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Your Horse from the Ground Up - The Lower Leg - Part 2
 By mosquito
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8th Sep 2012 •
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2 comments
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We’ve taken a look at the solid structures of the lower leg – the bones – now let’s see what makes those bones move. First of all, remember that there are no muscles below the knee or the hock, so there’s no actual ‘engine’ to move these bones. It all comes from a network of ligaments and tendons that connect to muscles higher up the leg. The neat part of that is it makes them easy to see, but the down side it that these delicate structures have very little protection. If you take a close look at your horse’s leg, you’ll see that the cannon bone runs right down the front; this way it can shield the soft tissue structures at the back of the leg. Okay, that’s great when the horse is moving forward – it’s unlikely that a horse will injure its tendons and ligaments rapping a jump rail or falling down. However, impact to the back of the leg is another matter, and plenty of injuries occur when a horse ‘overstrikes’ or steps on the back of its front legs with the hooves of its hind legs, or if it falls or gets cast and gets its feet tangled up. Even so, impact injuries are not all that common to these structures; a much bigger concern is injuries from overuse or conformation defects. To understand this better, let’s see what’s going on under the skin of the lower leg.
 Before we start, here’s a quick explanatory note about the difference between tendons and ligaments. Simply put, tendons connect muscle to bone, and are directly involved in movement. As the muscle contracts, the tendons pull on the joints and create the flexion (bend) or extension (straightening back out again) that allows movement. Ligaments are the fibers that hold bones together in the joints. Their job is to keep everything straight and prevent over flexion or over extension in the joints, so they also protect tendons from tearing or straining.
Now, think about how a horse moves. To lift the leg, the horse has to contract the tendons to bend the joints and pull them upward. This uses the ‘flexor’ structures. There are four flexor tendons, all of which attach to the muscles of the upper leg and stretch down to parts of the lower leg and even into the hoof, of which three are really significant. The ‘superficial’ flexor tendon isn’t superficial at all – it runs all the way down and connects to the pastern bones. It’s called superficial because it is very exposed, running down the back of the leg where it has room to move but little protection. The ‘deep digital’ flexor tendon, another important structure, runs over the navicular bone and attaches deep in the hoof to the pedal bone – this is the culprit that gets irritated in the case of navicular disease. The ‘lateral’ flexor tendon runs down the side of the leg to the long pastern bone, this one is less involved in movement but is easily bruised.
 Lets’ take a closer look at the superficial flexor tendon, as this is the one most often injured. This large round tendon is very conspicuous at the back of the leg, and runs right down to the fetlock where it splits into two and flattens out, wrapping over the deep flexor tendon. These two tendons are actually pretty robust, and impact injuries are uncommon. The big concern with the superficial flexor tendon is ‘bowing’. Bowed tendons occur as a result of overflexion of the tendon, which stretches the tendon and pulls it away from the cannon bone. A bowed tendon is painful, takes a long time to heal, and usually leaves the horse with a visible long bump along the back of the leg. It almost never occurs in the hind leg, since overflexion of the pasterns of the hind leg is difficult – the back legs don’t take as much weight or impact, and the bones are longer enabling more flexion to occur without damage. In the front leg however, especially in horses that gallop and jump, or have very ‘upright’ anatomy, bowed tendons are a serious risk. Once they occur, it is very difficult - sometimes impossible – to get a horse sound enough to work again.
How do you prevent bowed tendons? Well, avoid fast work or jumping on very hard – or very soft – ground, be especially protective of your horse is he has very straight, upright conformation in his lower leg, and use a good farrier to ensure that bad trimming or shoeing doesn’t exacerbate the flexion in the lower leg. And if your horse bows a tendon? Rapid icing of the injury is necessary, as inflammation can make it much worse very quickly, and get immediate veterinary attention. Expect a long recovery period for your horse, and in most cases, limited work at best once he recovers. Of all the lower leg injuries, this is the one that – in my opinion – can be most debilitating and yet occurs all too often.
But what about the ligaments of the back of the leg, and how do they protect these tendons? There is a complicated network of ligaments that keep the bones together and moving in the right direction. A collection of ‘check’ ligaments connect the bones at various points from the knee though the splint bones, and down to the pasterns, and these ligaments also hold the tendons in place. The most significant ligament of the back of the lower leg is the suspensory ligament, which runs down the back of the cannon bone, then splits and wraps around the pastern bones to the sesamoids, then reaches right around to the extensor tendons that come down the front of the leg. It is arguably the single most important structure in the lower leg as it holds all these bones and tendons in place. It is something of a flaw in the horse’s evolution that so much relies on one structure, and that it is so interconnected. This may be partly because the suspensory ligament – which is highly elastic – also operates like a rubber band. As the horse moves, it stretches the ligament, which ‘snaps’ back, helping to propel the horse’s movement. Remember, without muscles down there, the horse needs all the help it can get to keep these joints moving and generate power to run and jump.
