How to Take Farrier Friendly Radiographs. Presented at OSU CVM fall conference 2014

How to Take Farrier Friendly Radiographs

Sammy L. Pittman, DVM

Innovative Equine Podiatry and Veterinary Services, Pllc

            Gaining relative information within the hoof capsule, that helps the vet/farrier team make decisions, requires a consistent and detailed approach.  Most of us learn radiographic technique that concentrates a study on bony structures.  The same radiographic views that detail the coffin joint or navicular bone are essentially useless to designing a therapeutic shoeing program.  It gives us no reliable information with regards to the mechanical properties that are in play. 

            Informative radiographs are relative to the answers we seek.  Being attentive to the many details will allow consistent repeatable and comparative images.  Following the guidelines below will give you the ability to produce consistent and reliable radiographic exams helpful in evaluating the mechanical properties affected by trimming and shoeing. 

1)  Place both hooves on blocks that are designed to allow the primary beam to penetrate the hoof between the palmar rim of the coffin bone and ground surface.  Wire embedded into the surface of the blocks is helpful when measuring relevant angles.  To determine height of blocks set your xray generator on the ground and measure to the center of the crosshairs on the collimator then subtract 1/2 to 3/4 of an inch.  This will consistently place your beam just below the coffin bone in most barefoot and shod horses with the exception of large padded packages.  If the horse toes out then the blocks toe out as well and the same for  a toed in conformation.  Blocks should be about the same width apart as the gap in between the upper forearm at the level of the sternum.  This will be about one hands width in most light breed horses.  The hoof should be set to the medial and palmar/plantar edge of the block to allow the radiograph cassette/plate to be touching the hoaof in the lateral view and as close as possible in the dorsopalmar views.  Aligning the frog stay or central sulcus with the sagittal wire marker embedded into the block will aid in appropriate beam alignment for the Dp and lateral view. 

2)   Beam orientation must be centered on the area of interest.  Trying to identify important measurements relative to therapeutic shoeing and trimming requires a low beam orientation.  This is consistently obtained by setting up your blocks as previously described.  A perpendicular beam to cassette/plate orientation should always be obtained to prevent distortion of your image. 

High beam

High Beam

             

Low beam

Low Beam

  












3)  The cassette/plate should be touching the hoof on the medial side in the lateral view to prevent as much magnification as possible. 

4) Use radiographic paste to mark the dorsal hoof wall in the sagital plane in all lateral views.  The paste should start where the last hair exits and extend to entire length of the hoof capsule.  This allows accurate measurement of coronary band to extensor process distance, horn-lamellar zones, and allows definition of every ripple, defect or growth ring. 

5)  Focal film distance should be always consistent and can range from 24" to 28".  Typically with today's smaller units closer to 28" allows visualization of just below fetlock.  Just keep it consistent. 

6)  A calibration instrument should be placed in the sagittal plane for the lateral and the transverse plane for the dorsopalmar view.  Most digital radiographic software allows for calibration based on a known measurement in the radiograph.  Metron software has a built in calibration component and a specific calibration instrument embedded into their blocks or an autoscaler.  However, simply placing a known length of wire or metal bar stock in the plane of interest will allow you to calibrate your radiograph regardless of software.  Calibration is important to correct for magnification that occurs. This magnification will be consistent if you your radiographic technique is consistent but it is important to document this detail.  Typical magnification is around 10 percent.  This factor is important if you are taking measurements on the radiograph and transferring them to the hoof.   Correction of magnification without software can be accomplished by a simple math equation. Where (actual foot measurement)={ (Length of Calibration tool) x (radiographic measurement of concern)} / ( radiographic measurement of the calibration tool).

Good Bone Detail but not good for soft tissue

7)  Radiograph technique can vary widely but typically  low Kvp  and higher Ma produces better soft tissue detail.  Today's digital units often give us a good representation of bone and soft tissue detail but it is still worth playing with your settings to find greater grey scale within the hoof capsule.  Seeing the dermo-epidermal junction is a good guideline to know if you are seeing enough soft tissue detail.  I also like to see the deep flexor tendon in my foot films.       

