Arterial flow and bone alignment

Above is an x-ray of a horse with a severely rotated (broken forward) P3, with separation from the hoof capsule. It's pretty obvious that the heels are very high, there are compression areas of the horn below the coronet at the toe indicating long standing pressure there, and the bone has moved away from its normal anatomical position of being parallel to the hoof wall.

The P-angle is very extreme, with the bone almost standing on its tip.

Strasser argues that the arrangement of the bones of the coffin joint in such a hoof, mimics the situation that occurs for a split second as the fetlock joint descends under load.

Rotating the photograph so that P3 is about ground parallel and digitally removing the flared capsule, shows how this might occur.

The arrangement of the pastern bones relative to the navicular bone at the back of the coffin joint is the same as occurs under load. Strasser says this means that the momentary reduction in arterial flow that occurs normally on pastern extension, is permanent in such a hoof.  Obviously blood flow is not completely shut off as happens in full pastern extension for a split second, but it is impaired, which increases pressure in the arteries and results in a loss of bone around the artery entrance points.  

She also argues that, because the heel is supplied by an artery which branches off above the cut off point, the enervation of the heel region is unaffected, but the enervation of the toe region is affected which explains why a horse may continue to load even a badly damaged toe in preference to its heel.