The distal phalanx, also known as the third phalanx, coffin or pedal bone, seems fragile, lying suspended above the sole, encapsulated by the outer hoof wall and supported by an intricate web of anatomical structures. It can be likened to the tip of one finger or digit and is a critical piece of the equine anatomical puzzle.
Role As A Pump
The distal phalanx is not only a major contributor in the weight-bearing process, but also functions as a catalyst in the hoof mechanism supporting blood flow.
Some research suggests the blood flow provided by this hoof mechanism alleviates much of the work the cardiac system otherwise would have to do. Since horses have small hearts in relation to their size, its importance in this respect is critical. The foot, in effect, acts as an auxiliary heart pump, expanding with each stride and allowing the coffin bone to descend and the solar corium to fill with blood, which is then forced from the hoof and up the leg as the weight-bearing foot leaves the ground.
For a horse to stride freely, the hoof must be able to withstand the majority of concussion. The frog first contacts the surface and absorbs the initial shock; the heel buttresses and bars follow, transferring more of the energy to the flexible lateral cartilages, so that when the sole starts to shift energy to the coffin bone, much of it already has been absorbed. By the time the hoof wall at the toe is engaged, only negligible energy remains.
Since the sole maintains a key relationship to the coffin bone, it is essential to recognize and treat hoof distortions before they become pathologies that affect welfare and performance.
Things To Keep In Mind
Dallas Goble is a member of the International Equine Veterinarians Hall Of Fame and a former director of Equine Clinics at the University of Tennessee, and head veterinarian for the Budweiser Clydesdales, National Herd Health Program. He says these are some of the things farriers need to keep in mind about the role the distal phalanx plays when trimming and shoeing in order to maintain optimal health and balance.
Goble describes the coffin bone as "shovel-shaped," and the first supporting bone in the horse's anatomy to begin weight bearing as the hoof touches. "It is a critical link in the ability of a horse to perform athletically and comfortably," he says. He goes on to explain that the foot is made up of two intimately entwined major structures, the hoof capsule and its associated soft-tissue structures.
"The hoof capsule includes the hard hoof wall, sole and frog of the foot. The coffin bone then fits within the hoof capsule, with the binding attachment between the two major structures being the laminae bond," Goble explains. "The hoof capsule supplies the insensitive laminae (without blood or nerve fibers), while the coffin bone and its associated soft-tissue structures supply the sensitive laminae (which have both blood and nerve fibers). This binding is strengthened by the formation of primary and secondary laminae (finger like projections from the primary laminae) which inter-digitate and form the cohesive forces that maintain the coffin bone and hoof capsule in their respective positions.
"This junction between the sensitive and insensitive laminae we recognize as the white line when we examine the bottom of the foot.
"Needless to say, the harmonious relationship between the hoof capsule and the coffin bone must be maintained for the horse to be an effective athlete, as well as maintaining comfort with freedom of pain when performing. Happily this occurs most of the time, but there are those times when problems develop in this relationship which may affect all components of the foot. That is when the horse may experience lameness and, depending on the problem or problems present, may have serious consequences in his future comfort and use."
Anything that disrupts the close working relationship between the hoof capsule and the coffin bone may result in lameness, according to Goble. Examples could include a puncture through the sole, which may result in an infection and cause acute lameness and the possibility of osteomyelitis in severe cases.
"We are all aware of the perils of laminitis, and the catastrophic separations that can develop between the hoof capsule and the coffin bone, at times even calling for euthanization," he says. "Other associated disparities, such as arthritic involvement within the coffin joint (distal interphalangeal joint), comprised of the navicular bone and the second phalanx (short pastern bone) can also occur."
Different Healing Process
The distal phalanx, like the sesamoid bones, does not have a periosteal covering, as do most other bones. As a result, its response to injury is somewhat different. Most other bones form a callous and primary union. But in the case of severe injuries, such as a fracture, the coffin bone heals by a fibrous union that infiltrates with calcium over time, making a full recovery somewhat tenuous.
In some instances, Goble notes that the fracture line will remain visible on radiographs for years, "explaining why the horse may remain chronically lame."
The coffin bone is also sensitive to shoeing changes, changes in balance of the foot, the length of the toe of the hoof capsule or dorsal hoof wall, and the position of the heel of the hoof in relation to the bearing surface of the ground.
The deep digital flexor tendon attaches to the bottom of the coffin bone beneath the sole and near the point of the apex of the frog.
"There are those horses who have underrun heels or a negative palmar angle of the coffin bone who will tear or strain the attachment of the deep digital flexor tendon, another cause for lameness in the horse," he says.
The Farrier's Role
Goble stresses the importance of proper hoof care as it relates to the coffin bone.
"As a reliable and knowledgeable farrier, you can often correct potential problems before they actually materialize," he says, "and in the event that the horse is lame, you are a mandatory participant with your veterinarian in a solution to any foot problem."
Jeff Myrick, an American Farrier's Association certified journeyman farrier from Sandgate, Vt., echoes this sentiment.
"Fortunately, we seldom actually see the coffin bone, but we all should attempt to visualize it within the hoof as we work," he says. "However far afield the arguments on balance go, balance itself — as the horse feels it — pertains to the location of the coffin bone within the hoof and its relationship with the ground."
Myrick says there are a few simple guidelines for trimming and shoeing and can help the coffin bone maintain the protections nature provided for it.
"The first simple protection for the coffin bone is the sole — don't steal it. It belongs on the bottom of the foot, not in a pile," he says. "The method of trimming that involves knifing out the sole until it moves to thumb pressure will not leave enough sole to protect the bone. If the sole yields to thumb pressure, it is very likely that you have gone too far in the search of shorter hoof or greater breakover. No amount of philosophy will put the sole back where you should have left it."
Myrick's rule of thumb is that it is far better to err on the side of caution and leave a greater vertical depth to the foot. He suggests trying to achieve a correct hoof pastern axis by setting the shoe back half the thickness of the wall at the toe.
"This is an old axiom of shoe fitting, so it should sound familiar," he says. "Why only halfway? Because the arch of the horn at the toe is the coffin bone's best defense against trauma, just as it is the keystone to a hoof's strength."
Myrick notes one of the problems farriers deal with is that when a hoof in not balanced, it seems obvious, but defining and achieving correct balance is difficult.
"The hoof should mimic the contour of the coffin bone. Hooves that deviate from the profile of the bone show their displeasure by cracking or flaring, both symptoms of uneven weight bearing," he says. "And that may be the only clue as to what true balance is: even weight bearing.
"Another rule of thumb is to sight down a lifted leg to see if it is at 90 degrees to the centerline of the cannon bone, but the conformation of many horses simply will not allow such a hard-and-fast rule. This rule is difficult to apply to the hind feet as well, because you cannot sight down them the same way you can the front limbs. As an added difficulty, if the heels are not trimmed to an equal length, distortion of the hoof capsule can make reading the depth of sole almost impossible."