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First published online October 30, 2007
British Journal of Radiology (2008) 81, 20-24
© 2008 British Institute of Radiology
doi: 10.1259/bjr/66470309
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Full paper

Anomalous insertion of the medial meniscus into the anterior cruciate ligament: the MR appearance

J-G Cha, MD1, K-D Min, MD, PhD2, J-K Han, MD3, H-S Hong, MD, PhD1, S-J Park, MD, PhD1, J-S Park, MD, PhD1 and S-H Paik, MD1

Departments of 1 Radiology and 2 Orthopedics, Soonchunhyang University Bucheon Hospital, 1174, Jung-dong, Wonmigu, Gyeonggi-do, 420-021 and 3 Department of Radiology, Soonchunhyang University Cheonan Hospital 23-20 Bongmyung-Dong, Cheonan-Si, Chungnam-Do, 330-721, Republic of Korea

Correspondence: Jang Gyu Cha, Department of Radiology, Soonchunhyang University Bucheon Hospital, 1174, Jung-dong, Wonmi-gu, Gyeonggi-do, 420-021, Republic of Korea. E-mail: mj4907{at}schbc.ac.kr


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results
 Discussion
 References
 
The aim of this study was to determine the characteristic MR findings of the anomalous insertion of the medial meniscus (AIMM) into the anterior cruciate ligament (ACL), and to document potential pitfalls in its interpretation. We reviewed 1326 consecutive knee arthroscopic studies to identify patients with an AIMM. 30 knees of 26 patients (4 females, 22 males; average age, 31.3 years; range, 14–50 years) were included in this study. We evaluated the presence of an AIMM and analysed the MR findings, including the ACL attachment sites of the AIMMs, the absence of the transverse ligament, meniscal tears and a discoid meniscus. AIMMs were detected by MR imaging in 16 knees of 18 patients (60%, 18/30). The AIMMs showed a linear band with low signal intensity on T2 weighted sagittal images running parallel to the ACL. The AIMMs were inserted into the lower portion of the ACL in eight knees, the middle portion in five knees, and the intercondylar notch in five knees. Meniscal tears (10 medial, 10 lateral) were found in 20 knees of 16 patients. Six knees of five patients showed a discoid meniscus. 15 knees of 14 patients showed no transverse ligament on MR imaging. An AIMM has the potential to be misinterpreted as a meniscal tear, ACL tear or infrapatellar plica on knee MR imaging. Familiarity with the characteristic MR findings can aid in the detection of an AIMM into the ACL.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results
 Discussion
 References
 
Several meniscal anomalies have been reported, including discoid variants [1, 2], hypoplasia of the anterior horn [3], complete absence of the meniscus [4] and anomalous insertion of the medial meniscus (AIMM) into the anterior cruciate ligament (ACL) [5–12]. Among these anomalies, an AIMM into the ACL is rare. It runs from the anterior horn of the medial meniscus to either the ACL or the intercondylar notch (Figure 1Go). A Finnish arthroscopy-based study reported a prevalence of 1.2% for AIMMs [11]. Several case reports [9, 10, 13] have described the MR imaging of AIMMs; however, a focused description of the MR findings of AIMMs has not been reported. In this study, we describe the MR appearance of an AIMM into the ACL and characterize its relationship to meniscal anomalies, meniscal tears and infrapatellar plicae.


Figure 1
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  		Figure 1. Schematic drawing of knee flexion from the anterior view shows the anomalous insertion of the medial meniscus(AIMM) anterior into the anterior cruciate ligament (ACL) as it courses from the anterior horn of the medial meniscus to the anterior aspect of the ACL.

 

   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results
 Discussion
 References
 
Study group
We reviewed (C.J.G) 1324 surgical records of knee arthroscopy procedures performed at the Department of Sports Orthopaedics, Soonchunhyang University Bucheon Hospital between February 2001 and March 2004 to identify patients for this retrospective study. For each patient, knee arthroscopy was performed to evaluate knee injury or knee pain by the same surgeon with six years' experience in knee arthroscopy (M.K.D). Patients who had undergone previous knee surgery were excluded. Our study included 30 MR images obtained from 26 patients — 4 females and 22 males with a mean age of 31.3 years (range, 14–50 years) — with an arthroscopically confirmed AIMM. The patients were referred by the orthopaedic clinic for knee pain of a mean duration of 3 months (range, 2 weeks–6 months) in 18 patients; the remaining 8 patients were admitted because of proximal tibial fracture (n = 3), bone contusion (n = 1), patellar dislocation (n = 2) or sports injury (n = 2). This study was approved by the institutional review board of our hospital. Informed consent was not required for review of the records.

