A case of a paraarticular chondroma in the Hoffa\'s fat pad

Clinical History

A 57-year-old man presented with a 3-month history of pain in his right knee. He reported also a lack of extension of the joint. He did not report a history of trauma. Physical examination showed a painful mass under the patella.

Imaging Findings

Radiograph in the lateral view shows a soft-tissue mass in the Hoffa’s fat pad; some central high density due to the calcification may be seen inside (Fig. 1).

The subsequent MRI examination confirms the presence of a well-defined lesion in the infra-patellar fat pad. It presents intermediate signal intensity on sagittal T1-weighted images (Fig. 2).
On the axial fat saturation T2-weighted images the lesion shows heterogeneous, mainly high signal intensity (Fig. 3). On both T1 and T2 sequences the lesion presents some small areas of low signal intensity probably due to the calcifications.
After contrast administration a peripheral enhancement is seen (Fig. 4).

Discussion

Para-articular chondroma is a rare, benign soft tissue tumour [1, 2]. It arises from the capsule or the para-articular connective tissue of a joint. The pathogenesis of these tumours is still debated: it seems to be related to the cartilaginous metaplasia of the mesenchymal cells [1]. This lesion affects both men and women, between 10 and 80 years of age.
Although some cases have been described in the ankle, elbow and the hip joint, the knee is most frequently affected; this tumour is more frequently intra-capsular, involving the infra-patellar fat pad and only few cases have been described with an extra-capsular location [1, 2, 3]. Clinically, the tumour appears as a painful mass. The patient suffers from a lasting pain, with a time span from several months to many years, associated with a limitation of joint movement. On physical examination the lesion may be stable or mobile.

The plain radiographs may show a soft tissue mass; calcifications are visible in about 30-70% of cases. The pattern of calcification is mainly that of curvilinear and ring-like densities. Rarely, a peripheral rim ossification is seen. Secondary changes of the adjacent bone, such as cortical erosions and reactive sclerosis, have been reported in the literature.
On MRI the para-articular chondroma appears as a well-circumscribed mass isointense or hypointense relative to skeletal muscle on T1-weighted images. On T2-weighted images the tumour mainly appears hyperintense; on both T1 and T2 sequences the lesion may appear heterogeneous because of the central ossification that causes signal voids. After contrast injection the lesion exhibits more frequently a peripheral enhancement [3].

Para-articular chondromas must be differentiated mainly from other benign intra-articular and juxta-cortical calcified lesions, such as synovial chondromatosis [4] (in which lesions are multiple) periosteal chondroma (which shows severe cortical erosion) localized nodular synovitis and old haematomas. In differential diagnosis malignant lesions, such as synovial sarcoma and synovial chondrosarcoma, must be considered [1, 2].
The certain diagnosis must be confirmed by histopathologic examination.

Surgical excision is the treatment of choice, being careful not to injure the joint integrity. Despite the benign nature of the lesion, local recurrence has been reported; malignant transformation has never been referred. The small size of the lesion despite its long history, the nature of calcifications and the location itself, help to make the correct diagnosis in order to avoid whatever kind of unnecessary and aggressive surgical treatment.

Differential Diagnosis List

Final Diagnosis

Para-articular chondroma

URL:	https://www.eurorad.org/case/11267
DOI:	10.1594/EURORAD/CASE.11267
ISSN:	1563-4086