The extensor tendons of the fingers have a central band and lateral bands. If the central band (also called central slip) is injured and the lateral bands remain intact, then the PIP joint cannot be fully extended, and the DIP joint becomes hyperextended. Over time this will cause a boutonniere deformity. To assess for a central slip injury you can check an Elson test or modified Elson test.
Have the patient curl their fingers around a table or box with fingers flexed at the PIP. Have the patient extend the finger while you apply pressure at the middle phalanx. If the central slip is intact you will feel tension as the finger is extended.
Modified Elson test:
This exam maneuver compares the injured finger to the same finger on the patient's other hand. The patient touches together their thumbs and the middle phalanx of the finger being tested. They should attempt to touch their fingernails together and should NOT be able to do it, because the central slip holds tension. In a central slip injury the DIP will straighten compared to the uninjured finger.