What exactly is founder? Founder is when the capsule wall and the coffin bone become detached; this detachment occurs when the sensitive and insensitive laminae are separated due to inflammation, bacterial/fungal disease, or mechanical forces. This blog will illustrate founder due to mechanical forces caused by poor trimming which is called mechanical founder.
When flaring occurs at the toe the toe area’s capsule wall is pried away from the front of the coffin bone. This causes the laminae at the front of the hoof to become stretched/detached. In a healthy hoof with no toe flaring the capsule wall provides support to the sole tubules—keeping them upright and compact. When the toe area’s capsule wall flares forward the sole tubules at the toe lose the support provided by capsule wall and begin bending and moving forward as well. This results in the sole tubules becoming less vertical and less compact at the front of the coffin bone which decreases the thickness of the sole at the front of the coffin bone. In extreme cases, the sole will become so thin at the toe the front of the coffin bone itself touches the ground. This also means the front of the hoof loses concavity. There are more visuals and descriptions in "Part II" of this blog series if you need a refresher on how flaring reduces both concavity and sole thickness.
As the sole becomes thinner at the front of the coffin bone this also results in the angle of the coffin bone changing. Most hoof care providers describe this as the coffin bone “sinking.” I feel it would be more accurately described as the sole becoming thinner at the toe which results in the coffin bone becoming closer to the ground at the toe. In other words, the change in angle occurs because the sole tubules towards the heel of the hoof are still upright while the sole tubules towards the toe area are being bent and pulled forward—making the sole thicker near the heel and thinner near the toe. This angle of the coffin bone is referred to at the palmar angle. In a healthy hoof the palmar angle is 0° meaning it is parallel to the ground. A hoof with mechanical founder will have a positive palmar angle meaning the coffin bone is closer to the ground at the toe than the heel (our illustration has a 5° palmar angle).
The toe is where the horse breaks over every stride which makes it a prime area for flaring due to the leveraging action of break over—remember flaring becomes painful to the horse as soon as it starts stressing the sensitive laminae (causing laminitis) which means toe flare is painful to the horse well before it is classified as mechanical founder.
Next, I’ll share some tools to help you evaluate your own horse’s hooves for flaring!