Medial Meniscus Extrusion on Knee MRI: Is Extent Associated with Severity of Degeneration or Type of Tear?
Abstract
OBJECTIVE. The meniscus is considered “extruded” when it extends beyond the tibial margin. We hypothesize that severe degeneration, large radial tears, complex tears, and tears involving the meniscal root would alter meniscal stability and cause more substantial extrusion.
MATERIALS AND METHODS. The knee MRI database at Thomas Jefferson University Hospital was searched for reports describing meniscal extrusion; MR images were reviewed retrospectively. On mid coronal images, extrusion of the medial meniscus was quantified in millimeters. A separate, independent review of the meniscus evaluated degeneration severity and tear (type and extent). Radial tears were divided into those involving more (large) or less (small) than 50% of the meniscal width. Tears that involved the “root” at the tibial spine were recorded. Chi-square analysis compared these findings with extrusion extent, divided into minor (≤ 3 mm) and major (> 3 mm) extrusion.
RESULTS. One hundred five knees were reviewed (12 men and 93 women; age range, 34–83 years; mean age, 56 years). Distribution of medial meniscus extrusion was 2 mm (n = 17), 3 mm (n = 17), 4 mm (n = 27), 5 mm (n = 14), 6 mm (n = 16), and 7–10 mm (n = 14). Mild, moderate, or marked degeneration was seen in 47%, 26%, and 27% with minor extrusion, respectively, and in 17%, 41%, and 42% with major extrusion, respectively (p = 0.003). Tears were seen in 59% (20/34) with minor extrusion versus 89% (63/71) with major extrusion (p = 0.001). Tears involved one third, two thirds, or all of the meniscus in 75%, 25%, or 0%, respectively, with minor extrusion and 46%, 40%, or 14% with major extrusion, respectively (p = 0.014). Longitudinal (nonradial) and horizontal tears were not associated with extent of extrusion (p = 1.0). Oblique tears were significantly associated with minor extrusion (minor, 26% [9/34]; major, 4% [3/71]; p = 0.003). Radial tears were seen in 9% (3/34) with minor extrusion versus 21% (15/71) with major extrusion (p = 0.20). All three radial tears with minor extrusion were small; conversely, 87% (13/15) of radial tears with major extrusion were large (p = 0.019). Complex tears were seen in 18% (6/34) with minor extrusion versus 59% (42/71) with major extrusion (p < 0.001). Tears involving the meniscal root were seen in 3% (1/34) with minor extrusion and 42% (30/71) with major extrusion (p < 0.001).
CONCLUSION. Substantial medial meniscus extrusion (> 3 mm) is associated with severe meniscal degeneration, extensive tear, complex tear, large radial tear, and tear involving the meniscal root.
Introduction
The meniscus is an integral part of the complex biomechanical system of the knee, crucial in sharing the force load by increasing the contact surface area and providing uniform distribution of weight bearing across the articular surfaces [1–3]. Damage to the meniscus, resulting in alteration of this function, modifies the pattern of load distribution, contributing to compartmental instability [1]. The meniscus is considered “extruded” when it extends beyond the tibial margin; this results from disruption of collagen fibers within the meniscus that provide hoop strength and may be associated with the presence of osteoarthritis [4].
We hypothesized that severe degeneration and certain tears that significantly alter meniscal mechanics would cause more substantial extrusion. Our purpose was to determine if the extent of meniscal extrusion is associated with the type of meniscal abnormality.
Materials and Methods
Sample Selection
We selected patients using an automatic key word search of the radiology information system at our institution (from January 2001 to February 2002) for knee MRI reports describing meniscal extrusion. Reports referring to prior surgery or artifact were excluded from the initial search results. The search methodology yielded 105 examinations, which constituted the study sample. There were 12 men and 93 women with an age range of 34–83 years (mean age, 56 years).
The medial meniscus was chosen as the subject of this study for a number of reasons: The mechanics of the medial and lateral meniscus are very likely different, and we sought to achieve a more homogeneous group. We wanted to exclude variation in measurement of extrusion from the tibial margin, which might occur if medial and lateral measurements were combined. Meniscal extrusion is more functionally significant medially because the medial compartment sustains more weight-bearing stress and medial compartment abnormality is more common than lateral compartment abnormality.
