Extramedullary devices are used extensively to stabilize fractures in long bones. The type of pin-bone anchorage is a determining factor in fixation properties, which differ between mono-cortical and bi-cortical stabilizations. This computational study compares the effects of mono-cortical and bi-cortical pins of a unilateral uniplanar external fixator on the construct stiffness, early phase of bone healing, and pin loosening. Eight finite element models were established for a simple transverse tibia fracture, treated with a unilateral uniplanar external fixator with surgical variations in the pin-bone anchorage. Each model was subjected to a partial body weight, and axial stiffness was calculated. A deviatoric strain-based mechano-regulation algorithm was applied, and tissue differentiation in the callus was predicted. Finally, a strain-based failure criterion was employed to assess the risk of pin loosening. The axial stiffnesses of bi-cortical structures were slightly larger than the results of the mono-cortical sets. Regardless of the number of pins, bi-cortical systems produce a more uniform distribution of differentiated tissue than the corresponding mono-cortical constructs. Finally, both mono-cortical and bi-cortical groups held the critical strains of the pin-bone interface within the acceptable ranges and provided a protected construct against the risk of pin loosening. Based on the findings of this study, mono-cortical pins could be considered potential alternatives to bi-cortical fixations at the early stage of healing. Nevertheless, successful management of diaphyseal fracture through mono-cortical fixation needs to be assessed over the full period of healing, in further studies.

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