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1
Department of Radiology, University of Wisconsin Hospital and Clinics, E3/311
CSC, 600 Highland Ave., Madison, WI 53792.
2
Present address: Erskine Diagnostic Center, 1948 First Ave., Cedar Rapids, IA
52402.
Received April 5, 2000;
accepted after revision June 12, 2000.
Presented at the annual meeting of the American Roentgen Ray Society,
Washington, DC, May 2000.
Abstract
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MATERIALS AND METHODS. We identified 59 consecutive patients who underwent both knee MR imaging examinations and knee arthroscopy. Thirty patients had lateral meniscal tears, and 29 had intact lateral menisci. We reviewed paired sagittal proton density- and T2-weighted MR images from these 59 patients for abnormal superior popliteomeniscal fascicles and edema surrounding the posterolateral capsule.
RESULTS. The superior popliteomeniscal fascicles were abnormal in nine of the 30 patients with torn lateral menisci but were normal in all 29 patients with intact menisci (p = 0.001). Abnormal fascicles were apparent only when the lateral meniscal tear involved the posterior horn. Posterior pericapsular edema was seen in 10 patients with a torn posterior horn and in one patient with an anterior horn tear of the lateral meniscus, but in only two patients with intact menisci (p = 0.006).
CONCLUSION. The presence of superior popliteomeniscal fascicle abnormalities and of posterior pericapsular edema is significantly associated with a tear of the lateral meniscus, most commonly in the posterior horn. Noting the presence of these findings may help improve the accuracy of MR diagnosis of lateral meniscal tears.
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The popliteomeniscal fascicles are the superior and inferior struts that anchor the posterior horn of the lateral meniscus to the capsule [4]. Absence of the popliteomeniscal fascicles is an indirect sign on knee arthrography of a lateral meniscal tear [5].
A recent study reported that the popliteomeniscal fascicles are routinely seen on MR imaging of the knee when the lateral meniscus is intact [6]. On the basis of that study, we postulated that failure to visualize the popliteomeniscal fascicles is an indirect MR imaging sign of a lateral meniscal tear. As we were considering this project, we also noted that posterior pericapsular edema was often seen with lateral meniscal tears. As a result, we decided to investigate whether such edema might be another indirect sign of a lateral meniscal tear. We are now reporting our findings about the association between these two signs and lateral meniscal tears.
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The patient cohort included 14 males and 16 females between the ages of 14 and 71 years (mean age, 36.5 years) with a lateral meniscal tear and 18 males and 11 females between the ages of 16 and 69 years (mean age, 38.4 years) with intact lateral menisci.
Each MR imaging examination was performed with a standard protocol on the same 1.5-T scanner (Signa; General Electric Medical Systems, Milwaukee, WI) using a phased array knee coil. The knee was fully extended with 10° of external rotation. The paired sagittal proton density—weighted and T2-weighted sequences were obtained with the following parameters: 2000/20, 80 (TR/first-echo TE, second-echo TE); 14-cm field of view; 256 x 192 matrix; one excitation; and 3-mm slices with 1.5-mm interslice gap.
Two musculoskeletal radiologists reviewed the sagittal images using a standard scoring sheet. The observers noted whether the superior popliteomeniscal fascicle was normal or abnormal (Figs. 1A,1B,1C,2,3,4A,4B) and whether edema was present around the posterior capsule of the lateral capsule (Figs. 1A,1B,1C,2,3,4A,4B) using the following grading system: 1, normal; 2, probably normal; 3, equivocal; 4, probably abnormal; and 5, definitely abnormal. A superior popliteomeniscal fascicle was considered abnormal if it was disrupted or absent (Figs. 2 and 3). Edema could be present anterior to, posterior to, or on both sides of the capsule (Fig. 4A). We did not assess the inferior popliteomeniscal fascicle because we found it more difficult to confidently identify than the superior fascicle.
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These MR imaging observations were then correlated with the presence or absence of a lateral meniscal tear using the two-sided Pearson chi-square statistic. A statistically significant difference was present if p was less than 0.05. For statistical analysis, we considered the superior popliteomeniscal fascicle to be disrupted and pericapsular edema to be present if graded as 4 or 5.
