Evaluating the Navicular Bone

Indepth Equine Podiatry Symposium Notes Written and presented January 2010 by R.F. (Ric) Redden, DVM

Much has been written about this small but very significant bone found in the heel area of the horse's foot. The term navicular disease sounds benign enough and many believe it to be a specific disease that is easily diagnosed. However, the contrary would be more exact. In spite of intense efforts by researchers, anatomists, pathologists, surgeons and diagnosticians, this area continues to hold many secrets. The syndrome itself is very complex as it involves not only the bone itself, but also adjacent support structures and anatomical zones that house or protect this very sensitive area. Therefore, diagnosing navicular disease is not straightforward and should never be approached with a black and white attitude. Many years ago it was thought that increased pain identified by hoof testers over the center frog across the heels, a positive response from a PD nerve block and a lollipop or two in the 65° DP radiograph was all that was needed for a navicular diagnosis. However, there are many horses with very significant appearing lesions that never develop soundness problems and remain competitive at high levels and others that have no demonstrable lesions but have chronic heel pain.

Treating navicular disease can be just as confusing as diagnosing it. Often the coffin joints and/or bursa may be injected with hyaluronic acid and/or corticosterioids. The horse is then sent to the farrier, who is expected to provide a solution given only a very generic prescription: raise the heel and rocker the toe and/or apply an egg bar shoe with wedge. This can be a difficult task without a point of reference. Treatment can be further complicated if the horse is on a tough training schedule, and the owner, trainer, etc. are reluctant to rest him unless absolutely necessary. You must ask yourself, how can such a complex, mysterious syndrome have such a vague treatment plan? Examining the unique characteristics of each case and determining the mechanical needs of the foot in question, rather than the syndrome as a whole, can provide a host of treatment options that can set the stage for successful treatment and management of this syndrome, lifting some of the stigma that has long been attached to it.

Examining the Foot

Developing an astute eye for detail can help all concerned detect unique characteristics that can alert us to potential problems, prevent lameness issues and show the effect of chronic problems. When evaluating the exterior of the foot, avoid lumping all feet into the same category as other feet. As we well know, feet are not born equal and certainly don't remain in a static state. They are constantly changing due to growth, use, wear, trimming, shoeing and pathology. The unique characteristics of feet become more noticeable as you develop your eye for detail. Studying photos taken perpendicular to the subject is a great way to train your eye to see these characteristics. Radiographs taken in the same plane can help us tie the external characteristics with what is happening inside. Given time and keen focus, we can begin to see the imaginary radiograph as we look at the foot and vice versa.

The basic points of interest that make feet appear different are hoof angle, growth rings and shape of the toe, quarters, heel and ground surface. There are several stereotype feet within all breeds with a large range of foot sizes, shapes and growth patterns. The majority of horses regardless of breed will have a high/low syndrome (one foot has a steeper hoof angle with more heel mass). Even though it can be very subtle, the highest profile foot will have characteristics of a low grade club foot. These small differences influence the overall health of the feet. Growth rings that are quite close together indicate slow growth. This may occur at the toe with club feet, the heel with the negative PA foot, the medial quarter for horses that naturally list to the medial quarter, etc. Wide growth rings indicate accelerated growth and can occur at the heel in club and laminitis feet and at the toe in negative PA feet.

The amount of sole depth can greatly affect the way we evaluate and treat navicular cases, therefore it is important to note this important measurement and identify what category it falls under:

  • 20-25mm: very thick

  • 15-20mm: adequate

  • 12-15mm: moderately thin

  • 5-12mm: thin5mm or less: super thin

Wall thickness goes along with the thickness and durability of the sole. The thinner the sole, the thinner and more fragile the wall and vice versa.

Pastern alignment in the healthy foot will lie in the same plane as the face of the hoof wall. The club foot pastern sets forward on the hoof, which puts the linear face in the same plane as the hoof. The heel and buttress shape varies greatly depending on PA and foot mass. The club or upright foot has adequate to excessive heel mass, which creates a thicker, more durable appearing heel. The bar size, shape and overall strength is relative to hoof shape, PA and overall health of the foot.

