involved the posterior horns, and were horizontal (40%) and complex (37%). Ninety percent of patients with severe osteoarthritis had meniscal damage. In another study by Bergkvist and colleagues (all ages included), horizontal and complex tears encompassed 68% of all tears. In general, horizontal and complex tears are considered degenerative tears.
Radial tears are oriented perpendicular to the tibial plateau and the meniscal axis. They disrupt the longitudinal collagen bundles and hoop strength of the meniscus, resulting in increased meniscal extrusion and greater cartilage damage compared with horizontal tears. In the Framingham study (ages 50–90 years), radial tears accounted for 15% of all tears, compared with 7% in the study by Bergkvist and colleagues (all ages). Lateral radial tears are more common in younger patients and are associated with ACL injury. In the study by Bergkvist and colleagues, 36% of lateral radial tears had an associated ACL injury, compared with 8% with medial radial tears.
Vertical (longitudinal and bucket-handle) tears extend perpendicular to the tibial plateau and parallel to the meniscal axis. Similar to radial tears, they occur in a younger population. In the Framingham study, 7% of patients had a longitudinal tear, versus 25% in the study by Bergkvist and colleagues. The average age was 36 years for vertical tears. Vertical tears are also associated with ACL injury. Forty-five percent of ACL injuries present with vertical tears (including bucket-handle tears) to the medial meniscus, whereas only 28% occur on the lateral side.
Meniscal root tears occur within 1 cm of the bony tibial attachment and lead to significant meniscus extrusion, accelerating the development of arthritis. Compared with other tear types, they occur infrequently, at a rate of 1%. In degenerative arthritis, posterior root tears often occur on the medial side and are present in up to 6% of patients with osteoarthritis. Although root tears are not traditionally thought of as degenerative tears, medial meniscus posterior root tears (MMPRTs) occur with increased age, higher body mass index (BMI), increased varus, and lower sports activity level compared with other tear types. In the acute setting, meniscal root tears have been reported in multiligamentous knee injuries and in cases of hyperflexion and squatting.
Ramp lesions are meniscocapsular lesions in the posterior horn of the medial meniscus, often disrupting the meniscotibial ligamentous attachment. They occur frequently with ACL injury, occurring in 9% to 17% of all ACL tears. The MRI sensitivity of ramp lesions is only 48%, making the arthroscopic examination the gold standard. If ramp lesions are not recognized and treated, they can result in continued pain, dysfunction, and instability caused by altered knee joint kinematics.
Risk Factors
Modifiable risk factors
Both modifiable and nonmodifiable risk factors exist for meniscus injury ( Table 1 ). Increased activity level and BMI are considered the most significant modifiable risk factors. Meniscus injuries occur in a variety of sports but are most frequent in football, soccer, basketball, and wrestling. These activities require cutting, pivoting, and quick and unanticipated direction changes, which are associated with the mechanism of acute traumatic meniscus injury. Concomitant injuries to the knee joint, such as ACL tears and tibial plateau fractures, are also associated with meniscus injury. A more thorough discussion of modifiable and nonmodifiable risk factors for meniscal injuries is provided later.
| Risk Factor | Description |
|---|---|
| Modifiable | |
| Activity level | Increased activity level for most tears; decreased activity level for MMPRT |
| BMI | BMI>25 kg/m 2 |
| Nonmodifiable | |
| Age | Increased age; younger age may be protective, especially in cases of delayed ACL reconstruction |
| Gender | Men at increased risk; among gender-comparable sports, women may be at greater risk; women at risk for MMPRT |
| Anatomy | |
| PTS | PTS>13° may increase risk in ACL-deficient knees |
| MMS | MMS>3.5° may increase risk of ramp lesion in ACL deficiency |
| Biconcave medial tibial plateau | Biconcave plateau may increase risk of complex medial meniscus tears |
| Knee malalignment | <178° or >182°; higher risk of meniscal extrusion with valgus knees |
Activity level
Increased activity level is a recognizable risk factor for meniscus injury. This finding is evident from numerous studies in athletes, showing increased rates of meniscus injury compared with the general population. A good example of this comes from the military population, in which the incidence of meniscus injury is 8.27 per 1000 person-years. In the civilian population, the rate is 0.61 to 0.70 per 1000 person-years. This effect is magnified in cases of knee instability, such as ACL tear, in which cases of recurrent instability result in increased rates of meniscal damage.
Although increased activity level has been linked with most meniscus tears, lower sports activity level has been associated with MMPRT. Note that this factor may be confounded by increased BMI as a result of a sedentary lifestyle. Ultimately, in some instances of MMPRT, increased activity level may be protective against meniscus injury.
Body mass index
In athletes and nonathletes, BMI has been identified as a factor associated with meniscus injury. In a recent biomechanical study by Achtnich and colleagues, BMI correlated positively to medial meniscus extrusion. With meniscal extrusion, the normal function of the meniscus is impaired, leading to altered loading conditions within the knee. In a study of the general population, the probability of having meniscus surgery increased with a BMI greater than 27.5 kg/m 2 in men and 25 kg/m 2 in women. Patients with a BMI greater than 40 kg/m 2 had 15 times greater odds of having meniscus surgery. In a study of middle-aged individuals, meniscus lesions were found nearly twice as often in overweight and obese individuals. Of various meniscus injuries, MMPRT may have the greatest association with BMI, with a 4.9-fold increase with BMI greater than 30 kg/m 2 .
