by Scott Morrison DVM
As many of us have witnessed the foot is a common source of lameness. It has been estimated in multiple retrospective studies that the heel region of the foot alone accounts for more than one third of all chronic lameness in the horse. It shouldnâ€™t be a surprise that the only region of the horseâ€™s body in constant contact with the ground is most susceptible to trauma and injury. The foot is the part of the body that first receives the forces generated during ground impact. A healthy foot can dampen the vibrations generated during ground impact by as much as 80-90%. The role this small structure plays in shock absorption to protect the structures above it is amazing.
The heel region in particular is designed for this purpose and houses the structures most responsible for absorbing shock such as the frog, digital cushion, collateral cartilages, bars and an extensive vascular system. All these structures work together to absorb and dissipate shock. Therefore, for the heel to accommodate this task it needs to have healthy and fully functional structures.
At higher speeds, when shock absorption is most needed, the heel should contact the ground first. Horses with poor heel structures and those which land toe first, donâ€™t utilize the heel to absorb shock and as a result commonly present with lameness issues further up the limb. Repetitive toe first landing is an inefficient and dangerous landing pattern. It can be seen in horses that are fatigued or lazy and donâ€™t fully extend the shoulder; it can also be seen in the long toe, low heel syndrome or in cases which have preexisting heel pain and land on the toe in an effort to protect the heel. It is however, normal for horses to land toe first when landing from a jump. Besides bypassing the heelâ€™s shock absorption function, toe first landing has been shown to actually put more strain on the deep digital flexor tendon and navicular apparatus. A normal heel first landing puts a gradual increase in strain on the deep digital flexor tendon, whereas toe first landing puts abrupt high peak strains on the tendon, almost like a snapping action. Visualize a normal horse galloping, as the limb is fully extended the heel impacts the ground, as the horses body passes over the limb (and the limb is now in the vertical position) the toe region sinks into the ground and the fetlock displaces downward (full extension of fetlock joint). This pattern slowly increases tension on the tendon; in fact as the limb is fully loaded the toe is sinking into the ground creating a heel wedge effect, which helps take some strain off of the tendon during this position. In a toe first landing pattern the toe would impact the ground, then as the body passes over the limb the heel rocks backward as the fetlock descends downward. This "out of phase rotationâ€ of the foot and fetlock has been shown to put high strains on the deep digital flexor tendon and navicular apparatus.
Horses which are used for jumping tend to be predisposed to "navicular region painâ€. MRI studies of horses with navicular region pain often show the damage and pain is to the deep digital flexor tendon. Treatment of these cases requires shoeing to decrease strain on the tendon, rest and proper physical rehabilitation. Acute injuries may benefit from intralesional injections such as stem cells, platelet rich plasma and extracellular matrix products.
More importantly, how do we reduce or help minimize the occurrence of such injuries? First, make sure the foot is properly balanced and shod. Ideally the coffin joint should be in the center of the footâ€™s weight bearing surface. Many feet develop low heels and a longer toe. This puts the foot out of balance, the coffin joint is placed more towards the back of the foot and a lengthy toe lever is created, putting more strain on the deep digital flexor tendon and potentially creating a toe first landing (image at right shows a foot with poor balance). Some feet naturally have a low heel and longer toe and canâ€™t be changed, but they can be managed and helped with trimming and shoeing. Rolled toes, rockered toes and fitting the heels with adequate support are ways to compensate these feet and keep the coffin joint joint in the center of the footâ€™s weight bearing surface.
Besides good farriery and foot balance, the ground condition or footing is also an important factor. Footing that is too soft or too hard can interfere with the dynamics of limb loading. Ideally, the footing should be soft enough to allow the toe to sink in. If you look at a hoof print in a grass field, you will notice the imprint is slightly deeper at the toe. Footing that is too soft will put more stress on soft tissue structures and footing that is too firm can cause foot bruising and generally causes more wear and tear on the boney column.
It is important to keep the feet on a strict shoeing schedule and not allow the toe to get too long. Working a horse that is overdue for trimming/shoeing increases the risk of injury. Finally, keeping the horse fit and well conditioned for his/her job can eliminate fatigue as a source of abnormal foot landing patterns. Understanding the biomechanics of the equine foot, and good management practices play an important role in the prevention of foot lameness.
Understanding Ground Impact (PDF)