The extensor process of the coffin bone is located at the top of the coffin bone, just inside the top of the hoof capsule, behind the coronary band. The common digital extensor tendon attaches here. Functionally, this tendon flips the foot forward in movement. 

Excessive tension on this tendon may ultimately pull off (fracture) or change the shape of the extensor process of the coffin bone. This condition occurs more commonly in horses of "club foot" conformation (high heeled, steep hoof).

Fractures can result from direct trauma or hyper-extension of the joint causing contact with P2. In some cases, this condition can be seen in young, sound horses. In these cases, it is thought to be a congenital or developmental defect.

In chronic, severe cases, fracture of the extensor process is seen in conjunction with a change in the shape of the hoof called Buttress Foot or Pyramidal Disease.

In Buttress Foot, the foot eventually becomes triangular on the ground surface. Buttress foot, and the bony changes seen in the extensor process probably develop due to excessive pull on the digital extensor tendon in a club foot.

Horses with significant extensor process fracture or bony change associated with the extensor process are usually lame. Horses with obvious Buttress Foot are usually chronically and severely lame.

Surgical (arthroscopic) removal of the fragment is usually the treatment of choice. In cases of very recent fracture, a large fragment and with little arthritis, the surgeon may elect to screw the fragment in place.