Swan neck deformity caused by the weaking of the volar plate (supportive ligament in front of the PIP joint that normally keeps the PIP joint from hyperextending) and as a result, the PIPJ goes into hyperextension due to the strength of the extensor tendons (from the extensor digitorium). The extensor tendon gets out of balance, which allows the DIP joint to get pulled downward into flexion.
Mallet finger is where there is injury to the extensor tenden thus the DIPJ goes into flexion, by the unopposed action of the DIPJ.
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