            Currently I measure and monitor several distances and angles to follow the health of the foot, design therapeutic shoeing plans and monitor disease processes.  Below is a description and diagram of each measurement, how to measure and a short discussion about each measurement.

            Coronary band to extensor process (CE) is measured from top of paste which is applied at most proximal aspect hoof wall at the point of the last hair follicle down to the extensor process of the coffin bone.  This will range from 8 to 30 mm in most healthy hooves.  This number does not give you much information as a single measured parameter.  However, when monitored and compared in serial radiographs, especially when monitoring an acute laminitis case, it is extremely valuable. For example, an acutely laminitic patient that measures 8mm on day 1 of clinical signs and then measures 18mm on day 4.  This is a 10mm  distal displacement which is usually accompanied by a 10mm decrease in sole depth as well and varying degrees of rotational displacement.  

            Horn-Lamellar zone (HL) is measured in two areas, one proximal just below extensor process and one distal just above apex of coffin bone.  This will most commonly measure 15 mm in most light breed horses but can be as high as 20mm in larger breeds, mules and donkeys.  This measure is expressed as proximal HL/Distal HL (15/15).  Instead of measuring only rotation this will give you a measurable displacement that is more definitive than a generic rotation.  Evaluating the dermal-epidermal junction is also of great importance as it should split the horn lamellar zone further defining each.  This allows more specific interpretation of changes in the HL zone.  For example with laminitis the L component of the HL zone will change not the H component.  Early in laminitis this may be the only notable change and an increase of 3-4 mm is a significant finding and may have no measurable rotation.  Several important disease processes can be discovered in this zone and many foot diseases such as clubs, chronic/acute laminitis, white line disease, keratomas and abscesses have very unique qualities that can be shown here.

            Sole Depth (SD) is measured from the tip of the coffin bone down to most distal aspect of the sole.  The cup is also of importance as it is present to different degrees depending on health or pathology and can also be falsely created.  This measurement is expressed as SD/Cup.  Healthy feet with no pathology will most commonly carry 12-15mm of sole and a 2-3mm cup (15/3). This should be of upmost concern of the vet/farrier team when striving to obtain soundness and health of the foot.  This should be the measurement at the day of the farrier visit.  Often thin soled horses are at 6-7mm of sole 8 weeks into the cycle and this is a sign of a compromised foot that requires a different approach to increase foot mass and health. Two measurements can be made to give you more information, one at tip of coffin bone and one under wing of coffin bone.  Venogram findings suggest that a depth of 15mm is required to maintain a healthy appearance to the solar vascular bed with robust and correctly aligned terminal papillae

            Digital Breakover (DB) is measured from the tip of the coffin bone to where the foot or shoe if shod would leave the ground.  Healthy hooves that maintain adequate SD and good digital alignment will commonly maintain a DB of 20-25mm.  Many times in perimeter fit shoes, depending on type of foot, bone angle, and toe lever this number is considerably higher than ideal at the day of the farrier visit and continues to lengthen throughout the cycle due to hoof growth. This gives us a measurable lever arm that applies its force to the deep digital flexor tendon and its subsequent force impacts on apex of the coffin bone, dorsal hoof wall and navicular apparatus.  Below I discuss toe lever (TL) that in my opinion gives a more accurate understanding of the lever arm involved. 

            Bone Angle (BA)  is the angle of the coffin bone when viewed in a lateral radiograph.  Average BA will be 50 degrees.  In my practice I have measured BA's as low as 36 degrees in very low heeled and long toed horses to 70 degrees in club feet.  The shape of the coffin bone determines the shape of the hoof.  Most of the time the horses that have low heel long toe conformation will have a less than 50 degree bone angle with a long measurable toe lever (see below) and the opposite is true for upright club feet. Granted, horses that have overgrown unkempt feet may have crushed heels and a long toe but may have a good BA.  I feel that monitoring this parameter early in life could potentially identify feet that may have a common sequalae with regards to lameness later in life.  For example, a horse with a 42 degree BA and a 70mm Toe lever may be at higher risk of hyperextension injuries of the pastern, coffin and fetlock joint and increased tension strain on deep digital flexor tendon, and navicular apparatus when compared to a coffin bone with a lower bone angle and shorter toe lever. If we could identify this early in a horse's career and change the shoeing protocol to better manage this handicap maybe we could reduce the  amount of wear and tear to some degree.  

