Updated: Apr 22, 2020
Indepth Equine Podiatry Symposium Notes Written and presented January 2010 by R.F. (Ric) Redden, DVM
Many speed and sport horses are plagued with quarter cracks, crushed heels and soft tissue injuries that are all too often chalked up as bad luck and seldom receive the respect they deserve. But what many people do not realize is that quarter cracks and similar foot injuries are the end result of a series of cascading events that occur inside the foot long before the actual injury occurs. By the time these external changes happen, internal damage has already occurred. Most farriers treat and repair these commonly occurring problems to the best of their ability, however any treatment performed without regard to what is happening within the foot often does not address the underlying cause. Therefore these cases often get caught up in a vicious cycle that plagues them throughout their career and sometimes the rest of their lives.
The first sign of digital cushion and heel tubule demise is a decreasing palmar angle (PA). While the PA is easily influenced with a rasp, other factors can also cause a decrease in PA. It is often thought that a lower PA is caused by farriers removing too much heel, but this is not so in most cases as the heel can also come off from within the foot, not just from the outside. As crushing begins to collapse the digital cushion, the heel tubules are subjected to excessive load. They begin to fray, then fold inward and forward. Once the cycle has started it can proceed very quickly, especially when horses are training at top speeds. Therefore using the PA as an overall health monitor of the foot helps trainers, vets and farriers have a quantitative means of tracking equilibrium and balance and offers a means of setting a benchmark that reflects optimum health of the foot. By identifying the PA and its relationship to other key parameters such as sole depth and medial/lateral balance, we can develop shoeing protocols that can enhance the health of the foot and maintain the mass, toughness and balance required for intense training programs. Developing our knowledge of the internal mechanics of the foot and how they function allow us not only to manage problems that already exist, but also develop foot care programs that can prevent or minimize their occurrence in the first place.
Measuring the Palmar Angle
The PA is measured by drawing a line along the palmar rim and connecting it with a line along the ground surface. This angle can vary greatly in healthy feet, depending on age, breed, use, etc. Therefore there is no standard written in stone that can be referred to as a normal PA. A zero PA occurs when the palmar rim is parallel to the ground surface. The negative PA occurs when the wings are lower than the apex. This is also referred to as caudal rotation.
When shod with a flat shoe, the PA with the ground surface is the same as it would be if the horse were barefoot. However, when a shoe and/or pad alters the angle of the PA with the ground surface the PA can be referred to as the shod PA. Wedge shoes, wedge pads, rocker shoes and heel elevation shoes create a different PA than what we find in the bare foot. Using the barefoot PA and the very flexible shod PA we can develop a large range of shoes that meet the mechanical requirements of many pathological syndromes.
Characteristics of the Negative PA
The crushed heel front foot is normally found opposite a club foot. The club foot does not have to be a high grade, as a horse with a low grade 2 club can have a very noticeable low heel on the opposite foot with a thin digital cushion and zero to negative PA. The rear foot that is directly behind the club foot will also have a shallower digital cushion and often a moderate to excessively low PA that can be as much as 10-12 °s negative. The hind foot that is diagonal to the club will be the strongest foot of the four, and is invariably the easiest to maintain with a healthy sole depth of 15+mm and a positive PA.
The foot has a very characteristic appearance when in the negative plane, though quick assessment is easier in hind feet than in front feet. The hind foot will have a slight bulge along the dorsal face and the growth ring pattern will be much wider at the toe than at the heel. When a horse with a healthy toe-heel growth ratio is standing with the hind cannon bone perpendicular to the ground, a line drawn along the coronary band slope will strike the front leg at the knee or below. In the negative PA foot, this line will strike the abdomen or flank.
Having reviewed literally thousands of images before and after shoeing, it is obvious to me that many of our thoughts and concepts of how the bone relates to what we see on the outside of the foot are often contrary to radiographic evidence. Radiographic information is vital to managing horses with crushed digital cushions and a negative PA. To develop an eye for what is happening inside the foot we must study the radiographic image and develop a working knowledge of how it correlates with external landmarks to offer key information concerning imbalance and the cascading series of events that occur as the heel goes into the crushed mode. Therefore farriers need to develop a base for reading and interpreting the relationship of the bone to the hoof and hoof to ground surface. To do this it is important to review farrier friendly radiographs (lateral, low beam with opaque wall marker) on as many horses as possible before and after shoeing and/or trimming. I find this is the only way we can continue developing an eye for the little details that describe the state of health of the foot and the only way we can fine tune our mechanical protocol.
