The meniscus is a fibrocartilaginous structure between femur and tibia with the important task of load distribution but potentially also shock absorption [1–3]. About 70% of the load passes through the medial tibiofemoral compartment and 30% through the lateral compartment in normally aligned knees [4]. While there is limited biomechanical studies of effects of different meniscus position per se, there is evidence that a displaced meniscus, e.g., due to a root tear, will no longer provide optimal load transmission in the knee, thus result in increased cartilage contact stress in similar manner as after partial or total meniscectomy [5, 6].

Substantial extrusion of the meniscal body, i.e., when the peripheral part of the mid portion of the meniscus is markedly located outside the tibial joint margin, is considered a structural feature that is strongly related to the incidence and progression of knee osteoarthritis (OA) [7–9]. A high degree of meniscal body extrusion on knee magnetic resonance (MR) imaging has been shown to be strongly associated with development of radiographic knee OA, cartilage loss, and the development of ipsilateral bone marrow lesions [10–12]. Potentially it may also be causing knee symptoms, e.g., by affecting the often pain sensitive knee capsule and synovial tissue [13]. Meniscal extrusion has also been reported to be associated with meniscal degeneration and extensive tears [14–16]. However, when discussing pathological meniscal position, one first needs to know more about meniscal position in the asymptomatic knee. For instance, medial meniscal extrusion of up to 3 millimetres (mm) is common in persons 40 years of age or older without knee symptoms [7]. Still, there is limited data published on this topic and also knee sizes and the sizes of menisci may vary considerably [17, 18]. Hence, the relative degree of meniscal extrusion may be very different between subjects even if the absolute measure of extrusion is identical. Also, to the best of our knowledge there are no studies of meniscal position over time in individuals free of knee OA.

We have provided reference values for medial and lateral meniscal body position in knees of asymptomatic persons without radiographic knee OA and without common risk factors for knee OA using a two-dimensional measurement technique on coronal MR images. Interestingly, we found that there was a small tendency to increased medial meniscal body extrusion over a 4-year period. However, most meniscal position parameters remained relatively stable. We also provide data for a relative measure of meniscal extrusion, i.e., which takes into account the fact that knee size (and size of menisci) may vary between subjects and an absolute measure of extrusion may not give justice to these size differences.

Since physiological degradation of meniscal tissue and its displacement may be a slow process, there is a need of more sensitive measuring methods of meniscal position than semi-quantitative scoring, which is incorporated in the most commonly used knee MR imaging scoring systems of knee OA [24, 25]. For instance, in subjects with knee OA or with risk factors for knee OA, within-grade changes in semi-quantitative MR imaging assessment are valid and may increase sensitivity in detecting longitudinal changes of cartilage and bone marrow lesions [26]. It is plausible that this is true also for meniscal changes. Full segmentation of meniscal volume and its relation to the tibial plateau can provide very detailed information but is time consuming, hence challenging for larger sample sizes or in daily clinical practice [27–29].

To our knowledge, there is not much written about the physiological meniscal position in the asymptomatic knee free of radiographic OA [30] and in particular its potential change over time. Several earlier studies distinguished between “minor” and “major” extrusion and in radiologic literature, it seems to have become common to define a threshold of 3 mm or more on coronal images as “pathological” extrusion [14]. Consequently, an extrusion of the meniscus body less than 3 mm is often regarded as of less clinical importance. This value is supported by results for meniscal extrusion measurements in a control group of asymptomatic persons, which however included subjects with evidence of radiographic OA changes [7]. The informative value about pathology of a “minor” extrusion is ambiguous. In contrast to the above-mentioned 3 mm cut-off is the 2 and 5 mm separation, which is used in the semi-quantitative MR imaging scoring systems BLOKS and MOAKS [24, 25]. Instead of an absolute measure, we would like to advocate for the use of a relative measure, similar to the early work addressing meniscal extrusion as seen on MR imaging [8]. In the report by Kenny, the ratio was constructed by dividing by the meniscus width. However, we propose to rather use tibia width (or ipsilateral tibia plateau width) in the coronal plane because meniscal body width may be affected by degeneration, tear or meniscus surgery. Thus, we are currently addressing this question in knees with and without radiographic OA to more accurately determine the most suitable measure to divide by and to define cut-offs for clinical and research use. Interestingly, in relative terms women tended to have more medial meniscal extrusion at the baseline exam compared to men, which also has been previously described by Bloecker et al. [18]. This tendency in our study was not evident unless taking into account knee size, i.e., using our proposed extrusion index construct. This finding may be important as knee OA is more common in women, and meniscus position and collateral ligament laxity may play important roles in pathogenesis.

