Posterior Medial Meniscus Root Ligament Lesions: MRI Classification and Associated Findings

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1 Musculoskeletal Imaging Original Research Choi et al. MRI of Posterior Medial Meniscus Root Ligament Lesions Musculoskeletal Imaging Original Research Ja-Young Choi 1 Eric Y. Chang 2,3 Guilherme M. Cunha 4 Monica Tafur 3 Sheronda Statum 3 Christine B. Chung 2,3 Choi JY, Chang EY, Cunha GM, Tafur M, Statum S, Chung CB Keywords: medial meniscus tear, MRI, posterior root ligament DOI: /AJR Received January 19, 2014; accepted after revision March 21, Department of Radiology, Seoul National University Hospital, Seoul, Korea. 2 Department of Radiology, Veterans Administration San Diego Healthcare System, La Jolla, CA. 3 Department of Radiology, University of California, San Diego Medical Center, 3350 La Jolla Village Dr, MC 114, San Diego, CA Address correspondence to E. Y. Chang (ericchangmd@gmail.com). 4 Department of Radiology, Clínica de diagnóstico por Imagem (CDPI), Rio de Janeiro, Brazil. AJR 2014; 203: X/14/ American Roentgen Ray Society Posterior Medial Meniscus Root Ligament Lesions: MRI Classification and Associated Findings OBJECTIVE. The purposes of this study were to determine the prevalence of altered MRI appearances of posterior medial meniscus root ligament (PMMRL) lesions, introduce a classification of lesion types, and report associated findings. MATERIALS AND METHODS. We retrospectively reviewed 419 knee MRI studies to identify the presence of PMMRL lesions. Classification was established on the basis of lesions encountered. The medial compartment was assessed for medial meniscal tears in the meniscus proper, medial meniscal extrusion, insertional PMMRL osseous changes, regional synovitis, osteoarthritis, insufficiency fracture, and cruciate ligament abnormality. RESULTS. PMMRL abnormalities occurred in 28.6% (120/419) of the studies: degeneration, 14.3% (60/419) and tear, 14.3% (60/419). Our classification system included degeneration and tearing. Tearing was categorized as partial or complete with delineation of the point of failure as entheseal, midsubstance, or junction to meniscus. Of all tears, 93.3% (56/60) occurred at the meniscal junction. Univariate analysis revealed significant differences between the knees with and without PMMRL lesions in age, medial meniscal tear, medial meniscal extrusion, insertional PMMRL osseous change, regional synovitis, osteoarthritis, insufficiency fracture (p = 0.017), and cruciate ligament degeneration (p < 0.001). CONCLUSION. PMMRL lesions are commonly detected in symptomatic patients. We have introduced an MRI classification system. PMMRL lesions are significantly associated with age, medial meniscal tears, medial meniscal extrusion, insertional PMMRL osseous change, regional synovitis, osteoarthritis, insufficiency fracture, and cruciate ligament degeneration. T he meniscal roots are ligamentlike structures that serve to anchor the fibrocartilaginous menisci onto the tibial intercondylar fossa or intercondylar eminence [1, 2]. The role of the meniscal root is paramount; it prevents the meniscus from being extruded and allows the meniscus to generate hoop stress. This enables the menisci to effectively transfer the load from the femur to the tibia while protecting the articular cartilage of the knee from excessive load [2]. Therefore, complete tearing of a meniscal root results in complete loss of hoop stress, which is nearly functionally identical to that of total meniscectomy [3] and a critical risk factor of early osteoarthritis of the knee [4 6]. The posterior medial meniscus root ligament (PMMRL) attaches to the posterior intercondylar fossa between the attachments of the posterior root of the lateral meniscus and posterior cruciate ligament [1]. Several recent reports have suggested that PMMRL tears are relatively common, particularly in the middle-aged or elderly population, and may be a cause of medial tibiofemoral osteoarthritis of the knee, which is the most common source of total knee arthroplasty [7, 8]. MRI is the primary diagnostic tool for PMMRL tears and has been regarded as both reliable and accurate for detection [4, 9]. Despite the growing orthopedic and imaging literature on tears of the PMMRL, little information is currently available about the incidence of PMMRL lesions, classification of the lesion types, site of the failure, and other associated findings on MR images. The purposes of this study were to determine the prevalence of altered MRI appearances of PMMRL lesions, introduce a classification of lesion types, and report associated findings. Materials and Methods Patient Selection This retrospective study was approved by the institutional review board of our institution, which 1286 AJR:203, December 2014

