Balancing the Normal Foot: Hoof Preparation, Shoe Fit and Shoe Modification in the Performance Horse (Part 2)
© O.K. Balch, D. Butler and M.A. Collier
published in ANVIL Magazine, September 1998
Reprinted with permission from Equine Veterinary Education, 1997
Equine Sports Medicine Laboratory and Comparative Orthopaedic Research Laboratory, Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, Oklahoma State University, Stillwater, Oklahoma 74078-0107 and Butler Publishing and Farrier Services, PO Box 1390, Laporte, Colorado 80535, USA
Ideal shoe characteristics and research on different shoe types
Ideal shoes properly fitted to horses' hooves and serving as extensions of hoof capsules would have the following characteristics in typical, short-footed (nongaited) performance horses:
1) Perpetuate the normal anti-concussion mechanisms of the hoof 2) Position the ground-bearing surface of the hoof in the correct spatial relationship to the axes of upper limb and cannon bone
3) Prevent excessive wear of epidermal hoof
4) Prevent excessive trauma to dermal hoof
5) Minimally lengthen the hoof
6) Minimally weight the hoof
7) Alter traction characteristics of the hoof in a manner appropriate for the horse's athletic activity
Standardbreds, Tennessee Walking horses, American Saddlebreds, Arabians, and Morgans are examples of breeds of horses and individuals within breeds that are trained and shod differently than typical short-footed performance horses to accentuate gait. The hooves of these horses are often trimmed (and shod with pads) to augment natural hoof length and weighted with heavier shoes to exaggerate limb kinematics (Fig. 10).
Fourteen years ago, Doug Leach (1983) wrote "The lack of specific information on how different shoe types affect equine locomotion is of great concern." Unfortunately, published research investigating the specific effects of different shoe types continues to be limited to a few subjects:
1) Effects of altered shape (squared, rockered, or rolled) of the toe on breakover in trotting horses (13)
2) Effects of a compressible plastic shoe - the Seattle Shoe - on the kinematics of the strides of galloping Thoroughbred horses (51)
3) Metacarpophangeal and phalangeal joint kinematics in horses shod with hoof caulks (47)
4) Effect of toe-weights on stride characteristics of Standardbred trotters (49)
5) Correlation between the presence of toe grabs on aluminum racing plates and catastrophic musculoskeletal injuries on California Thoroughbred racing tracks (20)
6) Shoe-weight induced changes in kinematic variables in trotting horses (8)
SHOE FIT, HEEL EXPANSION AND "THROWN" SHOES
Fitting the shoe to the hoof requires that the hoof be properly trimmed and dressed as described here earlier. A shoe is then shaped so that its outside circumference matches the outside circumference of the hoof at the toe and quarters. As described earlier, normal weight bearing during the stance phase of locomotion will cause the sole to descend and commonly cause the heels to expand abaxially. Logically, this heel expansion should be accentuated at higher velocities; force-plate studies confirm that the force exerted by the hoof on a measuring plate increases as the velocity of the horse increases. (42) Also, gaits which are characterized by weight-bearing on a single limb at a time during the stance phase (i.e., four-beat gaits such as a walk or gallop) versus weight-bearing on multiple limbs (i.e., two-beat gaits such as a trot or pace) should accentuate hoof expansion. Heel expansion is affected by the geometry of the hoof capsule; horses with upright quarters and heels, prominent bars, small frogs, and relatively narrow heels demonstrate less heel expansion than horses with sloping quarters and heels, diminished bars, and prominent frogs. (23)
The outside circumference of the shoe should be shaped so that when the heels do expand maximally, contact is maintained between the ground surface of the hoof and foot surface of the shoe. Unfortunately, no studies quantifying hoof expansion at various velocities and gaits or with different hoof conformations have been published. The American Farriers Association has devised a series of skill tests for farrier certification. (29) While these skill tests are not intended to be appropriate guidelines for the shoeing of all horses, the skill test evaluating heel expansion states that the branches of the shoes should extend beyond the wall the "thickness of a dime" or approximately 1.5 mm to compensate for expected hoof expansion. (29) These same
skill tests state that heel length of the shoe is appropriate if it extends beyond the most palmar/plantar aspect of the wall a distance equal to that reserved for expansion.
