How to Manage a Quarter Crack
Reprinted with permission from the American Association of Equine Practitioners.
|Fig. 1. Palmar view of a foot with a sheared heel. Note the disparity between lateral and medial heel length and the compression of the structures above the medial heel.|
|Fig. 2. An acute quarter crack. Note the bleeding.|
2. Foot Conformation
Sheared heels can be defined as a hoof-capsule distortionresulting from displacement of one heel bulbproximally relative to the adjacent heel bulb (Fig.1).5-8 This disparity between the lateral and medialheel bulb is generally 0.5 cm or more.9 Whenthe weight of the horse is not distributed uniformlyover the entire hoof during the landing phase of thestride, one focal area of the foot, usually a heel andaccompanying heel quarter, receives a disproportionateamount of the total load. This resultantforce leads to a remodeling of the affected heel bulb.The degree of distortion in the affected heel is dependenton the amount of the load sustained by theindividual foot. Sheared heels can occur in thehind as well as the forefeet. This continual disproportionateload and the increased compressive stresses placed on one side of the foot predispose thefoot to injury such a quarter crack (Fig. 2).5
3. Structural Changes to the Foot
The equine hoof capsule is a viscoelastic structurethat has the unique ability to deform when weight isaccepted uniformly.10 However, if the unequal loadis continually placed on one quarter/heel, over time,structural changes will become apparent. The increasedload on one side of the foot causes the hoofwall to assume a steeper angle (that is, the wallbecomes straighter). Along with the increasedhoof-wall angle, other changes, such as contractureof the heel subjected to the greater load, will soonfollow. This decreases the ground surface of thefoot, resulting in a lack of expansion on that side ofthe foot and making the solar surface in the palmar/plantar section of the foot asymmetrical. Overtime, the hoof wall begins to roll under on the affectedside, which further decreases ground surfaceunder that area of the foot. The side of the foot thatfirst impacts the ground develops a flare because ofbending of the hoof tubules.
Over time, the stresses placed on the overloadedside of the foot exceed the ability of the hoof wall todeform.10 The submural tissue on the affected side will be subjected to excessive compressive forcesthat result in hemorrhage along with stretching ortearing of the lamellae. It is thought that the exudationof fluid in the submural tissue increases pressureand will eventually disrupt the coronarycorium, consequently leading to the formation of aquarter crack. Furthermore, a recent study of agroup of horses with quarter cracks showed the freemargin of the ungual cartilage above the coronetresulting from the displaced quarter/heel to be lessthan 15 mm.11 This lack of free margin seems toprevent lateral expansion of the ungual cartilage,leading to increased pressure in the sheared heeland trauma to the adjacent coronet.
The presence of a sheared heal indicates a disproportionateweight distribution over a section ofthe hoof that anatomically cannot resist the additionalstresses without distortion or displacement.Horses with conformation that causes the limbs toland and load asymmetrically across the hoof will besubject to this type of deformation. The growthrate around the circumference of the hoof is usuallyapproximately uniform, but regional disturbances ingrowth rate can occur to either increase or decreasegrowth. The position of the coronary band is relatedto the balance between hoof wall growth at thecoronary band and the rate of migration of the hoofwall distally. Furthermore, the rate of migration ofthe hoof wall is a balance between an active processoccurring in the lamellae to cause them to movedistally and the force on the wall from the groundreaction force. Clinical evidence suggests that hoofwall growth is at least in part, if not predominantly,inversely determined by the force of weight bearingat the ground surface of the wall. If the rate of hoofwall growth exceeds the rate of migration distally,the coronary band displaces proximally. This appearsto be the mechanism in horses with shearedheels/quarters. Due to the fact that the growthrings below the coronet are usually very close togetherwhere the hoof wall is displaced and that theslow hoof wall growth is most likely related to increasedweight bearing (force) by the wall; thiswould suggest that the wall is forced proximally.Whether or not this is a real phenomenon as suggestedby clinical experience has not been confirmedin a scientific manner.
