Evaluation of the Structural Behaviour of a Unilateral External Fixator for Osteosynthesis

Mara Terzini1, 2, Simona Sicuranza1, 2, Flavia Alberghina3, Laura Ravera3, Domenico Costantino Aloj3, Cristina Bignardi1, 2, *
1 Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy
2 Polito BIOMed Lab, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy
3 Orthopedics and Traumatology Complex Structure AslVC, Sant’Andrea Hospital, Corso Mario Abbiate, 21, 13100 Vercelli, Italy

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© 2021 Terzini et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the Polito BIOMed Lab, Politecnico di Torino, Corso Duca degli Abruzzi, Torino, Corso Duca degli Abruzzi 24, 10129 Italy;



External fixation is an osteosynthesis technique particularly useful in trauma surgery and Damage Control Orthopedics (DCO). However, complications, such as pin loosening and pin tract infections, are fairly common. For reducing thermal damage and infection rates, monocortical pins have been proposed as an alternative to the most used bicortical pins. However, there is a lack of studies regarding their mechanical properties.


The aim of the study is to assess the static and dynamic stability of a unilateral external fixator experimentally when applied through monocortical pins for the reduction of femur and tibia fractures.


A modular unilateral external fixator was used and a total number of 6 pins were used per test. The static tests were performed in displacement control by applying a vertical displacement to the upper fixture at 1 mm/min until a tension load of 380 N was reached. The dynamic tests were performed by applying a sinusoidal displacement. During each test, forces and crosshead displacements were acquired. Two different stiffness indexes were assessed.


By comparing the two anatomic regions, it was found that the fixator behaves stiffer when mounted on the femur, regardless of the pins used, while stiffnesses comparable to the femur ones are reached by the tibia when 4 mm diameter pins are used. Static analysis revealed excellent fixator stability when implanted with 4 mm diameter monocortical pins on both anatomic regions. On the contrary, two tibia and one femur samples showed failures at the bone-pin interface when 3 mm diameter pins were used.


Dynamic analysis showed no substantial difference between the tested configurations and confirmed the fixator's ability to sustain cyclic loading without further damage to the sample.

Keywords: Skeletal fractures, Osteosynthesis, External fixation, Monocortical pins, Stiffness, Orthopedics.