Abstract
Menisci congenital anomalies are rare morphologic abnormalities including accessory meniscus, discoid meniscus, double-layered meniscus, hypoplastic meniscus and ring-shaped meniscus (RSM). In a 35 year-old male patient, MR arthrography showed a bridging accessory bundle connecting the anterior cruciate ligament and posterior cruciate ligament with the posterior horn of the lateral meniscus. Arthroscopic examination showed a fan-like obstacle embracing the posterior horn of the lateral meniscus. It would be important to correctly identify this anatomical variant, because the bundle connecting the external meniscus to the ligaments of the central pivot can be misinterpreted as a meniscal fragment.
Keywords: Knee, MRI, MR-arthrography and anatomical variants
Abbreviations: RSM, ring-shaped meniscus; MRI, magnetic resonance imaging; ACL, anterior cruciate ligament; PCL, posterior cruciate ligament
Background
The menisci act as shock absorbers and are important stabilizers of the knee joint [1].
The menisci in human embryos develop from an intermediate mesodermal blastema, which has thick-plate shape, and are fully defined approximately eight weeks after conception [2], [3], [4].
The lateral meniscus is an incomplete oval-shaped structure more mobile than the medial meniscus, as it lacks capsular attachments [3].
Congenital anomalies of the menisci are rare morphologic abnormalities including accessory meniscus, discoid meniscus, double-layered meniscus, hypoplastic meniscus and ring-shaped meniscus (RSM), which is usually limited to the lateral meniscus [5], [6], [7], [8], [9], [10], [11], [12], [13].
We present a case report of an anatomical variant of the lateral meniscus.
Case presentation
A 33 year-old male was referred to our department due to a 2 month history of right knee pain after a weight-bearing valgus stress injury occurred during exercise.
Magnetic resonance imaging (MRI) revealed a "double posterior cruciate ligament”, without morphological lesions of the lateral meniscus, with anomalous appearance at tibial insertion.
A differential diagnosis between meniscal tibial attachment lesion and anatomical variant was considered. Subsequently an MRI-arthrography was performed showing an accessory "bridging" bundle that connected both the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) with the lateral meniscus posterior horn (Fig. 1A-D, Fig. 2A-F, Fig. 3A-C).
Fig. 1.
(A-D). a: MRI. Sagittal DP fat saturation weigthed image. MR pattern of double PCL (white arrows). (B) MRI.Coronal DP fat saturation weigthed image. Abnormal insertion of the lateral meniscus posterior horn (empty arrow). (C) MR arthrography. Sagittal GE T1 fat saturation weigthed image. Accessory "bridging" bundle that connected the ACL with lateral meniscus posterior horn (thin white arrow). (D) MR arthrography. Coronal SE T1 weigthed image. Abnormal bridge connection between lateral meniscus posterior horn and PCL (curved arrow).
Fig. 2.
(A-F): MR arthrography. Sagittal GE T1 fat saturation weigthed image, from the lateral compartment to the medial compartment. An accessory "bridging" bundle like MR pattern of double PCL (white arrows) that connected ACL (empty arrow) and the PCL (curved arrow) with the lateral meniscus posterior horn. Lateral meniscus anterior horn (thin white arrow)
Fig. 3.
(A-C) MR arthrography. axial SE T1 weigthed image at the level of the meniscal plane and intercondylar space. Abnormal bridge connection between lateral meniscus posterior horn (arrowhead) and PCL (curved arrow). "bridging" bundle (white arrow) that connected the ACL (empty arrow) with PCL (curved arrow)
Arthroscopic examination confirmed the anatomical variant previously detected by MRI. In particular, the postero-medial approach showed a barrier from the posterior cruciate ligament, embracing the posterior horn of the lateral meniscus (Fig. 4).
Fig. 4.
Arthroscopy. The postero-medial approach showed a posterior cruciate ligament barrier embracing the posterior horn of the lateral meniscus
The structure was like a reparative scar due to a damage to the posterior cruciate ligament. The anamnestic data excluded the reparative finding.
Discussion and literature review
Each meniscus can be anatomically divided in three parts: anterior horn, body and posterior horn. The function of both anterior and posterior horns is to secure the menisci to the tibial plateau; these are critical structures for their biomechanical function [14,15].
Meniscal variants are frequently identified on routine MRI and most commonly involve the lateral meniscus [16].
The discoid meniscus is the most frequent meniscal variant and has been subclassified into the modified Watanabe classification as complete discoid meniscus, incomplete discoid meniscus, Wrisberg type meniscus and the RSM [17,18]. Through a literature review, there's a number of variants of intermeniscal ligaments, including the anterior and posterior (transverse) ligaments and the oblique intermeniscal ligament [17].
Iqbal et al reported a series of RSMs in six patients including a description of two cases of medial RSM and a case of bilateral lateral RSM [17].
Fujikawa et al report a case of ring-shaped lateral meniscus with a small vertical tear of the posterior part and two cases of patients with lateral meniscal malformations of the knee that were found incidentally. MRI of both cases showed a fragment-like meniscus structure located in the intercondylar notch, very similar to the "fragment in the intercondylar notch sign" observed in displaced bucket-handle tears. Arthroscopic examinations revealed a ring-shaped lateral meniscus in one case, and "flipped-over" lateral meniscus in the other. In the latter, a similar lateral meniscal malformation was observed contralaterally at MRI-arthrography [19].
Esteves C et al report a case of a 25 year-old girl with progressive knee pain whose MRI demonstrates a ring-shaped lateral meniscus and an hypoplasic anterior cruciate ligament [20].
Sadigursky D et al report a case of a 36 year-old Hispanic man presenting several symptoms, including knee pain, locking and effusion.
MRI showed anatomical abnormality of the menisci corresponding to the junction of the posterior horns of the menisci in tandem with the insertion of the posterior meniscus fibers in the anterior cruciate ligament [21].
The meniscal variation presented in this report is not described in any other study on meniscal abnormalities. Additional investigation of similar cases is required so that a suitable description can be added among the possible anatomical variations of the menisci.
Conclusion
This is the first study about a meniscal anatomical variant with junction of the posterior horn with both ACL and PCL. The recognition of meniscal variants is important as they can be misinterpreted for more significant pathology on MRI. MR arthrography better defines and depicts intra-articular anatomic variants than MRI.
Patient consent
Informed consent was obtained from all individual participants included in the study.
Footnotes
Competing Interest: The authors declare that they have no conflict of interest.
Funding: No grant or funding were received for this study.
Ethical statement: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Human and animal rights: This article does not contain any studies with animals performed by any of the authors.
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