The menisci are C-shaped disks of fibrocartilage that are believed to represent extensions of the tibia that serve to deepen the articular surfaces of the tibial plateau to better accommodate the condyles of the femur. The meniscus has been shown to play a vital role in load transmission across the knee joint [1]. Biomechanical studies have shown that at least 50% of the compressive load of the knee joint is transmitted through the meniscus in extension, whereas approximately 85% of the load is transmitted in 90° of flexion [2]. The absence of the meniscus increases the peak pressure in the knee joint. Resection of as little as 15–34% of the meniscus increased contact pressures by more than 350% [3].

Reports in the literature about meniscal subluxation focus on meniscal tears, meniscal root avulsion, and hypermobility of the anterior horn associated with meniscal extrusion [4, 5]. The terminology in the literature is varied, with “meniscal subluxation” and “meniscal extrusion” used interchangeably. In our study, we used only the term “meniscal extrusion” to refer to displacement of 3 mm or more of the meniscus from the central margin of the tibial plateau as measured in the coronal plane. Extrusion of the meniscus after trauma has also been reported [5]. In older patients with knee pain, reports have suggested an association between effusion, osteoarthritis, and meniscal extrusion [6].

This study was prompted by the observation of meniscal extrusion on an MRI examination of rugby players performed before contract signing and questions on its clinical significance by one of the authors and the orthopedic surgeons. The aim of this study was to assess the rate of meniscal extrusion in young athletes and an association between meniscal extrusion and common knee joint abnormalities.

Two musculoskeletal radiologists retrospectively and independently evaluated the MR images of a total of 76 knees (47 patients). The study group consisted of 24 professional athletes (15 soccer players and nine rugby players; all men) who are currently free of pain in the knee, impaired mobility, and joint swelling and 23 age-matched active individuals (18 males, 5 females) with no history of knee problems who had been referred from the orthopedics department. All players were on a professional team and had undergone MRI as part of an examination before signing a contract. Meniscal tears were suspected in some, but no clear diagnosis was made after evaluation of the case history. The mean age of the athletes was 29.5 years (range, 26–36 years) and that of the control group was 26.2 years (range, 15–41 years).

The MR studies were performed on a 1.5-T unit (Signa Horizon LX, GE Healthcare). A knee coil was used in all the athletes and subjects. The examination protocol included sagittal and coronal T1-weighted spin-echo sequences (TR range/TE range, 500–600/15–20) and a sagittal T2-weighted gradient-echo sequence (TR range/TE, 450–700/20; flip angle, 30°), with a 4-mm section thickness and a 0.4-mm gap. The field of view was 18 cm, with a matrix size of 256 × 256.

In the coronal plane, measurements were made using the technique described by Breitenseher et al. [6]. The criterion for meniscal extrusion was a distance of 3 mm or more between the peripheral border of the meniscus and the central margin of the tibial plateau as measured in the coronal plane (Fig. 1). A distance of less than 3 mm was not considered as meniscal extrusion. An average value from the three central images for the distance of each meniscus from the edge of the tibial plateau was calculated for the distances from the medial and lateral menisci to the edges of the tibial condyles.

An internal meniscal signal extending to the articular surface was considered a meniscal tear. Meniscal extrusion was correlated with degenerative changes, meniscal tears, joint effusion, and other knee joint injuries. Degenerative change was defined as subchondral sclerosis, marginal osteophytes, or cartilage loss in our study. Only cases of moderate or large joint effusion were considered in our study; cases of small physiologic joint effusion were not considered in our study. Only high-grade or transected anterior cruciate ligament (ACL) tears were considered as ACL tears in our study.

Statistical analyses of the measurements were obtained using STATA software (version 7, StataCorp), and a Fisher's exact test was used to assess the significance of the associations between meniscal extrusion and knee joint abnormalities. For analyses, the knees were treated as independent of each other.

Forty knees of athletes and 36 knees of the control subjects were imaged in total. Forty-eight percent of the athletes' knees (13 athletes, 19 knees) showed evidence of meniscal extrusion. In the athletes' knees, 12 lateral menisci (63%; mean distance, 4.04 mm) and 15/20 medial menisci (75%; mean distance, 4.29 mm) were extruded.

In the athletes, a significant association between joint effusion and meniscal extrusion was found (p = 0.004). Among the knees with effusion, 79% (11 knees) shown meniscal extrusion, whereas only 21% (three knees) did not contain meniscal extrusion. A significant association was found between meniscal tears and meniscal extrusion (p = 0.01), with 87% of knees in athletes with meniscal extrusion and only 13% of knees of those without meniscal extrusion containing a meniscal tear. A significant association was found between ACL tear and subluxation (p = 0.04), with 20% (four knees) in athletes with meniscal extrusion containing ACL tears.

