Podotrochlear Syndrome (also referred to as navicular syndrome or navicular disease) affects the podo­trochlear apparatus (PTA) of the equine foot and typically occurs in the forelimbs. [1]

The condition can cause a variable degree of lameness. Horses affected by Podotrochlear Syndrome may display changes in their gait including shorter stride length and a general decline in performance.

There is no single cause of Podotrochlear Syndrome. Multiple structures including bones, tendons, and ligaments within the foot can be affected. [1]

A definitive diagnosis of Podotrochlear Syndrome requires a veterinary clinical evaluation and testing modalities including diagnostic analgesia and radiographic imaging. [1]

Although there is no cure for Podotrochlear Syndrome, the condition can be treated with corrective trimming and shoeing, medications, and non-surgical and surgical interventions. [2]

What is Podotrochlear Syndrome?

Podotrochlear Syndrome is a leading cause of lameness in sport horses and those used for recreational riding. The condition occurs most commonly in the front legs and rarely in the hind legs.

There is no single cause of Podotrochlear Syndrome. The condition is believed to result from multiple injury and disease processes. [3]

Horses with Podotrochlear Syndrome experience pain in the heel of one or more of their feet. The condition affects the podo­trochlear apparatus (PTA) of the equine anatomy.

Navicular Bone in Horses

The PTA includes the navicular bone, the navicular bursa (the fluid-filled sac between the deep digital flexor tendon (DDFT) and the navicular bone), the coffin joint, the collateral ligament, the impar ligament, the suspensory ligament of the navicular bone, and DDFT. [1]

The navicular bone is often affected in horses with Podotrochlear Syndrome, which is why this condition is commonly referred to as Navicular Disease.

This bone is an important component of the distal interphalangeal (DIP) joint which plays an integral role in hoof function and serves as a fulcrum for the DDFT. [1]

Development & Effects in the Equine Foot

The rear portion of the foot in horses affected by Podotrochlear Syndrome can undergo a range of changes due to the condition.

Podotrochlear Syndrome often involves a disruption to the normal bone remodelling processes that occur in the navicular bone. When the rate of bone absorption exceeds the rate of bone rebuilding, degeneration and lesion formation occurs in the navicular bone. [4]

The compact and spongy components of the navicular bone and fibrocartilage tissue located at the insertion points of ligaments and tendons may be eroded in horses with Podotrochlear Syndrome.

The condition may also cause thickening to occur in the compact bone. [1][3]

The altered bone remodelling processes associated with Podotrochlear Syndrome are believed to possibly result from trauma to the bone or compromised blood supply to the bone. [5]

Podotrochlear Syndrome can cause the development of inflammation, lesions, and adhesions in the DDFT and lesions in the collateral and impar ligaments. Inflammation may also occur in the synovial membranes of the DIP and in the navicular bursa. [1][3]

Research demonstrates that the flexor tendons of horses with Podotrochlear Syndrome display a higher proportion of non-aligned collagen compared to horses without the condition. This is thought to occur as part of a process caused by excessive tension and compression forces in the forelimb. [6]

Horses affected by Podotrochlear Syndrome have demonstrated abnormalities in specific biochemical markers associated with collagen, cartilage, and bone formation. [3]

Symptoms of Podotrochlear Syndrome

Horses with Podotrochlear Syndrome may experience unilateral or bilateral lameness that develops slowly or has a sudden onset. Affected horses may have an increased digital pulse, although this symptom is not necessarily indicative of the syndrome.

Some horses with Podotrochlear Syndrome show similar symptoms to those with primary lesions of the DDFT. For example, they may point their lame limb or stand with their forelimbs in a camped-out position rather than positioning them underneath their body.

Podotrochlear Syndrome causes a variable degree of lameness in affected horses depending on how severe the disease process is and if the horse is affected unilaterally or bilaterally.

Horses with two affected legs may demonstrate a shortened stride, whereas horses with one affected leg may show extreme lameness in that limb.

Horses with Podotrochlear Syndrome typically have difficulty making turns. [2] Their lameness may be especially evident when lunging on a circle and when the affected limb is on the inside of the circle.

In some horses with Podotrochlear Syndrome, their lameness is more noticeable on hard surfaces. [2] Affected horses may also have intermittent lameness. [2]

Testing

Compression of the heel or frog with the use of hoof testers may cause pain in horses with Podotrochlear Syndrome, although not all affected horses are reactive.

Flexion testing of the distal limbs in horses with Podotrochlear Syndrome may produce different responses.

Prolonged extension of the DIP joint may accentuate lameness in a limb affected by Podotrochlear Syndrome. [1]

Lameness may appear more severe when the toe is elevated using a board or wedge to incline in. [1] However, this test is unspecific because increased lameness results from the test may be due to a primary injury to the DDFT or the collateral ligament of the DIP joint. [1]

Causes and Risk Factors

Multiple risk factors are associated with the development of Podotrochlear Syndrome.

