Sheared heels and the correlation to spontaneous quarter cracks
Reprinted with permission from Equine Veterinary Education (EVE). Original published in Equine Veterinary Education Vol 23 May 2011.
S. E. O'Grady* and H. H. Castelijns†
The strong association between sheared heels and aspontaneous quarter crack is hard to ignore. Althoughinappropriate farriery may play a role, limb conformationand the landing pattern of the horse appear to be thedominant factors causing this type of hoof capsuledeformation. The importance of determining theunderlying cause and implementing the appropriatefarriery cannot be over emphasised when managing aquarter crack associated with a sheared heel. The repair ofspontaneous quarter cracks will be of little value, and thedefect will have a tendency to recur, unless the cause isidentified and rectified.
Sheared heels as a clinical entity and a cause of lamenesswere first described in the veterinary literature 35 years ago(Moyer and Anderson 1975). A sheared heel is definedas a hoof capsule distortion resulting in a proximaldisplacement of one quarter/heel bulb relative to thecontralateral side of the hoof (Turner 1992). The disparitybetween the lateral and medial quarter/heel bulb isgenerally 0.5 cm or more and is measured from thecoronet to the ground or to the shoe. When the weight ofthe horse is not distributed uniformly over the entire hoofduring the landing and/or weightbearing phase of thestride, one section of the foot, usually a heel bulb andaccompanying quarter, receives a disproportionateamount of the total load. This repetitive disproportionateload causes the proximal displacement of the heel/quarterof the hoof capsule while the increased compressivestresses placed on the submural tissue in this areapredispose the foot to various injurious conditions includinga quarter crack (O'Grady 2002, 2005). While the diagnosisof a sheared heel is straightforward, the aetiology of the condition may be misleading and the farriery employed inthe treatment is often based on opinions. Sheared heelsappear to develop as an adaption-distortion of the hoofcapsule as a consequence of limb conformation thatresults in an abnormal strike and loading pattern of thefoot on the ground. Prevention or treatment of abnormallimb conformation is only possible in the foal; therefore inmature horses, therapy is directed toward managing thedistortion of the hoof capsule. Spontaneous quarter cracksare a common cause of decreased athletic performancein competition horses and frequently lead to foot lameness(O'Grady 2001a; Moyer 2003; Castelijns 2006) (Fig 1). A truequarter crack originates at the coronet, extends distallythrough the full thickness of the hoof wall into the dermis,leading to instability, inflammation and/or infection. Thesecracks can be painful due to infection or, more commonly,the 'pinching' of the underlying dermis as a result of themovement of the unstable hoof wall. This 'pinching' occursdue to the vertical movement of the heel bulb and theoutward movement of the entrapped ungual cartilage,axial to the origin of the quarter crack, during the loadingof the foot. The recurrent nature of quarter cracks involvingperformance horses presents a challenging and oftenfrustrating problem for equine veterinarians, farriers andhorse owners, as these horses often need to continue tocompete.
Many causes of quarter cracks have been described,such as trauma to the coronet, pre-existing damage to thedermis from infection, abnormal hoof conformation, shortshoes, inappropriate farrier practices or an abnormallanding pattern when the foot strikes the ground (O'Grady2001b). Yet the most consistent finding in all quarter crackcases is a foot conformation with a sheared heel on theside of the hoof with the defect and an abnormal strikepattern observed during the impact and loading phase ofthe stride. In fact, it is extremely rare to find a spontaneousquarter crack (as opposed to a crack due to outsidetrauma, such as a wire cut) that is not associated with thistype of hoof capsule distortion. Various materials and techniques exist for stabilising and repairing hoof cracks,but none will be successful in the case of spontaneousquarter cracks, unless the cause of the hoof wall defect isdetermined and addressed through basic farriery, as theseoriginate from the coronet, from the inside outwards(Moyer 1983; O'Grady 2001b; Moyer 2003; Castelijns 2006;McKinlay 2009). This paper will discuss the proposedaetiology leading to the hoof capsule distortion termedsheared heels and its correlation with quarter cracks, alongwith the farriery methods used in the authors' jointpractices to address sheared heels.
