Your show jumper lands off a five-foot vertical and is cutting right, thinking a few fences ahead. The footing is just a little bit sticky from the rain storm last night, but not to worry--you have a nice set of those European show jumping studs securely set in the shoes. As the leading foot grabs the ground, the shoe seats firmly in the muck, but the foot starts to rotate clockwise atop the shoe as some of the nails pull through the wall. As the foot comes off the ground, it goes one way and the shoe (and unfortunately a large chunk of the hoof wall firmly clinched to it) heads the other. Your horse's head bobs a few strides, you pull up with concern and look down to see a shoeless foot with a quarter of the wall missing.
This is not an uncommon way to lose part of the hoof wall, and it definitely is one situation where hoof repair materials will come in handy.
There are many ways by which a hoof might get to the state where some sort of hoof repair material is necessary to reconstruct the foot. As always, the best approach is to do everything in your power to lessen the chance of your horse ever having to benefit from some of this unique material. Regular shoeing (or trimming for those which go barefoot), keeping the foot balanced, promptly fixing loose shoes, early detection of "white line" problems or cracks, and correct use of caulks/studs are some of the ways to prevent losing a chunk of hoof wall.
The purpose of this article is not to discuss in depth the problems in which hoof repair material aids in the treatment, but to look at the materials and techniques with which I have worked. I do think it is useful to review the anatomy of the foot to get us started.
The wall of the hoof grows down from the epithelium covering the coronary dermis. This consists of horn tubules that are embedded in the intertubular horn, which in turn is attached to the coffin bone and hoof cartilage.
The three basic layers of the wall structure consist of the stratum externum, stratum medium, and stratum internum. The stratum externum consists of horn tissue produced by the perioplic dermis, which lies directly below (proximal to) the coronary dermis. The stratum externum near the coronary band is only a few millimeters thick and is somewhat pliable, but it becomes harder and drier over the lower (distal) hoof wall. The purpose of this wax-like coating is to prevent dehydration of the hoof structure; it is important not to remove too much of this protective layer, which might be caused through use of caustic chemicals such as turpentine to remove hoof polish or through excessive removal of the material by rasping during the shoeing/trimming process.
The bulk of the hoof wall is made up of the pigmented horn tubules of the stratum medium. The non-pigmented stratum internum consists of the approximately 600 laminae that interdigitate (connect tightly) with the sensitive laminae of the laminar dermis.
The dermis of the sole is attached firmly to the undersurface of the coffin bone and produces an admixture of horn tubules and intertubular horn. The junction between the sole and the wall is the so called "white line" (zona alba). The white line includes some of the non-pigmented stratum medium of the wall, the distal ends of the horny laminae, and, between these, pigmented horn produced over the terminal papillae of the laminar dermis.
There are several nutritional factors that are important to normal hoof growth. The protein structure of the hoof is loaded with sulfur-containing amino acids, so elemental sulfur is important to normal hoof growth. The sulfur-containing amino acids methionine and cystine are two of the more important ones. In addition to sulfur, protein, calcium, and the vitamin biotin are very important to normal hoof growth.
There are many supplements on the market specifically formulated to promote normal or "better" hoof growth--many containing methionine, biotin, sources of sulfur, and other trace minerals important to the enzyme process that makes hoof material. I must note here that it has never been demonstrated that the supplementation with any of the necessary building blocks for healthy hoof will increase hoof growth or its quality if a horse is getting the required amounts of all of the essential components from his diet. If the nutritional requirements of your horse are inadequate with respect to these essential components, that's another situation.
There is strong evidence that a nutritionally deficient diet with respect to protein and other essential components will have a negative effect on hoof growth. It has been shown in weaning-age ponies being fed a nutritionally deficient diet that there can be as high as an 80% reduction in hoof growth. Supplementing a horse's deficient diet with components targeting hoof growth is safe and could ensure that nutritional requirements are being met.
The mature horse's foot grows an average of 0.25-0.35 inches per month. This growth rate varies seasonally and is the slowest when the weather is the hottest or the coldest.
Hoof repair material has proved to be an innovative and progressive technology. Repair materials are marketed by several companies, such as Equilox (Equilox International Inc.), Grand Circuit material (Grand Circuit Products Inc.), HOOF-it (HIMG Inc./HOOF-it Technology), and one of the first products available, Technovit (JarVet Inc.). All of these products have their own advantages and disadvantages, and I won't promote one over the other. However, depending on the extent of injury, in most instances of hoof wall repair, the veterinarian/farrier will have to delay application of such materials until the underlying tissue is prepared to accept such materials. This usually means that the underlying tissue (exposed laminae) have to cornify (harden somewhat) or the application can cause tissue damage.
The chemical composition of these products shares the basic composition of the polymethylmethacrylates (the same as many of the super-glues) with similar chemical base compounds used as industrial contact cements. A common factor is that these products come as a "part-A" and a "part-B" that are either two thick liquids or a liquid and a powder. The products that start out as two pastes (such as Grand Circuit Products) come with a unique and special applicator that mixes the two components in the exact concentrations with the push of a plunger. The products mix and come out one applicator tip "ready to go." When the two parts are mixed together, there is an "exothermic" (heat-generating) chemical reaction, and the resultant paste hardens into a solid product that ranges in hardness from similar to the natural hardness of the hoof to considerably harder.
