Introduction
Quarter cracks are a common cause of foot lameness and/or decreased athletic performance in race and sport horses. They typically originate at the coronary band and continue distally. They are full thickness, extending into the dermis of the hoof, which leads to instability, inflammation and/or infection. Quarter cracks are painful due to infection and/or the result of instability caused by movement of the hoof wall posterior to the crack. The vertical movement of the heel bulb on the affected side further complicates this instability. Causes of quarter cracks may include trauma to the coronary band, preexisting damage to the corium from infection, abnormal hoof conformation; especially the long toe underrun heel, focal foot imbalances, short shoes or an abnormal landing pattern when the foot strikes the ground.

Often the problem facing equine practitioners is that many of the horses that develop quarter cracks must continue to perform. For this to occur, it is essential for the repair to provide strength and stability to the hoof wall defect, allowing the horse to perform while at the same time promoting healing so that the crack will grow out. Various techniques for repairing hoof cracks have been described. They include corrective shoeing, stabilizing the crack by placing implants across the defect and covering the crack with an acrylic material ¹. This report describes a simple, consistent method of repairing a quarter crack that, when combined with appropriate shoeing, gives superior stability.

Correct the Cause
For a successful repair, it is necessary to try to determine the underlying cause of the quarter crack and to correct it where possible. Horses with long toe underrun heel foot conformation appear to be prone to quarter cracks. This type of heel may be weak due to insufficient hoof wall growth and there may be insufficient ground surface to adequately support the palmar/plantar part of the foot. In addition, there may be increased pressure in the quarter and heel during the stance phase as a result of delayed breakover caused by the long toe. On the other hand, upright hoof conformation with high heels promotes heel first landing, which increases pressure through the heels and quarters often leading to a hoof wall defect.

Short shoes leave the heels unsupported and put the weight-bearing surface in front of the vertical axis of the limb. In such instances, a vertical line drawn from the origin of the quarter crack will invariably coincide with the end of the shoe (Figure 1). The use of an aluminum plate with a toe grab as used in racehorses raises the toe, creating a broken-back hoof pastern axis. This would also appear increase the pressure in the posterior part of the foot.

Of utmost importance is the landing phase of the stride. Many horses will contact the ground asymmetrically, landing first on one side of the hoof and then impacting with the opposite side. This type of strike pattern may be related to conformation but can also be caused by inappropriate trimming. This type of stride may place excess forces on one side of the hoof wall, and may cause enough force to proximally displace the heel bulb. When coupled with abnormal foot conformation, this landing pattern may lead to a shearing force in the lamina. The landing pattern and the conformation of the foot may also perpetuate quarter cracks caused by trauma to the coronary band or a previous abscess.

Methods and Materials
When infection is present, it is characterized by marked lameness; pain on palpation and a swollen discolored coronary band above the defect. Occasionally, exudate can be expressed when digital pressure is applied to the coronet. If infection is present, the crack should be opened and bandaged with 2% iodine, or a similar disinfectant agent, for at least 48 hours before the repair.
Before beginning the quarter crack repair, the shoes should be removed and the feet trimmed appropriately. In horses with the long toe underrun heel conformation, attempts to move the breakover back is helpful. The hoof wall is always lowered on the affected side. Lowering the hoof wall of the quarter and heel beneath the quarter crack will decrease the amount of impact when the foot strikes the ground.

When starting the repair, the hoof wall should be thoroughly cleaned and dried. The quarter crack is opened its entire length using a Dremel ^a tool a with a tungsten carbide bit, being careful (especially near the coronary band) not to create any unnecessary hemorrhage. All necrotic and loose horn should be debrided in the area of the crack. The hoof surface on either side of the defect should be sanded using the Dremel tool with a coarse grit drum sander. Two sets of paired 3/64-inch holes one quarter inch apart are then drilled on either side of the crack beginning at least ? inch from the margin of the crack and ending within the depths of the opened area. Stainless steel (21-gauge) wire is bent in a "hair pin" shape 2.5 inches long and a small steel tab is placed on each wire unit (Figure 2). One wire unit is passed through the holes from palmar to dorsal direction and another wire unit is passed through the opposing holes in a dorsal to palmar direction into the depth of the crack. The ends of all the wires are pulled tight and bent outwards. The tab placed on the wire unit will now lie against the outer hoof wall. This prevents the wires from cutting into the hoof wall. Additional sets of these wire units can be used according to the length of the defect or until the desired stabilization is achieved.

If the crack leads into the dermis, it cannot be completely eliminated. In all such cases, a drain is placed. . To place a drain, a small amount of medicated putty ^b is rolled into a tubular shape and placed over the length of the remaining defect within the debrided crack. One-eighth inch hollow rubber flexible tubing is pressed into the putty and will exit at the coronary band to form the drain (Figure 3). The ends of all the opposing wires are now joined together and twisted until resistance is felt. The excess wire in front of the twist is cut off within the defect (Figure 4). There should be no movement in the hoof wall posterior to the crack when digital pressure is applied.

