The Coffin Bone AKA Pedal Bone AKA P-3 AKA Distal Phalanx

Skeleton of the Horse

Adult riding horses weigh anywhere from 1000 to 1600 lbs. That along with the weight of a load in the form of a rider or other, is totally supported by the four feet. Each foot has a small bone in it, which is the distal phalanx or coffin bone. 65% of the total weight is carried by the front feet. Landing from a jump or gallop stride, the impact is increased hundreds of times on just one of those feet. The hoof and it's internal structures of bone, cartilage, blood vessels, tendons, ligaments and nerves are all very important! All parts of the foot must be healthy to function properly.

Bone is not dead! (Or at least it shouldn't be!)

Bone is made up of collagen fibers (similar to scar tissue) impregnated with calcium hydroxapatite (an inorganic salt) and tendon tissue. It is the collagen which gives the bone it's tensile strength while the mineral salts give it resistance to compression.
The collagen fibers align themselves to force. So as bone grows, the collagen fibers line up like a handful of pixie sticks.
Bone can be cancellous or cortical. Cortical bone is the dense, hard outer part of the bone while the cancellous bone is the spongy irregular internal part.

Cross-section Thru the Foot

Coffin Bones Dissolve

When the foot receives uneven weight distribution, as is the case with high-heeled hooves, the coffin bone's dorsal surface is pressed forward against the dorsal wall of hoof horn. This compresses the laminar corium which eventually dies due to lack of blood supply thru the blood vessels that run thru the coffin bone. the laminar corium can no longer produce quality laminar horn. If there is also excess sole pressing upwards on the solar surface of the coffin bone, the solar corium too dies. This corium can no longer produce healthy sole horn.
When a dead hoof is trimmed to reintroduce blood flow to these damaged areas of corium, the blood flow is uncontained by destroyed vessels. The dead corium and bone cells are dissolved and either abscessed out or resorbed. This "house cleaning" would be like coming into a burned out house to rebuild it from the inside out. All the old dead stuff has to be removed before the body can begin restructuring. To the degree that damage is extensive, there may be a permanent loss of bone density.
Interestingly, prior to a rehabilitative trim, dead bone appears the same on an x-ray as does live bone. When a trim is applied to produce blood flow, this dead bone dissolves.
This is a serious consideration when thinking about restoring blood flow to a damaged hoof.

Dissolved Coffin Bone Compared to a Relatively Healthy Coffin bone

A Healthy Coffin Bone is Parabolic

This is the sole view of a healthy coffin bone. Compare this to the oval contracted ones.

Shoeing and/or Contraction Can Deform the Coffin Bone

The palmar processes or "wings" of the coffin bone start out as cartilage. Because the back half of the hoof is designed to spread out with the frog acting as an expansion joint, much of the shock of footfall is taken up thru this design.
The palmar processes do not fully form until the horse is around 5 years of age. If the the foal is raised in a stall or it is not provided adequate movement on hard ground, the hoof horn doesn't receive the amount of force required to start the hoof growing in a normal shape. The hoof can retain the contracted form with which it was born, retarding and deforming the development of the palmar processes. It is not unusual for owners of foals to neglect any hoof care until the horse reaches "training age". By then, it can be difficult to correct the feet with trimming and rehab. If the deformity is ignored (due to a lack of recognition), and a shoe is added onto this deformed young foot, a lifetime of lameness issues usually follows. This seems to be more prevalent in "halter" and "show" horses where the foal is "protected" and kept in a stall. (Quarter horse people call them "lead 'em and feed 'em" halter horses) Without lots of movement on hard ground, not only do the feet suffer deformity, but bones in the legs will be weak and subject to break. Witness the vast number of broken legs of race horses.
As a horse becomes older, these unnatural stresses on the ligaments to the palmar processes cause the ligaments to calcify and become what is referred to as "side bone". Pieces of these boney growths can break off, abscess and exit out the coronary band. "Ring bone" both high and low, are calcifications of ligament attachments on the bones of the foot due to unnatural stress from incorrect alignment. Often these are cited as the cause of lameness. However, when the foot is trimmed properly and the bones realigned, the joint will dissolve the extra bone if it is not excessive. 

Loss of Concavity=Game Over

When inflammation has dissolved the coffin bone to the point there is no longer any concavity in the bone itself, the horse is no longer able to support the rider's weight. This coffin bone shows the transverse arch nearly perforating the sole!

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Coffin Bone from a Newborn Foal

The palmar processes of the coffin bone have not yet formed.

Understand Bone Health

The coffin bone is unique, but it has some of the same properties of other bones in the body.
All bones are covered by a membrane containing blood vessels and nerves called a "periosteum". In all bones, the periosteum nourishes the bone cells.
The periosteum of the coffin bone is called the corium. It is highly modified to contain a dense supply of blood vessels and nerves. Different parts of the corium perform different functions and so, are shaped differently.
The corium influences the growth and health of the coffin bone. If it is strangulated by pressure from shoes or contracted hoof horn, it can bruise or die. If the corium that feeds the bone cells die, the bone cells may also die.

The Importance of Bone-forming Cells

Bone is produced by specialized cells called osteoblasts. These osteoblasts live in the periosteum and, in the case of the coffin bone, the corium, surrounding the bone. The osteoblasts later become osteocytes living within the bone itself.
The osteoblast produces the collagen-based substance called osteon. The osteon cells are impregnated with hydroxapatite which then crystalizes into the calcium hydroxapatite salts.
When the osteoblast is finished producing the bone, it becomes an osteocyte within the bone and continues to maintain bone chemistry by receiving nourishment from small blood vessels.
If the osteocytes die due to lack of blood supply, the bone looses it's tensile strength, becomes brittle and easily fractures. Coffin bone fractures in shod horses are very common. This is due in large part to the fact that shoes reduce the temperature to a level that cannot sustain cellular metabolism AND the shoe restricts circulation and normal horn growth. Because the shoe fixes the bottom of the growing hoof, not allowing the natural bell shape, it causes the hard horn to constrict around P3. While the hoof horn has insulation properties, it cannot completely protect the internal parts of the foot from damage due to continuous conduction of cold by a steel shoe and nails.

X ray of a Foundered Horse

This horse above x-ray) had been shod with it's coffin bone tipped forward for so long that the dorsal laminar wall had begun to abscess. Instead of removing the cause (the shoes and the high bars) the vet excavated the dorsal wall in what is know as a "resection".  The farrier reshod the horse, still leaving the coffin bone tipped forward. This is not an isolated incidence of ignorance. There were approximately 25 vets and farriers in attendence on this case.

Contracted Coffin Bones with Palmar Processes Dissolved and Malformed

Founder Dissolved Coffin Bone

Due to repeated inflammation, the coffin bone has dissolved in this cadaver hoof.

Contraction and Shear can lead to fragile coffin bones

Because the hard horn can become very tight, restricting blood flow thru the dorsal laminar corium, the blood flow thru the coffin bone can build up pressure. This pressure dissolves the bone from within making it fragile.
The x ray below shows the coffin bone's network of openings for blood vessels.

Loss of Concavity

It is important that the coffin bone is concave on it's palmar surface. If this concavity is ever lost, it can never be restored. NEVER. 
A horse that has foundered and has a "ski-tipped" profile to it's coffin bone has lost that concavity. While dorsal attachment can be improved, a coffin bone that has lost concavity has also lost the surface height dorsally that provides for laminar attachment. The amount of surface on the dorsal area of the coffin bone in a normal sized horse is about 2 and 1/2 inches. A coffin bone that has lost concavity will have lost at least 1/2 inch of attachment.