Our finding of more medial meniscal extrusion in knees with medial meniscal tear is in agreement with previous cross-sectional reports, suggesting that tears are associated with increased meniscal extrusion [14–16]. Knees with meniscal tears in our study also tended to have less tibial plateau coverage compared to knees without tears. Between 40% and 70% of load has been reported to be transmitted by the menisci; the rest through direct contact of articular cartilage [31]. Being exposed to this great stress, its position (and coverage) plays an often critical role in maintaining a healthy knee. Our measurements suggest a mean coverage of 34% of width medially and of 31% laterally. While the lateral meniscus coverage is the same as suggested for patients with symptomatic knee OA, the medial coverage is 13% greater in asymptomatic knees compared to the results of symptomatic OA knees using the same measurement technique [12]. The wider range of coverage in sample of asymptomatic individuals most likely reflects the variety of anatomical shapes and dimensions of the tibial plateau and meniscus body [17, 18].

While meniscal coverage diminishes in OA affected subjects due to meniscal destruction and radial displacement [12], the coverage in asymptomatic knees at baseline stayed about the same during a 4-year time period, both medially and laterally. The tendency in the present study to increased extrusion of the medial meniscus body with time, not affecting coverage and overlap distance to the same extent, might potentially indicate increased bulging of the peripheral meniscal margin (more convex shape), i.e., not so much radial displacement (shift in position). Of note, meniscal extrusion is a combined construct of radial displacement and potential change in meniscus width, e.g., due to bulging of the peripheral meniscus margin and/or meniscus hypertrophy [32, 33]. The clinical relevance of our longitudinal findings is unknown, and is a question for forthcoming studies.

This study has important limitations that we would like to point out. Due to funding limitations and time constraints our aims included only the study of meniscal body position. Hence, the study of potential anterior and posterior horn movements is a question for future work. The number of knees with meniscal tear is small to provide robust estimates of association. Since knee alignment have great impact on biomechanical force transmission which may influence the degree of extrusion, a larger sample allowing enough numbers in different alignment categories would be valuable in future work [34]. For this study, we used a relatively straightforward two-dimensional quantitative measurement technique on the mid-coronal slice, i.e., a technique that does not consume as much time or needs specialized software as full volume segmentation, and therefore is more suitable for clinical practice. However, it does not provide as much detailed information as techniques doing full segmentation of the meniscus body [27–29]. The reading software we used (Efilm 3.4) only allowed measurements to the closest millimetre. As we are dealing with small measurements such as meniscal extrusion, measurements to a tenth of a millimetre would have been preferable using alternate imaging software. Our calculated coverage represents the % width of the ipsilateral tibia plateau that is covered by meniscus in the mid-coronal MRI slice. Although this variable is not representing the whole area of the meniscus covering the plateau, we suggest it likely provides a fairly representative proxy of the cartilage coverage of the meniscal body. Finally, the reader being familiar with the time sequence in which the images were taken, sensitivity to eventual changes in size and position was increased but potential bias cannot be excluded [35]. Any change in rotation between exams or misclassification of mid-coronal slice is expected to be non-differential which will only bias findings towards the null.

Using 3-Tesla MR imaging data from the OAI “unexposed” reference cohort, we provide data of meniscus medial and lateral body position in asymptomatic subjects including a new relative measure of extrusion as a ratio of extrusion by the tibia width. These measures may be useful tools in future studies of meniscus position in OA knees and for clinical practice. Further, our findings suggest that over a 4-year time frame there is on average a small increased medial body extrusion (mainly due to increased bulging) even in asymptomatic middle-aged or elderly subjects without knee OA at baseline. The association of these alterations with incidence of cartilage changes, radiographic OA etc. is a topic for further research as well as potential meniscal changes in position in younger adults.