Chapter 6

### INTRODUCTION

This chapter aims to serve as a basic introduction to the radiographic evaluation of arthritis. There are reference texts and books devoted to the subject, including those from Resnick1 and Brower,2 which serve as excellent references for those readers interested in further study. This chapter is rich in images, focusing on radiographs, with some attention paid to CT and MRI. A brief section on musculoskeletal infection then follows the discussion on arthritis.

#### Osteoarthritis

Osteoarthritis is the most common joint disease worldwide, with well over half the elderly population affected. Osteoarthritis is primarily a disease of cartilage, with bony changes representing later sequelae of damaged cartilage. The disease is diagnosed by both radiographic and clinical findings, such as joint pain and stiffness. There are no specific serum biomarkers. Osteoarthritis has traditionally been subdivided into primary and secondary forms based on cause. In primary osteoarthritis, no specific underlying cause for the joint disease is identifiable. When a causative factor can be identified, the term secondary osteoarthritis is used. Any event or disease that causes cartilage loss and destruction can result in secondary osteoarthritis, including trauma or inflammatory arthritides such as rheumatoid arthritis. Often times, the underlying cause in cases of secondary osteoarthritis can be deduced through careful examination for old traumatic deformity or erosions from inflammatory arthritides. Despite the subtype, the radiographic characteristics of asymmetric joint space narrowing, subchondral sclerosis, osteophytosis, and subchondral cyst formation remain the same.

Joint space narrowing, resulting from progressive loss of the articular cartilage, is nonspecific to osteoarthritis but helps classify disease severity. In most osteoarthritic joints, cartilage loss is not uniform, with regions of the joint responsible for weight-bearing or exposed to abnormal forces most significantly narrowed. With progressive cartilage loss, the subchondral bone is exposed to abnormal forces that result in remodeling, manifested as sclerosis radiographically. Subchondral cysts or geodes may form, which are areas of focal subchondral bone resorption. One explanation for the formation of these cysts in osteoarthritis is that as fissures or pits form in the articular cartilage, synovial fluid is extruded through these defects into the subchondral bone, forming a cyst. Subchondral cysts, or geodes, are not unique to osteoarthritis. Other arthropathies that can lead to geodes include rheumatoid arthritis, calcium pyrophosphate deposition disease, and hemophilic arthropathy. Osteophytes form at the margins of degraded articular cartilage and act to increase the surface area and provide stability to a damaged joint. As the disease progresses, the osteophytes increase in size.

Osteoarthritis of the hand (Figure 6-1) typically involves mostly the distal interphalangeal (DIP) joints and to a lesser extent the proximal interphalangeal (PIP) joints as well as the joints at the base of the thumb (Figure 6-2), including the first carpometacarpal (CMC) joint (so-called trapezium–trapezoid first metacarpal joint), and scaphoid–trapezium–trapezoid (STT) joints. The more proximal joints of the hand and ...

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