Structural changes to the foot
The equine hoof capsule is a viscoelastic structure that hasthe unique ability to deform when weight is accepteduniformly (Parks 2003). However, if an unequal load iscontinually placed on one quarter/heel, over time,structural changes will become apparent. The increasedload on one side of the foot causes the hoof wall toassume a steeper angle, i.e. the wall becomes straighter.This is a predictable response to increased load. Along withthe increased hoof wall angle, other changes such ascontracture of the heel subjected to the greater load willsoon follow. The narrow heel will decrease the groundsurface of the foot resulting in a lack of expansion on thatside of the foot, making the solar surface in the palmar/plantar section of the foot asymmetrical. Over time, thehoof wall begins to 'roll under' on the affected side, whichfurther decreases ground surface under that area of thefoot. The side of the foot that first impacts the grounddevelops an outward flare due to bending of the hooftubules (Fig 2).
Over time, the stresses placed on the overloaded sideof the foot exceed the ability of the hoof wall to deformand a distortion will occur (Parks 2003). This overload resultsin the coronet at the heel quarter and heel to be displacedproximally. Not only is the coronet displaced proximally butthose structures located axially from the coronet to themiddle phalanx are also displaced proximally andcompressed, and this section of the foot becomes narrow(Fig 3). The submural tissue on the affected side may besubjected to excessive compressive forces that lead tostretching or tearing of the lamellae resulting inhaemorrhage. It is thought that the exudation of fluid in thesubmural tissue increases pressure and will eventuallydisrupt the coronary corium contributing to the formationof a defect. Furthermore, a recent study of horses withquarter cracks, showed the free margin of the ungualcartilage above the coronet at the site of the crack to be<15 mm, as a result of the proximally displaced quarter/heel (Castelijns 2006). This lack of free margin appears tointerfere with the abaxial expansion of the ungualcartilage when the foot is loaded, leading to increasedpressure in the sheared heel and trauma to the adjacentcoronet.
The presence of a sheared heel indicates adisproportionate weight distribution over a section of thehoof that anatomically cannot resist the additional stresseswithout distortion or displacement. In this area, there isdorsal migration of the reflection of the wall at its junctionwith the bar and there are densely packed growth ringsbelow the coronet. On gross dissection, the coronarygroove, instead of being circular on a cross section,becomes disto-proximally elongated and narrow in thedisplaced quarter/heel. The narrower coronet produces athinner hoof wall in this area (Fig 4).
The growth rate around the circumference of the hoofis usually approximately uniform, but regional disturbancesin growth rate can occur that will either increase ordecrease growth. The position of the coronary band isrelated to the balance between hoof wall growth at thecoronary band and the rate of migration of the hoof wall distally. Furthermore, the rate of migration of the hoof wallis a balance between an active process occurring in thelamellae to cause them to move distally and the force onthe wall from the ground reaction force. Clinical evidencesuggests that hoof wall growth is at least in part, if notpredominantly, inversely determined by the force ofweightbearing at the ground surface of the wall (A.H.Parks, personal communication 2010). If the rate of hoofwall growth exceeds the rate of migration distally, thecoronary band displaces proximally. This appears to be themechanism in horses with sheared heels/quarters. Tightlyplaced growth rings below the coronet coupled with slowhoof growth would suggest that the wall is forcedproximally. Whether or not this is a real phenomenonsuggested by clinical experience has not been confirmedin a scientific manner.
In order to formulate a rational approach to management,it is necessary to discuss the aetiology of sheared heels. Thepresence of a sheared heel when a spontaneous quartercrack occurs provides ample evidence that this type ofhoof capsule distortion plays a role in the cause of thedefect. It was assumed for years that inappropriate farrierpractices may lead to this type of hoof capsule distortionwhen trimming methods, such as leaving the heels long orexcessively lowering one side of the foot, would result inexcessive forces/stresses being placed on a given sectionof the foot. The term used to describe this type of hoofcapsule distortion was a lateral medial imbalance.Although this may indeed contribute, in the authors'experience, it does not predominately influence this typeof foot conformation. To substantiate this theory, oneauthor (S.E.O.) reviewed 50 dorsopalmar 0° radiographs onhorses that had a foot with one heel bulb displacedproximally ≥0.5 cm. In all cases, it was clearly shown thatthe solar surface of the distal phalanx was approximatelyhorizontal (parallel) with the ground. There was also anappropriate amount of sole depth under both the lateraland medial side of the distal phalanx. This would indicatethat the disparity in heel height was not originating fromthe hoof wall and sole located distal to the distal phalanx(Fig 5). Anatomically, the distal phalanx occupies thedorsal two-thirds of the hoof capsule while the majority ofthe space in the palmar/plantar foot is occupied by softtissue (Fig 6). The displacement of the heel thus seems tooccur palmar/plantar to the body of the distal phalanx inthe section of the hoof comprised of soft tissue.
