Synovial osteochondromatosis is a rare benign neoplastic disorder resulting from synovial proliferation and cartilaginous metaplasia, leading to the formation of multiple hyaline cartilage nodules that may ossify and detach into the joint or surrounding soft tissues. While it typically affects large appendicular joints such as the knee, hip and elbow, spinal involvement is exceptionally uncommon. ^{Reference Littrell, Inwards, Sim and Wenger1} The cervical spine is the least frequently affected segment, with fewer than 20 documented cases in the English-language literature. Recognition of this entity is important because its clinical and imaging features may overlap with more common extradural or foraminal lesions.

A 74-year-old woman presented with several months of progressive neck pain and decreased cervical spine mobility, without symptoms of radiculopathy or myelopathy. Neurological examination remained normal throughout. MRI and CT of the cervical spine (Figures 1 and 2) revealed lobulated extradural masses within the left C1–C2 neuroforamen and centered at the left C4–C5 facet joint. The lesions demonstrated focal areas of signal drop on gradient-echo (T2) sequences, consistent with susceptibility effects related to calcified or ossified components. CT confirmed internal calcified/ossified foci.

Figure 1.

MRI of the cervical spine. T1-weighted images (A, D, G), T2-weighted images (B, E, H), Short Tau Inversion Recovery (STIR) (C, F), gradient-echo (T2*) (I) and post-contrast fat-saturated T1-weighted images (J). Lobulated extradural lesions within the left C1–C2 neuroforamen (blue arrows) and left C4–C5 facet joint (orange arrows). The lesions demonstrate high signal intensity on both T1-and T2-weighted images, with signal suppression on STIR, T2* and fat-suppressed T1 sequences – findings consistent with internal fatty marrow calcified/ossified foci. No significant contrast enhancement was observed, except for a questionable peripheral rim enhancement in the lesion at the C1–C2 level.

Figure 2.

CT of the cervical spine. Sagittal images (A and B), axial (C and D) and coronal (E). CT confirms the presence of calcified/ossified lesions within the left C1–C2 neuroforamen (blue arrows) and the left C4–C5 facet joint (orange arrows). The latter lesion is larger in size, extending into the lateral neuroforamen and adjacent paravertebral musculature. Notably, the lesions show no true corticomedullary continuity with adjacent osseous structures, excluding osteochondroma from the differential diagnosis.

Although the imaging findings were suggestive of synovial osteochondromatosis, the differential diagnosis included other extradural lesions such as extradural meningioma, cartilaginous neoplasms (including synovial chondrosarcoma and chondroma), ossified sequestered disc fragments and, less likely, melanocytic schwannoma. It is important to emphasize that, unlike the more common intradural spinal meningiomas, extradural meningiomas are rare but well-documented entities and should be specifically considered in the differential diagnosis of calcified extradural spinal masses. In the present case, the pattern of dense peripheral ossification with relative central lucency favored an ossified rather than mineralized lesion, making melanocytic schwannoma – typically associated with coarse, amorphous calcifications – a less probable diagnosis. Key imaging features favoring synovial osteochondromatosis included the intimate relationship to the facet joint, chronic extrinsic osseous remodeling without aggressive destruction and heterogeneous signal intensity with peripheral enhancement.

Spinal synovial osteochondromatosis typically arises adjacent to a facet joint and remains extra-osseous. Littrell et al. ^{Reference Littrell, Inwards, Sim and Wenger1} described the largest series to date, identifying 29 histologically confirmed spinal cases, 16 (55%) of which involved the cervical spine. Lesions are extradural and typically centered at the neuroforamen, without osseous invasion. Up to 30% of cases may lack visible calcification on CT, particularly in early disease stages, when the nodules are predominantly composed of non-mineralized hyaline cartilage. In this phase, lesions may appear isoattenuating to surrounding soft tissues on CT, limiting detectability. As the disease progresses, chondral maturation leads to dystrophic calcification and ossification, resulting in increased conspicuity on CT. MR characteristics vary according to chondral maturation, fat content and mineralized nodules. As demonstrated in our case, an increased T1 signal may correspond to fatty marrow within maturing cartilaginous nodules. A recent report by Yamauchi et al. ^{Reference Yamauchi, Nakashima, Inoue, Nojima and Imagama2} described the youngest known patient with cervical spine involvement – a 6-year-old child – underscoring the broad age range and variable presentation.

Histologically, synovial osteochondromatosis is categorized as primary – arising de novo in normal joints – or secondary, associated with degenerative or inflammatory arthropathy. ^{Reference Shaw, Zibly, Prasad, Ikeda, Boue and Governale3} Primary lesions demonstrate hypercellular cartilage nodules with mild atypia and occasional binucleation, while secondary lesions are less organized and lack atypia. Although generally benign, malignant transformation into chondrosarcoma has been reported in up to 6% of extraspinal cases; ^{Reference Littrell, Inwards, Sim and Wenger1,Reference Unni, Inwards, Bridge, Kindblom and Wold4} however, no malignant transformation involving the cervical spine has been described to date.

Management depends on symptoms and neural compression. Surgical resection with synovectomy is the preferred treatment, although complete excision may be technically challenging in the cervical spine due to proximity to the vertebral artery and nerve roots. Recurrence is possible even after gross total resection, underscoring the importance of individualized surgical planning and long-term follow-up imaging. ^{Reference Gallia, Weiss, Campbell, McCarthy, Tufaro and Gokaslan5}