Foot Pain

Stephen E. O'Grady, DVM, MRCVS

Navicular disease / navicular syndrome are the most commonly diagnosed causes of foot pain in the horse. True navicular disease needs convincing evidence on radiographic examination of bone damage to arrive at this diagnosis. Navicular syndrome on the other hand has no useable definition of what it is and one cannot accurately pin down the exact location of the pain with local anesthetics or radiographs.

True navicular disease cases localized to or originating from the bone are the minority. Navicular disease would be attributed to lesions of the navicular bone such as navicular impar fractures, one large cystic lesion or multiple large cystic lesions within the body of the navicular bone or a lytic area located on the flexor surface of the navicular bone as noted on the skyline radiographic view. These horses are chronically lame and the disease will always be accompanied by some type of hoof capsule distortion (damaged heels, contracted heels, narrow prolapsed frog, etc.). Bone spurs on the wing of the navicular bone (usually lateral) are hard to use in the diagnosis of navicular disease as they are seen on many sound horses with off-set feet. Treatment for navicular disease generally involves a low palmar digital neurectomy accompanied by suitable therapeutic shoeing to promote the health and proper mechanics of the foot. Therapeutic shoeing alone seldom resolves the lameness with true navicular disease.

Navicular syndrome (I prefer to call it foot pain) presents with many of the same symptoms as navicular disease but without radiographic lesions involving the navicular bone. Ultrasonographic examination can be used to access the soft tissue structures in the palmar portion of the foot and may yield a diagnosis in a limited number of cases. The horse will become sound with palmar digital anesthesia and often with a coffin joint block. Hoof capsule distortion or some type of damage to the heel area of the hoof capsule will generally accompany palmar foot pain. We then have to consider whether this is the cause or result of the foot pain. The heels of the hoof will generally be low or under-run and result in a broken back hoof pastern-axis (Figure 1).

Fig. 1. Note the short shoe and the negative palmar angle.

 Damage to the structures of the hoof capsule in the heel area along with the abnormal hoof pastern-axis lead to further problems such as:

• Weight bearing is concentrated palmarly to where it is placed on and overloads the compromised structures in the heel rather than weight bearing being placed along the entire solar surface of the third phalanx (coffin bone).
• As the heels collapse, weight bearing is shifted to the soft tissue structures in the heel portion of the foot namely the frog, digital cushion and the deep digital flexor tendon.
• Loss of protection in the heel area due to damage of these structures leads to constant bruising
• Damage to the structures in the heel allows the palmar process of the coffin bone to move downwards creating a negative palmar angle relative to the angle at the toe.
• Animal will begin to land toe first due to the discomfort in the heels.

So called navicular syndrome (foot pain) as compared to navicular disease is treated with therapeutic shoeing and analgesics. As discomfort is thought to originate from the soft tissue structures of the foot; improving the mechanics of the foot and restoring weight bearing to those structures for which it was intended will often result in soundness. Figure 2 shows pastern axis and heels corrected with glue-on shoe.

Fig. 2. This is the foot shown on the radiograph in Fig 1 after shoeing

Now so called navicular “changes” on the ventral border of the navicular bone - what are these?

Wouldn't these be "wear and tear" changes from one set of radiographs to another based on or related to what the horse does as an athletic pursuit? To use these so called “changes” found on radiographs when taken as either baseline films or as part of a pre-purchase examination for their predictive value regarding future soundness, we would have to take into consideration the age, breed, athletic activity (how much), surface on which the horse is worked, hoof conformation, position of P3 within the hoof capsule, how the horse lands, farrier care, etc. If we start with a healthy functional foot on the initial set of radiographs and then note a compromised or distorted foot with damaged heels on subsequent radiographs (Figure 3a-b); wouldn't this account for the "changes" seen on the latest radiographs in most cases?

Figure3a. Note conformation of hoof capsule and position of P3	Figure3b. 6 months later. Note distortion of hoof capsule and negative palmar angle of P3

And what do these changes mean? How should veterinarians interpret these “changes”? Wouldn’t it be of more value to make a prediction based on the structure of the foot rather than the radiographs? Perhaps it would be beneficial to take a set of digital photos of the feet each time it is radiographed? Lots of questions with few answers.