© Kent M. Thompson, PhD
published in ANVIL Magazine, February 1998.
An introduction to basic mechanical properties which govern movement in the lower limb, primarily those which are exerted by the tendinous/ligamentous structures
To understand the mechanical functioning of the lower limb, we must first discuss the forces which initiate or prevent movement. Several structures are involved in the function of the lower limb including the deep digital flexor tendon, superficial digital flexor tendon, suspensory ligament, digital extensor tendon, extensor branch of the suspensory ligament, laminae and the ground reaction force. To model the lower limb, forces are generated by muscle contraction and relaxation which are transferred by the elastic tissue of tendon and ligament.
Each tendon or ligament generates a turning force (moment) about one joint or several joints. One example of this is the coffin joint which is acted upon by the ground reaction force, deep digital flexor tendon, extensor branches of the suspensory ligament and common extensor tendon. In the standing horse, the following mathematical relationship can serve to define the equilibrium which exists. However, during movement, the equation would change to represent different parts of the stride and it would not be in equilibrium. To simplify the present mechanical definition of the lower limb, we will consider only the forces as they are in the standing horse. If at any point part of the equation reaches its threshold level, then damage to the associated structure will likely occur. For example, if the deep digital flexor tendon component increases by a change in the toe angle, then that structure or related structures may be prone to damage under normal loading conditions.
An illustration is a case where the toe angle has changed to increase the force present in the deep digital flexor tendon. In order to maintain equilibrium, the other tendinous/ligamentous components must change their relative distribution of forces.
Once a mathematical relationship like this is formulated, empirical or derived data must be collected which will either support or disprove the hypothesis. To do this, it is now possible to measure the contribution of each tendon, or ligament structure in the lower limb with devices which measure strain. From these we can calculate the direction and magnitude of the forces which can be used to build a mathematical model of the lower limb. One use of this model is to compare the effect that different farriery techniques have on the relative loads that associated structures must carry.
Hoof balance is a term frequently used, yet the question still remains regarding how to precisely define a properly balanced foot. Balance in the sagittal plane has been associated with the shoulder and pastern angle, yet medio-lateral balance is somewhat more difficult to quantify. Alterations in hoof balance through the addition of wedge pads have been used for many years for therapeutic and performance reasons. In recent years, largedegree wedge pads have been used successfully in the treatment of laminitic horses. However, limited studies are available that quantify the influence of toe angle on the normal kinetic forces of the lower limb.
Equine tendon and ligament injury occurs in many horses which are in race training. It has been reported that injuries due to tendon or ligament damage occur in 10-15% of the horses in training. The etiology of tendon damage is not well defined and likely is multi-factorial. The damage to the tendon or ligament results from a separation or tearing of collagen fibers due to a load applied to the tendon which is above its point of failure. Hoof angle is frequently implicated as one cause of lameness in horses. A large toe angle (>55 degrees) is not generally associated with an increased incidence of musculo-skeletal damage. However, a low toe angle has been implicated as a factor in the onset of lower limb disorders including degenerative bone disease, navicular disease, bone chips, tendon tearing and ligament strains. Once the tendon is damaged, the treatment administered is the subject of debate. However, one part of the treatment should include the removal of as much of the load borne by the damaged structures as possible. This can be done by a combination of splints, bandaging and changes in the toe angle.
The study which I will discuss here was designed to determine the influence that changes in toe angle had on strain in the suspensory ligament, extensor branch of the suspensory ligament, deep digital flexor tendon, superficial digital flexor tendon and the surface of the hoof wall. This information will help us to determine normal mechanical function of the lower limb as well as to determine treatment of lameness disorders including laminitis and various tendon injuries. For this study, each leg was trimmed to a constant angle (55 degrees). Heel wedge pads were then added to create the following toe angles: 58 degrees, 61 degrees, 64 degrees, 67 degrees, 70 degrees, and 78 degrees.
Strain on the deep flexor tendon was reduced with increases in toe angle. The reduction in strain followed a linear pattern with the increasing toe angle at two locations on the deep flexor tendon. Deep digital flexor tendon strains decreased from 2.49 for the 55 degree toe angle to 1.42% and 1.02% for the 70 degree and 78 degree toe angles, respectively. This represented a 59% decrease in tendon strain between the 55 degree and 78 degree treatments. Strain decreased in a similar fashion at the interphalangeal site, as there was a 64% decrease between the 55 degree and 78 degree angles. Strain measured in the superficial flexor tendon was not affected by an increase in toe angle. Within the range of toe angle used in this study, there was no change in strain on the superficial flexor tendon. Initial strain for the control treatment was 2.90% and varied very little with the increase in toe angle. This is similar to that observed by others. The suspensory ligament acted very similarly to the superficial flexor tendon; there was not any change in suspensory ligament strain with the addition of the toe wedges. Strain in the suspensory ligament had a small numerical increase in strain as the toe angle was increased, but the change was not significant. However, a large increase was noted for the extensor branch of the suspensory ligament. There was very little strain present on the extensor branches with the 55 degree, 58 degree, 61 degree and 64 degree toe angles. However, when the toe angle was increased to 67 degrees, extensor branch strain increased dramatically. At the 55 degree toe angle, strain on the extensor branches was .02% and when toe angle was increased to 78 degrees, strain increased to 1.40%.
Strains on the surface of the hoof wall remained in compression during loading and the magnitude of compression increased on the medial and lateral walls with increases in toe angle, and decreased on the dorsal hoof wall. A similar but opposite trend was observed for the dorsal hoof wall as strain decreased 52.8% as toe angle increased. Strain on the medial hoof wall followed the same pattern as the lateral hoof wall. The trend in this case was for a higher strain on the hoof wall as toe angle increased. The difference was noted between the 78 degree toe angle and the 58 degree and 55 degree toe angles.
Heel wedges had no effect on reducing strain in the superficial flexor tendon and suspensory ligament. However, heel wedges are recommended in cases where reduction of strain on the deep flexor tendon is needed, such as laminitis and tendon flexor tendon injury. One other effect noted with the addition of a heel wedge is the large increase in strain of the extensor branch of the suspensory ligament. This may be of particular interest in the treatment of laminitic horses, as reduction of deep flexor tendon strain and increased extensor branch strain both occur with heel wedges and will work concurrently to stabilize the coffin bone in normal and laminitic horses. Heel wedges also raised compressive strains on the surface of the hoof wall at both the medial and lateral quarters, which may affect the occurrence and treatment of quarter cracks.
Thus, heel wedges have a great influence on redirecting the forces that are generated by the tendon and ligament structures in the lower limb. The use of heel wedges will be of great benefit in treating laminitic horses, due to their effect on reducing strain in the deep flexor tendon and increasing strain in the extensor branch of the suspensory ligament.