Based on the individual athlete's conformation and sport, increased musculoskeletal demands may require modification of the above shoe-fit guidelines. American Saddlebreds often have long pasterns; the branches should extend even with the bulbs of the heels to position the shoe's most posterior support for the limb under Russell's imaginary line dropped from the fetlock. Conversely, American Quarter Horses used in roping events will often trap and loosen front shoes by stepping on them with another shod foot if the hooves are shod "full" with conventional expansion and coverage of the heels. Because of their athletic activities, these horses are most successfully shod "tight" with minimal room for heel expansion and then reset more frequently, if necessary, to avoid the consequences of the quarters and heels overgrowing the shoe. Providing room for heel expansion inadvertently exposes a prominent ledge (the squared edge of the shoe from the last nail hole to the heel) to be stepped on by another shod foot; slightly rolling or "boxing" this squared edge of the shoe reduces the possibility of the shoe being trapped. (10,22) In circumstances where the loss of a shoe would be devastating, such as the endurance portion of the Olympic equestrian competitions, flexible methylmethacrylate (Equilox) has been used to fill the space between the shoe and hoof, completely removing the ledge. (18)
Farriers who are shoeing horses with a history of "throwing" or pulling shoes are often tempted to "tuck" the medial branch of the shoe toward the frog to prevent the horse from pulling off ("stepping off") the shoe and physically damaging the wall. (31) Tucking the medial branch is also used to prevent interference injuries associated with
the hoof striking the contralateral limb. In the United Kingdom, a three-quarter shoe (the medial branch caudal to the 2nd nail hole is removed) is used to prevent similar interference injuries; (22) unfortunately, the lack of medial support predisposes the suspended hoof to collapse. Tucking the medial branch of the shoe (as opposed to correctly executed "tight" shoeing) loads the white line and angle of the sole rather than properly transferring force to the quarters and heels of the wall. If non-wall epidermal structures are inadvertently loaded by tucking the medial heel of the shoe, then the routine repetitive concussion that is normally associated with equine locomotion may bruise the underlying sensitive layers. (33) If the necrosis of the sole occurs at the junction of the wall and bars (the angle of the sole), the resulting bruise is called a "corn."
ATTACHING THE SHOE TO THE WALL
Shoes are traditionally attached to hooves with rectangular, steel nails, although glue-on shoes are available commercially. Glue-on shoes will undoubtedly increase in numbers and diversity as their attachment becomes more secure and user-friendly. However, the use of steel nails to attach shoes to hooves will likely remain the predominant method because of the relatively low cost of nails, the ease of preparation of the hoof to be nailed, the brief time required to attach the shoe with nails, and the overall successfulness of the procedure.
As few nails as possible (usually 6) should be used to attach a shoe. Nails come in a multitude of sizes and head shapes for different horseshoeing circumstances. When nails are driven through the wall and clinched, the wall is potentially weakened
because of the splitting of the stratum medium. The smallest size nail which will fasten the shoe securely to the wall for the required length of time should be used to minimize splitting. (10) However, unless the stratum medium is very thin, fractured, or weak, periodic nailing at the regular intervals necessary for shoes to be reset does little or no long-term damage.
While not proven, traditional farriery protocols direct that shoes should be nailed to hooves with heel nails placed no further palmar (plantar) than the widest part of the hoof so that the normal expansion of the hoof wall is not hindered (Fig. 11) (29) Nails ideally should exit the wall one-third the height of the wall at the heel nail and form a line that parallels the shoe; (29) in normal-sized saddle horses, the nail exits about 2 cm (3/4 inch) from the ground surface of the wall. (10) Nail clinches, the bent-down ends of the nails that exit the wall, should be square-shaped and embedded smoothly in the wall. (29) Care should be taken not to excessively rasp the nail clinches; the strength of the connection of the shoe to the wall largely depends on the friction of the nail shaft in the stratum medium and protuberance of the nail clinch over adjacent hoof wall. A properly clinched hoof will be so smoothly finished that the clinches will not feel rough. (10) The use of clips (metallic projections from the foot surface of the shoe that overlap the wall) stabilizes shoes on hooves by reducing shear forces on horseshoe nails and are especially useful in horses with weak walls. (Fig. 12)
COMMERCIAL AND HAND-FORGED SHOES: Adding refinements
Traditionally, commercial or pre-manufactured ("keg") horseshoes have lacked the refinements of hand-forged shoes. Nail holes in many commercial shoes are punched a uniform distance from the outer circumference of the shoe and perpendicular to the branch, while nail holes in hand-forged shoes may be punched in the web (the width of the branch) at locations which reflect the differing position of the white line and the differing thickness of stratum medium at the quarters and the toe. (Fig. 13) Attempting to minimize mechanical stresses associated with changes in directions when nails are driven and clinched, nail holes in hand-forged shoes should be punched to match the differing slopes of the wall at the toe and quarters (Fig. 14). Most commercial shoes are manufactured flat on the foot surface of the shoe, while handmade shoes can be easily concaved (and commonly are) on the portion of foot surface that sits adjacent to the white line and sole. Careful use of the hoof knife to concave the white line and adjacent sole that is covered by the shoe will additionally protect the sensitive sole from chronic bruising. Properly concaved shoes (and concaved soles) are particularly important in performance horses such as jumpers and hunters; these athletes may periodically load their forelimbs sufficiently to bump the relieved foot surface of routinely prepared shoes.(7) Perhaps the most serious shortcoming of most commercial shoes (and, unfortunately, many handmade shoes) is the beveling of the heel of the shoe. (Fig. 15) This traditional beveling reduces the ground-contact surface of the shoe and moves the resultant bearing surface dorsally. Unfortunately, many farriers cut and bevel the heels on their handmade shoes in a similar fashion, (29) presumably because of tradition and aesthetics of the finished product.
