Lesions of the Menisci of the Knee: Value of MR Imaging Criteria for Recognition of Unstable Lesions

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1 B. C. Vande Berg 1 P. Poilvache 2 F. Duchateau 1 F. E. Lecouvet 1 J. E. Dubuc 2 B. Maldague 1 J. Malghem 1 Received June 12, 2000; accepted after revision August 24, Department of Radiology, Cliniques Universitaires St. Luc, Université Catholique de Louvain, 10 Ave. Hippocrate, 1200 Brussels, Belgium. Address correspondence to B. C. Vande Berg. 2 Department of Orthopedic Surgery, Cliniques Universitaires St. Luc, Université Catholique de Louvain, 1200 Brussels, Belgium. AJR 2001;176: X/01/ American Roentgen Ray Society s of the Menisci of the Knee: Value of MR Imaging Criteria for Recognition of Unstable s OBJECTIVE. This study aimed to determine the value of four MR imaging criteria for evaluating unstable meniscal lesions. MATERIALS AND METHODS. Criteria for unstable meniscal lesions were the presence of a displaced meniscal fragment, visibility on more than three 3-mm-thick coronal and two 4- mm-thick sagittal images, having more than one orientation plane or more than one pattern (contour irregularity, peripheral separation, tear), and having intrameniscal high signal intensity on T2-weighted spin-echo images. Sensitivity, specificity, and positive and negative predictive values for recognition of instability among all meniscal lesions were determined for the presence of each individual criterion and for the presence of at least one criterion in 50 consecutive patients (mean age, 46 years) who underwent MR imaging and subsequent arthroscopy. RESULTS. Sensitivities and specificities of these four criteria ranged between 18% and 54% and between 94% and 100%, respectively. Positive and negative predictive values ranged between 92% and 100% and between 39% and 52%, respectively. The presence of at least one criterion enabled recognition of unstable lesions with a sensitivity and specificity of 82% and with positive and negative predictive values of 90% and 70%, respectively. CONCLUSION. The four MR imaging criteria have high specificities and positive predictive values and low sensitivities and negative predictive values when evaluating unstable meniscal lesions. M anagement of meniscal lesions of the knee encompasses nonoperative care, partial or total meniscectomy, and meniscus repair or grafting [1, 2]. stability is an important criterion in deciding whether to resect, repair, or leave alone a meniscal lesion [3, 4] and is best determined by direct visualization and palpation at arthroscopy [2, 5, 6]. Unstable lesions are lesions in which the meniscus or a fragment of it can be inappropriately displaced by a probe into the femorotibial joint. The ability of MR imaging to detect meniscal lesions has been extensively studied. Little attention has been given to the preoperative determination of meniscal lesion stability using MR imaging. A high positive predictive value of MR imaging to recognize unstable meniscal lesions would mean that MR imaging could help recognize patients who are most likely to benefit from surgery. To the best of our knowledge, no established MR imaging criteria for unstable meniscal lesions exist. Theoretically, these criteria should be based on the presence of a displaced meniscal fragment and on lesion shape, size, and signal, as proposed by Carpenter et al. [7]. The purpose of our study was to define and assess the value of four MR imaging criteria for recognizing unstable meniscal lesions. Materials and Methods Patients The study population consisted of 50 consecutive patients who underwent MR imaging of the knee between January and November 1999 and subsequent arthroscopy at our institution but without prior arthroscopy on that knee. The 36 male and the 14 female patients of the study population had a mean age of 46 years (range, years). All patients were referred by two orthopedic surgeons from our institution for MR imaging of the right (n = 24) or left (n = 26) knee because of a clinical suspicion of a meniscal lesion. Twentyfive patients had a history of knee trauma that had occurred 4 weeks to 5 years before the MR imaging (mean delay, 10.9 months). Twenty-five patients had a history of spontaneous knee pain without a recognizable triggering event. AJR:176, March