            Palmar angle (PA) also known as solar angle of the distal phalanx or ventral angle is measured from the wings of the coffin bone in comparison to a level ground surface or embedded wire in block.  It can be tricky to measure in some feet with considerable bone remodeling.  Using the wings will offer the  most consistent measurement. This gives us a manner in which to evaluate flexor tendon engagement. In general lowering the PA increases tendon tension and raising should decrease the tension. This angle will average 3-5 degrees in the horse that maintains adequate sole depth and is free of lameness but can vary greatly.  PA should be evaluated in this manner:  Is this PA healthy for this foot?  The answer comes from evaluation of sole depth, clinical exam and digital alignment.  For example, PA measures 8 degrees and maintains a SD of 15/3 and good digital alignment.  This case is higher than what is ideal but currently considered healthy for this case.  On the other hand PA measures 3 degrees and sole depth is 7mm.  This is not likely a healthy PA as a higher PA with less deep digital flexor tendon tension will unload the solar corium and vital growth center of the sole.  This angle is also of great value to monitor in a preventive podiatry program.

Toe Lever (TL) can be expressed as static toe lever or shod toe lever.  Shod TL is  measured from center of center of rotation of the coffin joint to where the hoof/shoe would leave the ground and static TL is measured from the center of rotation to the tip of the coffin bone.  Shod TL we can effect and static we cannot.    Lower BA coffin bones typically  have a longer TL than higher degree.  In my practice I see static TL as short as 45mm to as long as 75mm in adult horses.  Monitoring this at a young age may allow us to apply orthotics that will decrease the effective lever arm that antagonizes the lower limb.  Therapeutic shoe packages can be evaluated with regard to amount of lever arm relief.  Simply setting the shoe back only effects this measurement a few millimeters and sometimes many lameness issues respond to a TL that is 3-4 times less than what is measured on their bare foot. 

Tendon Surface Angle (TSA) is measured on this distal part of the navicular bone compared to a level ground marker. This is relative to the course of the deep digital flexor tendon takes at turns to attach to the coffin bone.  Monitoring the change of TSA with your applied orthotic is of value especially cases that show navicular bone lesions in this region.   Simply changing DB  may be beneficial in many cases however raising PA and TSA is often required to be therapeutic.

References:

            1.  Redden, R.F. Clinical and Radiographic Examination of the Equine Foot. In Proceedings  Am. Assoc. Equine Pract. 2003;49:174.

            2.  Merit, K.  How to take foot radiographs. In proceedings Am. Assoc. Equine Pract. 2008.

            3.  Floyd, A. Mansman, R.  2007  Equine podiatry, Radiology and Radiography of the Foot.  pg 141

Theory of two major loads article

Theory of Two Loads

            I have struggled with what forces are involved in the hoof and how they changed with different palmar angles and varying degrees of deep digital flexor tension (DDF). So to aid in my understanding I consider two extreme examples to help describe my simplified idea of two major loads within the hoof capsule.  First I will describe tendon load (TL) and the extreme example to be used is a high grade club.  Next we will discuss bone load (BL) or ram load with the extreme example of a post ddf tenotomy laminitis case. 

             I think we can all agree that there is a significant pull from the DDF in club foot cases.  Lets consider the action of the DDF.  As weight is applied to the limb or the DDF muscle contracts the  pulling force is transferred to the coffin bone via the semilunar crest at the DDF tendon insertion.  This pulls the coffin bone around its articulation with the distal end of the second phanlanx (P2) and the DDF tendon also is pressed against the flexor surface of the navicular bone.  Extraction forces are apparent at the horn-lamellar interdigitation and compression forces on the solar corium directly beneath the apex of the coffin bone.  Club feet are affected by a shortened musculotendonous unit via increased neurologic stimulation of the flexor muscle.  This tranfers load to the apex of the coffin bone and the horn-lamellar interface at the toe.  So for simplicity sake consider two lengths of rope both attached above carpus and at the semilunar crest of coffin bone.  The shorter length will transfer more load to the apex than the longer when weight is applied to the limb.