Evidence from venograms of healthy shod feet suggests that most horses require a minimum sole depth of 15-18mm to adequately protect the sensitive structures of the foot. This measurement and its relationship to PA are very important and demand great respect from farriers and vets who are focused on pathological shoeing solutions. When in harmony, this relationship provides adequate sole depth and healthy equilibrium between DDF tension, apex compression and lamellar stress. When equilibrium has been lost, a series of cascading events takes place that can quickly put the foot in jeopardy.
Many horses and vets disagree with my recommended sole depth dimension as they see a lot of top horses with very thin soles. I would have felt the same way before obtaining information from venograms. Shod feet with a consistent growth pattern heel to toe and medial to lateral that maintain 7-10mm of new horn growth every 30-45 days will have a cupped sole of 15+mm in depth and a slightly positive PA. Venograms of these feet will consistently reveal a full 10mm of vascular supply distal to the palmar rim and remarkable solar papillae that will be in the same plane as the dorsal hoof wall. This benchmark is consistently confirmed with venograms performed with proper technique. Other horses that are highly competitive with 10mm or less sole do not have a healthy vascular supply. Venograms reveal severe compression of the circumflex vessels and solar papillae. Solar papillae, if present, are very short relative to the foot with 15+mm of sole, and the medial quarter will usually have stark loss of solar perfusion in the palmar zone. These horses may appear sound, but they are training with a major vascular deficit. These feet can bruise easily, often develop quarter cracks on the deficient quarter and heal slowly.
A close observation of a thin-soled foot will reveal a thin and often shelly wall. Medial quarter growth rings are often stacked up very tightly together, which means the quarter is dormant with no growth. The wall will have 2-4 sets of nail holes as the farrier has run out of places to nail. That is when composites are used to hold the shoe on. When the shoe is removed, the foot side will have deep creases in the heel area in the same place that folded heel tubules sit on the shoe. This high friction area triggers a vicious cycle, as the wall rolls under the foot. The horse actually walks on the outside of his wall instead of the end horn tubules, further crushing the tissue in the heel area. The frog starts to protrude lower than the heel tubules that are constantly being crushed. This foot, commonly found in many training stables, is headed for trouble. This is why I maintain that 15+mm of sole and a positive PA are required for a foot to stay healthy. This is the depth that the horse should have immediately post shoeing, not 4-6 weeks post shoeing. Heavier breeds such as warm blood crosses, etc. require several millimeters more sole depth to provide adequate protection to the sensitive structures. This may not be easy to achieve, but it should be our goal.
When in medial/lateral balance, the PA of the medial and lateral wing of PIII will be identical. However, when one wing lists out of balance this wing will have a different PA. Medial wing listing can be found in a large majority of horses. While it has been generally thought to be abnormal, I am not so certain this is correct, as it appears to be influenced by conformation and development. The majority of feet that have remarkable imbalance also have stark differences in the shape of the medial wing (observed on the low beam DP radiographic view) relative to the lateral wing. This could be due to remodeling along the lines of stress and/or a genetic linkage. The significance of this finding is relative to the incidence of medial quarter bruising, heel pain, slower than normal horn growth and subsequent quarter cracks that often plague speed and sport horses. The horse that has significant medial listing seldom has adequate foot mass and most often has a crushed digital cushion that produces a negative PA that can be quite severe. This is often the precursor for quarter cracks. It is common to find a negative 6-10° PA in front as well as in hind feet in training stables worldwide.
Treating the Low PA Foot
Feet are not born equal by any means and do not respond the same to a set standard of shoeing and trimming. Some are destined to be stronger than others within a certain breed and among different breeds. There are numerous ways to aid mass production and mass maintenance, all the while striving to improve balance. We should be more focused on mass than balance as it must come first in order to re-establish a healthier foot. Creating the illusion of balance without accelerating mass often results in a vicious cycle, as the foot becomes weaker rather than stronger. Growth centers must be stimulated in one fashion or another if we want the weak foot to become strong. Simply making it look more appealing to the eye is not conducive to a progressive foot program. Unfortunately, farriers are often expected to fix the weak foot by taking away everything that makes it look weak and sanding or polishing what is leftover. If we want stronger feet we must be focused on mass rather than balance.
Much like the club foot we do not have means to cure the low heel, crushed cushion foot, but we do have options that can greatly enhance the health of the foot and help prevent the ill effects that cause all sorts of