2 MRI of Posterior Medial Meniscus Root Ligament Lesions waived the requirement for informed consent. For enrollment of study subjects, we reviewed 500 consecutive knee MRI examinations at our institution (from January to June 2012), which were performed on symptomatic patients. We excluded nine potential subjects with infectious or inflammatory arthritis and benign or malignant tumors of the knee. In addition, we excluded 47 patients with a history of knee surgery or fractures around the knee. Twenty-five incomplete MRI studies were also excluded from the review. This left 419 MRI studies in 404 patients. The mean age of the patients was 45.7 years with an age range of years. There were 194 men (mean age, 40.9 years; range, years) and 225 women (mean age, 50.0 years; range, years) and 216 right and 203 left knees. MR Image Acquisition Patients underwent knee MRI on 1.5-T (Signa EchoSpeed HDX or TwinSpeed Excite, GE Healthcare) and 3-T (TwinSpeed, GE Healthcare) systems with a knee coil. MRI protocols incorporated the following sequences: sagittal fast spin-echo intermediate-weighted (TR range/te, /15 33; echo-train length, 8; mm section thickness; mm interslice gap; matrix; cm FOV; and 1 2 signal averages), sagittal spin-echo T1-weighted ( /11 15; echo-train length, 2 4; mm section thickness; mm interslice gap; matrix; cm FOV; and 1 signal average), sagittal fast spin-echo T2-weighted with fat saturation ( /60 75; echo-train length, 12; mm section thickness; mm interslice gap; matrix; cm FOV; and 1 2 signal averages), coronal fast spin-echo T2-weighted with fat saturation ( /60 75; echo-train length, 8; 4-mm section thickness; 1-mm interslice gap; matrix; 14-cm FOV, and 1 signal average), coronal spin-echo T1-weighted ( /11 15; echotrain length, 2; 4-mm section thickness, 1-mm interslice gap, matrix, 14-cm FOV, and 1 signal average), and axial spin-echo intermediate-weighted with fat saturation ( /25 40; echo-train length, 8; 4-mm section thickness; 1-mm interslice gap; matrix; 15-cm FOV; and 2 signal averages). MR Image Analysis The PMMRL was defined as extending from the tibial attachment point to just lateral from the articular cartilage inflection point of the medial tibial plateau [10] (Fig. 1). The PMMRL was evaluated on all examinations, and lesions were classified into three groups on the basis of the following definitions: degeneration, characterized by thickening of the root with intrasubstance hyperintensity not contacting the articular surface; partial tear, characterized by abnormal signal intensity extending to the articular surface or abnormal morphology of the root with partial root discontinuity; and complete tear, characterized by complete discontinuity of the affected root (Fig. 2). When a PMMRL tear was present, the tear pattern was also categorized into one of two types: radial or transverse, characterized by a vertical tear perpendicular to the longitudinal axis of the root ligament or longitudinal cleavage tear, characterized by a linear tear parallel to the longitudinal axis of the root ligament, which could be horizontally or vertically oriented (Fig. 3). Additionally, the point of failure was characterized as entheseal when it occurred at the tibial attachment point of the root ligament, midsubstance when it occurred within Fig. 1 Drawing shows anatomy of posterior medial meniscus root ligament (PMMRL). PMMRL was defined as from tibial attachment point of root ligament to just lateral from articular cartilage inflection point of medial tibial plateau. Ligament was subcategorized into three zones: transition to posterior horn of medial meniscus at junction between root and posterior horn (a), midsubstance within root ligament proper (b), and entheseal at tibial attachment point of root ligament (c). the root ligament proper, or transition to the posterior horn of the medial meniscus (PHMM) when it occurred at the junction between the root and posterior horn (Fig. 1). We recorded all points if the tear occurred at more than one location. All images were assessed for the presence of a medial meniscal tear in the meniscus proper, medial meniscal extrusion, insertional PMMRL osseous changes, regional synovitis, insufficiency fracture, and cruciate ligament degeneration and