Month: May 2024

A 58-year-old female presented with a chief complaint of focal lateral knee pain and swelling. She had a remote history of anterior cruciate ligament repair, as well as medial collateral ligament injury and meniscal injuries. The patient stated that she had a history of recurrent intermittent effusions. However, that day she noted focal swelling. She reported playing tennis a few days before and noted a pain in her knee with pivoting. No knee instability. No fever.

Vitals: BP 118/60 | Pulse 52 | Temp 97.8 °F (36.6 °C) | Resp 12

On physical examination of the knee, there was a 2 cm x 3 cm firm, immobile round mass at the inferior lateral aspect of the left knee with associated pain, mild warmth, and swelling. Decreased flexion of the joint was observed, with some pain beyond 100 degrees of flexion.

A bedside ultrasound was performed. What do you see?

Discussion

In these images, we see classic findings of a parameniscal cyst. There is a joint effusion seen superior with Hoffa’s fat pad evident. In the lateral knee, we see extrusion of the meniscus on ultrasound, along with several small fluid collections within the meniscus (meniscal cysts). Beyond the margin of the meniscus, we see the larger parameniscal cyst situated more superficially and featuring a thick wall. Corresponding MRI images from a few months prior also show the anterior cyst forming on the meniscus. Meniscal cysts are found in 1% of MRIs obtained for knee pain [1]. They are a rare pathology most often associated with a synovial leak of fluid through degeneration of the meniscus, secondary to tears of the meniscus. Physical exam findings associated with meniscal cysts include localized pain, with a firm fluid collection at the joint line, anterior or posterior, often accompanied by a joint effusion. However, only 20% of cysts are palpable on examination with the average size being 1-2 cm [2]. Patients will present with local pain, but may also present with peroneal nerve palsy, or foot drop, if the cyst is located inferior and laterally. Point-of-care ultrasound confirms the diagnosis with an accuracy of 94%, a sensitivity of 97%, and a specificity of 86% [3]. The ultrasound appearance of the cyst may be multiloculated and contiguous with the knee joint along the meniscus or within the meniscus. The fluid removed from the cyst is often thick and gelatinous and therefore requires a large gauge needle and ultrasound guidance for successful aspiration [4]. A steroid injection is often performed following fluid aspiration. If there is recurrence, the cyst can be surgically removed. CT or MRI can also confirm diagnosis [5]. Point-of-care ultrasound is a useful tool to distinguish meniscal cysts from other cystic and solid masses at the knee joint [6]. The differential diagnosis of meniscal cysts includes osteophytes associated with degenerative joint disease, traumatic bursitis, lipoma, ganglion cyst, or, rarely, synovial sarcoma.