Optimizing the biomechanical compatibility of orthopedic screws for bone fracture fixation.

Progressive loosening of bone fixation screws is a well-documented phenomenon, induced by stress shielding and subsequent adaptive bone remodeling which results in bone loss around the screw. A set of two-dimensional computational (finite element) models was developed in order to test the effect of various engineering designs of fixation screws on the predicted screw-bone stress transfer, and consequently, on the biomechanical conditions for osteosynthesis. A dimensionless set of stress-transfer parameters (STP) was developed to quantify the screw-bone load sharing, enabling a convenient rating to be given of the biomechanical compatibility of practically any given screw design according to the nature of the simulated mechanical interaction. The results indicated that newly proposed screw designs, i.e. a "graded-stiffness" composite screw with a reduced-stiffness-titanium core and outer polymeric threads and an "active-compression" hollow screw which generates compressive stresses on the surrounding bone, are expected to provide significantly better biomechanical performances in terms of the STP criteria, compared with conservative screw designs. Accordingly, the present work demonstrates that finite element computer simulations can be used as a powerful tool for design and evaluation of bone screws, including geometrical features, material characteristics and even coatings.