You might hear the word 'concavity' thrown around quite a bit in the hoof world.
Concavity describes the dome shape of a horse's hoof.
|Here is an example of moderate sole concavity. You can see the gentle shape of the dome from the edges of the frog all the way out to the laminae line (except for that lumpy bar).|
|A blurry but more obvious example of concavity.|
|Imagine the ruler is a hard smooth ground surface - ta da! The solar concavity becomes evident.|
Concavity (whether it be mild or more noticeable) is the sign of a healthy hoof. Perfect concavity that has a gentle slope all the way to the laminae line is a sign of adequate sole thickness.
The hoof has a concave shape because the solar surface of P3 is also concave. With some horses it is very much so, others the concavity is shallower.
|Front pedal bones as viewed when you are holding the hoof as if to pick it - i.e. from the sole. Note the largest one has a shallow curve, while others (particuarly the smaller ones) have a more pronounced concavity.|
|A diagram of the pedal bone showing the digital cushion (the light blue part). Note the concavity, the curve of the bone. Imagine how it would shape the sole of the hoof if the sole were allowed to 'do it's thing'.|
|Shows the direct correllation between the pedal bone (top of photo, here covered in corium) and the sole. I love this pic, it is very very educational!|
|Aaaaaaand one more. Is it clear in your mind now? :)|
A concave sole is flexible and will sink (be pressed flat) and cause the hoof to expand when loaded (the degree of which depends on about 30,000 factors including hoof, terrain, load force etc etc etc) allowing P3 (which, if you remember, has a slight rotation of 3-5 degrees in the back) to flatten out and become ground parrallel.
Achieving solar concavity is not something you can trim into a hoof - you must certainly NOT cut it out of the sole even if the sole is very thick (except in very special circumstances, as always never say never with hooves!) - but it is a by-product of proper trimming and a sign that you are doing the right thing.
A horse who is developing concavity and sole thickness will often develop a toe callus on the sole in front of the frog. It looks like a flat callused area in a crescent moon shape.
|My old OTTB Beckham had a toe callus - highlighted in red.|
|Side shot of the same hoof.|
|Again, highlighted in red in case it isn't clear above.|
The toe callus is a good indication that you are 'half way' to complete transition (from shoes or prior poor hoof form). You know you are all the way there when the toe callus dissapears, and the solar concavity reaches in a gentle slope all the way to the laminae line.
The hoof throws down a toe callus as a priority because this is the launching pad of the hoof - it takes an incredible amount of force to push the horse into the next step and all that force is directed at the point of breakover - where the toe callus forms. This is also often where your toe will end up once the hoof shortens - but that is another post for another day.
You will find that the horse's sole will change relative to the terrain it is conditioned to - on hard ground, the hoof will loose concavity as it reaches toward the ground for contact. On soft ground, the hoof will cup more as the hoof sinks into the ground and still achieves sole contact.
So that is sole concavity in a nutshell. I hope that gives you a sufficient insight!