|Fig. 3. DP radiograph of a foot with sheared heels. Markers placed at the coronary band of the heels show the different heel height while the distal phalanx remains parallel with the ground. Note that the distal phalanx is offset to the lateral side.|
It was assumed for years that inappropriate farrierpractices may lead to sheared heels when trimmingmethods such as leaving the heels long orexcessively lowering one side of the foot would resultin excessive forces/stresses being placed on a givensection of the foot. The term used to discribe thistype of hoof capsule distortion was a lateral medialimbalance. To substantiate this theory, the authorreviewed 50 dorsopalmar 0° (DP) radiographs onhorses that had a foot with one heel bulb displacedproximally 0.5 cm or greater. In all cases, it wasfound that the solar surface of the distal phalanx was approximately horizontal (parallel) with theground, indicating that the disparity in heel heightwas not originating from the hoof wall and solelocated beneath the distal phalanx in the heel (Fig.3). Also, the distal phalanx occupies the dorsal sectionof the hoof capsule anatomically, whereas themajority of the space in the palmar/plantar foot isoccupied by soft tissue (Fig. 4). The displacementof the heel occurs palmar/plantar to the body of thedistal phalanx in the section of the hoof comprised ofsoft tissue. This may account for the proliferationof soft tissue and the additional hoof-wall growthoccurring above the ground surface of the foot.
|Fig. 4. This illustration shows the ratio of bone to soft tissue in the foot. The hoof-capsule distortion noted in a sheared heel will involve the soft-tissue structures palmar to the distal phalanx. Note the dotted line. (Courtesy of Dr. Andrew Parks.)|
|Fig. 5. Foot shows focal proximal displacement of coronet at the origin of crack, which coincides with the illustration in figure 4. This is the point of maximum stress.|
To formulate a rational approach to management, itis necessary to discuss the etiology of sheared heels.Traumatic wounds to the coronary band in the palmar/plantar region of the foot often lead to a quartercrack. Treatment here involves appropriate woundcare and stabilization of the coronet to promote healing.Inappropriate farrier care or lack of appropriatefarriery over time may contribute to theformation of a quarter crack. Inappropriate farrierymay lead to hoof-capsule distortions where the forces/pressure on a given section of the foot becomeexcessive, leading to a hoof-wall defect. Conformationalfaults in the upper limb that change thehorse's flight phase of the stride lead to a hoofcapsuledistortion termed sheared heels. The presenceof a sheared heel combined with a spontaneousquarter crack seems to provide ample evidence thatthis type of hoof-capsule distortion plays a majorrole in the etiology. Sheared heels seem to resultfrom unequal loading of the foot as it impacts theground. In this instance, the altered flight patterncauses the horse to impact the ground with one sideof the foot before full weight-bearing on other side ofthe foot. This focal disproportionate weight-bearingdisplaces the heel bulb proximally, creating theunequal heel height. In the conformationally predisposedhorse, the horse will generally have a narrowchest, and the carpus will be rotated laterally.When viewed from the front, although the entirelimb faces outward or in some instances, medially,the axial alignment of the limb from the carpus tothe ground surface of the foot forms a straight line,indicating a rotational deviation of the limb. Withthe knee facing outward, it changes breakover suchthat it occurs in an outward or lateral direction, thuschanging the optimal straight flight path of the footduring the stride so that the foot is unable to landunder the horse evenly on both heels. As the horseapproaches the landing phase of the stride, thisflight pattern forces the foot to contact the ground onone side of the foot and then sustain excessive loadon the opposite side. Using a slow-motion videocamera, one can actually distinguish the pointwhere the foot impacts the ground on one side andthe point where the hoof loads the surface on theother side. Quarter cracks usually occur directlyabove the point of the greatest load and most forcewithin the hoof wall. This point can be readilyobserved, because there will be a small focal displacementin the coronet proximally above the quartercrack (Fig. 5). Furthermore, there seems to be acorrelation between an offset distal phalanx andsheared heels. Most commonly, the distal phalanxis offset laterally within the hoof capsule rather than directly under the proximal and middle phalanges,causing the medial side of the hoof capsule toassume more load.
The evaluation of sheared heels begins with visualassessment of the hoof and limb conformation withthe horse standing on a hard, level surface. Thegross changes noted in the foot are proportional tothe amount of continual load sustained, the extent ofstructural damage, and the duration of the condition.When sheared heels are present, the heelbulb on the affected side is displaced proximallywhen viewed from behind the horse. When viewedfrom the front, the hoof wall on the affected side isstraighter and in chronic cases, will begin to rollunder the foot. There is a marked flare of the hoofwall present on the side opposite the affected heel.When viewed from the side, the coronary band isdisplaced proximally above the damaged heel insteadof having a gradual uniform slope from the toeto the heel. The solar surface of the foot reflectschanges elsewhere in the hoof capsule. The footwill be less symmetrical; the sole in the quarter andheel area will appear wider on the side with the flareand narrower on the side with the sheared heel.It is important to view the horse in motion, again ona hard, level surface, from the front and rear. Thisshould be done at a walk and a trot. When viewedfrom behind, this should determine which section ofthe foot is contacting the ground initially and whichportion of the foot is receiving the impact. Thedirection of breakover should be noted when viewedfrom the front.
