Male with painless enlarged knee

A 66-year-old male with a history of knee infection with surgical debridement, presents with a long-standing painless enlarged left knee.

Axial proton density (PD) magnetic resonance imaging (MRI) of the left knee shows two heterogeneous intermediate-signal irregular oval-shaped lesions located in the lateral patellofemoral recess and adjacent to the medial collateral ligament (MCL), which erode the patellar and femoral surfaces (Figure 1a). Sagittal PD MRI of the same knee demonstrates the patellofemoral hyperintense lesion extending to the suprapatellar recess and showing no fat suppression (Figure 1b).

Axial computed tomography (CT) of the same patient shows the same lesions demonstrated by MRI, which are irregular and heterogeneously hyperdense, with better demonstration of cortical erosions (Figure 2a). Axial dual-energy CT (DECT) with color mapping shows uric acid deposits (orange) in the aforementioned lesions (Figure 2b). Three-dimensional DECT with color mapping shows uric acid depositions (blue) within knee joint and soft tissues (Figure 2c).

Background and clinical perspective

Gout is a chronic disease of monosodium urate (MSU) crystal deposition and is one of the most common forms of inflammatory arthritis in adults, especially men [1].

Diagnosis of gout is traditionally based on characteristic clinical symptoms combined with elevated plasma urate levels and preferably joint/tophus aspiration with microscopical verification of MSU crystals [2]. However, aspiration is not always possible in routine clinical practice, and imaging techniques have gained an increasing role in the diagnosis of gout patients [2].

Imaging perspective

Various non-invasive imaging modalities such as radiography, sonography, conventional computed tomography (CT) and magnetic resonance imaging (MRI), have been used for the evaluation and diagnosis of gout [3]. Conventional CT can demonstrate erosions and hyperdense tophi with high sensitivity, though these findings remain of insufficient specificity for the diagnosis of gout. MRI can depict cortical erosions, marrow edema and gouty tophi, which may have variable signal characteristics depending on the amount of calcium present [4]. These imaging features, however, are not specific for gout, and often the diagnosis can only be inferred by correlating with disease distribution and other clinical features.

Dual-energy CT (DECT) offers the unique capability for the non-invasive detection of these crystals earlier in the course of the disease, as it can automatically color-code MSU depositions, based on predefined software settings. [3, 5]. It has been used to reveal the distribution and quantity of deposited MSU crystals in gout [4].

Outcome

Urate-lowering therapy (ULT) is a long-term management of gout to reduce serum urate levels, which can lead to dissolution of MSU crystals deposition, reduction or prevention of gout attacks, and joint damage [2, 6]. When left untreated, recurrent gout attacks and chronic gout inflammation can lead to severe structural damage to the painful bones, causing great harm to patients [1].

Take-home message / Teaching points

Gout is a chronic disease of MSU crystal deposition and is one of the most common forms of inflammatory arthritis in adults, especially men. Imaging findings play an important role in its diagnosis, especially when joint/tophus aspiration cannot be performed and as it provides specific information about the distribution and quantity of deposited crystals in soft tissues and joints.

All patient data have been completely anonymized throughout the entire manuscript and related files.