Objective Assessment Hoof Balance
Tracy A. Turner, DVM, MS, Dipl. ACVS
St. Paul, Minnesota 55108

The importance of a balanced foot in the treatment of equine lameness is well documented.^1-4 However, the assessment and choice of options for correction of an imbalanced foot can be quite subjective.^1-3,5,6 Balance is defined as the harmonious adjustment of parts. For the hoof, balance has been defined as the equal distribution of weight over the foot.^2,5,6 This must be more precisely defined as equal medial to lateral distribution of weight since more weight is normally placed on the caudal half of the foot.⁵ Caudal to cranial imbalance has been defined as deviation in the hoof alignment or as problems with heel support.

Effective communication between the veterinarian and farrier is imperative for proper treatment of most lamenesses in the horse. Unfortunately, it is often difficult for both to attend the horse at the same time. There can also be a difference in semantics between the professions which can lead to misinterpretation of clinical findings and recommended corrections which can in turn lead to resentment. It is important therefore, to develop acceptable definitions of problems and methods of communicating precisely.

Six hoof balance abnormalities have been described⁷: broken hoof axis, underrun heels, contracted heels, shear heels, mismatched hoof angles, and small feet. Some authors have attempted to define these hoof abnormalities objectively.^2,3,7 A broken hoof axis exists when the slopes of the pastern and hoof are not the same. This condition is further defined as broken-back, when the hoof angle is lower than the pastern angle, and as broken-forward when the hoof angle is steeper than the pastern angle. Underrun heels have been defined as angle of the heels of 5^o less than the toe angle. Contracted heels was defined as frog width less than 67% of the frog length. Sheared heels was defined as a disparity between the medial and lateral heel lengths of 0.5cm or more. Small feet (small feet to body size) was defined as a weight to hoof area ratio of greater than 78 pounds per square inch. The purpose of this paper is to show how measurements of the feet and radiographs can be used to graphically document the presence of hoof abnormalities.

Eleven measurements are made of each foot. The horse's weight is determined with a weight tape or scale. Seven measurements are made of the hoof length with a tape measure: medial and lateral heel lengths, medial and lateral quarter lengths, dorsomedial and dorsolateral toe lengths, and sagittal toe length. These measurements are recorded on a graph to illustrate the general shape of the foot. In addition, the frog's length and width are measured at their longest and widest points. The hoof circumference immediately below the coronary band, and the hoof angle (using a hoof gauge) are also measured. From these measurements, two additional measurements can be calculated: the frog ratio (frog width divided by length) and the body size to hoof area (horse's weight (pounds) X 12.56 / square of the hoof wall circumference(C) (inches)).

A dorsopalmar(plantar) and a lateral radiograph of the hoof can also be used to determine valuable information about hoof balance. The horse must be standing with the metacarpus (tarsus) perpendicular to the ground which can most easily be determined by either the use of a level placed against the cannon bone or the use of a weighted string to align the leg. The radiographic beam should be horizontal and centered on the hoof. Resting the horse's foot on a block to raise the hoof off the ground facilitates these exposures (the opposite limb should be similarly elevated). For easy identification of landmarks, radiopaque markers can be attached to the hoof. A wire placed sagittally (midline) along the toe from the coronary band to the ground, a thumb tack in the apex of the frog and thumb tacks in the most caudal point of the ground contact of each heel emphasize these areas on the radiographs, making their identification much easier.

After plotting the hoof wall lengths, one should have a curve that reflects the shape of the hoof. For a hoof of average hoof angle (48^o-55^o), flattening of the plotted curve indicates that the heels are underrun. A flat curve would also be expected for very upright hooves (>60^o). Generally speaking, the 3 measurements at the toe should be equal. The measurements at the quarter are usually 1-2 cm shorter than the toe (for the average hoof). The heel length should generally be about one-third of the toe length. The remaining measurements are used in the previously described formulas to determine weight to body size and frog ratios for determination of a contracted foot.

Examination of the lateral and dorsopalmar(plantar)(DP) radiographs provides excellent pictorial evidence of imbalance. The lateral radiograph should be evaluated for P2 and P3 alignment which gives insight into the presence of a broken-hoof axis. In addition, the alignment between P3 and the hoof wall should be assessed. If the hoof wall and dorsal surface of P3 are not parallel the functional hoof angle can be determined by measuring the angle of the dorsal surface of P3 with the ground. Usually the slope of the heels can be seen on the radiograph and can also be used to determine whether the heels are underrun.

The DP radiographic projection should be assessed for joint alignment, medial and lateral hoof wall lengths, and foot symmetry. Joint alignment is determined by examining the symmetry of the joint space. Malalignment is present if one side of the joint is more narrow. This phenomenon can also be caused by poor positioning in which case all three of the lower leg joints (fetlock, pastern, and coffin) will be affected. The hoof wall length can be measured directly from the film. The symmetry of weight bearing can be predicted in a similar manner.

Numerous factors contribute to the balance of an equine hoof.^1-7 Toe length is important because it determines the length of the lever arm over which the limb rotates and the timing of hoof lift.⁴ Hence, a long toe, that would delay breakover, could be expected to increase the pressure of the deep flexor tendon over the navicular bone, increase the tension on the proximal suspensory ligament of the navicular bone, and increase the dorsal rim pressure on the joints of the leg.^4,8 The optimal toe length has not been determined. Toe length to a certain extent will be dictated by the horse's use as well as the horse's height and weight. Guidelines have been described that relate toe length to body weight: 3 inches (7.6cm) for 800 to 900 pound horses, 3.25 inches (8.25cm) for 950 to 1,050 pound horses, and 3.5 inches (8.9cm) for horses weighing 1,150 to 1,250 pounds.⁴ A graph of hoof measurements will document disparities in hoof wall length between feet. Lengthening one hoof over its opposite has been suggested as a treatment for limb length disparity in the horse.⁹ However, this condition has not been scientifically documented in the horse. It has been the author's experience that apparent limb length disparities are more commonly due to mild flexural deformities (contracted tendon) rather than actual differences in limb length and that this condition is most commonly manifested as mismatched hoof angles. One study indicated that 28% of normal performance horses may be affected in this manner.³ This can most easily be documented utilizing the lateral radiographs.