tearing. A meniscus tear was designated when intrasubstance signal intensity in the meniscus proper extended to an articular surface or abnormal meniscal morphology was noted on two or more images [11]. Medial meniscal tears were also subclassified into those with or without continuous involvement into the PMMRL. Medial meniscal extrusion was designated present when the medial meniscal body extended greater than 3 mm peripherally from the medial tibial plateau on the A B Fig. 2 MRI classification of posterior medial meniscus root ligament (PMMRL) lesions on sagittal fast spin-echo (FSE) intermediate-weighted (A, C, and E) and coronal FSE T2-weighted with fat saturation MR images (B, D, and F). A and B, MR images in 55-year-old man show degeneration. Localized intrasubstance hyperintensity within thickened root ligament (arrows) is seen with smooth surface. Outer smooth contour and total continuity of affected root ligament are preserved on consecutive sagittal images (not shown). (Fig. 2 continues on next page) AJR:203, December

3 Choi et al. coronal images [12]. Insertional PMMRL osseous changes were designated present when there was focal bone marrow signal alteration, such as edemalike or cystlike changes just beneath the tibial attachment point of the PMMRL (Fig. 4). Regional synovitis was designated present when intermediate-hyperintense villous synovial proliferation on fluid-sensitive images was seen in the medial tibiofemoral compartment. Insufficiency fracture was defined as a low-signal-intensity focus beneath the articular surface in the weightbearing area of the tibiofemoral compartment on T1-weighted images and central focal linear subchondral low-signalintensity or focal subchondral bone plate impaction with marked surrounding bone marrow edema on fat-suppressed T2-weighted images [13] (Fig. 5). Cruciate ligament degeneration was diagnosed when the ligament showed increased signal C E F Fig. 2 (continued) MRI classification of posterior medial meniscus root ligament (PMMRL) lesions on sagittal fast spin-echo (FSE) intermediate-weighted (A, C, and E) and coronal FSE T2-weighted with fat saturation MR images (B, D, and F). C and D, MR images in 52-year-old woman show partial tear. Marked decrease (arrow) in cross-sectional area of root ligament is shown on sagittal image (C), but there is preserved continuity on consecutive sagittal images (not shown). Coronal image (D) shows focal deep defect (arrow) of affected root ligament at transition to posterior horn of medial meniscus (PHMM) with wavy residual root ligament. E and F, MR images in 33-year-old woman show complete tear. Root ligament is completely absent at transition to PHMM on sagittal image (E). Fluid-filled full-thickness cleft (arrow) is also seen on coronal image (F). intensity, giving rise to an appearance similar to a stalk of celery [14] or associated cruciate ganglion cyst with fluidlike signal intensity in the substance of the ligament [15]. Cruciate ligament tear was designated when the ligament was discontinuous, thick or thin, amorphous, or hyperintense. To determine the interobserver reliabilities of the MRI assessments of PMMRL lesions, two board-certified radiologists with 11 and 18 years of experience in knee MRI retrospectively reviewed 40 selected knee MRI studies independently that included 10 normal knees, 10 degenerations, 10 partial tears, and 10 complete tears of the root ligament and were randomly mixed. Intraobserver retest for reliability was determined by comparing the repeat MRI classification made by a single observer with a 12-week interval between evaluations. The interobserver and intraobserver reliability for the presence of PMMRL lesions was evaluated using kappa statistics. The kappa coefficient of interobserver and intraobserver reliabilities for PMMRL lesion was and 0.930, respectively, indicating almost perfect agreement. Thus, MRI analyses taken by a single observer were used in the analyses. Radiographic Assessment of Knee Osteoarthritis Radiographic assessment of knee osteoarthritis was carried out using weightbearing anteroposterior and supine lateral knee radiography interpreted by a musculoskeletal radiologist. On the basis of the Kellgren-Lawrence (KL) radiographic osteoarthritis grading system [16], degrees of osteoarthritis in the tibiofemoral joint of the knee were assigned as five osteoarthritis grades: none (KL grade 0), doubtful (KL grade 1), minimal (KL grade 2), moderate (KL D 1288 AJR:203, December 2014

