BackgroundIt has been shown that there is an anatomical link between the distal interphalangeal joint (DIP) extensor tendon enthesitis and the nail changes in psoriatic arthritis (PsA) [1].ObjectivesTo evaluate the relationship between nail involvement and enthesopathy at DIP level in PsA patients using ultrasonography (US).MethodsWe included patients with PsA. According to OMERACT, the following elemental lesions were dichotomously (presence/absence) evaluated on greyscale US assessment at extensor tendon insertion at distal phalanx of DIP joints: abnormally hypoechoic, enthesophytes, and erosions. We also measured the tendon thickness. Increased abnormal vascularization at entheseal level was also assessed using PD technique. We also performed an US assessment of the finger nails that included the study of morphological changes and measurement of the thickness of nail bed (NBT), nail plate (NPT) and adjacent skin (ST).ResultsWe included 33 PsA patients (323 DIP). Eleven patients (34 %) presented psoriatic onychopathy (45 fingernails) with a mean NAPSI 7 (IQR (25,75)) [2 -18]. US study of the nails revealed dystrophy in 75 nails (23%).At patient level, the mean NPT, NBT and SK were 1.90±0.22, 0.38±0.09, and 2.33±0.62, respectively.None of the patients had clinical involvement of DIP.Using US, we examined 325 extensor tendon. The mean thickness of the tendon was 0.63mm ±0.1 mm. The tendon was abnormally hypoechoic in two fingers (0.61%).Erosions were present in 16 DIP (4.9%).We found enthesophytes in 82 DIP joints at insertion of extensor digitorum tendon (25.23%).We did not observe increased abnormal vascularization at entheseal level.At finger level, the extensor tendon thickness was higher in the presence of US nail dystrophy (0.70 mm vs 0.60, p=0.01). Erosions were more common in fingers with US nail involvement (14.6 % vs 1.9%, p=0.00).Osteophytes were present in 20% of fingers with US nail involvement and 26.3% of fingers without US nail involvement (p=0.166).The thickness of DIP digital extensor tendons was correlated with the NBT(r = 0.412, p = 0.00), the NPT (r = 0.310, p = 0.00), and the thickness of the adjacent skin (r = 0.509, p = 0.00).ConclusionThe presence of US nail dystrophy was associated with thicker extensor tendon and more erosions. The thickness of the tendon was correlated with the thickness of the nail parts.This might be explained by the close relationship between nail and enthesis and supports the theory of the enthesis as an extended organ beyond the tendon bony attachment.