Biomechanical analysis of four external fixation pin insertion techniques

Having multiple external fixation pin designs and insertion techniques has led to debate as to which combination creates the stiffest construct. This study sought to biomechanically evaluate construct strength using self-drilling (SD) and self-tapping (ST) pins inserted with either bicortical or unicortical fixation. SD and ST 5.0 mm stainless steel pins were used in combination with bicortical self-drilling (BCSD), bicortical self-tapping (BCST), unicortical self-drilling (UCSD), and unicortical selftapping (UCST) techniques. Pre-drilling for the self-tapping pins was completed with a 4.0 mm drill bit using ¾ inch polyvinyl chloride (PVC) pipe as the insertional medium. The PVC pin constructs were then loaded to failure in a cantilever bending method using a mechanical testing system. Ten trials of each technique were analyzed. BCSD insertion technique had the highest maximum failure force and stiffness of all tested techniques (P<0.0001). SD pins were significantly stronger to bending forces than ST pins in both the unicortical and bicortical setting (P<0.0001). Three point bending tests of the 5.0 mm SD and ST threaded area showed that threaded portion of the SD pins had a 300 N greater maximum failure force than the ST pins. Biomechanical analysis of external fixation pin insertion techniques demonstrates that bicortical fixation with SD pins achieved the greatest resistance to bending load. Despite both pins being 5.0 mm and constructed from stainless steel, ST and SD behaved differently with regard to maximum failure force and stiffness. This study demonstrates that insertion technique and pin selection are both important variables when attempting to achieve a stiff external fixation construct.