Due to the complex biomechanics of the human body as well as the individual loading of implants in different body regions and overloading events such as stumbling or falling, load-bearing endoprostheses and in particular their fixation elements (mostly bone screws) can be exposed to very high stresses. These often lead to complications such as screw loosening or material failure (fracture) of the fixation element, resulting in loss of function. This inevitably leads to revision surgery. For example, 34% of all implant failures in the spinal region are due to material fatigue of the pedicle screws. In general, it can be stated that research and product-related development is mainly focused on the qualification of implant structures. Development efforts in the area of fixation elements are undertaken only to a limited extent. In particular, design optimization of pedicle screws has so far been based on screw pull-out analyses and corresponding numerical simulations with regard to screw loosening. Dominant bending stresses leading to fatigue failure have received little attention during the design process. This paper presents a proposal for a density-adaptive design of bone screws using pedicle screws for spinal fixations as an example. The basis is the analysis and categorization of currently available variants of bone screws, which differ in principle in their thread design because of different application areas (cortical or cancellous bone). These screw variants are investigated in FEA simulations for pullout and bending with regard to occurring stresses. A corresponding simulation model is presented for this purpose. The precise design models for these screws are generated in a CAD system using a self-developed configuration tool. Based on the FEA evaluation, the proposal for a new pedicle screw design, consisting of several thread types merged into each other, is described in detail. By integrating different thread types over the shaft, the respective properties of the bone can thus be optimally utilized.

Proposal for an Adaptive Bone Screw Design Based on FEA Studies Exemplified by Pedicle Screw

Paper Proceedings: https://asmedigitalcollection.asme.org/IDETC-CIE/IDETC-CIE2021/volume/85369

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