Mechanical Characterization of a Totally Intramedullary Gradual Elongation Nail

Abstract

An intramedullary gradual elongation nail (Albizzia) capable of fulfilling the function of traditional intramedullary nails while providing the gradual, controlled distraction of an external fixator was designed for progressive lengthening of lower limbs. In this study, the biomechanics of the gradual elongation nail were compared with several intramedullary nails: Grosse & Kempf, Russell-Taylor, AO, and Laffay. Bending stiffness, torsional stiffness, ultimate bending strength, and torsional strength were determined using the American Society for Testing and Materials standard F383-73 as a guide. The results show that in unextended and elongated conditions, the gradual elongation nail has torsional stiffness (1-5 Nm2) comparable with the AO nail (2 Nm2) and bending stiffness (41-89 Nm2), ultimate bending (246 Nm), and torsional (28-37 Nm) strengths within the ranges obtained for other intramedullary nails (27-105 Nm2, 167-298 Nm, and 2-100 Nm, respectively). Additionally, the low torque required to lengthen the device under a 500 N load (3 Nm) and the low longitudinal stiffness because of the active dynamization system with bimodal load deformation characteristics (80-120 N/mm initial, 600-800 N/mm secondary) produce a device with almost no torsional and longitudinal stress shielding. From a biomechanical point of view, this single, completely implantable device is a safe, viable, and efficacious alternative to external fixation for progressive lengthening of lower limbs.