This page is meant to be used as an aid to measuring helpful parameters from a podiatry style/farrier friendly radiographs. Strict guidelines with regards to radiographic geometry must be adhered too in order to obtain consistent findings between cases. The effort in defining the parameters is not too give us the ideal foot that we should make every foot into but give us tools to evaluate each individual mechanical setup with regards to identifying handicaps or likely areas of pain. There is no specific guideline that every foot should have these exact set of parameters. Just think about the set of parameters and ask yourself "are these parameters healthy considering soundness, history and external hoof characteristics for this foot". These measurements will vary with breeds and one must create his/her own set of commonly encountered measurements. Your own set of common findings will likely be very close to what I will present below. Measuring these parameters will give the practitioner a common terminology and the capability to communicate with precise details without actually viewing the radiograph. This is helpful when I am consulting with farriers. I can give them the parameters and they can draw that exact image on a piece of paper. They now have an internal understanding of the foot that is in front of them without seeing the radiograph
Both hooves should be placed on blocks that preferably have wire embedded into them to give us a level reference point for measuring angles. These images below do not and shame on me.
Blocks should be turned to fit the horse. If toes in or out then blocks will be turned in or out. This will allow a perpendicular alignment to the xray beam and cassette.
Block height should be that it places the xray beam 3/4 to 1 in above the ground surface so that center of the beam shoots directly under solar aspect of coffin bone dead center of the hoof from dorsal to palmar/plantar.
The focal distance, which is measured from the xray generator to the xray cassette/plate is preferably 24 inches with a traditional system and likely closer to 28 with digital systems. It is important that with digital systems that a calibration has been performed and checked. Your equipment supplier should be able to help you with that.
The cassette or plate should be touching the hoof as to reduce as much magnification as possible. A radio-opaque paste is applied to the dorsal hoof wall to allow a perfect outline of all irregularities with the outer hoof wall and a measuring reference. The paste starts at the most proximal (upper) portion of hoof wall and extends to ground surface on a sagittal center line.
Lastly the technique should reveal a soft tissue like densities which will not be the best radiograph to see bony changes. Below is a diagram depicting placement of horse, cassette, and generator. Further discussion of parameters measured and there importance will be below each image.
Coronary band to extensor process (CE) is measured from top of paste which is applied at most proximal aspect hoof wall down to the extensor process of the coffin bone. This will range from 8 to 15 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.
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 60 degrees in very upright clubby 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 is 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. 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 measure. This gives us a manner in which to evaluate flexor tendon engagement. 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) is measured from center of articulation to where the hoof/shoe would leave the ground and is relative to BA. Lower BA coffin bones typically have a longer TL than higher degree. In my practice I see as short as 45mm to as long as 75mm. 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.
I learn more and more everyday from the radiographs I take and there is much more to be discussed and much more that can be found. This discovery process is endless and only requires some thought.