Biomechanical analysis of different types of pedicle screw augmentation: A cadaveric and synthetic bone sample study of instrumented vertebral specimens
Introduction
The pedicle screw system is commonly used for spinal reconstruction, fixation, and other spinal disorder surgeries [1], [2], [3]. When the vertebral bodies are seriously affected by osteoporosis, most pedicle screws cannot provide sufficient strength at the screw-bone interface, and such unions carry a high risk of loosening or failure [4], [5], [6]. Previous research has demonstrated that the pull-out strength of the pedicle screw in osteoporotic bone decreases with lower bone mineral density (BMD) values [7], [8]. Hence, the screw holding strength is not easy to improve during orthopedic surgery. Some in vitro studies have indicated that using cement-augmented pedicle screws significantly increases both pull-out strength and transverse bending stiffness [9], [10], [11], [12]. In clinical practice, a cement-augmented screw enhances the holding strength of screws in osteoporotic bone have been demonstrated by orthopedic surgeons. This technique has been used to reduce interoperative screw-bone interface failure and to reinforce its strength [4], [13], [14], [15].
Although the advantages of cement-augmented screws are clear, reports of their clinical use are limited. Worrisome side effects include cement leakage into the spinal canal, neural foramina and paravertebral veins. Bone cement and cannulated pedicle screws have been developed to provide cement augmentation primarily in the distal end of the screw. As this technique and screw design evolved, there are several potential problems existing, such as risk of cement leakage outside the vertebral body, the difficulty of injection with cannulated screw (it requires high-pressure cement injection using low-viscosity cement via cannulated screw) and a difficulty of removing the fixed screw if there are complications after the initial surgery [16].
Recent research has focused on the perforated screw with cement augmentation, which allows cement injection through the perforation to improve the holding strength of the screw [17]. Although studies [9], [10], [11], [12] have shown increased pull-out strength with various types of screw augmentation, none yet compare cannulated screws pre-filled with cement and those with cement injected through the perforation. Comparing to cement injected through the perforation, cement pre-filled method is more convenient, and due to solid screw is the only equipment to prepare. If the biomechanical strength of pre-filled with cement is comparing to cement injection through the perforation, the method of cement pre-filled have more advantages.
In addition, the revision characteristics of these two types of screws remain unknown. Some have hypothesized that pre-filled screws provide more uniform cement distribution, resulting in better pull-out strength and revision characteristics than screws with cement injected through the perforation. Hence, the primary purpose of this study is to determine the pull-out strength, stiffness and failure pull-out energy of cement-augmented, cannulated-fenestrated pedicle screws, both pre-filled with cement and those with cement injected through perforations, in an osteoporotic cadaveric thoracolumbar model, as well as to determine the extraction torques of these screws in synthetic bone samples.
Section snippets
Cadaver sample preparation and pull-out test
A total of 32 human vertebrae from four female thoracic and lumbar spines (mean age 62.3 years, range 55–71 years), spanning T10 to L5, were used. Prior to testing, each explanted spine was vacuum sealed, frozen and submitted for BMD testing with dual-energy X-ray absorptiometry (DEXA, Hologic QDR 4500C; Hologic Inc., Waltham, MA). In addition, each specimen received radiographic evaluation to exclude any tumors or significant disease. Before implantation, the sizes of the pedicles in both
Pull-out strength and energy of pedicle screw on cadaver specimen
No statistical differences were found in the BMD (mean ± SD) of the control group (group 1, n = 13, 0.42 ± 0.12 g/cm2, T score = −5.2), the cement pre-filled group (group 2, n = 14, 0.39 ± 0.11 g/cm2, T score = −5.4), and the cement-injected group (group 3, n = 13, 0.43 ± 0.12 g/cm2, T score = −5.1) (p > 0.05). The tested vertebral levels of three groups were showed in Table 2. The average pedicle heights (PDH) of all segments were 15.4 ± 1.0 mm, 15.3 ± 0.9 mm, and 15.4 ± 0.5 mm in group 1, 2, and 3. The average pedicle widths
Discussion
For the elderly population and patients with osteoporosis who need surgical treatment, improvements in bone-screw fixation are desirable because the complications of screw loosening, pull-out and implant failure are dire [5], [19], [20]. Hence, the techniques of perforated screw with cement augmentation were developed to enhance the holding strength of screws in osteoporotic bone [9], [10], [11], [12]. In clinical use, cement-augmented screws may cause problems, especially during revision,
Conclusion
Clinically, quick setting of bone cement for maximal fixation strength is necessary when assembling the spinal structure, especially when compression and distraction force is needed to correct spinal deformity. If the pedicle screw is not rigid enough, the structure can immediately loosen, especially in patients with severe osteoporosis. In summary, we found better initial fixation strength (including pull-out strength, stiffness and failure pull-out energy) and better revision characteristics
Funding
This work was supported in part by the administrative bureau of Southern Taiwan Science Park under Grant Number: BY-03-04-17-98 and the UST-UCSD International Center of Excellence in Advanced Bioengineering sponsored by the Taiwan National Science Council I-RiCE Program under Grant Number: NSC-100-2911-I-009-101.
Conflict of interest statement
We declare that we have no conflict of interest in the authorship or publication of this contribution.
Ethical approval
The ethical approval is not required in this study.
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