Intramedullary nails

Intramedullary nails, used to repair fractured femurs, are currently made from stainless steel or a titanium alloy. The nail is slightly curved (typically a 1 cm bow over a 30 cm length) and hollow. Some designs have a longitudinal slit and holes at either end in which to locate fixing screws. Ideally, the nail should have similar mechanical properties to those of cortical bone, be capable of some bending deformation and have sufficient strength to allow the insertion of fixing screws. The mechanical properties for current materials are not ideally suited to this application, their stiffness being an order of magnitude higher than that of bone. This difference in rigidity means that there is not a uniform load distribution in the region of bone and implant and this can impede healing. Bone is a natural composite material and its properties vary with direction and water content. Typical bone has the following longitudinal and transverse properties:

• longitudinal modulus 17.11 GPa
• transverse modulus 11.5 GPa,
• axial Poisson's ratio 0.64
• transverse Poisson's ratio 0.58
• tensile strength 148 MPa
• transverse strength 49 MPa
• compressive strength 193 MPa
• transverse compressive strength 133 MPa

The bending and shear stiffness of a human femur are found to be 1.7 x 10² < EI < 3.1 x 10² N m² and 1.5 x 10² < GJ < 2.1 x 10² N m². The variation is due to the changing geometry of the femur. The objective of any design, therefore, is to match these stiffness characteristics as closely as possible. Typical geometrical constraints of a nail are inside and outside diameters of 5 and 12 mm respectively. A good way to manufacture a composite component with a substantial degree of axial symmetry is by filament winding. The proposed lay-up of the carbon fibre was, starting at the inside wall, a +q  layer of 1.25 mm thickness, a -q layer of 1.25 mm thickness and a 1 mm thick hoop layer. The latter was included to assist in compressing the ±q layers and to improve transverse strength and the ability of the structure to take fixing screws. The following properties were assumed for a 60 vol.% unidirectional carbon fibre epoxy composite: longitudinal modulus, E₁ 140 GPa, transverse modulus, E₂ 9 GPa, in-plane shear modulus G₁₂ 5.5 GPa, and Poisson's ratio, n₁₂ 0.18, and a laminate analysis carried out to determine the elastic properties of a [± 0, 90], lay-up. The results are shown below.

Elastic properties for possible laminate configurations

It is clear that it is not possible to obtain both E₁  and E₂  in the desired range together with an appropriate value of G₁₂ to match the properties of bone. For simplicity of manufacture, however, it was decided not to choose a more elaborate winding pattern. That selected was [ ± 35, 90], as this gave substantially similar longitudinal and transverse moduli, higher than that of bone, but lower than those of stainless steel or titanium.