Effect of geometrical design variables on implantation configuration and fixation stiffness of titling bone anchors: A parametric finite element study

The mechanical interaction of a tilting anchor and cancellous bones of various densities was simulated using finite element modeling. The model enjoyed a sophisticated representation of the bone, as an elasto-plastic material with large deformation capability. The anchor's tilting action during implantation phase, as well as its fixation stiffness during pull-out test, were predicted by the model and a parametric study was performed to investigate the effects of the anchor's distal width and corner fillet radius, on these measures. The model predictions were validated against the results of an experimental test on ovine humerus specimens. The model could reasonably reproduce the tilting action of the anchor during the implantation phase. Comparison of the model predictions with the experimental results revealed similar trends during both the implantation and the pull-out phases, but smaller displacement magnitudes (end points: 1.4 vs. 2.1 mm and 4.6 vs. 5.2 mm, respectively). The results of the parametric study indicated substantial increase in the fixation stiffness with increasing bone density. Reducing the distal width and increasing the fillet radius improved the anchor's implantation configuration and fixation stiffness in low-density bones. For high-density bone applications, however, a larger distal width was favored for improving the fixation stiffness.

Trabecular deformations during screw pull-out: a micro-CT study of lapine bone

The mechanical fixation of endosseous implants, such as screws, in trabecular bone is challenging because of the complex porous microstructure. Development of new screw designs to improve fracture fixation, especially in high-porosity osteoporotic bone, requires a profound understanding of how the structural system implant/trabeculae interacts when it is subjected to mechanical load. In this study, pull-out tests of screw implants were performed. Screws were first inserted into the trabecular bone of rabbit femurs and then pulled out from the bone inside a computational tomography scanner. The tests were interrupted at certain load steps to acquire 3D images. The images were then analysed with a digital volume correlation technique to estimate deformation and strain fields inside the bone during the tests. The results indicate that the highest shear strains are concentrated between the inner and outer thread diameter, whereas compressive strains are found at larger distances from the screw. Tensile strains were somewhat smaller. Strain concentrations and the location of trabecular failures provide experimental information that could be used in the development of new screw designs and/or to validate numerical simulations.

A Computational Approximation to Model Variation in Cancellous Bone Screw Pull-Out

Cancellous bone screws are used to achieve good pull-out characteristics when connected to cancellous bone. This study examines some screw characteristics, including pitch and inner diameter, using a model of cancellous bone with a range of bone apparent densities (ADs). This was achieved using bone geometry based on microCT-scanned cancellous bone and converted into a geometric model using mimics® software. The finite element (FE) models were produced in ansys®. The calculated reaction force for pull-out of 0.2 mm shows the influence of design parameters. Change in the proximal half angle increased the stiffness by about 15% in line with the experimental findings of others. An increase in pull-out reaction force with an increase in bone AD was also observed. However, when a particular screw geometry in lower AD bone was modeled and then rotated through 180 deg on plan, a significant reduction in reaction force was noted. Further models with screws of similar geometry in the same location showed similar reductions in reaction force and hence pull-out stiffness. Examination of the geometry of the bone/screw interface indicates that in certain positions there is little cancellous bone to support the implant—leading to low pull-out reaction forces, which is very difficult to predict. The study also examined the effect of increasing the bone stiffness adjacent to the implant, and concluded that, even in bone of low AD, increases in pull-out stiffness might be achieved.

Variability of the pullout strength of cancellous bone screws with cement augmentation

BACKGROUND

Orthopaedic surgeons often face clinical situations where improved screw holding power in cancellous bone is needed. Injectable calcium phosphate cements are one option to enhance fixation.

METHODS

Paired screw pullout tests were undertaken in which human cadaver bone was augmented with calcium phosphate cement. A finite element model was used to investigate sensitivity to screw positional placement.

FINDINGS

Statistical analysis of the data concluded that the pullout strength was generally increased by cement augmentation in the in vitro human cadaver tests. However, when comparing the individual paired samples there were surprising results with lower strength than anticipated after augmentation, in apparent contradiction to the generally expected conclusion. Investigation using the finite element model showed that these strength reductions could be accounted for by small screw positional changes. A change of 0.5mm might result in predicted pullout force changes of up to 28%.

INTERPRETATION

Small changes in screw position might lead to significant changes in pullout strength sufficient to explain the lower than expected individual pullout values in augmented cancellous bone. Consequently whilst the addition of cement at a position of low strength would increase the pullout strength at that point, it might not reach the pullout strength of the un-augmented paired test site. However, the overall effect of cement augmentation produces a significant improvement at whatever point in the bone the screw is placed. The use of polymeric bone-substitute materials for tests may not reveal the natural variation encountered in tests using real bone structures.