FIGURE 1. Balance as viewed from a solar aspect.
Humans seem to be drawn toward simplifying rules of thumb in all aspects of life. In farriery, such rules of thumb tell us that certain aspects of the hoof should be “balanced 50-50” or “align in a straight line” or “should be parallel.” Whether or not there are careful measurements to back up such statements, such assertions have a certain allure and “sound right” to many people. After a while, such beliefs become second nature to many practitioners.
Meet John Craig. He doesn’t profess to understand why people believe certain things, but he does believe — after measuring hundreds of hooves — that a perfectly symmetrical hoof with “aligned bones” might not be the goal you’re after.
In a talk delivered at the 2005 International Hoof-Care Summit, the Creston, Calif., researcher takes a look at six widely accepted beliefs in horseshoeing.
“I’m not attacking these teachings,” he says. “I think they’re still valid, but I prefer to call them ‘rules of thumb.’ We want to get to the next level, measure a little more carefully and see how valid they are.”
With the availability of digital images and measuring software, it is now relatively easy to look at these things more closely and with more accuracy. The research reported in his talk was co-authored by Monique Craig, his wife and partner in equine hoof research, whose company, EponaTech, developed the Metron software used for measuring the hoof.
Belief No. 1: Balance The Foot From Side To Side. The principle behind this teaching lies in the idea of drawing a line down the center of the frog and striving to make the foot symmetrical, side-to-side (Figure 1). “Are horses really like that?” Craig asks. “Should they be? And why would you consider this ideal? I don’t necessarily have the answers.”
But he does have a lot of research. In a study regarding balance, Craig reviewed 80 feet. His study shows that 66 of the 80 were narrower on the medial side of the foot. It wasn’t by a large amount — about 2 percent.
“This one isn’t too controversial,” he admits. “The medial wall is typically steeper and the lateral wall less so, which translates to the solar aspect such that the medial side is typically a little narrower. Not a huge deal. But in my opinion, I wouldn’t be thinning the lateral wall to bring it to perfect symmetry, as nature isn’t necessarily symmetric.”
Belief No. 2: The Hoof Angle Should Match The Heel Angle. “There’s absolutely nothing wrong with this happening,” Craig says, “but I don’t think it’s the normal situation.”
FIGURE 2. Common theories of balance hold that the hoof angle should match the heel angle.
In a study involving 230 feet, Craig measured each hoof, subtracting the heel angle from the hoof angle. The study showed that most horses have somewhat underrun heels. The average was an 8.3 degree difference.
“If you start to get away from the average we found, like say a 24-degree difference, then yes, it’s something to worry about,” Craig explains. “But if you have a difference of 8 or 10 degrees, I think it’s normal, at least according to this study.”
Craig continues urging farriers to think twice before applying any mechanical contraptions to horses who may fall within the 8-degree average. “In my opinion,” he says, “horses tend to be this way, and it is not a problem.”
“Having a rule of thumb when you’re out shoeing a horse and have a million things to think about isn’t a bad thing. Indeed, the hoof and heel angles should be roughly equal, but the trend is that the heel is slightly less inclined than the hoof.”
Belief No. 3: Strive For A 50-50 Dorso-Palmar Balance. “What we’re talking about is this business of taking the center of articulation and dropping it straight down so it bisects the hoof in half,” he says. This theory has been noted in a number of textbooks, including a recent book written by a PhD in biomechanics. The theory claims that a “well-balanced” foot shows this relationship relative to the center of articulation of the coffin joint (Figure 3).
FIGURE 3. Balance theories suggest this 50-50 relationship between the coffin joint and the rest of the hoof.
To measure feet for this theory, Craig selects three points at the distal end of P2, where his software fits a circle to those three points. At the center of the circle is the center of articulation, which is where P3 tends to rotate around P2. The measurement Craig is after is the percentage of foot support ahead of the center of articulation. Craig verified the method by studying cadaver legs and inserting pins—concluding that fitting a circle in the lateral radiograph successfully finds the center of articulation, within 1 millimeter.
“When we look at these feet, we always see more than 50 percent ahead of articulation,” he explains. “There’s an innate desire to make things parallel and line up, or to make them 50-50 by some measure. But it’s not that simple in a biological system.”
Craig’s study examined 90 feet. In this study, none of them measured at 50 percent ahead of articulation. In fact, the majority of the feet showed a 65 percent support ahead of the center of articulation.
“We believe this is normal,” he concludes. “We don’t believe in ‘trailers’ or other attempts to alter the foot in attempts to achieve 50 percent ahead of articulation. It doesn’t seem nature intended it that way.”
Belief No. 4: The Three Phalanges Of The Digit Should Align In A Straight Line. “This is the most difficult one to get at, this business of bones aligning,” Craig says (Figure 4). “Of course, you’ll see this in every book you pick up. I’m not saying it’s wrong. You don’t want them completely misaligned, but let’s look at them a little more closely.”