The single most frequently encountered abnormal foot conformation has been identified as underrun heels, often accompanied by a long toe. (36) The potential for this conformation to promote chronically bruised dermal structures within the palmar (plantar) part of the hoof capsule and musculoskeletal injuries elsewhere in the limb is easily underestimated because of the prevalence of this conformation in many performance horse breeds. As the musculoskeletal demands of these horses increase, the absence of proper heel support becomes a more limiting factor. Decreasing the ground-contact surface of the hoof or shoe unnecessarily should be avoided. The authors would encourage manufacturers of commercial horseshoes to discontinue beveling the heels of their shoes at angles that approximate expected dorsal hoof angles. For sound biomechanical reasons, the heels of horseshoes should be finished perpendicular to the branch of the shoe.
Farriers are challenged to provide optimal heel expansion and coverage in performance horses that minimizes the possibility of shoes being pulled off and maximizes the period of time between resets that the quarters and heels are properly supported by the branches of the shoe before the hoof overgrows the branches. Farriers are often unfairly criticized when shoes are lost on horses that were shod with appropriate expansion and heel coverage.
Successful athletic performance rests on sound trimming and shoeing techniques. Guidelines for appropriate hoof preparation, shoe fit, and shoe modification for performance horses have been provided with the admonition that individual conformation and the specific athletic activity must be considered. It is important to remember that the hoof capsule is constantly growing and consequently the ground-contact position of the hoof is continually moving dorsally, abaxially, and distally, relative to the fetlock. Most horses whose hooves grow 6 - 9 mm per month should be trimmed (and shod, if shoes are necessary) every 6 - 8 weeks. For athletes, reshoeing (trimming and fitting new shoes) or resetting (trimming and refitting previously used shoes) is necessary at regular intervals that may vary from as little as 4 - 5 weeks for some hunters, performance horses, and racehorses shod with minimal expansion to as much as 2 - 3 months for some gaited horses whose hooves grow very slowly and uniformly.
Veterinarians must be knowledgeable about fundamentals of farriery and should impress upon owners and trainers the necessity of regular and timely hoof care. For the successful resolution of many forms of lameness, veterinarians must be educated regarding the diverse effects of farriery on the limb kinematics and kinetics, and the importance of encouraging normal hoof conformation and recognizing abnormal foot configuration. In 1980, William Moyer wrote: "If the configuration of the foot is abnormal, it is either the direct cause, a complicating factor, or an effect of the basic [lameness] problem. An imbalanced foot or an improper shoe will create or increase the signs of musculoskeletal disease."
Appreciation is expressed to Drs. B. Grant, K. White, H. Clayton and M. Alberts. Farriers R. Luikart, S. Alley, O. Batt, and M. Chance provided technical advice and assistance.
Innovative Animal Products, 6256 - 34th Avenue, N.W., Rochester, Minnesota, 55901.
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23. Hill, C. and Klimesh, R. (1996) Shoeing for soundness. In: Horseowners Guide to Lameness. Ed: T.S. Stashak. Philadelphia, Lea & Febiger, p 377.
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27. Lessiter, F. (1996) Check Shoeing Rules and Regulations from 191 Horse Organizations. Am. Farriers Assoc. 22(6), 260-277.
28. Lockner, F.K., Milne, D.W., Mills, E.J., and Groom, J.J. (1980) In vivo and in vitro measurement of tendon strain in the horse. Am. J. Vet. Res. 41, 1929-1937.
29. Luikart, R. (1993) Standards for Judging Farriery. Lexington, Kentucky, American Farrier's Association Publishing, pp 10-13.
30. Lungwitz, A. (1891) The changes in the form of the horse's hoof under the action of the body-weight. J. Comp. Pathol. Therap. 4(3), 191-211.
