Biomechanics as a Guide to Therapeutic Farriery

Biomechanics as a Guide to Therapeutic Farriery

Andrew H. Parks, MA, VetMB, DACVS

Biomechanics provides at least two different types of information relevant to therapeutic shoeing. First, it provides information about the relationship between the distal phalanx and the hoof capsule, and the relationship between the phalanges, navicular bone, and metacarpus/metatarsus. Second, it provides diagnostic information that can identify which structure is injured and the nature of the injury. This discussion will focus on the latter. Additionally, radiography is a valuable aid for placement of a shoe in relation to the position of the distal phalanx.

Every therapeutic shoe uses a few biomechanical principles:

Changing the distribution of force across the ground surface of the foot, either symmetrically or asymmetrically (Figure 1A)
Changing the center of pressure (also called point of force) (Figure 1B)
Changing the moments about the distal interphalangeal joint (Figure 1C and 1D)
Changing the rate of deceleration as the foot lands (Figure. 1E)

For a review of the basic biomechanics of the equine foot…please see here.

Figure 1A. Distribution of force. 1 shows ground reaction force GRF concentrated on the distal hoof wall. 2 shows that a pour-in pad will distribute the GRF symmetrically across the solar surface of the foot. 3 shows asymmetrical distribution of force.

Figure 1B. The ground reaction force (red arrow pointing upwards) is the force exerted by the ground on a body (the foot). The center of pressure (CoP) it that point through which the ground reaction force acts on the foot. The CoP can be changed by various farriery maneuvers as noted by the change of position of the arrows on the ground surface of the foot which will change the distribution of stresses within the structures of the foot.

Figure 1C. Changing the moments about the distal interphalangeal joint (DIPJ). As an example, when heel elevation is added, it shortens the deep digital flexor tendon and thereby decreases the tension in the tendon and changes the position of the joint in relation to the center of the foot, both of which change the moments about the DIPJ. This is effective at rest and in motion.

Figure 1D. The moments about the distal interphalangeal joint may also be altered during the breakover phase of the stride by adjusting the position of breakover in the shoe; in the schematic on the left, the point of breakover is fixed whereas the foot breaks over a series of points in the schematic on the right; therefore, breakover starts with a shorter moment arm in the image on the right.

Figure 1E. Changing the rate of deceleration. Placing a pad that compresses between the foot and the shoe will supplement the natural hoof mechanism during impact. An example may be the popular Castle Impak® pad.

Symmetrical or asymmetrical distribution of load (weight) on the ground surface of the foot can be used to either decrease or increase pressure on any given area of the foot. For example, pour-in pads distribute the ground reaction force broadly across the ground surface of the foot whereas using a shoe with a narrow web will concentrate the ground reaction force on the distal surface of the wall. Asymmetrical distribution is used to apply pressure to a focal section on the solar surface of the foot (Figure 1A ). Asymmetrical distribution of the ground reaction force on the ground surface of the foot will cause the center of pressure to change and is likely to have a variety of effects depending on the structure (Figure 1B). For example, in the frontal plane, placing a wedge on one side of the foot will move the center of pressure towards the side that is elevated. The moments about the distal interphalangeal joint in the sagittal plane can be changed by elevating the toe or heels, a maneuver that is effective both at rest and at exercise (Figure 1C). The moments about the distal interphalangeal joint during the break over phase of the stride are affected by the position of break over on the shoe (Figure 1D). The distal interphalangeal joint exhibits limited movement in the frontal plane, so there are also moments about the joint in the frontal plane. Changing the rate of deceleration as the foot lands is primarily directed at decreasing maximum deceleration (Figure 1E), which may be achieved by incorporating a material between the foot and the shoe that compresses during impact. It should be noted that changing any one of these principles is likely to change at least one other. Additionally, it should be noted that while there are only a few biomechanical principles to follow; however, there are many ways to implement them.

Determining a therapeutic shoeing strategy requires several sequential questions to be considered and addressed:

What is the structure affected?
Is the structure stressed under tension or compression?
Is the tension or compression affected by weight bearing at rest and/or by movement?
What biomechanical principles should be applied to decrease the tension/compression in the affected structure?
What shoeing (or other appliance) manipulations (modifications?) will achieve the desired result?

The affected tissue is identified by a combination of the physical examination, diagnostic imaging, and diagnostic analgesia. Ligaments and tendons are stressed under tension while bone and joints are primarily stressed under compression. Additionally, the lamellae are stressed under tension and the tissues of the sole are stressed under compression. The application of biomechanical principles and shoeing manipulations are best demonstrated by examples.

Desmitis of the collateral ligament of the distal interphalangeal joint is not an infrequent MRI diagnosis. As a ligament, it is stressed under tension, that is anything that tends to lengthen the ligament will increase the tension in it. Therefore, the therapeutic goal is to shorten the ligament and limit how much it can be stretched. At rest this is accomplished by moving the center of pressure towards the side of the ligament which will minimize the opening of the joint on that side and thus minimize lengthening the ligament. The joint on the affected side will also tend to open and the ligament lengthen when a horse is turning away from that side. To minimize lengthening the ligament as the horse turns, the associated moment in the frontal plane can be decreased. The simplest way to move the center of pressure to the affected side if the horse is standing on a deformable surface is to make the branch of the shoe on the affected side wider than the contralateral branch. To ease movement away from the affected side, the opposite branch of the shoe is rounded (Figure 2).

Figure 2. Collateral ligament injury. Illustration A shows the concept of moving the center of pressure toward the affected side. B shows a wider branch of the shoe that effectively moves the CoP. C is an example of an aluminum shoe with a unilateral wide branch which will have a flotation effect on a deformable surface.

Asymmetrical bone bruising in the subchondral bone or loss of cartilage on either side of one of the interphalangeal joints that are identified on MRI should logically be treated in the opposite manner to collateral ligament injury. That is, the structures are stressed under compression and therefore, the center or pressure should be moved away from the affected side and break over eased on the affected side.

Figure 3. Unilateral compression of the distal proximal phalanx. Illustration A shows the concept of moving the center of pressure away from the affected side. B shows a wider branch of the shoe that effectively moves the CoP toward the contralateral side of the foot. C is an example of an aluminum shoe with a unilateral wide branch.

Deep digital flexor tendon injury within the foot is also a relatively common MRI diagnosis. As a tendon, decreasing its length decreases its tension (force). Therefore, the goal is to shorten the tendon when standing at rest and the stance phase of the stride and limit its tension at break over. The tendon can be shortened at rest by applying a wedge to the heels and at break over by reducing the lever arm as break over begins by moving the point of break over in a palmar direction (Figure 1C & 1D).

There are several reasons why therapeutic shoeing might be ineffective or fail:

The disease is too severe for any therapeutic shoeing to be effective.
The wrong principles have been applied.
The correct principles have been applied, but to an insufficient degree to be effective.
The correct principles have been applied, but so severely that other structures are damaged.

Additionally, there are occasions when an injury is present, but the severity of the disease or severity of the symptoms are sufficiently mild that the injury is likely to heal regardless.

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