Unexplained knee pain in children and adolescents: Dr. Cordelia Carter reviews the common presenting symptoms and treatment of the discoid lateral meniscus.
Left knee pain
SS is a 9-year-old girl who presented to our ambulatory clinic with left knee pain. Per her mother's report, she had sustained an injury to the left knee while playing hide-and-seek nearly one year prior to her initial presentation. The pain from this injury resolved without formal intervention, but has recurred atraumatically over the last several months and has not resolved despite symptomatic treatments, including rest and oral anti-inflammatory medication. In addition to the patient's complaint of knee pain, her mother has noted swelling in the knee and an activity-related limp. SS and her mother endorse occasional mechanical symptoms, including locking and "giving way" of the knee. Aside from the isolated left knee pain, a review of systems is entirely negative. She has no significant past medical or surgical history, and there is no family history of either infectious or inflammatory arthritis.
On physical examination, SS is a healthy, well-appearing child. Her iliac crests are level. She has a minimally antalgic gait. Passive range of motion (ROM) of both hips is supple, painless and symmetric. Passive ROM of the unaffected right knee is 0 to 140 degrees. Passive ROM of the left knee is 10 to 135 degrees, with pain at the extremes of this motion. Bringing the knee passively from flexion to extension results in an audible clunk. The skin of the left knee is intact, and a moderate effusion is present. Patellar tracking is normal. The knee is ligamentously stable, with a 1A Lachman maneuver, and negative posterior and anterior drawer signs. She has discrete lateral joint line tenderness and a positive McMurray's maneuver.
Figure 1: A representative sagittal T1 image of the left knee reveals abnormally thickened "block-like" meniscal tissue, with confluence of the anterior and posterior meniscal horns. Increased signal within the mensical substance is suggestive of associated tearing.
Anteroposterior, lateral, skyline and notch views of the left knee were performed and did not demonstrate any osseous abnormalities. Magnetic resonance imaging (MRI) of the left knee (Figure 1) revealed a lateral discoid meniscus with increased intrasubstance signal suggestive of an associated meniscal tear.
Torn lateral discoid meniscus
Figure 2: Intraoperative arthroscopic view of the lateral joint space, revealing an abnormal "block-like" discoid meniscus and reactive synovitis.
Because of the persistent pain, swelling and "giving way" episodes in the knee, surgical intervention - consisting of left knee arthroscopy with saucerization of the abnormal discoid tissue and concomitant repair of the torn meniscus - was undertaken (Figures 2 and 3).
Figure 3: Intraoperative arthroscopic view of the lateral joint space, following saucerization of the abnormal tissue and repair of the torn posterior horn.
Care was taken at the time of arthroscopy to evaluate the peripheral rim of the meniscus for instability, in addition to evaluating the residual meniscal tissue for unstable tears requiring repair.
Figure 4a: Intraoperative arthroscopic view of the lateral joint space, revealing a Type II (partial, stable) discoid meniscus, before saucerization.
The discoid meniscus is a well-described anatomic variant of the lateral meniscus, with an estimated prevalence ranging from less than 1% to upwards of 20%. There does not seem to be a predilection for gender, although multiple studies have demonstrated racial disparities in the incidence of discoid meniscus, with Asian populations having the highest reported rates. The incidence of discoid meniscus in the American population ranges from 3% to 6%.
Figure 4b: Intraoperative arthroscopic view of the lateral joint space, revealing a Type II (partial, stable) discoid meniscus, after saucerization.
Unlike a morphologically normal lateral meniscus, the discoid meniscus has both an increased width and an increased thickness, and may be described as "block-like." The discoid meniscus may also have irregular ligamentous connections, and as a result, the stability conferred by the normal meniscocapsular ligaments may be compromised. Interestingly, it is not simply the gross morphology of the discoid meniscus that is anomalous, but its histomorphology is abnormal, too: significant disorganization of the normal circumferential collagen network has been demonstrated in discoid menisci, and it is believed that this disorganization weakens the ultrastructure of the discoid meniscus, ultimately predisposing it to tear.
Patients with a discoid meniscus have highly variable presentations. Younger children may present with a painless knee clunk and lack of terminal knee extension. Older children, by contrast, more typically present with pain and physical examination findings consistent with a torn meniscus. Myriad classification systems have been proposed for the discoid meniscus, but perhaps the one most widely used is that of Watanabe, who described three subtypes: 1) Type I - complete (disk-shaped) and stable; (2) Type II - partial (semilunar-shaped) and stable (Figure 4); and 3) Type III - the so-called Wrisberg variant, which lacks normal ligamentous connections peripherally and is inherently unstable.
Historically, the preferred treatment for symptomatic discoid menisci was total or subtotal meniscectomy. In recent years, however, the evidence against performing this type of procedure in children and young adults has continued to mount, with the majority of mid- and long-term outcomes studies demonstrating early degenerative changes radiographically following these procedures. While surgical salvage techniques such as meniscal allograft transplantation do exist, they tend to be more technically demanding with poorly understood outcomes. As a result, methods for treating the discoid meniscus have shifted away from the surgically aggressive to the more conservative current standard of care. Today, the primary goal of surgical intervention is to maximize knee stability and function by restoring anatomic dimensions to the lateral meniscus and repairing torn or unstable residual meniscal tissue. Meniscal saucerization with stabilization as deemed necessary intraoperatively has effectively supplanted meniscectomy in the treatment of the discoid meniscus.
After an initial period of postoperative immobilization and protected weightbearing, SS did a short course of focused physical therapy to regain knee motion and strength. At her six-month postoperative follow-up, SS had no residual knee pain or effusion, passive knee ROM of 0 to 135 degrees, and had returned asymptomatically to unlimited activities.
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