31. Moyer, W., and Anderson, J. (1975a) Lameness caused by improper shoeing. J. Am. Vet. Med. Assoc. 166, 47-53.
32. Moyer, W., and Anderson, J. (1975b) Sheared heels: Diagnosis and treatment. J. Am. Vet. Med. Assoc. 166, 53-55.
33. Moyer, W. (1980) Corrective shoeing. In: The Veterinary Clinics of North America, Large Animal Practice: Symposium on Equine Lameness. Ed: W. Moyer. W.B. Saunders Company, Philadelphia, USA. p 15.
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40. Riemersma, D.J., Van Den Bogert, A.J., Jansen, M.O. et al. (1996) Influence of shoeing on ground reaction forces and tendon strains in the forelimbs of ponies. Equine Vet. J. 28(2), 126-133.
41. Russell, W. (1903) Scientific Horseshoeing for Leveling and Balancing the Action and Gait of Horses and Remedying and Curing the Different Diseases of the Foot. Robert Clark Co., Cincinnati, OH, p 117.
42. Schryver, H.F., Baretl, D.L., Langrana, N. et al. (1978) Locomotion in the horse: kinematics and external and internal forces in the normal equine digit in the walk and trot. Am. J. Vet. Res. 39, 1728-1733.
43. Simpson, J. (1968) The theory of shoeing and balancing. In: Care and Training of the Trotter and Pacer. Ed: J. Harrison. United States Trotting Association, Columbus, OH, USA. pp. 293-372.
44. Stashak, T.S. (1987) Adams' Lameness in Horses, ed. 4. Philadelphia, Lea & Febiger, pp 91-94, 98, 799.
45. Stashak, T.S (1996) Examination for lameness. In: Horseowners Guide to Lameness. Ed: T.S. Stashak. Philadelphia, Lea & Febiger, p 85.
46. Stephens, P.R., Nunamaker, D.M., and Butterweck, D.M. (1989) Application of a Hall-effect transducer for measurement of tendon strain in horses. Am. J. Vet. Res. 50, 1089-1095.
47. Thompson, K.N., and Herring, L.S. (1994) Metacarpophangeal and phalangeal joint kinematics in horses shoed with hoof caulks. J. Equine Vet. Sci. 14(6), 319.
48. Turner, T. and Stork C. (1988) Hoof abnormalities and their relationship to lameness. Proc. Am. Ass. Equine Practnrs. 34, 293-297.
49. Willemen, M.A., Salvelberg, H.H.C.M., Bruin, G., and Barneveld, A. (1994) The effect of toe weights on linear and temporal stride characteristics of Standardbred trotters. Vet. Quar. 2, S97-S100.
50. Wilson, P.D., Ratzlaff, M., Grant, B.D., Hyde, M.L. and Balch, O.K. (1992) The effects of a compressible plastic shoe - the Seattle Shoe - on the kinematics of the strides of galloping Thoroughbred horses. J. Equine Vet. Sci. 12(6), 374-381.
51. Wilson, D. and Keegan, K. (1995) Pathophysiology and diagnosis of musculoskeletal disease. In: The Horse: Disease & Clinical Management. Eds: C.N. Kobluk, T.R. Ames, and R.J. Geor. W.B. Saunders Company, Philadelphia, USA. p. 608.
- Fig. 1 Absence of dorsal alignment of the hoof and pastern creates broken-forward and broken-backward hooves.
- Fig. 2 Hoof-flight patterns: Presumed and actual. Drawings A, B, and C are adapted from A. Dollar's 1898 classic depiction of the effects of hoof angle alterations on hoof flight. These drawings were not based on cinematographic studies. Unfortunately, these quite inaccurate basic patterns are still displayed in current textbooks. Drawing A represents the presumed hoof flight that is produced by a normal hoof angle and characterized by a uniphasic parabolic curve with its maximal height centered on the parabolic curve. Drawings B and C display the presumed effects that either lowering or raising the hoof angle would shift the point of maximal height of the parabolic curve either caudally or cranially, respectively. Cinematographic and videographic studies of equine locomotion have demonstrated definitively that hoof flight patterns are biphasic; the first maximum which occurred shortly after toe off is always higher than the second maximum, which occurred shortly before initial ground contact. Drawing D represents seven limb positions in a stride representing 6 horses trotting at 4 m/s and shod with normal angle and hoof length. (Balch et al 1996)
- Fig. 3 Effects of hoof angle modifications on the joints, tendons, and ligaments of the distal portion of the forelimb. Raising the hoof angle flexes the distal interphalangeal joint, flexes the proximal interphalangeal joint slightly, and extends the metacarpophalangeal joint very slightly. Lowering the hoof angle has the opposite effects on these joints. Raising the hoof angle decreases tension in the deep digital flexor tendon and increases tension in the superficial digital flexor tendon. Lowering the hoof angle increases tension in the deep digital flexor tendon. (Adapted with permission from Balch et al., 1995) Hoof balance and lameness: Improper toe length, hoof angle, and mediolateral balance. Compend. Contin. Educ. Pract. Vet. 17(10), 1275-1283)
- Fig. 4 Hoof protractors, calipers, and rulers are used to measure quantitatively hoof angle and hoof length.