Specific chemical compositions (as well as how the "part-A" and "part-B" are mixed together) alter the amount of heat produced during the curing process and the final hardness of the material. In addition, the time from when the two parts are mixed together and when the final product "sets-up" varies with not only the specific chemical composition, but the ambient temperature and humidity. In addition, some of the available repair materials are of an acrylic base.
The benefits of some of the more chemically "tweaked" repair materials is more control over the curing time--too fast and it hardens before you get it all out of the mixing cup and on to the foot; too slow and it could be impossible to achieve the desired results with a fidgety horse. Another factor is the hardness--it is desirable to obtain something similar to the hardness of the natural hoof. If the repair material is too hard, there might be difficulty in driving nails through it and instead of joining the existing hoof and functioning in shock absorption, it might concentrate the forces on the foot at the junction between the material and the natural hoof.
When the ambient temperature is too cold, the use of special heat guns (high-output hairdryers) can aid in regulating the set-time, but only the inherent chemical composition can alter the heat generated and the final set hardness. The HOOF-it company has been promoting their HOOF-it II material, which has been adjusted chemically to provide a "set-up" hardness similar to natural hoof with lower curing temperatures and an "easy to work within" set-up time. One potential benefit of lowering the curing temperature comes when working on a hoof where a large portion of the insensitive tissue is absent and the repair material is very close to sensitive tissue (blood vessels and nerves). If a great amount of heat is produced very close to the sensitive tissue, it is possible to burn that tissue.
As A General Filler
If the wall has broken off for whatever reason, it is beneficial to be able to replace the missing horn tissue with the repair material. It might be necessary to fill in missing areas so that nails can be driven into something. In addition to having a place to nail safely, the repair material can fill in the gaps between the shoe and the missing hoof wall, which allows for the even distribution of forces placed on the foot. Maintaining the even distribution of force can allow the horse to continue training or competing and decrease the potential for further breakage of the foot.
Again, the materials that are simulating the natural hardness of the foot show great promise because they absorb forces placed on the foot and distribute them in a more natural manner.
There is a growing number of commercially made "glue-on" shoes for corrective and routine shoeing. One very common use of glue-on shoes that comes to mind is on the foal with contracted tendons--the application of a shoe with a toe extension can act as a lever and keep the foot parallel to the ground. That places tension on the tendons and ligaments, giving them impetus to elongate and stretch. With the neonatal foot too sensitive to allow for nailing, hoof repair material makes a perfect mechanism onto which these shoes can be attached. In addition, some of the newer materials that cure at a lower temperature reduce the risk of thermal damage to the neonatal foot. Glue-on shoes also are useful in cases where there is very little foot remaining into which nails can be driven. This allows for the protection of the foot with the shoe minus the risk of doing more harm with the nails.
There are a variety of metal and high-tech plastic glue-on shoes. Another use is the emergency placement of a shoe in the middle of a long trail ride situation where there is no farrier. If supplies are at hand (plan ahead) an "emergency shoe" can be glued on by someone who has had a little instruction, giving the horse foot protection until a farrier can replace the shoe.
Medicated Repair Material
In addition to hoof repair material, the polymethylmethacrylates are used in orthopedic surgery for the repair of bones; these types of cements are used in "gluing" in the ball and socket of an artificial hip joint. There has been extensive research looking into the antibiotics impregnated into this material, and specifically if they can be added to the hoof repair material and provide some degree of protection when in the body. There are several factors involved, making this slightly more complicated than it really is.
First, the antibiotic must survive the heat of the curing process; some antimicrobials are destroyed by heat and thus inactivated. Second, the addition of the antimicrobial (liquids or powders) must not interfere with the curing of the repair material, thus reducing its strength. Third, the antimicrobial incorporated into the hardened material must be able to "leak" out and be an effective antimicrobial.
There are many studies looking at the aminoglycoside antibiotics (such as gentamicin and amikacin) for this use. They fare better in the incorporation into the cement well and provide an antibacterial effect against susceptible bacteria. The antibiotic "metronidazole" has been used in hoof repair material and has been marketed as a medicated hoof repair. Our laboratory did several experiments determining that metronidazole does survive the incorporation into the hoof repair materials and comes out as an effective antibacterial.
The question remains regarding the role of bacterial infection in true hoof wall disease. Most of my research over the past five years (supported by Steve Bloom, Grand Circuit Products, and Bob Peacock of Farrier Science Clinic) indicates that the so called "white line disease" is most likely caused by fungal organisms (see shelly feet article in The Horse of January 2000). This fungal organism has been called onychomycosis (meaning fungal infection of the hoof). The antimicrobial metronidazole is an antibiotic and does not have the ability to kill fungi. I currently am looking at the ability of "miconazole," a true antifungal drug, to survive the incorporation into hoof repair materials and be effective (supported by Farrier Science Clinic and HOOF-it Inc.). The preliminary results are quite promising, but a good deal more research is needed.
So, when your farrier and/or your veterinarian discuss how to solve the hoof problems you are encountering with your horse--be it from injury or disease--you will be better able to understand your options, and get your horse back on his feet in the shortest amount of time.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.