The presence of collapsed heels and a hoof capsule that is "running forward," as evidenced by examination of the solar surface
as described above, should be a red flag. Regardless of whether the horse is lame at the time, it is important to obtain lateral
radiographs of the feet. Use a radiographic technique that yields good soft tissue detail, and center the beam at the solar
margin of the third phalanx (P3) to ensure a true lateral. These radiographs are important both to confirm the presence of
a negative palmar or plantar angle (PA) and to develop an appropriate treatment plan (see "All about PA").
Photo 2: Relative sole depth is reversed in a foot with NPAS, so there is more depth under the tip of the third phalanx than
under the wings.
Although several subtle abnormalities may be evident on a good soft tissue radiograph in feet with NPAS, the two most obvious
and most important are a negative PA and abnormal digital alignment (DA).
PA. When looking at the area beneath P3 on a true lateral radiograph of a healthy foot, there is more sole depth under the wings
of P3 than under the tip, thereby creating a positive PA. Relative sole depth at these two locations is reversed in a foot
with NPAS, so there is more sole depth under the tip of P3 than under the wings (Photo 2).
DA. On a lateral radiograph that includes the pastern, the long axes of the three phalanges (and their dorsal surfaces) can be
represented by a straight line in a well-trimmed horse with healthy feet. This normal alignment is broken in a horse with
NPAS by the displacement of P3 as a consequence of distortion and ultimate collapse of the hoof capsule in the heel region.
In fact, it is not uncommon to find osteophytes in the region of the extensor process of P3 in these feet, as the distal displacement
of the wings of P3 tips the extensor process closer to the dorsal surface of P2.