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Understanding the disease progression of abnormal hoof anatomy, Part 1
The mathematical needs of lower grades of laminitis.
I have devoted the last several decades of my life to understanding and treating laminitis. It has been, at times, an exercise in futility as I fought to understand the progression of the disease in the untreated or undertreated equine foot. During all of these years, there have, fortunately, been more successes than failures, and I have grown to understand what to expect and how to correct it before it becomes life-threatening. I have also taken every one of my failures personally.
I have no crystal ball nor long distance x-ray vision (or short distance for that matter), but after so many years of watching this disease progress in hundreds of individuals, I do know this:
- A full recovery is more likely if the horse starts receiving treatment while it is still in the acute stages. (Acute stages describes the period within the first six to eight weeks of the appearance of the condition.) The acute stages may include prolapse of the solar corium through the hard outer sole, rupture of the coronary band and the appearance of a "sinker" line above the coronary band. None of these signs are irreversible.
- The chances of a positive prognosis are much higher if the client or attending veterinarian raises the heels 10 to 15 degrees at the first signs of laminitis.
- Never be afraid to ask for help, trust your intuition or try something different if it makes good sense, especially if what you are doing is not working.
- All treatment options should be based on a sound understanding of mathematics and how it applies to the hoof.
What is the role of mathematics in abnormal hoof anatomy, and how does it apply to the treatment of laminitis?
Laminitis starts with a deterioration or destruction of the basement membrane of the primary and secondary laminae of the foot. This causes the distal phalanx (P3) to pull away from its strong connection with the hoof wall. This damage—the laminae being stretched and torn and their circulation altered—leads to space-filling seromas that we often call the laminar wedge, which can be observed on lateral radiographs. The edema and extravascular fluid buildup in these spaces cause pain and will eventually need to either escape through the coronary band or the sole. When this occurs, coronary band ruptures or solar prolapses (which look like abscesses) occur. Meanwhile, with the loss of one half of the check-and-balance system of the dorsopalmar or dorsoplantar stability of P3 (laminar separation), the second half of the system or deep digital flexor tendon continues to do its job of pulling the P3 in a palmar/plantar direction. The horse's body does not tell the deep digital flexor to stop doing its job, so we, as veterinarians need to. But how? We start by measuring the palmar angle of the foot—the angle the bottom of P3 makes with the ground.
Doing the math
If in a light-breed horse, for example, the palmar angle is typically a to 5-degree palmar angle and the palmar angle has become 12 degrees, then there is an abnormal rotation (excessive palmar angle) of 7 to 9 degrees. Adding 10 degrees of heel wedge to the foot will stop the pull of the deep digital flexor (12 degrees minus 3 degrees = 9 degrees, so adding 10 degrees gives us an extra degree to play with). If the case has rotated to a palmar angle of 18 degrees and should be 3 degrees, then at least 15 degrees of heel wedge is needed to stop the deep digital flexor pull.
The next concern is the SDT, or sole depth under the tip of P3. The SDT needs a total of 15 mm—8 mm of solar corium space for normal circulation without compression and an additional 7 mm minimum of sole for comfort. When the palmar angle increases in the higher Obel grades of laminitis, the sole thins and severe circulatory compression ensues. When circulatory compression is present, new bone or sole can't grow, and it becomes a vicious circle of bone resorption and thinning sole until the sole prolapses and exposes the circulation rich corium (Grade IV).
At this point, the trim of the foot prior to the placement of the heel wedges should be considered. As with putting the deep digital flexor tendon into a neutral state of tension, the solar circulation can be put into a neutral state of compression by trimming the foot to a "0" palmar angle.
On horses with sufficient heel, the heels can be rasped back from the quarters to the back of the frog into a "0" palmar angle before applying the wedges. This is going to cause two things to happen: 1) the bottom of the foot will now be in two planes and 2) you will need to measure how much heel you are rasping off and add that measurement to the amount of heel wedge you will need to apply.