MRI Protocol
MRI examinations of 89 knees were performed with a 1.5-T superconducting magnet (Signa, General Electric Medical Systems) using a quadrature extremity coil. MRI protocol incorporated the following sequences: sagittal spin-echo intermediate-weighted (TR range/TE, 1,000–1,200/20; 4-mm section thickness; 1-mm interslice gap; 256 × 192 matrix; 14-cm field of view; and 1 signal acquired), sagittal and axial fast spin-echo T2-weighted with fat saturation (TR range/TE range, 2,400–6,000/60–75; echo-train length of 8; 4-mm section thickness; 1-mm interslice gap; 256 × 192 matrix; 14-cm field of view; and 2 signals averaged), and coronal fast spin-echo intermediate-weighted (2,000–3,800/35–45, echo-train length of 8, 4-mm section thickness, 1-mm interslice gap, 256 × 192 matrix, 14-cm field of view, and 2 signals averaged). The other 16 knees were imaged with a 0.3-T scanner (Airis II, Hitachi Medical Systems America) using a quadrature extremity coil. The following sequences were used: sagittal spin-echo intermediate-weighted (1,000–1,200/20–25, 3-mm section thickness, 1-mm interslice gap, 256 × 256 matrix, 16-cm field of view, and 2 signals averaged), sagittal and axial fast spin-echo T2-weighted (3,000–5,000/90–120, 4-mm section thickness, 1-mm interslice gap, 256 × 256 matrix, 16-cm field of view, and 4 signals averaged), coronal spin-echo T1-weighted (500–600/12–15, 3-mm section thickness, 1-mm interslice gap, 256 × 256 matrix, 16-cm field of view, and 3 signals averaged), and coronal STIR (2,000–4,000/20–30; 4-mm section thickness; 1-mm interslice gap; 256 x 256 matrix; 16-cm field of view; 2 signals averaged; and inversion time, 80 msec).
MRI Review
The process of reviewing the 105 MR images was performed retrospectively and was divided into two independent phases. In the first phase, one reviewer evaluated the extent of medial meniscus extrusion (Fig. 1) using a PACS (picture archiving and communication system) workstation. On the coronal image at the mid point of the medial femoral condyle, extrusion of the medial meniscus from the margin of medial tibial plateau was quantified in millimeters. The measurement was performed by first drawing a vertical line intersecting the peripheral margin of the medial tibial plateau at the point of transition from horizontal to vertical; the length of another line extending from the first line to the outer margin of the meniscus was defined as the measurement of meniscal extrusion. Osteophytes were excluded for determining the medial margin.
In the second phase, a separate, independent review of the medial meniscus was made by two experienced musculoskeletal radiologists in consensus for the presence of degeneration and tear. The severity of degeneration (defined as central intrameniscal signal not communicating with the superior or inferior margin) was graded as none, mild (< 25% of the meniscus involved), moderate (26–50% of the meniscus involved), or marked (> 50% of the meniscus involved) using sagittal or coronal short TE (T1- or intermediate-weighted) images. Degeneration was graded for the meniscus overall.
“Positivity” for a meniscal tear was defined using well-accepted criteria. The two major criteria applied for the diagnosis of meniscal tears were abnormal signal intensity and abnormal morphology on T1-weighted or intermediate-weighted imaging sequences. Intrameniscal signal is considered to be a “tear” when it unequivocally extends to the articular surface. “Unequivocally” was defined as signal intensity contacting the surface on two adjacent sections. The meniscal morphology displayed on MR images is, in part, section-dependent. On sagittal sections both menisci have a bow-tie configuration on the most peripheral sections. The more central sections reveal a separate anterior and posterior horn with a triangular configuration showing sharp margins. On coronal sections, the anterior and posterior horns have a similarly well-defined triangular configuration, whereas the body is diminutive, reflecting the C-shaped configuration. Deviation from this expected morphology was considered a tear.