If you watch a horse jumping, or in piaffe, or another athletic movement, take a close look at the hind legs. You’ll see lots of flexion in the fetlock, and you might wonder why. I mean, if there’s no muscle there, there’s no point in all that flexion, and there’s nothing to push off with, right? Well, that’s where the suspensory ligament comes in. All that flexion is pulling that rubber band of a ligament, and as the horse pushes off with its hindquarters, releasing the pressure on the hind feet, the ligament snaps back and propels the horse forward and upward. Now you see why it matters. And this is why suspensory ligament injuries are all too common – flex the pastern too much, and you risk tearing – or even breaking – that rubber band.
Suspensory ligament injuries – like flexor tendon injuries – are extremely serious, and need rapid treatment. If you suspect a suspensory tear or strain, ice and anti-inflammatories are critical and must be immediate. Once again though, a major injury to the suspensory may never heal completely, and once it’s happened a horse will need lots of careful management to prevent a recurrence. Unlike the flexor tendons, suspensory injuries can happen just as easily in the hind legs as the front – so if your horse is doing a lot of very athletic work, keep a close eye for heat and swelling in the back legs too. Tears can occur high up the leg, although most suspensory injuries happen near the fetlock – any sign of heat or swelling and you need to act fast!
Down the front of the leg are a couple of very exposed structures, but fortunately they aren’t often a source of problems, since they act more to straighten the leg back to a ‘recovery’ position after the flexors have initiated all that movement. The common extensor tendon runs down the front of the leg, attaching to the pedal bone, and works with another smaller extensor tendon to help pull the bones of the pastern back toward where they started – or even further in the case of elegant extended paces - before the flexor structures bent them to move the horse. The extensor tendons are held in place at the top of the cannon bone by the annular ligaments, which unlike the ligaments at the back of the leg, actually sit horizontally like straps around the front of the cannon bone. The extensor tendons also meet the suspensory ligament down deep at the base of the pastern. Because they don’t have to do anything that involves carrying the horse’s weight, they don’t often get injured or overstretched. Impact injuries to the fronts the leg can cause inflammation and bruising, but these injuries are usually minor and heal quickly.
 So, what can you do to make sure your horse’s delicate lower legs stay healthy and injury free? First, protect these delicate structures from impact injuries with boots or bandages if you are doing very athletic work, or anything where the horse is likely to knock himself or into a jump pole or other obstacle. Watch your horse move from the front and the back; if your horse’s conformation means that it swings it feet to the outside or inside, rather than moving its hooves in a straight line, then protective gear like brushing boots is probably in order.
Second, protect your horse’s flexor tendons and suspensory ligaments from injury with thoughtful riding. Take a close look at your horse’s conformation. If it is very ‘upright’, meaning a shallow angle in its pastern and fetlock, or very ‘deep’, meaning a steep angle, then you will need to take extra care. An angle of about 45 degrees at the fetlock is about right, and you want the pastern to flow into the hoof in a straight line. Correct trimming and showing is important for all horses to ensure the angles of the pastern and fetlock are as stable as possible – poor hoof structure is a leading cause of tendon and ligament injury. For all horses - but especially those with poor lower leg conformation – be very careful of rising hard and fast in very deep, soft ground (watch out when riding on plowed ground!) which can cause the foot to land at awkward angles, or on very hard, packed surfaces, where the horse’s weight can push down and overflex the soft tissue. Both of these extremes can lead to over flexion and injury.
Jumping, galloping, collection – all of these athletic movements can cause flexion injuries, especially when the surface isn’t just right. If your horse is an athlete or does a lot of work where the lower leg has to bend a lot or take a lot of impact, keep a close eye on the legs. You can prevent chronic problems by hosing your horses legs with cold water after hard work, or applying cooling products like clay or liniment after exercise. Most of all, if you suspect an injury to the soft tissue structure, act fast – time is critical. Treat the area immediately with ice or cold water, and get veterinary attention right away. Of all of the structural parts of your horse, these delicate structures are the most important, and yet the most vulnerable – take care of them! |
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