Farriery is directed to unloading the hoof wall anddecreasing the forces on the side of the foot with thequarter crack. This is accomplished by improvingthe conformation of the hoof, trimming methods,and applying the appropriate shoe. When a horsedevelops a full-thickness quarter crack, it is advisableto take the animal out of training to allowhealing, but this is not always an option with competitionhorses. There will also be constraintsplaced on the farriery because of the training andcompetition schedule of the horse. For example,the author likes to remove the shoes and stand thehorse on a hard surface for 24 h before trimming andshoeing the horse. This alone allows the affectedside of the foot to settle into a more acceptable conformation.Furthermore, when possible, the authorwill perform the farriery and wait for thecoronet to settle into a more acceptable position orslope before a repair is considered. If the repair isperformed immediately, the defect is fixed with thecoronet in a displaced position.
When infection is present, it is characterized bymarked lameness, pain on palpation, and a swollendiscolored coronary band above the defect. Occasionally,exudate can be expressed when digital pressure is applied to the coronet. Infection oftenoccurs shortly after a defect is patched with sometype of composite that generates heat. If infectionis present, the patch (if present) is removed, thecrack should be opened for drainage, and the foot isbandaged with a suitable disinfectant agent for atleast 48 h.
|Fig. 6. This illustration shows proportions of a well-trimmed foot. A foot with a sheared heel shows increased ground surface on the affected side after trimming (blue line).|
Farriery is initiated by removing the shoes andagain observing the horse walking on a hard surface,noting the strike pattern of the foot. The foot istrimmed appropriately using the guidelines of a parallelhoof-pastern axis, and the center of articulationand heels of the hoof capsule are trimmed to includethe base of the frog. To start, a line can be drawnacross the widest part of the foot with a felt-tip pen.The frog is trimmed to where it is pliable, and thequarters and heels of the hoof capsule from themiddle of the foot are rasped palmarly so that theheels of the hoof capsule and the trimmed frog are onthe same plane. An attempt is made to create asmuch ground surface under the affected heel as possible,which will result in that side being marginallylower than the other side of the foot. The toe andquarters are reduced appropriately so when the trimis completed, the surface area on either side of theline drawn on the widest part of the foot will beapproximately equal (Fig. 6). Lowering the heel onthe displaced side of the foot is logical, because it isthe taller heel and it increases the ground surface ofthe foot on that side. Bearing in mind that themechanism that accounts for this foot conformationis not completely understood, the trim is based onclinical impressions having successfully treated alarge number of cases. After the trim, the horse isagain walked on a hard surface, and some improvementin the landing pattern will be noted.
Any horse with a full-thickness quarter crackshould be placed in a bar shoe, if possible. Variousconfigurations of bar shoes, such as a straight-bar,heart-bar, or Z-bar shoe, can be used. All of theseshoes effectively increase the surface area of thefoot, provide palmar/plantar support, and decrease the independent vertical movement at the bulbs ofthe heels. No nails are placed palmar to the defectin the foot.
|Fig. 7. Straight-bar shoe with a leather pad.|
|Fig. 8. Straight-bar shoe with heel unloaded. Note the taper from the quarter to the heel.|
|Fig. 9. Hole being drilled into the ledge created in the debrided quarter crack.|
The author's choice is a wide web-steel straightbarshoeb fitted symmetrically to the trimmed foot,and a leather pad is attached to the shoe (Fig. 7).Before applying the shoe, the medial quarter andheel of the affected side of the foot is further loweredin a tapered fashion with a rasp. Impression materialcis placed in the palmar section of the foot fromthe apex of the frog palmarly, except under the medialheel. The shoe is then attached to the foot.Lowering the hoof wall at the quarter/heel will createa space between the shoe and the hoof wall onthe medial side (Fig. 8). This improves the footconformation, the landing pattern, unloads the medialheel, and allows the heel bulb to settle down andassume a more acceptable position.
