Primary cystic arthrosis of the hip
Primary cystic arthrosis of the hip (PCAH) is a rare condition consisting of non-degenerative cystic changes within one or both acetabula. This condition typically presents clinically as unexplained severe hip pain in a young adult. Because the cystic changes are pathologically identical to degenerative subchondral cysts, this is a diagnosis of exclusion based on the combination of radiographic and clinical features.
Clinical presentation
This condition classically presents insidiously in young adulthood to middle age (20-50 years) with acute to subacute onset of severe, debilitating hip pain.
Pathology
The pathophysiology of the cystic changes seen in primary cystic arthrosis of the hip is unclear but may be similar to that of routine degenerative subchondral cysts with an unknown predisposing factor.
Radiographic features
Plain radiograph
The subchondral cysts seen in PCAH are identical to those seen in other locations, but with a notable absence of other signs of degenerative joint disease.
MRI
On MR imaging, the subchondral cysts in PCAH are again similar to those seen in typical osteoarthritis.
They typically show well-defined margins, some degree of lobulation, low to intermediate signal intensity on T1- weighted images, and high signal intensity on fluid-sensitive images.
Following administration of intravenous gadolinium, they may show variable patterns of enhancement including diffuse, irregular, heterogeneous, or rim enhancement. There may be considerable bone marrow edema surrounding the lesion that may also enhance. MR imaging often will depict a connection between the joint and the cyst, especially if the intraarticular contrast is utilized. There may be regions of abnormal cartilage near the lesion, often with a focal full-thickness defect.
Treatment and prognosis
Because of the severity of symptoms and typical large size of the cysts in PCAH, these patients often move on to more aggressive therapies such as joint arthroplasty if conservative care fails.
Differential diagnosis
General imaging differential considerations include:
