Indepth Equine Podiatry Symposium Notes
Identifying and Treating the Negative Palmar Angle
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 lameness issues in sport and speed horses. Genetics and conformation play a role in the demise of the heel, otherwise it would be quite easy to fix these cases and keep them fixed. Since this is not the case, we must enhance the mass and then the balance of feet that are programmed to fall apart under the stress of training.
The Rocker Motion Shoe
Depending on the degree and chronicity of the crushed damage, several options are available for the horse that is out of training. Horses with low scale crushing respond nicely to a 4 point trim, hot sear and a few days protection before going barefoot. For more severe cases, the rock 'n roll concept is a great way to quickly enhance mass. Using the rocker motion shoe to enhance blood flow to the deprived solar corium aids quick recovery of all but non-functional solar papillae, subsequently accelerating sole proliferation. However, the degree of mechanics, how quickly we need the mass and the state of training and/or other ability that is expected of the horse must always be taken into consideration. I prefer to pull a horse out of training for 2-3 shoeing periods in order to gain optimum foot mass over the shortest period of time. Simply eliminating daily concussion to the heavily compromised heel and buttresses starts the natural healing process.
The higher the mechanical score of the shoe (1 point for every two degrees it raises the PA), the greater the response. Therefore, if a horse can back out of fast work for a few weeks and use a higher mechanical shoe, a 12-15° PA rocker rail would be my choice. It is not unusual to double or triple sole depth in the first 4-6 weeks following application of the shoe. Many speed horses typically have 5-8mm of total sole depth with zero to negative PA and little or no signs of new growth month after month. The mechanics of this degree of rocker shoe can produce 12-15mm of sole in one shoeing period on this type of foot. Once the growth centers are stimulated the foot starts to quickly reconstruct itself.
When healthy foot mass has been re-established, most horses can go back into slow work wearing the rocker rail. However, the new growth is immature and needs conditioning time to become strong and durable. The foot may require mechanical aids to enhance perfusion for several months depending on the response, chronicity and age of the horse. This is the difficult part of foot reconstruction. As soon as sole is 15+mm, PA has improved and the horse is moving great, most trainers and owners want to go back to the flat shoe. But as soon as the DDF is solidly engaged by the action of the flat shoe, sole growth stops and within 30 days the 15mm of sole can be reduced by several millimeters from within. This sounds impossible, but that is the reality. I have witnessed this sudden loss of sole mass many times. Many think that the only way to lose sole is from wear or the rasp, but simply shutting down solar papillae can deteriorate existing sole very rapidly.
Weaning the mechanics off of the foot as it heals works well for many cases. I like to drop the self-adjusting PA 3-5° between shoeings until I can use the full rocker shoe, which normally increases the PA 3-5° from the barefoot static image. The next step down is a flat shoe with rocker. This is not a rocker toe shoe; the belly is a smooth radius from toe to heel with the peak of the belly slightly behind the widest point of the foot on the low heel and slightly in front of the widest point on the club or steeper foot. Many horses can wear this style of shoe for many months and compete at any level. This can make everyone happy as it looks more like a regular shoe. However, I have many cases that slowly return to a state of no growth and thin sole when everyone thinks the foot is back to normal and will hold forever. Once the digital cushion has been crushed and the heel tubules have folded under the heel there will always be a tendency for the foot to revert to this state given good reason. Therefore, we must seek to maintain this condition rather than cure it.
Note the PA on the bare foot is 3-4°. The shod PA is 10-12° using the full rocker shoe with no added mechanics. This would make the shoe a 4 score shoe, as we have raised the PA 8°.
Many styles of shoe can be used to enhance the rocker motion. The basic points of interest that I hold strongly to regardless of style or discipline are:Know how much PA you can use to meet the desires of the client, all the while gaining optimum recovery.Crushed heels must be pushed back to solid tubules that can be loaded on end. This may put the heels well above the ground surface of the frog. So be it. When the wall is rolled under and the shoe is resting on the outside of the wall, the angle of the foot (between the bars and heel tubules) is under tremendous load and is deprived of adequate circulation, putting it in shutdown mode.Frogs that extend well below the heels can be used as load zones as long as the frog is dry and tough. I put a positive frog pressure bar in shoes for rehab horses, but never speed horses unless the foot is well along in the recovery process.Rocker the toe forward of the apex of PIII. No sole is removed under the apex. I even leave the dirt in this area.The shoe rests on the anterior pillars and heel tubules. When a frog pressure bar is used, the frog carries the majority of the heel load. The tubules just touch when under maximum load.An air space should be seen between the toe and shoe when fitted properly as well as between the peak of the belly and the wall. Do not be concerned about this gap as it will fill quickly with wall and sole. You can fill it if you like; the shoe does not have to fit every contour of the foot to offer accelerated foot mass.Soften the heel branches of the shoe, creating a nice, smooth blend with the heel tubules. This is a very important step as excessive pressure to the buttresses and heel can cause a painful response if the heel is allowed to push heavily into the tender area.Always leave as much foot mass as possible when applying these shoes. The goal is to gain foot mass, not take away from it.Always nail in the center of the belly, not the toe as you would a flat shoe. Center belly nailing allows the shoe to move at the same speed as the horse. Flat shoes often stay on the ground microseconds after the horse has moved. Note excessive nail hole wear with flat shoes.Horses that can be turned out for free exercise while training have far better quality feet and horn growth rate than horses that are stabled for 23 hours a day. The foot needs constant demand for toughness if it is going to become tough.
