Meniscofemoral ligaments: patterns of tears and pseudotears of the menisci using cadaveric and clinical material

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1 Skeletal Radiol (2007) 36: DOI /s SCIENTIFIC ARTICLE Meniscofemoral ligaments: patterns of tears and pseudotears of the menisci using cadaveric and clinical material Marcelo R. de Abreu & Christine B. Chung & Debbra Trudell & Donald Resnick Received: 21 July 2006 /Revised: 2 January 2007 /Accepted: 19 February 2007 / Published online: 5 May 2007 # ISS 2007 Abstract Purpose The purpose of the study was to determine the different types of pseudotears of the posterior horn of the lateral meniscus caused by the nearby meniscofemoral ligaments (MFLs), and to correlate the presence of these ligaments with patterns of meniscal tear. Design Retrospective clinical study with patients and prospective observatory study with cadaveric material. Patients Magnetic resonance imaging studies of the knee in 49 patients who had subsequent arthroscopy of the knee performed over a 1-year period at a single institution were reviewed by two readers in consensus for the presence and morphology of the MFLs of Humphry (LH) and Wrisberg (LW). Ten cadaveric knee specimens were used for MRI, anatomic, and histologic study. Results The LH was present in 55% of patients, the LW in 94%, and both were present in 44.9%. The thickness of the LH and LW ranged from 1 3 mm (mean 1.9, SD 0.61), and from mm (mean 1.8, SD 0.65) respectively (p> 0.05). A pseudotear in the posterior horn of the lateral M. R. de Abreu : C. B. Chung : D. Trudell : D. Resnick VA Health Care System, University of California San Diego, La Jolla Village Drive, San Diego, CA 3655, USA M. R. de Abreu Hospital Mae de Deus, Porto Alegre, Brazil M. R. de Abreu PPG Clinica Medica, Universidade Federal Rio Grande do Sul, Rio Grande do Sul, Brazil M. R. de Abreu (*) Rua Costa 128, CEP Porto Alegre, RS, Brazil marcelorad@hotmail.com meniscus was present in 63% of patients. In 13% the pseudotear was vertically oriented, and in 87% the pseudotear had an anterosuperior to posteroinferior orientation, ranging from 37 to 87. There was no association between the presence of one or both MFLs and the occurrence of medial or lateral meniscal tears (p>0.05). Conclusion Meniscofemoral ligaments are frequent anatomical structures that are found in the majority of knees with MRI. They commonly cause a pseudotear of the posterior horn of the lateral meniscus that can be simple, double, or complex in appearance, with vertical or anterosuperior to posteroinferior orientation. Keywords Meniscofemoral ligaments. Magnetic resonance imaging. Knee. Joint Introduction The meniscofemoral ligaments (MFLs) are structures that attach the posterior horn of the lateral meniscus to the femur. The most common of these, the posterior meniscofemoral ligament (of Wrisberg), serves to attach the posterior horn of the lateral meniscus to the intercondylar area of the femur, passing behind the posterior cruciate ligament (PCL; Fig. 1). Less common, the anterior MFL (of Humphry) serves to attach the posterior horn of the lateral meniscus to the intercondylar area of the femur, passing in front of the PCL [1]. Whereas the attachment sites and course of the ligaments are relatively uniform, their incidence is not. Persons may have both, one, or none of the MFLs in each knee [1, 2]. The accessory ligaments of Humphry and Wrisberg are believed to increase the congruity between the meniscoti-

2 730 Skeletal Radiol (2007) 36: horn of the lateral meniscus and other soft tissue structures can be effectively investigated. By virtue of their anatomy, the MFLs have been implicated in the false diagnosis of a tear of the posterior horn of the lateral meniscus during MRI [7]. The purpose of this study was to determine the different types of pseudotears of the posterior horn of the lateral meniscus caused by the insertion of nearby MFLs, and to correlate the presence of these ligaments with patterns of meniscal tear using MRI in a series of patients and cadaveric specimens. Materials and methods Both the clinical and cadaveric part of the study were approved by the Institutional Review Board. Informed consent was obtained from