Wrisberg-Variant Discoid Lateral Meniscus: Current Concepts, Treatment Options, and Imaging Features With Emphasis on Dynamic Ultrasonography

First described by Young¹ in 1889, discoid lateral meniscus covers a spectrum of meniscal disorders of varying morphology and stability. Determining the true incidence of discoid lateral menisci is difficult because of the large number of asymptomatic cases, though published estimates range from 1% to 17%^2-4 of the population, with bilaterality occurring in up to 20%.⁵ The most commonly used classification system for discoid lateral menisci—reported by Watanabe and colleagues⁶—describes 3 types of meniscal pathology based on stability to probing and arthroscopic appearance. Type I is stable to probing, has normal tibial attachments, and is “block-shaped,” with increased thickness spanning the entire lateral tibial plateau. Type II is stable to probing and has normal tibial attachments as well, but covers less than 80% of the lateral tibial plateau. Type III (the Wrisberg variant) is unstable because it lacks a posterior meniscotibial (coronary) ligament and has only 1 posterior attachment, the posterior meniscofemoral ligament, or Wrisberg ligament. Wrisberg-variant discoid lateral menisci are rare; estimated incidence is 0.2%.⁷

Pathophysiology

The normal lateral meniscus, with its flat tibial and concave femoral surfaces, is crucial to load transmission across the knee joint.⁸ Embryologically differentiating from mesenchymal tissue within the limb bud during fetal development, a normal lateral meniscus never has a discoid shape.^8-10 The implication, that discoid lateral menisci represent a congenital anomaly, is further supported by ultrastructural studies involving transmission electron microscopy. These studies have demonstrated that discoid menisci have fewer collagen fibers with a more disorganized course compared with normal menisci.¹¹

With considerable variability, the average normal lateral meniscus is 12 mm wide and 4 mm thick.² The blood supply to the lateral meniscus recedes during growth, with only the peripheral third remaining in adulthood⁸ and the inner two-thirds receiving nutrients by diffusion from the intra-articular fluid.⁵ In comparison, discoid lateral menisci often have poorer vascularity than normal menisci and therefore are more susceptible to tears.^8,12,13

Ligamentous attachments to the lateral meniscus include the lateral meniscocapsular ligament, which attaches to the lateral joint capsule. In addition, 70% to 100% of people have accessory meniscofemoral ligaments, which insert anterior (ligament of Humphrey) or posterior (ligament of Wrisberg) to the posterior cruciate ligament.¹⁴ There are no ligamentous attachments at the popliteus hiatus or lateral collateral ligament, allowing for 9- to 11-mm excursion of the lateral meniscus during knee flexion and extension.³ Morphologically, the lack of a meniscotibial (coronary) ligament in the setting of a discoid lateral meniscus (Wrisberg variant) results in meniscal hypermobility. During knee range of motion, compressive forces between the femoral condyle and the tibial plateau spread through the peripheral portion of the meniscus and, without ligamentous attachments, allow it to displace anteriorly into the femoral intercondylar notch. This displacement results in impingement between the femur and the tibia^15-18 and leads to the characteristic symptoms of “snapping knee syndrome.”¹⁰

Clinical Features

Snapping knee syndrome was first described by Kroiss¹⁹ in 1910.⁵ Multiple authors have described patients’ primary complaints as pain, swelling, locking, and a palpable or visible snap at terminal extension. Sudden movement of a soft-tissue structure across a bony prominence during a provocative maneuver is the source of the snapping. The syndrome has many etiologies. Extra-articular causes of lateral snapping knee syndrome include iliotibial band friction syndrome, soft-tissue tumors, hypermobile popliteus tendons, and abnormal anterior insertions of the biceps femoris tendons.^20,21 Common intra-articular etiologies include ganglion, synovial, and parameniscal cysts; intra-articular loose bodies; lateral meniscal tears; and discoid lateral menisci.²² Patients with discoid lateral menisci often present with knee pain, popping, range-of-motion limitations, and snapping.^23,24 However, the symptoms are quite variable and depend on type of discoid meniscus, presence of a tear, and stability of the rim.^2,7,18

Obtaining a thorough history is essential in evaluating patients with suspected discoid lateral menisci. Physical examination should include evaluation of the lateral joint line for bulges, effusion, and tenderness. Patients may experience knee pain with flexion to 30° to 40° when varus or valgus stress (modified McMurray maneuver) is applied.¹⁰ In addition, a clunk may be appreciated with McMurray testing as a result of subluxation of the unstable lateral meniscus.¹⁰ The contralateral knee should be carefully evaluated, given the frequency of bilateral discoid menisci.¹⁰

The figure-4 test, a maneuver developed by LaPrade and Konowalchuk²⁵ to detect peripheral meniscal tears or tears of the popliteomeniscal fascicles, is performed with the patient in the supine position, with the foot of the affected extremity placed on the contralateral knee. Normally, the popliteus tendon pulls the meniscus out of the joint when the knee is brought into the figure-4 position. However, without popliteomeniscal fascicles, the meniscus subluxes into the joint and becomes impinged. With the patient in the figure-4 position, reproduction of symptoms over the lateral joint line is a positive test and suggests peripheral meniscal tears and/or tears or absence of the popliteomeniscal fascicles.²⁵