Design and prototype production of a new external fixator pin that can dampen movement to reduce pin tract inflammation

Background

External fixation is a commonly used method in fixation of bone fractures. In this method, bones are positioned and secured by pins of the external fixator system. Although the system provides a reliable solution for fixing the fracture site, the most common problem in clinical practice of this method is pin site failure due to pin micro motions. This problem leads to the loosening of the pins which results in infection at the pin-bone interface that's called pin tract inflammation. The aim of this study is to reduce pin motions by altering the design of conventional Schanz pin so, a new pin, called Micro-Motion Damping Pin (MDP).

Methods

MDP was compared with the Schanz pin in two methods. The first method compared inter-pin motion behaviors on a same external fixator bar that was set on polyethylene rods. The second method compared bending behaviors. A universal axial test machine was used to apply force in the experiments, and a non-contact video extensometer was used to capture inter-pin motions between the first and the third pins on the proximal fragment.

Results

Although, the results showed that there was not a statistically significant differences between two pin designs in reducing the inter-pin motions (p = 0,240), the measured displacement of the MDP was less than the Schanz pin when compared. Displacements were 0,117 ± 0,02mm and 0,142 ± 0,065mm for the MDP and the Schanz pins respectively.

Conclusion

The results point out the MDP absorbed some amount of the inter-pin motion when compared to Schanz pins. Moreover, the MDP's mechanical results were superior in bending tests, and the difference was statistically significant (p = 0,002). In conclusion, the new pin can be used in an external fixation treatment, since its mechanical features are as good as the Schanz pin, however it should be evaluated on human cadaver subjects in future studies.