Discoid Lateral Meniscus in Children Limited Knee Extension and Meniscal Instability in the Posterior Segment

Background: Limited knee extension is a well-known sign in children with discoid meniscus, but its causative mechanism is controversial. The purpose of this study was to characterize discoid meniscus with this clinical manifestation with a focus on instability of the posterior segment and its morphologic features. Methods: The authors retrospectively reviewed magnetic resonance imaging and intraoperative arthroscopic videos of 34 knees in 32 consecutive patients (age range, 4.5-15.0 years) who underwent arthroscopic treatment for symptomatic discoid lateral meniscus. Knees were classified into 2 groups, depending on the presence (n = 19) or absence (n = 15) of an extension block, which was defined as a limitation of knee extension by 10 degrees or more under general anesthesia. Meniscal instability in the posterior segment was determined based on arthroscopic findings, and its correlation with knee extension block was analyzed. Using magnetic resonance imaging, the thicknesses of anterior and posterior segments were measured to assess significance of the thickness differences by a presence of knee extension block and by the posterior segment instability. Results: There were 13 knees with a stable posterior segment and 21 knees with an unstable posterior segment. Meniscal instability in the posterior segment was observed in the knees with a surface tear (n = 12), in a hypermobile posterior rim despite the absence of a surface tear (n = 5), and in knees with a freely movable posterior rim without posterior tibial attachment (n = 4). Knee extension block occurred exclusively in complete type menisci and was significantly correlated with meniscal instability in the posterior segment. Magnetic resonance imaging observations suggested that morphologic features characteristic of thick anterior and thin posterior segments were significantly associated with meniscal instability in the posterior segment. Clinically, knee extension block disappeared in all cases within 4 months of arthroscopic treatment. Conclusions: A redundant anterior segment associated with meniscal instability in the posterior segment is likely to cause limited knee extension in children with discoid lateral meniscus. This correlation mandates that discoid menisci be thoroughly evaluated during arthroscopic treatment to identify any instability that might require stabilization. Level of Evidence: Level III case-control study.