                                                             

                                              Figure 1 short rope/high pa/club

                                                         

                                            Figure 2 Longer Rope/low pa/slam dunk

           

These forces and the changes implied are noted on radiograph's of club feet, as a remodeled tip of coffin bone, a small bump midway down on the face of P3 and often smaller,and a less dense navicular bone.  These changes follow Wolfes law of bone remodels along lines of tension and compression.  Now consider the external characteristics of this extreme example:  Atrophied frog, deep central sulcus, wider growth rings at heel than toe, bulging or flat sole at and around apex of frog.  These characteristics are created by the excessive DDF tension which allows for an unbalanced load distribution between tendon load and bone load.  This excessive TL prevents loading and stimulation of the palmar portion of the hoof and leaves the frog and heel suspended in the air.

.              

Figure 3 bone remodeling on tip of coffin bone

            The second load to consider is bone or ram load (BL).  This is the weight that is transferred through the bony column directly to the ground.  If no DDF was present then all load is distributed through this manner and forces are increased in the heel region.  Consider the case of a post deep digital flexor tenotomy when all TL has been negated due to severing of the   DDF tendon. All weight and forces are concentrated in the heel region and has more of a table leg distribution of forces.  I feel that many of the crushed heel, low to negative palmar angle hooves  have a similar situation. Just as the club foot is born with shortened musculotendinous unit the low Palmar angle/crushed heel or slam dunk foot may have a longer than ideal musculotendinous unit allowing a greater bone load that will allow more weight or load through the bony column to the palmar/plantar aspect.  I think it is possible to create a negative palmar angle and crushed heels with poor mechanics in many of our everyday shoeing practice that could possibly take a normal healthy foot with good sole depth and palmar angle to thin soles and negative palmar angle, however many are destined for that path from a very early age due to conformation.  It is impossible to take a  club foot caused by shortened musculotendinous unit and create a negative palmar angle and the same may be true for the slam dunk foot as many will revert back to crushed and under run heels once orthotic devices have been applied to increase hoof quality, sole depth and aid in treatment of lameness. 

         Consider a heel sore horse that is landing toe first, this is evidence to me that the horse can use the tendon to transfer load to the front of the foot to unload the painful buttress, digital cushion and many related soft tissue structures.  Many horses compensate quite well by transferring load to the front of the foot via DDF with initial heel soreness but it is not long until the extra workload by the tendon creates inflammation within the tendon itself  and many of soft tissues and ligaments associated with the palmar/plantar aspect and fatiguing the flexor muscle group.  This is when a trip to the vet usually occurs as they are now unable to effectively transfer load to a non painful region and show obvious signs of lameness.  The increased load transferred to the front by the toe first landing and often long digital breakover in these cases decreases blood supply to vital growth centers and adds to the further compromise of hoof and sole quantity and quality.  Radiographs would show very thin soles below wings of coffin bone, low to negative palmar angle, a very low tendon surface angle, as scallop of bone remodeling in palmar/plantar aspect of solar margin of coffin bone and upright pasterns.  External characteristics noted are:  Wider growth rings at toe than heel, flat and thin soles, 2-3 sets of nail holes, wide robust frog, and under run heels.

       

                                     Figure 4Low Pa bone remodeling/low ddft tension

           

For further understanding let us consider treatment of these two scenarios and why they are successful            l in increasing soundness and quality of hoof mass.  For the club foot syndrome, lower grades that are not surgical candidates, increasing palmar angle and lengthening the heel base will allow more BL and less TL.  Decreasing the TL will decrease the amount of load being transferred to the toe and allow more bone or ram load to push into the heels.  The easiest and most successful approach I have found, is using rocker shoe mechanics.  The heels are trimmed to the widest part of the frog parallel to the wings of the coffin bone and toe is trimmed perpendicular to the frog axis at a low rocker toe style angle.   