The hoof angle should be the same as the hoof axis.^2-7,10,11 Utilizing the lateral radiographic projection, the ideal hoof angulation to properly align the second and third phalanges can be measured accurately. The appropriate correction can be determined by measuring the degree of malalignment (flexion or extension) present in the coffin joint and raising or lowering the hoof angle that amount.¹¹ For instance, if the lateral radiographic projection showed 4^o flexion of the coffin joint, then the hoof angle should be lowered 4^o. In most cases, the aligned hoof axis is 52^o+2^o for the front feet and 55^o+2^o for the back feet.^2,4,5,7,8 Intentional lowering of the hoof angle has been used to increase stride length in racehorses but studies have shown that this is not true⁸; therefore, there is no reason not to shoe for a correct hoof axis and a broken hoof axis can predispose to lameness problems^2,4,5,7 and it has been associated with a greater risk of breakdown in racehorses.¹²

In addition to hoof axis deviations, the lateral radiographic projection can be used to document problems of heel support, i.e. underrun heels. In horses with hoof angles between 50 and 55 degrees, the hoof length graph also documents underrun heels if toe length to heel length ratio is less than 3:1. The proper position of the heels can be determined by either drawing a bisecting line through the metacarpus to the ground.^4,5 or measuring the appropriate position on the radiograph.⁷ Where these lines contact the ground is the point where the heels should be. From a practical point, the heel-ground contact should be even with the base of the frog.² Underrun heels are the most commonly encountered hoof abnormality. In one study of foot related lameness it was found in 77% of the horses² and in another study of normal performance horses this condition was found in 52% of the horses.³ The necessity of correcting underrun heels has been well documented.^1-7 If left uncorrected underrun can cause alterations in hoof wall growth that can be very difficult to correct and it can predispose to lameness problems that range from bruised heels to navicular syndrome.^1,2,7

One of the most difficult parameters to assess is the hoof's ability to expand.^13,14 Applied clinical studies have shown that the frog length to width ratio is useful for this purpose; when the frog's width is at least two-thirds its length, the hoof has normal expansive abilities.^2,3,7 When the frog is narrow, hoof expansion is thought to be reduced. Whether this is a function of frog pressure is not known, although both reduced and excessive frog pressure have been shown to cause hoof contracture.^13,14 Identification of a narrow frog should alert the clinician that steps need to be taken to promote hoof expansion.^2,3,7 These may vary from simply ensuring proper heel support to encouraging hoof expansion through the use of slipper heels.

Medial/lateral imbalance or shear heels has been shown to cause, or predispose to, a number of hoof related lamenesses.^2-4,6,7,15 Medial/lateral balance can be assessed by both the hoof measurements and the radiographic examination. The graph of hoof wall measurements will clearly show if one side of the hoof is longer than the other.a The obvious correction is to make the walls equal, although it is not always that simple.^2-4,6,7,15 The dorso-palmar radiograph will also clearly demonstrate any imbalance. Since this projection will also show the effect the imbalance has on the coffin joint, this radiograph can be used to emphasize the need for correction. The magnification in most radiographs makes even subtle disparities more obvious. It is accepted that conformation can alter this balance.^4-6,15 The radiograph will help determine if the imbalance is hoof related or conformational. Hoof related imbalances will show medial/lateral hoof length disparities, and the first and second phalanges can be bisected equally. If the medial/lateral disparity is conformationally related the first and second phalanges will appear oblique on the DP radiograph.

The final assessment of balance is the weight of the horse in proportion to its feet. Small feet have been a commonly described problem, particularly in Quarter Horses, that predispose the horse to lameness.^2,3,7,16 One study identified small feet as an indicator of poor prognosis in the treatment of navicular syndrome.² Most descriptions of what actually constitutes a small foot are quite subjective. However, studies have been performed utilizing simplistic formulas to make this assessment objective.^2,3,7 These studies measured the circumference of the hoof immediately below the coronary band. This was done to get a rough idea of the hoof cross sectional area. This was then compared to the horse's weight and statistical analysis was performed. Seventy-eight pounds per square inch was determined to be the maximum weight to hoof area ratio for a normal performance horse. The steps to determine this number have been simplified to the following formula:12.56 X wt(lbs)/C2(in2). Once identified, a high weight to hoof area ratio can be used to show a client that their horse should lose weight. In addition, it can be used to show the necessity of fitting a shoe as fully as practical in order to produce the largest surface area as possible for that particular horse's hoof.

Assessment of a horse's hoof balance utilizing hoof measurements is a method that can be easily incorporated into the routine examination of the hoof. It actually takes very little time (about 2 minutes) to make the necessary measurements. Radiographic examination of the feet is a routine diagnostic assessment already performed. By assuring proper positioning and alignment of the hoof during radiographic examination, additional information that can be invaluable in the case assessment can be attained. The information will help show problems of imbalance, can be used to graphically illustrate the problem to an owner or trainer, and can be used to clearly show the shoeing needs of the horse, thus improving overall communications between veterinarian, owner, and farrier.

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