Knee effusion: ultrasound as a useful tool for the detection of calcium pyrophosphate crystals

The objective of this study was to evaluate the sensitivity and specificity of ultrasound (US) and conventional radiography (CR) for the detection of calcium pyrophosphate (CPP) crystals in patients with knee effusion. Consecutive patients ≥50 years old with knee effusion were included. All patients underwent arthrocentesis with aspiration of synovial fluid (SF) and subsequent analysis of CPP crystals using plain light and polarizing light microscopy. US and CR of the involved knee were performed immediately after arthrocentesis. CR results were read by an experienced rheumatologist, searching for chondrocalcinosis. US examinations were carried out by an experienced rheumatologist blinded to all clinical and imaging data. The following US abnormal findings were considered indicative of CPP crystals deposition (CPPD): (1) hyperechoic bands within the femoral hyaline cartilage layer, and (2) hyperechoic sparkling spots in meniscal fibrocartilage. A total of 75 knees were evaluated in the same number of patients. Analysis of SF revealed CPP crystals in 15 out of 75 (20 %) knees: all (10) patients with previous diagnosis of CPPD, 3 patients with previous diagnosis of primary knee osteoarthritis (OA) and 2 patients without previous definitive diagnosis of a rheumatic condition. Using SF analysis as reference method, sensitivity and specificity for US findings was 60 and 96.7 %, respectively, while CR showed a sensitivity of 40 % and a specificity of 83.3 %. US results showed high specificity with acceptable sensitivity to detect CPP crystals in patients with knee effusion. Compared with CR, US results had better specificity and sensitivity. US may be used in daily rheumatologic practice when CPPD is suspected.