In athletes, this risk remains relevant. In a study of National Basketball Association (NBA) players, BMI was a significant predictor of meniscus injury risk. For players with a BMI greater than 25 kg/m 2 , the risk of meniscus injury increased from 1.6 to 2.2 injuries per 100 athlete-seasons. In a study of ACL reconstructed patients, increased BMI (>25 kg/m 2 ) was associated with a greater number of medial and lateral meniscus tears. Although this association exists in many studies, some studies have not shown an association between meniscus injury and BMI in athletes ; however, these studies may be underpowered for BMI given their homogenous patient populations.
Nonmodifiable risk factors
Age
Similar to rotator cuff tears, the prevalence of meniscus tears increases with age. In a population aged 50 to 90 years, the prevalence of meniscus tears was 31%, whereas, in a younger, asymptomatic population aged 18 to 39 years, the prevalence was 5.6%. In asymptomatic individuals, there is an increased prevalence of meniscal abnormalities and degenerative tears on MRI with increasing age. Although the prevalence increases with age, the incidence of acute, traumatic tears decreases with age. In boys and men, the peak incidence occurs between 12 and 30 years of age, compared with 11 to 20 years in women. This finding is likely caused by decreased activity level with increasing age. In contrast, in young, active populations with similar activity requirements, the incidence of meniscus tear seems to increase with increasing age. In a recent study, military service members more than 40 years of age experienced 4 times as many meniscus injuries as those less than 20 years old.
In cases of ACL injury, advancing age increases the risk of chondral damage and meniscus injuries. There is evidence to suggest that, in patients age 22 years and younger, there is a lower rate of meniscus injury in cases of delayed ACL reconstruction. As a result, younger age may have a protective effect on meniscus injury in cases of ACL injury.
Gender
The incidence of meniscus injury differs among male and female individuals. At the high school level, across multiple studies, there is a higher prevalence of meniscus injuries in boys than in girls. However, among gender-comparable sports, injury rates may be higher in girls. A recent study analyzed sex differences among high school and collegiate sports. Overall, the injury rates were slightly higher for male athletes at 0.68 per 10,000 AEs versus 0.53 per 10,000 AEs for female athletes. In the military, men are 20% more likely to sustain a meniscus injury. In patients undergoing ACL reconstruction, male sex is associated with more meniscus injuries than female sex at the time of surgery.
Although male athletes may be at greater risk for meniscus injuries than female athletes, adult women are at greater risk for MMPRT. In 1 study, female sex was associated with a 5.9-fold increase in risk for MMPRT, even higher than BMI greater than 30 kg/m 2 , which had a 4.9-fold increase in risk.
Anatomy
Anatomic factors may increase the risk of medial meniscus injury. These factors include posterior tibial slope (PTS), medial meniscal slope (MMS), a biconcave medial tibial plateau, and knee malalignment. The discoid meniscus as a risk factor is discussed later. In theory, the shape of the tibial plateau and meniscus affects force transmission across the menisci, especially in cases of ACL deficiency.
Posterior tibial slope and medial meniscal slope
In cases of ACL deficiency, every 10° increase in PTS leads to an additional 6 mm of anterior tibial translation. In turn, increased PTS has been associated with an increased incidence of posterior horn medial meniscus tears. In 1 study, PTS greater than 13° was a risk factor for secondary meniscus tears in ACL-deficient knees. At present, PTS remains a controversial risk factor for meniscus injury, partially because of the challenges associated with measuring PTS, which lead to substantial variability. Similarly, medial meniscal slope may also be associated with an increased risk for meniscus tears in combination with ACL injury. Patients with an ACL injury and ramp lesion had an MMS of 3.5°, which was larger than that of patients with an isolated ACL tear (2°). This effect was greater in cases of delayed ACL reconstruction (>6 months from injury). There was no association between increased PTS and ramp lesion. It is hypothesized that an increased MMS may predispose the posterior horn of the medial meniscus to higher stress, increasing its risk for injury.
Biconcave medial tibial plateau
A biconcave medial tibial plateau is characterized by a coronal plane tibial plateau ridge separating the anterior and posterior portions of the tibial plateau. The biconcavity can either be cartilaginous or bony. In a study by Barber and colleagues, patients with biconcave medial tibia plateaus had more complex medial meniscus tears (69%) than those without biconcavity (53%). The group with biconcave plateaus had a lower average BMI, making confounding unlikely. Given the paucity of literature on this topic, more research is needed to confirm these findings.
Knee malalignment
Knee malalignment increases load transmission in the medial or lateral tibiofemoral compartment, resulting in increased meniscal extrusion with greater malalignment. Meniscal extrusion increases the risk for meniscus injury. In the Multicenter Osteoarthritis Study, both varus and valgus malalignment (<178° or >182°) was associated with meniscal extrusion. Valgus malalignment was associated with a higher risk of meniscal extrusion than varus malalignment. Moreover, there was a strong association with increasing malalignment and progressive meniscal damage, especially in valgus knees. Some studies have found that mild malalignment (<3° from neutral) does not increase meniscal extrusion ; however, these studies do not apply to cases of moderate to severe malalignment.
Activities Associated with Meniscus Injury
Sports
High school sports
In high school sports, the rate of knee injuries is 2.98 injuries per 10,000 AEs. Meniscus tears occur at a rate of 0.51 per 10,000 AEs, accounting for approximately 10% to 20% of all knee injuries. They occur more frequently in competition and during player-on-player contact. In a large study of more than 21 million AEs, boys’ football, girls’ soccer, girls’ basketball, and boys’ wrestling had the greatest incidence of meniscus injuries ( Table 2 ). Cheerleading, track and field, and swimming had the lowest incidence of injuries. Although the incidence of meniscus injury was greater in boys, among gender-comparable sports, the incidence was higher in girls. In girls’ soccer, basketball, softball, lacrosse, and track and field, meniscus injury risk was twice that of their male counterparts.
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