A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model
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%).
References (33)
- et al.
The mechanical properties of surgical bone screws and some aspects of insertion practice
Injury
(1972) - et al.
Investigation of fixation screw pull-out strength on human spine
J Biomechanics
(2004) - et al.
Effects of bone materials on the screw pull-out strength in human spine
Med Eng Phys
(2006) Optimizing the biomechanical compatibility of orthopaedic screws for bone fracture fixation
Med Eng Phys
(2002)- et al.
Increase of pullout strength of spinal pedicle screws with conical core: biomechanical tests and finite element analyses
J Orthop Res
(2005) - et al.
Titanium alloys in total joint replacement—a materials science perspective
Biomaterials
(1998) - et al.
Complications associated with the technique of pedicle screw fixation. A selected survey of ABS members
Spine
(1993) - et al.
Surgical complications of posterior lumbar interbody fusion with total facetectomy in 251 patients
J Neurosurg (Spine)
(2006) - et al.
Complications and problems related to pedicle screw fixation of the spine
Clin Orthop Relat Res
(2003) - et al.
Loads on a telemeterized vertebral body replacement measured in two patients
Spine
(2008)
Implant–bone interface
Biomechanics of spine stabilization
Actual holding power of various screws in bone
Ann Surg
Factors affecting the pullout strength of cancellous bone screws
J Biomech Eng
Cancellous bone screw thread design and holding power
J Orthop Trauma
Biomechanical evaluation of the pullout strength of cervical screws
J Spinal Disord Tech
A study of some factors which affect the strength of screws and their insertion and holding power in bone
J Biomechanics
Cited by (56)
Homogenized finite element models can accurately predict screw pull-out in continuum materials, but not in porous materials
2021, Computer Methods and Programs in BiomedicineA novel parameter for the prediction of pedicle screw fixation in cancellous bone - A biomechanical study on synthetic foam
2020, Medical Engineering and PhysicsHybrid composite pedicle screw - finite element modelling with parametric optimization
2020, Informatics in Medicine UnlockedCitation Excerpt :Thus, a good design yields low pull-out displacements, a bad design results in higher pull-out displacements. Generally, an excellent shaft and thread design is crucial to prevent the screw from pulling out [34,54–60]. As mentioned in section 2.1.1.1.,
Design factors of lumbar pedicle screws under bending load: A finite element analysis
2019, Biocybernetics and Biomedical EngineeringCitation Excerpt :Pedicle screws are primarily responsible for retaining the stability of the posterior spinal devices [39]. Loosening and breakage of lumbar pedicle screw are the most common complications affecting the spinal stability [1,2,42]. Around 5% screws in 20% patients, specially older age (65 ± 9 years), suffered loosening in dynamic spinal stabilization systems [3].