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We discovered an association between the location of the tear and the presence of a superior popliteomeniscal fascicle abnormality, with nine of 22 tears that involved the posterior horn having abnormal fascicles but none of the eight anterior or midportion tears having an abnormal fascicle (p = 0.031). Although posterior pericapsular edema was more common with posterior horn tears, the difference was not statistically significant: 10 of 22 tears involving the posterior horn had edema, but one of the eight tears not involving the posterior horn had posterior edema (p = 0.098). The location of a tear in the inner two thirds versus the outer one third of the lateral meniscus was not associated with either popliteomeniscal fascicle abnormality (p = 0.523) or pericapsular edema (p = 0.510).
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We also found in our own practice that our sensitivity for the diagnosis of lateral meniscal tears has been consistently lower than that for tears of the medial meniscus. For this reason, we undertook this study looking for indirect signs of a lateral meniscal tear that might alert the interpreter of an MR examination to the presence of a subtle tear.
Our study confirmed our hypothesis that an abnormal superior popliteomeniscal fascicle and the presence of pericapsular posterolateral edema were associated with lateral meniscal tears. We believe this association exists because the same mechanism of injury that causes a lateral meniscal tear also causes an injury to the popliteomeniscal fascicle and capsule.
The superior popliteomeniscal fascicle can be abnormal because of either disruption or absence. We do not know if failure to visualize a popliteomeniscal fascicle on MR imaging is caused by disruption of the fascicle or distortion due to edema. Pavlov and Goldman [5] speculated that edema and adhesions are responsible for the failure to visualize the popliteomeniscal fascicles on knee arthrography. Although all nine of our patients with superior fascicle abnormalities had lateral meniscal tears, we cannot conclude that a fascicle abnormality on MR imaging examination is a definitive sign of a lateral meniscal tear. Johnson and De Smet [6] reported that the superior popliteomeniscal fascicle was not seen in two of 66 patients with intact lateral menisci. Therefore, an abnormal superior popliteomeniscal fascicle should be viewed as suggesting a lateral meniscal tear but not as a pathognomonic sign. Assessing the value of this indirect sign as a way to diagnose a lateral meniscal tear will require a prospective study with a larger number of patients.
Although the presence of pericapsular edema is also associated with a lateral meniscal tear, it is not as specific, because two of 13 patients with such edema did not have a lateral meniscal tear. This edema is presumably caused by hemorrhage resulting from capsular stretching or tearing. Although a common mechanism of injury often tears both the capsule and lateral meniscus, it may tear only the capsule and create this edema without causing a lateral meniscal tear.
The other association we found was between these indirect signs and the location of the tear. The superior popliteomeniscal fascicles were abnormal in nine of the 22 patients with lateral meniscal tears involving the posterior horn but were normal in the eight patients whose tears did not involve the posterior horn. Similarly, posterior pericapsular edema was seen in eight of the 22 patients with posterior horn tears but in only one of the eight patients whose tears did not involve the posterior horn. This difference between tear location and the association of these indirect signs suggests that tears involving the posterior horn have a different mechanism of injury than tears involving the body and anterior horn. Tears of the posterior horn of the lateral meniscus are presumably caused by a compression, distraction, or rotational injury that affects the posterolateral meniscal attachments.
A limitation of this study is that we did not have sufficient clinical information to determine if all the meniscal tears were caused by a specific episode of trauma or if they might be degenerative in origin. We also were not able to determine the interval between the injury and the MR imaging examination in a sufficient number of cases to analyze whether a fascicle abnormality or pericapsular edema is more common in patients with acute injuries. We suspect that both findings would be more common in acute injuries, but a new study is necessary to address this issue. With time, fascicles may heal and the edema may clear.
In conclusion, we have shown that abnormalities of the superior popliteomeniscal fascicle and posterolateral pericapsular edema are indirect MR imaging signs of a lateral meniscal tear. If either of these signs is present, those interpreting MR imaging examinations should pay special attention to the lateral meniscus, because a tear is likely present in the posterior horn.
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