Meniscal tears were classified by type (longitudinal, radial, horizontal, oblique, and complex) and by location (anterior horn, body, and posterior horn). The tears were further characterized as follows: longitudinal tear, a vertical tear parallel to the outer margin of the meniscus; radial tear, a vertical tear perpendicular to the meniscus circumference, beginning in the inner margin and extending peripherally; horizontal tear, dividing the meniscus into superior and inferior portions; and oblique tear, intermediate between vertical and horizontal. A complex tear consists of a combination of longitudinal, radial, and horizontal tears, presenting as more than one cleavage plane [5–7]. Additionally, radial tears were subdivided into those involving less than 50% of meniscal width and those involving 50% or more of meniscal width. Tears of the posterior horn were also subclassified into those involving the meniscal “root” near the tibial spine (i.e., the posterior central attachment).
Statistical Analysis
We determined the frequency distribution of measured meniscal extrusion compared with the various types of graded meniscal abnormality. Statistical analysis was performed to determine whether there were significant differences in the proportion of substantial meniscal extrusions, depending on meniscal lesion type. Meniscal extrusion was dichotomized into minor (≤ 3 mm) and major (> 3 mm). The null hypothesis tested was that the number of major extrusions is independent of the meniscal lesion type. Several types of comparisons were performed. For meniscal degeneration, a 2 × 4 contingency table was created, and the presence of an association was determined using a chi-square test for trend (because the hypothesis is that more severe degeneration is associated with more substantial extrusion). For meniscal tears, pairwise comparisons were performed for any type of tear and for the subsets, on the basis of locations and types (as defined previously). For each pairwise comparison, 2 × 2 contingency tables were generated, with the columns indicating the type of meniscal lesion of interest (dichotomized as present vs absent) and with the rows indicating the type of extrusion (minor or major). Odds ratios (ORs) with confidence intervals (CIs) were calculated to determine the measure of association. Fisher's exact test was applied to each 2 × 2 contingency table separately to determine if nonrandom associations existed between the two variables of interest. A p value of less than 0.05 was considered to indicate a statistically significant difference for any statistical test.
Results
The frequency distribution of medial meniscus extrusion extent with degeneration and type or location of tear is summarized in Table 1. The distribution of medial meniscus extrusion was 2 mm (n = 17), 3 mm (n = 17), 4 mm (n = 27), 5 mm (n = 14), 6 mm (n = 16), and 7–10 mm (n = 14). Minor extrusion (≤ 3 mm) was found in 34 knees, and major extrusion (> 3 mm) was present in 71 knees.
| No. | Extrusion (mm) | Degeneration | Tear | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mild (%) | Moderate (%) | Marked (%) | Total (%) | Location | Type | ||||||||||
| Posterior Horn (%) | Body (%) | Anterior Horn (%) | Root (%) | Longitudinal (%) | Horizontal (%) | Oblique (%) | Radial (%) | > 50%a (%) | Complex (%) | ||||||
| 17 | 2 | 8/17 (47) | 4/17 (24) | 2/17 (12) | 10/17 (59) | 9/17 (53) | 1/17 (6) | 0/17 (0) | 1/17 (6) | 0/17 (0) | 0/17 (0) | 5/17 (29) | 2/17 (12) | 0/2 (0) | 3/17 (18) |
| 17 | 3 | 8/17 (47) | 5/17 (29) | 3/17 (18) | 10/17 (59) | 6/17 (35) | 4/17 (24) | 0/17 (0) | 0/17 (0) | 1/17 (6) | 1/17 (6) | 4/17 (24) | 1/17 (6) | 0/1 (0) | 3/17 (18) |
| 27 | 4 | 3/27 (11) | 17/27 (63) | 6/27 (22) | 22/27 (81) | 14/27 (52) | 8/27 (30) | 1/27 (4) | 7/27 (26) | 2/27 (7) | 0/27 (0) | 2/27 (7) | 8/27 (30) | 6/8 (75) | 11/27 (41) |
| 14 | 5 | 5/14 (36) | 5/14 (36) | 2/14 (14) | 12/14 (86) | 11/14 (79) | 1/14 (7) | 0/14 (0) | 7/14 (50) | 0/14 (0) | 1/14 (7) | 0/14 (0) | 4/14 (29) | 4/4 (100) | 7/14 (50) |
| 16 | 6 | 3/16 (19) | 5/16 (31) | 7/16 (44) | 14/16 (88) | 2/16 (13) | 8/16 (50) | 4/16 (25) | 7/16 (44) | 0/16 (0) | 0/16 (0) | 0/16 (0) | 1/16 (6) | 1/1 (100) | 13/16 (81) |
| 14 | 7-10 | 1/14 (7) | 2/14 (14) | 11/14 (79) | 14/14 (100) | 2/14 (14) | 8/14 (57) | 4/14 (29) | 9/14 (64) | 0/14 (0) | 0/14 (0) | 1/14 (7) | 2/14 (14) | 2/2 (100) | 11/14 (79) |
a
More than 50% of the meniscal width.
