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1Principles

When the central slip is detached, the lateral bands are palmarly displaced and pull the DIP joint into hyperextension.

Deforming forces

Boutonnière deformity
When the central slip is detached, the lateral bands are palmarly displaced and pull the DIP joint into hyperextension.
The flexor digitorum superficialis (FDS) pulls proximally on the middle phalanx, forcing the PIP joint into flexion.

Swan-neck type deformity
If the middle phalanx is palmarly dislocated at the PIP joint by the energy of the trauma, the flexor digitorum profundus (FDP) pulls the DIP joint into flexion.