Introduction: For years traditional intramedullary nails (IMNs) have been used with great success to treat long bone fractures, however, based upon our clinical observations, we hypothesise that design changes incorporated into newer femoral IMNs reduces fracture stability resulting is a higher incidence of non-union.

AIMS: To biomechanically test the factors that may reduce fracture stability.

Materials and methods: The fracture fixation model consisted of custom made stainless steel IMNs of different wall thicknesses and outer diameters, cylinders manufactured from stainless steel, aluminium or HDPE of differing inner diameters and wall thicknesses, and 5mm rods made from stainless steel or titanium. The dimensions of the cylinders were chosen to resemble those commonly observed in the distal femur. The test nails and cylinders were connected using a single rod. Axial loading was undertaken up to 2KN (constant rate of 0.5KN/sec) and repeated a minimum of three times. The effects of various factors such as IM nail wall thickness and outer diameter, the alloy from which the rods were manufactured, and, the diameter, wall thickness and material properties of the cylinders were studied.

Results: The factors that most affected stability were the diameter, wall thickness and the material properties of the cylinders, with the least stable configuration being a HDPE cylinder with a diameter of 75mm and a wall thickness of 3mm. By reducing the diameter of the cylinder to 50mm combined with increasing the wall thickness to 5mm, stability increased considerably even when HDPE was used. The stability of each fracture fixation system was further reduced by using titanium rods.

Discussion: In clinical practice, new femoral IMNs permit longer cross screws to be inserted in the distal femur where the diameter is greatest and the cortical bone is thinnest. Since cancellous bone offers little resistance, screws effectively span from one cortex to the other gaining limited purchase in the bone. As a result, the newer IMN systems are more likely to displace regardless of the direction and force applied. This effect is exaggerated by using titanium. Overall the combination of screw length, choice of alloy and cortical thickness could easily explain our unsatisfactory clinical observations.