A 48-year-old woman with seropositive rheumatoid arthritis of 15 years' duration presents with progressive hand deformity and difficulty gripping. Examination reveals symmetrical small-joint synovitis, ulnar deviation at the MCP joints, and swan-neck deformity of the index finger. The structure marked **A** in the diagram shows boutonnière deformity of the middle finger. Which of the following best explains the mechanism of PIP flexion with compensatory DIP hyperextension in this deformity?
A. Rupture of the central slip of the extensor expansion, allowing lateral bands to subluxate volar to the PIP joint axis and convert from extensors to flexors of the PIP
B. Erosion of the PIP joint articular cartilage with mechanical blocking of extension by osteophytes
C. Contracture of the flexor digitorum superficialis tendon with secondary shortening of the intrinsic muscles
D. Attenuation of the volar plate at the PIP joint with primary flexor tendon contracture
Explanation
Why option 1 is correct
The boutonnière deformity shown at A is pathognomonic for rheumatoid arthritis and results from rupture of the central slip of the extensor expansion over the PIP joint. This rupture allows the lateral bands to subluxate volar (palmar) to the axis of rotation of the PIP joint. Because they now lie volar to the joint axis, the lateral bands—which normally extend the PIP—are converted into flexors of the PIP. Simultaneously, these subluxated lateral bands become abnormal extensors of the DIP joint, producing the characteristic compensatory DIP hyperextension. This mechanism is well-established in Maheshwari Essential Orthopaedics and is the defining pathophysiology of boutonnière deformity in RA.
Why each distractor is wrong
Option 2 (FDS contracture): While flexor contracture may occur secondarily in chronic boutonnière, the PRIMARY mechanism is central slip rupture, not FDS contracture. FDS involvement is not the initiating pathology.
Option 3 (Volar plate attenuation): Volar plate laxity is the mechanism of swan-neck deformity (PIP hyperextension), not boutonnière. Boutonnière is defined by PIP flexion, which is the opposite.
Option 4 (Erosive arthritis with mechanical blocking): While RA does cause marginal erosions at the PIP joint, erosions alone do not explain the specific mechanism of PIP flexion with DIP hyperextension. The deformity precedes severe erosive changes and is driven by soft-tissue pathology (central slip rupture), not bone destruction.
