Background Calcium pyrophosphate disease (CPPD) is a common disease characterized by deposition of calcium pyrophosphate (CPP) crystals in joints and tendons. The diagnosis is made by identification of CPP crystal in synovial fluid analysis or by imaging such as X-rays or ultrasound (US). However, the performance of the two imaging procedures in the diagnosis of CPPD was poorly studied.
Objectives The aim of the study was to investigate the reproducibility of US and its ability to detect CPP deposition in comparison to conventional X-rays.
Methods To be included in this single-centre case-control study, patients needed to have painful knee effusion. The final diagnosis of CPPD was proven by identification of CPP crystals in synovial fluid analysis.
All patients underwent a clinical evaluation, X-rays of knees in both anteroposterior and lateral views and US evaluation. All US exams were bilaterally performed by 2 rheumatologists blinded to clinical, laboratory, US and radiographic results. Knees cartilage was explored on the transversal and longitudinal suprapatellar plane in maximal flexion. Lateral and medial menisci of the knees were assessed in moderate flexion (30°) and complete extension. Presence of hyperechoic spots in cartilage and menisci were assessed for both knees.
Interobserver agreement between the 2 sonographers was estimated using the k coefficient.
Results A total of 32 patients (56% of males, mean age: 66.5±15.6 years) with knee effusion were included. Synovial fluid analysis revealed CPP crystals in 16 (50%) patients. Control patients were diagnosed gout (n=6), osteoarthritis (n=6), rheumatoid arthritis (n=2) and spondyloarthritis (n=2).
Among the 16 patients with CPP crystals, US revealed hyperechoic spots in menisci and/or cartilage in all patients (Se: 100%, Sp=87.5%) whereas X-rays showed CPPD in 9 patients (Se: 56%, Sp: 100%) (P=0.002). Accuracy of US and X-rays was 93.8% and 78.1%, respectively.
Calcifications of menisci were detected by US in 15 (93.7%) of the 16 CPPD patients and in 2 (13.5%) of 16 control patients. Hyperechoic spots of cartilage were noted among CPPD and control group, in 12 (75%) and 1 (6.2%) patients, respectively.
Agreement between the 2 sonographers was almost perfect for US CPPD (menisci and/or cartilage calcifications) diagnosis (k=0.87), menisci (k=0.81) and cartilage calcification (k=0.81).
Conclusions Our study confirms that US of knees is more sensitive than X-rays for the diagnosis of CPPD despite lower specificity. Additionally, the reproducibility of US was good. Thus, US appears to be useful for the diagnosis of CPPD.
Disclosure of Interest None declared