Combination of Navicular Bone Shape and Workload Predicts Disease

The shape of a horse’s navicular bone is a distinct factor in the development of navicular disease, according to a new study from a team of Dutch researchers.

Further, the study provides new evidence that navicular disease results from biomechanical overload of the navicular structures. In other words, a horse with a certain shape of navicular bone and that works too much is likely, over time, to develop navicular disease.

“This study confirms and elucidates the results of our [1995] study,” says Kees J. Dik, DVM and Ph.D., at the University of Utrecht, The Netherlands. In that study, Dik and his colleague, J. van den Broek, defined a standardized classification system to describe the shape of a navicular bone and used a standardized grading system to classify the radiological navicular bone condition.

Researchers have long known that radiographs of the navicular bone of a horse with navicular disease typically show a differently shaped bone than those of horses without the disease. But whether a horse gets navicular disease because its navicular bone shape is weak and so can’t withstand workload, or whether workload changes the shape of a normal navicular, causing navicular disease, has been a point of controversy. Dik’s work indicates the former is the case.

“The relationships between the shapes and the radiological changes indicate a shape-dependent distribution of the biomechanical forces exerted on the navicular bone,” Dik says. According to Dik, the proximal articulating edge of the navicular bone shows one of four shapes: it may be concave (shape 1), undulating (shape 2), straight (shape 3), or convex (shape 4). These shapes are genetically determined and appear to determine the horse’s susceptibility to developing navicular disease.

Shape 4, for example, distributes load well and is the least susceptible to navicular disease. In comparison, shape 1 does the poorest job of distributing load, leading to early breakdown of the navicular structures. This breakdown eventually results in changes to the navicular bone that are apparent in radiographs that Dik classifies according to “grade.”

Grades 0 and 1 indicate a normal navicular bone. Grade 2 represents a “fair” condition, and grades 3 and 4 reflect more severe conditions.

The navicular bone lies at the junction of the second and third phalanges (the short pastern bone and the coffin bone) and is fairly readily x-rayed. But the disease involves the navicular bone as well as the associated soft-tissue structures. Radiographs show changes to bone, only.

“Particularly in the early stage of the disease, the overload may be limited to these soft tissues,” Dik said. “Therefore, in such cases significant radiological changes will be absent.” But the shape of the horse’s navicular bone could provide an early-warning sign for a high probability of development of navicular disease.

In addition, Dik has found certain breeds of horses are correlated with a higher prevalence of shape 1 and grades 3 and 4—in other words, with the changes associated with navicular disease. Dutch Warmbloods, for example, show a higher prevalence of shape 1 and grades 3 and 4 than do Friesans or Finnhorses.

But while a horse may be predisposed to navicular disease due to the shape of the navicular bone, the researchers wondered if age was also a factor. If a horse has a particularly shaped navicular bone, is it more likely to develop navicular disease under regular use by a certain age?

Dik and his colleagues looked at 746 normal and 174 clinically affected Dutch Warmbloods. In both normal and affected horses, shapes 1 and 2 (the shapes most susceptible to navicular disease) had the highest prevalence in the horses aged 3 to 4. In normal horses, shape 1 significantly decreased in the older age classes.

“The relative increase of the shape 4 prevalence in elderly normal horses is a process of selection, i.e. the survival of individuals with the best shape conformation,” says Dik.

Not surprisingly, in the affected horses prevalence of shapes 1 and 2 was significantly higher at all ages.

Overall, only 15 percent of normal horses had either grade 3 or 4 lesions present—which represents the most severe navicular condition, yet with no sign of disease. But 85 percent of affected horses had grade 3 or 4 lesions.

“This demonstrates the diagnostic value of this standardized radiological grading of the navicular bone condition,” Dik says.

In the normal horses, the researchers found a significant, age-independent association between shape and grade. The risk for grades 3 and 4 decreased as the prevalence of shape 4 increased.

In the clinically affected animals, grade was associated with age across all four shapes—for example, grades 3 and 4 were significantly more prevalent in older animals.

“This indicates that biomechanical overload may result from age-related accumulation of workload,” he says.

In affected horses aged 5 to 9, the researchers found a sharp increase in the percentage of affected horses with grades 3 or 4 lesions and shape 3. This percentage remained high in the older age groups (ages 5 to 9 years, 69 percent; 10 to 14 years, 83 percent; 15 to 19 years, 100 percent).

A similar jump for affected horses with shape 4 didn’t show up until age 10 to 14 years. (From 20 percent of five horses at ages 5 to 9 to 83 percent of six horses at ages 10 to 14.)

