Pyrophosphate arthropathy

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In patients with pyrophosphate arthropathy, crystals are detectable within the substance of the hyaline cartilage (Fig. 4.7) [11-13]. The sparkling reflectivity of pyrophosphate crystals allows for clear depiction of even minimal aggregates within cartilage. Crystal deposition can be focal or diffuse -leading to the development of a 'double contour', which is created by the permeability of the crystal layer, allowing US to penetrate and depict the bone profile beneath.

This is typically seen in the articular cartilage of the femoral condyles and should not be confused with meniscal calcification [9]. One striking feature of this deposition pattern is the apparent geometric location of the crystal layer within the middle portion of

Pyrophosphate arthropathy.a Longitudinal US scan of the ulnar aspect of the wrist. b X-ray.Calcification of the triangular ligament of the carpus (arrowheads) is evident. t = extensor carpi ulnaris tendon; u = ulna; tr = triquetrum

Pyrophosphate arthropathy.a Longitudinal US scan of the ulnar aspect of the wrist. b X-ray.Calcification of the triangular ligament of the carpus (arrowheads) is evident. t = extensor carpi ulnaris tendon; u = ulna; tr = triquetrum the articular cartilage,which may help to understand why cartilage is damaged in pyrophosphate arthropa-thy, leading to secondary degenerative changes.

Calcific deposits in pyrophosphate arthropathy appear as hyperechoic rounded or amorphous shaped areas and their location within the fibrocar-

tilage can be confirmed by dynamic assessment of the joint during real-time scanning. These aggregates can be identified in the menisci of the knee and in the triangular ligament of the wrist. There is close correlation between the appearance of these crystal deposits on X-ray and US (Figs. 4.8 a, b, 4.9 a, b).

Pyrophosphate arthropathy of the knee joint. a, b US images. c X-ray. Lateral (a) and medial (b) longitudinal US scans demonstrate the presence of calcification of both the menisci (arrowheads). f = femur; t = tibia

Pyrophosphate arthropathy of the knee joint. a, b US images. c X-ray. Lateral (a) and medial (b) longitudinal US scans demonstrate the presence of calcification of both the menisci (arrowheads). f = femur; t = tibia

4.2 Synovial cavity

Ultrasound is a highly sensitive technique for the detection of even minimal fluid collections and it still represents a particularly useful diagnostic tool to quantify fluid and to monitor its evolution. This latest application is considerably helpful in rheumatological therapy because it constitutes a valid method of evaluation of efficacy. The considerable sensitivity of the identification of synovial fluid collection, the highly detailed anatomical depiction and the real time visualization of tissues make US the ideal imaging technique for interventional guided procedures, such as arthro-centesis. Thanks to US, the aspiration of synovial fluid is even possible even when the joint collection is minimal.

Pathologic conditions that can be assessed within the synovial cavity with US include hydrarthro-sis, pneumohydrarthrosis, pyarthrosis, hemarthro-sis, lipohemarthrosis, bursitis, tenosynovitis and synovial thickening.

US may occasionally detect the presence of synovial ganglia, joint mice and synovial calcification.

Intracavitary synovial fluid collection

A collection of fluid within the synovial cavity causes the swelling of the involved joint.

In hydrarthrosis, US shows fluid collection within the cavity, which has an anechoic appearance with dorsal acoustic enhancement (Fig. 4.10 a,b).

The amount of fluid within the joint is directly proportional to the severity of the synovial inflammation and to the capability of the capsular wall to expand. In some cases the anechoic appearance of the fluid collection can be inhomogeneous because of the presence of dot-like echoes scattered within the collection itself [14-16]. This more complicated appearance of the collection may depend on the presence of a fibrinous component within the inflammatory exudate, which can be particularly abundant in relapsing collections and can be

Fig. 4.10 a,b a US scan of medial paracondylar recess.Anechoic reactive fluid collection containing a thin septum (physiological medio-patellar plica, arrowheads). b Axial fat suppression sequence magnetic resonance (MR) scan confirms the presence of mediopatellar plica (arrowheads),which appear as low signal bundle within the hyperintense articular fluid collection

Fig. 4.10 a,b a US scan of medial paracondylar recess.Anechoic reactive fluid collection containing a thin septum (physiological medio-patellar plica, arrowheads). b Axial fat suppression sequence magnetic resonance (MR) scan confirms the presence of mediopatellar plica (arrowheads),which appear as low signal bundle within the hyperintense articular fluid collection visualized as arranged echogenic and inhomoge-neous clusters, with a scirrhous conformation.

