Improvement of the shear fixation stability of intramedullary nailing.

Background: The healing outcome of long bone fractures is strongly influenced by the mechanical environment. High interfragmentary movement at the fracture site is detrimental to the fracture healing process. Long bone fractures stabilized with thin intramedullary nails commonly used for unreamed intramedullary nailing might be very flexible in shear direction and therefore critical for the fracture healing outcome. The aims of this study were to simulate the shear interfragmentary movement during gait for a human tibia treated with intramedullary nailing and to investigate if this movement could be lowered by implant design modifications.

Methods: The shear movement was calculated with a 3D finite element model based on computer tomograph images of a cadaver bone-implant complex of a transverse tibia fracture treated with a Stryker T2 Standard Tibial Nail. This model was validated through in vitro test results under pure shear, axial, bending and torsional loading.

Findings: High shear movements of approximately 4mm were calculated during gait. These shear movements could be reduced by approximately 30% either by implant modifications or the use of a 1mm thicker nail. Combining the implant modifications with a 1mm thicker nail, the shear movements could be reduced by 54%.

Interpretation: The increase of the fixation stiffness by using an implant material with a high Young's modulus in combination with an angle-stable nail-screw fixation helps to reduce the shear movement during gait and possibly to lower the risk of a prolonged healing time with unreamed intramedullary nailing.