Page 170 - Speedhorse February 2020

EQUINE HEALTH
SPLINTS IN HORSES
by Heather Smith Thomas
The horse has three bones in each lower
leg between the knee/hock and fetlock joint. The cannon bone is the largest, and the main support for the limb. The two small splint bones, which are finger size in diameter, are long and slender and are attached to the cannon bone on each side and toward the
rear. In the front legs, the splint bones are called the second and fourth metacarpals (the cannon is the third metacarpal). In the hind legs, the splint bones are called the second and fourth metatarsal bones (the cannon is the third metatarsal).
Mild irritations of the splint bones may cause heat, swelling, and mild lameness, and then resolve on their own.
THE SPLINT BONE
Each splint bone is held firmly against
the cannon bone by a strong, thin ligament called the interosseous ligament. This forms a groove for the suspensory ligament, and also gives some protection to the deep flexor tendon that runs down the back of the leg behind the suspensory. The interosseous ligament between the splint bones and the cannon bone can sometimes be confused with the suspensory ligament because the latter is also occasionally referred to as the interosseous ligament.
The splint bones are broadest at the top, forming part of the base for the joint above it. From there on down, the splint bone tapers to
a small, rounded knob. The bone ends about
2/3 of the way down the cannon, just above
the fetlock joint. In an earlier time, these splint bones were probably larger leg bones. The horse’s primitive ancestors had four toes, and these bones were probably attached to the outer toes.
In the modern horse, these vestigial small bones in the front legs act as braces and support for the knee, keeping it from collapsing backward by serving as anchors for connective and supporting structures that attach to the small bone at the rear of the knee joint. This small, rounded bone (accessory carpal bone, also called the trapezium) protrudes behind the knee and acts as a brace point and fulcrum for supportive sheets of ligament that pass behind the knee from the forearm to the cannon bone.
The splint bones are not firmly attached to the cannon bone until the horse is about three years old, and sometimes as late as five years of age. Until the splint bones are completely attached, there is always the possibility of getting a splint whenever there’s too much trauma, stress, movement or rubbing between the bones. Lameness from splints is most common in two and three year olds in hard training, but has also been known to occur in horses as old as four or five that are worked exceptionally hard or have extra stress on these bones due to poor conformation.
Gary Baxter, VMD, MS, Hospital Director at University of Georgia in Athens, Georgia, says that most of what are called true splints— inflammation and subsequent bone growth due to tearing of the ligament attaching the splint bone to the cannon bone—occur in younger horses. “In an older horse with swelling in that
Gary Baxter, VMD, MS,
Hospital Director at University of Georgia
region and lameness, you’d be more apt to suspect some external trauma,” he says.
Traditionally, it was thought that the splint bone’s primary function was weight-bearing support for the knee, alongside the cannon bone. A recent study looked at how the head
of the inside splint bone interacts with the carpus (knee joint), assessing the rotational and compressive stability of the carpus. “Essentially, we found that when the top 20% of the medial splint bone was removed, it did not really affect the stability of the carpus when downward pressure was applied, but it made the carpus very unstable when the carpus was twisted. The stability was only affected when you remove the top 20% of the bone, where all the ligaments attach to the splint bone,” says Baxter.
“The very top part of the splint bone appeared to be more important to rotational stability of
the carpus than it was to weight-bearing and downward compression. The head of the splint bone is essential for support of the knee during twisting actions of the limb,” he explains.
“We are not sure whether this has anything to do with the development of splints, but whenever the limb takes weight, the splint bones are pushed toward the back of the leg and probably put pressure on the interosseous ligament. Splints would be related to the movement of the splint bone and the amount of movement is probably related to the amount of work the carpus is doing. The harder the work, the more likely the interosseous ligament will tear,” he says.
168 SPEEDHORSE, February 2020