Pyarthrosis occurs in bacterial arthritis, which is usually rare in patients with normal immune systems, while it is common in children, in immuno-suppressed patients, in diabetics and in patients on dialysis. In acute infections with joint fluid collection, it is necessary to sample the fluid in order to prescribe the most appropriate antibiotic therapy. In chronic infections the fluid collection is usually poor and it is often associated with considerable synovial thickening. In infections the fluid is usually hypoechoic, but it may appear hyperechoic in more superficial joints. In such cases, the synovial hyperemia can be well-depicted with the use of Doppler techniques as a complement to gray scale US [17,18]. However, it should be kept in mind that synovial hyperemia in bacterial arthritis is not a mandatory finding, because it depends on the patient's age, on the duration of the infection and on the immune status. Therefore, since there is no certainty in differentiating septic from aseptic inflammation and it is more suitable to perform a biopsy when clinical suspicion is high.

Hemarthrosis exhibit a peculiar US pattern that changes with time similar to hematoma. Hemor-rhagic fluid collections are in fact homogeneously echogenic within the first two to three days from onset, due to the presence of a corpuscular content. After the third day, the hemarthrosis shows a progressive reduction in echogenicity due to lytic enzyme release. Eventually, US shows echogenic branches, corresponding to fibrinous clots, crossing the anechoic-appearing zone [14,15].

Occasionally, the post-arthrocentesis follow-up examination demonstrates the presence of pneu-mohydrarthrosis. The presence of gas in the joint cavity produces a highly reflective mist within the anechoic fluid collection, forming an air-fluid level that changes together with the patient's position. When assessing hydrarthrosis and pneumohy-drarthrosis, color and power Doppler techniques do not demonstrate significant vascular changes [3,17,18].

Lipohemarthrosis is easily identified by means of US and it appears as a dual-phase collection, showing a fluid-fluid level. The overlying echogenic fraction corresponds to the lipid content, while the underlying fraction is hemorrhagic. When lipohe-marthrosis is found in a post-traumatic limb, the presence of a joint fracture can be suspected.

Synovial thickening

Hypertrophic or hyperplasic synovial thickening is a condition found in several long-standing inflammatory arthropathies and it can be the cause of bone and cartilage erosion in the joint.

US nowadays can identify inflammatory synovial thickening more accurately than clinical examination, especially when small joints such as the metacarpophalangeal and interphalangeal joints are affected, commonly observed in chronic pol-yarthropathies. Synovial thickening is characterized by heterogeneous echotexture varying from hypoechoic to hyperechoic, depending on the amount of water contained in the synovial tissue

Longitudinal sonogram of wrist, dorsal side in a patient affected by rheumatoid arthritis. Synovial proliferation appears hypoechoic (*). T=extensor tendons

Longitudinal sonogram of wrist, dorsal side in a patient affected by rheumatoid arthritis. Synovial proliferation appears hypoechoic (*). T=extensor tendons

Longitudinal sonogram of wrist, dorsal side. Patient affected by rheumatoid arthritis. In this case, synovial proliferation (arrowheads) has a hyperechoic appearance

(Figs. 4.11,4.12). In larger joints, such as the knee, the synovial thickening appears as a succession of irregularly proliferating branches, mildly echoic, jutting out from the synovia into the articular cavity; the assessment of synovial pannus is considerably easier when associated with a fluid collection because it works as a contrast agent [1,14-16] (Fig. 4.13).

In pigmented villonodular synovitis, the synovial hypertrophy is usually overabundant, made of thick fusiform villi and gross nodules, with a winding outline surrounded by abundant fluid collection. A similar appearance can be observed in joints affected by relapsing hemarthrosis in hemo-philic arthropathies. The continual presence of hemorragic effusion irritates the synovial membrane and determines the formation of pannus that starts as a simple thickening and then turns into villous hypertrophy. The sonographer should always

Longitudinal US scan of supra-patellar recess showing large amount of anechoic fluid collection with hyperechoic synovial proliferation (*). TQ = quadricipital tendon; F = femur
Villous Tenosynovitis

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