Ultrasound Differentiation of Pyrophosphate Arthropathy (CPPD) and Gout: Crystal Localization
Crystal Topography as the Principal Differential Feature
According to the literature, differentiation between CPPD and gout is based primarily on the localization of crystalline deposits relative to cartilaginous structures.
| Feature | CPPD (calcium pyrophosphate) | Gout (monosodium urate) |
|---|---|---|
| Localization in hyaline cartilage | Within cartilage — echogenic midzone band (e.g., femoral condyle) | On cartilage surface |
| Localization in menisci | Centrally (meniscal chondrocalcinosis) | Primarily on meniscal surface |
| Characteristic findings | Chondrocalcinosis | Tophi, erosions |
Ultrasound Semiotics of CPPD
On longitudinal scanning, an echogenic midzone band is visualized within the thickness of articular cartilage due to calcium pyrophosphate deposition (for example, in the cartilage of the femoral condyle posteriorly). Meniscal chondrocalcinosis may be detected medially. On radiographs in the same case — mild to moderate chondrocalcinosis of the medial and lateral menisci.
Ultrasound Semiotics of Gout
Monosodium urate crystals are initially deposited on the surface of cartilage and meniscus. In chronic tophaceous gout, a dense tophus obturating the joint is visualized (for example, distal to the diaphysis of the first metatarsal bone in the region of the MTP joint); underlying erosions may not be visible in such cases. On CT in these situations — large, well-demarcated cortical erosions and calcified soft tissue mass.
Important Methodological Consideration
Isolated echogenic foci in the synovium may represent synovial proliferation per se and do not necessarily indicate crystal arthropathy. When effusion, synovial hypertrophy, and hyperemia on color Doppler are present, these findings are interpreted in the context of inflammatory activity rather than automatically attributed to crystal deposits.
The reversibility of the ultrasound findings has been demonstrated: monosodium urate deposits on hyaline cartilage may disappear after achieving sustained normouricemia.
Frequently asked questions
Where are crystals deposited in CPPD and gout?
In CPPD — within hyaline cartilage (echogenic midzone band) and centrally in the menisci; in gout — on the surface of cartilage and meniscus.
How does pyrophosphate deposit appear on ultrasound?
An echogenic midzone band within the thickness of articular cartilage, for example in the femoral condyle, often with meniscal chondrocalcinosis.
Do echogenic foci in the synovium always indicate crystals?
No. Isolated echogenic foci may reflect synovial proliferation per se and do not necessarily indicate crystal arthropathy.
Are urate deposits reversible on ultrasound?
Yes, according to the literature, monosodium urate deposits on hyaline cartilage disappear after achieving sustained normouricemia.
What is visualized in chronic tophaceous gout in the MTP joint region?
A dense tophus obturating the joint distal to the diaphysis of the first metatarsal bone; underlying erosions may not be visible on ultrasound in such cases.