Conformational faults in the upper limb that changethe horse's flight phase of the stride appear to be the majorfactor leading to this type of hoof capsule distortion. Whensuch faults occur, the altered flight pattern causes thehorse to impact the ground with one side of the foot priorto full weightbearing on the contralateral side of the foot. In the conformationally predisposed horse, the horse willgenerally have a narrow chest and the carpus will berotated laterally. When viewed from the front, although theentire limb faces outward, or in some instances, inward,the axial alignment of the limb from above the carpus tothe ground surface of the foot forms a straight lineindicating a rotational deviation of the limb. For example,with a lateral rotational deformity, the knee faces outward,this moves breakover to an outward or lateral direction,thus altering the flight phase of the stride such that the footis unable to land under the horse evenly on both heels. As the limb approaches the landing phase of the stride, thisflight pattern forces the foot to contact the ground on oneside and then sustain excessive load on the contralateralside. There is a far greater incidence of a sheared heeloccurring on the medial side of the hoof but sheared heelson the lateral side are not uncommon. Using a slow motionvideo camera, one can actually distinguish the pointwhere the foot impacts the ground on one side and thepoint where the hoof loads the surface on the other.Spontaneous quarter cracks generally occur directlyabove the point of maximum load and the largest forcegenerated within the hoof wall. This point can be readilyobserved as there will be a focal proximal displacement inthe coronet located directly above the quarter crack(Fig 7). Furthermore, there appears to be a correlationbetween an offset distal phalanx and sheared heels. Mostcommonly the distal phalanx is offset laterally within thehoof capsule rather than being directly under the proximaland middle phalanges causing the medial side of the hoofcapsule to assume more load.
The evaluation of sheared heels begins with visualassessment of the hoof and limb conformation with thehorse standing on a hard level surface. The gross changesnoted in the foot are proportional to the amount ofcontinual load sustained, the extent of structural damageand the duration of the condition. When sheared heels arepresent, the heel bulb on the affected side is displacedproximally and the structures above the heel bulb will becompressed when viewed from behind the horse. Whenviewed from the front, the hoof wall on the affected side isstraighter and, in chronic cases, will begin to roll under thefoot. There is generally a marked flare of the hoof wallpresent on the side opposite the affected heel in the toequarter. When viewed from the affected side, the coronaryband is displaced proximally above the damaged heel and will assume a horizontal contour, or a focaldisplacement, instead of having a gradual uniform slopefrom the toe to the heel. The solar surface of the footreflects changes elsewhere in the hoof capsule: the footwill be less symmetrical; the sole in the quarter and heelarea will appear wider on the side with the flare andnarrower on the side with the sheared heel.
The vertically displaced heel is often affected byrecurrent spontaneous quarter crack. When this occurs,the free margin of the ungual cartilage is usuallydiminished in feet with sheared heels due to upwarddisplacement of the hoof wall at the heel. This is animportant parameter to keep in mind when addressing aquarter crack. On palpation and measurement of a footwith a sheared heel and a spontaneous quarter crack, itis not uncommon to find the proximal border of theungual cartilage at or below the coronary band. When aquarter crack is present, palpation of the ungualcartilage and moving the cartilage outwards (abaxially)by hooking a finger axially to it, tends to elicit pain andopening of the proximal margins of the crack (Fig 8). Thepainful reaction is usually not elicited on the contralateralside (to the sheared heel) by the same manipulation. Theproximal margin of the quarter crack is always near thehighest point of the vertical distortion of the coronaryband when observed from the side. Measurement of thefree ungual cartilage margin above the quarter crack bymeans of metric calipers reveals that when spontaneouscracks are present, this distance is ≤15 mm (2-15mm)(Castelijns 2006) (Fig 9).
