CASE 11365 Published on 19.12.2013

Synovial osteochondromatosis of the anserine bursa secondary to a tibial exostosis

Section

Musculoskeletal system

Case Type

Clinical Cases

Authors

Luís Duarte Silva, Miguel Oliveira e Castro, Bruno Santos, Carlos Bilreiro, Francisco Aleixo

Centro Hospitalar do Barlavento Algarvio,
Radiologia;
Sítio do Poço Seco
8500 Portimão, Portugal;
Email:luisdsilva49@gmail.com
Patient

13 years, female

Categories
Area of Interest Musculoskeletal bone, Musculoskeletal joint, Musculoskeletal soft tissue ; Imaging Technique CT, Conventional radiography, Digital radiography, MR
Clinical History
A 13-year-old female patient presented at our hospital with a history of painless tumefaction on the medial side of her left knee, starting a few weeks before.
Imaging Findings
Radiographs (Fig. 1) showed soft tissue densities with chondroid-like calcifications adjacent to the medial tibia. CT examination (Fig. 2) additionally evidenced a small thorn-like exostosis at the medial tibia, representing either a pes anserinus bony spur or an osteochondroma. This lesion was surrounded by a distended anserine bursa, which contained four loose bodies with “ring-and-arc” type calcifications. On MR imaging (Figs. 3, 4), these loose bodies were isointense on T1WI and hyperintense on T2WI, with hypointense areas in all pulse sequences related to calcifications. After contrast administration, peripheral enhancement of the loose bodies was evident.
Surgery and histologic analysis were performed. The surgical specimen included four chondroma-like well-defined cartilaginous nodules, some with calcified matrix. No mention of synovial chondrometaplasia was made. The exostosis was not resected.
This combination of radiologic and pathologic findings is consistent with the diagnosis of bursal synovial osteochondromatosis secondary to the exostosis.
Discussion
Exostoses occurring in the pes anserinus region may or may not be osteochondromata [1, 2, 3]. In our case, there was a pedunculated lesion angled away from the joint, with cortical and medullary continuity with the parent bone, thus suggesting osteochondroma. Nevertheless, it arose in the pes anserinus location and was very small and thorn-like, a classic appearance for a pes anserinus spur [1, 3, 4].
Synovial osteochondromatosis (SOC) is an uncommon disorder characterized by the formation of multiple cartilaginous nodules or osseous loose bodies within the synovium. It can be classified as primary (benign neoplastic process) or secondary (associated with joint abnormalities, such as mechanical or arthritic conditions that cause intraarticular chondral bodies). Rarely, it may involve extraarticular sites, arising in synovium about the tendons or bursa [5, 6].
Imaging features of SOC are frequently pathognomonic [5, 6]. Radiographic features include multiple intraarticular (or intrabursal, as in this case) chondral bodies with “ring-and-arc” chondroid mineralization and extrinsic erosion of bone. CT is the optimal radiologic modality to identify and characterize these calcified intraarticular fragments and extrinsic erosions. MR imaging appearance is variable owing to the degree of mineralization and ossification of the chondral bodies, although the extent of involvement is exquisitely depicted. The noncalcified regions of hyaline cartilage typically demonstrate hyperintensity on T2WI because of the high water content of this tissue.
In the unusual case presented here, we believe that the presence of a tibial exostosis played a significant role in the development of bursal SOC [7, 8, 9, 10]. Despite its small size, it is possible that the exostosis in this case, given its specific location, caused chronic mechanical impingement of the anserine bursa. During this process, trauma to the bursal wall could have produced synovitis and subsequent fibrin coagula within the bursal cavity, which, in turn, could have undergone chondrification. An alternative potential source for intrabursal osteochondral bodies is synovial chondrometaplasia [8]. Also, if the exostosis represents an osteochondroma, cartilaginous debris from its tip could have been shed into the bursa, where, nourished by bursal fluid, they continued to grow and proliferate [7].
The pathologic appearance of SOC may be misleading because of significant histologic atypia and radiologic correlation is vital for correct diagnosis, as it allows the distinction of this condition from more aggressive chondroid neoplasms [5, 7, 8]. A multimodality imaging approach is desired to fully characterize this condition and to allow for optimal patient management.
Differential Diagnosis List
Synovial osteochondromatosis of the anserine bursa secondary to a tibial exostosis
Extraskeletal chondroma
Malignant transformation of osteochondroma
Periosteal chondroma
Calcific bursitis
Synovial chondrosarcoma
Long-term bursitis with formation of rice bodies
Final Diagnosis
Synovial osteochondromatosis of the anserine bursa secondary to a tibial exostosis
Case information
URL: https://www.eurorad.org/case/11365
DOI: 10.1594/EURORAD/CASE.11365
ISSN: 1563-4086