Horse breeds including Quarter horses, Thoroughbred crossbreds, and warmbloods are most often affected by Podotrochlear Syndrome. [5]

Genetics may play a role in the development of Podotrochlear Syndrome. In a study of Hanoverian warmblood horses, researchers discovered several areas of the genome that are associated with the condition. [7]

Some horses may be genetically predisposed to have irregularities in the nutrient channels of their navicular bones. The nutrient channels in their navicular bones had abnormal sizes and shapes and were not in typical locations within the bones. [7]

Other risk factors that may predispose horses to Podotrochlear Syndrome include being 10 years of age or older and having a weight-to-height ratio of 3.45 or higher.

Horses performing in the sport of showjumping are at a greater risk of developing the syndrome. [1]

Horses with injuries that result in poor blood supply to the rear of the foot may have an increased risk for developing Podotrochlear Syndrome.

Horses born with a divided (bipartite) navicular bone may be more prone to developing Podotrochlear Syndrome and associated pain. A divided navicular bone can also promote injury to the DDFT, thus promoting the syndrome. [1]

Diagnosis of Podotrochlear Syndrome

Horses suspected of having Podotrochlear Syndrome require a veterinary diagnosis.

To determine if a horse has Podotrochlear Syndrome, veterinarians typically perform a digital nerve block on the palmar digital nerves which traverse the back of the pastern and are involved with sensory perception in the equine foot. [1]

If a horse becomes sound following a digital nerve block to the palmar nerves in the foot, he is likely experiencing pain in his heel and may have Podotrochlear Syndrome.

A digital nerve block that fails to eliminate lameness entirely may result if there are adhesions of the DDFT to the navicular bone. However, a digital nerve block that is unsuccessful in relieving pain may also indicate the source of the pain may be higher in the leg.

X-ray & Ultrasound

Radiographs (x-rays) are useful for identifying degenerative changes to the navicular and surrounding bones in horses with suspected Podotrochlear Syndrome.

Lateromedial, dorsoproximal-palmarodistal oblique, and palmaroproximal-palmarodistal oblique images of the foot should be taken. [1]

Ultrasound is used to examine the DDFT and the navicular ligaments in horses with suspected Podotrochlear Syndrome.[8]

MRI

The most reliable method for diagnosing Podotrochlear Syndrome is Magnetic resonance imaging (MRI). This procedure enables a comprehensive evaluation of the bones and soft tissues in the hooves that may be affected.

However, an MRI is a costly procedure and horses must be under general anesthesia when they undergo the test.

Before the use of MRI to diagnose Podotrochlear Syndrome, it was assumed that horses with heel pain and suspected navicular disease were subject to degeneration in their navicular bone.

MRI has facilitated an increased understanding of Podotrochlear Syndrome and the numerous additional structures of the foot that are now known to also be commonly involved with the condition.

Treatment Options for Horses

Although a treatable condition, Podotrochlear Syndrome is not typically cured since it is a progressive condition.

Approximately 65 to 75% of horses treated for the condition show improvement in performance, whereas 40 to 50% of those treated remain sound for one to two years. [3]

Some of the most common treatment options for the condition include the following:

Restricted Movement

Horses diagnosed with Podotrochlear syndrome often require stall rest or limited movement to allow the damaged structures in their feet to recover, if recovery is possible.

Horses with damaged ligaments and tendons may require several months of rest in a stall or small area.

Improving Foot Balance

Corrective trimming and shoeing are critical in horses with Podotrochlear Syndrome to improve the balance of the foot and ensure a level footfall.

A rolled toe egg bar shoe is helpful for easing the breakover of the toe and reduces forces on the heel while providing support to it.

Wedge shoes are used to reduce tension on the DDFT by improving the angle of the pastern.

Managing with Medications

Non-steroidal anti-inflammatory drugs (NSAID’s):

Pain relieving NSAIDs such as phenylbutazone are useful for reducing pain in horses with Podotrochlear Syndrome.

Other medications used in the treatment of the condition include isoxsuprine, aspirin, and warfarin which help to improve the blood supply to the navicular bone.

Corticosteroids:

Some horses with Podotrochlear Syndrome benefit from injections of anti-inflammatory corticosteroids into their affected hooves to control pain. Injections of these drugs into the coffin bone also help to reduce pain in approximately one-third of horses with the condition. [3]

Corticosteroids may be injected into the navicular bursa in horses with Podotrochlear Syndrome, although this procedure increases the risk of rupturing the structure.

Bisphosphonates:

Drugs including Tildren and Osphos may be used to help prevent bone loss in some horses with Podotrochlear Syndrome whose bones of the foot are involved. These drugs work by slowing the processes involved with bone degradation.