- Fig. 5 Attaining mediolateral balance through the commonly used geometric-limb-axis-oriented technique. Line A represents the axes of the cannon bone and phalanges. The yellow wedge labeled C is the portion of the hoof necessary to be trimmed if the solar surface of the hoof is to be made perpendicular to line A. (Adapted with permission from Balch et al  Factors involved in the balancing of equine hooves. J. Am. Vet. Med. Assoc. 198, 1980-1989)
- Fig. 6 Attaining mediolateral balance through result-directed trimming. The left front hoof lands first on the lateral aspect of the wall. Result-directed trimming dictates that the lateral side of the hoof be trimmed shorter so that the medial and lateral heels land simultaneously. (Adapted with permission from Balch et al  Factors involved in the balancing of equine hooves. J. Am. Vet. Med. Assoc. 198, 1980-1989)
- Fig. 7 Relative position of the hoof in relationship to the rest of the limb. Line B represents the "angle of incidence" defined by the axes of the phalangeal bones. Line C bisects the metacarpus (metatarsus) and extends distally to brush the palmar (plantar) border of the heels on the ground surface of the wall. (illustration adapted from Russell, W.  Scientific Horseshoeing for Leveling and Balancing the Action and Gait of Horses and Remedying and Curing the Different Diseases of the Foot. Robert Clark Co., Cincinnati, OH, p. 99)
- Fig. 8 Use of a shoe to extend the ground-bearing surface of a limb with angular deformities. Illustration A depicts a weanling with multiple limb angular deformities: mild carpal valgus with lateral rotation and severe fetlock varus with medial rotation. The angular deformity of the fetlock displaces the force vectors that are associated with the horse's weight laterally, relative to the ground surface of the hoof; the upright, heavily loaded lateral heel and the flare of the wall at the medial toe are consequences. Illustration B uses red vertical lines adjacent to the medial and lateral borders of the fetlock to suggest the appropriate ground support for the limb. Illustrations C and D depict the appropriately trimmed hoof and the trimmed and shod hoof with a lateral extension, respectively. As suggested by the contours of the fetlock, osteoarthritis was confirmed radiographically. Shoeing with a lateral extension was palliative, not curative.
- Fig. 9 Nail placement based on traditional farriery protocols. Heel nails should be no further palmar (plantar) than the widest part of the hoof (represented by line A). Areas delineated by B are the portions of the shoe that extend abaxially to provide support for the expected normal expansion of the hoof.
- Fig. 10 Clips stabilize shoes by reducing shear forces on horseshoe nails.
- Fig. 11 Nail hole placement in hand-forged and commercial shoes. The nail holes in these recently manufactured commercial shoes are punched too far caudally. However, the nail holes are punched a varying distance from the outer circumference of the shoe to compensate for the differing position of the white line and the differing thickness of stratum medium at the quarters and the toe. Older commercial shoes often lacked this refinement. Nail holes in the hand-forged shoe are appropriately placed.
- Fig. 12 Angle of nails in hand-forged and commercial shoes. Relative to the branch of the shoe, nail holes in hand-forged shoes should be punched to match the differing slope of the wall at the toe and quarters. In contrast, nail holes in many commercial shoes are punched perpendicular to the branch of the shoe; the absence of slope to match the contour of the wall unnecessarily increases mechanical stress on the wall when the nails are driven and clinched.
- Fig. 13 Beveling of the heels of commercial and handmade shoes. All shoes are displayed with their ground surface facing upwards. The heels of the first four shoes, commercial shoes of various types, are beveled in a fashion that decreases the ground-bearing surface of the shoe. The heels of the handmade shoe are finished perpendicular to the branch and do not decrease ground-surface area.
Table 1 - Guidelines for hoof length based on the weight of the horse:
Small 360 - 400800 - 9007.6 3.0
Medium 425 - 475950 - 10508.253.25
Large 525 - 5751150 - 12508.93.5
These guidelines apply to most breeds of nongaited horses; however exceptions
occur. If the horse will be barefoot in a turnout environment, an additional
0.5 to 0.6 centimeter (3/16 to 1/4 inch) of wall and a thicker sole is