2 Vande Berg et al. MR Imaging Thirty-four MR imaging studies were performed using a 0.5-T magnet (.5 NT; Philips Medical Systems, Best, The Netherlands), and 16 MR imaging studies were performed using a 1.5-T magnet (Signa; General Electric Medical Systems, Milwaukee, WI) with dedicated phased array quadrature knee coils. The imaging system used was randomly selected depending on the examination schedule. Patients were imaged in the conventional supine position. With the 0.5-T magnet, the following protocols were used: coronal spin-echo T1-weighted (TR/TE, 509/20) and gradient-echo T2*-weighted (800/28, 45 flip angle) sequences; parasagittal dual-echo spin-echo intermediate- and T2- weighted (TR/first-echo TE, second-echo TE, 2670/ 14,120) sequences; and axial gradient-echo T2- weighted (1000/28, 45 flip angle) sequences. Fields of view were 140 mm for coronal sequences and 160 mm for sagittal and axial sequences. Matrix size was Numbers of signal acquisitions were four for coronal T1-weighted sequences, two for gradientecho sequences, and one for parasagittal dual-echo sequences. Slice thickness was 3 mm for coronal and axial images and 4 mm for parasagittal images, with a 0.1 slice thickness interval. With the 1.5-T magnet, the following protocols were used: coronal spin-echo T1-weighted (520/15) and gradient-echo T2*-weighted (800/25, 30 flip angle) sequences; parasagittal fast spin-echo intermediate-weighted (3000/ 25) and T2-weighted (3000/120) sequences; and axial gradient-echo T2*-weighted (800/25, 30 flip angle) sequences. Slice thickness was 3 mm for coronal and axial images and 4 mm for parasagittal images, with a 0.1 slice thickness interval. Fields of view were 180 mm for parasagittal sequences and 160 mm for coronal and axial sequences. Matrix size was Two acquisitions were used for all sequences. Image Analysis A musculoskeletal radiologist with 9 years experience who was unaware of the arthroscopic findings reviewed the 50 MR imaging studies to determine the presence or absence of meniscal lesions, including contour irregularity, peripheral separation, and tear. Contour irregularity was defined as an altered meniscus shape with truncation, blunting, flattening, or rounding of the inner borders on coronal T1- or T2*- weighted or sagittal intermediate- or T2-weighted MR images. Peripheral meniscus separation was defined by the presence of high signal intensity on T2- weighted images between the meniscus and the capsule. Meniscal tear was defined as an abnormal intrameniscal signal that extended to the articulating surface of the meniscus on more than one coronal T2*-weighted or sagittal intermediate- or T2- weighted MR image. Questionable extension of abnormal signal to the free meniscal surface when viewed on only a single image was not reported as a tear [8, 9]. Meniscal lesions were identified as involving the medial or lateral menisci. The same musculoskeletal radiologist determined the presence or absence of each of the following four MR imaging criteria that were adapted from the initial proposal of Carpenter et al. [7]. Criteria for unstable meniscal lesions were the presence of a displaced meniscal fragment; visibility on more than three 3-mm-thick coronal and two 4- mm-thick sagittal images; having more than one orientation plane or more than one pattern (contour irregularity, peripheral separation, tear); and having intrameniscal high signal intensity on T2-weighted spin-echo images. A displaced meniscal fragment consisted of a fragment that had lost its anatomic location and was found in the intercondylar notch or in a meniscal recess [2, 10, 11]. The presence of a displaced meniscal fragment was considered to indicate an unstable lesion. size was determined by counting the number of contiguous coronal gradient-echo T2*- weighted or sagittal intermediate-weighted MR images on which contour abnormality, peripheral separation, or tear were seen at the meniscal surface. s that were visible on more than two 4-mm-thick sagittal and more than three 3-mmthick coronal images were considered unstable. shape was determined by noting the presence of one or more of the three lesion patterns just described (contour irregularity, peripheral meniscus separation, and meniscal tear) and the different cleavage plane orientations within the meniscus on coronal T2*-weighted or sagittal intermediate-weighted MR images. s that included more than one lesion pattern or more than one cleavage plane orientation in the same meniscal area were considered unstable. The presence of more than one lesion pattern or more than one cleavage plane orientation that were not in continuity with one another was not considered to indicate an unstable lesion. signal intensity was determined on spinecho T2-weighted MR images. The presence of high signal intensity identical to that of joint fluid in the meniscus on T2-weighted images was considered to indicate an unstable lesion. High signal intensity adjacent to the meniscus caused by a meniscal cyst was not considered indicative of instability. Size, shape, and signal intensity of the meniscal lesions were determined within the meniscus, and the presence of a displaced meniscal fragment was not taken into account for determination of these parameters. Arthroscopy All knee arthroscopy was performed by two orthopedic surgeons of our institution after a mean delay of 