PsA? OA? Cases of Joint Confusion

McGonagle D, Hermann KGA, Tan AL. Differentiation between osteoarthritis and psoriatic arthritis: implications for pathogenesis and treatment in the biological therapy era. Rheumatology (2014) doi: 10.1093/rheumatology/keu328 First published online: September 16, 2014

Yuan Z, Ieong FH, Jiang H, Sobel ES. Osteoarthritis and psoriatic arthritis: Findings in three-dimensional biophotonics imaging.Biomed Mater Eng. (2014) 24:3063-71. doi: 10.3233/BME-141128

While osteoarthritis (OA) and psoriatic arthritis (PsA) are regarded as completely different diseases, both can start in similar regions of joints where ligament meets bone and it may be hard to distinguish the two in their early stages, according to a new review.

These researchers in the UK and Germany say that generalized forms of OA and PsA cause degeneration in the cervical spine and the synovial-entheseal complexes in the finger and knee joints.

The resulting misdiagnosis of a primary degeneration-related pathology as being part of PsA may explain why some biologic disease modifying antirheumatic drugs (DMARDs) appear to fail in PsA patients. “This is not a reflection of poor clinical acumen,” the authors stress, “but rather a failure to appreciate that the pathological process overlaps in the two diseases.”

Most PsA joints show classical inflamed entheses with diffuse bone edema on MRI, while classical degenerative entheses are typical of advanced OA joints, the review points out. However, there’s a subset of joints where there are overlapping features, “often with some degree of inflammation or degenerative changes that can be accepted as either OA or PsA.”

For example, finger dactylitis and psoriatic nail changes in PsA may appear similar to dystrophic nail changes and swollen joints in people with OA -- and both diseases can produce new bone formations that look similar on X-rays. PsA generally attacks the distal interphalangeal (DIP) finger joints first, and some OA patients develop severe disease in the DIP joints.

Fusing two imaging technologies--X-rays that can't distinguish soft tissues very well and 3D diffuse optical tomography (3D DOT) that can do so accurately--researchers at the University of Florida, Gainsville, may have identified a sensitive tool to help distinguish between the two conditions in commonly affected joints. They described the concept MistMistin a bioengineering journal last January.

In a preliminary case study, the “fused” X-ray/DOT imaging was used to examine the DIP finger joints in six volunteers – two diagnosed with OA, two with PsA, and two healthy controls.

The resulting 3D images reveal significant differences between OA and PsA -- notably the narrowed joint space of OA and the “pencil in cup” bone changes and deformation in the DIP joints.

While the Florida findings need to be replicated in larger studies, the multimodality imaging approach may help to avoid  inappropriate use of biological DMARDs in some PsA patients.