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Intraosseous tophaceous gout - femur

Case contributed by Fabien Ho
Diagnosis almost certain

Presentation

Knee mechanical pain. Past history of uric acid urinary tract stones

Patient Data

Age: 70
Gender: Male

Arthrography of the left knee

x-ray

Frontal radiograph after arthrography procedure showed a lytic lesion within the lateral condyle epiphysis of the left femur, with sclerotic margins on the metaphyseal and lateral borders (Lodwick IA).

Arthro-CT

ct

Replacement of the lateral condyle subchondral bone medulla by a dense tissue mass of 230 H.U. (mean density, SD +/- 131 H.U., [-131; 1055]), slightly calcified, although without the ring and arc pattern of a cartilaginous matrix. Intraarticular deposits of the same density were also seen adjacent to the cruciate ligaments. The cartilage was relatively well-preserved in the lateral compartment of the left knee.

The lytic lesion featured frank hypointense signal on T1w sequence, mild and heterogeneous hyperintensity on Fat Sat Proton Density sequence, both compared to muscle signal. Contrast-enhanced Fat-Sat T1w sequence showed synovitis and peripheral enhancement of the intraosseous lesion, consistent with inflammation in contact with bone medulla of the lateral femoral condyle.

Hands Xray: On the proximal interphalangeal joint of the 2nd right digit, there was a paraarticular soft tissue dense mass with subchondral lytic areas with sclerotic margins, and preserved joint space. These anomalies were consistent with mild gout of the hands, in contrast with the knee features. (This patient also had amputation of the 3rd phalange of the 3rd left digit decades ago due to trauma.)

Case Discussion

Diagnosis

Complementary biology revealed hyperuricemia (960 µmol/L). Joint fluid analysis found negative birefringent and needle-like crystals consistent with urate crystals.

Our final diagnosis was intraosseous gouty tophus. This patient has been given colchicine and non-steroid anti-inflammatory drugs and has been relieved from pain. No surgical treatment has been planned as of date.

Discussion

Bony hyperlucency on standard radiographs is the first step to a wide range of diagnoses. In epiphyseal hyperlucencies, radiologists discuss degenerative, infectious, inflammatory, metabolic and tumoral changes as differential diagnoses. Within the degenerative category, one should consider the subchondral lucencies found in primary osteoarthritis. Intraosseous lucencies can also be seen in rheumatoid or psoriatic arthritis. An infectious etiology, such as primary osteomyelitis or cystic tuberculosis, might also explain the findings of a focal lytic lesion. Possible benign neoplastic entities from the knee epiphysis include simple bone cyst, aneurysmal bone cyst, giant cell tumor, chondroblastoma, enchondroma, osteoblastoma, brown tumor...

Tophi do not usually appear on physical or radiographic examination until 10–12 years after onset of gout. They are most commonly found in the hands, wrists, and feet 1. Resnick, in 1981, reported on six cases of intraosseous tophaceous calcifications of the hands, wrists, and feet, stating that the majority of these lesions of the feet appeared in metatarsals, especially the first, as well as in the proximal phalanges of the digits and, less commonly, in the tarsal bones. In each of the six cases additional typical radiographic findings of severe gouty arthritis were evident 2.
However, lytic lesions of bone as described here, although rare, have been reported to occur without prior symptoms suggestive of gout 1.Therefore, when a past history of gout is not known, intraosseus tophi may mimic neoplasic processes or infection and the correct diagnosis may be challenging. Intraosseus tophi are a rare, unusual presentation of tophaceus gout which have previously been reported within the patella 3, tibial plateau 4, within the talus and midfoot bones 5.

The radiologic manifestations of gout are well known. Radiographs remain the examination method of choice in the diagnosis of gouty arthritis, with large paraarticular “punched-out” erosions with overhanging edges and preserved joint space, as well as soft tissue dense masses.
CT has been used extensively to investigate gouty arthritis. Tophi are seen as discrete masses with a density of 160–170 HU. Dual energy CT also facilitates recognition of paraarticular calcifications when present 6. However, the fact that our lesion is more dense than the expected 160-170 UH is most likely related to intraosseous urate deposits that have calcified 2.
The MRI signal intensity of intraosseous tophus described here was hyperintense and more heterogeneous than that of muscle on DP FS sequence, while being frankly hypointense on T1 compared to muscle. In other series, tophaceous deposits show variable appearance on MRI 4. The deposits may have low to intermediate signal intensity on T1 and low signal intensity (if the tophi are calcified) or high signal intensity on T2 images, depending on the degree of hydration of the tophi and crystals. An inflamed joint usually has the appearance of arthritis, including joint effusion and paraarticular edema. Variable degrees of gadolinium enhancement of tophi, related to increased hypervascularity of the affected synovium and of granulated tissue surrounding it, have also been reported. Therefore, tophus signal intensity is not pathognomonic of gout.

In summary, even if a history of gout is absent, tophaceous gout should be included in the differential diagnosis of intraosseous lesions, as its CT and MRI manifestations are non-specific.

Doctors Lasfar O., Storey J, and Cauvy C contributed to this case.
This case was submitted to two journals which refused to publish it.

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