Biomechanical properties of a structurally optimized carbon-fibre/epoxy intramedullary nail for femoral shaft fracture fixation

• A previously optimized composite intramedullary (IM) nail was mechanically tested.
• The new IM nail is significantly more flexible in compression than in bending.
• The CF/epoxy IM nail is sufficiently strong in all clinical-type loading.
• The proposed nail has a negligible moisture absorption rate (~0.5%).
• The new nail could potentially provide a preferred mechanical environment for healing.

Intramedullary nails are the golden treatment option for diaphyseal fractures. However, their high stiffness can shield the surrounding bone from the natural physiologic load resulting in subsequent bone loss. Their stiff structure can also delay union by reducing compressive loads at the fracture site, thereby inhibiting secondary bone healing. Composite intramedullary nails have recently been introduced to address these drawbacks. The purpose of this study is to evaluate the mechanical properties of a previously developed composite IM nail made of carbon-fibre/epoxy whose structure was optimized based on fracture healing requirements using the selective stress shielding approach. Following manufacturing, the cross-section of the composite nail was examined under an optical microscope to find the porosity of the structure. Mechanical properties of the proposed composite intramedullary nail were determined using standard tension, compression, bending, and torsion tests. The failed specimens were then examined to obtain the modes of failure. The material showed high strength in tension (403.9±7.8403.9±7.8 MPa), compression (316.9±10.9316.9±10.9 MPa), bending (405.3±8.1405.3±8.1 MPa), and torsion (328.5±7.3328.5±7.3 MPa). Comparing the flexural modulus (41.1±0.941.1±0.9 GPa) with the compressive modulus (10.0±0.210.0±0.2 GPa) yielded that the material was significantly more flexible in compression than in bending. This customized flexibility along with the high torsional stiffness of the nail (70.7±2.070.7±2.0 N m2) has made it ideal as a fracture fixation device since this unique structure can stabilize the fracture while allowing for compression of fracture ends. Negligible moisture absorption (~0.5%~0.5%) and low porosity of the laminate structure (< 3%) are other advantages of the proposed structure. The findings suggested that the carbon-fibre/epoxy intramedullary nail is flexible axially while being relatively rigid in bending and torsion and is strong enough in all types of physiologic loading, making it a potential candidate for use as an alternative to the conventional titanium-alloy intramedullary nails.