Fracture Types and Treatments
Of all the emergencies requiring veterinary intervention, a fracture requires some of the most intensive medical attention--from first aid, to repair, and, finally, to rehabilitation and recovery. Fortunately, even a serious fracture is not the death sentence it once was for horses.
According to Carl Kirker-Head, MA, Vet MB, Dipl. ACVS, ECVS, Marilyn M. Simpson Professor at the Tufts University Cummings School of Veterinary Medicine and director of the Orthopedic Research Laboratory, the last 10 to 15 years have brought a much better understanding of equine bone healing that is driving improvements in fracture treatment. These include the use of minimally invasive surgical techniques and novel implants, as well as the use of growth factors and tissue engineering to speed bone healing.
Biology of a Fracture
Bone is a dynamic tissue that continually renews itself throughout an animal's life. Specialized cells, produced in the bone and delivered by the surrounding soft tissues, continually remove old bone and deposit new material through a normal process called bone remodeling. Bone remodeling is orchestrated by an array of biochemical factors, including hormones in the bloodstream and specialized molecules at the site of bone development. When a bone breaks, this elaborate self-regulating process--from the capillaries that provide access for the building blocks of new bone, to the bone structure itself--is disrupted.
The horse's body responds rapidly to a fracture with an accelerated bone production cycle that repeats some aspects of embryonic bone development. Injury to the surrounding tissues triggers a cascade of biochemical signals that recruit stem cells for the healing process. Mesenchymal cells (stem cells arising from the same embryonic tissue that develops into bone and muscle) migrate to the injury site and quickly begin to divide before developing into cartilage cells; this cartilage is laid down at the site in attempt to quickly bridge the gap in the bone. Bone replaces the cartilage slowly over several months. Eventually, the normal remodeling process takes over, restoring normal bone stability.
Many cells and chemical mediators are involved in the fracture healing process. The major ones are:
- Mesenchymal stem cells These undifferentiated cells (those that have not yet assumed a specific role within the body), as mentioned above, are capable of developing into cells that can generate new bone, carti-lage, or fibrous tissue.
- Fibroblasts These specialized mature cells deposit fibrous tissue at the injury site. They are generally undesirable and tend to be most common in adverse fracture environments (e.g., areas of high motion and poor blood supply).
- Osteoblasts These cells make bone.
- Osteoclasts These cells resorb (break down) bone.
- Growth factors These molecules control the complex fracture repair process. The ones receiving the most attention are bone morphogenetic proteins (BMPs), but many others are similarly involved.
- Osteocytes These are mature bone cells that can sense mechanical stress and secrete growth factors when required.
The Role of Soft Tissue
The body's soft tissues (e.g., muscle, skin, and subcutaneous tissues) are critical to bone healing. "When a bone is fractured, the immediate blood supply to that part of the body is often sufficiently traumatized that we must rely more on the surrounding soft tissues to provide nutrients," says Kirker-Head. The horse is at a tremendous disadvantage because there is very little muscle where fractures usually occur--the lower limbs. "This is why we very frequently run into complications, including infection and non-union, because we have an inadequate blood supply and relatively poor protection for the healing fracture."
Soft tissues provide a barrier between the external environment and the fracture, and they deliver the blood supply to repairing tissue. "For this reason," he says, "it is important to consider the soft tissue damage that occurs at the time of injury."
Bone Regeneration Growth Factors
Following injury, the body releases growth factors into the local fracture environment to expedite bone healing. Until recently, it had been impossible to harness these potent molecules' ability to speed bone healing, but clinicians have started to use BMPs in both horses and humans.
For example, doctors have used BMPs in soldiers returning from Iraq with multiple fractures from IED trauma, notes Kirker-Head. Growth factors accelerate bone healing by supporting traditional bone graft techniques. Augmenting the bone regeneration process is particularly helpful in the horse, since weight-bearing must be established as quickly as possible.
Scientists in the Cummings School's Orthopedic Research Lab are investigating how they can most effectively deliver growth factors to injured bone. "Delivery methods are important to make sure that growth factors remain where they are needed at the fracture site and do not cause problems in other parts of the body," says Kirker-Head. Ongoing research includes the use of bioabsorbable delivery vehicles made from silk and collagen, novel nanoscale technologies that minimize inflammation, and genes that might be able to control the activity of local growth factors.
Surgical techniques are less invasive than they were even 10 years ago. "Today, when we can, we are making much smaller incisions, we're not exposing as much of the fracture as we would have done previously, and we are reducing the amount of bone fragment manipulation and subsequent use of implants," he says.
That said, implants remain a necessary part of fracture treatment. Kirker-Head explains: "Veterinarians are still faced with the problem that the horse, when it comes out of surgery, must be essentially pain-free and able to move around. We use comparatively large amounts of metal to make a sufficiently strong repair so things don't fall apart on us during the recovery process or shortly thereafter."
Veterinary implant technology is largely derived from human implant systems. Equine surgeons use the locking compression plate, a comparatively new, minimally invasive bone plate. It restores the bone's weight-bearing ability while freeing the surrounding tissues of direct pressure.
"A lot of exciting things are happening in tissue engineering and repair," says Kirker-Head. Natural and synthetic resorbable materials can be made in a solid, but porous format to serve as a support for the mechanical and biochemical bone healing process. Technologies also include resorbable implants and bone plates.
Surgeons can treat even a complex fracture in a weight-bearing limb successfully under the right circumstances. A better understanding of the bone healing process at the molecular level is driving the development of a growing armamentarium of minimally invasive treatments that involve both mechanical and biological support.
"The bottom line," says Kirker-Head, "is that with our use of bone growth factors, increasing use of novel implants, a better understanding of how to rehabilitate horses, and, to a certain extent, our improved understanding of complications like laminitis, we are well ahead of where we were even 10 years ago in the treatment of fractures in horses."
Kirker-Head, C.A.; Boudrieau, R.J.; Kraus, K.H. Use of Bone Morphogenetic Proteins for Augmentation of Bone Regeneration. JAVMA, Vol. 231, No. 7. October 1, 2007.
About the Author
Nancy Zacks holds an M.S. in Science Journalism from the Boston University College of Communication. She grew up in suburban Philadelphia where she learned to ride over fields and fences in nearby Malvern, Pa. When not writing, she enjoys riding at an eventing barn, drawing and painting horses, volunteering at a therapeutic riding program, and walking with Lilly, her black Labrador Retriever.
POLL: Rehabbing the Injured Horse