patients or relatives as required. Clinical study Patients Fig. 1 a Computer drawing (posterior view of the right knee) demonstrating the meniscofemoral ligaments (MFLs) of Humphry and Wrisberg, and the pseudotear zone (P). b Corresponding coronal T1-weighted spin-echo MR image (600/14). PCL posterior cruciate ligament, MM medial meniscus, LM lateral meniscus bial socket and the lateral femoral condyle. This occurs primarily during weight-bearing with the knee flexed and the foot firmly fixed. The resultant anterior femoral translation serves to tighten the meniscofemoral ligament and draw the posterior horn of the lateral meniscus anteromedially [3]. The MFLs are often the sole attachment of the posterior horn of the lateral meniscus when there is no posterior meniscal root [4]. The lateral meniscus, unlike the medial meniscus, has no firm tibial or femoral peripheral attachments, which is one of the reasons for its increased mobility compared with its counterpart [5]. Although it has been generally thought that the MFLs always insert into the femoral condyle, the MFL can insert into the PCL [6]. With magnetic resonance imaging (MRI), the identification of the MFLs and their relationship with the posterior Forty-nine MRI studies of the knee in 49 patients (42 men, 7 women; age range years; mean age 47.2 years, SD 12.8) who had subsequent arthroscopy of the knee performed over a 1-year period at a single institution were reviewed for the presence and morphology of MFLs. One or both of these ligaments were considered present based on the following imaging criteria: a ligamentous formation greater than 0.5 mm in thickness originating from the posterior horn of the lateral meniscus with a course adjacent to the posterior cruciate ligament (PCL) and insertion into the medial portion of the medial femoral condyle. If the course of the ligament was anterior to the PCL it was designated a ligament of Humphry and, if posterior, a ligament of Wrisberg. Imaging Images were acquired with a 1.5-T MR imager (Signa; GE Medical Systems, Milwaukee, WI, USA). In all cases, imaging was performed in three standard planes with a phased array knee coil with a 16-cm square field of view, 3-mm slice thickness with a 1-mm gap, matrix of , and number of excitations between 2 and 3. The following sequences were acquired: coronal spin echo (SE) T1- weighted ( ms/12 25 ms), coronal fat-suppressed fast SE T2-weighted (3,200 4,400 ms/67 85 ms, echo train length of 4 to 6), axial fat-suppressed fast spin echo (FSE) Proton Density weighted (2,000 2,200 ms/20 40 ms), sagittal fast spin echo (FSE) Proton Density weighted (2,000 2,200 ms/20 40 ms), and sagittal short inversion

3 Skeletal Radiol (2007) 36: time inversion recovery (STIR; 3,000 4,585 ms/34 60 ms/ 150 ms [inversion time msec]) images. In 6 cases, SE T1- weighted MR images with and without fat suppression were acquired before and after intravenous administration of 0.1 mmo/kg gadopentetate-containing contrast material (Magnevist; Berlex, Wayne, NJ, USA). Image interpretation The two musculoskeletal radiologists who retrospectively analyzed the MRI studies of the knee in consensus were asked to evaluate several parameters. Readers determined the presence or absence of each MFL, its thickness, and the presence or absence of a pseudotear in the posterior horn of the lateral meniscus. The criterion used to establish the presence of a pseudotear was the visualization of a linear intermediate or high signal intensity zone in T1, T2 or proton density-weighted images in at least 1 sagittal slice in the posterior horn of the lateral meniscus extending to the articular surface of the meniscus that, when followed in subsequent slices, was seen to originate from the MFL. The pseudotear generated by one or two MFLs was analyzed in more detail regarding its extent, its angulations in the sagittal plane with regard to the tibial plateau (see Fig. 2), and the presence or absence of high signal intensity inside the pseudotear in fluid sensitive images (T2-weighted, STIR). Arthroscopy reports were recorded with regard to the presence and location of meniscal tears. Cadaveric study Specimens Eleven cadaveric knee specimens, harvested from unembalmed cadavers, were