Let’s start with the bones again.
First things first. These bones are not puzzle pieces, they don’t have tongue and groove slots like hardwood flooring. They fit together but they need something to HOLD them together. Imagine going in with tape and taping all of these together, just to hold them in place. Those are ligaments. You can’t move your bones yet, but at least they are stuck together with little pieces of elastic tape, like tiny Ace Bandages.
Mr. Pony Bones needs some ligaments (tape, tape, tape!):
There are no muscles lower than the “knee” on the horse. When the muscle contracts above, it pulls on lower limbs like puppet strings. These puppet strings are TENDONS. Look carefully at the lower limbs. See any muscles? Nope. Just ropey little tendons. The entire lower limb is puppet strings. The upper limb is the puppet master.
The tendons connect muscle to structure. The muscle exists all above the knee. Muscle by itself, does not have the spidey sense to attach to anything. It must first bond to a tendon and the tendon, like spider webbing, attaches out. Tendons attach to bones and travel alllll the way up the limb to find the muscles that will control them.
For that reason, there are really two main functions of tendons for the lower limb: tilt the coffin bone (and thus hoof) up and tilt the coffin bone (and thus hoof) down. That’s all she wrote. Like a wing flap on a plane that can only tilt up or down, you have a tendon that runs down the front of the leg, to tilt the coffin bone UP and you have two that run down the back of the leg to tilt the coffin bone down.
Just like you can lift your foot up or point your toes down. Flippy flappy. I’ll note here that they only flip-flap, they don’t rotate like our ankles or wrists. If your horse’s hoof could twist around like an owl’s head, we’d have a lot more issues!
OK! I’ve marked the green flappy tendon in the front (Extensor Tendon, or coffin bone rotate UP). The dark blue markings are the flappy tendon in the back (Deep Digital Flexor Tendon, Superficial Flexor Tendon, or coffin bone rotate down. Think of a cat “flexing” its claws: they pull “down”). Up, down. Extend and Flex.
Where the flexor tendon runs along the back of the leg, it slides down and attaches to the Coffin Bone. With it going up-down only, you would think material could be pinched in the bones. That’s where the Navicular Bone steps in. Like a tiny footbridge, the Navicular Bone spans the “plates” where the Pastern Bone and Coffin Bone join.
We always see cut-aways from the side and it’s hard to picture the hoof in 3D. You see the Coffin Bone and it looks like a weird triangle pointing down, the sides of the bone appear hazy and possibly like they aren’t even part of it. You see the Navicular Bone and it looks like a weird peg from the side, instead of a wide, flat pad of a bone. In an x-ray, a horse hoof looks more like a bird toe than a croissant-shaped “foot”. So feel free to look at the 3D set of shots above, to get the fuller picture.
The Navicular Bone has two “grooves” that you can feel, very subtly, with your fingers and they are about the width of your fingertips (almost like an ergonomic computer mouse, actually).
The Navicular Bone is an important pivot point on the leg, giving a bend to the Deep Digital Flexor Tendon (DDTF). The more bends (like pulleys) you have in a rope system, the more the strain is carried by equal parts. You can see that the topside of these bones each has some part sticking out. This guards the DDFT from getting snagged in between the bones. They are like the wheels on the cable system of a ski lift, carrying the cord away from the frame of the bones.
We’ve seen the tendons and the bones that work in tandem with the tendons. Let’s look at the ligaments.
You can see the light blue lines are the ligaments (the tape); just holding bones to bones. In the left image you can see, in purple, the Suspensory Ligament. The Suspensory is a LIGAMENT: it connects bones to bones. Its function is to SUSPEND. The same ligament is noted in green in the image on the right.
Near the top of the cannon bone is where The Suspensory “sprouts” or originates. It travels down the back of the cannon bone and then splits into two, it travels down both sides of the pastern bone and wraps to the front. It keeps the fetlock from bottoming out during high activity: Jumping, sliding, running. The DDFT (that tilts the Coffin Bone “down” or pointing/flexing) also wraps around the back of the pastern and also works to suspend the hoof during stress and workload. It is noted in medium blue on the left image and dark blue on the right image. The right image does such a great job of showing how these two rubber bands hold up and suspend that leg. The Navicular Bone is the sliding bridge that supports where the DDTF connects to the Coffin Bone. It is the last "pulley" or friction point for the tendon.
The Suspensory is not a muscle. It will not get “worked out” and beef up and be stronger. It can either be tight, or it can be loose. It’s a rubber band. Stretch it too far and it will snap. Stretch it just shy of snapping and hold it there and it can get loose and floppy. You hear about suspensory injuries: that’s where it is and what it’s for. If your horse has injuries to its Navicular Bone, its DDFT and its Suspensory Ligament, it would seem that the “shock system” of your horse’s leg is not working 100%.
We know the horse needs to have his foot land so that his frog and digital cushion can be the first shock absorbers.
The horse will need to see the ground, assess his footing and move his hoof up or down to best meet the ground to absorb shock. The next point of shock absorption is the suspensory ligament and the Deep Digital Flexor Tendon, where the fetlock is, then up to the shoulder. Normally the knee is locking at the point of impact and doesn’t help absorb the shock. (Mental note: I should teach my horse how to do more squats, maybe that would help.)
Super sleuths out there: if something higher up is having trouble, could it be because something beneath it is not holding up its end of the deal? Go down the leg. If the suspensory is fried, if you have tears in your DDTF, if the Navicular Bone has deteriorated… something beneath it might not be doing its job.
Let’s look at the HOOF again. (The Hoof? AGAIN? Awww Mom, do we have to?!?!)
Next: What came first, the chicken or the egg: How does the hoof grow? It’s always just “THERE”.