The discoid meniscus is an uncommon anatomical meniscal variant that may present with pain, snapping, motion loss, swelling, and locking of the affected knee, typically during childhood or adolescence.1 Although the etiology of discoid meniscus is not completely understood, it is considered a congenital anomaly with a possible genetic component.2, 3 Incidence is estimated at 0.4% to 5.2% in the Western European population but is reportedly much higher (roughly 17%) in Asian populations.4-6, Discoid menisci, when present, are almost always in the lateral compartment, although cases of medial discoid meniscus have also been reported. The overall incidence of medial discoid meniscus has been estimated to be between 0.06% and 0.3% while that of lateral discoid meniscus, as detailed above, is significantly higher.7-11
ANATOMY AND MORPHOLOGY
The menisci differentiate from mesenchymal tissue early during fetal development with a clear definition by 8 weeks and a mature anatomical shape by 14 weeks in utero.12 Interestingly, menisci never possess a discoid shape during the normal course of development.13,14 The meniscus is fully vascularized at birth, with the central one-third becoming avascular by 9 months as the vascular supply regresses.14 By 10 years, only the peripheral one-third maintains its vascular supply, and the menisci have adult-like characteristics with tissue composed mainly of circumferential collagen fibers.4,14-16
A normal meniscus is wedge-shaped in the coronal plane and crescent-shaped in the axial plane. The medial meniscus is shaped like the letter “C,” is connected firmly to the joint capsule, and covers approximately 50% of the medial tibial plateau. The normal lateral meniscus covers 70% of the lateral tibial plateau and has firm anterior and posterior attachments while the lateral portion is less securely tethered.17 A discoid meniscus is characterized by an atypical shape—the center being partially or completely filled in (or “disc-like”)—often associated with anomalous attachments to the tibia, femur, and surrounding joint capsule. In addition to an atypical morphology, the discoid meniscus may also be much thicker than normal, resulting in a “block-like” appearance. The increased thickness may be responsible for the pathognomonic snapping of the knee as it is brought passively into flexion or extension (hence, the term “snapping knee” was traditionally used to describe a symptomatic discoid lateral meniscus). While the discoid shape is relatively straightforward to address, the increased meniscal thickness may be quite difficult to correct surgically.4-6,18
In addition to the macromorphological differences characteristic of discoid menisci, histological differences have also been described, including differences in collagen density and disorganization of the circumferential collagen network.19-21 Taken together, these differences may compromise the ability of the discoid meniscus to withstand normal forces placed across the knee and predispose it to tear. This is important because the normal meniscus plays a critical role in facilitating load distribution across the knee joint, in addition to assisting with functions including shock absorption, proprioception, and stabilization of the knee. Torn and/or unstable discoid menisci are unable to perform these functions adequately, which may ultimately result in degenerative joint disease and progress to end-stage osteoarthritis (OA).
CLASSIFICATION
The traditional Watanabe classification of discoid meniscus consists of 3 variants: type I, a complete discoid shape; type II, an incomplete discoid shape; and type III, the Wrisberg variant.22 The Wrisberg variant is described as a more normal-appearing meniscus but lacks normal peripheral attachments.5, 6, 15 This classification system was expanded by Monllau and colleagues, 23 who described a ring-type meniscus variant with normal posterior tibial attachments. Although the Watanabe classification is commonly used, its utility in assisting with clinical decision-making may be limited.
Continue to: Several other classification schemes...