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