a flexible inramedullary fixation rod for fractured bones is provided having a length sufficient to extend a substantial distance on either side of a diaphyseal fracture site and a substantially circular core with radially disposed fins extending substantially along the full length thereof . the radii of the core and fins are such that the rod can pass through the bone &# 39 ; s medullary canal and engage the inner , or endosteal , surface of the bone along line contacts , thereby allowing normal endosteal healing and intramedullary revascularization . the rod is constructed of a flexible , bio - compatible material , such as titanium , stainless steel , resorbable polymers or carbon fiber - polysulfone . the radii of the fins are selected to be substantially equal to the minimum radius of the medullary canal . the core radius is selected in accordance with the young &# 39 ; s and shear moduli associated with the bio - compatible material to provide a rod structural rigidity preferably between 0 . 1 % and 10 % of the inherent bone rigidity with the relationship between the rod bending and torsional rigidities preferably being substantially the same as the corresponding relationship for the intact bone . this causes natural stresses to be maintained at the fracture site , thereby promoting natural and therefore stronger healing of the fractured bone .