This abnormality, therefore, may not only increase overall navicular pressure, but result in differences in pressure from one side to the other.Īffected horses often have a history of intermittent thoracic limb lameness that improves with rest. Since the navicular bone lies across the back of the foot, it experiences differences in forces from one side of the foot to the other. where the heels are at different levels relative to the ground surface. Underslung heels, therefore, result in increased concussive pressure on the navicular bone. Frequently, the navicular bone lies just above the point where the heel meets the ground and the force is the greatest. Underslung heels are farther from the center of gravity of the limb, and are therefore subject to greater weightbearing or concussive forces.
The greater the distance between the foot and the center of gravity, the greater the weightbearing force on the back of the foot. Weightbearing force is therefore greatest on the palmar aspect of the foot closest to the limb’s center of gravity.
Due to the anatomy of the fetlock joint, a horse’s center of gravity is actually behind (palmar to) where the foot rests on the ground surface. The more DDF tension, the more pressure on the navicular bone. The more delayed the breakover, the greater the DDF tension prior to advancing the limb. This conformation delays the breakover of the limb. Increased DDF tension results in increased pressure across its fulcrum (the navicular bone). Since this tendon stretches very little, increasing its traveling distance increases its tension. This conformation results in an increased distance between the DDF tendon’s origin in the DDF muscle along the back of the radius and its insertion on the palmar aspect of the third phalanx. Since it is our goal to keep your horse performing at its intended level, we will only discuss factors which are treatable. can all play a role in the development of navicular inflammation. The horse’s breed, size, type of work, frequency of work, intensity of work, conformation, etc. Horses can have excessive DDF tension for a number of reasons. Both may also occur as a result of excessive DDF tension.
Other contributors to navicular inflammation include navicular bursitis (inflammation of the navicular bursa) and venous congestion/hypertension within the navicular bone. However, if the pressure is of high intensity and/or persistent, inflammation (and pain) may ensue. In this situation, horses will remain asymptomatic. If the pressure is of low intensity and/or of limited duration, navicular bone remodeling may successfully absorb, diffuse, and redistribute the load. This results in a higher hard bone:soft bone ratio which increases the bone’s stiffness and decreases its compressibility. During remodeling, osteoblasts (hard bone cells) are laid down along the palmar surface of the marrow cavity. However, if the pressure is persistent and/or severe, the navicular bone will begin to remodel in an attempt to adapt to the biomechanical stimulus. In the normal situation, the navicular bone responds by compressing. Since the DDF tendon uses the navicular bone as a fulcrum, increased DDF tension results in increased pressure across the navicular bone. Simply, inflammation in the palmar foot or navicular area develops as a result of excessive tension of the deep digital flexor (DDF) tendon. The navicular bursa is a synovial “sac” between the navicular bone and DDF tendon and serves to lubricate the tendon as it slides over the back of the navicular bone. The soft marrow interior allows the navicular bone to compress when under pressure. The navicular bone consists of a hard cortical exterior and a soft cancellous interior (medulla or marrow).
It serves as a fulcrum for the deep digital flexor (DDF) tendon as this structure passes around the back of the foot to attach on the third phalanx (P3 or “coffin bone”). The navicular bone resides palmar to the coffin joint in the back 1/3 of the foot. Although the pelvic limbs may be affected, for all intents and purposes it is a disease of the thoracic limbs. Navicular inflammation, often referred to as “Navicular Disease” or “Podotrochleosis”, is one of the most common causes of intermittent, often-shifting, thoracic limb lameness in horses between 4 and 15 years of age.
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