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Coffin Bone Fractures

January 5, 2006     E-mail this page to a friend!

by Heather Smith Thomas

The coffin bone inside the hoof (sometimes called the pedal
bone, but now more commonly called the third phalanx or PIII) is a
somewhat delicate structure. For protection, it depends on the
support and elasticity of the hoof wall, the digital cushion behind
and beneath it, and the sensitive laminae (inner part of the hoof
wall). If any part of that support system and protection is
hindered or disrupted, the coffin bone becomes susceptible to
injury. Trauma and stress are the major causes of injury--such as
concussion (fast gaits on hard surfaces), forces applied to the
foot unevenly, too great a force, a hard blow (kicking a solid
object), or weakening of the bone from an infection (such as from
a puncture wound in the foot).

Racing on firm dirt tracks is a major cause of coffin bone
fractures. The left foot is most vulnerable in horses racing
counterclockwise. Factures can also occur when doing fast work on
hard surfaces when the horse is putting a lot of force on the feet
along with some twisting motion--as when performing at speed with
stops and turns.

Fracture is not uncommon in racehorses, since the feet are
subjected to a great deal of stress and concussion at high speeds.
It occurs most often in the front feet, since they carry more
weight. The fracture occurs as the foot strikes the ground,
especially if there is a twisting motion as the foot is put down.

If the bone is fractured through the center, involving the
joint surface where the coffin bone interfaces with the short
pastern bone (second phalanx or PII) the lameness is usually quite
severe. In some cases the horse may refuse to put the foot on the
ground for several days, hobbling around on three legs. The foot
will be warm, and hoof testers will reveal pain over the entire
sole area. The horse will be more lame when walking on soft ground
than on hard ground or other firm surfaces. This is because the
soft ground will put more pressure on the sole (the most painful
area) than on the wall of the hoof. The hoof sinks into the soft
ground; the ground makes contact with the sole, creating more pain.

Any fracture in the area of the joint between coffin bone and
pastern bone is serious and may take awhile to heal due to movement
in the fracture line as the foot takes weight. Arthritis (chronic
inflammation of the joint) is a common result. In some instances
this can leave the horse permanently lame and unsound.

Sometimes the coffin bone will break near one of the outer
wings of the bone. If this is the case, the horse is usually not
so lame, and the outcome and chance for proper healing is usually
more favorable. Hoof testers may show pain over the entire sole,
but severe pain over the affected quarter. In some cases the horse
will not react to hoof testers, but x-rays will reveal the location
of the fracture. A positive diagnosis is best achieved with x-rays
to determine not only whether the bone is actually fractured, but
also to pinpoint where the break is located. It may be necessary
to take several x-ray views of the wings, to find the crack. With
the advent of bone scan (nuclear scintigraphy) many cases of stress
related injury to the coffin bone (when the horse first starts
showing foot soreness) can be diagnosed before the fracture shows
up on x-rays. These horses can be treated early, often preventing
a serious fracture.

Surgery has been tried for repairing some fractures, screwing
the two broken pieces together. But in most cases the bone can be
immobilized enough with special shoes. This is often done with a
full bar shoe and quarter clips. The bar should be placed so that
it is recessed a little from the frog, so that no frog pressure
will result. Quarter clips can be welded to the outside of the
shoe branches at the back of the quarters (toward the heel) to
prevent the quarters from expanding when weight is placed on the
foot. Sometimes an aluminum hoof plate (hospital plate) can be
used for the same purpose, keeping any weight and pressure off the
frog and sole of the foot.

Preventing quarter expansion and eliminating frog pressure
keeps the hoof from expanding, limiting the movement of the bone
inside the foot and halting all motion in the fracture line while
it heals. The horse should be shod this way for 3 to 6 months, the
shoe being reset every 2 to 4 weeks. In some cases, 6 to 8 months
of special shoeing may be needed before the lameness disappears.

After the horse is moving soundly again, and shows no pain in
the sole area when hoof testers are used, he should still be shod
with quarter clips or a bar shoe for awhile longer. Some horses
will require continual use of special shoeing in order to stay
sound, or the bone may fracture again when subjected to stress.

After a fracture, the horse should not be worked hard again
for about 6 months. In some cases, pain and lameness will still be
noticed after 6 months' rest; a longer period may be needed before
healing is complete. The length of time needed for recovery will
often depend on the location and severity of the fracture. Chances
for good recovery are best if the horse is rested adequately.

Fractures that extend into the coffin joint sometimes have
less chance for full recovery. Fractures that do not involve the
joint surface often become painless within a few months and heal
fairly quickly. Perfect healing in many cases may be difficult to
obtain, however, since the bone is very porous. Even though the
solid hoof wall acts as a natural cast to help immobilize the bone
inside it, there is still some movement and hoof expansion as the
foot takes weight. Absolute immobility is nearly impossible to
obtain, and the closer the fracture is to the wing, the greater the
movement along the fracture line will be, since the hoof wall
expands most in the area of the quarters (where the wings of the
bone are located).

But if the fracture does not involve the joint, the movement
associated with weight-bearing is not so detrimental; it encourages
the growth of insensitive and somewhat flexible scar tissue which
can eventually bridge the fracture gap. The flexible scar tissue
formation is actually preferable to a fragile and incomplete bony
union. If the fracture involves the joint, however, a solid bony
union with perfect alignment of the joint surfaces at the break
give better healing and long-term soundness.

Proper diagnosis and treatment can make a lot of difference in
whether or not the horse will heal properly or remain sound when he
goes back to work. Neglect in treating a coffin bone fracture can
lead to more arthritis and risk for permanent lameness.