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Biomechanical Analysis of Various Insertional Techniques in External Fixation Pin Insertion
Dillon Arango, MD1; Michael Narvaez, MD1; Nathan Tiedeken, MD2; William Smith, MD3; James Raphael, MD1
1Einstein Medical Center, Philadelphia, PA, 2University of Pittsburgh Medical Center, Pittsburgh, PA, 3Temple University Hospital, Philadelphia, PA
Introduction: Throughout the evolution of external fixators, the principles affecting stability of a construct remain unchanged. While this stability is affected by numerous variables, the quality of the pin-bone interface has proven to be the most important. With multiple pin design/insertion technique combinations, lack of a standardized technique has led to debate as to which combination creates the stiffest construct. The aim of this study was to evaluate self-drilling and self-tapping pins inserted with either bicortical or unicortical fixation. This study is the first to biomechanically evaluate pin insertion techniques for one of the oldest and most common forms of fracture fixation.
Methods: Techniques were evaluated with the use of 5.0 mm self-drilling and self-tapping pins. Bicortical self-drilling (BCSD), bicortical self-tapping (BCST), unicortical self-drilling (UCSD), and unicortical self-tapping (UCST) were tested (Figure 1). Pre-drilling for the self-tapping pins was completed with a 4.0 drill bit using ¾ inch diameter PVC pipe as the insertional medium. PVC was used to reduce the variability encountered when using bones. The PVC – external pin constructs were then tested in a cantilever bending mode at a loading rate of 5 mm/min using a servo-electric mechanical testing system. Ten trials of each technique were analyzed.
Results: Bicortical self-drilling (BCSD) insertion technique had the highest yield strength and stiffness compared to all other insertion techniques (P < 0.0001). Self-drilling pins were significantly stronger to bending forces compared to self-tapping pins inserted with the same techniques (P < 0.0001). Three point bending tests of the pin threaded area showed that 5.0 mm self-drilling pins had yield strength of at least 300 N higher than the 5.0 mm self-tapping pins (Figure 2).
Discussion: Biomechanical analysis of various insertional techniques for external fixation demonstrate that bicortical fixation with self-drilling pins achieved the greatest resistance to bending strength. Surgeons must evaluate the clinical situation and determine what technique will allow the greatest biomechanical fixation, while understanding the surrounding soft tissues and local biology. Depending on the pin used, self-tapping and self-drilling pins may behave differently with regard to ultimate yield strength and stiffness. This could be attributed to the thread design. This is the first study to evaluate the biomechanical properties of insertion techniques for external fixation.
Figure 1: Biomechanical setup (left) and examples of unicortical and bicortical pin insertion techniques.
Figure 2: Comparison of failure strength from four different pin insertion techniques
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