Dynamic simulations of cancellous bone resorption around orthopaedic fixative implants

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 screw profiles on the predicted extent of bone resorption. An algorithm simulating local bone adaptation to mechanical stimuli was developed and subsequently used to evaluate the biomechanical performances of the different screw profiles analyzed, i.e., triangular, rectangular and trapezoidal thread shapes. This remodeling algorithm predicted local bone gain or loss in the vicinity of the screw as a response to the resulted mechanical stress distribution. A dimensionless set of stress intensity parameters (SIP) was developed to quantify the bone-screw stress transfer, enabling a convenient rating of different screw performances according to the nature of expected adaptation of the surrounding bone. The results indicated that a wide rectangular screw profile is of superior biomechanical compatibility with bone compared to the other profile types. The present work demonstrated that bone remodeling computer simulations can be used as a powerful tool for evaluation of different design parameters of fixative screws, such as geometry, material characteristics and even coatings.