When altering horses’ traction, farriers are applying lessons that they learned when they were but knee-high to a grasshopper. It’s called the Goldilocks principle.

Applying too little traction causes the horse to slip too much, thus risking serious injury. Too much traction severely alters the horse’s natural grip, which also heightens the possibility of serious injury. That makes finding traction that’s “just right” so critical for the health and performance of the horse. When the effects of the principle are observed — too little traction vs. too much traction — you are witnessing the Goldilocks effect.

A recent study published by the Journal of Biomechanics titled, “Hoof position during limb loading affects dorsoproximal bone strains on the equine proximal phalanx,” by Ellen Singer, Susan Stover and Tanya Garcia illustrates that importance.

Singer is an equine veterinarian and senior lecturer in equine orthopaedics at the University of Liverpool. Stover is a veterinarian and professor of anatomy, physiology and cell biology at the University of California-Davis. Garcia is a biomedical engineer and manager of the musculoskeletal biomechanics research, imaging services and gait lab at UC-Davis.

How Do P1 Fractures Occur?

Singer became intrigued by fractures of the first phalanx (P1) and why they occur. The trio studied P1 strains, as well as evaluated how much axial rotation and collateral motion it had in relation to the third metacarpal bone (MC3). The result was an article published by the Journal of Biomechanics in 2013 titled, “How do metacarpophalangeal joint extension, collateromotion and axial rotation influence dorsal surface strains of the equine proximal phalanx at different loads in vitro?” That led to the most recent study.

Farrier Takeaways

Restricting forward slide of the equine foot during loading results in high bone strains during hyperextension of the metacarpophalangeal joint.

While there is no direct evidence that links calks to sport horse injuries, toe grabs are associated with injuries.

The study’s author advises the use of studs only when absolutely necessary.

Borium applied to a horseshoe should not be too tall.

“Since P1 fractures seem to occur more commonly in horses that race on turf, in the U.K. anyway,” Singer says, “we thought that maybe we could mimic different surfaces by altering the ‘foot slip,’ which in our system is the dorsal translation of the foot plate.”

To achieve this, the team mea­­­sured P1 bone strains and metacar­pophalangeal joint (MCPJ) angles during in vitro loading to simulate walk, trot and gallop loads, while hoof motion was either facilitated or constrained in the horizontal plane.

“Our goal was to demonstrate the potential effects of alterations in hoof slip, as represented by hoof position, on P1 bone strains and MCPJ angles during in vitro limb loading,” the study states. “We hypothesized that restricting forward slide of the foot during load­ing would result in higher bone strains during MCPJ hyperextension.”

Testing Cadaver Limbs

The tests were conducted at the Veterinary Research Laboratory at UC-Davis, which is run by Stover.

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A new study finds that decreasing hoof slide can cause saggital fractures of the proximal phalanx. Ellen Singer, an equine veterinarian and the study’s author, advises that Borium applied to horseshoes should not be too tall.

A serohydraulic material testing system (MTS) from Minneapolis, Minn.-based MTS Systems Corp. was used to mechanically load the cadaver limbs. The system is equipped with an axial-torsional load transducer. Two orthogonal translation on a linear bearing system allowed the hoof to translate in the horizontal plane in dorsopalmar and lateromedial directions, which was dependent on hoof position while the limb was loaded.

“The MTS at Sue Stover’s lab is unique,” Singer explains, “and one of the few that can load an equine limb to the force that mimics what occurs in nature.”

During the testing, they employed five forelimbs from five mature Tho­roughbred or Thoroughbred cross horses that were euthanized for reasons unrelated to forelimb pathology.

“Limbs were transected mid-radius and the proximal end was potted in a cylinder with polymethylmethacrylate [PMMA] while the limb was loaded in a standing position,” according to the study, which noted that bone markers were placed in MC3 and P1. “Two video cameras, dorsomedial and dorsolateral to the limb, recorded marker motion. A rosette and uniaxial strain gauge were applied to the dorsoproximal surface of P1 and zeroed with the limb hanging (Figure 1).”

Figure 1
Foot_Forces_Fig1.jpg

Lateral (A) and dorsal (B) views illustrate bone instrumentation and hoof translations for foot conditions within the materials testing machine. The inset (C) illustrates the location (percentage length of P1) of the rosette and uniaxial strain gauges on the dorsoproximal aspect of P1.

Illustration: Journal of Biomechanics 48 (2015) 1930-1936

The trio studied three foot conditions by allowing or restricting motion of the translation. The hoof was permitted to move in the forward condition 70 mm dorsally during loading. It was permitted to move 40 mm dorsally in the restricted condition. It was allowed to move 70 mm dorsally and 40 mm laterally in the free condition.

“We were trying to determine what happens in the bone with different de­grees of movement of the hoof,” Singer says. “The forward and restricted probably mimic different degrees of foot slip. The free position allowed for freedom of movement from lateral-medial, which may occur on certain, possibly very slippery surfaces in the live horse.”

The researchers secured the foot to the translation table with the radius and metacarpal bones aligned parallel to the axis of loading with a physiologic palmar fetlock angle with a load of 700 newtons (N).

The newton, named for famed English scientist Isaac Newton, is the International System of Units derived unit of force. “A force of 1 newton will accelerate a mass of 1 kilogram at the rate of 1 meter per second squared,” according to the University of North Carolina’s Dictionary of Units of Measurement.

The research team determined load ranges to capture known peak vertical forces for stance, 1,800 newtons (N); walk, 3,600 N; trot 5,400, N; and gal­lop, 10,500 N.

Test Results Confirm Hypothesis

Researchers found that the mag­nitude of P1 bone strains and MCPJ angles increased as limb load increased over all foot conditions when averaged.