examined with radiography to ensure integrity of the specimen and to exclude pathologic bone or soft-tissue conditions. Ten fresh human knees (8 men, 2 women; age range at death years; mean age at death 78 years) were subsequently selected for the MRI and anatomic study. The cadaveric specimens were immediately deep-frozen at 60 C (Bio-Freezer; Forma Scientific, Marietta, OH, USA). None of the specimens had evidence of surgical intervention or previous injuries in or about the knee. The cadaveric specimens were allowed to thaw for 18 h at room temperature prior to MRI. Subsequently, the cadaveric specimens were prepared according to methods described in the literature [8]. Imaging Magnetic resonance images were acquired with a 1.5-T superconducting MR imager (Signa; GE Medical Systems, Milwaukee, WI, USA) with two 5-inch standard flexible surface coils (Flex Coil; Medical Advances, Milwaukee, WI, USA) positioned posteromedially and posterolaterally at the menisci level. All cadaveric knees were placed in a neutral position and immobilized with foam pads. Imaging Fig. 2 a Sagittal T1-weighted spin echo MR image (600 ms/ 20 ms) of a cadaveric knee specimen obtained after the injection of a gadopentetate-containing contrast material demonstrating the pseudotear (white arrowhead) generated by the connection of the ligament of Wrisberg and the posterior horn of the lateral meniscus and the measurement of its angulation with regard to the tibial plateau. b Corresponding Faxitron of the same cadaveric slice showing fat density at the pseudotear. Meniscus (black arrow), ligament of Wrisberg (white arrow), and pseudotear (curved arrow)

4 732 Skeletal Radiol (2007) 36: was performed in the coronal, transverse, and sagittal planes. The MRI protocol consisted of T1-weighted spinecho (SE) sequences (repetition time ms/echo time ms= 600/20 23). To acquire a higher signal-to-noise ratio, a section thickness of 2.5 mm with a space gap of 0.5 mm was used. The field of view was cm in the sagittal, coronal, and axial planes, and the data acquisition matrix was pixels. SE T1-weighted MR images in sagittal, coronal, and axial planes with and without fat suppression were acquired before and after intrarticular administration of 15 ml of a gadopentetate-containing contrast agent solution at 1% concentration (Magnevist). MRI studies were evaluated by the consensus of two musculoskeletal radiologists with emphasis on the MFLs of the knee. Knee sectioning After MRI was performed, the cadaveric specimens were frozen again at 60 C for more than 96 h and were subsequently sectioned in the sagittal plane with a band saw into 3-mm-thick slices. Each sagittal slice (23 slices per patient; 10 patients) was photographed and imaged with high resolution radiography (Faxitron; Hewlett Packard, McMinnville, OR, USA). To determine the anatomic relationships of the MFLs with the meniscus and surrounding structures, the findings at MRI were correlated with those derived from inspection of cadaveric sections. The cadaveric study was analyzed by the consensus of two musculoskeletal radiologists, the same ones who performed evaluation of the clinical study. Table 2 Number of sagittal slices in which the pseudotear was observed with MRI in 49 patients Number of sagittal slices Ligament present (%) 1 6 (19) 2 15 (48) 3 8 (26) 4 4 (6) Total pseudotears 31 (100) were stained using the hematoxylin-eosin technique. Histologic sections were analyzed at light microscopy (magnification, 34 to 3,100) in consensus by a musculoskeletal radiologist and an experienced orthopedic pathologist (30 years experience in orthopedic pathology). The examiners recorded in consensus the presence of the pseudotears and its tissue composition. Results Clinical study The frequency of MFLs in the study patients is shown in Table 1. A pseudotear in the posterior horn of the lateral Histology To analyze the tissue composition of the pseudotears generated by the MFLs and its intimate relationship with adjacent structures at its insertion site, histologic samples of this area were collected in 3 knee specimens. Samples were suspended in a 10% formalin solution for histologic analysis immediately after inspection of the cadaveric specimens. Specimens