Bone Fragments of the Coffin Bones of Foals

Livestock Update, April 1995

Larry A. Lawrence, Animal and Poultry Sciences Department, Virginia Tech

Bone fragments in the heel region of the foot have been found in a large number of foals, but previously, researchers had not been able to identify a cause. Although these bone fragments had not been evaluated microscopically, they had been considered by many to be either fractures, isolated sites of new bone formation (secondary ossification centers) or osteochondrosis (an abnormality of cartilage and bone maturation). Researchers from the University of California at Davis Equine Research Laboratory in collaboration with the International Equine Podiatry Center in Versailles, Kentucky have determined that these bone fragments are fractures, probably caused by a strong pull of the deep digital flexor tendon.

A surprisingly high incidence of these bony abnormalities was found by Dr. R. F. Redden, a research collaborator from the International Equine Podiatry Center. He examined the front feet of 149 Thoroughbred foals between the ages of six and 49 weeks on six different farms in 1990. Using radiographic examinations, he found 119 foals to have one or more bone fragments of the wings of the coffin bone. Many of the foals he examined had only a slight lameness which usually resolved with stall rest. Additionally, six foals that were referred to the U.C. Davis Veterinary Medical Teaching Hospital (VMTH) for lameness between 1985 and 1992, were also identified as having abnormal bone fragments of the heel region. These foals ranged from six to 52 weeks of age and included Quarter Horses, Thoroughbreds and Appaloosas.

The high incidence of bone fragments involving the coffin bone from the Thoroughbred farm survey and the lack of previous microscopic evaluation of these abnormalities encouraged researchers from the U.C. Davis Equine Research Laboratory to conduct a study to characterize these bony abnormalities. Drs. Andris Kaneps, Timothy O'Brien, R. F. Redden, Susan Stover and Roy Pool designed the study to use radiographic examinations, high-detail tissue radiography and microscopic tissue evaluation of the feet of 32 Thoroughbred foals from Kentucky and California. The foals ranged from three weeks to six months of age and had died of various other causes.

The radiographic studies revealed that 16 percent of the feet examined had bone fragments from the coffin bone. High-detail tissue radiographs revealed 51 percent of the feet examined had fractures of the coffin bone. Microscopic tissue evaluation revealed 56 percent of the feet examined had fractures or a healing fracture of the coffin bone. The microscopic evaluations also revealed that these fractures were not associated with new bone formation or osteochondrosis.

The researchers have determined a probable cause for the formation of these fractures based on several consistent findings from the microscopic examination of these tissues. A notch in the bone that corresponds to a normal anatomic structure called the parietal sulcus was always found associated with the fracture site. The deep digital flexor tendon was consistently found to one side of the fracture line and not attached to the bone fragment. The forces of weight bearing act in opposition to the pull of the deep digital flexor tendon attachment, creating a shear force. This force appears to be concentrated by the parietal sulcus, causing a fracture of the wing of the coffin bone.

Characteristics of the hoof and forelimb conformation may also contribute to the development of these fractures. Club-footed conformation or poor conformation can contribute to causing excessive forces associated with weight bearing which act in combination with increased tension of the deep digital flexor tendon, possibly resulting in fractures of the coffin bone. Excessive trimming of the hooves may also subject the coffin bone to increased forces of concussion as a result of decreased sole thickness, and to increased strain of the deep digital flexor tendon due to decreased hoof angle.

This study concluded that the bony abnormalities in the heel region of the foot are, in fact, fractures that may or may not cause varying degrees of lameness and that many of these fractures may not be evident on radiographic examinations. Additional research will be conducted using high-detailed computed tomography and magnetic resonance imaging to develop more sensitive diagnostic procedures to identify these fractures. The prognosis for healing appears to be good as the fractures heal with stall confinement for four to 12 weeks. How these fractures of the coffin bone affect hoof wall strength and growth, developmental abnormalities and future soundness is not known at this time and is the focus of continued research efforts.

Credit: Laurie Fio, University of California at Davis Equine Research Laboratory

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