The restricted condition resulted in significantly larger strains for the three arms of the rosette gauge — proximomedial-distoletral, proximodistral and proximolateral-distalmedial — in comparison to the free and forward conditions. It also resulted in a significant­ly lar­ger MCPJ extension angle for the restricted compared to the forward condition. Al­though the free condition allowed lateral transition, there was no difference in bone strains and angles in the forward and free conditions.


Restricting forward slide during loading affects dorsoproximal P1 bone strains during simulated gallop load …


The gallop load produced the greatest differences in P1 bone strains between foot conditions.

“Restriction of dorsal translation resulted in higher axial compressive, maximum shear and engineering shear strains compared to the forward condition,” the report states. “The forward condition had higher compressive principal strain, lower tensile principal strain and lower transverse tensile strain compared to the free condition at 10,000 N for all three arms of the rosette gauge and for compressive principal strain.

“The forward condition had higher compressive principal strain, lower tensile principal strain and lower transverse tensile strain compared to the free condition at 10,000 N.”

When analyzing the testing results, the researchers found that their initial premise regarding P1 bone strains holds up under testing.

“The results support the hypothesis that restricting forward slide of the equine hoof during loading affects dorsoproximal P1 bone strains during simulated gallop load in cadaveric forelimbs,” the report summarizes. “The re­striction of forward foot movement re­sulted in higher axial compressive and shear strains. Lateral foot translation resulted in lower compressive principal and higher tensile principal strains.

“The significant alteration in P1 bone strains with different foot conditions supports previous studies that demonstrated different fetlock joint kinematics in horses running on surface materials with different hoof-surface interaction characteristics.”

The MCPJ in a galloping horse ex­periences the largest forces and major dissipation of energy from foot strike to mid-stance. Impact dissipates when the hoof slides as it decelerates when contacting the ground. The researchers found that foot slide was 30 mm shorter in the restricted condition than the forward.

“The foot slip restriction in this study is greater than the difference observed between turf and an all-weather wax surface (10-27 mm) or which resulted from the use of heel studs in horses galloping on turf (13-25 mm),” the study reports. “The limitation of hoof slip in the restricted condition led to a significant increase in axial compressive, maximum shear and engineering shear strains on dorsoproximal P1.”

The MCPJ is loaded more rapidly to maximum extension when foot slip is limited on turf. How­ever, the duration of stance is decreased. As a result, “the kin­ematic events [occur] ear­lier, in absolute time and per­centage of the stance phase.” Horses galloping on turf likely will cause higher MCPJ forces initially over a shorter duration than on synthetic surfaces.

“This feature of MCPJ biomechanics on turf surfaces may contribute to the higher incidence of fatal sagittal P1 fractures for horses racing on turf,” according to the study. “The current study and the cited research indicate that the effect of surface on the equine digit, MCPJ kinematics and P1 bone strain is important, since the foot:ground interaction is one of the viscoelastic elements of the mechanics of limb loading.”

Traction Recommendations

While traction devices aid a performance horse, particularly while galloping and jumping, they also can be det­ri­mental.

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Toe grabs likely alter foot slip and there’s evidence that they cause injury, including catastrophic harm to the fetlock.

“While direct evidence is lacking to connect injury in the sport horse to the use of horseshoe calks, the use of toe grabs predisposes Thoroughbred racehorses to injury, particularly catastrophic fetlock injuries,” the study cites a 1996 paper by equine veterinarian Albert Kane titled, “Horseshoe characteristics as possible risk factors for fatal musculoskeletal injury of Thoroughbred racehorses.”

Restricting foot slip goes beyond the interaction of the foot and ground, causing alterations to strain in the fetlock region, Singer says.

“I think this is an argument for performance horses to have stud holes with studs placed when necessary, rather than having studs present all the time,” she says. “Although not proven in the study, I think that the study could also be used as an argument to limit the number of stud places since it seems that decreasing foot slip increases strain, in particular shear strain, in the fetlock region.

“Therefore, it is likely that more studs will result in more restriction of foot slip and in­crease in strain at the fetlock joint possibly increasing the risk of injury.”

Yet, Singer isn’t advocating the elimination of the traction devices. Rather, she’s suggesting that they should be limited.

“Obviously, studs are useful — or they are perceived to be useful by competitors,” she says. “So, it would be difficult to recommend that they stop using them, but caution against too many in each foot or studs that are relatively large.”

The veterinarian suggests a lower profile while using traction devices.

“Where Borium is used as a per­manent stud to decrease slipping on potentially icy roads,” Singer says, “they should not be too tall of the shoe.”

Singer’s advice could help you limit the Goldilocks effect and find traction that’s just right for the horse.

References

Kane, A.J., Stover, S.M., Gardner, I.A., Case, J.T., Johnson, B.J., Read, D.H., Ardans, A.A., 1996. Horseshoe characteristics as possible risk factors for fatal musculoskeletal injury of Thoroughbred racehorses. Am. J. Vet. Res. 57 (8), 1147–1152.

Singer, E., Garcia, T., Stover, S., 2013. How do metacarpophalangeal joint extension, collateromotion and axial rotation influence dorsal surface strains of the equine proximal phalanx at different loads in vitro? J. Biomech. 46 (4), 738–744. http: //dx.doi.org/
10.1016/j.jbiomech.2012.11.028, S0021-9290(12)00680-X [pii].

Singer, E., Garcia, T., Stover, S., 2015. Hoof position during limb loading affects dorsoproximal bone strains on the equine proximal phalanx. J. Biomech. 48 (2015) 1930–1936. http://dx.doi.org/10.1016/j.jbiomech.2015.04.014.