were embedded in paraffin and sectioned further into 5-μm-thick slices. Histologic sections Table 1 Frequency of meniscofemoral ligaments (MFLs) in 49 clinical patients Presence (%) Mean thickness, mm (range) Average angle of pseudotear Humphry 27 (55) 1.9 (1 3) 63 Wrisberg 46 (94) 1.8 (1 3.8) 58 Both ligaments 22 (45) Total patients 49 No statistic difference between thickness or average angle (p>0.05). All patients had at least one MFL Fig. 3 Sagittal proton density (PD)-weighted fast spin-echo MR image (2,200 ms/40 ms) in a 36-year-old woman demonstrating the anatomic configuration of the posterior compartment of the knee between the PCL and the lateral meniscal root (mr), with the MFLs of Humphry (white arrow), Wrisberg (white arrowhead), and joint capsule (black arrowhead)

5 Skeletal Radiol (2007) 36: meniscus was present in 63% of patients (31 out of 49). The numbers of sagittal slices in which the pseudotear was observed varied from 1 to 4 and are summarized in Table 2. Fluid in the pseudotear was observed in 10% of cases (5 out of 31). In 13% (4 out of 31) the pseudotear was vertically oriented, and in 87% (27 out of 31) the pseudotear had an orientation from anterosuperior to posteroinferior, ranging from 37 to 87 (mean 60.6, SD 17.6). The angle of the pseudotears generated by ligament of Wrisberg (63 ) and ligament of Humphry (58 ) did not differ statistically (p<0.05). When both ligaments were present (45%, 22 out of 49) 1 pseudotear was visualized in 14 patients, 2 pseudotears in 2 patients (see Fig. 3), and no pseudotear in 6 patients. Two pseudotears were observed in 2 additional patients caused by 1 MFL and the joint capsule. When both ligaments were present and there was 1 pseudotear, the pseudotear corresponded to the ligament of Wrisberg in 39%, and to the ligament of Humphry in 15%; in 46% of cases, the ligament causing the pseudotear could not be determined. On arthroscopic examination, medial meniscus tears were seen in 65% (32 out of 49) of patients in one or two locations: in the anterior horn in 3% (1 out of 33), in the body in 9% (3 out of 33), in the posterior horn in 72% (24 out of 33), and as a bucket handle tear in 15% (5 out of 33). The lateral meniscus was torn in 25% (12 out of 49): a tear in the body was seen in 42% (5 out of 12) and in the posterior horn in 58% (7 out of 12). In 1 case a tear of the posterior horn of the lateral meniscus was noted to happen at the insertion site of the MFL of Wrisberg (see Fig. 4). There was no association between the presence of one or both MFLs and the occurrence of medial or lateral meniscal tears (p>0.05). Cadaveric study The ligament of Humphry was present in 5 knees and the ligament of Wrisberg in 7 knees. Both MFLs were present in 5 knees. There was no knee joint without at least 1 meniscofemoral ligament. A pseudotear of the posterior horn of the lateral meniscus was present in 6 joints (see Fig. 5); in 1 knee there were 2 pseudotears, 1 caused by the ligament of Wrisberg and the other by the ligament of Humphry (see Fig. 6). The angle of the pseudotears ranged from In 3 knee specimens the pseudotear was seen in only 1 section, in 2 specimens in 2 sections, and in 1 specimen in 3 sections. In specimen number 2, there was no evidence of a meniscal root and the meniscofemoral ligaments were the only insertion structure of the posterior horn of the lateral meniscus; in specimen number 3, the ligament of Wrisberg inserted into the joint capsule and not into the lateral meniscus; and in specimen number 6 the ligament of Wrisberg was bifid. Results of histology On histological study of the site of the pseudotear in the posterior horn of the lateral meniscus, the MFLs showed architecture and composition very similar to the adjacent meniscal tissue. Collagen fibers were responsible for the majority of MFLs, with their orientation very similar to the fibers of the external margin of the adjacent meniscus. All histologic samples collected in 3 knee specimens showed a pseudotear caused by the MFLs. The space between the MFL and the meniscus (the pseudotear) was composed of a gap in the density of the collagen fibers of the MFL and of Fig. 4 a Sagittal PD-weighted fast spin-echo MR image (2,200 ms/40 ms) in a 30-yearold man with an arthroscopically proven tear at the MFL insertion site, demonstrating a wide space between the ligament of Wrisberg (white arrowhead) and the posterior horn of the lateral meniscus (white arrow). b Lateral slice of the same knee demonstrating a vertical tear (arrowhead) at the pseudotear location