MR imaging techniques
All images were acquired with a Signa 1.5-T MR imager (GE Medical Systems, Milwaukee, WI). By using a dedicated knee coil (Quadrature coil; GE Medical Systems), fast spin-echo T2 weighted images were obtained in coronal and sagittal planes (4000 ms/85 ms (repetition time (TR)/echo time (TE)), 5 mm section thickness, 1 mm intersection gap, two signals acquired, 320x256 matrix, 14 cm field of view). Proton density images were obtained in the coronal plane (3000 ms/12 ms TR/TE, 4 mm section thickness, 0.5 mm intersection gap, two signals acquired, 320x256 matrix, 14 cm field of view). Proton density images with fat saturation were obtained in the sagittal plane (3000 ms/12 ms TR/TE, 5 mm section thickness, 1 mm intersection gap, two signals acquired, 320x256 matrix, 14 cm field of view). T1 weighted images were obtained in the sagittal plane (600 ms/10 ms TR/TE, 4 mm section thickness, 1 mm intersection gap, two signals acquired, 512x192 matrix, 14 cm field of view). Gradient two-dimensional T2* images were obtained in the axial plane (517 ms/15 ms TR/TE, 5 mm section thickness, 1 mm intersection gap, two signals acquired, 512x224 matrix, 20° flip angle, 16 cm field of view).

Image interpretation
For image analysis, all knee MR images were analysed by two radiologists with four (C.J.G) and three (H.J.G) years' experience in musculoskeletal radiology. Each MR imaging examination was evaluated for the presence or absence of an AIMM from the anterior horn of the medial meniscus to the anterior part of the ACL. The insertion site of the AIMM into the ACL was classified as Type 1 (inferior third), Type 2 (middle third) or Type 3 (superior third; intercondylar notch). We also evaluated the presence of the transverse ligament, infrapatellar plica and a discoid meniscus, and determined whether the meniscus was injured. A discoid meniscus is defined as being present if three or more 5-mm-thick contiguous sagittal images demonstrate continuity of the meniscus between the anterior and posterior horns [14].


   Results
Top
 Abstract
 Introduction
 Methods and materials
 Results
Discussion
 References
 
In this study, the incidence of AIMMs was 2.27% (30/1324). Low-signal band structures were detected in 18 out of 30 knees (60%) in 26 patients. The AIMMs showed a linear band with low signal intensity on T2 weighted sagittal images running parallel to the ACL (Figure 2aGo). Through their course, the AIMMs showed an isointense signal intensity to the meniscus and the ACL, with a high signal intensity gap between the ACL and the AIMM (Figure 2aGo).


Figure 2
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  		Figure 2. A 36-year-old male with acute knee injury. (a) Sagittal T2 weighted image of the left knee joint reveals an anomalous insertion of the medial meniscus (AIMM; arrows) attached to the middle portion of the. At the tibial attachment site of the AIMM, it mimics a tear of the anterior horn of the medial meniscus (arrowhead). (b) Sagittal T2 weighted image of the contralateral right knee joint also shows an AIMM with a high-signal-intensity gap between the AIMM (arrowheads) with the fan-shaped meniscal insertion (arrows). (c) The arthroscopy from the lateral portal view shows that the AIMM covers the anterior margin of the anterior cruciate ligament (ACL) in the left knee joint.

 
Descending to the tibial level, the AIMM attaches to the anterior horn of the medial meniscus with a broad base (GoFigures 2b and 3aGo). The AIMMs were inserted into the lower part of the ACL (Type I) in eight knees (Figure 2AGo), the middle part (Type 2) in five knees (Figure 3bGo) and the intercondylar notch (Type 3) in five knees on sagittal MR images.


Figure 3
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  		Figure 3. A 19-year-old male with a sports injury. (a,b) Consecutive sagittal T2 weighted images show the anomalous insertion of the medial meniscus (AIMM; arrows) inserting from the anterior horn of the medial meniscus and attaching to the lower portion of the anterior cruciate ligament (ACL). (c,d) Arthroscopy from the anterior lateral views reveals the AIMM linked with the anterior horn medial meniscus (arrowheads). The AIMM can be separated from the ACL (arrows) by the hook.

 
15 out of 30 knees (50%) revealed a transverse ligament on coronal and sagittal imaging. We also evaluated accompanying lesions in all 14 knees on MR imaging. 20 knees (66.67%, 16 patients) with AIMMs showed meniscal tears. In 10 knees of 7 patients, tears of the medial meniscus were seen. In 10 knees of 9 patients, tears of the lateral meniscus were revealed (all posterior horn).

Discoid menisci were seen in two medial (2/30, 6.67%) and four lateral menisci (4/30, 13.33%) in patients with AIMMs. One patient had a discoid meniscus on the medial meniscus of the right knee and on the lateral meniscus of the left knee (Figure 4a–cGo). In four knees of three patients, arthroscopic excision of the AIMMs was performed (GoFigures 2c and 3c,dGo). In post-operative follow-up, recurrent knee pain or limitation in the range of motion was not found in any of the patients in either the excision or non-excision groups. In patients with AIMMs, infrapatellar plicae were found by arthroscopy in all knees. Histologically, the anomalous band in two knees was fibrocartilaginous tissue, which was morphologically identical to meniscal tissue.


Figure 4
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  		Figure 4. A 18-year-old male with pain in both knee joints. (a) Coronal proton density-weighted image reveals a medial discoid meniscus (arrows) in the right knee joint. (b) Sagittal T2 weighted image of the controlateral left knee joint shows the anomalous insertion of the medial meniscus (AIMM) inserting into the anterior horn of the medial meniscus (arrows). (c) Coronal T2 weighted image of the left knee joint demonstrates a lateral discoid meniscus (arrows).

