A negative plantar angle can sometimes develop in the pelvic (back) feet, meaning that the coffin bone (or third phalanx/P3) is "tipping upward" along its solar margin (i.e. along the front of the bone). Some professionals refer to this abnormality as counter-rotation of the coffin bone.
There are a few indicators that a negative plantar angle is present:
Since the plantar angle of the coffin bone is accurately represented by the angle of the frog, increased distance between the apex of the frog/ground surface as compared to the base of the frog/ground surface suggests negative plantar angle.
In chronic cases, "bowing" of the dorsal hoof wall occurs as the coffin bone gradually tips upward (just as "dishing" of the hoof wall occurs with downward rotation of the coffin bone).
Negative plantar angle dramatically increases the challenge to the stifle joints. For example, a person would feel stress to the knees while wearing wedges under the toes. In an attempt to facilitate stifle function, the horse will often attempt to increase hoof angle by sticking the toes down into soft footing during exercise.