Continued Evolution of ESF Concepts 

by IMEX Veterinary

Since the 1980’s, external skeletal fixation has become an increasingly popular method of veterinary fracture repair. Over that same time period, one can follow a steady progression away from simple ESF frames toward more complex ESF frames, often utilizing multiple external rods and full-pins. For those of us who’s early external fixation experience was based on simple frames with smooth pins, the evolution to complex frames with threaded pins was a blessing. No longer would so many of our patients suffer from poor limb use and premature pin loosening. Evolution to complex frames with positive threaded pins was based on clinical experience and mechanical testing, and assisted in revitalizing the use of ESF. With more consistent, repeatable clinical success, practitioners became willing to utilize external fixation on more and more fractures, not just on open, contaminated injuries.

With the evolution of more stable frame constructs utilizing positive-profile threaded pins, application of the KE device became more and more time consuming, hardware intensive, and expensive. However, time and effort put into building complex frames and targeting pins yielded such improved clinical performance that “modern” ESF concepts were widely accepted and applied. Mechanical aspects of these “modern” concepts included: utilize full-pins whenever possible; complex frames decrease morbidity; and stack or augment external rods whenever full-pins are not used. Development of acrylic and pin external fixators, utilization of full-pin aiming devices, availability of rod augmentation systems, and development of ESF clamps that could be added after the fact, were additional developments that somewhat simplified the application of “modern” ESF constructs.

Simple half-pin frames are the most powerful way to simplify the ESF technique

IMEX Veterinary, Inc. developed the SK ESF system to address the limitations and frustrations associated with the KE device. Design criteria for the SK ESF clamp included the ability to effectively grip a wide range of different pin diameters, capacity to pass positive threaded pins directly through the clamp bolt, ability to add or subtract a pin or clamp after the fact, independence from expensive instrumentation to apply fixation pins and frames, and the capability of the clamp to act as an aiming device. Such design criteria facilitates the construction of “modern” ESF frames regardless of their degree of complexity.

However, the most powerful way to simplify the ESF method lies in mechanical frame improvements that are not dependent on complex frames for consistent and predictable clinical success. It has been shown that with the KE device, the external rod is the weak link in simple frame constructs (see figures 1, 2, and 3). To overcome the mechanical weakness of the KE device, surgical techniques evolved to the use of complex frames. While most new ESF devices have been designed to simplify construction of complex frames, the SK ESF system is the first veterinary ESF device to recommend and mechanically support simpler frame constructs.

While simplifying frame construction, the SK ESF system also addresses the weak rod problem with connecting rods that are significantly larger in diameter than those historically used in veterinary medicine. With external rods no longer being such a weak link, it is not necessary to build complex frames or to incorporate full-pins to achieve frame strength. In fact, a simple study of frame stiffness comparing common frame geometries of the medium KE and the large SK frames (both with identical pin number and pin size) was performed to further outline the ramifications of incorporating more rigid external rods into ESF frames (Bronson, D.G., Ross, J.D., Welch, R.D., Proceedings of Veterinary Orthopedic Society Annual Meeting, 1999). Results of that study indicate that the large SK Type I-b provides 60% of the axial strength of the most complex Type III KE frame. In cranio-caudal bending the simplest Type I-a, large SK frame exceeds all KE frames except the Type III. Comparable Type I-a and Type I-b frames exhibit a 4-fold increase in stiffness when constructed utilizing large SK rods instead of medium KE rods.

As one can see, by utilizing the strong external rods of the SK fixator, one can expect simple frame constructs to perform without loss of mechanical integrity. Since introduction of the SK system in 1998, our customers utilizing the SK fixator have progressively gained confidence in utilizing simple SK frames instead of the typical, more complex frames required with KE components.

Simple half-pin frames are the most powerful way to simplify the ESF technique because:

• Half-pins breach the integument only once-therefore reducing soft tissue morbidity and simplifying postoperative pin tract care.
• Half-pins require only one ESF clamp-therefore reducing clamp number, frame cost, and application time.
• Half-pins allow the surgeon to center the pin in the bone optimally, without concern over targeting the pin to intersect a second connecting rod, and accurate pin centering reduces the likelihood of premature pin loosening.
• Use of half-pins rather than full-pins facilitates their placement in safe, low morbidity soft tissue corridors.

• Using simpler frames as outlined above. Continue to strive for three to four pins per major bone fragment, but utilize half-pin frames whenever possible.
• If using stiff frames to promote early limb use and fracture re-vascularization, do so with a defined, staged disassembly strategy. For example, a minimal Type II frame or a Type I-b frame could be converted to a Type I-a frame as part of a destabilization strategy if necessary. Likewise, some pins could be removed from a Type I-a frame, assuming it was initially constructed with an optimal number of fixation pins and the patient shows evidence of healing (this strategy is NOT recommended with KE and similar frames). Another destabilization option is to replace a stiff connecting rod with a less stiff connecting rod (6.3 mm titanium rod replaced with 6.3 mm carbon fiber composite rod) or to downsize the frame (removal of large SK clamps and rods, replaced with small SK clamps and rods). This strategy is particularly effective with the SK ESF device due to the pin diameter overlap between different clamp sizes inherent in the design of the system. When approaching each patient, an evaluation will need to be made to determine whether or not staged disassembly is necessary.