Definition and Reliability Assessment of Elementary Ultrasonographic Findings in Calcium Pyrophosphate Deposition Disease: A Study by the OMERACT Calcium Pyrophosphate Deposition Disease Ultrasound Subtask Force

Objective. To define the ultrasonographic characteristics of calcium pyrophosphate crystal (CPP) deposits in joints and periarticular tissues and to evaluate the intra-and interobserver reliability of expert ultrasonographers in the assessment of CPP deposition disease (CPPD) according to the new definitions. Methods. After a systematic literature review, a Delphi survey was circulated among a group of expert ultrasonographers, who were members of the CPPD Ultrasound (US) Outcome Measures in Rheumatology (OMERACT) subtask force, to obtain definitions of the US characteristics of CPPD at the level of fibrocartilage (FC), hyaline cartilage (HC), tendon, and synovial fluid (SF). Subsequently, the reliability of US in assessing CPPD at knee and wrist levels according to the agreed definitions was tested in static images and in patients with CPPD. Cohen's kappa was used for statistical analysis. Results. HC and FC of the knee yielded the highest interobserver k values among all the structures examined, in both the Web-based (0.73 for HC and 0.58 for FC) and patient-based exercises (0.55 for the HC and 0.64 for the FC). Kappa values for the other structures were lower, ranging from 0.28 in tendons to 0.50 in SF in the static exercise and from 0.09 (proximal patellar tendon) to 0.27 (triangular FC of the wrist) in the patient-based exercise. Conclusion. The new OMERACT definitions for the US identification of CPPD proved to be reliable at the level of the HC and FC of the knee. Further studies are needed to better define the US characteristics of CPPD and optimize the scanning technique in other anatomical sites.