Medial meniscus degeneration was graded as mild in 28 knees, moderate in 38 knees, and marked in 31 knees. Of the 34 knees with minor medial meniscus extrusion, degeneration of the meniscus was graded as mild in 16 (47%), moderate in nine (26%), and marked in five (15%). In the 71 knees with major extrusion, degeneration of the medial meniscus was classified as mild in 12 knees (17%), moderate in 29 (41%), and marked in 26 (37%). The trend of association between severity of degeneration and extent of extrusion was significant (p = 0.003).
Medial meniscus tears were found in 83 of the 105 knees. Medial meniscus tear was observed in 20 knees (59%) with minor meniscal extrusion versus 63 knees (89%) with major meniscal extrusion (p = 0.001; OR, 5.5 [95% CI, 1.8–17.1]).
Three knees had isolated vertical longitudinal tears; one (3%) was observed in the 34 knees with minor extrusion and two (3%) were in the 71 knees with major extrusion (p = 1.0; OR, 0.96 [95% CI, 0.06–27.7]). Similarly, there were few isolated horizontal tears: two were seen, one (3%) in the minor extrusion group and one (1%) in the major extrusion group (p = 1.0; OR, 0.47 [95% CI, 0.01–17.9]). Oblique tears were observed in nine (26%) of the 34 knees with minor extrusion (Fig. 2A, 2B, 2C) and three (4%) of the 71 knees with major extrusion. Oblique tears were significantly associated with minor extrusion (p = 0.003; OR, 0.12 [95% CI, 0.02–0.55]).
Radial tears were seen in 18 of the 105 knees; five were small, involving less than 50% of meniscal width, and 13 were large, involving 50% or more of meniscal width. There were three radial tears (9%) in the 34 knees with minor medial meniscus extrusion versus 15 (21%) in the 71 knees with major medial meniscus extrusion (p = 0.198; OR, 2.77 [95% CI, 0.65–13.1]). All three radial tears in knees with minor extrusion involved less than 50% of the meniscal width; conversely, 13 radial tears in knees with major extrusion (87%) involved more than 50% of meniscal width (p = 0.017; OR, 5.0 [95% CI, 1.3–23.1]). Therefore, although radial tears as a group were not associated with extent of extrusion, large radial tears were significantly associated with major extrusion (Fig. 3A, 3B, 3C).
Complex medial meniscus tears were present in 48 knees. In the 34 knees with minor medial meniscus extrusion, complex tears were seen in six cases (18%), whereas in the 71 knees with major medial meniscus extrusion 42 (59%) had complex tears (Fig. 4A, 4B). This association was significant (p < 0.001; OR, 6.8 [95% CI, 2.3–21.0]).
Concerning the location of the tears, 44 (53%) involved the posterior horn; 30 (36%), the meniscus body; and nine (11%), the anterior horn; of the 44 posterior horn tears, 31 (37% of the 83 tears) involved the posterior meniscal root. Fifteen tears (44%) located in the posterior horn were found in 34 knees with minor meniscal extrusion, and 29 (41%) were present in 71 knees with major extrusion (p = 0.916; OR, 0.88 [95% CI, 0.35–2.2]). Five tears (15%) were located at the meniscus body in knees with minor medial meniscus extrusion versus 25 (35%) in 71 knees with major meniscal extrusion (p = 0.051; OR, 3.2 [95% CI, 0.99–10.6]). In 71 patients with major meniscal extrusion, nine (13%) tears involved the anterior horn, whereas none were found at that location in knees with minor meniscal extrusion (p = 0.072; OR could not be calculated because of the presence of a zero value). Tear involving the meniscal root (Fig. 5A, 5B, 5C) was observed in one (3%) of the 34 knees with minor medial meniscus extrusion and in 30 (42%) of the 71 knees with major medial meniscus extrusion (p < 0.001; OR, 24.1 [95% CI, 3.2–500.3]). Radial tears were particularly common at the posterior horn; of the 18 radial tears, 15 (83%) occurred at the posterior horn, with five (33%) of these 15 at the root. Tears involved one third, two thirds, and all of the meniscus in 75%, 25%, or 0% with minor extrusion, respectively, and 46%, 40%, or 14% with major extrusion, respectively; extent of tear was significantly associated with extent of extrusion (p = 0.014).
