Nearly 30% of patients with cutaneous psoriasis (PsO) developed psoriatic arthritis (PsA). Among these patients 20 % will have severe destructive arthritis. The risk of developing PsA is significantly higher in patients with nail involvement (OR = 2.24; 95% CI [1.26-3.98]). The risk is particularly high for the peripheral form of PsA and onycholysis (OR=2.80; 95% CI [1.34-5.85]).

Thus the investigators wanted to test the hypothesis that onycholysis, in patients without PsA, is a potential clinical marker of subclinical distal enthesopathy and, by extension, of bone micro-structural alterations.

Patients and Methods

The investigators will recruit 4 groups of subjects:

1. Patients with peripheral PsA,
2. Patients with psoriatic nail onycholysis,
3. Patients with PsO only
4. Healthy match control subjects. The investigators will assess the presence of enthesopathy by ultrasonography and bone structural damages (by HR-pQCT) in all subjects at baseline and 4 years.

Inclusion Criteria:

• adults over 18 years and until 65 years
• both gender
• covered by the French National Insurance
• subjects entering one of the 4 groups.

Exclusion Criteria:

• Treatment by biological agents are an exclusion criteria for PsO, onycholysis and control patients.
• Pregnant women

PSUPSO is a prospective study. Diagnosis of PsA is based on the CASPAR criteria (Taylor 2006). PASI, NAPSI and NAPSI target are recorded. PsO patients are gender and age matched with onycholysis patients patients.