Biomechanical assessment of composite versus metallic intramedullary nailing system in femoral shaft fractures: A finite element study

AbstractBackgroundIntramedullary nails are the primary choice for treating long bone fractures. However, complications following nail surgery including non-union, delayed union, and fracture of the bone or the implant still exist. Reducing nail stiffness while still maintaining sufficient stability seems to be the ideal solution to overcome the abovementioned complications.MethodsIn this study, a new hybrid concept for nails made of carbon fibers/flax/epoxy was developed in order to reduce stress shielding. The mechanical performance of this new implant in terms of fracture stability and load sharing was assessed using a comprehensive non-linear FE model. This model considers several mechanical factors in nine fracture configurations at immediately post-operative, and in the healed bone stages.ResultsPost-operative results showed that the hybrid composite nail increases the average normal force at the fracture site by 319.23N (P<0.05), and the mean stress in the vicinity of fracture by 2.11MPa (P<0.05) at 45% gait cycle, while only 0.33mm and 0.39mm (P<0.05) increases in the fracture opening and the fragments' shear movement were observed. The healed bone results revealed that implantation of the titanium nail caused 20.2% reduction in bone stiffness, while the composite nail lowered the stiffness by 11.8% as compared to an intact femur.InterpretationOur results suggest that the composite nail can provide a preferred mechanical environment for healing, particularly in transverse shaft fractures. This may help bioengineers better understand the biomechanics of fracture healing, and aid in the design of effective implants

Biomechanical analysis using FEA and experiments of metal plate and bone strut repair of a femur midshaft segmental defect

© 2018 Jason Coquim et al. This investigation assessed the biomechanical performance of the metal plate and bone strut technique for fixing recalcitrant nonunions of femur midshaft segmental defects, which has not been systematically done before. A finite element (FE) model was developed and then validated by experiments with the femur in 15 deg of adduction at a subclinical hip force of 1 kN. Then, FE analysis was done with the femur in 15 deg of adduction at a hip force of 3 kN representing about 4 x body weight for a 75 kg person to examine clinically relevant cases, such as an intact femur plus 8 different combinations of a lateral metal plate of fixed length, a medial bone strut of varying length, and varying numbers and locations of screws to secure the plate and strut around a midshaft defect. Using the traditional “high stiffness” femur-implant construct criterion, the repair technique using both a lateral plate and a medial strut fixed with the maximum possible number of screws would be the most desirable since it had the highest stiffness (1948 N/mm); moreover, this produced a peak femur cortical Von Mises stress (92 MPa) which was below the ultimate tensile strength of cortical bone. Conversely, using the more modern “low stiffness” femur-implant construct criterion, the repair technique using only a lateral plate but no medial strut provided the lowest stiffness (606 N/mm), which could potentially permit more in-line interfragmentary motion (i.e., perpendicular to the fracture gap, but in the direction of the femur shaft long axis) to enhance callus formation for secondary-type fracture healing; however, this also generated a peak femur cortical Von Mises stress (171 MPa) which was above the ultimate tensile strength of cortical bone