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 3⁄4 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. Introduction External fixation is a temporizing, and at times definitive, fixation method used in a wide array of orthopaedic scenarios. Consisting of pins, connecting bars and clamps, this construct can be rapidly applied with minimal insult to surrounding soft tissues.1,2 External fixation constructs have evolved to include a multitude of applications and forms, yet the main principles that affect the overall stability of the construct remain the same. Factors affecting stability include pin number/size/placement, the distance between the bone and connecting rods, the number of connecting rods, and the pin to bone interface.3,4 While many factors affect the stability of external fixators, the pin-bone interface has been noted to be the most important in determining both long-term strength and survivability of the construct.4 This interface depends on the bone quality, pin design, and insertion technique. While the quality of the bone is not controlled by the surgeon, the insertion technique and pin selection are modifiable variables that affect the stability and longevity of an external fixation frame.5,6 The insertion technique is influenced by pin design with the two most common pin types being SD and ST pins. These half threaded pins include built in taps and flutes to allow for egress of bone dust. SD and ST pins can decrease operative time allowing for a single insertion which bypasses the need for predrilling or tapping. Non SD and non-ST pins require pre-drilling or pre-tapping before final pin insertion. Pin insertion technique is one of the most important biomechanical principles to maximize the pin-bone interface and help prevent pin loosening.7 With the existence of multiple pin insertion techniques and no standardized insertion method, there has been debate as to which technique creates the stiffest construct. Current concern focuses on the theoretical increased risk of loosening with bicortical pin fixation. The suggested mechanism of loosening occurs after the near cortical hole is drilled. As the pin engages the second cortex and begins drilling the far cortex, the near cortical hole enlarges due to threads constantly spinning until the far cortex is engaged with the pin threads. This results in the far cortex being the only site of thread engagement, which may potentially increase the risk for pin loosening.8-11 To prevent this, some surgeons advocate for engaging the tip of the half threaded pin in the far cortex without over drilling the near cortex threads. Arbeitsgemeinschaft für Osteosynthesefragen (AO Trauma Foundation) recommends that an external fixation pin be placed with minimal purchase in the far cortex when constructing an external fixation frame.12 The aim of this biomechanical study was to evaluate construct stiffness using SD and ST pins that were inserted with either bicortical or unicortical fixation. To the authors’ knowledge, this study is the first to biomechanically evaluate pin insertion techniques for one of the oldest and most common forms of orthopaedic fracture fix-