Results
Construct Design
Three different construct designs were evaluated using our testing protocol. The mean stiffness differed across all constructs as seen in Figure 6. Of the constructs using the 11-mm–diameter bars, construct 2 had the highest mean stiffness (32.1 +/- 3.7 N/mm), and this stiffness was significantly greater than the mean stiffness for construct 1 (15.3 +/- 1.5 N/mm; P < .05) and construct 3 (18.4 +/- 2.9 N/mm; P< .05). There was no statistically significant difference in stiffness between construct 1 and construct 3.
Of the constructs using 8-mm–diameter bars, construct 2 had the highest mean stiffness (11.5 +/- 2.4 N/mm), and this stiffness was significantly greater than the mean stiffness for construct 1 (5.0 +/- 0.9 N/mm; P < .05). There was no statistically significant difference in stiffness between construct 2 and construct 3 (7.8 +/- 1.9 N/mm) or between construct 1 and construct 3.
Discussion
Although numerous investigators have examined the biomechanical properties of external fixator systems, the effect of pin-to-bar clamps on frame stiffness is unknown. Biomechanical studies have found that uniplanar constructs with multiple bars can provide adequate strength for temporary fixation.3-9 With multiple options within a particular external fixator set, it is ideal to understand the benefit of using one component instead of another.
The main results from this experiment are: (1) constructs with pin-to-bar clamps and 2 crossbars are stiffer than those using multipin clamps and 2 crossbars; (2) constructs with a single crossbar and pin-to-bar clamps are as stiff as constructs using 2 crossbars and multipin clamps.
Figure 6 shows the average stiffness differences between the 8-mm and 11-mm–diameter bar constructs tested in this study. As expected, each 11-mm diameter–bar construct had a higher average stiffness compared with the 8-mm–diameter bar constructs. Across both the 8-mm and 11-mm–diameter bar constructs, construct 2 had a higher stiffness than that of constructs 1 and 3. Furthermore, there was no difference in the stiffness between constructs 1 and 3.
To improve external fixator stiffness, number of pins and optimization of pin spread can improve the strength of the construct.7 When using pin-to-bar clamps, 1 pin should be as close to the fracture as possible, with the second pin as far from the fracture as possible. 7 Multipin clamps, by design, prevent any optimization of pin spread and require a clustered-pin arrangement.
Bar configuration also plays a critical role in construct stiffness. Bar-to-bone distance should be approximately 2 fingerbreadths from the skin to maximize the stiffness of the construct.4,10-14 Multipin clamps use “bull horn” extensions that tend to elevate the bar away from the skin, increasing the distance between the bar and the bone.
A temporary spanning knee external fixator is commonly used for treating high-energy periarticular tibial or femoral fractures. To hold the fracture in an adequately reduced position, the frame must resist the deforming forces inherent with all fractures. A frame that is not adequately stiff will not hold the fracture in the reduced position, even at the time of initial surgery, which negates one of the benefits of placing the patient in the frame. Hence, adequate stiffness of the spanning-knee fixator is critical to the effectiveness of temporary stabilization before permanent fixation.
The results of this study provide evidence for the superiority of pin-to-bar clamps over multipin clamps in optimizing external fixator construct stiffness. At our institution, we almost exclusively use the single pin-to-bar clamps for spanning-knee external fixation. Based on the results of this study, we often use only a single crossbar. The ability to use a single bar greatly reduces the cost of the construct because crossbars can cost from $100 to $150, depending on the manufacturer.
A recent cost analysis of spanning-knee external fixators showed that construct costs can range from $8,000 to $19,000.15 The lower-cost constructs included 2 crossbars while the more expensive constructs had additional bars and multipin clamps. The authors noted that constructs with larger diameter bars and higher overall stiffness resulted in an improved cost per stiffness ratio. The results of this study support our conclusions regarding bar diameter. Additionally, our results show improved stiffness of constructs with pin-to-bar clamps instead of multipin clamps. By limiting the need for an additional bar, using pin-to-bar clamps and a single large diameter crossbar can create a very cost-efficient and rigidly stable construct.
One criticism of this study is the testing of used equipment. All external fixator manufacturers must evaluate and carefully examine any used equipment prior to the resterilization process and potential release to the practitioner for re-use. Our rationale for using used equipment is based on the assumption that the vast majority of patients do not have their external fixators removed because of failure but because of definitive surgical treatment, and the timing of removal does not necessarily follow a predetermined protocol. For example, timing of definitive surgery is usually set by the patient’s general health status, status of the soft tissues, and surgeon availability. Therefore, this equipment was tested with the presumption that the equipment was in the same state as if the patient continued to wear the frame 1 more day. A study testing unused equipment would be the next step in evaluating external fixators.