Abstract
Previous studies have demonstrated that bone morphological properties are a significant determinant of orthopaedic fastener fixation strength. The authors previously tested a new design of unthreaded expandable fastener (UEF) prototype against screws and demonstrated a significant increase in pull-out strength. However the effect of bone morphology on the pull-out strength of the UEF and expandable fasteners in general is unknown. This study assessed the correlation between failure force and maximum force against five microstructural parameters. The failure force of the UEF was correlated to the trabecular bone volume fraction, as with screws. Unlike screws, however, the maximum force of the UEF has an inverse relationship with cortical volume. No correlation was found between failure force and the Structural Model Index (SMI). Additionally the critical volume of interest (VOI) for the UEF is around the bottom of the fastener where the expansion occurs, whereas for the screw a full height VOI is critical. Furthermore, we observed that screw mechanical performance may be affected more by bone morphological parameters that are associated with poorer quality bone. Therefore the UEF may perform better than screws in low quality osteoporotic bone.
Highlights
Orthopaedic screws have long been used to affix orthopaedic implants to bone in order to stabilise fractures and fuse joints
Trabecular bone volume fraction demonstrated a high degree of positive correlation with failure force for the screw over all full height volume of interest (VOI) diameters
For the unthreaded expandable fastener (UEF) a high degree of positive correlation was only demonstrated for failure force over a small range of VOI diameters (6-8mm) and VOI heights (2-4mm)
Summary
Orthopaedic screws have long been used to affix orthopaedic implants to bone in order to stabilise fractures and fuse joints. Screw loosening and pull-out are still significant problems [1, 2]. In order to address these problems, the authors are developing a new design of expandable fastener to provide superior fixation strength and reduce the incidence of loosening. Low fastener pull-out strength can lead to screw loosening in–vivo, causing injury and potentially requiring revision surgery. Often the cause of this failure is low quality bone due to osteoporosis [1, 2]. In a previous study [3], the authors have demonstrated a 41% higher failure strength for a new design of unthreaded expandable fastener (UEF) prototype compared to conventional orthopaedic screws of equivalent length and diameter in a non-osteoporotic ovine bone model. King and Cebon have demonstrated favourable results for expandable fasteners in osteoporotic bone but not in normal bone, indicating that expandable fasteners may perform better relative to screws in poor quality bone [4]
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