Christopher T. Ritchlin, MD, MPH, Editor

A combination of events over the last 15 years, however, has transformed our understanding of disease mechanisms, clinical features, and treatment options for psoriatic arthritis. The development of the Classification of Psoriatic Arthritis Criteria (CASPAR) greatly improved selection of patients for clinical trials. Efforts by members of the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) and Outcome Measures in Rheumatology (OMERACT) resulted in the development of validated and reliable outcome measures that included not only skin and joints but also enthesitis, and dactylitis, coupled with assessments of function and quality of life. These advances were triggered by the development of new biologic therapies with great efficacy for psoriasis and the many facets of psoriatic musculoskeletal inflammation. Over the last several years, new discoveries centered on key mechanisms that underlie skin and joint inflammations emerged and have proven to be quite distinct from pathways in rheumatoid arthritis. In parallel, new biologic agents and small molecules have been recently developed that show efficacy in psoriatic but not rheumatoid arthritis. These new findings have generated great excitement and expectations regarding the discovery of more effective and safer therapies for psoriatic arthritis.

This issue of Rheumatic Disease Clinics of North America addresses the landscape of advances, many of them recently described in psoriatic arthritis. The first three articles present the epidemiology, clinical features, and natural history of psoriatic arthritis along with prognostic considerations. The next two articles outline imaging modalities important for diagnosis and monitoring in psoriatic arthritis and the critical elements important for early diagnosis and treatment. The next articles highlight genetic, epigenetic, and pharmacogenetic aspects and recent advances in our understanding of the cause and pathogenesis of psoriatic arthritis and psoriasis with emphasis on new disease paradigms that have unveiled new treatment targets. The next two articles discuss the critical importance of comorbidities in the diagnosis and management of psoriatic arthritis and the use of newly developed outcome measures in clinical trials and in the office setting. The next two articles provide an evidence-based review of disease management in psoriatic arthritis that includes traditional disease-modifying rheumatic agents, small molecules, and biologic agents. The concluding article provides a detailed overview of novel treatment concepts in psoriatic arthritis.

The authors who contributed articles to this volume are leaders in the field, and their contributions have advanced the science, diagnosis, and treatment of psoriatic arthritis over the last fifteen years. They have unique perspectives that provide a comprehensive overview of psoriatic arthritis. I want to express my sincere appreciation for their efforts.