FIGURE 4. It is almost universally accepted that these bones should align.
Since an accurate way of measuring bone alignment hasn’t been found, Craig created his own method. First, he fits a circle at the pastern and coffin joints and assigns a line on the dorsal surface of P3. He then constructs an axis of P1 that comes through the center of articulation of the pastern joint. He then connects another line between the two centers of articulation and extends it down. This method allows a geometrical reference to be assigned to the bones.
“Imagine rotating the coffin joint, you’ll see the angle between the lower two lines gives a measure of the angle,” he says. “If you turn only the pastern joint, the coffin joint angle won’t turn. They’re independent measurements, but because of the dependent apparatus, they tend to move together.”
Craig continues by explaining that making statements about joint alignment can be touchy as it has a lot to do with how the horse has been positioned in its stand.
“Vets are taught to make the horse stand with its cannon bone vertical,” he explains. “Sometimes it’s hard to make a horse do that.”
Craig suggests that horses should be stood on blocks of equal height.
“You have to control this or you’re not going to learn anything meaningful about the bone alignment,” he says.
Another factor is how the horse is shod. If wedge pads are used, for example, the angles of the bones change dramatically. These factors, Craig argues, need to be evaluated prior to assessing bone alignment.
Craig conducted a study to measure bone alignment. Part of his process was determining the average palmar angle, which is the angle between the bottom of P3 and level. In the study, the average angle was around 4 degrees. In a study where farriers were more inclined to use wedge pads, the average was 6.5 degrees.
“Here’s what we found,” Craig begins. “The average coffin joint angle was not zero, which would be the lined-up position, but was 8.6 degrees. The average pastern joint angle also wasn’t zero, but about 6 degrees.
“The point I’m making in this study was the average was not a straight line. It’s a little bit upright and turns upright as you go up the bone column. To the eye, it’s not far off straight, hence the ‘rule of thumb’ is correct, but to be a little more precise, the bones tend to want to be a little bit to the upright side of straight.”
FIGURE 5. Should the coffin joint be parallel to ground? How about the fetlock joint?
Belief No. 5: Coffin Joint “Parallel to the Ground” Is Proper Medial-Lateral Balance. “It’s generally accepted that the coffin joint should be parallel to the ground for good medial-lateral balance,” Craig says (see Figure 5). “I’d pretty much concur that this is the proper thing to do. But I want to put out a little food for thought.”
The food for thought, Craig says, comes when you consider the P1 bone itself. While many assume P1 is symmetrical, Craig’s research indicates it virtually always has a tilt. In 31 horses, Craig found that in every case, the P1 bone was lower on the lateral side, by an average of 2 1/2 to 3 degrees.
“What’s the point?” Craig asks. “Why are we so convinced that the coffin joint should be parallel to the ground? Perhaps it’s more important that the fetlock joint be parallel to the ground. Due to the asymmetry of the P1 bone we can’t have both. Or you can split the difference — and neither joint axis would be parallel. While I still think you should aim to make the ground parallel with the coffin joint, we’ve begun to feel that ‘medial too high’ is somehow worse than ‘medial too low.’ ”
Belief No. 6: The Tip Of The Frog Is A Reliable Marker Relative To P3. “You’re always looking for external cues on the hoof and relating them to internal geometry,” Craig says. “The tip of the frog is a pretty good marker, but how good is it? A lot of theories as well as practical methods revolve around locating the tip of the frog, so this is worth a closer look.”
FIGURE 6. Live frog locations are shown in red. External frog locations are in blue.
In conjunction with farrier-researcher Michael Savoldi, of Shandon, Calif., who is currently in the midst of a large dissection study of hundreds of legs, holes were drilled at the tip of the visible frog on cadaver legs. Next, the foot was dissected and a second hole drilled at the tip of the live frog. When the P3 bone was removed, these two holes in the palmar surface of the P3 bone were examined. The result? The external frog was found to be farther out toward the toe than the internal frog. But Craig learned even more.
“The frog moves,” he explains. “In our study of 31 feet, it moved around 1/2 inch, relative to the bone. So, while it is still a good reference point, it certainly is far from perfect.” (Figure 6.).
What’s The Point?
The point of Craig’s studies isn’t to trivialize the work done by countless farriers and veterinarians over hundreds of years. But he believes by combining modern measurement devices with computer software and digital cameras, we can gain new insights into hoof conformation that were not easily studied before.
“I don’t mean to be attacking the books,” he says. “I would say they’re reasonable and they set good ‘first-level’ guidelines. But now we can make more careful measurements and I’d like to get people to think. The point is to use this information as a tool and not to get too carried away with any one formula.”
He is also realistic about the scope of his study:
“I don’t claim this study ‘proves’ anything other than what has been presented for this set of horses,” he says. “But at the same time, I strongly suspect that the same trends would be seen in any sufficiently large population of horses.”