It comes as no surprise that when a horse’s feet aren’t even, neither are the forces placed on those important structures. But exactly what effects do uneven hooves have on horses in motion?
A team of European researchers recently investigated whether unevenness influences loading patterns during movement and whether foot conformation or the difference between feet is more important in limb loading. Sarah Jane Hobbs, PhD, the research lead in equine biomechanics at the University of Central Lancashire in England, presented her team’s study results at the 2017 Annual International Hoof-Care Summit, held. Jan 24-27 in Cincinnati, Ohio. She worked alongside Willem Back, DVM, PhD, Dipl. ECVS, Cert. KNWvD, Spec. KNMvD, and his students from Utrecht University, in the Netherlands, and Sandra Nauwelaerts, PhD, from the University of Antwerp, in Belgium, on the project.
In previous studies, researchers found a connection between uneven feet and pain avoidance as well as postural and loading preferences when standing. In one study of elite performance horses, researchers revealed that horses with uneven feet typically retired earlier than those with even feet.
In Hobbs’ study, her team evaluated 34 shod and unshod horses that came into the clinic for routine examinations. Thirteen of the horses had even feet while 21 had uneven feet. The team used proximal and distal (top and bottom) markers on the dorsal hoof wall (the outer hoof wall at the toe) to measure the differences between the left and right hoof wall angle in the front feet. They classified horses with a difference in hoof angle of more than 1.5 degrees as having uneven feet and those with 1.5 degrees or less as even. They also labeled feet as flat (less than 50 degrees), medium (50-55 degrees), or upright (more than 55 degrees).
The researchers set up infrared three-dimensional motion analysis cameras for gait analysis along a corridor lined with rubber mats placed over a force platform. They also placed a cluster of markers along each horse’s cannon bone and long pastern bone and on various points along the hooves to collect kinematic data, which they analyzed with 3-D software. Handlers trotted the horses down the corridor across the force plate so the researchers could evaluate forces at the ground and lower limb movement.
The team also measured the following conformational parameters to see which were better predictors of foot unevenness:
- Hoof area;
- Hoof width;
- Heel height (unloaded);
- Hoof angle;
- Long pastern length;
- Long pastern inclination;
- Cannon bone length; and
- Fetlock angle.
The team found that, of these parameters, hoof angle was the best predictor of uneven feet, followed by unloaded heel height. Hoof width was not a good predictor of uneven feet, Hobbs said. They also determined that the difference in hoof angle between the two front feet had a greater effect on the results than the hoof angle of each individual foot.
When comparing the measurements between the two front feet in the horses with uneven feet, Hobbs discovered several differences in movement patterns and the forces produced at the ground: In the limb with the flatter foot, braking force produced when the hoof was in contact with the ground was greater than braking force in the limb with the steeper foot. In the middle of the stride, when the body sinks to its lowest point, the fetlock joint sunk lower and the vertical force was bigger in the limb with the flatter foot compared to the limb with the steeper foot. The lowest point of fetlock sinking occurred before the force reached its maximum.
After this, the horse starts to rise up and push forward, and the point where the horse stops braking and starts pushing forward happened at a later time in the limb with the flat foot. This was not because the flatter foot was on the ground for longer than the steeper foot, Hobbs said; both hooves spent the same amount of time on the ground.
Because the fetlock sinks more in the limb with the flatter foot and it occurs before the vertical force reaches its maximum, the limb is less stiff during loading. This corresponds to the ‘mechanical lameness’ that veterinarians describe at the walk, where the flatter foot’s fetlock joint visibly sinks more. Because we can’t see this in the trot, researchers have previously suggested that the condition is not associated with pain or pathology (disease). Hobbs’ measurements, however, suggest this might not be the case. The difference was not visible, but the alteration in stiffness and the difference in maximum force might indicate that underlying pathologies do exist. Future studies should be able to confirm this.
The fact that the hooves are in contact with the ground for the same amount of time is not surprising, as the horses were graded sound at the trot. The differences in the timing of the force measurements relate to how big those forces are and where they are acting on the limb and body while the foot is on the ground. So, in the flatter-footed limb the force vector (the arrow coming out of the ground that describes the total force and its direction) is bigger and points more toward the tail for a longer time, which suggests the rear of the limb is loaded longer during each step. As the force vector moves toward the nose (as the limb goes from braking to pushing) there is less time on the ground for this to happen, so pushing forward and breakover occur quicker in this limb. The opposite occurs in the limb with the steeper foot. These differences will load the internal structures of the limbs differently, which is likely to influence the integrity of the hoof and limb tissues and also exacerbate changes in foot shape.
Horses with uneven feet experience different loading patterns during locomotion, which might be partly responsible for changes in foot shape along with habitual asymmetrical standing postures. It’s important to monitor these horses’ hooves, as asymmetrical feet could lead to lameness and possibly early retirement.