It is important to view the horse in motion, again on ahard level surface from the front and rear. This should bedone at a walk and a trot. When viewed from behind, thisshould determine which section of the foot is contactingthe ground initially and which portion of the foot isreceiving the impact. The direction of breakover should benoted when viewed from the front. As the human eye isincapable of observing events with a duration of <0.02 s,and foot landing and loading can be quite different atdifferent speeds and gaits, slow motion review of highspeed film is highly recommended.
Farriery is directed toward unloading the hoof wall anddecreasing the forces on the displaced side of the footwith the quarter crack. This is accomplished by improvingthe shape of the hoof and the landing pattern and theapplication of the appropriate shoe. When a horsedevelops a full thickness quarter crack, it is advisable totake the animal out of training to allow healing, but this isnot always an option with competition horses. Constraintsmay be placed on the farriery due to the training andcompetition schedule of the horse. For example, theauthor (S.E.O.) likes to remove the shoes and stand thehorse on a hard surface for 12-24 h prior to trimming andshoeing. This alone allows the affected side of the foot tosettle into a more acceptable conformation (Fig 10). If asevere sheared heel hoof capsule distortion is present, theunshod foot can be stood on some form of frog supportand the foot is placed in a soak bandage for 24 h (Snowand Birdsall 1990). This results in a profound change inhoof shape and the distance between the coronet andthe middle phalanx will widen. If a quarter crack ispresent, when possible, the authors prefer to perform thefarriery and then wait for the coronet to settle into a moreacceptable position or slope before any type of repair isconsidered. If the repair has to be performedimmediately, due to the competition schedule, thedefect will be repaired with the coronet in a displacedposition.
Farriery is initiated by removing the shoes and againobserving the horse walking on a hard surface noting thestrike pattern of the foot. The authors use a doubletrimming method in an attempt to improve and unload thedistorted quarter/heel. The foot is trimmed appropriatelyusing the guidelines of a parallel hoof-pastern axis, thecentre of articulation and heels of the hoof capsuletrimmed to include the base of the frog (O'Grady 2009). Tostart, a line can be drawn across the widest part of the footwith a felt tip pen. The frog is trimmed to where it is pliable and the quarters and heels of the hoof capsule from themiddle of the foot are rasped palmarly so that the heels ofthe hoof capsule and the trimmed frog are on the sameplane if possible. An attempt is made to create as muchground surface under the affected heel as possible, whichwill often result in that side being marginally lower than theother side of the foot. The toe and quarters are reducedappropriately so when the trim is completed, the surfacearea on either side of the line drawn or the widest part ofthe foot will approximate each other (Fig 11). Lowering theheel on the displaced side of the foot is logical as it is thetaller heel and it increases the ground surface of the footon that side. Following the trim, the horse is again walked on a hard surface and some improvement in the landingpattern should be noted.
It is one author's opinion (S.E.O.) that, when initiallymanaging a sheared heel, especially with a quarter crack,the horse should be placed in a bar shoe if possible. Barshoes effectively increase the surface area of the foot,allow the palmar/plantar section of the foot to beunloaded, and decrease the independent verticalmovement at the bulbs of the heels. The author's choice isa wide web steel straight bar shoe (Kerckhaert Shoes)1fitted symmetrically to the trimmed foot (Fig 12). Beforeapplying the shoe, a second trim is performed under theproximally displaced quarter/heel, which goes from 0 mmat the ipsilateral toe (e.g. inside toe for medial shearedheel) to an average of 7 mm at the affected heel. Theamount of heel that can be taken off in the second trimdepends on the sole depth at the seat of corn and on theseverity of the proximal displacement of the coronaryband at the sheared heel. The amount of heel under thesheared heel that can be taken off with this second trimideally corresponds to the difference in length/heightbetween the 2 heels. Lowering the hoof wall at thequarter/heel will create a space between the shoe andthe hoof wall on displaced side of the hoof (Fig 13). Thisimproves the landing pattern, unloads the affected heeland allows the heel bulb to settle down and assume amore acceptable position. Feet with a low palmar/plantarangle rarely have enough sole depth under the affectedheel for the second trim, in these cases the rest of the hoof wall can be raised with a rim pad or with a full leather padand impression material. When a full pad is used,impression material (Equilox Pink)2 is placed in the palmarsection of the foot from the apex of the frog palmarlyexcept under the displaced heel where the second trim was performed. Only the first 2 nails should be placed inthe toe of the shoe on the side with the sheared heel toeffectively allow the displaced heel to settle into a moreacceptable position. After the shoe is attached to the foot,the affected heel will rapidly descend onto the shoe,making the original space created by the second trimbetween the hoof wall and the shoe disappear. The extentof the second trim at the heel will determine the increaseof free margin of the ungual cartilage above the coroneton the affected side of the hoof. This can be observedvisually, palpated and measured with calipers. As mosthorses with a sheared heel have a predisposing limbconformation (e.g., a rotational deformity), these feethave a tendency to continue to deform the affected heelproximally and the double trim method usually has to beapplied to some degree at each consecutive shoeing.Horses with this type of hoof conformation should be resetat 4-6 week intervals.
