Causes of Ulnar Drift in Rheumatoid Arthritis

Anatomical

Direction of pull of extensor & flexor tendons
Shape of head of metacarpal (relative ulnar deficiency)
Ulna collateral stronger than radial collateral
Presence of extensor indicis & digiti minimi on ulnar side of communis tendon

Physiological (Functional)

Gravity
Force of opposition from thumb onto fingers
Turning taps & door handles etc
Pressure on radial side of hand on rising from chair etc

Pathological

Synovitis

attrition of radial collaterals

Articular cartilage erosios
Extensor tendon subluxation
Capsular, ligamentous & volar plate disruptions
Intrinsic tendon shortening
Radial deviation of wrist

Z deformity & ↑ displacing force of extensor & flexor tendons

Trigger finger releases
- distorted pull of flexors as well