Bare area of synovial joint
What is the “bare area”? Why do the earliest erosions begin here?
In synovial articulations, hyaline articular cartilage covers the ends of both articulating bones. The articular capsule envelops the joint cavity and is composed of an outer fibrous capsule and a thin inner synovial membrane. The synovial membrane typically does not extend over cartilaginous surfaces but lines the nonarticular portion of the synovial joint and also covers the intracapsular bone surfaces that are not covered by cartilage. These unprotected bony areas occur at the peripheral aspect of the joint and are referred to as “bare areas”.
In these areas, the bone does not have a protective cartilage covering. Consequently, the inflamed synovial pannus, which occurs in inflammatory arthritides such as RA, comes in direct contact with bone, resulting in marginal erosions. These “bare areas” are where you should look for the earliest evidence of erosions. Specialized views including the Norgaard view (ball-catcher’s view) of the hands are optimal for demonstrating the earliest erosive changes in an inflammatory peripheral arthritis, whereas the anteroposterior Ferguson view of the sacroiliac joints demonstrates the earliest changes in an inflammatory axial arthropathy. With progression of disease, the pannus proliferates to cover the cartilage surfaces, resulting in cartilage destruction (joint space narrowing) and more diffuse bony erosions.