It appears that the enthesis is where psoriatic arthritis begins, where Th17 and Th22 mediate bone remodelling: paradoxically both bone erosion as well as new bone formation, that is so characteristic of PsA.
The concept of "deep Koebner phenomenon" is proposed, where stress/strain injury sets off the self-perpetuating inflammation beginning at the enthesis, resulting in bone erosion as well as new bone formation.
The entheseal inflammation subsequently "spills over" to the adjacent joint, causing synovitis as a secondary event; unlike in RA, where synovitis is primary. Clinical detection of joint swelling and tenderness is thus a late event in PsA compared to RA.
Therefore, in order to detect PsA early so as to intervene more effectively, imaging by MRI or ultrasound to detect enthesitis is key.
However, it is not cost-effective to screen every enthesis/joint of every patient with psoriasis for enthesitis/synovitis at every clinic visit, even if ultrasound is available at the point of care. Such multiple joint sonography or whole body MRI are only undertaken in research settings, not in the clinics.
The practical approach is to sound only the joints/entheses which the patient highlights as stiff or painful.