6 734 Skeletal Radiol (2007) 36: Discussion Fig. 5 Sagittal cadaveric slice acquired at the level of the posterior horn of the lateral meniscus showing the pseudotear (arrowheads) caused by the MFL of Wrisberg after its connection with the meniscus. M meniscus, W ligament of Wrisberg orientation of these fibers in a different direction. A small quantity of fat intermixed with the collagen fibers was also noted at the site of the pseudotear. The high incidence of MFLs shown in our clinical study (Humphry ligament, 55%; Wrisberg ligament, 94%) is in agreement with the results of two recent anatomical studies [9, 10]. Other studies have reported an incidence of MFLs that varied from 71 to 80% of knees [1, 3]. Although some reports suggest that the ligament of Humphry is thinner than the ligament of Wrisberg [3], others suggest that when one ligament is thin or absent, the other is thick [10]. Our results showed no difference between the sizes of the MFLs on cross-sectional measurement. The pseudotear caused by the high signal band located between the MFLs and the posterior horn of the lateral meniscus was seen frequently (63%) on the MR images of our patients. The lack of visualization of a pseudotear in 37% of patients with MFLs was probably related to the slice thickness used in MR imaging and also to the presence of thin structures traveling in oblique planes. When present, the pseudotear was visualized in 1 (19%), 2 (48%), 3 (26%), or 4 (6%) sagittal slices. When seen in only 1 sagittal slice, the pseudotear could be caused by a volume average artefact of the MFL connection with the meniscus. The visualization of the pseudotear in multiple continuous sagittal slices was Fig. 6 a Sagittal T1-weighted spin echo MR image (600 ms/ 20 ms) of a cadaveric knee specimen obtained after the injection of a gadopentetate-containing contrast material demonstrating the connection of the MFLs of Humphry (H) and Wrisberg (W) with the posterior horn of the lateral meniscus generating two pseudotears. b Corresponding sagittal cadaveric slice acquired at the same level. c Corresponding Faxitron showing the collagen fibers structure of the ligament of Humphry (black arrow) and Wrisberg (white arrow). d Corresponding histology (hematoxylin-eosin stain; objective magnification 10) at the same level

7 Skeletal Radiol (2007) 36: dependent on the type of insertion of the MFL. In cases in which it inserted with a vertical orientation, it was seen on just 1 image; in cases in which it paralleled the posterior horn of the lateral meniscus, it was seen on more than 1 sagittal image. As an estimate, with a slice thickness of 3 mm and an interslice gap of 1 mm, visualization of the pseudotear on 4 sagittal images implies that the MFL extended parallel to the posterior horn of the lateral meniscus for about 16 mm. Steep angles of the MFLs create more pseudotears. The orientation of the pseudotears was always from an anterosuperior to a posteroinferior direction with angulation varying from 37 to 90, and a few cases with a vertical angle (90 ). The degree of angulation of the pseudotears did not differ according to which MFL was present. The visualization of just one pseudotear when both ligaments were present (14 out of 22) suggests that the MFLs connect with each other before they connect to the lateral meniscus Although some authors consider that the ligaments of both Humphry and Wrisberg are anterior and posterior branches of one meniscofemoral ligament [11], we prefer, as do other authors [3, 12], to consider the MFLs as individual ligaments. In support of this, we observed in 2 patients 2 pseudotears in the same sagittal slice, each corresponding to 1 individual MFL. This appearance has not yet been described in the literature. In 2 patients and in 1 cadaveric specimen, however, the presence of two pseudotears in one sagittal slice was related to a single MFL and the insertion