11.8 weeks (range, 1 30 weeks) after MR imaging. MR studies were available to surgeons on films. Original reports made by three radiologists, including the one who retrospectively reviewed the MR studies of the knees that underwent surgery, included descriptions of meniscal lesions and of displaced meniscal fragments. Initial reports by the three radiologists did not mention the precise extent, shape, or signal of the lesions and did not determine whether lesions were considered stable or unstable. At the time of arthroscopy, meniscal lesion patterns, including contour irregularities, peripheral separations, and tears, were noted on charts. Drawings that showed the location and configuration of all meniscal abnormalities were made. Meniscal lesions were considered unstable if they were displaced or if mobile meniscal fragments were found at inspection and palpation during arthroscopy, according to standard criteria [2]. Meniscal lesions without flaps or displaced fragments were considered stable. Statistics Operative notes and drawings were used as the gold standard for determining meniscal integrity and meniscal lesion stability. Sensitivity, specificity, and positive and negative predictive values with 95% confidence intervals for the detection of meniscal lesions were calculated. Sensitivity, specificity, and positive and negative predictive values for detecting unstable lesions among all lesions were calculated for each of the four MR imaging criteria. Finally, sensitivity, specificity, and positive and negative predictive values for detecting unstable lesions among all lesions were calculated for the presence of at least one of the four criteria. s that were missed at MR imaging were considered stable. Results Meniscal s At arthroscopy, 23 unstable and seven stable medial meniscal lesions and 10 stable and 10 unstable lateral meniscal lesions were found. Twenty-nine of 30 medial and 19 of 20 lateral meniscal lesions found at arthroscopy were observed on MR images. Among the 20 medial and 30 lateral menisci that were normal at arthroscopy, 18 medial and 27 lateral menisci were found to be normal on MR images. One medial and one lateral meniscal abnormality at arthroscopy were missed on MR images. These two lesions were stable at arthroscopy. Two normal medial and three normal lateral menisci at arthroscopy were considered abnormal and stable in MR imaging (Table 1). Sensitivity and specificity of MR imaging for the detection of meniscal abnormality were 96% and 90%, respectively (Table 2). Displaced Meniscal Fragment Seven medial and five lateral meniscal fragments were observed in the intercondylar space (n = 9) or in the medial inframeniscal recess (n = 3) (Fig. 1). The presence of a displaced meniscal fragment had sensitivity and specificity for the detection of an unstable tear of 36% and 94%, respectively, with positive and negative predictive values of 92% and 43%, respectively. Size Fifteen medial and three lateral meniscal lesions were visible on more than three T2*- 772 AJR:176, March 2001

3 MR Imaging of Knee Meniscus s weighted coronal and two intermediateweighted sagittal MR images. The presence of lesion visibility on more than three T2*- weighted coronal and two intermediateweighted sagittal MR images had a sensitivity and specificity for the detection of an unstable lesion of 54% and 94%, respectively, with positive and negative predictive values of 95% and 52%, respectively. Shape Fourteen medial and one lateral meniscal lesions consisted of more than one lesion pattern or more than one cleavage plane (Figs. 1 and 2). The presence of more than one lesion pattern or more than one cleavage plane in the same meniscal area had a sensitivity and a specificity for the detection of an unstable lesion of 45% and 94%, respectively, with positive and negative predictive values of 94% and 47%, respectively. Signal Intensity Six medial and no lateral meniscal lesions displayed intrameniscal high signal intensity on T2-weighted images (Fig. 3). The presence of high signal intensity in the medial or lateral meniscus had a sensitivity and specificity for the detection of an unstable lesion of 18% and 100%, respectively, with positive and negative predictive values of 100% and 39%. TABLE 1 Arthroscopy Presence of Any MR Imaging Criterion Twenty-one of 23 medial and six of 10 lateral menisci had at least one MR imaging crite- Distribution of Normal Menisci and Stable and Unstable s of the Medial and Lateral Menisci Found at Arthroscopy and at MR Imaging Normal Medial Meniscus Stable Unstable MR Imaging Normal Lateral Meniscus Stable Unstable Normal Stable lesion Unstable lesion Note. Stable lesions on MR imaging presented none of the four MR imaging criteria for unstable lesions. Unstable lesions presented at least one of the four criteria. TABLE 2 Fig year-old man with