Teaching Case

A 77-years-old male patient presented insidious pain and swelling of the left knee joint, especially in the popliteal fossa, for the last eleven months. On clinical examination, he had a limitation in range of motion of the knee. There was no history of traumatic event.

Plain radiograph of the knee was the first radiological investigation performed, which showed a large mineralized mass behind the knee, with typical ring-and-arc appearance of chondroid tissue, associated with some round bodies with peripheral rim cortex of calcification (target appearance) (Fig.1).
Therefore, magnetic resonance imaging (MRI) was performed to better understand the nature and extension of these findings, demonstrating the presence of Baker's cyst (Fig.2), which contained many bodies of variable sign, most of them with low signal intensity on all pulse sequences due to calcification (corresponding to the mineralized mass on radiograph) and others with central fat intensity on all pulse sequences and low signal intensity peripheral rim cortex due to ossification (Fig.3 and Fig.4). In addition, there were bodies of identical characteristics adjacent to the posterior cruciate ligament and along the popliteal tendon sheath. These references are compatible with the diagnosis of synovial osteochondromatosis (SO).

Synovial osteochondromatosis is a disorder characterized by the formation of small cartilaginous nodules that protrude on the inner surface of the synovial membrane, undergo pedunculation and finally break off, released into the joint space[1-3]. They can become calcified and progress to ossification. Rarely, it may involve extra-articular sites, arising in synovium about the tendons or bursa[4].
SO can be classified as: Primary – a rare benign neoplastic process that commonly is mono-articular and mostly affects the large joints, with knee as being predominantly affected; Secondary – associated with joint abnormalities, such as mechanical or arthritic conditions that cause detachment of (osteo)cartilaginous fragments from the articular surface. Involvement of the Baker`s cyst is exceedingly rare[1, 4, 5].

The clinical diagnosis of SO may be difficult, since the patients may either be asymptomatic or present non-specific symptoms, like pain, swelling and movement restriction[2].

The imaging studies that may be used in assessing include:
-Plain film radiography is the most commonly used diagnostic method. It can identify mineralized bodies, which often have a uniform size and typical chondroid “ring-and-arc” pattern of mineralization. They can progress to enchondral ossification with a peripheral rim cortex and inner trabecular bone (target appearance). In the secondary form of the disease, the osteochondral bodies are fewer in number and more variable in size. In rare cases, the individual chondral bodies coalesce to form a larger mineralized mass[2, 4, 6].
-Computed tomography is the optimal radiologic modality to identify and characterize calcification (like the "ring-and-arc" pattern of mineralization) and ossification with central yellow marrow (fat attenuation)[4].
-MRI is helpful in unusual presentations and to evaluate the lesion extension. The appearance and signal of synovial osteochondromatosis are variable - the non-calcified chondral nodules demonstrate high signal intensity on T2; the calcified chondral nodules show low signal intensity on all pulse sequences; and the ossified nodules are isointense to fat (due to fatty marrow) and show low signal intensity peripheral rim (bony cortex)[4-6].

The treatment is surgical and often consists of excision of any intra-articular body and resection of the synovial membranes involved[5, 7].

This case reveals a rare location of SO (within Baker's cyst) presented like a large mineralized mass. It shows typical signs that should alert the radiologist for this diagnosis, namely the target appearance on radiograph, representing the peripheral rim cortex of calcification, and its sign on MRI depending the phase of calcification or ossification.

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