Gout of the extensor mechanism of the knee

Patient with leukemia (note abnormal marrow signal) who presented with acute knee pain and pre-patellar soft tissue swelling. Imaging shows gouty tophi involving the extensor tendons of the knee, with nodules in the distal quadriceps and proximal patellar tendons (yellow arrows). Lateral conventional radiograph does not show any calcifications. Patients with cancer are pre-disposed to gout due to hyperuricemia in the setting of high cell turnover or treatment-related tumor lysis.

Tophaceous masses of gout at the knee are most commonly located on the medial aspect of the infrapatellar fat pad and anterior joint recess (~90% of cases), at the lateral femoral condyle at the attachment site of the popliteus tendon (~80%), and the intercondylar fossae (~70%). Involvement of the extensor mechanism (distal quadirceps tendon and the patellar tendon) is less common, but characteristic.

Gouty tophi present as lobulated or amorphous masses. On MRI, they are isointense on T1-WI and heterogeneously intermediate-to-hypointense on T2-WI, with variable enhancement. Well-defined erosions of the patella can be seen with large tophi. Large erosions can mimick malignancy. Calcifications, when present can help narrow the differential diagnosis; however, as in the case above, they may not always be present.

Differential considerations include:

• Post-traumatic or reactive enthesopathy, hydroxyapatite deposition: Will have calcifications.
• Tenosynovial giant cell tumor: Can have low signal due to hemorrhage. No calcifications.
• Amyloid deposition: Typically low signal. Can have calcifications.
• Sarcoma: Gouty tophi can get very aggressive and erode into the patella, mimicking a soft tissue sarcoma.

References

• Kester C, Wallace MT, Jelinek J, Aboulafia A. Gouty involvement of the patella and extensor mechanism of the knee mimicking aggressive neoplasm. A case series. Skeletal Radiol. 2018 Jan 14. [Epub ahead of print]
• Ko KH, Hsu YC, Lee HS, Lee CH, Huang GS. Tophaceous gout of the knee: revisiting MRI patterns in 30 patients. J Clin Rheumatol. 2010 Aug;16(5):209-14.
• Wadhwa V, Cho G, Moore D, Pezeshk P, Coyner K, Chhabra A. T2 black lesions on routine knee MRI: differential considerations. Eur Radiol. 2016 Jul;26(7):2387-99.