Circular ring external fixators (cESF) have grown in popularity in veterinary medicine over the last 10 years. They open up many new possibilities for the veterinary surgeon, but also offer many challenges. The application of cESF is difficult conceptually and technically, and the components are expensive. This presentation will provide an overview of the uses of cESF and the various components used in these situations. Although training in the actual application of the fixators is beyond the limits of this format, some comments will review the basics of cESF use.
The use of cESF devices was originally pioneered by Gavriil Ilizarov. His contribution includes the discovery of the concept of distraction osteogenesis. This principal is one of the most important ideas that make cESF useful. Distraction osteogenesis is the principal that accelerated bone formation can be induced by providing a constant traction force across a bone gap. This traction force is generated by making small adjustments to the device on a regular basis, thus moving the bone incrementally. As a segment of bone is moved away from another segment, a type of callus, or regenerate, is formed. The finding was first used to treat dwarfism in people, but has since found many other applications. If the movement is made too rapidly, the new bone will not form, resulting in a conventional nonunion. If the movement is made too slowly, the bone fragments will heal and prevent further motion.
Another valuable aspect of cESF is that very small wires are used. Although the wires have limited strength in bending, they have good strength in tension. When placed across a ring and anchored at both ends, wires placed under tension provide good axial strength. This is because any axial movement of the bone is quickly translated into tension on the wire. The stiffness is a nonlinear characteristic and depends on the tension of the wire, diameter of the wire and diameter of the ring. Additional variables in the strength of a cESF are the shape of the rings, the position of the wires, the number of wires and rings etc. Some of these variables are under the control of the surgeon, while others are dictated by anatomy and circumstances.
One by-product of the use of small diameter wires is that there is a dynamization across the fracture. Dynamization is the process of allowing small amounts of axial motion across a fracture site to stimulate healing. The term is frequently misused to refer to the staged disassembly of an ESF device. Dynamization can be performed actively through the use of motors, but occurs passively during weight-bearing with the use of cESF because the wires flex slightly before reaching a high enough level of tension to provide axial resistance and stability.
These fundamental characteristics of cESF devices lead to the most common applications and advantages of the type of ESF. The first indication is in fracture repair. Although most any fracture can be repaired using cESF, the method is particularly suited to fractures with an extremely small distal or proximal piece. Where there might only be room for one pin of a conventional ESF, there will often be room for two or three cESF wires. The pins are relatively easy to place due to their shape and small size. Mistakes do not leave as large a defect in the bone.
A second common use of cESF is to take advantage of the ability to perform distraction osteogenesis. This is useful in correcting angular limb deformities because the correction can be made over a longer period of time as the animal grows. This allows adjustment to eventual adult size and allows soft tissues to adapt to the changes. Correcting angular limb deformities is very complex and requires extensive planning and experience with cESF methods. Distraction osteogenesis is also useful in bone transport. Bone transport involves moving a small segment of normal bone through a large defect in order to quickly fill that defect with regenerate bone. The defect could be due to a highly comminuted fracture or due to tumor resection as in limb-spare procedures. Once the transported segment is moved across the entire defect, time must be given to allow it to "dock", or heal, to the opposite fragment. Often bone grafting is necessary at that point.
The most commonly used cESF system is marketed by IMEX Veterinary Inc. This system will be used to describe the principles and components of cESF techniques in this presentation. There are other systems marketed around the world that differ in a variety of ways from the IMEX system. The use of the IMEX system for discussion here is not meant to imply that other systems are necessarily inferior.
At a conceptual level, the components of cESF are the same as in conventional ESF: pins that penetrate the bone, connecting bars, and a clamping system. In the case of cESF, the "pins" are actually the wires as mentioned earlier. The wires are placed all the way through the limb as in a Type II ESF. The wires are connected to complete rings of various shapes, or arches, and then tensioned across the frame. Some wires have balls of metal in the middle which allow them to anchor against a bone. They can then be used to move the bone by applying traction to the wire. These wires are called olive wires.
There are several methods of affixing the wires to the rings, but usually a type of slotted bolt or washer is used that clamps on to the wire. Wires may be placed on either side of the ring, but only one wire will fit on a given side. Wires may be suspended from posts in order to place additional wires in a segment. Ideally, wires attached to a ring are placed at ninety degrees to each other, but this is not possible in every case. When the relationship of the wires to each other strays too far from ninety degrees some stability is lost in one direction of bending.
These arches or rings are connected in a variety of ways. Most frequently, threaded rods are used to span between rings on opposite sides of the fracture line and also between rings on the same fracture segment. The rings are connected to the bars using conventional hex-nuts. Alternatively, the rings may be connected using adjustable devices called motors. The motors are used to distract fragments by turning a nut several times daily. Furthermore, rings can be linked to conventional ESF pins using small posts or hybrid bars that screw into the rings but are of a size that corresponds to ESF clamps.
Brief introduction into application
Application of cESF is challenging and should be approached gradually and conservatively. As with any new technique, the surgeon should begin with simple cases and work up toward more complex uses. Laboratory sessions are offered at some veterinary meetings or as stand-alone courses. Alternatively, gaining experience by observing someone who has higher skill with cESF could be helpful. In all cases, careful planning before surgery is vital. In many cases the frames can be constructed beforehand based on radiographs and then modified slightly at surgery (the frames can be wrapped and sterilized after being assembled).
Wires should be driven by use of a power drill, not by hand. It is helpful to have an assistant since wires on the same ring should optimally be tightened simultaneously. When intra-operative fluoroscopy is available, it greatly facilitates accurate placement of fixation wires. Post-operative radiographs will reveal malalignments which can often be corrected by adjusting where a pin is located on a ring. Olive wires are also used to move fragments slightly.
It is important to have at least two wires in a fracture fragment. Additional wires or pins will add to the stability. More rings, complete rings, or smaller rings will be stronger but may not be possible given the location of the fracture.
The use of hinges and other techniques to accomplish angular corrections is beyond the scope of this discussion.