Virginia Therapeutic Farriery

Aspects of Functional Anatomy of the Distal Limb

Reprinted with permission from the American Association of Equine Practitioners.
Originally printed in the 2012 AAEP Convention proceedings

Andrew H. Parks, MA, Vet MB, MRCVS, Diplomate ACVS

1. Introduction
Every day, clinicians change the point of breakoverof a foot, add a wedge or a pad to a shoe, or resectpart of a hoof capsule. All of these change the waythe foot functions in some manner. We make someof these changes because that is what we were instructedto do in school, because it may have workedfor us before, or because it makes sense to us-buthow often do we actually think about the underlyingbiomechanical principles involved? This report discussessome aspects of function of the front foot ofthe horse. It is written from the perspective of aclinician trying to incorporate scientific developmentsin the biomechanics of the horse's foot intoclinical practice. It is not intended as a comprehensivereview of all the biomechanical studies thathave addressed the function of the equine digit.Readers who would like an in-depth review of digitalbiomechanics are referred to several recent excellentarticles.1-4

Understanding how the foot works in this mannerundoubtedly improves one's ability to treat the morecomplicated foot problems in horses; however, thesubject is fascinating in its own right. The mostimportant aspects of foot function are related to thehoof and the distal interphalangeal joint. Therefore,this article will briefly discuss the anatomy ofthese structures, aspects of foot function at rest andat the trot, and briefly explore how some commonmanipulations used therapeutically may affect function.As clinicians, we tend to be very good at qualitativeideas such as the biomechanical conceptsexplored in this article. Fortunately for us, it appearsto work well much of the time. However,there are occasions when the outcome of a biomechanicalevent is the result of two different determinantsthat function in an opposite manner to eachother, and therefore, the result is the balance of thetwo. In such circumstances, without quantifyingboth effects, it is not possible to determine the netresult. Therefore, excessive reliance on qualitativeconcepts can lead to overinterpretation or misinterpretationof the facts.

The majority of studies have examined the kineticsand kinematics of locomotion of horses at rest, atthe walk and trot, and measured strains present invarious tissues. These scientific studies have examinedthe position of the different elements of thedistal limb in relation to each other and the ground,the force applied to the ground surface of the foot,strains in the hoof capsule and in major tendons andligaments, and the biomechanical properties of someof the tissues in the foot. From this information,given specified assumptions and other static measurements,more information may be calculated.Last, using experimental data of the forces appliedto the foot and the biomechanical properties of thetissues, finite element analysis models have beendeveloped to determine what is happening with respectto movement and forces within some of thosetissues that cannot currently be directly measured.

Glossary of Biomechanical Terms

Ground Reaction Force

The ground reaction force is the force exerted bythe ground on a body that is in contact with theground. It is depicted as a vector that representsthe sum of all individual forces on the surfaceof the body in consideration.

Distribution of Force

Anywhere there is contact between the body andthe ground, there is a force between the body andthe ground. However, the force is not necessarilyevenly distributed. For example, if a horse isstanding on sand, the pressure is primarily distributedacross the middle of the foot, includingthe sole and part of the frog. However, if thehorse is standing on a flat, unyielding surface,most of the pressure is distributed around theperimeter of the foot at the interface of theground and wall.

Center of Pressure

The center of pressure is that point throughwhich the ground reaction force acts. Therefore,the center of pressure is the point about whichthe forces from all the different areas of contactare evenly distributed. It is also called the pointof zero moment because it is that point at whichall the moments created by forces on the object,in this case the horse’s foot, cancel each otherout. The center of pressure is static only if ahorse is standing still. When a horse is moving,the location of the center of pressure is dynamic.When the center or pressure is movedto one side of the foot, the bone and joints willbe subject to increased compressive stress, thecollateral ligaments to reduced tensile stress,and the lamellae to greater tensile/shear stressand vice versa. When the center of pressure isshifted in a palmar direction, tension in the deepdigital flexor tendon is reduced, the weightbearingby the palmar hoof wall is increased, andvice versa.

Moment (Torque)

A moment is the tendency of force to cause rotationabout an axis. It is calculated as the productof the length of the lever arm and thecomponent of the force that is at right angles tothe lever arm. If there are two equal but oppositemoments acting around an axis, no movementoccurs.

