Design and preliminary testing of an active intramedullary nail.

PURPOSE To enhance bone healing through controlled interfragmentary movements, numerous experiments have been conducted in animal models employing external fixation devices to apply mechanical stimulation to the fracture site. However, the efficacy of these fixators has been questioned. On the other hand, intramedullary nailing is a widely established clinical practice for reducing closed tibial fractures. MATERIAL AND METHODS In an effort to enhance bone healing, to overcome the disadvantages of external fixators (i.e., non-uniform linear movement), and to enhance the advantages of intramedullary nailing (i.e., reduced risk of infection), an active intramedullary nail has been designed and fabricated. Active nail will provide controlled in-situ stimulation (simultaneously axial and shear) from a selectable acceleration (0.35 to 8.17g - axial and 0.44g to 10.46 g - shear), associated to a discreet set of high-frequency values (29.82 - 172.05 Hz - axial and 29.68 to 172.13 - shear). RESULTS Five active intramedullary nails were fabricated, capable of producing average acceleration between 0.35 and 10.4 g. Acceleration is applied simultaneously by all three axes (x, y, and z), resulting in axial and shear stimulation. For each acceleration level, there are a limited number of frequencies that can be selected. For each frequency, there are a limited number of acceleration levels that can be delivered. Bone morphology produces different levels of acceleration in each axe. Acceleration levels are controlled externally only by the variable power source (1.5VDC to 6VDC). Accelerated in-vitro testing showed that the life of the device exceeded the required active period. Mechanical test showed that in case of failure of the active component, the active intramedullary nail will act as a standard nail, allowing bone healing to continue its normal course. Ex vivo experiments were conducted inserting one active intramedullary nail in two intact adult sheep tibia. Results indicate that the strain induced by the active intramedullary nail (from 18.62 με to 38.13 με) has been reported to be osteogenic. Additional experiments are required in order to statistically validate the strain that can be induced in vivo by the active intramedullary nail. Also, in vivo experiments using simple fractures of the tibial shaft need to be conducted in order to assess if effectively, applying active mechanical stimulation in situ enhances bone healing.