The central slip of the extensor tendon inserts into the base of the middle phalanx. Together with the lateral slips of the extensor tendon, the central slip extends the finger at the proximal interphalangeal joint (PIPJ).
Disruption of the central slip produces a boutonniere deformity. The finger can flex but cannot extend at the PIPJ.
The injury may be open for example as a result of injury from a knife or glass; or may be closed, for example, from sudden forced flexion.
A boutonniere deformity may be seen acutely but more commonly develops after a few days or weeks. With time, the lateral slips diverge and the proximal phalanx slips upwards through the gap as if through a buttonhole; hence, this injury may be described as a buttonhole deformity. Eventually, the classical boutonniere deformity is seen. The finger is held flexed at the PIPJ but hyperextended at the distal interphalangeal joint.
In rheumatoid arthritis, the tendon may rupture spontaneously.
An xray should be obtained to look for any fracture particularly at the base of the middle phalanx.
Closed injuries should be managed by splintage. The PIPJ is held extended whilst allowing movement at the metacarpophalangeal joint and the DIPJ.
Open injuries require operative intervention.
Beware of the patient who presents immediately with a swollen PIPJ and full extension. Review them in 2-3 days. Initially, extension may be attained using the lateral slips and disruption of the central slip not appreciated.
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