Delta external fixation as a suitable option for definitive fixation of osteoporotic pilon fractures in biomechanics: a finite element analysis.

Introduction: Pilon fractures are high-energy traumatic injuries characterized by complex metaphyseal fractures that involve the distal tibial articular surface, often accompanied by severe soft tissue damage. External fixation represent a crucial treatment treatment modality, particularly for elderly patients with osteoporosis (OP). This study aimed to identify the most suitable external fixation strategy for OP-associated Pilon fractures by comparing the biomechanics of three common used external fixations strategy. Methods: A finite element model was developed to simulate Ruedi-Allgower III/AO43C Pilon fractures combined with fibular fractures. This model was used to simulate gait cycles (swing and stance phases) for analysis of the key biomechanical parameters, including maximum von Mises stress (Max_VMS) in both bony structures and external fixation, as well as micromovements of fracture fragments, distal tibial fragments, the fibula, and the tibio-talar articular surface. These analyses were performed under both normal and OP bone conditions. Results: For the same external fixation and phase, the Max_VMS in bony structures was higher in normal bone than in OP bone, whereas the Max_VMS in the external fixation was significantly higher under OP conditions. Across all fixation, OP bone consistently exhibited greater fracture fragment micromovement than normal bone. Among the three fixations evaluated, the Delta external fixation showed the lowest Max_VMS on the tibio-talar articular surface (1.87 MPa in OP during the swing phase; <1.83 MPa in OP during the stance phase) and the minimal fragment micromovement. Specifically, during the swing phase, the Delta fixation restricted fragment micromovement to 0.15-0.40 mm (the optimal range for fracture healing) in both bone conditions, compared to 0.40-2.00 mm range observed with Unilateral and Ilizarov fixations. During the stance phase, Delta fixatior maintained fragment micromovement below 2 mm (the threshold for fixation failure) in both bone conditions, a threshold that the other two fixation methods exceeded. Additionally, the Delta fixation prevented excessive stress concentration on lateral distal tibial fragments and reduced anterior fragment micromovement via K-wires in the first phalanx. Discussion: Among the three external fixation simulation, The finite element analysis result shows the Delta external fixation have superior biomechanical stability-characterized by reduced stress concentration and optimal fragment micromovement-for Pilon fractures fixation, particularly in OP. Its ability to restrict micromovement facilitates healing during the swing phase, while keeping micromovement below the threshold for fixation failure during the stance phase establishes it as the preferred fixation method for OP-associated Pilon fractures, which may promote early ambulation and fracture healing.