Next, elastic adhesive tape is placed around the coronary band to prevent irritation from contact with the composite. Three sections of a strong structural fabric that resembles fiberglass, called "spectra", ^c is cut 2x3 inches. The previously sanded hoof is rinsed with free-flowing denatured alcohol. The polymethylmethacrylate composite (PMMA) ^d. is mixed thoroughly and a layer applied to the prepared area of the foot, being sure to fill the quarter crack ². The spectra fabric is impregnated with the PMMA composite and placed over the prepared area followed by a layer of composite. This procedure is repeated until three sections of fabric have been placed over the defect. Finally, the hoof wall is wrapped in plastic wrap and an elastic adhesive bandage is pulled tightly around hoof wall, compressing the fabric. Upon completion of the cure cycle, which takes two to three minutes, depending on the ambient temperature, the rubber drain is removed and the repair is sanded to remove excess composite.

Choice of Shoes
Any horse with a full thickness quarter crack that warrants repair should be placed in a bar shoe if possible. Various configurations of bar shoes are a straight-bar, egg-bar, heart-bar or Z-bar shoe. All of these shoes effectively increase the bearing surface of the foot, provide palmar/plantar support, and decrease the independent vertical movement at the bulbs of the heels. No nails are placed into the repaired area of the foot.

Recently, this author has glued the shoes on most horses with quarter cracks. Gluing the shoe offers several advantages. First, it allows the foot to be trimmed and shod to a more suitable foot conformation. Second, gluing on the shoe allows for more expansion to be provided under the quarter and heel thus increasing support. Finally, when shoes are glued on, there is no concern with nail placement in the affected quarter ³.

Discussion
Combining an implant and a composite in repairing a quarter crack increases the strength and durability of the repair. Types of implants described previously include fiberglass and screws, screws and wires, various suture patterns and clamps that apply tension across the crack ^1,4,5. These methods have been somewhat cumbersome; the screws present a risk as they often impinge on the dermis and uniform tension is rarely achieved across or within the crack.

The advantage of the method described in this report is that the wires are placed from opposing sides of the defect and joined together within the defect creating uniform tension within the crack. The wires are also incorporated into the composite as part of the repair. Another advantage of this technique is that, since the implant is contained within the hoof wall, there is very little metal on the surface of the hoof wall to interfere with the adhesion of the composite.

Many failures in quarter crack repair appear to result form the rigidity of the acrylic material used and its resultant inability to maintain a consistent bond to the hoof wall long enough to allow healing. The polymethylmethacrylate used in recent years has excellent adhesive characteristics with the horn wall, while having a flexibility that approximates that of the hoof itself, thus more closely approximating normal function ⁶. The addition of spectra to the composite, instead of fiberglass, increases the strength of the overall repair due to its high tensile strength. Placement of a drain effectively decreases the risk of infection under the repair.
The importance of determining the underlying cause, proper trimming of the feet and applying the correct shoe cannot be over emphasized when repairing a quarter crack. The palmar/plantar support provided by the appropriate shoe is as important as the repair itself. This technique described here for quarter crack repair has produced consistent results, and horses are able to return to almost immediate soundness. In addition, horses treated in this manner have been able to return to successful competition following the repair, and no infection, reoccurrence or bond failures have been noted.

References

1. Moyer, W.: Repairing hoof cracks in the horse: A review and a report of a new technique. Comp. Cont. Ed. 1983 ; 5:495-497
2. Moyer W, Sigafoos R. Preliminary experience and uses of composite hoof wall repair, in Proceedings. 37th Annu Conv Am Assoc Equine Practnr 1991; 681-686.
3. O'Grady S.E., Watson E., How to glue on therapeutic shoes. Proc. 45th Annu. Conv. Am. Assoc. Equine Practnr. 1999; 115-119.
4. Butler, J.: The repair of hoof cracks using fiberglass and screws. Proc. 22nd Annu. Meet. Am. Assoc. Eq. Practnr 1976; .235-237.
5. Blackford JT, Blackford LAW, Latimer FG: Adjustable tension band stabilization of hoof cracks in horses. Proc. 37th Annu Conv Am Assoc Equine Practnr 1991; 497-512.
6. Moyer W, Sigafoos R. Equine hoof wall repair. Trenton: Veterinary Learning Systems, 1993, 6.

Footnotes:
a. Dremel® Dremel Tool Co. Emerson Electric, 4915 21st., Racine, WI 53401
b. Keratex® putty - Advance Equine P.O. Box 54 Versailles, KY 40383
c. Spectra® Equilox Int'l 110 NE 2nd Street, Pine Island, MN 55963
d. Equilox® - Equilox Int'l 110 NE 2nd Street, Pine Island, MN 55963