Understanding the disease progression of abnormal hoof anatomy, Part 1
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:
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.