Focusing on ligamentous soft tissue inflammation for the future understanding of early axial psoriatic arthritis

Imaging has transformed the understanding of inflammatory and degenerative arthritis in both peripheral and axial disease. In axial inflammation, fat suppression magnetic resonance imaging (MRI) has unravelled the role of sub-fibrocartilaginous osteitis in axial spondyloarthritis and the role of peri-entheseal vertebral body osteitis and subsequent spinal new bone formation. Established or late-stage axial psoriatic arthritis (PsA) cases often exhibit impressive para-marginal or chunky syndesmophytosis on conventional X-ray that pathologically represents entheseal soft tissue ossification. However, the spinal entheseal soft tissue and contiguous ligamentous tissues are poorly visualized on MRI in subjects with early inflammatory back pain including those with axial PsA. In this article, we highlight the need for imaging modalities to discern the crucial soft tissue "ligamentous" component of axial PsA towards diagnosis, prognosis and therapy validation. We issue a clarion call to focus advanced imaging methodology on spinal ligamentous soft tissue that represents the last hidden backwater of PsA immunopathology that needs visualization to fully decipher axial PsA pathogenesis. This in combination with the existing ability to visualize ligamentous bony anchorage site osteitis is needed to define a gold standard test for axial PsA.