4 MRI of Posterior Medial Meniscus Root Ligament Lesions grade 3), and severe (KL grade 4). Radiographic tibiofemoral osteoarthritis was considered present if the KL grade was 2 or higher. Statistical Analysis The prevalence of PMMRL lesions and proportion of each lesion type were computed. The various clinical and radiologic parameters were statistically tested using univariate analysis. The Student t test was performed for quantitative variables, and a Pearson chi-square test was used for qualitative variables. All statistical analyses were carried out using SPSS for Windows, version 18.0, and p values of < 0.05 were considered significant throughout. TABLE 1: Characteristics of Knees With and Without Posterior Medial Meniscus Root Ligament (PMMRL) Lesions Parameters Results PMMRL lesions were found in 120 (28.6%) of 419 knees: degeneration in 60 (14.3%) knees, partial tear in 49 (11.7%), and complete tear in 11 (2.6%). Of the 60 PMMRL tears, radial or transverse tears were seen in 41 (68.3%) knees and longitudinal cleavage in 13 (21.7%), and miscellaneous tears in 6 (10.0%). In terms of the point of failure, most of the tears (56 tears, 93.3%) involved the junction to the PHMM: 33 (55%) involved only the junction to PHMM whereas 23 (38.3%) involved both the junction and midsubstance of the root ligament. The tears at the midsubstance and entheseal portions were only found in two (3.3%) and two (3.3%) knees, respectively. Results of assessments for concurrent MRI findings are summarized in Table 1. Medial meniscal tears in the meniscus proper were found in 166 (39.6%) of 419 knees and, interestingly, most PMMRL lesions (104 of 120 lesions, 86.7%) were accompanied by a tear in the meniscus proper. Moreover, 39 (65%) of 60 PMMRL tears were directly continuous with an adjacent medial meniscal tear. Univariate analysis revealed a significant difference between the knees with and without PMMRL lesions in patient age, medial meniscal tear, medial meniscal extrusion, insertional PMMRL osseous change, regional synovitis, osteoarthritis, insufficiency fracture, and cruciate ligament degeneration (Table 1). Discussion Tears of the PMMRL continue to receive increasing attention in the literature, largely PMMRL Lesions No (n = 299, 71.4) Yes (n = 120, 28.6) p Mean age ± SD (y) 41.4 ± ± 15.7 < Sex (male:female) 143:156 51: Medial meniscal tear in meniscus proper 62 (20.7) 104 (86.7) < Medial meniscal extrusion 23 (7.7) 61 (50.8) < Insertional PMMRL osseous changes 17 (5.7) 43 (35.8) < Regional synovitis 23 (7.7) 56 (46.7) < Osteoarthritis 27 (9.0) 51 (42.5) < Insufficiency fracture 11 (3.7) 12 (10) Anterior cruciate ligament tear 47 (15.7) 14 (11.7) Anterior cruciate ligament degeneration 14 (4.7) 29 (24.2) < Posterior cruciate ligament tear 11 (3.7) 3 (2.5) Posterior cruciate ligament degeneration 2 (0.7) 11 (9.2) < Note Data in parentheses are percentages. Statistically significant values (p < 0.05) are shown in bold. due to wide recognition of the importance of this structure. Nevertheless, detailed information on the PMMRL in terms of frequency, types, and associated factors is not well established. This study aimed to address these unclear issues related to the PMMRL lesion. In our study, PMMRL lesions, including degeneration and tearing, were frequent findings. Among 419 consecutive knees undergoing MRI for knee symptoms, we found root ligament lesions in 28.6%, including tears in 14.3%. Previous studies have shown wide variability in prevalence of PMMRL tears, ranging from 0.4 to 29.5% [7, 9, 12, 17 20]. It is likely that the variability in PMMRL A Fig. 3 Two patterns of posterior medial meniscus root ligament (PMMRL) tears on sagittal fast spin-echo (FSE) intermediate-weighted (A and C) and coronal FSE T2- weighted with fat saturation MR images (B and D). A and B, MR images in 67-year-old woman show radial or transverse tear. Nondelineation of root ligament (arrow) is noted on only one sagittal image (A). Full-thickness defect at transition to posterior horn of medial meniscus (arrow, B) is seen, direction of which is perpendicular to longitudinal axis of root ligament. (Fig. 3 continues on next page) B AJR:203, December