2. Anatomy of the Hoof and Distal Interphalangeal Joint
The hoof is the integument of the foot, and as such,it is composed of three layers: the epidermis, dermis,and subcutaneous tissue. It is also divided into 5/6regions: the limbic (perioplic), coronary, parietal (lamellar),solar, and cuneate/bulbar regions. Thehoof capsule is formed by the stratum corneum ofthe epidermis of all these layers. The wall isformed by the stratum corneum of three layers-thelimbic, coronary, and parietal-and these layers arecalled the stratum externum, stratum medium, andstratum internum, respectively. Each region of thehoof is highly specialized. The stratum medium ofthe wall is formed from tubular and intertubularhorn. The structure, size, and density of the horntubules vary with which zone of the wall they arelocated. The moisture content of the wall similarlyvaries, being drier more superficially and more hydratedin the deeper layers. The interdigitations ofthe lamellae are highly specialized and provide avery large surface area of contact between the epidermisand adjacent dermis. The frog is muchsofter than the wall and sole, and the underlyingsubcutaneous tissue is greatly modified to form thedigital cushion. Therefore, the wall is well adaptedto weight-bearing, the sole adapted to protecting theunderlying soft tissues and weight distribution, andthe frog and digital cushion adapted to permit expansionof the foot and participate in damping ofvibrations.

The distal interphalangeal joint (DIPJ) is a complexjoint with three articulations: (1) between themiddle and distal phalanx, (2) between middle phalanxand the distal sesamoid (navicular bone), and(3) between the distal phalanx and the distal sesamoid.There is very little movement between thedistal phalanx and the distal sesamoid,5 so they arefrequently treated as one unit and will be so for theremainder of this discussion. The distal interphalangealjoint is a ginglymus joint. However, becausethe saggital groove on the middle phalanx isvery shallow and the opposing ridge on the distalphalanx very low, it also permits significant rotationand movement in the frontal plane.

3. Aspects of Distal Forelimb Function in theStationary Horse
In a standing horse, the weight (mass times accelerationof gravity) borne by the limb is supported bythe ground, which opposes the weight with an equaland opposite force. This force exerted on the hoofby the ground is the ground reaction force (GRF).At rest, both of these forces are approximately vertical.The weight of the horse is not uniformly distributedacross the ground surface of the foot. Byusing a complex pressure transducer system, thedistribution of the GRF on the ground surface of thefoot has been examined under various conditions.6It has been shown that in a shod horse, the weight isborne relatively evenly over the area that the shoecontacts firm ground. In a barefooted horse thathas just been trimmed and is standing on firmground, weight-bearing is increased compared to theuntrimmed state and ground contact is present over the frog but is not necessarily evenly distributedaround the perimeter of the foot. In a barefootedhorse that has been at pasture and then stood onfirm ground, the weight-bearing is primarily at theheel and toe. The pattern of weight-bearing at thetoe has been shown to vary; it is either spreadbroadly across the toe from the toe-quarter junctionon one side to the other or restricted to the toequarterjunctions without any weight-bearing at thecenter of the toe. When a horse is placed on asurface that deforms to the shape of the foot, theweight-bearing area becomes much larger and isbroadly distributed across the center of the groundsurface of the foot.

The mechanical interaction between the horse andthe ground is measured with force plates that do notdifferentiate between weight-bearing by differentparts of the foot but renders a single value. It isrepresented as a vector (GRFV). Vectors have adirection and magnitude. This vector representsthe summation of all the forces acting on the foot.Measurements made this way can be broken intothree components representing the three orthogonalplanes: vertical, craniocaudal, and mediolateral.As such, they have a point of action. This point ofaction is given several names: point of force, point ofzero moment, and center of pressure. This articlewill use the latter because its meaning is more intuitiveto most people. At rest, the vertical componentof the GRFV is much greater than either of thetwo horizontal components.

