Primary cystic arthrosis of the hip

Case contributed by Dr Matt Skalski

Presentation

Insidious onset of progressive, severe right hip pain and now unable to bear weight on that side. Patient reports no history of trauma or instigatory activity.

Patient Data

Age: 30
Gender: Female

A well-defined, lucent lesion is observed in right supraacetabular region with a thin rim of sclerosis. There is no evidence of osseous expansion, cortical violation or periosteal reaction. Soft tissues are unremarkable. 

The patient was referred for an MRI of the right hip to better characterize the lesion and further investigate the patient's unexplained severe hip pain. 

The lesion identified previously is lobulated with predominately iso to hypointense  signal on T1, hyperintense on STIR, with surrounding bone marrow edema. Mild peripheral enhancement of the lesion is observed.

Incidentally, two adnexal cysts are identified on the left; a small simple cyst and a larger complex cyst. The larger cyst demonstrates a heterogeneous increased T1 and STIR signal with no evidence of enhancement.

At biopsy, the initial localizing scan showed that there was gas within the lesion. This indicates that this must be continuous with the joint space and  is likely just a subchondral cyst, as none of the other differential considerations would show gas. After seeing this finding on the initial scan, the biopsy was aborted.

A 6 month follow-up MR arthrogram was recommended to confirm the stability of the lesion and demonstrate the communicating cystic nature of the lesion.

MR arthrogram shows that the right supra-acetabular lesion is stable in size and appearance. Gadolinium based contrast clearly fills a portion of the cystic lesion, confirming communication with the joint space. A full-thickness cartilage defect at this site.

Relatively unchanged from previous imaging, the large heterogenous  adnexal cyst is again noted.  The simple adnexal cyst seen in the initial imaging is no longer present. 

Case Discussion

Subchondral cysts become a challenging diagnostic entity when they present as an epiphyseal lucent lesion in the absence of other readily identifiable degenerative changes, and/or in a younger patient. In such instances, this pathology has been considered by some to represent primary cystic arthrosis of the hip when it occurs about the hip with severe, inexplicable pain and limited range of motion. Further imaging workup may be necessary in these scenarios to exclude other differential possibilities, as was done in this case.

Cystic joint changes are most commonly associated with arthritic processes such as osteoarthritis, rheumatoid arthritis and calcium pyrophosphate dehydrate crystal deposition disease (CPPD), or with osteonecrosis, all of which commonly affect the femoroacetabular joint. Occasionally, large and/or solitary subchondral cysts occurring in these diseases may be confused with a neoplastic process, especially given the right clinical scenario. Periarticular/epiphyseal tumors include chondroblastoma, giant cell tumor, metastasis, histiocytosis X and aneurysmal bone cyst. In the acetabulum specifically, fibrous dysplasia and pigmented villonodular synovitis (PVNS) could also be considered.

The radiographic and pathological appearance of subchondral cysts and intraosseous ganglia are essentially indistinguishable, and arguably represent the same lesion in many cases. 

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Case information

rID: 31640
Case created: 19th Oct 2014
Last edited: 8th Feb 2016
Inclusion in quiz mode: Included

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