In the higher palmar angle rotations, there is no way to add the amount of heel wedge needed without causing the horse to knuckle forward. (The higher grades will be discussed in Part 2.)
Once you understand the mathematical needs of the foot—putting the deep digital flexor tendon in neutral and achieving a "0" palmar angle under P3—you will find you are stopping further rotation by the pull of the deep digital flexor tendon and achieving free flowing blood under the P3 through a "0" palmar angle.
Below, I discuss the lower grades of laminitis and their mathematical needs. In future articles in this series, I will discuss the higher grades of laminitis and their mathematical or surgical correction.
The lower grades of laminitis
By looking at a standard or digital radiograph (note: digital radiographs are generally not true to size, so you will need to adjust your images to 100 percent true image), you can see and measure abnormalities of the horn-laminar zone (HLZ), digital alignment, palmar angle, SDT, the sole depth at the wing of P3 (SDW) and extensor process-coronary band (EP/CB) (Figure 1).
Figure 1:(Photos courtesy of Dr. Floyd)
The Grade I laminitis foot is considered pathologic when it has a palmar angle of 5 to 9 degrees (Figure 2). In the lower palmar angle (PA) cases of Grade I (e.g., 5 to 6 degrees), the laminitic foot is most easily diagnosed by the uneven HLZ, as there may be little else obvious (Figure 3).
Figure 2: Grade I laminitis. (Photos courtesy of Dr. Floyd)
The HLZ is the perpendicular distance between the hoof wall and the coffin bone. It is measured in two places, just below the extensor process and at the apex. The HLZ in adult light-breed horses is usually 17 to 19 mm (e.g., 17/17 mm, 18/18 mm, or 19/19 mm) (Figure 1). The more disparity between your first number and second number, the greater the laminar wedge or rotation away from your hoof wall. Note that an HLZ of greater than 20 mm in a light-breed horse represents serious edema.
Figure 3: Grade I laminitis with HLZ disparity.
The digital alignment, which is normally considered to be a straight line or 180 degrees when measured with a goniometer, will appear bent at the distal interphalangeal joint and will have a lower goniometer reading (e.g., 165 to 170 degrees). I like to measure the digital alignment from the dorsal distal face of P1 through the dorsal face of P2 and P3.
A Grade II laminitic foot will have an increased palmar angle of 10 to 14 degrees. The disparity in the HLZ will be obvious, as will a decrease in the SDT measurement and an increase in both the EP/CB and SDW. The digital angle will be lower as well. A lamellar wedge is usually present on lateral radiographs (Figure 4).
Figure 4: Grade II with lamellar wedge.
Following are some important facts to keep in mind if you are fortunate enough to see a horse in the first stages of the disease process:
- 10 degrees of heel lift is always a good place to start, but then obtain radiographs and measurements and adjust the heel lift accordingly.
- Always rasp the heels back from the quarters to the bulbs of the heels before applying a heel lift appliance.
- Obtain lateral radiographs weekly, because a Grade I might become a Grade IV. Weekly radiographs will also help you determine if treatment is helping. The foot should grow 1 mm of sole a week. If the sole does not grow, it requires more help.
- It takes six to eight weeks for the final pathology to demonstrate itself through radiographs.
Trying to figure out the mathematical needs of the foot is fun. Pull out a few old radiographs, judge the needs of those feet, and then look up your history to see what you did and how the horses responded. Grade I and II laminitis cases generally respond well to the therapy outlined here and should return to soundness, if treated properly.
Have fun with the calculations and maybe you will start enjoying these cases the way I did so many years ago.
Andrea E. Floyd, DVM, has specialized in equine podiatry for more than 25 years. She is the owner of Serenity Equine, Evington, Va., and the author of Equine Podiatry. Dr. Floyd is a member of the American Veterinary Medical Association, American Association of Equine Practitioners and the American Farriers Association.