Understanding the mechanics of the front leg and the stress put on the deep flexor tendon and thus on the navicular bone, will help you immensely with your comprehension and mainly the prevention of this injury caused for most part by human ignorance.
The name navicular is from the Latin word “navicula,” which means, “boat”. The navicular bone is shaped like a little ship, hence the term for it. In reality it would best compared to either some kind of pulley or even better, a wooden peg around which a rope slides. If we would slide a rope around a wooden peg back and forth, it would wear out the wood in time as well as the rope itself would wear out. The deep flexor slides and presses on the navicular bone and during the final phase of the front leg carrying function (pic.1), the stress is at its high point. Any abnormal stress to the area will result in the damage of the navicular bone. The bone itself has porcelain like texture at the area where the flexor tendon is sliding and pressing. The unusual and unnatural stress will cause the porcelain like texture to become rough (porous like), which will irritate the tendon by increasing the friction between the two. This is the most common initial injury (damage) to the area, which is slow coming (progressing), very seldom noticed or often mistaken for stiff shoulders, since the horse seems to work out of it. It is important to know that the pain in navicular lameness in most cases is more soft tissue related. Knowing this will help you in the management and care for the afflicted horse.
I am amazed at the fact that many of the veterinarians believe that the injury to navicular bone is often caused during the landing phase (impact) of the front leg. The pictures here show the various stresses on the bones, tendons and ligaments during deferent phases of the front leg carrying functions. The picture B shows the forelimb of a horse during the high point of carrying stress. See, that most stress is on the suspensory ligaments and the superficial flexor tendon. Here is where the horses suffer injuries to these hereby stressed anatomical parts besides others, like injuries to the knee sesamoids etc. It is the picture number 1 that shows the maximum stress to the deep flexor and thus the resulted maximum stress to the navicular bone. It is in this final stage of carrying function, just before the front leg “breaks over”, when these anatomical parts of the front leg are stressed to the maximum. This is the key element not only to understanding of this lameness but also to the prevention of it.
Many people unfortunately believe, that the afflicted horse does not wish to step on his heel and that is why he is trying to travel in a short stride. Here is where the misunderstanding comes in effect. The fact is that the horse is actually trying to prevent the stretching of the deep flexor, which is at its high point before the front leg’s “breaks over”. Hence, the further under the horse is the leg in full contact with the ground, the more it hurts. The horse is trying to pick the supportive limb as soon as possible, thus trying to bring the opposite leg that is in the air to the ground as soon as he can. This will result in short-stepping and landing on his toes, as he stretches them toward the ground to reach it. This is the reason for short stride when afflicted by navicular injury and it is not that he doesn’t want to step on his heel as believed. It is only in the more advanced stage, that the horse can feel the pain in his heal due to the soft bone-tissue growth which will then reduce even more the blood supply to the foot (by that time he will be also sore in many other parts of his body as the secondary effects come to existence). If the horse would be preventing the stepping on his heel, thus preventing the stress on it, the lifting of the heel like the use of wedge pads would make things worse, especially when it comes to landing and impact since the heels would land first. Well, the opposite is true, which supports the theory in this entire paragraph and proves the erring of majority of veterinarians.
There are some people that seem to understand this, but they are looking for answers in the wrong places. This lameness was not as frequent in the older days and it afflicted more often the horses that worked in draft. The picture 2 shows the reduction of the stress in the deep flexor tendon by lifting the heel. Horses in heavy-draft, when pulling, have very little stress during the landing, but an immense stress during the final phase of the front leg carrying function just before the so-called break over. (They had to keep leaning into the harness when moving their hind legs under). This cannot be reduced by some training of these horses but only by a proper shoeing adjustment to the work as the picture # 2 shows one example.
In riding horses it is however a different matter that has actually nothing to do with shoeing for the most part, but with the way we ride horses today. Just about all horses in riding industries other than racing, are traveling very heavy and unnaturally on the forehand, thus unnaturally overstressing the deep flexor tendon and often causing the lameness associated with navicular. The racehorse for most part runs and travels in his natural balance, which is somewhat on the forehand. The dressage horse is supposed to present a balanced horse, while in fact the opposite is true in this day and age when these horses perform extended gaits on very overweight forehand. A lame dressage horses was something that almost did not exist when the riding was at its peak, which was about one hundred years ago. The basic school-work (which some people consider as certain levels of dressage) actually served as therapeutic riding, to remove any secondary soreness in recovering from injury by the riding stock. It is inconceivable to cause injury to someone during his therapeutic recovery, unless it is done improperly. To add to this problem of riding out of balance and very heavy on the forehand is the unsuitable surface in many riding arenas. Usually too hard, thus the break over is delayed even further and causing more stress on the critical anatomical part. The dressage today as well as most competitive riding is in the hands of incompetent people who do not understand the nature, the horse and his basic movement and balance factors.