Osphos is an intramuscularly administered drug, whereas Tildren is intravenously administered. Each of these drugs must be prescribed by a veterinarian and has associated risks including colic and kidney damage.

A research study involving 11 horses with Podotrochlear Syndrome that were treated with the bisphosphonate drug clodronic acid demonstrated that six horses displayed an improved gait and reduced lameness scores. [9]

At 90 days following treatment, six horses showed a mean reduction of two degrees in their lameness score.

Bisphosphonates cannot be used in association with NSAIDs. They should also not be used for extended periods of time at high doses as bones may become weaker over time.

Extracorporeal Shockwave Therapy

Some horses with Podotrochlear Syndrome may benefit from receiving extracorporeal shockwave therapy treatments, specifically those with damage to their tendons or ligaments.

According to a research study involving 27 horses that were treated and followed up with, extracorporeal shock wave therapy was considered effective in decreasing the lameness associated with navicular syndrome. [10]

Upon evaluation, lameness level was observed to decrease in 81% of the horses as determined by an unmasked evaluator (an observer who had seen the horse prior to treatment) and in 56% of the horses with masked evaluators (observers who had not seen the horse prior to treatment). [10]

Horses with ligaments and tendons affected by Podotrochlear Syndrome are most likely to respond favourably to shockwave treatments since they help to reduce inflammation and pain in the treated area.

Shockwave treatments enhance cell metabolism and blood supply to the targeted region through the application of pressure waves.

Because shockwave therapy produces some level of discomfort during administration, horses must be sedated during the treatment.

Surgical Intervention

Neurectomy is performed on some horses with Podotrochlear Syndrome to desensitize the back portion of the foot to reduce pain sensation.

This procedure is considered as a last resort as it can result in complications including rupture of the DDFT, the development of neuromas, and damage to the foot due to a lack of sensory perception.

A study of 17 horses with Podotrochlear Syndrome that were treated with bilateral navicular suspensory desmotomy, 12 horses became sound following surgery and one improved. [11] Four of the horses treated with the surgery were lame at six months or later following the procedure. [11]

Neurectomy as an intervention for Podotrochlear Syndrome may not prove to be successful long term in some cases. The nerves may reconnect following a neurectomy procedure thus enabling pain perception again which can result in the recurrence of lameness.

Prevention of Podotrochlear Syndrome

A complex condition, there is no specific strategy to prevent Podotrochlear Syndrome. However, to reduce the risk of the condition developing, horses should receive adequate exercise to maintain healthy body weight and to encourage normal blood flow in the foot.

Horses require proper farrier care to keep their hooves in balance to reduce the risk of developing Podotrochlear Syndrome. Good nutrition also encourages healthy foot development in all horses.

If your horse is affected by this condition, it is recommended to work with a nutritionist to ensure your horse’s diet is providing the necessary nutrients to support hoof, bone and ligament health.

Horses affected by Podotrochlear Syndrome benefit from receiving adequate amino acid, trace mineral and biotin intake from their diet to support the structures of their PTA.

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References

  1. Dyson, Sue. Navicular Disease and Injuries of the Podotrochlear Apparatus. Veterian Key. Accessed: 9th September 2021.
  2. Carson, D. Navicular Syndrome in Horses. Accessed: 9th September 2021.
  3. Rijkenhuizen, A. Navicular disease: A review of what’s new. Equine Veterinary Journal. 2006.
  4. Pool, R.R. et al. Pathophysiology of Navicular Syndrome. Veterinary Clinics of North America: Equine Practice. 1989.
  5. Alex Bishop and Peggy Auwerda. Navicular Disease in Western Pleasure Horses. Iowa State University. Equine Science. Accessed: 9th September 2021.
  6. Salinas, P. et al. Navicular Syndrome-related changes to collagen proportion of different cross-sections of the flexor tendons in equine distal forelimb. Research in Veterinary Science. 2021.
  7. Diesterbeck, U.S. et al. Genome-wide search for microsatellite markers associated with radiologic alterations in the navicular bone of Hanoverian warmblood horses. Mamm Genome. 2007.
  8. Grewal, J.S. et al. Assessment of the ultrasonographic characteristics of the podotrochlear apparatus in clinically normal horses and horses with navicular syndrome. Journal of the American Veterinary Medical Association. 2004.
  9. Argüelles, D. Clinical efficacy of clodronic acid in horses diagnosed with navicular syndrome: A field study using objective and subjective lameness evaluation. Res Vet Sci. 2019.
  10. McClure S.R. et al. Extracorporeal Shock Wave Therapy for Treatment of Navicular Syndrome. In: AAEP Annual Convention of the American Association of Equine Practitioners. 2004.
  11. Bell, B.T.L. Surgical treatment of navicular syndrome in the horse using navicular suspensory desmotomy. New Zealand Veterinary Journal. 1996.