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
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.
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.
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 ligament.
Once I have found lesions in the above mentioned views, I confirm them with the following views:
65° DP with high beam: This view will reveal lesions within the main body of the bone and associated ligaments. Note the TSA and how it lines up on the 65° flexed lateral view relative to the ground surface. This helps us better understand the way the image was made. This view can reveal very significant lesions, but it may fail to reveal large, thin layer cortical flaps that remain attached to the cortex. Shoes should always be pulled for this view as the branches of the shoe can obscure the wings of the navicular bone and cause unwarranted scatter radiation. Even aluminum shoes create significant scatter radiation and reduce resolution.
The moisture content of the frog and buttress plays a major role in just how much you can see on this view. The hard, dry foot requires increased MAS to penetrate the deep structures and even then the resolution will not be as good as on the foot with higher moisture content. I trim the central frog sulci, creating a smooth surface. The deep, dry fissure will always be superimposed over the frog on the 65° DP view and can cause problems with interpretation. Get rid of the shadow with a farrier knife. I do not pack feet for the sake of getting rid of airspace created by the sulci of the frog. All feet have gas density associated with the frog and cup of foot. Therefore I prefer to read it, as I can actually see the anatomy that creates the gas density. When you pack the foot, resolution and detail suffer tremendously. Pack one side and see which has the better resolution.
Tangential (or skyline) taken with grid: This view has been recommended for many years as a reliable means of seeing the tendon surface. Unfortunately this can be very misleading. Unless the TSA is 40° or better, this view with the foot sitting on a tunnel does not reveal the tendon surface, as it is well below the prominence that appears to be tendon surface but is actually the most proximal point of the central crest. Also, when the beam strikes the cassette at any angle other than 90° distortion is automatically part of the image. Therefore using the tunnel guarantees distortion. The farther from 90° the beam hits the cassette the more distortion is created. TSA of 40° or greater offers a better view of the actual tendon surface than the lower TSA that is frequently found with the lower profile foot. The skyline tunnel view of the low TSA and bone is not representative of the tendon surface and the bone is grossly distorted due to the oblique beam angle. Yes, a few lesions in the DDF and along the proximal aspect of the navicular bone can be identified as well as a few lucent lesions superimposed over the bone, but all are distorted when the beam strikes the cassette at an angle less than 90°. The sclerotic cortex and pre-navicular cortex that is often diagnosed is most often the product of distortion and can be misleading and result in gross misinterpretation.
45° 65° DP: When taken with a 6:1 grid This view can be helpful to describe location, size and significance of smaller lesions as well as those found in adjacent tissue. Magnification will occur as the plate is approximately 4-5 inches from the bone itself.
The gold standard when using traditional film and a 6:1 grid is to strive to take all film with a perpendicular beam/film relationship. Center the beam on the subject of interest, use adequate KV and MAS to penetrate the bone and optimum resolution and detail can be obtained.
I like to think we are managing navicular disease instead of treating it. Normally speaking, navicular disease can't be cured, but the painful tissue can heal given the proper environment and the horse can most often be useful once again. Squaring or rockering the toe, raising the heel and applying an egg bar shoe remains the prescription for most farriers. There is some merit in this very vague description of how the horse should be shod, as some will respond favorably and continue doing their work for a while. However, the tough, non-responsive cases require more mechanics (less DDF tension).
In order to design a more detailed, specific treatment plan, both vets and farriers should ponder these questions: Where is the pain? When is it painful? What part of the mechanical formula within the foot is no longer in harmony? Let's assume the pain occurs as the DDF tightens along the inflamed bursa and bone during peak load. Reasons for pain may include a cyst that has suddenly become painful, a cortical surface ulcer, collapsed cortex and extensive abrasions to the tendon as it courses over the rough surface. Fractures of the navicular bone are relatively rare compared to more commonly occurring lesions, but they do occur, as do congenital bi and tripartite navicular bones. Any one or all of the above can be associated with a routine run of the mill case. Horses experiencing pain associated with the navicular bone and support ligaments prefer not to drop their heel during peak load regardless of what lesion is causing the pain. Watch closely and you can see this compensated travel pattern. Some will actually jog sounder on a hard surface than they will on a very soft one, as the heel sinks into the ground on a soft surface, increasing DDF tension and therefore pain.
Soft tissue parameters also need to be considered as they influence our management protocol. The farrier needs to know sole depth, PA, extent of the damage and whether rest is recommended or the horse is to continue light training. All of these factors influence how the horse should be shod so that everyone is happy. The DP and lateral low beam views will reveal how much sole depth we have to work with as well as heel mass and medial/lateral balance. The horse with 20mm of sole and a 3 to 5° PA has options not available to horses with 10mm of sole and a negative 5° PA. If the horse can be shut down (no training) for at least 6-10 weeks, the shoeing protocol will be totally different than the one we would elect for a horse that remains in training.
