How to Evaluate the Equine Hoof Capsule

1. Introduction

The hoof capsule is comprised of the hoof wall, sole, frog, and bulbs of the heels, which, through the unique continuous bond between its components, form a casing on the ground surface of the limb that affords protection to the soft tissue and osseous structures enclosed within the capsule.¹  The hoof wall is a viscoelastic structure that has the ability to deform under load and then return to its original shape when the weight is removed. It is well accepted that abnormal weight distribution on the foot or disproportionate forces placed on a section of the hoof will, over time, cause it to assume an abnormal shape.^1–4  These abnormal stresses within the foot will also predispose the foot to injury or disease. Increased stress or weight-bearing placed on a section of the hoof capsule may originate from a single source or it may be from multiple contributing factors such as abnormal limb conformation, strike pattern, amount of work, type of footing, and inappropriate farrier practices. Excess stress placed on one section of the hoof capsule can manifest itself in a variety of ways, such as compressed growth rings, flares or under-running of the hoof wall, dorsal migration of the heels, and either focal or diffuse displacement of the coronary band.^6,7  Distortion of the hoof capsule of the forelimbs appears to be related to limb alignment and load, whereas deformation in the hind feet appears to be different and related to propulsion. Because the hoof capsule distortion of the forelimbs is commonly associated with lameness and various disease processes, only the forelimbs will be considered in this report. Because the “normal” foot has never been defined, each view will begin with what is perceived to be an ideal, good, or healthy foot.^1,8  Palpation of the hoof capsule often complements the visual examination, and the areas where palpation is relevant will be included. The goal of evaluating the hoof capsule is to identify deformation and changes in growth pattern that indicate abnormal distribution of forces (stresses) on the foot. Because hoof capsule distortion and abnormal loading usually accompany lameness, farriery will form part of or sometimes the entire treatment. Farriery is used to help redistribute the load and help improve or resolve the hoof capsule deformation. 

2. Mechanism of Distortion

Evaluation of the hoof capsule morphology will indicate where the hoof wall is unduly stressed; however, the evaluation must be coupled with an understanding of the abnormal distribution of forces that lead to hoof capsule deformation. Understanding the biomechanical forces leading to hoof capsule distortions is also helpful for the clinician in applying the appropriate farriery to modify these stresses. There are many excellent reviews of basic biomechanics of the hoof in the veterinary literature.^1-5 Increased load or weight-bearing by a portion of the wall has three consequences: (1) it may cause deviation of the wall outward (flares) or inward (under-running) from its normal position; (2) it may cause the wall to move proximally; or (3) it may decrease hoof wall growth. A reduction in load or weight-bearing generally has the opposite effect. Briefly, in the standing horse, the weight of the horse borne by the limb is supported by the ground, which opposes the weight with an equal and opposite force. The force exerted on the foot by the ground is termed the ground reaction force. The term center of pressure (COP) is the point on the ground surface of the foot through which the ground reaction force acts on the foot. The center of pressure varies among horses but is approximately located in the center of the solar surface of the foot in the standing horse. However, when the horse is moving, the location of the COP changes dynamically. The position of the COP at any point in the stride determines the distribution of forces between the medial and lateral and the dorsal palmar aspects of the foot. When the center of pressure is moved to one side of the foot, that side of the foot will be subject to increased forces. If the COP is moved in a palmar direction, the weight-bearing or load on the palmar hoof wall is increased. Relating this to hoof capsule distortions, if the COP is located more medially, over time, a medial hoof wall flare (bending) and a lateral under-running will develop. Or, if the COP is located more dorsally because of increased tension in the deep digital flexor tendon, the hoof capsule will develop a higher heel with a flare in the dorsal hoof wall. Farriery is used to change the location of the center of pressure (to some extent) and change the distribution of forces on the ground surface of the foot.

3. Limb Conformation

When evaluating hoof capsule deformation, limb conformation should be considered. Abnormal limb conformation affects the landing pattern and stance phase of the stride. Few horses have ideal limb conformation, and any change in conformation is going to change the distribution of forces within the hoof capsule, leading to deformation. In the frontal plane, the forelimbs should be of equal length and size and bear equal weight. A line dropped from the scapulohumeral joint to the ground should bisect the limb. Certain types of abnormal limb conformation have been described.⁹ In the frontal plane, abnormal conformation is described as valgus (the limb’s segment distal to the affected joint will deviate laterally) or varus (the distal segment of the limb will deviate medially). The joint most often affected is the carpus, and, to a lesser degree, the metacarpalphalangeal joint. Here, there will be excess load placed on the hoof opposite the direction of the deviation. If a line dropped from the metacarpalphalangeal joint through the digit to the ground does not bisect the hoof capsule, the foot is considered offset to one side (usually laterally) and therefore increased load is placed on the opposite side of the foot (Fig. 1). In the transverse plane, conformation abnormalities are characterized by axial rotations of the limb or its segments, either laterally or medially. For example, a horse with a narrow chest and a lateral axial rotation will land on the lateral side of the hoof and then load the medial side resulting in proximal displacement of the quarter /heel on the medial side and causing the hoof deformation termed “sheared heels”^10,11 (Fig. 2). 
 

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