Rheumatoid arthritis is a chronic, auto-immune, inflammatory disorder affecting multiple organs, It most affects synovial joints, commonly the hands and feet. In the hand, it is seen as an inflammatory arthritis with characteristic bilateral, symmetric, multi-articular involvement. 

The inflammation initially is seen on X-ray as soft tissue swelling around affected joints. This progresses pathologically to formation of destructive synovial pannus, leading to marginal erosions seen on radiographs. As the disease progresses, central erosions and concentric joint space loss due to cartilage loss, are also seen. Articular surface breach with subchondral cysts may be seen.

Inflammatory pannus causes some typical deformities and subluxation at the destroyed articular surfaces.

Swan-neck deformity is initiated by a flexor synovitis  that increases the flexor pull on the metacarpophalangeal joint.  PIP  hyperextension may be due to lax volar plate, tethered collateral ligaments and/or flexor tendon rupture.  Flexion at the DIP may be reciprocal or due to extensor tendon inflammation. 

Boutonniere deformities occur when the central slip of the extensor digitorum tendon is torn or stretched resulting in PIP joint flexion. Increasing PIP joint flexion causes further extensor retinaculum damage resulting in "buttonholing" of the proximal phalanx between the lateral bands of the extensor tendon. Secondary DIP joint extension then occurs,

Note: Grateful acknowledgement of images and case kindly contributed by my colleague at Chesterfield, Dr Ian Bickle.