5 Choi et al. C D Fig. 3 (continued) Two patterns of posterior medial meniscus root ligament (PMMRL) tears on sagittal fast spin-echo (FSE) intermediate-weighted (A and C) and coronal FSE T2-weighted with fat saturation MR images (B and D). C and D, MR images in 63-year-old man show longitudinal cleavage tear. Linear fluid-equivalent hyperintensity (arrow) splitting root ligament into superior and inferior halves is seen on sagittal image (C). This lesion (asterisk, D) involves nearly whole length of root ligament, parallel to its longitudinal axis. tear prevalence is a result of a combination of factors, including different patient populations; differences in diagnostic methods (imaging vs arthroscopy); and differences in definition, including location and inclusion of partial tears. Most studies have used binary classification systems for meniscal root tears: normal or tearing and, most frequently, either a radial tear or avulsion at the insertion of the meniscus [18, 21, 22]. However, besides those types, our assessments have revealed Fig year-old woman with tibial posterior medial meniscus root ligament (PMMRL) insertional osseous change. Sagittal fast spin-echo T2-weighted MR image with fat suppression shows tiny cystic lesion (solid arrow) in subcortical bone marrow of tibia near root ligament tibial attachment. Small less hyperintense area (arrowhead) considered as bone marrow edemalike change is also noted in anterosuperior aspect of aforementioned cystic change. Partial tear (open arrow) of root ligament is seen. Fig year-old woman with posterior medial meniscus root ligament (PMMRL) tear and femoral insufficiency fracture. On coronal fat-suppressed fast spin-echo T2-weighted MR image, focal dark line (solid arrow) considered as insufficiency fracture is seen in subchondral area of distal femur. Associated large bone marrow edemalike change (arrowheads) surrounding fracture is noted. In addition, meniscal extrusion (> 3 mm) (open arrow) is observed. PMMRL tear is not shown. that PMMRL tears can be further classified into additional subtypes, such as longitudinal cleavage tear. We believe that longitudinal cleavage tears of the PMMRL may result from direct extension of preceding PHMM tears into the adjacent root ligament. However, this hypothesis warrants future independent longitudinal studies for confirmation. In terms of the point of failure, we observed that PMMRL tears mostly commonly occur at the junction with the PHMM followed by the midsubstance, and they least commonly manifest as an avulsion at the enthesis. A previous animal study had similar results regarding the most frequent point of failure [23]. Possibly, this is because the PMMRL is the stiffest meniscal attachment among all of the root ligaments and thus would be uniquely predisposed to injury at the junction with the PHMM during activities [24]. Moreover, the junction between the posterior horn and the root ligament may be structurally weaker because of the differences in morphology and collagen distribution between the fibrocartilagenous meniscus, root ligament, and enthesis [1, 25]. Additionally, PMMRLs are exposed to more stressful conditions in both tensile and compressive loads, whereas the anterior root ligaments are mostly exposed only to tensile loads [25]. We believe that a detailed description of PMMRL pathology would be helpful for orthopedic surgeons to manage PMMRL tears in clinical practice. Conventionally, PMMRL tears have been treated by meniscectomy [26, 27]. However, because recent biomechanical studies have revealed significant changes in contact pressure and knee joint kinematics due to PMMRL tears [3, 28], several techniques for meniscal root repair to restore joint biomechanics to near normal conditions have been developed. These techniques fall into two broad categories: transosseous suture repairs and suture anchor repairs [29 34]. In the case of transosseous suture repair, suture capturing should be performed approximately 2 3 mm from the edge of the tear; however, if a PMMRL tear occurs at the junction with the PHMM, capturing the posterior horn may create excessive tension on 1290 AJR:203, December 2014

6 MRI of Posterior Medial Meniscus Root Ligament Lesions the meniscal body and placing the sutures too close to the tear edge may result in suture cut-out and loss of fixation [29]. In addition, the further away the PMMRL tear is from the entheseal portion, the less favorable characteristics it could have for healing because the PMMRL is vascularized and amenable to repair whereas the paracentral posterior horn region is not [21, 35]. Thus, we believe that reporting more detailed information about the tear location, pattern, and adjacent PHMM would be more valuable to establish a management plan, such as repair, meniscectomy, or observation with conservative management, compared with simply reporting root tear. On the basis of our results, age, medial