REITER'S ARTHRITIS:

1. Distribution: The MTP joints, first IP, and the calcaneus are the most frequently involved joints in the foot. Initially there is a monoarticular involvement which may lead to the misdiagnosis of septic arthritis. The calcaneus is involved in 50% of patients.

2. Erosion pattern:

Like psoriatic arthritis, ill-defined erosions with uniform joint loss and bone proliferation are observed. In addition, periostitis along bone shafts is present. There may be fusiform soft tissue swelling involving a single digit, giving appearance of a sausage. Early juxta-articular osteoporosis is present however there is usually a re-establishment of normal mineralization.There is erosion and bone production at the attachment of the Achilles tendon and the plantar aponeurosis. Soft tissue swelling and fluffy periostitis involving the distal ends of the tibia and fibula are characteristic for ankle involvement.

3. Differential diagnosis:
Radiographic changes characteristic of arthritis of Reiter's disease are identical to those of psoriatic arthritis. However, the differences in distribution between the two allows for more accurate diagnosis. Reiter's arthritis predominately involves lower extremity, primarily feet, ankles, knees, and SI joints in bilateral asymmetric distribution. Hands, hips, and spine are less frequently involved.