Rheumatoid hand deformities
Untreated rheumatoid arthritis, an autoimmune disease in which the joint lining is chronically inflamed, can cause deformities, abnormal positions of the fingers and thumb that can become fixed over time.
Remember
A swan-neck deformity has the middle joint of the finger bent backwards beyond straight (hyperextended) and the joint nearest the fingertip bent forwards (flexed), boutonnière is the reverse, and BUS Z recalls the four classic rheumatoid hand deformities.
In rheumatoid arthritis, long-standing synovitis (inflammation of the joint lining) stretches the joint capsule and ligaments. The extensors (muscles that straighten the finger) and the flexors (muscles that bend it) then fall out of balance, and over time the joint can become fixed in an abnormal position. These severe deformities are much rarer now, because disease-modifying drugs, which slow the progression of the disease, are used.
The proximal interphalangeal (PIP) joint is the middle knuckle of a finger, and the distal interphalangeal (DIP) joint is the knuckle nearest the fingertip. In a swan-neck deformity the PIP joint is hyperextended while the DIP joint is flexed, so the finger takes on the curve of a swan's neck and making a fist or gripping is hard. A boutonnière deformity is the mirror image, with PIP flexion and DIP hyperextension.
The metacarpophalangeal (MCP) joint is the knuckle at the base of the finger or thumb. In ulnar deviation the fingers drift towards the little-finger side, because the swollen MCP joints are partly dislocated, which is called subluxation. In the Z-thumb, sometimes called a duck-bill deformity, the MCP joint is flexed while the interphalangeal (IP) joint, the thumb's end joint, is hyperextended, giving a zigzag shape.
The thumb has just one interphalangeal joint, so PIP and DIP joints are not there to deform.
Key facts
- Swan-neck deformity
- the PIP joint is hyperextended and the DIP joint is flexed.
- Boutonnière deformity
- the PIP joint is flexed and the DIP joint is hyperextended, the reverse of swan-neck.
- Ulnar deviation
- the fingers drift towards the little-finger side at the MCP joints, which are swollen and partly dislocated.
- Z-thumb
- the MCP joint is flexed and the IP joint is hyperextended.
- BUS Z
- boutonnière, ulnar deviation, swan-neck and Z-thumb are the four classic deformities.
- Osteoarthritis
- bony lumps at the finger joints point to osteoarthritis, the wear-and-repair arthritis, whereas these deformities change the position of the joints.
Exam traps
- Boutonnière deformity is not the pattern of PIP hyperextension with DIP flexion, because it is the opposite pattern, with PIP flexion and DIP hyperextension.
- Z-thumb deformity does not describe a finger, because it affects the thumb, with MCP flexion and IP hyperextension, and the thumb does not have PIP and DIP joints.
- Ulnar deviation does not describe the bend at the PIP and DIP joints, because it changes the direction the fingers point, as a sideways drift at the MCP joints.
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