                                    

                                                                 Figure 5 Grade 3 club

    

              Figure 6 Grade 3 club with Rocker Rail

 The trim will vary based on such parameters as palmar angle, sole depth and digital breakover but the basic approach will stay the same.  The next step is to determine what shoe to shape to fit our specific needs.  In general low grade clubs will do fine in a rockered flat shoe as higher grade clubs may require starting with a wedged shoe that has greater mechanical potential.  Consider a flat shoe that is rockered can alter pa 2-4 degrees and a 5 degree rail shoe is starting with 5 degrees, so any added rocker will increase potential to alter palmar angle.  So the question to be answered is how much PA increase do I need to create less tendon load and more bone load?  Low grade clubs require less than higher grades.  This approach will allow more ram or bone load, more heel loading that will result in less atrophy of the frog, decreasing depth of the central sulcus, increased sole depth below the tip of coffin bone and more even toe to heel growth patterns.  With less TL comes less H/L zone extraction force and less solar corium compression. 

Now consider a case of acute lamintis with extensive H/L detachment and venogram shows decreased perfusion at the coronary waterfall, compromised vasculature down face of the coffin bone,  tip of coffin bone has displaced 3 mm below the circumflex artery, and terminal papillae are horizontal versus being in normal orientation with the face of p3.  This gives us a picture of severely compromised dorsal portion, including the horn-lamellar attachment and solar corium below the tip of coffin bone.  A DDF tenotomy may be indicated in many cases such as this. This will completely unload the forces of the DDF and allow all weight to be transferred down through the bony column into the palmar/plantar region of the foot and unloading much of the compromised areas in dorsal aspect.    This can be shown by post tenotomy radiographs and venograms.    This release and increased load now through the bony column to heels will often push the coffin bone up closer to its original placement prior to laminitis episode and displacement and radiographs will show measurable decrease in distal h/l zone and increase in sole depth just from the unloading that occurs from complete release of DDF.

                          Figure 7 laminitis with rotation

Figure 8 Post tendon cutting and derotation shoeing  

Figure 9Acute laminitis venogram

 In the above drawings (Figure 7 and 8) shows the pull of the tendon with detached bone to horn attachments and a post tenotomy with derotational shoeing.  Without a healthy lamellar attachment there is no antagonistic force to counteract the pull of the ddft (TL) and the coffin bone rotates around its articulation compressing solar corium at the tip of the coffin bone.  Figure 9 shows an acute laminitis case in which the bone is compressing the blood supply at the tip of coffin bone due to lamellar detachment.  You can see the tip of coffin bone below the circumflex artery.  This area is heavily loaded secondary to the TL and loss of the bone to horn attachment.  The image on the right is of the same horse 2 weeks after derotational shoeing and deep digital flexor tenotomy.  The tenotomy negates all TL and its forces applied to the damaged areas (lamellar zone, sole under tip of p3) and heavily loads the palmar/plantar aspect of the hoof through BL only.  Note the restructuring of the blood vessels under and around the tip of the coffin bone in this short 2 week period. 

I have always considered that anytime we raise the palmar angle via wedges or rocker shoe mechanics that we increased the load on the heels but it really wasn't clear why until considering these two loads.  These examples are two extreme versions and most feet will fall somewhere in between.  When a healthy balance between TL and BL exist we find good  feet that are easy to maintain with adequate sole depth and a positive palmar angle but when loads sway more to one side of spectrum to overloaded portions become unhealthy and need our assistance in balancing the load via a well designed protocol based on and monitored by serial podiatry style radiographs and venograms.

Cyril's Hoof Spring

This is a technique I learned from a farrier from Switzerland (Cyril Zuber) while at Dr. Redden's advanced equine podiatry class last month.  This case experienced a severe heel bulb laceration about a year ago.  All has healed up but a severe contraction secondary to loss of heel mass from injury and lack of full load bearing has occured.  I can't say if the contraction component is related to any lameness but a more cosmetic hoof could be obtained.

Below is radiographs and photos of the hoof and spring application process.  At the very end is a video of the spring being released showing the action that it is applying to the hoof capsule.

I placed in a custom steel rocker rail to fully load heels and maintain a self adjusting palmar angle with greatly reduced toe lever.