Objectives Skin and joint involvement in psoriasis (PsO) and psoriatic arthritis (PsA) are thought to relate to the so-called Koebner response. Given that dactylitis is non-randomly distributed in the digits, this study tested the hypothesis that the accessory pulleys linked to the flexor tendons were thickened in PsA and thus exhibited koebnerisation.

Methods Ninety-six subjects (27 PsA, 27 rheumatoid arthritis (RA), 23 PsO and 19 healthy controls (HCs)) were enrolled. The A1, A2 and A4 pulley thickness was measured using a high-resolution probe (22 MHz). All patients were in remission or low disease activity with current dactylitis being excluded.

Results Within 864 pulleys investigated, patients with PsA had thicker pulleys in every digit compared with both RA (P<0.001 and P=0.003) and HCs (P<0.001). RA and PsO groups had some pulleys in some digits thicker than HCs whereas some others were comparable. The second digit A1 pulley thickness was higher in patients with PsA with previous dactylitis (P=0.020). More pulleys were thickened in the PsA group (165/243, 68%) than RA (41/243, 17%; P<0.001) and HCs (13/171, 7.6%; P<0.001).

Conclusions In established PsA, the accessory pulleys are thickened compared with RA, PsO or HCs and especially in subjects with a history of dactylitis. These findings implicate the involvement of pulleys in PsA-related tenosynovitis and dactylitis supporting the idea of deep koebnerisation in dactylitis and sites of high physical stress.