An amazing variety of fixators are available throughout the world, most developed for human use. Because of their size and cost, most are not practical for veterinary use, although some are adaptable, especially those designed for hand or forearm use in humans. In North America the traditional devices commercially available (IMEX Veterinary, Longview, Texas; Osteo-Technology International Inc., Hunt Valley, Md; Gauthier Medical, Rochester, Minn; Securos, Charlton, Mass) are based on the Kirschner-Ehmer adaption of the Roger Anderson splint.22 The earlier Stader
apparatus did not achieve lasting popularity.21 Similar devices are available from
various manufacturers in Europe. The clamps used in these fixators are rather sim- ple in design, and although they have some inherent deficiencies relative to stability, they still function adequately if properly applied, and they provide a relatively
economical method of stabilizing a wide variety of fractures. Unless otherwise noted, all frames depicted in this text are of the Kirschner-Ehmer variety.
Fixation Pins
Most fixator frames are fastened to the bone by stainless steel fixation pins that must penetrate both first and second cortices. If the fixation pin penetrates only one skin surface and two bone cortices, it is called a half pin, and it is the only pin used in type I frames (Figure 2-34, A and B). Those pins that penetrate two skin surfaces and two bone cortices are called full pins and are the basis of type II frames. A minimum of two pins is required in each major bone fragment to ensure stability, but more often, three or more pins are indicated (see section on biomechanical considerations). These pins may be smooth, partially threaded, or fully threaded in design; the latter are not widely used because of their lack of stiffness. Smooth pins are typically Steinmann pins of the appropriate diameter cut to length after inser- tion into the bone. Partially threaded pins are either end-threaded or center-threaded pins. Threads can be cut from the stock of the pin (negative-thread-profile pins) or can be built up (raised-thread, enhanced-thread, or positive-thread-profile pins) to a larger diameter than the pin shaft (Figure 2-34, C). Negative-thread-profile pins usually have a fine thread, that is, a relatively high number of threads per unit of length, and are designed for cortical bone insertion. Positive-thread-profile pins are available in both the cortical thread and a coarser, flatter pitch thread intended for insertion into cancellous bone, such as found in the metaphyses of long bones. Typically, a mixture of threaded and smooth pins is used in most frames. Kirschner- Ehmer clamps allow the use of pin diameters up to 5/64 inch (2.0 mm) in small
clamps, to 1/8inch (3.2 mm) in medium clamps, and to 3/16inch (4.8 mm) in large
clamps. An important consideration in the fabrication of frames using positive- profile-threaded pins is that the threads of these pins will not slide through Kirschner-Ehmer clamps, so they must always be inserted into the clamps from the unthreaded end unless oversized clamps are used. Two relatively new systems (Secur-U, Securos, Charlton, Mass; IMEX-SK, IMEX Veterinary, Longview, Texas) use clamps that accept positive-profile-threaded pins and allow attachment to the connecting bar between previously installed pins.
Miniature positive-profile end-threaded pins are available with a roughed central area of the pin shank to allow greater purchase of acrylic connecting bars (Figure 2-34, D). These pins have a cortical thread (IMEX Veterinary, Longview, Texas) and have shank sizes ranging from 0.35 inch (0.9 mm) to 3/32inch (2.4 mm).
Miniature positive-profile-threaded pins are very useful for tiny mandible and long- bone fractures.
Connecting Bars
The connecting bar, or rod, functions to connect the fixation pin clusters attached to the bone fragments. The resulting bone-frame construct provides enough stabil- ity to allow the bone to heal while maintaining functional use of the limb. The bars used for Kirschner-Ehmer clamps are typically solid stainless steel rods with a diam- eter between 1/8 and 1/4inch (3.2 and 6.5 mm) (Figure 2-34, E). Traditionally, the
connecting rod has been used only as a straight rod, but as illustrated later, in many cases it is useful to contour the rod. Although hollow rods are more rigid, they cannot be contoured, and their cost makes them impractical for veterinary use.
Mechanical stiffness of the connecting bar affects axial loads at the fracture site and shear loads on the fixation pins (see Biomechanical Considerations). One fixation sys- tem (IMEX-SK) uses larger connecting rods of increased stiffness (Figure 2-34, G). The SK system is available in three sizes.37The small SK connecting rod is 1/4inch
2—Fractures: Classification, Diagnosis, and Treatment 71
A
B
C
FIGURE 2-34. External fixator components (IMEX Veterinary, Longview, Texas). A, Fixation half pins with positive thread profile in large (3/16 inch; 4.8 mm), medium (1/8inch; 3.2 mm), and small (5/64inch; 2.0 mm) diameters. The upper three pins have cortical threads and the lower three pins have cancellous threads. B, Ellis pins are partially threaded pins with negative cortical thread profile. The threads penetrate the second cortex, and the smooth part of the pin rests in the first cortex. C, Fixation full pins with positive thread profile in large (3/16inch; 4.8 mm), medium (1/8inch; 3.2 mm), and small (5/64inch; 2.0 mm) diameters. The upper three pins have cortical threads, and the lower three pins have cancel-
E
F D
FIGURE 2-34. Continued D, Miniature positive-profile end-threaded pins with a roughed central area of the pin shank to allow greater purchase of acrylic connecting bars. These pins have a cortical thread and have shank sizes ranging from 0.35 inch (0.9 mm) to 3/32inch (2.4 mm). E, Kirschner-Ehmer double and single clamps. F, The Secur-U external fixation clamp (Securos, Charlton, Mass) may be applied to connecting bars between two previously installed clamps and allows installation of enhanced-thread pins at any location.
