OBJECTIVES: Anatomic constraints about the proximal tibia limit the ability to insert wires at the biomechanically optimum angle of 90 degrees, thus diminishing the potential stability of external fixators used for proximal tibial fractures. To overcome this problem, surgeons use more than 2 wires at a single level of fixation or a second level of fixation. This study evaluated the effect of transfixion wire number and placement of a second level of fixation on the stiffness of proximal tibial external fixation. METHODS: A fiberglass tibia fixed into an idealized ring external frame was tested. Load-deformation behavior was compared among the different wire numbers (1 level with 2, 3, 4, and 5 wires) and placement of a second level of fixation (2 wires first level and 1 wire second level, and 2 wires first level and 2 wires second level) at 2, 3, 4, 5, 6, 7, 8, 9, and 10 cm distance from the first level of fixation. Identical loading conditions of central axial compression, medial compression-bending, posterior compression-bending, posteromedial compression-bending, and torsion were used. Stiffness values were calculated from the load-displacement and the torque-angle curves. RESULTS: An increase in wire number at 1 level led to an overall increase in stiffness, whereas the addition of a second level of fixation and increased spread between these 2 levels increased bending stiffness. CONCLUSIONS: The addition of a second level of external fixation of the proximal tibia fragment with maximum possible distance between the 2 levels increases bending stiffness, whereas increasing the number of transfixion wires increases overall stiffness. Axial and torsional stiffness is proportional to the total number of wires regardless of the number of levels of fixation.
OBJECTIVES: Anatomic constraints about the proximal tibia limit the ability to insert wires at the biomechanically optimum angle of 90 degrees, thus diminishing the potential stability of external fixators used for proximal tibial fractures. To overcome this problem, surgeons use more than 2 wires at a single level of fixation or a second level of fixation. This study evaluated the effect of transfixion wire number and placement of a second level of fixation on the stiffness of proximal tibial external fixation. METHODS: A fiberglass tibia fixed into an idealized ring external frame was tested. Load-deformation behavior was compared among the different wire numbers (1 level with 2, 3, 4, and 5 wires) and placement of a second level of fixation (2 wires first level and 1 wire second level, and 2 wires first level and 2 wires second level) at 2, 3, 4, 5, 6, 7, 8, 9, and 10 cm distance from the first level of fixation. Identical loading conditions of central axial compression, medial compression-bending, posterior compression-bending, posteromedial compression-bending, and torsion were used. Stiffness values were calculated from the load-displacement and the torque-angle curves. RESULTS: An increase in wire number at 1 level led to an overall increase in stiffness, whereas the addition of a second level of fixation and increased spread between these 2 levels increased bending stiffness. CONCLUSIONS: The addition of a second level of external fixation of the proximal tibia fragment with maximum possible distance between the 2 levels increases bending stiffness, whereas increasing the number of transfixion wires increases overall stiffness. Axial and torsional stiffness is proportional to the total number of wires regardless of the number of levels of fixation.