Table 2 summarizes the comparisons between degree of meniscal extrusion and type of meniscal lesion.
| No. | Extrusion (mm) | Degeneration | Tear | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mild (%) | Moderate (%) | Marked (%) | Total (%) | Location | Type | ||||||||||
| Posterior Horn (%) | Body (%) | Anterior Horn (%) | Root (%) | Longitudinal (%) | Horizontal (%) | Oblique (%) | Radial (%) | > 50%a (%) | Complex (%) | ||||||
| 34 | 2-3 | 16/34 (47) | 9/34 (26) | 5/34 (15) | 20/34 (59) | 15/34 (44) | 5/34 (15) | 0/34 (0) | 1/34 (3) | 1/34 (3) | 1/34 (3) | 9/34 (26) | 3/34 (9) | 0/3 (0) | 6/34 (18) |
| 71 | > 3 | 12/71 (17) | 29/71 (41) | 26/71 (37) | 63/71 (89) | 29/71 (41) | 25/71 (35) | 9/71 (13) | 30/71 (42) | 2/71 (3) | 1/71 (1) | 3/71 (4) | 15/71 (21) | 13/15 (87) | 42/71 (59) |
| p | 0.003 | 0.001 | 0.92 | 0.051 | 0.072 | < 0.001 | 1.0 | 1.0 | 0.003 | 0.20 | 0.019 | < 0.001 | |||
a
More than 50% of the meniscal width.
Discussion
Important functions of the knee meniscus include facilitating load distribution, force absorption, proprioception, lubrication, and stabilization [8, 9]. It is responsible for supporting between 40% and 70% of load across the knee; the remainder is transmitted by direct contact of articular cartilage [1, 2]. During functional knee activity, the meniscus follows the movement of the tibia and femur, increasing the contact area and providing an effective distribution of forces across the joint surfaces, important for maintenance of articular integrity [10].
The medial meniscus has less inherent mobility than the lateral meniscus, in part because of its firm attachment to the medial collateral ligament, and it bears more force during weight bearing. Thus, it is more prone to injury (both the acute variety and the chronic repetitious type), with resultant degeneration and tear. The posterior horn in particular possesses a strong attachment to the tibia near the tibial spine known as meniscal root or central attachment [10].
During load transmission, the compressive forces applied on the wedge-shaped meniscus result in circumferential stress (hoop strain) that stretches the collagen bundles in a radial direction between the anterior and posterior attachments. The integrity and orientation of the meniscal collagen fibers, the attachments of anterior and posterior horns, and the presence of intermeniscal connections are some factors that influence resistance to hoop strain [10, 11]. Meniscal degeneration and most common tears result from trauma, being composed of radial (shearing) and axial (compressive) vectors applied between the meniscal and articular cartilage [12]. Electron microscopy studies of meniscal structure have revealed three collagenous layers. The main collagen fibril bundles are dense and are located in the central portion of the meniscus (the central main layer); they predominantly run in a circumferential orientation [12, 13]. It has been proposed that these fibers are primarily responsible for meniscal stability, preventing expansion when an axial load is applied [14].
Miller et al. [15] provided perhaps the earliest description of meniscal extrusion on MRI, concluding that extrusion greater than 25% of meniscal width was not significantly associated with meniscal tear. However, this study did not control for the confounding factor of meniscal degeneration, which can also disrupt meniscal mechanics. In our population, the degree of medial meniscus degeneration was significantly associated with the extent of meniscal extrusion. This finding corresponds to prior work on histology of the degenerated meniscus that showed increased size of the collagen fibers due to separation of fibrils and microcyst formation, with resultant reduction in resistance to hoop strain, which can stretch the fibers in a radial outward direction, causing meniscal extrusion [16].