8. Repairing the Defect
The use of a composite (with fiberglass) alone maynot provide sufficient stability for a quarter-crackrepair. Combining an implant with the compositein the repair increases the strength and durability ofthe repair. Types of implants described previously include fiberglass and screws, screws and wires, varioussuture patterns, and clamps that apply tensionacross the crack.12-15 These methods have beensomewhat cumbersome; the screws present a risk,because they often impinge on the dermis and uniformtension is rarely achieved across or within thedefect.
When a decision is made to repair the quartercrack, the hoof wall should be thoroughly cleanedand dried. The defect is explored through its entirelength using a Dremel toold with a tungsten carbidebit, being careful not to create any unnecessaryhemorrhage. All loose undermined horn should beremoved, the defect is widened, and the burr is usedto create a solid ledge on either side of the trough.The hoof surface on either side of the defect shouldbe sanded with a coarse sanding block. Two sets ofpaired 0.047-in holes spaced 0.25-in apart are thendrilled across from each other on either side of thecrack beginning at least 0.375-0.500 in from themargin of the crack and ending within the ledge ofthe trough (Fig. 9). Stainless steel wire formed in ahair-pin shape 2.5-in long with a small steel tab oneach wire unit is commercially available.e Onewire unit is passed through the holes from a palmarto dorsal direction, and another wire unit is passedthrough the opposing holes in a dorsal to palmardirection into the depth of the crack. The ends ofthe wires are pulled tight and bent outwards. Thetab placed on the wire unit will now lie against theouter hoof wall. This prevents the wires from cuttinginto the hoof wall. Additional sets of thesewire units can be used according to the length of thedefect or until the desired stabilization is achieved.
The internal length of the defect usually exceedsthe external length, and therefore, it cannot be completelyeliminated, thus allowing a portal of entryinto the submural tissue. This necessitates someform of drain or interface between the composite andthe floor of the defect. If the interior of the crack issoft and pliable or if hemorrhage has been encountered,a drain should be used. A drain or interface can be placed in the defect before it is covered withthe composite. A small amount of medicated puttyfis rolled into a tubular shape the length of thetrough and placed within the debrided defect to actas an interface. If a drain is desired, thin rubberflexible tubing is pressed into the putty. The tubingwill exit at the coronary band and below thecrack to form the drain. The ends of all the opposingwires are now joined together and twisted untilresistance is felt. The excess wire in front of thetwist is cut off within the defect (Fig. 10, A-C).There should be no movement in the hoof wall oneither side of the crack when digital pressure isapplied.
|Fig. 10. (A-C). A shows the wires joined together and being twisted. B shows the wires cut just above the twist. C shows the completed implant with a drain.|
The completed implant is now covered with a compositepatch. Elastic adhesive tape is placedaround the coronary band to prevent irritation fromcontact with the composite. The polymethylmethacrylate(PMMA)d composite is mixed thoroughlywith strands of fiberglass, the mixture isplaced on a section of plastic wrap, and it is appliedover the implant. The composite is molded into thedesired shape through the plastic wrap and thencovered with a cohesive bandage placed around thehoof wall to compress the composite. On completionof the cure cycle, which takes 2-3 min dependingon the ambient temperature, the rubber drain isremoved, and the repair is sanded to remove excesscomposite.
The importance of determining the underlying causeand implementing the appropriate farriery cannotbe overemphasized when managing a quarter crack.The strong association of sheared heels with limbconformation and the landing pattern of the horsewhen a quarter crack is present is hard to ignore.Assessing the limb conformation, improving the footconformation, and applying the appropriate shoeseem to be as important as the repair technique usedfor the defect. Inadequate attention to these factorsmay account for the many failures encounteredand the recurring nature of quarter cracks. Theuse of a composite (with fiberglass) alone may notprovide sufficient stability for a quarter-crack repairto heal. Combining an implant with the composite in repairing a defect increases the strength and stabilityof the repair. Types of implants describedpreviously include fiberglass and screws, screws andwires, various suture patterns, and clamps that applytension across the crack.12-15 These methodshave been somewhat cumbersome; the screwspresent a risk, because they often impinge on thedermis and uniform tension is rarely achievedacross or within the crack.
The advantage of the repair method described inthis paper is that the procedure is exceptionallystrong and increases stability while being relativelysimple and easy to learn. Additionally, the wiresare placed from opposing sides of the defect andjoined together within the defect, creating uniformtension within the crack. Another advantage ofthis technique is that, because the implant is containedwithin the hoof wall, there is very little metalon the surface of the hoof wall to interfere with theadhesion of the composite.
References and Footnotes