The Rocker Rail Shoe
The extent and chronicity of the lesion may require moderate to drastic DDF tension release to relieve the hot spot. I like to think of it as simply removing the idle pulley effect of the navicular bone, which has a primary function of reducing stress to the tendon and articular surface of the digits. This can be done with a self-adjusting PA rocker rail shoe that raises PA several degrees. This shoe has potential to drastically reduce friction from the tendon as it glides over the bone. How much raise is needed is relative to the existing PA, TSA and level of pain associated with the lesion or combination of lesions. The goal is to enhance the healing environment by reducing the overall friction reducing function of the navicular bone. This certainly does not cure the problem as the lesion remains and may become increasingly worse, but it can remove a large majority of the pain by drastically reducing the tendon force causing the hot spot. The inflamed tissue can heal as long as it is not being continually traumatized.
The horse with very small navicular changes that is off and on lame and seldom shows more than grade 2 lameness on a tight circle may require a much smaller PA increase and can often go in a full rocker shoe that only raises the PA 3-5°. When that is all that is needed there is no reason to go for the overkill mechanics. You may need that level of mechanics later if the lesion is progressive. The horse can often go back into competitive training following a few weeks of healing as long as the mechanics of the shoe maintain a happy balance between healing and preventing further damage to sensitive tissue.
While there are rare cases with large lesions that show no evidence of pain, most feet with large, distinct lesions will routinely be quite lame. As a rule I put these cases in a rocker rail shoe that allows the horse to establish a 15-18° PA. Turnout and slow trail riding is indicated as soon as the horse is comfortable. Often the pain response is minimal within a few days, even with horses that have been reported lame for many months to years. Daily slow exercise appears to be beneficial as the soft tissue is healing. The high scale case that becomes quite sound with 4-6 weeks of rest can be reset and begin a slow exercise program after the second reset. Many of these cases will remain quite sound in the rocker rail and remain in it when they go back into full training. From barrel racers to pleasure or halter horses, the mechanics of the shoe have not been a problem. Often owners and trainers want to know when the horse can go back into a flat shoe. Stop to think about this. As long as the lesion is there, putting the navicular bone back into a high tension mode will soon start the entire process over once again.
This horse has osseous changes associated with the extensor process and distal PII, as well as navicular lesions. The rocker shoe satisfied the mechanical demands for both problems, relieving the tension of the DDF across the navicular bone and reducing the pinch effect of the lesion at the coronary band, as the shoe prevents overextension. Note the parallel line between the wings of PIII and the heel branch of the shoe. The foot side of the heel branches has been forged to reduce pressure on the tender heel buttress.
Care is due when trimming and applying the shoe.
The positive PA foot needs to have the heel pushed back slightly, starting at the widest point of the foot. The goal is to create a parallel surface between the heel tubules and wings of PIII, all the while maintaining 20+mm of mass between these two planes. This is an important aspect to the mechanics as it increases the length of the heel load zone. Even though heel load is increased, the tension over the navicular bone is significantly decreased.
There is a very distinct landmark at the junction of the skin and frog just proximal to the central sulci of the frog. Passing an imaginary line through the point of the apex of the frog can give you a very good idea of what the PA is. Put a dot of paste on the little area that protrudes at this junction when taking lateral radiographs and observe the relationship. Farriers use this imaginary line to find the plane of the PA. Of course you don't rasp down to this line, as you would be out of foot and into the pink, but use this plane for all rockered shoes. In most feet there is very little hoof that comes off when setting a foot up for a rocker.
Rocker the toe forward of the apex, being careful not to involve the sole directly beneath the apex. Then forge the rocker into the rail shoe with breakover well behind the center of the shoe.
Start forging from the heel of the shoe and work forward. Using a rocker jig is helpful for creating a nice, uniform radius on the ground surface.
It is desirable to have the pivot point or center of the belly directly beneath the center of articulation of PII. Look at PII as if it was a wheel and PIII rotates around this axis. Then think of the belly of the shoe as a pivot point that allows the PA to rock forward. As this occurs, the coffin joint flexes around the center of articulation and the PA and TSA increase. On the low heel foot this pivot point should be slightly behind the widest point of the foot on the club or higher heel slightly forward of the widest point.
Note center of articulation and the path that PIII travels, all due to the pull from the DDF.
Getting the breakover too far forward forces the foot to tip back towards the heel. This is not good, and I see it far too often. The foot must rock forward of center balance to meet the requirements of reducing tension over the bursa and navicular bone.
