Introduction

Lameness originating in the foot is common. In many instances the cause can be ascribed to a "recognized" disease, such as pedal osteitis or navicular disease; this is based on physical examination supported by regional anesthesia and radiographs with little regard for the conformation or balance of the distal limb because, as useful they are, they do not change the diagnosis. However, in many other instances a lameness may be isolated to the foot, but further definition of the problem by radiography and nuclear scintigraphy is lacking; the best approach to treatment without a specific diagnosis is then in part based symptomatically on the balance and conformation of the distal limb.

The foot is anatomically defined as the structures within and including the hoof, and the digit describes the limb distal to the metacarpophalangeal joint. For practical reasons, this distinction is frequently blurred. The hoof is the integument of the foot and the hoof capsule is the cornified epithelium of the hoof.

Conformation and balance, both terms used to describe the distal limb, are unfortunately sometimes used interchangeably because of some natural ambiguity. For the purposes of this discussion, conformation is used to describe the size and shape of the distal limb excluding the hoof; conformation of the distal limb is therefore the sum of the shape and size of the individual anatomical structures and the relationship between them. Balance is considered to be the relationship between the hoof and the other anatomical structures of the distal limb, and between the hoof and the ground. Conformation is determined with the horse at rest and does not change much and then only slowly. In contrast, balance is both a static and functional concept that applies to the horse at rest and in motion, and is subject to rapid as well as slow change.

The form and function of the hoof can change in response to several influences. Rapid distortion of the hoof capsule occurs in response to prolonged steady alteration in intramural stresses because of its viscoelastic nature. Imbalance between hoof wear and growth directly alters the length of the hoof capsule and secondarily influences its angle and shape. Uneven growth around the circumference the circumference of the coronary band occurs in response to uneven stresses within the wall. Identification of these changes in form of the foot require an appreciation for what constitutes a normal foot.

The "normal" foot

The notion that there is a single pattern for the ideal foot is probably flawed because the ideal may vary with breed and use. Given this limitation, the size of the normal foot is related to the size of the horse, and although systematic documentation is lacking some practical guidelines are available for the forefeet. ^1,2

The shape of the foot is defined by a series of ratios that relate the size, and angles that relate the position of any one structure to the other structures of the distal limb. Though conformation and balance are three dimensional concepts, they are described from three viewpoints, laterally, dorsally and from the solar surface.

From the lateral aspect, the pastern and dorsal hoof wall are parallel, that is, the foot-pastern axis is straight. The angle of the foot-pastern axis is approximately 50-54 degrees. The angle of the heels is within 5 degrees of the pastern. The length of the toe is approximately related to the weight of the horse: 7.6 cm for a 360-400 kg horse, 8.25 cm for a 425-475 kg horse, and 8.9 cm for a 525-575 kg horse. An imaginary line that bisects the third metacarpus reaches the ground at the palmar border of the weight bearing surface of the foot. The coronary band slopes evenly towards the heels, at which point it may dip less acutely distally.
From the dorsal aspect, an imaginary line that bisects the third metacarpus bisects the pastern and foot. This same line is perpendicular to the ground and another line drawn between any two corresponding points on the coronary band. The highest point of the coronary band is at the center of the hoof and slopes gradually both medially and laterally. The medial wall is frequently slightly steeper than the lateral wall.

From the solar surface, the foot should be approximately as wide as it is long. The contour of the foot is smooth and almost circular. The foot should be approximately symmetrical about the axis of the frog; the lateral side may be slightly (5%) wider than the medial side. The width of the frog is 50 - 66% of its length. Both heels are of equal length. A line drawn across the weight bearing surface of the heels is perpendicular to the axis of the frog.

History

The signalment is useful to raise or lower the index of suspicion for a specific problem. The duration, speed of onset, progression of disease, treatment if any, and response to treatment are standard questions to be addressed for any lame horse.
The trimmin and shoeing history is important. In any horse, the shoeing intervals and the time since the last shoeing are important because the foot length and angle change between shoeing, and lameness soon after the last shoeing may be related to nail placement. Equally important is whether the onset of lameness coincides with a change in the way the horse is shod or with a change in farriers. The age the horse was first shod may give an indication to the relationship between shoeing and the lameness in young horses.