The weight of the body borne by the limb is transmittedthrough the limb by the skeletal system.The question that arises is, how is this force transmittedfrom the skeletal system to the ground?Based on clinical evidence, it has been assumed thatthe lamellae suspend the distal phalanx within thehoof capsule. In horses with laminitis in which thelamellae are severely damaged, the distal phalanxdisplaces within the hoof capsule. Additionally, itis possible to remove the majority of the sole in ahorse for therapeutic reasons, and the horse is ableto bear weight on the wall without the distal phalanxdisplacing. How does this correlate with thefact that in a shod horse on a firm surface the weightis distributed around the periphery of the foot, but ina barefoot horse standing on a yielding surface theweight is not distributed around the wall but acrossthe center of the foot?6 While it is not possible tomeasure where the force is going within the tissuesof the hoof capsule and lamellae, this has been modeledwith finite element analysis, which supportsthe intuitive position.7 When the weight is spreadover the center of the ground surface of the hoof, itindicates that the forces associated with weightbearingare directed to the wall through the sole,and then through the lamellae.7

Fig. 1. At rest, the ground reaction force (gray arrow) is dorsal tothe center of rotation of the distal interphalangeal joint. Assuch, it creates an extensor moment that is opposed by an equaland opposite moment, the flexor moment, generated by the forcein the deep digital flexor tendon so that the foot is stationary.

The center of pressure varies between horses butis approximately in the center of the ground surfaceof the foot (Fig. 1). This is dorsal to the center ofrotation of the distal interphalangeal joint. The force exerted through the skeletal system is actingthrough the center of rotation of the distal interphalangealjoint. Therefore, the GRF creates a momentabout the distal interphalangeal joint. Amoment is the tendency to cause rotation of a bodyabout an axis. This moment created by the GRFVwill cause the joint to dorsiflex (hyperextend) if unopposed;this moment is the extensor moment.In this case, the axis is the center of rotation of thedistal interphalangeal joint. The magnitude of themoment is the product of the force and the length ofthe moment arm. The force is the magnitude of theGRFV. The length of the moment arm is the shortestdistance between the line of action of the GRFVand the center of rotation of the DIPJ (i.e., the momentarm is perpendicular to the line of action of theGRF). Because the foot is in a stable position flaton the ground, the extensor moment must be opposedby an equal and opposite moment, which isthe flexor moment. The flexor moment is the productof the force in the deep digital flexor tendon andthe length of the moment arm, which is the shortestdistance from the center of rotation of the DIPJ tothe tendon.

4. Aspects of Distal Forelimb Function at the Trot
So far, this description has covered the dynamics ofthe foot of a horse standing at rest, but what aboutthe foot of a horse that is walking or trotting? Thestride is divided into flight and stance, and thisdiscussion will be confined to the stance phase.At the beginning of the stance, the limb is fullyprotracted, with the foot out in front and the limbalmost fully extended. After the foot comes to rest,the body continues to move forward and the trunkdescends. As it does so, the metacarpophalangealjoint (MCPJ) dorsiflexes (hyperextends) and the distalinterphalangeal joint flexes; that is, they arerotating in opposite directions. At midstance, thelimb is vertical and the MCPJ dorsiflexion and the DIPJ flexion have peaked. After midstance, thelimb moves toward full retraction. As it does so,the MCPJ decreases (but is still dorsiflexed) andthe DIPJ changes from flexion to dorsiflexion sothat at the beginning of breakover, both joints aredorsiflexed.

As the limb moves through the stance phase of thestride, the GRFV changes and these changes reflectthe different phases of the stride. The magnitudeof the vertical component of the GRFV is very lowimmediately after the foot contacts the ground, increasesas the horse bears more and more weight,and then decreases so that it is again very low whenjust before the foot leaves the ground. In the forelimbs,the horizontal component of the GRFV is, forapproximately the first 60% of the stride, in theopposite direction to the movement of the horse, thatis, it is a braking force.8 During the last 40% of thestride, the horizontal component of the GRFV is inthe same direction as the movement of the horse,that is, it is a propulsive force.

The strains in the flexor tendons, accessory ligamentof the deep digital flexor tendon, and suspensoryligament9 reflect the magnitude of the GRFVand the angulation of the metacarpophalangeal anddistal interphalangeal joints. The strains in thesuperficial digital flexor tendon and the suspensoryligament are greatest at the point of maximalweight-bearing and maximal dorsiflexion of themetacarpophalangeal joint. The strain in the deepdigital flexor tendon (measured proximal to its attachmentto its accessory ligament) does not increaseas much as that in the superficial flexortendon or the suspensory ligament because as themetacarpophalangeal joint dorsiflexes, the distal interphalangealjoint flexes, that is, the tendency forthe tendon to stretch around the metacarpophalangealjoint is offset, at least partially, by its tendencyto shorten around the distal interphalangeal joint.The accessory ligament of the deep digital flexor isunder greatest tension when both these distal jointsare dorsiflexed but before the magnitude of theGRFV has decreased markedly.