“Therefore, despite the different vulnerability of the four shapes, eventually biomechanical overload is also harmful to the stronger shapes 3 and 4,” Dik says.“Overall,” he adds, “the size, shape and balance of the foot, as well as the shape of the navicular bone are the most important conformation factors.” But is it possible to define the appropriate quantity of work for each shape class?

“Considering the many factors influencing the performance of sport horses, such a study probably will be a mission impossible,” Dik says. “Nevertheless, we are following a group of 40 young Dutch Warmblood horses submitted to a standardized training program.” Preliminary results should be available in 2004.

Navicular Disease and Heel Pain: A Damaging Synergy

Veterinarians and researchers have long known that a horse with navicular disease adopts a toe-first landing posture in an attempt to avoid putting weight on the painful heels of the affected front feet. New research indicates, however, that the toe-first landing increases the load on the navicular bone, which actually worsens the condition and sets up a continuous cycle of damage.

“Imagine you have a stone in the heel of your shoe,” said Dr. Alan Wilson, a leading researcher of the disease at the Royal Veterinary College (RVA) in Hertfordshire, England. “You will tiptoe, and you will especially not heel-strike, because a rapid impact would hurt,” he said.

But to land toe-first, a horse must contract the deep digital flexor muscle, which in turn flexes the joint between the coffin bone and the short pastern bone. When that joint, called the DIP joint (distal interphalangeal joint) is flexed, the horse lands toe-first. But a flexed DIP joint transfers the load of the horse’s stride onto the navicular bone—the very structure the horse is trying to protect. Wilson and his colleagues measured this load in horses with navicular disease and found it was high enough to cause damage to the cartilaginous structures that overlie the navicular bone and even reshape the bone itself.

Researchers still don’t know what triggers navicular disease. Does the horse adopt a toe-first landing because of other issues, thereby causing damage to the navicular bone? Or does the toe-first landing occur in response to navicular disease that has set in due to other factors such as heredity, poor blood flow to the area, conformation faults, improper shoeing or injury?

“A variety of factors will be involved,” Wilson said. “But perhaps any heel pain leads to increased navicular bone load.” And a higher load on the navicular bone is likely to reshape the bone and result in signs that can be seen radiographically, and in some cases lead to pathology and damage, he said.

Navicular disease has been identified for hundreds of years and today, researchers estimate it causes one-third of all chronic lameness conditions. It has been described as “osteo-arthritis-like” and involves degeneration of the navicular bone, the navicular bursa, the associated fibers of the deep digital flexor tendon (DDFT) and related ligaments.

The RVC researchers believe biomechanical overload of the navicular structures is responsible for navicular disease. In order to test their theory, they measured the stress on the navicular structures at the trot in six normal horses and eight with navicular disease. Previous research had only measured the force exerted on the navicular bone in healthy Dutch Warmblood horses. Force, however, is too general a measurement when considering the potential for damage to cartilaginous structures, according to Wilson. Stress, on the other hand, is a measurement of force per unit area, so it provides a picture of what the involved structures undergo during each stride. When they measured the peak force and stress on the navicular bone during the trot, both groups of horses had similar results.

“But the force and stress in the horses with navicular disease were approximately double the control group values early in the stance phase,” Wilson said. What’s more, the force on the bone remained high through stance to heel off.

In 10 of the 16 legs of the navicular horses, the highest force on the navicular bone was recorded at about 20 percent of stance while in all the normal legs navicular bone force peaked at around 85 percent of stance—in other words, highest force in navicular horses comes on much earlier in their step as they try to protect their heels. Zero percent of stance, Wilson said, is the foot just hitting the ground, while 100 percent is when the foot has left the ground.

In a separate study, Dr. Polly McGuigan and Wilson tested the theory of heel pain as the source of the toe-first landing. With kinematic and forceplate analysis, they measured the force in the front feet of seven horses with navicular disease at the trot before and after administering analgesia (or blocking) to the heel region. After analgesia, the force on the navicular bone was lower throughout the stance.

“It is interesting that the peak force for the navicular disease horses was no higher at the end of stance,” Wilson said. It may be that, as the force on the DDFT (flexor tendon) increases through stance, it exceeds the isometric capacity of the muscle and the muscle fibers extend transferring the DDFT force back to the accessory ligament,” he said. Contraction of the DDF muscle at least partially unloads the accessory ligament, according to Wilson. “This would return the bone force to the normal state in late stance.”

Proving this theory would require detailed individual measurements on horses with navicular disease, Wilson said. But such a mechanical response to heel pain could explain the development of boxy feet in horses with long-term navicular disease, he added, since unloading the heels may, over time, result in contraction of the heels and an upright hoof angle.

Wilson is now seeking funding to enable further investigation into the effect of different shoeing practices on the forces in horses with navicular disease.

Kathleen Hentcy