I normally clip my rocker rails whether glued on or nailed; this is just my preference.
Glue vs. nails:
Glue: When there is minimal foot mass, less than 15mm of sole and a low PA there is very little foot that can come off. Therefore, just rub the heel to create a horizontal plane and touch the toe ever so slightly, just a millimeter or two. Rocker the shoe and set it in a bed of Equilox® or similar composite. After the first six weeks there will be plenty of foot to nail for the next shoeing.
Nails: When nailing the shoe there may be a 1/16 to 1/8 inch gap between the shoe and foot in the center of the belly. This is not a problem. The shoe should sit square and solid on the two front pillars and the heels. A small air gap should be observed between the toe and shoe before nailing. The gap under the belly will soon fill with horn and sole. You can fill this gap with composite if desired, but it is not necessary.
You may need to weld a frog pressure bar in the rocker rail to prevent frog sag and further tubule crush on the low heel foot. If so, keep the frog as dry and tough as possible as it will just flatten out and compress when loaded. This foot will gain a lot of sole but little if any heel mass due to previously crushed heel tubules and crushed digital cushion. I seldom have to put a positive bar on the club foot as the frog is normally well recessed into the foot. The positive bar is technique sensitive and must be carefully fitted with each reset.
Note this horse has a slightly positive PA but a severely broken back hoof axis, which is due to heel pain diagnosed radiographically with significant navicular lesions seen in several views. The upright pastern is a response to heel pain and not the low PA. Note the immediate improvement in digital alignment, which is the result of reduced heel pain.
Fabricating the Positive Frog Bar
I prefer to trim the foot, rocker the shoe, fit the bar tightly in the shoe. Set the shoe on the foot so the bar sits on the posterior half of the frog and the heels of the shoe are approximately 1/16 to 1/8 inch off the heel tubules. Lift it off carefully and weld the bar in place. Soften the foot side heels of the shoe and set it on the foot while in the welding heat to make sure the bar did not move out of the desired position.
Positive frog pressure bar. When this shoe is applied, there should be a 1/16 to 1/8 inch gap between the heel tubules and the heel of the shoe. As load goes on the shoe, positive pressure is applied to the frog and buttress.
Clip the shoes and then cool. Put two nails in and set the foot down, observing the heels as they slowly descend to just touch the shoe surface. If the horse objects to the pressure, pull the shoe, reduce the gap and reapply until he likes it. This shoeing protocol has been a tremendous asset for treating a lot of heel pain cases as it stimulates accelerated foot growth and minimizes the crushing action of the higher PA.
Years ago we nerved everything that was unresponsive to medication. Most of us have found that the downside of nerving demands a lot of respect. There is increased risk of an abscess going undetected for several days to weeks, as the sole and heel are basically numb. Fulmagating abscesses can quickly become life threatening and are a major concern. Routine shoeing has inherent risk of an undetected hot nail, and thrown shoes or puncture wounds are a real threat to horses that are turned out. Think long and hard about nerving a navicular horse. There are better, more reliable and less risky options. When nerving works it is great, but if it goes sour you can be left with a major cripple and possibly a horse that must be euthanized.
Avulsion of the DDF
When the tendon surface lesions or craters are large and have eaten into the tendon, the DDF actively grows into the defect and becomes traumatized with each step, inherently weakening the tendon. Nerve this foot and the tendon can sustain irreparable damage. All of the sudden the toe will turn up and the horse will have an odd shaped heel/pastern profile. This presents the client with a life long problem that can be managed but never goes away. Avulsion of the tendon can also be due to excessive trauma to the heel subsequent to long term nerve blocks, such as absolute alcohol and cryo nerving.
Quarter Horses and Navicular
Having seen many breeds over the past 35 years, I find quarter horse to be far more prone to navicular problems that appear to originate within the bone itself. The quarter horse stereotype varies to some degree, but as a rule their feet are quite small relative to bone and body size. I often see 1,200 to 1,500 pound horses wearing a 000 shoe, which means the foot is less than four inches wide. The majority of the breed will have upright hoof confirmation with tight heels and a small frog that sits well up in the foot. The tendon surface angle (TSA) of the navicular bone is usually quite high, 30-45°, and the PA can be as high as 15°. This describes a low to mid scale club foot. The shape of the navicular bone is also quite small and appears to have a thinner cortex than that found on horses with lower PA and TSA. The steeper TSA apparently does not receive pressure from the DDF as would be expected with a lower TSA and PA. This could be the reason why the navicular bone does not appear to be as strong and mature as it is in feet with a lower TSA. Possibly there is a correlation as the incidence of navicular lesions and clinical signs appear to be much greater with this type of foot than with the low heel and crushed cushion with negative PA.