Abstract
Intramedullary nails are the primary choice for treating long bone fractures. However, the high axial rigidity of conventional nails, can significantly reduce compression at the fracture site, and thereby inhibit bone healing. It can also lead to subsequent bone loss upon fracture healing. Fibrereinforced composites have been suggested as an alternative material of choice in the design of fracture fixation implants to address these drawbacks. There are very few studies in the literature on the use of composite materials for intramedullary nails. In particular, there are no known studies which have considered the optimization of such implants to fulfill the requirements of a proper fracture healing. The purpose of the current thesis is to develop a composite intramedullary nail made of carbon-fibre/epoxy whose structure is optimized to provide a preferred mechanical environment for fracture healing. The thickness and stacking sequence of the composite tube were optimized using closed-form expressions for structural rigidities of a composite tube to minimize axial rigidity of the structure, while minimally sacrificing the bending and torsional rigidities. The actual performance of the best nail candidates inside the human femur was then examined in an experimentally validated finite element model. It was found that a composite nail with an outer diameter of 14 mm and a stacking sequence of [0 / -45 / 45 / -45 / 0 / -45 / 45 / -45 / 45 / -45 / 90 ] showed an overall superiority compared to the other configurations. The expressions for rigidity yielded an axial rigidity of 3.7MN , and bending and torsional rigidities of 70.3 and 70.9 2 N.m , respectively, which correlated well with the results of mechanical testing on the manufactured specimens (i.e. 3.74±0.05 MN, 66.9±1.0 2 N.m , and 70.7±2.0 2 N.m for axial, bending, and torsional rigidities, respectively). The manufactured composite specimens showed high strength in tension (403.9±7.8 MPa), compression (316.9±10.9 MPa), bending (405.3±8.1 MPa), and torsion (328.5±7.3 MPa). Moreover, a fatigue limit of 27 kN was obtained for the composite nail. The stiffness of the composite nail was found to remain almost constant versus the number of cycles. Overall, the findings of this thesis suggest that the proposed intramedullary nail is a potential candidate for use as an alternative to the conventional intramedullary nails.
Highlights
1.1 Background motivationThe femur is the longest and strongest bone in human body
In a fractured femur fixed with an IM nail, the stress shielding effect of the implant can cause the density in the bone to be lower than it was pre-operatively
A configuration used for carbon fiber (CF)/flax/epoxy in the literature [81], and a Ti bone plate were included in the table for the comparison purposes, resulting in 15 configurations in total (C1-C15)
Summary
1.1 Background motivationThe femur (thigh bone) is the longest and strongest bone in human body. Optimization of a fibre-reinforced composite bone plate was performed prior to optimizing a composite IM nail in order to test the hypothesis that a composite fixation device could be optimized based on the selective stress shielding approach to increase loading at the fracture site while maintaining the fracture stability. Both fracture plate and IM nail are used for treating femoral shaft fractures, the use of fracture plates is limited to those cases where nailing is technically impractical. It was hypothesized that the composite nail would have a high cycle fatigue strength (HCFS) that is much higher than the clinical-type loads experienced by an IM nail in physiologic activities
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