of the capsule into the meniscus. In 4 patients, high signal on fluid-sensitive sequences was observed in the pseudotear. The fluid between the MFL and the posterior horn of the lateral meniscus could be related to a groove in the proximal insertion of the MFL as the fluid was observed on only one sagittal image. Furthermore, the cadaveric and histological study revealed that there were collagen fibers and small amounts of fat between the MFL and the meniscus. In 1 cadaveric specimen there was an absence of the posterior root ligament attachment of the lateral meniscus. In this case the only insertion into the posterior horn of the lateral meniscus was the MFL. This variant has been described by other authors [4]. We believe that the absence of this posterior root ligament is more frequent than reported. Although there have been few reports regarding the function of the MFLs, these accessory structures are believed to increase the congruity between the meniscus and articular surface in the posterolateral aspect of the knee joint. According to Friederich and O Brien [12], the ligament of Humphry becomes tenser during knee flexion, stabilizing the posterior horn of the lateral meniscus during that motion. The ligament of Wrisberg is believed to provide meniscal stability during knee extension [12]. Also, during external rotation of the femur on the tibia, the posterior horn of the lateral meniscus is drawn anteromedially by the ligament of Humphry. It is believed that the combined action of the popliteus muscle through its aponeurotic meniscal attachment is important, improving controlled meniscal motion. As a safeguard against lateral meniscal injuries during knee motion, the MFLs probably have a similar function; the absence of one is compensated for by the hypertrophy of the other, as indicated by other authors [10]. The lack of correlation between the frequency of arthroscopically evident meniscal tears and the presence of one or other MFL reinforce this idea. We recognize that our study has several limitations. First, the number of patients was relatively small.. The study was also retrospective in nature and we did not have sufficient patient clinical information. Our MRI observations were confined to the sagittal plane. The anatomical study was done in a small number of cadaveric knees derived from elderly persons with some degree of osteoarthritis. The histological analysis was made only in a few selected cases. Meniscofemoral ligaments are frequent anatomical structures that are found in the majority of knees on MRI. They commonly cause a pseudotear of the posterior horn of the lateral meniscus that can be simple, double, or complex in appearance, with a vertical or anterosuperior to posteroinferior orientation. References 1. Radoievitch S. Les ligaments des ménisques interarticulaires du genou. Ann Anat Pathol 1931; 8: McCormack D, McGrath J. Anterior menisco-femoral ligament. Clin Anat 1992; 5: Heller L, Langmen J. The meniscofemoral ligaments of the human knee. J Bone Joint Surg 1964; 46B: Warwick R, Williams P, editors. Gray s anatomy of the human body. 35th British ed. Philadelphia: Saunders; p Seebacher JR, Inglis AE, Marshall JL. The structure of the posterolateral aspect of the knee. J Bone Joint Surg 1982; 64A: Cho JM, Suh JS, Na JB et al. Variations in the meniscofemoral ligaments at anatomical study and MR imaging. Skelet Radiol 1999; 28: Vahey TN, Bennett HT, Arrington LE, Shelbourne KD, Ng J. MR imaging of the knee: pseudotear of the lateral meniscus caused by the meniscofemoral ligament. Am J Roentgenol 1990; 154: Hodler J, Trudell D, Kang H et al. Inexpensive technique for performing magnetic resonance: pathologic correlation in cadavers. Invest Radiol 1992; 2: Wan AC, Pelle P. The menisco-femoral ligaments. Clin Anat 1995; 8: Yamamoto M, Hirrohata K. Anatomical study on the meniscofemoral ligaments of the knee. Kobe J Med Sci 1991; 37: Lee BY, Jee WH, Kim JM, Kim BS, Choi KH. Incidence and significance of demonstrating the meniscofemoral ligament on MRI. Br J Radiol 2000; 73: Fiederich NF, O Brien WR. Functional anatomy of the meniscofemoral ligaments. Presented at the Forth Congress of the European Society for Knee Surgery and Arthroscopy, June 1990.