meniscal tear, medial meniscal extrusion (> 3 mm), insertional PMMRL osseous changes, regional synovitis, osteoarthritis, insufficiency fracture, and cruciate ligament degeneration were significantly different between the knees with and without PMMRL lesions. PMMRL lesions were more frequent in elderly persons, similar to the results from previous reports [7, 8]. However, contrary to previous reports of female predominance, our results revealed no significant difference in between sexes. This discrepancy may result from the different characteristics between study cohorts. Regarding the association with a medial meniscal proper tear, a previous study reported that 18% of PMMRL tears were accompanied by medial meniscal tears [22], whereas our study found that approximately 83% (50 of 60 PMMRL tears) of PMMRL tears were accompanied by medial meniscal tears at the meniscus proper. In addition to sample differences, an explanation for our different rates could be the differences in definition of the root because Lee et al. [22] defined the root as occurring within 5 mm of the tibial attachment, whereas we defined the root as beginning lateral to the articular cartilage inflection point of the medial tibial plateau, similar to other authors [10]. As one of the main reasons, we postulate that medial displacement of the posterior horn after a PMMRL tear may cause the posterior horn to be injured by direct friction between the femoral and tibial articular surfaces. Thus, when a PMMRL tear is detected on MRI, attention should be paid to find a concomitant medial meniscal tear in the meniscus proper and vice versa. As for meniscal extrusion, prior research has indicated that it is associated with meniscal root tearing [5, 6, 12, 22], and this is consistent with the results of our study. Regarding the insertional PMMRL osseous change at the tibial intercondylar area, to date, little information is available about its incidence and the relationship with PMMRL lesion. Its cause is not clear. We presume that these insertional PMMRL osseous changes, including intraosseous bone marrow signal alteration or cystic change, may stem from repetitive abnormal stress, such as traction force on the bone at the root ligament attachment site that may cause focal necrosis with liquefaction and ganglion formation [36]. Didolkar and Vinson [37] also suggested that cysts located at the insertions of the medial meniscus had a statistically significant association with meniscal abnormality. Regarding regional synovitis, the higher frequency in the knees with PMMRL lesions may be partly attributed to the abundant vascular distribution in the root ligament, which may allow production of an inflammatory response. Osteoarthritis was more frequently present in the knees with PMMRL lesions, as previously described in the literature [12, 17, 21, 22]. The higher incidence of insufficiency fracture in the knees with PMMRL lesions in this study is correspondent with previous reports [19, 38, 39]. Interestingly, we observed that cruciate ligament degeneration was significantly more frequent in the knees with PMMRL lesions. To our knowledge, there have been no reports regarding this relationship. We believe that PMMRL lesions and cruciate ligament degeneration may be related to degeneration and aging. Some limitations of this study warrant consideration. First, we did not provide confirmatory arthroscopic findings for our MRI findings. Nevertheless, knee MRI has a wellknown high accuracy for diagnosis of meniscal root tear, with sensitivity of 85 96% and specificity of % [9, 22, 38]. Additionally, it is difficult to visualize the entire PMMRL on routine arthroscopy, and degeneration may not be visible. Second, this study provided information on the relationship between several factors and PMMRL ligament lesions. Nevertheless, because of the crosssectional nature of our study, we are not able to draw definite conclusions of causal relationships of associated factors and PMMRL lesions. Further longitudinal studies are required to reveal the risk factors for PMMRL lesions. Finally, our study population was not representative of the general population because we enrolled only symptomatic subjects warranting knee MRI. However, our population more closely represents that which would be encountered in daily practice at a tertiary care center. In conclusion, PMMRL lesions are commonly detected in symptomatic patients. We have introduced an MRI classification system. 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