Diagnostic Radiology/Musculoskeletal Imaging/Joint Disorders/Rheumatoid arthritis

Rheumatoid arthritis is the most common purely erosive inflammatory arthropathy. Rheumatoid arthritis is generally symmetric and affects proximal joints. There are no proliferative erosions as in erosive OA, psoriatic or Reiter's arthritides. Rheumatoid arthritis rarely includes any productive bone such as periostitis, enthesopathy, and osteophyte formation unless there is secondary osteoarthritis present. Rheumatoid factor is positive in 90-95% of the cases. Early in rheumatoid arthritis the RF may be falsely negative, and in older patients, the RF may be falsely positive. [MSK: The Requisites, 2nd edition, Manaster et al.]

The characteristic radiographic features of rheumatoid arthritis include soft-tissue swelling, periarticular osteoporosis, joint space narrowing, articular erosions, and marginal erosions. The marginal erosions are caused by the extension of pannus into synovial pockets overlying regions devoid of articular cartilage (which would normally protect against the toxic debris and inflammatory pannus in the synovial fluid). [Musculoskeletal Imaging: Case Review, Josephy Yu, 2001]

Within the Wrist
Early RA will demonstrate erosions in the distal radioulnar joint, ulnar styloid, radial styloid, and triquetro-pisiform joint (the latter are best seen on ball-catcher's view). Typically, the next erosions and joint space narrowing will occur in the carpal bones and MCP joints. Ligamentous rupture can lead to wrist instability resulting in ulnar translocation of entire carpus, scapholunate instability, and dorsal and volar flexion carpal instability (DISI and VISI).

Within the Elbow
Diffuse involvement of joint with extensive erosion in olecranon articulation and joint space narrowing with trochlea of humerus seeming to "dig into" olecranon. Joint effusion a

Within the Shoulder
Glenohumeral joint space narrowing with marginal erosions within the humeral head. Ligamentous/tendinous injury results in rotator cuff degeneration with a "high-riding" humeral head. Also look for lysis of the lateral clavicle and erosion at the insertion of the coracoclavicular ligament.

Within the Spine
The cervical spine is commonly affected; particularly look for C1-C2 involvement which can have catastrophic consequences. For instance, pannus formation near the odontoid can cause erosions in the odontoid and ligamentous laxity of the transverse ligaments. The latter results in increased predental space (greater than 2.5–3 mm which is upper limit of normal in adults) which can result in canal stenosis. Alternatively, the pannus itself can proliferate and directly compress the spinal cord. Also, ligamentous destruction can result in atlantoaxial impaction (i.e. dens goes up into the foramen magnum along with Mr. Cord—Mr. Cord doesn't like that). Other findings include erosions within the facet joints.

Within the Hip
Axial joint space narrowing leading to protrusio deformity at the severe end of the spectrum, diffuse osteoporosis, lack of osteophyte formation unless secondary OA occurs. In contradistinction, OA usually results in superolateral (i.e. weight-bearing) acetabular joint space narrowing, normal bone density and osteophyte formation

Within the Knee
Joint space narrowing, osteoporosis, marginal erosions, without osteophyte formation unless there is secondary OA (do you see a recurring theme?). There may be a joint effusion with fluid tracking into the semimembranosus-gastrocnemius bursa (AKA Baker's or popliteal cyst). Occasionally there will also be patellar tendon rupture.

Within the Feet
MTP joint erosions early on, particularly of the 5th MTP. Toe deformities that result include lateral deviation at the MCPs and hammer toes. Also look for retrocalcaneal bursitis with erosions in the posterior calcaneus; the retrocalcaneal fluid can obliterate the pre-Achilles fat pad (AKA Krager's fat pad).