Last Updated on March 23, 2022 by Allison Price
Navicular inflammation is a common cause of intermittent, often-shifting thoracic limb paralysis in horses aged between 4 and 15. It can affect the pelvic limbs, but it is most likely a disease of thoracic and thoracic.
Where’s the “Navicular Bone?”
The navicular bones is located in the palmar region of the coffin joint at the back third of the foot. It acts as a fulcrum to the deep digital flexor tendon (DDF), which wraps around the foot to attach on third phalanx (“P3” or “coffin bones). The navicular bones are composed of a hard, cortical exterior and soft, cancellous interior (medulla/marrow). The navicular bone can be compressed under pressure due to its soft marrow interior. The synovial “sac”, or navicular bursa, is located between the DDF tendon (navicular bone) and the tendon. It lubricates the tendon by sliding over the back of navicular bones.
What Causes Navicular Inflammation?
The DDF tendon causes inflammation in the palmar area and navicular region. The DDF tendon relies on the navicular bones as a fulcrum and therefore increases DDF tension causes increased pressure to the navicular. Normal circumstances will see the navicular bones compress. If the pressure is severe or persistent, however, the navicular bones will start to remodel to adjust to the biomechanical stimulus. Osteoblasts, which are hard bone cells, are placed along the palmar surface and marrow cavities during remodeling. This creates a higher ratio of hard bone to soft bone, which decreases bone’s compressibility and increases its stiffness. Navicular bone remodeling can absorb, diffuse, or redistribute pressure that is low in intensity and/or short-term. Horses will not be affected by this condition. If the pressure is persistent and/or intense, then inflammation (and possibly pain) could occur.
What Causes Navicular Inflammation?
Other contributors to navicular inflammation include navicular bursitis (inflammation of the navicular bursa) and venous congestion/hypertension within the navicular bone. These conditions can also be caused by excessive DDF tension.
There are many reasons horses can experience excessive DDF tension. Horse breed, size, type, frequency, intensity, conformation, and other factors can all play a role in excessive DDF tension. All of these factors can play a part in the development navicular inflammation. We will only talk about factors that can be treated, as it is our goal to keep horses performing at their best. Many of these factors are related to conformation. These are the most popular:
- Broken-back Distal Limb Axes This conformation causes an increase in distance between the DDF tendon’s origin in DDF muscle at the back of the radius, and its insertion on palmar aspect the third phalanx. This tendon is very short and increases its travel distance. An increase in DDF tension means that there is more pressure on its fulcrum (the navel bone).
- Low Heels/Long toes This configuration delays the breakover. The DDF tension before advancing the limb is greater the longer it takes for breakover to occur. The greater the DDF tension, it will put more pressure on the navicular bones.
- Underslung Heels. Because of the anatomy of the horse’s fetlock joint the horse’s center gravity is behind (palmarto) the point where the foot touches the ground. The palmar side of the foot closest the limb’s centre of gravity is where the weightbearing force is greatest. The distance between the foot’s center of gravity and the foot’s foot is greater, as is the weightbearing force on its back. Lower-leaning heels are closer to the center of gravity and therefore are subject to greater weightbearing and concussive forces. The point at which the heel touches the ground is often where the navicular bones lies, and this is the area where the greatest force is felt. Therefore, heels that are too low can cause increased pressure to the navicular bones.
- Sheared Heels Unbalanced heels are also known as. This is where heels are situated at different levels to the ground. The navicular bone is located across the back of the feet and experiences different forces from one foot to the next. This anomaly may cause an increase in overall navicular pressure and a difference in pressure on one side.
What clinical signs are there?
Many horses with a history of thoracic limb paralysis have an intermittent history that improves when they rest. The lameness can be seen in one limb, even though both the forefeet and hind feet are affected. Contralateral lameness can only be detected when perineural anesthesia has been used to anesthetize more severe feet. Perineural anesthesia of the contralateral leg often results in a sound horse. Here are some lameness and clinical characteristics that horses suffering from navicular inflammation may exhibit.
- High or contracted heels on the affected foot (often thrush).
- Grade 2-3/5 thoracic lameness of the limbs.
- Consistently point with the affected limb.
- May sit in the stall, with her toes in the bedding.
- Chronic bruising of sole or toe.
- Worse is trotting on the outside of a circle with the affected leg.
- Horses tend to land on their toes during movement.
- The frog of the affected leg is assessed by sensitivity to pressure (hoof testers).
- Palmar digital (PD), perineural anesthesia for the limb, ensures 100% soundness.
How can Navicular Inflammation be Diagnosed?
A clinical diagnosis is Navicular Inflammation. In other words, navicular inflammation is diagnosed by pain in the palmar side of the foot. Clinical examination, hoof testing and anesthesia response to PD perineural are all ways to determine if there is pain. Navicular inflammation is often assessed by radiographs. However, radiographs cannot show inflammation. Radiographic changes in this area are not indicative of navicular inflammation. To determine the presence or absence of pain and navicular inflammation, we must rely solely on clinical findings.
How can Navicular Inflammation be treated?
- CORRECTIVE SHOES
- To decrease DDF tendon tension, keep your distal limbs straight.
- To speed up breakover and reduce DDF tendon tension, trim, broaden and rocker your toes
- To stabilize the heels and prevent shearing, use full (egg-bar) shoes
- To move the point of weightbearing closer towards the limb’s center, extend the shoe palmarly
- MEDICATION DESIGNED FOR ENHANCEMENT OF PERIPHERAL BLOOD
- Aspirin: Prevents platelet accumulation and “thins the blood.”
- Isoxuprine (a vasodilator): Increases vessel diameter
- Phenylbutazone is a vasodilator
- SYSTEMIC ANTIINFLAMMATORY MEDICATION
- Phenylbutazone is a vasodilator as well as an anti-inflammatory
- Previcox is a long-term anti-inflammatory
- SYSTEMIC ARTHROTHERAPY
- Adequan, Legend, Cosequin, Flex-Free, etc. can normalize the synovial environment in the navicular Bursa and decrease inflammation/pain.
- LOCAL ARTHROTHERAPY
- We often perform intrasynovial injections in cases of severe or chronic navicular inflammation. This could involve direct injection of medication into the navicular bursa or indirect delivery via the distal interphalangeal (or “coffin”) joint.
- The product is very effective in relieving inflammation and other signs that can be associated with navicular inflammation.
- EXTRACORPOREAL SHOT WAVE THERAPY – (ESWT).
- Shockwave therapy has been shown to be a very effective treatment for navicular inflammation. This is especially true if there is concurrent deep digital flexor tendon disease (DDFT).
- PERMANENT ANESTHESIA/PD NEURECTOMY
- This is the ultimate “pull your wool over your eyes” technique. This technique reduces the horse’s pain perception, which is a good thing. However, it can also increase the chance of the horse injuring the numbed foot. This approach should not be used as a last resort.
Navicular inflammation is a condition that is triggered by excessive bone remodeling. The DDF tendon exerts excessive pressure on the bone, causing bone remodeling. An inheritance problem or acquired defect in conformation could cause excessive DDF tendon tension. It is possible to manage navigcular inflammation by early detection and correcting predisposing factors.