Psoriatic arthritis(PsA) and psoriasis(Ps) are two systemic inflammatory diseases linked with rash of psoriasis, but there's still great controversy regarding the exact relationship between them. Our study is to investigate the characteristics and differences of the ultrasonic imaging of enthesopathy in the lower extremity in patients with PsA and Ps, to explore the risk factors of Ps developing into PsA in the long term course.

As one of the most common chronic inflammatory skin diseases, psoriasis(Ps) can be intimately associated to psoriatic arthritis (PsA), affecting approximately 1% to 2% of the world's population. About 30% of them will develop into PsA with serious impact on their health-related quality of life.

PsA, characterised by different subsets with varying degree of severity, is a chronic, complicated inflammatory disease.Enthesitis is the prevalent and distinctive feature of PsA.

Musculoskeletal Ultrasound (MSUS) is a valid noninvasive modality, essential to evaluate entheses thickness, enthesophytes, bursitis and bone erosions as well as to disclose vascularisation on tendons through power doppler (PD) assessment. In recent years, more and more attention has been paid to the diagnosis and management of Ps and PsA, owing to its discrimination of identifying subclinical entheses and joint changes.

Inclusion Criteria:

• Clinical diagnosis of plaque psoriasis, psoriatic arthritis, ankylosing spondylitis, rheumatoid arthritis or osteoarthritis.
• Body Mass Index (BMI)<30 kg/m2.
• Absence of local glucocorticoid or other drugs injection or surgical treatment for entheses scanned in the previous 6 weeks prior to clinical and musculoskeletal ultrasound (MSUS) evaluation.

Exclusion Criteria:

• Other systemic rheumatic diseases or malignant neoplasms.
• Peripheral neuropathy of lower extremity.

A series of 44 patients with PsA, 44 age- and sex-matched patients of Ps and 44 HCs consecutively admitted from the outpatients clinic and ward from January 2016 to September 2016.

All individual participant data (IPD) are to be shared with other researchers, whenever it will be available, and it may be obtained by requesting from the principal investigator