 Introduction/Background Types of nondegenerative arthritides resulting in chronic extremity pain can be broadly divided into seropositive inflammatory arthritis, seronegative inflammatory arthritis (spondyloarthritis), and crystalline arthritis. Although erosive osteoarthritis is often classified as a subset of osteoarthritis [1], the severity of clinical symptoms often put it in the realm of inflammatory arthritis in terms of differential diagnosis. Although it is much less common than osteoarthritis, millions of Americans suffer from chronic extremity pain from inflammatory and crystalline arthropathies. The prevalence of rheumatoid arthritis (RA), the most common inflammatory arthritis, has been reported to be between 0.5% and 1% [2], and the prevalence of gout, the most common crystalline arthritis, is as high as 3.9% of adults in the United States [3]. Imaging plays an important role in the diagnosis and management of inflammatory and crystalline arthritis. It is important to consider global distribution of skeletal involvement, and correlating findings on multiple studies/modalities may be necessary. Furthermore, it is critical that imaging results are interpreted in the context of clinical and serologic results to add specificity because there is significant overlap of imaging findings amongst the various types of arthritis. Imaging can be used to identify osseous and/or soft tissues abnormalities that are seen with inflammatory and crystalline arthritis. In addition to joint space narrowing, the osseous abnormalities of the extremities that are assessed include erosions, bone proliferation in the form of enthesitis and periostitis, and bone marrow edema. The location of the erosions with respect to the joint (ie, marginal, para-articular, central, and subenethesial) is helpful in differential diagnosis.
Chronic Extremity Joint Pain Suspected Inflammatory Arthritis Crystalline Arthritis or Erosive Osteoarthritis PCAs.  Introduction/Background Types of nondegenerative arthritides resulting in chronic extremity pain can be broadly divided into seropositive inflammatory arthritis, seronegative inflammatory arthritis (spondyloarthritis), and crystalline arthritis. Although erosive osteoarthritis is often classified as a subset of osteoarthritis [1], the severity of clinical symptoms often put it in the realm of inflammatory arthritis in terms of differential diagnosis. Although it is much less common than osteoarthritis, millions of Americans suffer from chronic extremity pain from inflammatory and crystalline arthropathies. The prevalence of rheumatoid arthritis (RA), the most common inflammatory arthritis, has been reported to be between 0.5% and 1% [2], and the prevalence of gout, the most common crystalline arthritis, is as high as 3.9% of adults in the United States [3]. Imaging plays an important role in the diagnosis and management of inflammatory and crystalline arthritis. It is important to consider global distribution of skeletal involvement, and correlating findings on multiple studies/modalities may be necessary. Furthermore, it is critical that imaging results are interpreted in the context of clinical and serologic results to add specificity because there is significant overlap of imaging findings amongst the various types of arthritis. Imaging can be used to identify osseous and/or soft tissues abnormalities that are seen with inflammatory and crystalline arthritis. In addition to joint space narrowing, the osseous abnormalities of the extremities that are assessed include erosions, bone proliferation in the form of enthesitis and periostitis, and bone marrow edema. The location of the erosions with respect to the joint (ie, marginal, para-articular, central, and subenethesial) is helpful in differential diagnosis.