chronic right knee pain. A, Parasagittal intermediate-weighted spin-echo MR image shows horizontal tear (white arrow ) in posterior horn of medial meniscus. Note contour irregularity (black arrow ) of inner margin. B, MR image medial to A shows radial tear (arrow ) of meniscal body. Note meniscal fragment (arrowheads) flipped anteriorly and inferiorly. C, Coronal gradient-echo T2*-weighted MR image shows meniscal fragment (arrowheads) displaced in inferior recess. Unstable meniscal lesion with displaced fragment was found at arthroscopy. Sensitivity, Specificity, and Positive and Negative Predictive Values for Detection of Meniscal Abnormalities on Knee MR Imaging Compared with Knee Arthroscopy in 50 Patients Parameter Medial Meniscus Lateral Meniscus Medial and Lateral Menisci Sensitivity (%) 97 (87 99) 95 (85 98) 96 (90 98) Specificity (%) 90 (79 96) 90 (79 96) 90 (82 94) PPV (%) 93 (83 98) 86 (74 93) 90 (83 95) NPV (%) 95 (85 98) 96 (87 99) 96 (90 98) Note. Numbers in parentheses are 95% confidence intervals. PPV = positive predictive value, NPV = negative predictive value. A B C AJR:176, March

4 Vande Berg et al. rion indicative of instability and corresponded to an unstable tear at arthroscopy. Two medial and four lateral menisci without MR imaging criteria for instability were found to be unstable at arthroscopy. Sensitivity and specificity of the presence of at least one MR imaging criterion indicative of instability in medial and lateral menisci were both 82%, with positive and negative predictive values of 90% and 70%, respectively (Table 3). C A Fig year-old man with left knee pain. A, Parasagittal intermediate-weighted spin-echo MR image shows contour irregularity (arrow ) of apex of posterior horn of medial meniscus. B, MR image medial to A shows vertical tear (arrow ). C, MR image medial to B shows horizontal tear (arrow ). D, Coronal gradient-echo T2*-weighted MR image of posterior horn of medial meniscus shows focal substance loss (arrow ) and horizontal cleavage plane (arrowhead ) that extends to posteromedial aspect of medial meniscus. Presence of two lesion patterns and two cleavage planes, along with visibility of lesion on six coronal and five sagittal contiguous sections (not shown) suggests unstable tear, which was confirmed at arthroscopy. TABLE 3 Sensitivity, Specificity, and Positive and Negative Predictive Values of the Presence of At Least One of the Four MR Imaging Criteria for Detecting Unstable s on MR Imaging Compared with Knee Arthroscopy Parameter Medial Meniscus Lateral Meniscus Medial and Lateral Menisci Sensitivity (%) 91 (76 97) 60 (39 78) 82 (69 90) Specificity (%) 71 (53 84) 90 (70 97) 82 (70 90) PPV (%) 91 (76 97) 86 (65 95) 90 (79 96) NPV (%) 71 (53 84) 69 (47 85) 70 (56 81) Note. Numbers in parentheses are 95% confidence intervals. PPV = positive predictive value, NPV = negative predictive value. Discussion Our study shows that each of the proposed MR imaging criteria had high specificity but low sensitivity for the recognition of instability among all meniscal lesions when used retrospectively in a series of 50 patients in whom knee arthroscopy was performed. Moreover, when the presence of at least one of the four MR imaging criteria was considered, MR imaging was found to have sensitivity and specificity of 82% and positive and negative predictive values of 90% and 70%, respectively, for the determination of meniscal lesion stability. The high negative predictive value of MR imaging in recognizing meniscal lesions has taken on an important clinical significance with the possibility of conservative management of patients with clinically suspected meniscal lesions and negative results on MR imaging [12 14]. The positive predictive value of 90% for MR imaging in recognizing instability among all meniscal lesions that was shown in our study further increases the clinical role of MR imaging because it implies that MR imaging could help select patients with meniscal abnormalities who have B D 774 AJR:176, March 2001

5 MR Imaging of Knee Meniscus s Fig year-old man with right knee pain. Sagittal T2-weighted fast spin-echo MR image shows tear of posterior horn of medial meniscus with high signal intensity (arrow ) equivalent to that of joint fluid in meniscus. was found unstable at arthroscopy, with large meniscal flap of posterior horn of medial meniscus. unstable lesions that are more likely to benefit from surgery. Our study aimed at validating MR imaging criteria for unstable meniscal lesions. First, the presence of a displaced meniscal fragment at MR imaging was considered direct evidence of an unstable lesion. In the literature, sensitivity in the detection of displaced meniscal fragments on MR imaging was initially low [10, 15] and increased progressively with experience [7, 11, 16]. The fact that one fragment observed at MR imaging