To evaluate frequency and patterns, risk factors of MSU (monosodium urate) crystal deposition at lower extremity tendon by ultrasonography in gout patients, and to explore diagnostic value by ultrasonography.
Patients diagnosed with gout and age matched healthy controls had ultrasound scanning of both feet and knees including joints and tendons (achilles, quadriceps, and patellar tendon). Readers who scored the ultrasound scans for MSU crystal deposition were blinded to the patients' clinical diagnoses. Clinical characteristics were compared between positive and negative crystal deposition groups by US, and risk factors of MSU deposition in tendons were analyzed. Diagnostic values of MSU deposition were evaluated by ultrasonography according with positive MSU crystal in synovial fluid or tophi by polarized microscopy.
Eighty patients and eighty healthy controls were included. Thity-three patients (47.5%) had tophi by physical examination. The achilles tendon was the most commonly involved tendon site 41(51.2%), followed by the quadriceps tendons 22(27.5%), and patella tendon 10(12.5%). There were no MSU deposition in healthy control group at tendon by ultrasonography. Compared with negative MSU deposition at tendon site by ultrasonography, tendon MSU positive patients had longer mean gout duration [(87.3±40.9) months 
Tendon involvement at the lower extremity tendons in gout is very common. Long gout disease duration and high frequency of gout flare are both independent risk factors of tendon MSU deposition by ultrasonography. Ultrasonography had good sensitivity and specificity for detecting tendinous tophi and aggregates.