19/01/2022
This is such a great exploration and explanation of these two related but distinct issues!
Alignment versus Palmar/Plantar Angle
There has been a lot of recent awareness into negative palmar and plantar angles, as well as broken back HPA and phalangeal alignment. I think owner education into this is great, and more knowledge can help in identifying possible issues with our horses. That being said, some owners are getting the concepts a bit mixed up, so I wanted to clarify a few things.
I have had a handful of people (not clients) reach out to me and say their horse "is NPA," but once they send the rads along, I notice that the horse actually doesn't have negative palmar or plantar angles at all, but instead simply has a broken back phalangeal alignment.
So what is NPA?
A negative palmar (front feet) or plantar (hind feet) angle refers to the angle between the ground and the coffin bone. When drawing a horizontal line parallel to the ground plane, and another line parallel to the bottom of the coffin bone, these two lines will create an angle. If the back half of the coffin bone is closer to the ground than the front half, that angle is labeled negative. If the front half is closer to the ground, that angle is labeled positive. With a negative angle, a vet can diagnose NPA.
When it comes to phalangeal alignment, we name it based on the comparison to a straight alignment (although truly straight alignment is rarely found on radiographs). If you draw a line through the middle of the long pastern bone, or P1, parallel to the bone, a straight alignment would be if this line also was parallel through the short pastern bone (P2), and parallel to the dorsal aspect of the coffin bone (P3). A horse would have 'broken back phalangeal alignment' if P1/P2/P3 are not aligned within the limb/capsule, and to put it as simply as possible, the joints between them look as though someone has stood in front of the limb and pushed the joint back toward the tail. In more anatomical terms, P1 might have a more acute/upright angle than P2, and P2 would have a more acute/upright angle than P3.
I have attached 2 radiographs here to show these two issues.
In the radiograph on the left of a hind foot, this horse's coffin bone is tipped backwards in comparison to the ground plane, resulting in a -.21Ā° angle, which can be diagnosed as NPA. You can see the back half of the coffin bone is closer to the ground then the front half. Note that this horse also has broken back alignment.
In the radiograph on the right of a front foot (different horse), we can see the palmar angle is 9.12Ā°, showing a very clear positive palmar angle. That being said, P1/P2/P3 are out of alignment- if we started at the top crosshair and followed the green line to the ground, the bones do not continue parallel with that line and end up ahead of that line, at a shallower angle. This horse has a broken back alignment.
While some often address these two issues similarly, they aren't the same, and we should be careful with our comments and 'diagnoses' online and elsewhere- not all horses with broken back HPA or phalangeal alignment have NPA.
Radiograph mark ups are thanks to MetronHoof from EponaMind- Epona has done many studies with their radiograph database, and John Craig has some very interesting findings - you can hear more in the archives of talks on their website!