Based on the kinematic and kinetic events of thestride, stance is divided into three phases.4 Thefirst is the impact phase, which begins at first contactand is defined by the presence of shock wavespresent in the distal limb and is associated withlanding and initial loading of the limb. The supportphase begins at the end of the impact phase andends at heel lift-off. It is the phase of the stridewhen the limb bears maximal load and the periodbefore and after maximal loading. Its beginning isactually a continuation of the initial loading of thelimb that begins at first contact but without thevibrations of impact. Its ending at heel-off signifiesthe kinematic event because the unloading of thelimb continues until toe-off. Breakover begins withheel-off and ends with toe-off. The end of breakoveris like the impact phase in that it is associated with shock wave vibrations, but they are of considerablylower magnitude.

The impact phase, which lasts 25 to 50 milliseconds,is further subdivided into two parts associatedwith two collisions.4 The first is the impact of thefoot with the ground, which only lasts a few millisecondsfollowed by a second, which involves theimpact of the weight of the body and limb of thehorse with the foot. These two impacts overlap tosome degree and set up a series of irregular shockwaves associated with the deceleration of impact.

The moments about the distal interphalangealjoint of a horse trotting are a function of the magnitudeof the GRFV, the center of pressure, and thetension in the deep digital flexor tendon. Althoughthe center of pressure at first contact is often at theheels or lateral quarter, because it moves veryquickly toward the center of the foot and because themagnitude of the GRFV is low, this phase of thestride has received little attention. During the supportphase of the stride, when the center of pressureis in a relatively constant position in the center ofthe foot and dorsal to the center of rotation of thedistal interphalangeal joint, the extensor and flexormoment arms are relatively constant, and thereforethe force in the deep flexor tendon directly reflectsthe magnitude of the GRFV. Towards the end ofthe support phase the tension in the distal portionof the deep digital flexor tendon and its accessoryligament increases so that the flexor moment increases.At the same time the magnitude of theground reaction force is decreasing. Consequently,the center of pressure moves towards the toe, whichlengthens the extensor moment arm such that theextensor moment equals the flexor moment and thefoot remains flat on the ground. The heels lift offthe ground when the flexor moment exceeds theextensor moment (which occurs because the centerof pressure can move no further dorsally once it is atthe dorsal margin of the toe).

Fig. 2. Elevating the heels (depicted at rest in this schematic)causes the simultaneous decrease in tension in the deep digitalflexor tendon and movement of the center of pressure in a palmardirection toward the center of rotation of the distal interphalangealjoint. Thus, both moments are decreased equally, so thereis no net movement of the foot (except for the initial elevation ofthe heels). The decrease in tension in the deep digital flexortendon and the flexion of the distal interphalangeal joint causethe force on the navicular bone to be reduced.

The distal limb has developed to absorb the energyassociated with impact and the loading of thelimb. It is known that impact vibrations arelargely dampened by the time they have propagatedto the proximal phalanx.10,11 The evidence indicatesthat the tissues that absorb the energy are thesoft tissues of the hoof, for example, the lamellaeand underlying dermis/subcutaneous tissue and thearticular cartilage of the distal joints. Additionally,the structure of the digital cushion and thecollateral cartilages and their associated venousplexuses suggest that a hemodynamic dampingmechanism is present in the palmar half of thefoot.12 The impact associated with loading of thelimb by the weight of the trunk is also dampened bythe combined action of the musculotendinous structuresand the two distal joints in the limb, which, byextending the period over which the load is applied,decreases the maximum force on the distal limb.Dorsiflexion of the metacarpophalangal joint is associatedwith an increase in length of supporting tendons and ligament. The tendons are structuredto store energy as they stretch and release it as theyshorten, and the muscles are designed to dampenvibrations within the tendons.13 However, despitethese protective mechanisms, excessive, repetitivestrains can potentially damage tissues within thedigit during either impact or loading.