Among others, the western pleasure horses are also one of the most abused in this way, because the discipline requires forward movement with as little as possible impulsion. The entire horse is forced to travel and carry his weight and the weight of the rider on his front legs. On top of it, the arenas are very hard, because the horse in such movement would be tripping in deeper surface, similar as in dressage competition.
This widely spread, out of balance and heavy on the forehand style of riding today is also responsible for the wide spread of improper shoeing. Since most do not understand the mechanics of the horses movement, they do not understand the causes of this lameness thus they look for answers in shoeing. The rolled toe or rocker toe, the wedge pads or shoes, and finally the squared toes hysteria in the last decade testifies to general horsemanship incompetence in the horse world today.
Much nonsense is written in reference to navicular lameness and one of them associates a poor blood supply to the foot as one of the causes. Any two-bid horseman knows that when a horse favors one of his legs it will automatically reduce the blood flow to it (excluding fresh injuries and infections).
The picture B depicts the stress on the front leg during the middle stage of the carrying function. The tendons that are stressed most at this point are: the superficial flexor (5) and the suspensory (3) ligament. Note the deep flexor (4 dotted) is relatively lose during this stage. It is therefore a mistake to believe that the stress on navicular bone and the deep flexor is during the first stage (impact) and the middle stage (supporting) of the carrying phase of the front legs.
The right front is about in the same position of the stress as depicted in the above illustration (B).
The picture # 1 depicts the last phase of the carrying function of the front leg, just before the so-called break-over. It is in this stage that the deep flexor is stressed to maximum, thus the pressure against the navicular bone is also at its highest point. It is here where the navicular bone suffers the most, especially if the weight is increased by the rider and the horse being on the overweight front leg. The picture 2 shows an elevation of the heal, which will reduce the stress as well as it will reduce the stride and roominess of the gait. This is why the use of rolled toe, wedge pads or shoes, as well as an egg-bar shoes will help the afflicted horse. The elevation is not practical for riding horses as some form prevention of the navicular problem, it will distort the gait, its cadence and roominess. Read more on prevention on this site!
This photo depicts a horse very much on the forehand. Opposite of the above. The left front carries the entire weight of the horse and the rider in trot! Note the right hind is off the ground, while the diagonal leg is still solid in contact with the ground. One of main causes of lameness associated with navicular bone. The hard surface on which the horse is performing is also detrimental, especially in the turns, because it will delay the break over to the maximum stride, thus increasing the stress on the deep flexor as well as on the navicular bone. No one will tell you this folks, because the Olympic riders ride just as bad, and who am I or anyone else to tell them that. 🙂
The decrease of blood flow to the leg that is being favored, as well as the reduction in size of the hoof is normal when it is not used or stressed evenly as it should be. Again there is misinformation in many articles that associates the navicular problems with a small hoof. Any two-bid horseman knows, that when he encounters a horse with one hoof smaller and one larger it is for the most part caused by the uneven body weight distribution. In other words, the horse favors or favored the leg for a longer period of time, hence the foot got smaller in time and in many cases even when just a slight pain or discomfort is present for a long time. In reality, the reduction of blood supply to the hoof as well as it’s smaller size are actually side effects of the pain in the vicinity of navicular bone that went unnoticed for a longer period of time. Therefore the use of Isoxsuprine is totally useless for the afflicted horse and the only one who benefits from the prescription and the use of the drug is the veterinarian who usually adds more than the recommended 33% profit margin and of course the drug manufacturer as well. In some cases the Isoxsuprine will work with the Bute, more likely because of the drug interaction.
Understanding the hoof nature can be very helpful in understanding the growth and shape of the hoof.
Poor riding, mainly in the limited spaces of arenas and the improperly (on the forehand) extended gaits (trot gallop), especially on hard surfaces, cause most of navicular injuries. Galloping or extended trotting on the hard surface and riding on the forehand, especially in tight turns, are the leading causes of navicular injury in the working – riding horses today.