was associated with a stable lesion at arthroscopy could result from false-positive or false-negative findings at MR imaging or at arthroscopy, respectively. Second, lesion visibility on more than two 4- mm-thick sagittal and three 3-mm-thick coronal images corresponded to a lesion extent greater than 10 mm. This value was selected in agreement with the initial proposal of Carpenter et al. [7] and corresponded to the cutoff value used at arthroscopy to characterize unstable lesions [4, 17]. Third, lesion number was taken into account instead of lesion shape because exact fissure plane orientation was difficult to recognize. instability was recognized if more than one cleavage plane or more than one lesion pattern, including contour irregularity, peripheral meniscal separation, and meniscal tear, was found in the same meniscal area. Actually, oblique or parrot-beak tears that are the most frequent unstable meniscal lesions can be decomposed into focal contour irregularity because of meniscal substance loss or a radial or oblique tear at the meniscus free edge and longitudinal tear [5, 10]. A nondisplaced meniscal flap can be difficult to recognize on MR images. However, parrot-beak tears with a nondisplaced meniscal flap probably cause a lesion with multiple lesion patterns or plane orientations. Finally, the presence of fluidlike signal intensity in the meniscus on T2-weighted images also indicated unstable meniscal lesions. The presence of high signal intensity on T2- weighted images is a highly specific but poorly sensitive sign for a meniscal lesion [18]. The presence of intrameniscal high signal intensity on T2-weighted images likely occurred only in meniscal lesions in which torn edges were moderately separated and thus unstable, with subsequent accumulation of articular fluid in the large cleavage plane. Our study concluded that arthroscopy should not be performed in knees with meniscal lesions that lack MR imaging criteria for an unstable tear because the sensitivity and negative predictive value of the MR imaging criteria are relatively low. These criteria need to be refined to increase their sensitivity. Inter- and intraobserver reproducibility also need to be determined. Our purpose was not to determine with MR imaging whether the meniscal lesions were repairable because no lesions in this series were sutured. Neither did we correlate the meniscal lesion patterns on MR imaging with the arthroscopic findings because this correlation needs precise surgical data that were not available. The results of our study are limited by the imaging technique and the study design. First, coronal and sagittal imaging planes were used for lesion analysis. The performance of submillimetric axial imaging could have enhanced the results by providing better delineation of radial or oblique lesion components with subsequent direct visualization of parrot-beak tears and meniscal flaps, as previously shown [10, 19 21]. Second, we analyzed the MR studies that were obtained using mid (0.5-T) or high field strength (1.5- T) without taking into account field strength difference because of its lack of influence on MR imaging accuracy for meniscal tear detection [10, 22]. The influence of field strength cannot be excluded in assessing meniscal lesion stability. Third, selection bias existed in our study because only MR imaging studies from patients who underwent arthroscopy were retrospectively reviewed by a radiologist who knew that these patients had undergone surgery. Patients with negative findings on MR imaging or with subtle lesions probably did not undergo surgery and could be a group of potential false-negative findings on MR imaging. However, the two false-negative results on MR imaging were stable lesions at arthroscopy, and the five false-positive results at MR imaging showed no MR criteria for instability. Finally, other criteria independent of meniscal MR imaging appearance, including age of the patient and associated ligament lesions that could influence lesion stability [1, 3 6, 23], were not taken into account. In conclusion, MR imaging criteria for unstable meniscal lesions that are based on the presence of a displaced meniscal fragment and on determination of meniscal lesion extent, shape, and signal intensity have a high specificity and a high positive predictive value for the recognition of instability among all meniscal lesions. These criteria remain to be assessed in prospective clinical studies. References 1. Awbrey BJ. Arthroscopic management of meniscal injuries. Curr Opin Rheumatol 1993;5: Metcalf RW, Burks RT, Metcalf MS, et al. Arthroscopic menisectomy. In: McGinty JB, ed. Operative arthroscopy, 2nd ed. Philadelphia: Lippincott-Raven, 1996: DeHaven KE. Decision-making factors in the treatment of meniscus lesions. 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