Description

A 72-year-old woman with rheumatoid arthritis presented with a slow growing painful mass on the anteromedial aspect of the proximal right tibia over the past 2 years. She denied history of fever or local trauma, and acute phase reactants were within normal limits. She underwent knee ultrasound that showed a round well-defined homogeneous hypoechoic lesion connected to the medial meniscus, as well as mild synovitis and degenerative changes. Arthrocentesis of the knee revealed a non-inflammatory synovial fluid, negative for crystals and microbiologic study. Overtime, it fistulized and drained sero-hematic fluid, that was negative on culture swab. MRI of the knee demonstrated a lobulated cystic collection with internal fine septa located in the subcutaneous tissue, measuring 11.4 x 9.5 x 5 cm, and also signs of medial meniscal tear (Figure 1). Given the dimensions and functional impairment the cyst caused, the patient was refered to surgery.

Figure 1: Clinical image.

Discussion

The menisci play a crucial role in maintaining knee joint stability and distributing load forces during weight-bearing activities. Traumatic injury or degenerative changes can disrupt its integrity, sometimes leading to extrusion of synovial fluid through a meniscal tear, forming cystic structures known as parameniscal cysts. While relatively rare, parameniscal cysts are responsible for 4% of all knee injuries [1]. They are usually small and asymptomatic [2], but can grow larger and present as painful masses on either the medial or lateral aspect of the knee, leading to functional impairment and requiring surgical intervention [3].

Declarations

Funding: No specific funding was received from any funding bodies in the public, commercial, or not-for-profit sectors to carry out the work described in the manuscript.

Conflicts of interest: The authors have declared no conflicts of interest.

Informed consent: Written informed consent for publication was obtained from the patient.

References

1. De Smet AA, Graf BK, del Rio AM. Association of parameniscal cysts with underlying meniscal tears as identified on MRI and arthroscopy. AJR Am J Roentgenol. 2011; 196(2): 180-186.
2. Campbell SE, Sanders TG, Morrison WB. MR imaging of meniscal cysts: incidence, location, and clinical significance. AJR Am J Roentgenol. 2001; 177(2): 409-413.
3. Choi E H, Huang J, Bryk D, Bryk E. Giant medial parameniscal cyst in an osteoarthritic knee. 2009; 32: 978-84.