(COAD) intraarticular osteochondroma at SIJ ? (CT)

It is definitely not an osteochondroma.

Posterior pseudarthroses are common in the SIJ. They may be either
developmental or acquired. They occur more commonly in females and with
increasing frequency with age. They have a variety of shapes and occur
anywhere in the ligament compartment.

Then it is a matter of what word you choose to describe the bony prominence
that develops within the ligamentous part of the SI joint and how you
define ‘exostosis’ versus ‘osteophyte’ versus ‘bone spur’. I don’t like
‘exostosis’ here because my understanding is that the bone formation is
primarily within a previously existing ligamentous structure – and when
ligaments ossify, I don’t call them exostoses e.g. degenerative
ossification in DISH or post-traumatic ossification such as an ossified MCL.

So, I call it bone spurring with pseudarthrosis.

Rob