Identification of possible markers associated with the onset and / or progression of psoriatic arthritis through comparison in patients with psoriasis alone (Ps) and patients with psoriasis with joint involvement (PsA), plasma levels of a panel of cytokines referable to the Th17 pathway , together with serum levels of MMPs, TIMPs and markers of bone remodeling.

The primary objective of this study is to verify whether the arthropathy component has an effect on the metabolic activity of osteoblasts. This will be determined through the study of the differences in serum concentrations, between Ps and PsA subjects, of "CTx", the parameter best characterized in terms of analytical variability and intra- and inter-individual variability, in the field of bone metabolism.

The secondary objective is the verification of the differences in serum concentrations of the series of parameters specified above, in order to define a complete pattern of variations that can identify the molecular pathways involved in the definition of psoriatic arthropathy.

Inclusion Criteria:

• Age ≥18 years
• For Ps group:

plaque psoriasis no previous treatment with biological drugs

- For PsA group: plaque psoriasis with arthropathy no previous treatment with biological drugs but in treatment as per clinical practice or, alternatively, last treatment with drug suspended by a number of days double compared to the half-life of the drug

Exclusion Criteria:

• Age <18 years
• Pregnancy
• Neoplasia in progress or previous (<5 years)
• Other acute or chronic inflammatory diseases
• Other rheumatological diseases
• Metabolic diseases of the primitive bone
• Recent bone fractures (<6 months)
• Anxiety, psychosis, depressive disorders
• Inability to understand the information provided by the recruiter doctor
• For Ps group:

treatment with biological drugs