Some cases will present with displaced heels withrecurrent cracks and will resist lowering and widening ofthe sheared heel with the farriery methods describedabove. In these cases, one author (H.H.C.) treats thedistortion of the hoof capsule at the site of the shearedheel with a full wall thickness sub coronary groove, appliedwith a rasp or Dremel3 tool about 20 mm below andparallel to the coronet. Care must be taken to go all theway through the wall to the laminar corium, from the endof the heel forward to the most dorsal part of the hoofdistortion at the coronet. Horses may show discomfort fromthis procedure for a few days, especially if the laminarcorium has been reached, which results in tiny pinpointhaemorrhage being visible. After the procedure, anantiseptic combined with a compressive bandage shouldbe applied. The wall growth proximal to the groove willshow a totally new, wider, abaxial direction as it isdisconnected from the stresses being placed on thestraight distal wall.
The importance of determining the underlying cause andimplementing the appropriate farriery cannot be overemphasised when managing sheared heels with quartercracks. The strong association between sheared heels anda quarter crack coupled with limb conformation and thelanding pattern of the horse is hard to ignore. Thedebridement, stabilisation and repair of spontaneousquarter cracks will be of little value, and the defect willhave a tendency to recur, unless the cause is determinedand rectified.
Assessing the limb conformation, improving the footshape and applying the appropriate trim/shoe appear tobe as important as the repair technique used to stabilisethe defect. Inadequate attention to these factors mayaccount for the many failures encountered and therecurring nature of quarter cracks.
The heels of the horse's foot have a relatively largeamount of flexibility in the proximal to distal (vertical) axis.This can be explained by the anatomical features of thefoot: a discontinuity of the hoof capsule between theheels with highly mobile structures interposed betweenwhich are the frog, digital cushion, venous and arterialplexa and fibro-cartilaginous connective tissue. In otherwords, although the dorsal wall is intimately attached tothe parietal surface of the distal phalanx, the laminarattachment or suspension in the palmar/plantar section ofthe foot is far less rigid. This provides the flexibilitynecessary for function but also allows for proximaldisplacement of the heels when these receive excessivestress or a disproportionate load. Functionally thisarrangement serves the bare-footed horse well as thehoof capsule at the heels is able to adapt to the unevenfooting, but when shoes are applied this ability toadapt becomes modified. When trimming or shoeingmodifications in the sagittal plane of the foot are beingcontemplated, it is important to be aware of this verticalmobility, and the tendency for vertical displacement ofthe heels. A wedge pad placed under the heels, forexample, will cause proximal displacement of the heels(Castelijns 2006).
The prognosis for sheared heels is good, provided askilled, interested farrier is involved. It is also necessary tohave a committed owner as these cases often requireongoing maintenance. Theoretically, the prevention andtreatment of lameness and/or quarter cracks, causedby a hoof capsule distortion such as sheared heels issimple, but in practice it is often difficult to achieve. Beingaware that there is a strong correlation between shearedheels and hoof wall problems, such as quarter cracks,makes prevention and treatment not only logical butimperative.