Paget’s disease is a disease of osteoclasts that commonly affects the pelvis in an asymmetric distribution, more frequently on the right. Initially, the disease is predominantly lytic. Over time, disease extends through the bone and a coarse pattern of mixed lysis and sclerosis ensues. Bone expansion and softening occur, promoting osteoarthrosis and protrusio acetabuli respectively. 

Early Paget’s disease can mimic metastatic bone disease with bone pain, elevated alkaline phosphatase and increased bone turnover on isotope bone scan. Thickening of the iliopectineal line can be an early finding which helps to make the diagnosis of Paget’s disease. Of course both metastatic disease and sarcoma can occur in Pagetic bone and can present with pain.

In this patient, Paget’s disease involves both sides of the fused sacroiliac joints with confluent pagetic bone across the joints. Pre-existing partial ankylosis may have been present in the form of osteophytic bridging, a common feature of degenerative change. Altered bone contour in Paget’s disease promotes degenerative joint disease. In this patient there is no evidence for other causes of SIJ fusion such as ankylosing spondylitis.

Finally, late-stage Paget’s disease is relatively inactive and may be painless. Right groin pain in this patient was a consequence of a hemorrhage from his large renal cell carcinoma with a blood clot obstructing the proximal right ureter causing acute ureteric colic.