BiomechanicsBiomechanical Performance of Various Cement-Augmented Cannulated Pedicle Screw Designs for Osteoporotic Bones
Introduction
Spinal fixation is common treatment for certain conditions such as scoliosis, tumor including defects, vertebral fractures, and vertebral bone collapse. For fixation of vertebrae, pedicle screws are widely used [1]. However, most vertebral fractures are caused by osteoporosis, which is silently progressive. The number of the patients who have osteoporosis has increased owing to prolonged lifespan and an increase in the aging population [1]. Osteoporotic bones exhibit a high tendency to fracture because of low bone mineral density. Moreover, early-stage pullout is a common problem for surgeons during the fixation of osteoporotic bones. Poor bone quality limits the use of pedicle screws for patients with osteoporosis [2], [3].
To increase the initial pullout strength (before the occurrence of fusion), the researchers studied several parameters. The screw core design was investigated to determine whether the conical core or cylindrical type best fixes vertebra [4], [5], [6], [7], [8]. They concluded that conical core is more appropriate for higher pullout loads. In a recent study, radial drilled pedicle screws were investigated on synthetic foam and fresh-frozen calf vertebrae [9]. An optimum design was declared for tendency toward fusion.
Screw thread types were also investigated under design variables of flat overlap area [10], dual or single thread types [6], and tapping or non-tapping [11]. However, all of these design variables significantly affect the performance of screws more on healthy bones than on osteoporotic ones because the design parameters become critical after fusion. On the other hand, the main problem with osteoporotic bones is early-stage failure.
At this stage, expandable screws stand out as a brilliant alternative. There are several studies on expandable pedicle screws (EPS) [12], [13], [14], [15], [16], [17]. Expandable screws exhibit 30% and 50% higher pullout strength than pedicle screws (PS) on healthy bone and bone with low mineral density, respectively [12]. Wan et al. [14] reported newly formed bone tissue that grew through the center of EPS. This type of bone ingrowth makes revision operations more complex because of the resulting contra-conical geometry of EPS after expansion.
Avoiding the risks of revision surgery of EPS, researchers have focused on cement augmentation techniques [2]. Numerous studies addressed cement augmentation with polymethylmethacrylate (PMMA) [2], [15], [17], [18], [19], calcium phosphate [12], [20], [21], [22], and cyanoacrylate [23]. Calcium phosphate cement augmentation provides 3 times greater pullout strength on osteoporotic bones compared with bare PS [21]. Evans et al. [2] investigated the viscosity of PMMA and proved that it does not significantly affect pullout strength. The performance of cement types was compared in several studies; all concluded that PMMA is the superior alternative [15], [18], [24], [25]. The main disadvantage of cement augmentation is cement leakage into the spinal canal [4], [26], [27]. Wu et al. [28] showed that there was no leakage during cement augmentation on human cadavers.
There are also studies combining the EPS with cement augmentation [12], [17], [28]. Wu et al. [28] reported that EPS with PMMA provides 43% higher pullout strength on osteoporotic vertebrae. Cook et al. [17] reported that EPS with cement augmentation increased pullout strength 2.5 times compared with EPS alone. In addition, some researchers studied pedicle nailing systems as an alternative [29], [30]. All of these methods were tested on synthetic foam materials [1], [7], [20], [31], [32], living or fresh-frozen animals [2], [9], [14], [29], and human cadavers [4], [5], [12], [17], [18], [24]. However, only 1 study [33] investigated the type and number and position of holes that are normal to the cannula. This study provides critical information about the position of radial holes. Placing the holes more proximal to the screw tip caused cement leakage to the spinal canal. The position of radial holes is more important than the number of holes for cannulated screws.
This study investigated the effects of hole and gap position and type on the pullout strength of cannulated screws. Seven different designs were tested, including a control group. The researchers tested 2-mm cannulated screws with 2 or 3 holes that were gapped or non-cannulated (control group). In addition, holes and gaps were drilled or opened unilaterally and bilaterally. Synthetic foams were used as a test medium and PMMA was selected as the cement type. None of the designed screws has a self-tap. This work also investigated the torsional properties of designed screws to optimize the design regarding not only pullout but also torsional strength requirements.
Section snippets
Materials and Methods
Synthetic rigid polyurethane (PU) foam was used in this study to mimic trabecular bone. Polyurethane foam is widely used as a standard testing material for pullout studies of orthopedic implants [1], [7], [20], [31], [32], [34]. The properties of polyurethane foam are stated in ASTM F1839 [34]. According to the related standards and the literature, 2 different grades of PU foam were used: Grades 40 and 10, to simulate healthy and osteoporotic bones, respectively. Foams were produced and
Results
Table lists the pullout and torsion test results. For the torsion tests, the maximum torque values were exhibited by the control group (non-cannulated) at 20.3 Nm. The highest torsional strength among the tested cannulated screws was 13.54 Nm (p < .0001) for S2H. The minimum torsional strength was 9.45 Nm with a breaking angle of 39° (p < .005). Opening a gap or drilling screws bilaterally significantly decreased torsional strength. Similarly, opening a gap bilaterally or unilaterally decreased
Discussion
Cannula decreased the torsional strength owing to the reduced moment of inertia on the cylindrical cross-section of the screw. However, this torsional strength drawback was 9% for S2H samples. Opening a gap or radial drilling decreases the moment of inertia and increases the risk of crack initiation because of the machining parameters and tool beach marks. On the other hand, consideration of only the torsional parameters is not enough to assess the highly performing cannulated screw.
Grade 40 PU
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A novel calcium phosphate–based nanocomposite for the augmentation of cement-injectable cannulated pedicle screws fixation: A cadaver and biomechanical study
2020, Journal of Orthopaedic TranslationCitation Excerpt :Many studies have examined the biomechanical characteristics of primary osteoporotic posterior lumbar spinal instrumentation. As a primary approach, or a remedial approach after failure of spinal instrumentation in the osteoporotic spine, cement augmentation with CICPSs is an option to effectively secure screw with adequate robustness in the vertebral body [1,19,23–26]. Although it is widely accepted that PMMA augmentation is effective in improving screw fixation, reports describing its complications and shortcomings have gradually emerged in clinical practice over the past few years [9].
Cement Augmentation of Pedicle Screw Instrumentation: A Literature Review
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2023, Acta Orthopaedica BelgicaMechanical and Geometric Analysis of Fenestration Design for Polymethylmethacrylate-Augmented Pedicle Screw Fixation
2022, International Journal of Spine SurgeryDecompression and fusion surgery for osteoporotic vertebral fractures: WFNS spine committee recommendations
2022, Journal of Neurosurgical Sciences
Author disclosures: TT (none); AK (none); YO (none); DS (none); DT (none).
This study was supported by Osimplant (Bone Implant) and TUBITAK, Project 111M583.