This horse was presented with chronic heel pain. Radiographs revealed no demonstrable lesions other than unhealthy soft tissue parameters. Note the negative 6° PA, thin sole (13mm under the apex and 7mm under the wing), 25° TSA and severely broken back hoof axis. Cumulative ill effects of the extremely low PA coupled with the long digital breakover have contributed to the crushed digital cushion. This would be the low foot opposite the club. Note immediate improvement in digital alignment, which could be further improved with more mechanics in the shoe. Note the positive frog pressure bar.
Before and after photographs of the same foot. This thoroughbred race filly was taken out of full training and put in slow training while wearing this rehabilitation shoe. Normally this shoe requires three to six months to reestablish adequate digital cushion and buttress strength.
Moisture is a killer for unhealthy feet. Keep the foot dry. Mud buckets around race stables should be a thing of the past. A wet, soggy foot is one headed for maximum crush, and it doesn't take long.
I hot sear a lot of tender, shallow feet, especially in areas where moisture is a problem. Hot searing frogs works great to quickly toughen them.Keratex Hoof Hardner is a good product for daily use when moisture is a problem.Bedding on shavings instead of straw helps dry feet up.Venice turpentine on soles keeps them tough.Strong iodine works ok, but is not my preference as it dries out the sole too much and leaves its mark on all radiographs, which can result in a non-diagnostic image when superimposed over areas of interest.
There is a large range of options available, and not all can be covered here. Just remember that the following options are also available, and new concepts are being developed all the time.
Buttress wedgeShoes applied with nails along the outside of the foot, secured with Vettec Equi-Thane Super-Fast™Composite attachmentMechanical boots
The adult foot is the product of how it develops and matures from birth. Development is influenced by the environment, exercise, moisture, conformation factors, farrier care, management protocol, illness and injury. Therefore it is not only the farrier's responsibility to assure that all feet under their care reach and maintain optimum health. One of the most important aspects of a manager's role is to develop an eye for the unique characteristics of feet and the little things that make them so different. They should learn the basic silhouette of radiographs, especially soft tissue parameters and how measurements can influence decision making when it comes to enhancing or preserving the health and strength of the foot. With this base of knowledge, they can better communicate with their farrier and vet. Simply put, all three professionals must have a good understanding of what the foot needs to remain healthy and strong as well as the negatives that constantly eat at the mass and overall strength of the foot.
Managers should be alert to the following negatives influences:
Overzealous trimming of the frog, sole and horn capsule. Managers often insist on over trimming, thinking it helps avoid problems. We do not gain mass with a rasp.Constantly striving for a sale foot - small, smooth, well shaped and balanced - without regard to demands of developmental requirements, mass or pathology (such as club feet). This type of foot can be established when it is time for the sale, and there will be plenty of foot to work with.Excessive moisture.Lack of exercise and lack of demand for toughness. Manicured pastures with no rocks or abrasive surfaces put little demand on developing feet. There is a happy medium that can be designed into the program.Unruly or difficult to handle horses. These horses get the short end of the stick when it comes to foot care. The farrier's job is a very serious part of the overall foot program, not just something that should be done every 30 days. Training the horse to stand for the farrier is management's responsibility, not the farrier's, as it requires daily reinforcement training, which is vital for any and all optimum foot programs.
The most important goal for the farrier to achieve is adequate foot mass. Feet with optimum mass will have stronger, thicker walls and soles when mature. Unfortunately, most foot programs employ frequent trimming schedules that are usually designed to achieve ideal balance without regard to mass. This produces good balance but minimal mass and often results in thin walls and soles, weak bars and heel tubules. This can quickly put a foot into crushed heel mode, even before a horse goes into rigorous training.
Using the 4-point trim method to maintain optimum mass and balance with foals, weanlings and yearlings is a great way to help feet develop optimum strength right from birth. Remove only what the foot does not need and leave everything it does need. This is a great way to look at developing feet. The foal's foot really does not need to be trimmed but rather touched up, and a palm sander does a great job for the first few months of life. Young stock raised on manicured pastures that remain wet during early development often have excessive toe growth that does not exceed wear. Therefore breakover slowly moves forward, which can start heel crushing very early in life. I find that breakover is all that needs to be reduced with most foals. Nothing more needs to come off the foot. We don't gain mass with the rasp. If we only remove what a foot doesn't need and promote trimming that helps avoid excessive toe growth, horn breakage and subsequent cracks and abscesses we can enhance the health of all young feet.