External fixators are commonly used as a temporizing treatment for periarticular fractures about the knee. Since its inception with a claw used for patellar fractures by Malgaigne in 1853,1 external fixation has evolved to include pin–crossbar constructs. The stiffness of the construct directly affects the rate at which the frames are likely to fail.2 Most external fixation systems have the option for 2 types of pin–bar connectors, pin-to-bar clamps or multipin clamps. The multipin clamps rely on a cluster of multiple pins to connect the longitudinal supports. These clamps use the “bull horn” extensions to connect the pins to bars (Figure 1). The implant manufacturers recommend the use of 2 longitudinal bars when using these clamps. Conversely, single pin-to-bar clamps permit widely spaced pins but multipin clamps do not. Pin-to-bar clamps also tend to allow the longitudinal cross-bars to be placed closer to bone, improving frame stability.1
In the experience of Dr. Reisman, utilization of pin-to-bar clamps has resulted in improved external fixator construct stiffness compared with those using multipin clamps. He has recognized that, in his own practice, a busy level I trauma center where 4 to 5 spanning knee frames are applied daily, fracture stability is improved with the use of pin-to-bar clamps and often with only a single crossbar, resulting in a simpler, low-cost construct. Despite external fixators used for temporary fixation, frames need to be strong enough to maintain fracture length and stabilize the soft-tissue envelope for days to weeks. It is critical that the frame’s stability allows for patient transfers but controls fracture motion until definitive fixation. Despite having both options available in the external fixator set, there are no biomechanical studies that compare the effect of using pin-to-bar clamps or multipin clamps and bull horns on external fixator stiffness.
In this study, we compared the stiffness of 3 different types of spanning knee external fixator configurations, using multi-pin clamps and 2 crossbars, or pin-to-bar clamps with 1 or 2 crossbars. We compared constructs using 2 systems, 1 with 8-mm–diameter and another with 11-mm–diameter crossbars. We hypothesized that constructs assembled with pin-to-bar clamps would have improved bending stiffness compared with constructs using multipin clamps.
Materials and Methods
Three constructs were made under the supervision of Dr. Reisman, a trauma fellowship–trained orthopedic surgeon. The first construct (construct 1) used two 200-mm bars attached to pin-to-bar clamps with a single 450-mm–long spanning bar connecting the 2 segments (Figure 2). The second construct (construct 2) used 2 spanning bars with pin-to-bar clamps. The third construct (construct 3) used multipin clamps proximally and distally with two 450-mm–long spanning bars. Therefore, we tested 2 types of constructs using pin-to-bar clamps and 1 construct with multipin clamps. Four of each construct type were assembled with both 8-mm (Stryker) and 11-mm bars (Synthes), providing 24 testable constructs. For this study, we tested previously used and cleaned external fixation pins, bars, and clamps obtained from our trauma center. All equipment was examined thoroughly for any potential damaged parts.
To simulate the femoral and tibial attachments, two 5-mm–diameter pins were drilled into each of 2 steel cylinders and welded in place. The femoral cylinder (8.3×2.5 cm) had a pin distance of 55 mm, and the tibial cylinder (6.4×2.5 cm) had a pin distance of 32 mm (Figure 3). The pins were welded intosteel cylinders to help prevent any loosening or failure at the pin (ie, metal interface isolating stress to the components). Dr. Desai assembled the constructs and placed them on the cylinders with a distance of 25 mm between the fixator construct and the cylinder, with 306 mm between the femoral and tibial cylinders. The pin diameters, pin spread, pin number, and bar-to-cylinder distance were constant throughout testing with these specifications.
The assembled constructs were tested on a materials testing machine (MTS 858 Mini-Bionix Test System). A compressive force was applied, through a roller, to a flat plate (Figures 4, 5). This allowed the constructs to flex and bend freely without overly stressing the simulated pin-to-bone interface. Using this loading method, we could compare the stiffness of the different assembled constructs. Each assembled construct was tested 4 times sequentially on the MTS machine. There was no pin deformation when the load was applied through the roller to the flat plate, to the cylinder, to the pins, and onto the construct. It was possible to observe that the construct flexed when the load was applied. Load-displacement curves were produced for each test, and the stiffness was calculated from the slope of this curve. Each test was repeated 4 times, and the stiffness was measured from the load-displacement curve each time. The 4 stiffness measurements were averaged for each construct and compared across all constructs, using a Wilcoxon rank sum test for statistical analysis.
