An Infected, Extruded Lateral Meniscal Cyst as a Cause of Knee Symptoms

Abstract

A 62-yr-old male runner with known bilateral knee osteoarthritis presented to a Physical Medicine and Rehabilitation outpatient musculoskeletal practice because of a left lateral knee mass, with associated pain and pressure. The mass was clearly visible and palpable. X-rays revealed his preexisting severe tricompartmental osteoarthritis. Magnetic resonance imaging revealed a lateral meniscal tear with extrusion of the lateral meniscus through the lateral capsule, thus forming a large lateral meniscal cyst (Fig. 1).FIGURE 1: Coronal, T2 MRI image demonstrating extruded lateral meniscal cyst. MRI, magnetic resonance imaging.Initial aspiration of the cyst yielded 2 ml of very viscous fluid without crystals or infection. Repeated aspiration over subsequent months yielded up to 10 ml of similar fluid. Each aspiration improved his pain, pressure, and function. An orthopedist suggested postponing surgery for this cyst until his planned total knee replacement next year. He did well with nonsurgical management until he developed an acute infection of the cyst with associated left leg cellulitis (Fig. 2). It had been multiple months since any local injection/aspiration, and he had no recent systemic illness. Thus, the presumed source of infection was a local abrasion at the skin over the cyst, which had been a recurrent problem. Cyst cultures revealed Staphylococcus aureus with broad susceptibility to antibiotics. Based on possible underlying osteomyelitis at the proximal tibia, he underwent surgical irrigation, debridement, and antibiotic bead placement, in addition to intravenous antibiotics.FIGURE 2: Left knee with enlarged and erythematous lateral mass.Meniscal cysts are relatively uncommon and a rare cause of knee pain or mass, seen in approximately 1% of those undergoing meniscectomies.1 They more commonly arise from the lateral meniscus and are frequently associated with horizontal meniscal tears.2 The cause of meniscal cysts is unknown, but previous trauma may play a role.3 Magnetic resonance imaging is generally sufficient for diagnosis if an associated meniscal tear is visualized. Ultrasound can visualize the cyst, but it is inferior to magnetic resonance imaging for detecting underlying meniscal tears. Without a visualized meniscal tear, tissue diagnosis may be necessary to rule out malignancy.4 Treatment options include needle aspiration/corticosteroid injection vs. more definitive care via either arthroscopic or open surgical excision. An extensive search of the literature fails to reveal any other reports of an extruded meniscal cyst as a site of infection. Musculoskeletal physicians should nonetheless be aware of meniscal cysts and their complications, and diagnostic and treatment options.