The frog plays a major role in protecting the heel structure, aiding circulation to the foot and acting as a major energy sink when healthy. Note the width, depth and length and compare it to the frog on the opposite foot. Seldom are they identical. The higher heel will normally have a smaller frog set well into the depths of the heel. It will be quite small compared to that on the opposite, lower profile foot. The crushed heel foot will normally have a large, thick frog that may be well below the crushed heel tubules. Observing these basic characteristics every time you look at a foot soon helps you develop an astute eye for detail. The success of any and all therapeutic shoeing protocols is relative to the amount of foot mass and PA that we have to work with. Farriers need radiographic information and an eye for external characteristics that indicate what is available for them to work with.

Hoof Testers

Use hoof testers carefully when diagnosing a foot problem. A tough, dry foot with good mass will be unresponsive even when brewing a full blown abscess. A soft but fragile foot with minimal mass will be responsive even when the only pathology present is lack of foot mass. Young training horses almost always test sore over the frog and across the heel, which is more often than not just part of training soreness. Stop and ask yourself just how many sensitive structures are being influenced by the testers. Hoof testers are great for quickly locating commonly occurring abscesses, but be careful how you read the response when using them routinely for generalized foot lameness.

Radiographic Exam

Radiographs are certainly indicated when navicular syndrome is suspected. The views you take are relative to the information you seek. I never rely on one view to confirm a lesion. If I can't see it on at least three views I am not satisfied it exists. Note that if you use a digital unit you may find it difficult to penetrate the navicular bone enough to read the trabecula pattern. I prefer traditional film with grid over the majority of digitals I have used as I can design my exposure any way I need it. Digital does not allow this flexibility and the resolution and detail of portable digital units does not meet the standard consistently attained with my traditional film/grid combination. Yes, you can enhance the digital image by adjusting the contrast, but only within the range of the unit's programming. This can be a disadvantage, especially when you need to shift either well below or above the pre-set range of penetration. Before referring to traditional film grid techniques as dinosaur technology, take a very close look at the lesion that can easily be demonstrated with a versatile technique that you most likely will never see with your digital unit. My preferred radiographic views for identifying navicular lesions are as follows:

  • Lateral: When taking laterals for suspected navicular cases I use traditional medium speed film, asymmetric screens and 6:1 grid with high beam alignment centered over the navicular bone. If the wing of PIII has calcified well into the lateral cartilage it will be superimposed over the tendon surface of the navicular bone, obscuring the detail I seek. However, you don't know this unless you look. The tendon surface angle (TSA) is measured by drawing a line along the distal 1/2 of the navicular bone and connecting it with a line along the ground surface of the foot. It may be quite low, 10-15°, or extremely high, 35-45°. Concentrate on this area as most all navicular pathology occurs along the distal half of the bone and support ligaments. Being able to alter this angle offers a reliable means of managing the pain response.

Using the lateral low beam radiograph, measure the TSA of the navicular bone. Use this angle as a guide when designing your shoeing protocol for reducing tendon tension. Also note bone angle, as it can vary greatly between horses and on the same horse.

  • Flexed lateral: I take this view with a grid in the Redden Navicular Block™ and center the beam at the subject of interest. Make note of the TSA as it relates to the ground surface. When the tendon surface is perpendicular to the ground while positioned in a 65° DP block, the image produced will be a true DP representation of the bone. When the primary beam strikes the face of the navicular bone at a less than or more than 90° relationship with the ground, a large variety of images are produced. This gives rise to a large number of concepts and theories concerning radiographic lesions as there are few if any comparative views when the 65° DP is made without regard for the TSA. The image obtained will vary greatly simply due to a wide range of beam/subject positioning. Therefore by using the flexed lateral view we can observe the relationship of the primary beam to the navicular bone when the 65° DP view is produced. Cortical lesions and areas of calcification within the impar ligament can be described with this view. Remember to center the beam over the navicular bone to avoid image distortion.

  • DP: The straight on DP high beam is my third view. This view requires high MAS and a grid to produce the optimum image. PII must be all but burned out to see the cortical and trabecula pattern of the navicular bone. If you do not see a lesion with hard penetration and grid on this view, it is not likely you will see it on any other. However, deep penetration is required and your digital most likely will not allow you to go there. You will get a very generic look at bone shape and only enough detail to visualize the larger, more notable lesions.

  • 45° DP with grid: This view sets the wing of the navicular bone in relief and offers a nice view of lesions that may be located within the impar lig