An approximate model for cancellous bone screw fixation

Abstract

This paper presents a finite element (FE) model to identify parameters that affect the performance of an improved cancellous bone screw fixation technique, and hence potentially improved fracture treatment. In cancellous bone of low apparent density it can be difficult to achieve adequate screw fixation and hence provide stable fracture fixation that enables bony healing. Data from predictive FE models indicate that cements can have significant potential to improve screw holding power in cancellous bone. These FE models are used to demonstrate the key parameters that determine pullout strength in a variety of screw, bone, and cement set-ups, and to compare the effectiveness of different configurations. The paper concludes that significant advantages, up to an order of magnitude, in screw pull-out strength in cancellous bone might be gained by the appropriate use of a currently-approved calcium phosphate cement.

Bone Anchors—A Preliminary Finite Element Study of Some Factors Affecting Pullout

“…The interface between the bone and anchor is a frictional contact. Previous experience [23] has shown the results to be more appropriate to physical studies when compared to the alternative bonded model. This is because under bonded contact no contact shear stress or sliding occurs between the surfaces, resulting in a significantly stiffer structure.…”

“…In both high and low density structures they found a significant difference in bone-implant stiffness between models in which the bone/anchor interface is assumed to be contiguously connected, and concluded that continuum models are of limited use for peri-implant analyses. Brown et al [23] have also examined the fundamental phenomena affecting pullout with simplified FE models that represent the cellular structure.…”

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

“…Using the FE model shows that as the interdigitation of screw thread and trabecular strut is changed, quite significant changes in pullout stiffness are observed. This simplified model is not intended to produce exact numerical similitude, and not to model the effect of cement (this has been presented elsewhere Brown et al, 2013) but to indicate the order of changes possible from small changes of position. For a given pullout displacement, the range of pullout force as the initial screw position in the bone is changed radially is shown in Fig.…”

“…First, the methodology for the human cadaver study is detailed. Secondly, a brief outline of a simplified finite element model used to demonstrate relative values of screw pullout is presented; fuller descriptions and validation methods are given elsewhere (Brown et al, 2013).…”

“…3), while the trabecular strut thickness is 0.1 mm. The use of an axisymmetric (2-D) model is a simplification that represents the cancellous bone as a series of regularly-spaced interconnected thin-walled annular tubes (Brown et al, 2013). Nevertheless, this gives a better approximation to cancellous bone structure than the alternative use of a continuum (Wirth et al, 2011), as it defines the connectivity between screw threads and the surrounding medium.…”