Strain Analysis of Pin Insertion Sites in an External Fixation Model of the Ankle Joint Using Digital Image Correlation: Quantitative Assessment of Pin Loosening Risk

Abstract Background The use of temporary external fixation as initial treatment for fracture-dislocations of the ankle joint is well established. However, repeated loading at the interface between external fixation pins and bone may increase the risk of pin loosening. In the current study we used stereo digital image correlation, a strain measurement technique, to evaluate the risk of pin loosening by measuring strain on the bone surface at pin insertion sites. Methods An ankle joint model fitted with an external fixator was stabilized using three different calcaneal pin insertion configurations: (1) the parallel configuration, in which two transfixion pins are inserted parallel to each other through the calcaneus. (2) The cruciate configuration, in which a transfixion pin and a half-pin are inserted perpendicular to each other at 90°. (3) The crossed configuration, in which two half-pins are inserted at an angle of 45°. Loads were applied to the forefoot in the directions of dorsiflexion and plantarflexion, and strain distribution was analyzed based on displacement in three-dimensional images. Results In the cruciate configuration, strain was greater on the bone surface around the calcaneal half-pin, and stress concentration occurred at the insertion site. In the parallel configuration, strain was greater around the transfixion pins. In the crossed configuration, strain around the half-pins was the lowest. Conclusion This is the first quantitative evaluation of strain at external fixation pin insertion sites using stereo digital image correlation. Calcaneal pins in the cruciate and parallel configurations resulted in greater strain at the pin–bone interface, suggesting that these configurations may be risk factors for pin loosening. The crossed configuration resulted in lower strain and is considered to provide mechanically stable external fixation.