(6.3 mm) in diameter and available in carbon fiber or titanium. The larger SK connecting rod is 3/8inch (9.5 mm) in diameter and available in carbon fiber or alu-
minum. The mini-SK system is designed for use with the small stainless steel connecting rods (3.2 mm) of the Kirschner-Ehmer system. The use of carbon fiber, titanium, and aluminum allows for increased diameter and stiffness without significantly increasing weight. The carbon fiber rods are also radiolucent.
Acrylic materials can often be used to mold a connecting bar between the fixation pin clusters. This method is particularly applicable to bones such as the mandible and maxilla, as well as in transarticular applications, where it can be difficult to drive all the fixation pins in the same plane.24
Clamps
Traditional external fixator clamps are designed as part of the Kirschner-Ehmer system. Single clamps grip the fixation pin and connecting bar and are rotatable in
2—Fractures: Classification, Diagnosis, and Treatment 73
two axes: that of the fixation pin and that of the bolt. Single clamps are the only clamps required for one-plane frames (see following discussion). Where two con- necting bars need to be connected to each other, double clamps are used, in which two clamps rotate around the bolt axis (Figure 2-34, H). The design of the double clamp allows construction of multiplane frames, and these clamps are integral to the type IA double-clamp frame, the basic design used by Ehmer.22Because they are not
inherently as “stiff” (resistant to deformation when subjected to loading) as single clamps, double clamps have been relegated to an ancillary role in fracture fixation. However, they are still useful in fixation of corrective osteotomies in young dogs, in which bone healing is vigorous and the need for long-lasting stability is only mod- erate. The use of two connecting bars between the pin clusters significantly stiffens type IA double-clamp frames.
H G
FIGURE 2-34. Continued G, The SK clamp (IMEX Veterinary) is a split clamp, which allows the addition of clamps between two previously installed clamps. The SK clamp accom- modates a wide range of fixation pin diameters and allows the passage of enhanced-thread pins directly through the clamp bolt. The SK system uses larger connecting rods of increased stiffness made of titanium, aluminum, or carbon fiber. Carbon fiber is illustrated here. H, The original Kirschner-Ehmer splint utilizing both double and single clamps.
Two clamp designs have become available that offer distinct improvements over the Kirschner-Ehmer clamp. The Secur-U external fixation clamp (Securos) has several design advantages (see Figure 2-34, F). This clamp may be applied to connecting bars between two previously installed clamps and allows instal- lation of enhanced-thread pins at any location. Secur-U clamps are available in small and medium sizes and are attached to traditional stainless steel connecting rods, 1/8inch (3.2 mm) and 3/16inch (4.8 mm), respectively. The small Secur-U clamp
can accommodate fixation pins with a shaft diameter of 1/6 inch (1.6 mm) or 3/32 inch (2.3 mm). The large Secur-U clamp can accommodate fixation pins with
a shaft diameter of 3/32 inch (2.3 mm) or 1/8 inch (3.2 mm). Secur-U clamps have
been shown to be more resistant to pin-clamp slippage compared with the tradi- tional Kirschner-Ehmer clamp, and type II frames constructed with Secur-U clamps are 1.64 times more rigid in axial compression than the standard Kirschner-Ehmer system.38,39
The IMEX-SK system (IMEX Veterinary, Longview, Texas) is also a relatively new system of both connecting bars and pin-gripping clamps. The SK clamp (see Figure 2-34, G) is a split clamp, which allows the addition of clamps between two previously installed clamps in an existing fixator frame. The SK clamp can accom- modate a wider range of fixation pin diameters and allows the passage of enhanced- thread pins (except cancellous center threaded) directly through the clamp bolt. Therefore, placement of enhanced-thread pins at multiple sites is much easier than the traditional Kirschner-Ehmer system. The SK system is available in three sizes: small, large, and mini. The small SK system is designed to be used in cases in which a small or medium Kirschner-Ehmer fixator would have been used. The large SK sys- tem should be used in cases in which a medium or large Kirschner-Ehmer fixator would have been appropriate. The mini SK system is designed for the smallest patients. The clamps of the SK system cannot be used in conjunction with the stan- dard stainless steel connecting bars because of size variation, except for the mini SK system. The small SK clamp can accommodate fixation pins ranging from 3/32inch
(2.3 mm) to 5/32inch (4.0 mm) in diameter. The large SK clamp can accommodate
fixation pins ranging from 1/8inch (3.2 mm) to 3/16inch (4.8 mm) in diameter. The
mini SK clamp accommodates fixation pins from 0.035 inch (0.9 mm) to 3/32inch
(2.5 mm) in diameter.