We found that medial meniscus tears were also significantly associated with the extent of medial meniscus extrusion. In subset analyses, complex tears had a particularly strong measure of association with major extrusion (> 3 mm). The presence of extensive structural disruption with more than one cleavage plane through the collagen fiber bundles could explain the more significant alteration of the hoop strain resistance and resultant meniscal extrusion.
All three radial tears with minor extrusion involved less than 50% of the meniscal width; conversely, 87% of the radial tears with major extrusion involved more than 50% of the meniscal width. Large radial tears, which are oriented in a perpendicular direction to the circumferential collagen fibrils in the main portion of the meniscus, completely disrupt the ability to resist hoop strain, resulting in major meniscal extrusion.
Tears involving the meniscal root (central attachment) are also significantly related to the severity of meniscal extrusion, seen in 3% with minor extrusion and 42% with major extrusion. The meniscal root or central tibial attachment is more prone to injury, as noted by Vedi et al. [10] in a study of medial meniscus dynamics. Furthermore, meniscal root attachments present one of the primary factors in maintaining resistance to hoop strain during load bearing, preventing radial meniscal displacement. Tears at the posterior root were commonly (33%) radial type tears.
Oblique tears were three times more common in menisci with minor extrusion compared with major extrusion and were significantly associated with minor extrusion rather than with major extrusion. These types of tears are not oriented perpendicular to the collagen fibril network and do not disrupt the structure as much as tears across the bundles (e.g., radial tears); these findings are consistent with the previously described biomechanical models.
Detection of meniscal extrusion is important not only because it is associated with underlying tear but also because meniscal extrusion itself is thought to be related to development of osteoarthritis [17, 18]. Kenny [18] published a study relating radial displacement of medial meniscus and “Fairbank's” signs. These concern three radiographic abnormalities in the knee after meniscectomy, as a result of loss of meniscal function: an antero-posterior osseous ridge projecting downward from the femoral condyle, generalized flattening of the marginal half of the femoral articular surface, and narrowing of the joint space. Kenny concluded that these abnormalities could develop in knees with radial displacement (i.e., extrusion) of the medial meniscus and loss of meniscal function. Also, unstable meniscal tears have been associated with development of osteoarthritis [19]. Complete or subtotal meniscectomy was shown to induce fairly rapidly progressive osteoarthritis; thus, the current treatment goals include preserving as much meniscus as possible.
Some limitations of this study should be acknowledged. This was a retrospective convenience sample and not a prospectively derived cohort. A selection bias exists because these cases were selected on the basis of the presence of some degree of extrusion. This search methodology resulted in a study group with a predominance of women and an average age of 56 years; data acquired may not be transposable to a young, athletic population. There is not a consensus on how to determine meniscal extrusion, and other factors may contribute to displaced meniscal positioning, such as large joint effusions and marginal osteophytosis. We did not attempt to control for these potential confounders. It was not possible to blind reviewers to meniscal extrusion during the evaluation for medial meniscus degeneration and tear. We had no reference or gold standard because MRI was used as the surrogate documentation of abnormality. The absence of arthroscopic confirmation of our findings could also represent a limitation, although knee MRI has a well-documented high accuracy for diagnosis of meniscal tear, with sensitivities of 86–96% and specificities of 84–94% [6]. The relatively small number of knees with isolated horizontal tears (n = 2) and longitudinal tears (n = 3) is also a limitation. We did not correlate the imaging findings with specific clinical symptoms, although because these examinations came from a clinical repository, all patients were presumably symptomatic.
In conclusion, medial meniscus extrusion greater than 3 mm is significantly associated with extent of meniscal degeneration, extent of tear, complex tear, large radial tear, and tear involving the meniscal root. Therefore, when substantial meniscal extrusion is identified, a high likelihood exists that one of these lesions is present, resulting in disruption of meniscal stability.
Footnotes
Presented at the 2003 annual meeting of the American Roentgen Ray Society, San Diego, CA.
Address correspondence to W. B. Morrison ([email protected]).
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Submitted: November 7, 2003
Accepted: January 16, 2004
First published: November 23, 2012
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