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This chapter describes the main ultrasound findings that are detectable in patients with gout or CPPD disease.
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Monosodium urate and CPPD crystal aggregates can be clearly depicted by ultrasound in different anatomic regions and tissues.
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The shape and anatomic locaton of the CPPD crystal aggregates are the main features that help to identify them.
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Ultrasound is a useful imaging modality in crystal-related arthropathies.
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Ultrasound allows differentiation between monosodium urate and CPPD aggregates.
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The diagnostic role of ultrasound is increased by its success in guiding aspiration, especially when very small amounts of fluid are detected.
The crystal arthropathies are a group of disorders characterized by intra-articular and periarticular deposition of microcrystals. Monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals are the most common forms.
During the past decade, considerable evidence has been produced to support the sensitivity of ultrasound in the detection of soft tissue inflammation. Most of the investigations have been conducted in patients with rheumatoid arthritis. Some studies have tested the potential of ultrasound in revealing signs of crystal deposition in patients with gout and CPPD disease.
Soft tissue inflammation associated with crystal deposits leads to different and frequently overlapping ultrasound findings. These findings can be roughly divided into two main groups, those revealing aspecific signs of inflammation and those suggesting directly presence of crystal depositions. This chapter describes the main ultrasound findings that are detectable in patients with gout or CPPD disease.
Crystal Involvement of Hyaline Cartilage
The normal ultrasound appearance of the hyaline cartilage is characterized by a homogeneously anechoic layer delimited by two sharp hyperechoic margins ( Fig. 16-1 ). The superficial margin is thinner than the deeper one, and its visualization requires direction of the incident ultrasound beam perpendicular to the cartilage surface.
Gout
In patients with gout, MSU crystal deposition on the cartilage surface generates hyperechoic enhancement of the superficial (or chondrosynovial) interface and makes the superficial margin visualization independent of the ultrasound beam direction ( Fig. 16-2 ). Adhesion of MSU crystals to the cartilage surface can be focal or diffuse, and confirmation can be obtained by dynamic assessment during the joint flexion and extension movements.
Calcium Pyrophosphate Dihydrate Disease
In patients with CPPD disease, the pattern of crystal distribution at the hyaline cartilage is different from that of gout. The CPPD crystals lie within the cartilage layer, and their sparkling reflectivity makes their identification relatively easy, even for very small aggregates. They can be detected within the substance of the hyaline cartilage of different anatomic sites, including femoral condyles and metacarpal heads ( Fig. 16-3 ). When the ultrasound beam encounters even minimal crystal aggregates, relative echoes are visible even at very low levels of gain. The CPPD crystal aggregates can be widely variable. Expression ranges from isolated hyperechoic spots to extended deposits that may involve a wide portion of the hyaline cartilage ( Fig. 16-4 ).
CPPD crystal deposits usually do not have a sufficient compactness to stop the ultrasound beam progression, enabling the visualization of the underlying cortical bone profile. In most cases, there is a close correlation between the appearance of CPPD crystal deposits on the radiograph and on the ultrasound scan. Sometimes, small deposits of CPPD crystals can be detected by ultrasound when the radiograph is apparently normal ( Fig. 16-5 ).
Crystal Involvement of Fibrocartilage
The normal ultrasound appearance of the fibrocartilage has punctate echogenicity.
Gout
For gout, there is no typical ultrasound appearance of involvement of the fibrocartilage.
Calcium Pyrophosphate Dihydrate Disease
At the fibrocartilage level, CPPD deposits appear as hyperechoic and amorphous aggregates. Confirmation of their exact location within the fibrocartilage can be obtained by dynamic examination of the joint.
These aggregates can be identified in the menisci of the knee ( Fig. 16-6 ) and in the triangular fibrocartilage complex of the wrist ( Fig. 16-7 ). In most cases, there is a close correlation between the appearance of these crystal deposits on the radiograph and on the ultrasound scan ( Fig. 16-8 ). Sometimes, even minimal deposits of CPPD crystals can be detected by ultrasound when the radiograph is apparently normal ( Fig. 16-9 ). The lack of adequate acoustic windows does not allow proper ultrasound identification of CPPD crystals in all anatomic areas.