 

   Discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
References
 
Several studies on AIMMs have been reported since they were first described by Johnson [15]. The frequency of AIMMs in our study (2.26%, 30/1326 ) was much higher than that of a previous study [11]. Arjun et al [13] reported that AIMMs reveal a low-signal band-like structure extending from the anterior horn of the medial meniscus with a thin linear high-signal intensity band between the anomaly and the ACL on sagittal T2 weighted images; Nakajima et al [9] and Soejima et al [10] reported similar MR findings. The insertion sites of the AIMMs into the ACL, however, varied, with the lower insertion type being the most common (44.44%, 8/18). We believe that several reasons may explain why 40% of AIMMs were not detected on MR images: they may have been (i) too thin to be visualized by MR imaging, (ii) positioned improperly during MR imaging, or (iii) lacking a fibrotic portion in the AIMM. Nakajima et al [9] reported the absence of the transverse ligament on arthroscopy in a patient with an AIMM. We, however, found that 50% of the AIMMs had the transverse ligament present on MR imaging. A previous study on the frequency of the transverse ligament on MR imaging reported that this ligament was present in 53–58% of subjects [16, 17]. The frequency of the ligament in our study is close to this figure; therefore, we believe that there is no relationship between the absence of the transverse ligament and AIMMs.

We found that the prevalence of a medial discoid menicus in patients with AIMMs was 20–60 times greater than that reported in previous studies [1, 18]. In contrast, the prevalence of a lateral discoid meniscus in the AIMMs in our study was within the reported incidence range (0.8–16.6%) for Korean and Japanese populations [19, 20]. We believe that AIMMs have a closer connection with a medial discoid meniscus than a lateral discoid meniscus.

An infrapatellar plica looks similar to an AIMM, with a curvilinear appearance anterior to the ACL on sagittal T2 weighted images [21]. We postulate that the infrapatellar plica may be embryologically related to the AIMM because all cases showed infrapatellar plicae as thin membranes on arthroscopic studies. In addition, AIMMs and infrapatellar plicae have a chronological similarity in the developmental stage according to recent embryological studies. During the 10th and 11th week of embryologic development, the anterior horn of the medial meniscus attaches to the anterior aspect of the upper tibia near the joint. This may be a critical period for the development of the AIMM line [22], whereas the infrapatellar plicae may be formed when the synovial folds that compartmentalize the fetal knee joint fail to undergo regression during the same period [23, 24]. Therefore, we hypothesize that an AIMM may be one variation of infrapatellar plicae.

A tear of the anterior horn of the medial meniscus, ACL and infrapatellar plicae should be considered in differential diagnosis. Firstly, a tear in the anterior horn of the medial meniscus is prone to mimic an AIMM, but AIMMs can be confirmed by tracing the medial margin of the anterior horn of the medial meniscus in serial T2 weighted sagittal images. In addition, tears confined to the anterior third of the medial meniscus, particularly in its lateral part, are exceedingly rare, and therefore differentiating an AIMM from a tear is not a problem in practice. Secondly, the origin of the infrapatellar plica attaches not to the anterior horn of the medial meniscus but to Hoffa's fat pad or the inferior pole of the patella, which can be confirmed on MR images and arthroscopic studies. However, the previously documented characteristic MR findings, such as a linear low-signal-intensity structure anterior and parallel to the ACL on sagittal images, were not recognized in our study [21]. Furthermore, Arjun et al [13] reported that AIMMs could be mistaken for partial rupture of the ACL. However, this anomaly can be demonstrated by tracing from the anterior horn of the medial meniscus to the ACL on thin serial sections of T2 weighted images.

Controversy still exists as to whether AIMMs can lead to clinical problems. Kim et al [25] reported that 4 out of 77 patients with a discoid meniscus had an AIMM, and concluded that the anomaly bore no relation to the patient's symptoms. We could not find any difference between the excision and non-excision groups upon post-operative evaluation. Future studies will be required to ascertain the effect of the excision of AIMMs on a patient's symptoms.

In contrast, several reports highlight the potential for AIMMs to cause clinical symptoms. Santi and Richardson [8] reported the case of a patient who complained of anterior medial knee pain and showed bilateral AIMMs and Marfan's syndrome habitus. The pain disappeared after excision of the anomalous meniscal portion. To an orthopaedic surgeon, AIMMs may complicate the arthroscopic evaluation of the ACL, as is the case with infrapatellar plicae [21].

Limitations of this study include patient population bias, and a further study involving a control group is needed to assess the true incidence of AIMMs.

In conclusion, an AIMM may be misinterpreted as a tear of the ACL, meniscal tear or infrapatellar plica, and may complicate arthroscopy. Familiarity with the characteristic MR findings can aid in the prospective detection of AIMMs.

Received for publication December 23, 2006. Revision received March 12, 2007. Accepted for publication April 5, 2007.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 References
 

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This Article
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