Few studies have investigated the pathogenesis of sacroiliitis in axial spondyloarthritis (axSpA) and it is not well understood. Clinical, laboratory and imaging data were collected from 193 patients with axSpa at baseline and during follow-up. The most common feature in early sacroiliitis was subchondral pannus formation. Pannus invasion was found to be central to pathological degeneration of the cartilage. The researchers concluded that cartilage pannus invasion or endochondral ossification at baseline can predict radiological structural damage during follow-up.