This photo shows a dog with an ESF on the left, front leg. This device stabilizes the bone while it heals and allows the dog to maintain the use of its limb.
	These X-ray images show fractured dog legs being stabilized by different ESF configurations.
	Dr. Aron抯 research included laboratory testing on cadaver dog bones to study the bone/pin interference. The inset shows an IM pin within a dog bone. The irregularity of the medullary canal and the thinness of the cortex make an ESF necessary.
	A linear static stress analysis was performed on a model that represents a fractured dog bone, IM pin and KE ESF. Von Mises stresses at the bone/pin interface are especially important.
	Aric Applewhite, DVM, helped to develop the finite element model of the dog bone with an IM pin and ESF.

According to the 2003/2004 National Pet Owners Survey conducted by the American Pet Products Manufacturing Association, 65 million dogs and 77.7 million cats are found in at least one-out-of-three U.S. households. When pets break a bone in an accident, such as being hit by a car, veterinarians often mend the fracture with a combination of stabilization devices called intra-medullary (IM) pins and external skeletal fixators (ESFs), a technique that is employed daily across the United States. Research conducted at the University of Georgia under the direction of Dennis Aron, Doctor of Veterinary Medicine (DVM), using ALGOR finite element analysis (FEA) software is helping to establish better guidelines for how these stabilization devices can best be used to promote healing of animal fractures.

Stabilization Devices Help Fractures Heal

Physical trauma often results in the fracture of one or more of the long bones of the limbs. This type of bone consists of a dense cortex layer with a central cavity termed the medullary canal, which contains softer tissue. One technique for mending a fractured bone involves inserting an IM pin into the bone. When this technique is used on humans, the medullary canal is first hollowed out, or reamed, to achieve a perfectly cylindrical shape matching the diameter of the IM nail, as it is called when used on humans. The inserted nail achieves a tight press-fit within the bone, preventing bending, rotation and translation.

Dog and cat bones cannot be reamed, however, because the cortex of their bones is not as thick as human bones. Reaming is also made more difficult in dogs and cats because the long bones tend to not be as straight as human bones. This is not just a matter of pets being smaller; but rather is a species variation. While IM pins can effectively prevent bending when used on dogs and cats, they frequently are not effective as the only method of stabilizing a fracture because the pins do not achieve a tight fit within the bone.

Veterinarians, therefore, often combine the use of IM pins with ESFs. An ESF consists of a number of pins that penetrate the bone and exit through the skin to attach to rigid bars on the outside of the body. This device stabilizes the bone as it is healing while still allowing the animal to maintain the use of its limb.

Several different brands of ESFs are used in veterinary medicine, such as Kirschner-Ehmer (KE) and the IMEX SK?(SK). The variation in the brands of ESF devices involves different types of clamps that affix the pins to the bar, different types of pins that engage the bone and different material(s) that comprise the components (i.e., connecting bar and clamps). Additionally, veterinarians must select the number, type and configuration of ESF pins to provide adequate stabilization of the fracture.

Current guidelines for ESFs are based on small clinical studies that have looked at the effectiveness of different ESFs, both with and without an IM pin in various fracture scenarios. 揢sing software technology, such as finite element analysis, allows us to look at a greater number of ESF variations than is practical with clinical or laboratory testing,?said one of Aron抯 team members, Aric Applewhite, DVM. 揟he project underway at the University of Georgia will help us to compare different types of ESF devices, a variable number of ESF pins and the added stability provided by an IM pin in different fracture scenarios. Our goal is to determine if IM pins are necessary and which ESF configurations are best at stabilizing a fracture.?h4> Modeling a Fractured Bone

揂s non-engineers, we needed an FEA package that was easy to use and understand,?said Applewhite. 揥e chose ALGOR because of its reputation for being user-friendly and providing quality technical support. ALGOR抯 technical support staff was always available. They provided us with invaluable assistance throughout our project.?p> The finite element model created in ALGOR抯 Superdraw III consists of solid and beam elements. Solid brick elements comprise the IM pin, two pieces of bone and sections of the spongy material of the medullary canal, while the ESF frame is represented by beam elements.

揗odeling the IM pin in the medullary canal was the most challenging part of the whole modeling process,?said Applewhite抯 teammate, Heidi Radke, DVM. 揟hrough trial and error, we learned that actually modeling sections of the spongy material of the medullary canal provided more accurate results than approximating this region with contact elements.?p> The bone geometry was simplified to a hollow cylinder, the diameter of which was based on measurements of a large-sized (about 30 kg or 66 lbs) dog. The geometry was simplified to remove the variables of differences in breeds and different sizes of animals and, therefore, enabled the researchers to concentrate on the ESF and how it behaves in relation to an idealized bone/IM pin structure.

Published material properties for bovine bones, which are similar in strength to dog bones, were applied to the bone parts in the model. The properties of 316L stainless steel were used for the pins. Stainless steel was used to model the KE ESF and a carbon fiber composite was used for the SK ESF.

A force was applied at the femoral head to represent the weight of the dog. The model was fully constrained at the bottom of the bone and stabilizing elastic constraints were added to maintain spatial alignment while the model displaced vertically. Linear static stress analyses were performed on all of the models in the study.

In reviewing the analysis results, von Mises stresses at the bone/pin interface and deflections at the gap between the pin and bone were especially important. The first study that compares KE and SK unilateral ESFs with IM pins has been completed. The results were validated with mathematical methods, including convergence and patch testing, and against data from laboratory testing. The results of this first study were presented in January 2003 at the Annual Meeting of Bioengineering in Athens, Georgia. Comparing other variations in the IM pin/ESF configuration is an ongoing project that will provide veterinarians with improved solutions for treating pet fractures in the future.

Veterinary surgeon Dennis Aron oversees this research at the University of Georgia, where he is a professor. Veterinarians Aric Applewhite and Heidi Radke developed the finite element models for this project, assisted by engineers Mark Evans, Ph.D. and Guigen Zang, Ph.D.