Biomechanical analysis of unilateral external skeletal fixators combined with IM-pin and without IM-pin using finite-element method

Objectives - To determine if a unilateral external skeletal fixator (ESF) with a carbon fiber connecting rod (IMEX^™ SK) without an intramedullary (IM)-pin is mechanically comparable with a unilateral ESF with a stainless-steel connecting rod (IMEX^™ KE) with an IM-pin. Study Design - Finite-element method (FEM) - computer simulation. Methods - FEM models were validated by comparison against data from mechanical testing. Three-dimensional FEM models of a femur with a mid-diaphyseal fracture with a 20 mm gap were developed with 4 unilateral external skeletal fixator devices (6-pin KE, 6-pin KE IM-pin, 6-pin SK, and 6-pin SK IM-pin). A 300 N load was applied to the femur at the proximal end in a direction of θ= 10°distally and φ=10° laterally cranially. Relative displacements in x-, y- and z-directions at the gap were obtained and the overall stiffness was calculated as 300 N/total displacement. Load transfer at the pin-bone interface (PBI) was assessed by determining the von Mises stress maxima at the PBI-related nodes. Results - The 6-pin SK had superior mechanical performance compared with the 6-pin KE by exhibiting smaller displacements in all directions and higher stiffness. Compared with the 6-pin KE IM-pin, the 6-pin SK (without IM-pin) was superior in craniocaudal and lateromedial displacements, but inferior in axial displacements, overall stiffness and von Mises stress maxima. The 6-pin SK IM-pin was superior to the 6-pin KE IM-pin based on smaller displacements and higher stiffness. Conclusions - Although the SK device had superior mechanical performance compared with a KE device in a unilateral configuration, the addition of an IM-pin continues to be a powerful method of enhancing mechanical performance of either IMEX^™ SK or IMEX^™ KE unilateral constructs in clinical cases. Clinical Relevance - Based on the results of this FEM study we recommend the use of the "tied-in" IM-pin with the ESF clinically when striving for high rigidity. In less challenging situations, a unilateral SK ESF without IM-pin might provide sufficient rigidity for a successful fracture repair.