5. Effect of Common Manipulations on Foot Function
The effect of shoeing horses on foot function is welldocumented. In short, it is known that nailing onsteel shoes limits expansion of the foot14 and causesthe magnitude and frequency of impact vibrations toincrease.10,15 It is also known that some shoe andpad combinations can ameliorate these changes.15

Two adjustments to shoeing commonly performedfor horses with navicular disease are heel elevationand moving the point of breakover in a palmar direction.Our goal is usually described as taking thepressure off the navicular bone and making breakover“easier.” However, what do they really achievein terms of the biomechanics discussed above? Elevatingthe heels causes the distal interphalangealjoint to flex. At rest, elevating the heels moves thecenter of pressure toward the center of rotation ofthe distal interphalangeal joint. Therefore, itshortens the moment arm of the GRFV, whichmeans that the tension in the deep digital flexortendon is reduced (Fig. 2). The reduction in thedeep digital flexor tendon pressure tension in conjunctionwith the change in angle of the deep digitalflexor tendon around the navicular bone substantiallyreduces the pressure on the navicular bone.16The biomechanics of the middle of the stride aresimilar with respect to the position of the center ofpressure, but the magnitude of the GRFV and the tension in the deep digital flexor tendon aregreater. Therefore, elevating the heels would beprotective to the deep digital flexor tendon and navicularbone. However, moving the center of pressurein a palmar direction increases the load on theheels and increases intra-articular pressure. Additionally,flexion of the joint changes the distributionof pressure within the distal interphalangeal joint.Any of these effects of heel elevation are potentiallydeleterious.17

Fig. 3. Moving the point of breakover in a palmar directiondecreases the length of the moment arm at the beginning ofbreakover. This may cause breakover to occur slightly earlier,but it does not significantly reduce the duration of breakover orreduce the maximal force on the navicular bone.

Moving the point of breakover in a palmar directionis thought to improve distal limb function inhorses with diseases such as navicular syndrome.Moving the point of breakover in this manner doesshorten the extensor moment arm at the initiationof breakover18 (Fig. 3). Because the flexor momentexceeds the extensor moment when the GRFV canno longer move dorsally, breakover may occurslightly earlier. However, it does not shorten theduration of breakover. Furthermore, it does notdecrease the maximal pressure on the navicularbone as might be expected.18 This is because thepressure on the navicular bone is a function ofthe tension in the deep digital flexor tendon and theangle at which it bends around the bone. Therefore,maximal pressure on the navicular bone is abalance of decreasing tension in the deep digitalflexor tendon as the load on the limb decreasestoward the end of the stride and the increasingangle of the tendon around the navicular bone; thepeak pressure on the navicular bone occurs at approximately65% of the way through the stride,whereas breakover occurs at approximately 85% ofstance.16,18 These findings would argue againstthe effectiveness of moving the point of breakover ina palmar direction for horses with navicular disease.However, it appears that rolling the toe smoothesout the breakover process.19

In horses with laminitis, in addition to raising theheels and moving the point of breakover in a palmar direction, it is common practice to fill the spacebetween the branches of the shoe with a syntheticpolymer to promote weight-bearing by the sole.This procedure is also done for horses with otherclinical conditions of the foot. Clinical evidence ishighly varied, indicating that it appears to improvethe lameness in some horses but worsens it in others.However, the scientific evidence behind whatit does is minimal. This evidence, discussed above,suggests that it will distribute the force of weightbearingover a much wider area of the sole.6 However,finite element analysis suggests thatmovement of the quarters abaxially during foot expansionpulls at the margins of the sole, causing it toflatten out.7 Therefore, any device that limitsmovement of the sole in this manner may eitherlimit foot expansion or place excessive strain on thewhite line. Intuitively, the thickness and quality ofthe sole horn may also affect how the foot toleratesthis maneuver. However, much remains to be resolvedabout the effects of this procedure.

The farriery and veterinary professions havemade significant advances over the last 25 years intreating many conditions of the horse's foot, butundoubtedly we still have a long way to go. Thisprogress has received contributions from new scientificknowledge, but much of our progress has beenthe result of reasoning (sometimes good, sometimesbad) and experience. The author believes that thenext step in our progress requires more deliberatethinking by clinicians about concepts such as movingthe center of pressure and changing distributionof force into the development of therapeutic plansthat involve biomechanical manipulation of thehorse's foot.

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