A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model

https://doi.org/10.1016/j.medengphy.2009.11.003Get rights and content

Abstract

The present study aims to the design of a finite-element model simulating accurately the pullout behaviour of cylindrical pedicle screws and predicting their pullout force. Three commercial pedicle screws, subjected to pure pullout from synthetic bone, were studied experimentally. The results were used for the design, calibration and validation of a finite-element model. Special attention was paid to the accurate simulation of the failure inside the host material under shear. For this purpose, a bilinear cohesive zone material model was adopted, controlling the mode-II debonding of neighbouring elements in the vicinity of the screw. Comparison between experimental and numerical results proved that the implementation of this approach can significantly enhance the accuracy of the numerical simulation of a screw's mechanical behaviour under pure pullout loads. The numerical model was used for the parametric study of various factors affecting the pullout performance of a cylindrical pedicle screw. It was concluded that the major parameter influencing the pullout force is the outer radius (increasing its value by 36% increases the pullout force by 34%). The influence of the purchase length of the screw is of similar quantitative nature. The respective dependence on the thread inclination, depth and pitch was significantly weaker.

Introduction

Spinal fixation with pedicle screws is one of the most commonly used methods of instrumentation in the thoracolumbar spine. Its greatest advantage is the achievement of immediate rigid fixation with a minimum number of fused segments. Despite this advantage and the technological advances of the last decades, implant failures of pedicle screw fixation still occur. The most common problems are screw bending, breakage and loosening [1], [2], [3]. Infection is also another implant-related complication.

From a purely engineering point of view, the above mentioned failures and complications could be confronted by optimizing the design of the pedicle screws in order to achieve higher stability and mechanical strength reducing at the same time the implant-bone interface area.

The strength of the fixation of a metallic medical bone screw can be quantified by measuring its pullout force. Even though pure pullout is a rather simple load case, unlikely to be observed in vivo [4], it helps enlightening some controversial points concerning the mechanical behaviour of the metallic screw–bone tissue system. The main factors affecting the pullout force of a bone screw are its design, the material properties of the bone and the insertion technique followed by the surgeon [5].

Especially for the screw design and its influence on the pullout force many experimental [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and numerical [19], [20], [21], [22], [23], [24] studies have been carried out. Additionally some researchers have attempted to quantify the factors that influence the pullout force of a bone screw using equations developed for “machine” screws (i.e. screws for traditional mechanical engineering use) [7], [8]. However, the pullout phenomenon has been proven more complex than it was initially anticipated and even today there is no accurate and reliable way to predict the pullout force of a bone screw [5].

Without any doubt, the best approach for a rigorous study of the influence of the screw's design on its pullout force is the combination of experimental investigation and numerical simulation of the pullout phenomenon using the finite-element (FE) method.

In this context and for the purposes of the present study, the mechanical behaviour of three commercial cylindrical pedicle screws was investigated experimentally under pure pullout loads according to the respective standard (ASTM-F543-02). The experiments were performed using synthetic bone which simulated osteoporotic cancelous bone. The results of this experimental study were used for the design, calibration and validation of a reliable FE model which simulates the complex pullout failure mode and can accurately predict the pullout force of a pedicle screw. This FE model was employed for a thorough and detailed parametric study of the influence of the screw's design on its pullout force. The parameters studied were the outer radius, inclination, pitch and depth of the thread, as well as the screw purchase length. Another parameter that can influence a screw's pullout force is the conical angle of its core [22], [23]. This parameter was not included to the present study which is focused on cylindrical screws.

Section snippets

Experimental study

Three typical commercial pedicle screws were studied experimentally with respect to their pullout forces: the CDH7.5, CDH6.5 (Medtronic Sofamor Danek, Memphis, TN) and TL-Java5 (Zimmer Spine, Bordeaux, France). The basic quantities describing the geometry of the threads of these pedicle screws (Fig. 1) are the outer radius (OR), the core radius (CR), the pitch (P), the thickness of the thread tip (e) and the inclination of the thread (a1, a2). It is seen in Table 1 that the two CDH screws

Experimental study

The experimentally measured pullout forces for the CDH7.5, CDH6.5 and TL-Java5 screws were 438 ± 2, 382 ± 3 and 315 ± 4 N respectively, while typical load vs. displacement curves are shown in Fig. 6.

FE model—design, calibration, and validation

The results of the calibration procedure indicated that the pullout force increases linearly with increasing distance x. The value of x that minimizes the difference between experimental and numerical pullout force for the CDH7.5 screw was calculated to be equal to 0.61P.

Using the same value for the ratio x

Discussion and conclusions

Pedicle screws are sophisticated parts of composite structures (spinal fixation systems) which exhibit complex mechanical behaviour. The safest way to address this complexity is through the well known scheme of analysis and synthesis. In order to optimize the mechanical behaviour of pedicle screws one should first identify its constitutive aspects, study each one of them separately and at the end synthesize the general solution from the partial ones.

In this direction, it can be noted that

Conflict of interest statement

There are not any conflicts of interest at all.

Acknowledgements

This research project is co-financed by E.U.-European Social Fund (75%) and the Greek Ministry of Development-GSRT (25%).

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