Biomechanical Testing of Pedicle Screw Anchorage

Werner Schmoelz1, *, Richard Lindtner1, Anna Spicher1, Luis Álvarez-Galovich2, Javier Melchor Duart Clemente3
1 Department of Orthopaedics and Traumatology, Medical University of Innsbruck, Innsbruck, Austria
2 Spinal Unit, FJD, Madrid, Spain
3 Neurosurgery and Spinal Surgery Departments, Valencia General Hospital, Valencia, Spain

Abstract

This chapter provides an overview of biomechanical in vitro testing of pedicle screws. Several aspects, such as specimen selection, test setup, and loading modalities for the investigation of screw anchorage are discussed. In general, cement augmentation is an effective technique to improve pedicle screw anchorage. However, in clinical practice, it should be considered that augmentation is most effective in the osteoporotic bone while in healthy bone, the improvement of screw anchorage is only marginal.

Keywords: Augmentation, Loading protocol, PMMA cement, Pedicle screws, Screw loosening, Screw failure.

* Corresponding author Werner Schmoelz: Department of Orthopaedics and Traumatology, Medical University of Innsbruck, Innsbruck, Austria; E-mail: werner.schmoelz@i-med.ac.at

INTRODUCTION

In the last decades, the use of pedicle screws has become standard for dorsal instrumentations in modern spine surgery for many pathologies. Different conditions in morphology and bone quality in degenerative, deformity, trauma and tumor surgery have led to adaptions and modifications of the traditional pedicle screw concept. To enhance pedicle screw anchorage and reduce the risk of loosening, augmentation techniques with PMMA cement and alternative materials were developed and established in clinical practice. Other options to increase pedicle screw anchorage without increasing the overall rigidity of the instrumentation are modifications in the screw design, such as adaption of the thread, screw core diameter, expandable screws, or osteointegrative coatings of the screws [1-5].

In scoliotic deformities or hyperkyphotic spines, the research focus often shifts from the improvement of screw anchorage to the possibility of applying forces and moments with implanted pedicle screws to perform derotation, compression, and tension maneuvers to selected vertebrae, in order to correct the deformity. For this purpose, modified and long screw heads with reposition possibilities were developed.

In the implementation of design modifications and the development of novel pedicle screw designs, in vitro biomechanical investigation with cadaver specimens plays an important role in anticipating the effect and functionality of the implants and their later clinical performance. Therefore, in vitro biomechanical experiments are an important link between the development and clinical application of novel implants and surgical techniques.

The obvious advantages of biomechanical investigations prior to clinical trials are their relatively easy feasibility and the possibility of a direct comparison with current standard techniques using standardized protocols in a controlled lab environment with limited confounding factors. However, the clinical relevance of biomechanical in vitro investigations can vary with the experimental design and execution.

In the following lines, biomechanical testing methods for the evaluation of pedicle screw anchorage are briefly described and discussed. Additionally, selected studies investigating pedicle screw anchorage of various screw designs and augmentation techniques are presented, too.

MATERIALS

Specimens

Bone quality and donor characteristics such as age, sex, bone mineral density (osteoporotic, osteopenic or normal), and grade/state of degeneration can vary widely and may have a significant effect on the results. Therefore, selected specimens must be appropriate and suitable for the postulated hypothesis and study aim. Specimens of various origins as well as artificial bone surrogates or human cadaver tissue can be utilized for biomechanical testing. Due to the limited availability and legal handling requirements of human vertebral bodies, biomechanical testing is also conducted with ovine, bovine or porcine vertebral bodies. However, differences in the bone properties, anatomy, and morphology of animal specimens should be considered in the interpretation of the results and the transfer of the results to clinical practice [6].

With the use of human specimens, ethical considerations and specimen handling must be clarified and settled with the local ethical institutional review board prior to the start of testing [7]. Another relevant point to be considered with the use of human specimens is specimen preservation. It must be distinguished between fresh frozen and embalmed (e.g. Alcohol-Glycerin, formalin, Thiel fixated, etc.) specimens. In a study comparing the biomechanical properties of formalin-fixed and fresh frozen functional spinal units (FSU), it was reported that embalmed specimens do not resemble in vivo features and show significantly different biomechanical properties than fresh frozen specimens [8]. Regarding the effect of preservation methods on bone tissue, Unger et al. compared three preservation methods with fresh frozen bone tissue and concluded that embalming significantly alters the mechanical properties of bone tissue, and the use of embalmed specimens should be restricted to pilot tests [9]. In the literature, fresh frozen specimens are considered the gold standard. After slow thawing, they should be kept wet with saline solution during testing, and at room temperature. Also, test duration should be kept constant for reliable and reproducible results of the biomechanical experiments [10].

For clamping and fixation of the specimens in the test setup to enable mechanical loading, specimens are usually embedded in plastics (e.g. Poly-methyl- methacrylate (PMMA) or Epoxy -resin). The rigidity of the embedding on the tested structure as well as the stiffness of the embedding material should also be considered in the evaluation and interpretation of the measured physical parameters.

Biomechanical Testing

In the last decades, biomechanical test methods were continuously refined and adapted to implement new insights and knowledge in the engineering of material testing, in vivo measurements, and anatomy. This allowed a more realistic simulation of clinical conditions and to investigate relevant research questions as physiologically as feasible. In the following, two test methods to investigate pedicle screw anchorage are described.

Test Setups for Pedicle Screw Pull-Out Tests

Initial, simple, and quick experimental comparisons of varying screw designs or augmentation techniques of pedicle screws are often conducted with axial pull-out tests. They are carried out by applying an axial load with a displacement vector co-axial to the long screw axis while the vertebral body is fixed in the test setup (Fig. 1). After the complete pullout of the pedicle screw, the force-displacement curve is analyzed and a drop in the force plot (e.g. 25% of maximal force) is considered a failure of the screw anchorage.

This kind of experimental biomechanical testing is relatively simple and quick. However, the clinical relevance of pull-out tests for thoracolumbar pedicle screws is questionable. In particular, in pullout tests with cadaver specimens, a very high non-physiological failure load and a failure pattern not reflecting the clinical failure mode are often reported for cement-augmented pedicle screws. These high failure loads are due to pedicle avulsion fractures occurring while the cement cloud still attached to the screw is pulled through the pedicle. Additionally, in vivo studies of patients with instrumented implants have shown that during everyday activities, thoracolumbar pedicle screws are mainly subjected to a cranio-caudal axial loading with a superimposed bending moment; the absolute load magnitude varies strongly depending on the patients and the indication for the instrumentation [11, 12]. A mainly axial force co-axial to the long screw axis as applied in pull-out tests was not reported in the in vivo studies of patients with instrumented implants.

Fig. (1))
Test setup of a typical pull-out test with the vertebral body embedded in PMMA and fixed in a ball joint vice to align the long axis of the screw with the displacement vector applied by a material testing machine.

Test Setups for...