Physical Examination

Visual examination of the feet and distal limbs are made in conjunction with an assessment of the overall conformation of the horse to identify angular deformities, from base narrow/wide to varus/valgus, and rotational deformities arising at any level in the limb before more specifically examining the digit and foot. The appearance of the distal limb is compared to the ideal "normal" foot. In addition to examination from the three perspectives, the heels are evaluated from the palmar aspect with the foot on the ground, and the relationship between the ground surface of the foot and the axis of the limb examined by holding the metacarpus horizontally and sighting along the limb and down across the solar surface of the foot. Though balance and conformation are most frequently subjectively evaluated, evaluation by accurate measurement and recording enhances awareness, emphasizes small discrepancies and provides objectivity for future comparison.

Foci of pain are identified by careful palpation, methodical application of hoof testers, and flexion and extension of the interphalangeal joints. Paring the foot to expose clean horn allows evaluation of the white line for areas of separation, and the sole for hemorrhage in unpigmented areas. Inspection of the wall identifies the pattern of growth rings, regional decreases or increases in convexity, vertical and horizontal fissures, and hemorrhage originating from the coronary band.

The shoes themselves, and their positioning and attachment to the foot are examined. The type of shoe is evaluated for protection to the ground surface of the foot, amount of traction it is likely to provide, position of breakover, presence of supporting devices such as extensions and pads, and for potential effect on animation. The shoes should be examined for wear to indicate the direction of breakover, and the size and fit are examined in relation to the size and shape of the foot.

Observation at a walk should determine the manner the horse lands and breaks over, as well as the path of the foot during the flight phase of the stride. Toe first landing or excessively heel first landing indicate compensation for pain or dorsopalmar imbalance. Similarly, medial or excessively lateral heel/quarter first suggest mediolateral dynamic imbalance. The flight of the foot during the stride is correlated with rotational deviation of the limb and imbalance of the foot. The horse is observed at a trot for lameness including lunging, but evaluation of balance at gaits faster than a walk necessitate videorecording with or without a treadmill. Further evaluation with regional anesthesia may be required before radiography or nuclear scintigraphy.

Lameness originating in the foot is frequently associated with secondary foci of pain caused by compensation for the primary problem, e.g., suspensory desmitis in the forelimbs, and tarsal and back pain with the hindlimbs. These primary and secondary foci of pain need to be correlated.

Radiology

The radiographic findings of diseases affecting the osseous structures of the digit are well documented. Of equal importance, but given less consideration, is the relationship between the hoof capsule and the osseous structures. A range of parameters has been evaluated in a detailed study of young thoroughbreds, ³ and a second study has evaluated smaller numbers of a spectrum of different breeds. ⁴ A radiodense marker centered on the dorsal hoof wall and another at the apex of the frog aid correlation of the soft tissue features of the radiographs and the foot. A linear radiodense marker in the block the horse stands on for radiographs delineates the ground surface of the foot. To consistently evaluate this relationship accurately, a consistent radiographic technique is essential. There should be even weightbearing between both forelimbs. At least one lateral radiograph should be centered on the solar margins of the distal phalanx so that the 2 palmar processes are superimposed and. To evaluate the interphalangeal joints other laterals may be necessary. The most salient observations include, the relationship between the dorsal hoof wall and the parietal surface of the distal phalanx, the thickness of the soft tissues between the dorsal hoof wall and the parietal surface of the distal phalanx, the thickness of the sole, the angle of the sole of the distal phalanx to the ground, and the relationship between the center of rotation of the distal interphalangeal joint and the weight bearing surface of the foot.

A true dorsopalmar radiograph must be similarly taken centered on the solar surface of the distal phalanx with both forelimbs bearing weight evenly. The marker on the dorsal hoof wall at the center of the toe should be superimposed on the central sulcus of the frog. A block with a built in swivel has been recommended to allow the horse to stand in a natural position ⁵ because twisting the foot after it is planted on the block has been shown to introduce error in interpretation. The most salient features include the symmetry of the distal interphalangeal joint space, the relationship between the articular surface of the distal phalanx and the ground, and the relationship between the distal phalanx and the medial and lateral hoofwalls.