Seventy-eight percent of the control subjects' knees (18 knees) showed meniscal extrusion. No significant association was found between meniscal extrusion and joint effusion (Fig. 2), meniscal tear, or ACL tear (p = 1.00, 0.99, and 0.99, respectively). No significant association was found between subluxation and degenerative change or patellar tendinitis in either the athletes (p = 0.7378 and 0.1722, respectively) or the control subjects (p = 1.00 and 1.00, respectively). The results are summarized in Tables 1 and 2.

A review of the anatomy of the menisci is important to understand the mechanism of meniscal extrusion. In addition to firm attachments to the tibia by its anterior and posterior horns, the medial meniscus is also attached to the transverse ligament and the joint capsule throughout its length. At its midpoint, the medial meniscus is more firmly attached to the femur and the tibia through a condensation in the joint capsule known as the deep medial collateral ligament. In contrast, the lateral meniscus is attached to the tibia at its anterior and posterior horns with only a loose peripheral attachment to the joint capsule [7]. This difference in attachment to the capsule has led some to suggest that the medial meniscus is more prone to subluxation due to displacement of the medial capsule by fluid or osteophytes. Miller et al. [8] reported statistically significant differences in medial and lateral meniscal extrusions in patients older than those in our study group. There is little difference between the number of subluxed lateral and medial menisci in our study. Joint effusion does not appear to predispose athletes to medial meniscal extrusion over lateral meniscal extrusion. Miller et al. suggested that meniscal extrusion may be related to the presence of large osteophytes, but their criteria for meniscal extrusion included displacement in either the coronal or sagittal plane. Large degenerative osteophytes, however, were not present in our younger study group.

Studies have suggested that meniscal extrusion is not an abnormality itself in subjects with knee pain but instead is a secondary finding that is not rare and is indicative of other knee joint abnormalities, such as joint effusion and osteoarthritis [6, 8]. In our study, the lack of a significant association between meniscal extrusion and degenerative change or joint effusion in the control group suggests that this theory may not be so. Furthermore, as the knee passes through a range of motion, normal movement of the menisci is only in the anteroposterior plane and true radial displacement in the coronal plane does not occur [9]. All this suggests that meniscal extrusion may be an abnormal movement of the meniscus and could be a pathologic finding.

An important mechanism that contributes to the stability of the meniscus is its intrinsic structure. Circumferentially oriented collagen fibers in the meniscus provide significant resistance to hoop stresses. They counteract the compressive forces generated by the femur and tibia and the resultant tendency to radial meniscal displacement. A single cut or tear to the radial edge of the meniscus eliminates these hoop stresses and contributes to subluxation of the meniscus [10].

Meniscal tears with the elimination of hoop stresses and greater or more sustained compressive forces generated by athletes within the knee could explain the significant correlation found in our study between meniscal tears and subluxation in athletes. In the control population, a meniscal tear per se without the compressive forces may be insufficient to cause meniscal extrusion, as suggested by the poor correlation of meniscal tears and subluxation seen in our study. The lack of these forces, which may be generated in athletes playing a sport at a high level, could also explain the difference in correlation between ACL tears and meniscal extrusion in athletes and the control group (Table 3).

Meniscal extrusion has been reported in older subjects with symptomatic knee osteoarthritis, and data from reports suggest that meniscal extrusion precedes the development of degenerative joint disease [4, 10]. Laxity of the soft-tissue attachments, tears of the meniscus, and sufficient degenerative change within the meniscus leading to disruption of collagen fibers are believed to be contributing factors for meniscal extrusion in these subjects [11]. Another study of subjects with developing osteoarthritis suggested that it is after meniscal extrusion that unimpaired impaction of cartilage occurs and leads to the well-recognized changes of osteoarthritis in older patients [12].

The young age of individuals in our study could explain the lack of significant association between subluxation and degenerative changes in both the athletes and the control subjects. Follow-up studies need to be performed to determine whether uncorrected meniscal extrusion can lead to unimpaired cartilage impaction and early degenerative change in athletes, thereby shortening their careers.

Limitations of this study are the small numbers in the study group because of selection from only the local premier soccer and rugby league teams. Because meniscal extrusion was assessed on non-weight-bearing MR images, the true amount of meniscal extrusion may have been underestimated. The full extent of meniscal dynamics could not be assessed because of the constraints imposed by the bore of the magnet. The use of open bore magnets may help overcome these limitations. The significance of the type of peripheral meniscal tears (i.e., root tear, radial tear) and its association with meniscal extrusion were not assessed in this study and may warrant further analysis. Another limitation was the lack of a gold standard, such as results from arthroscopy. Further longitudinal studies are required to assess the association of meniscal extrusion and the onset of degenerative joint disease in athletes.

In conclusion, we suggest that meniscal extrusion is associated with meniscal tears and joint effusion in athletes. Its recognition in athletes may be significant.

Address correspondence to W. J. Rennie ([email protected]).