Mechanical and Geometric Analysis of Fenestration Design for Polymethylmethacrylate-Augmented Pedicle Screw Fixation

Abstract

Background: The practice of cement augmentation in pedicle screw fixation is well established. However, there is a lack of consensus regarding the optimal screw design or cement type. This remains a clinically important question given the incidence of cement augmentation-associated complications. While fenestrated screws have become widely used in clinical practice, the relationship between fenestration placement along the screw axis and cement plume geometry and pullout strength have yet to be clarified. This study was designed to evaluate the mechanical and geometric properties of different fenestrated screw designs and cement viscosities in pedicle screw fixation.Methods: Three different screw fenestration configurations and 2 different cement viscosities were examined in this study. Axial pullout tests were conducted in both foam blocks and cadaveric vertebrae. All vertebral specimens underwent tests of bone mineral density. In the foam blocks, 6 tests were conducted for each augmentation combination and also for nonaugmented controls. In the cadaveric testing, 36 lumbar vertebrae were instrumented with a cemented and uncemented control screw to compare features of fixation. Computed tomography (CT) images were taken to assess the geometric profile of the cement plumes in both the foam blocks and the cadaveric vertebrae.Results: In both foam blocks and vertebral specimens, cementation was shown to confer a significant increase in pullout strength. Significant correlations existed between the anterior-posterior and lateral cement plume diameters and pullout strength in cadaveric vertebra and foam blocks, respectively. Within instrumented vertebra, variables such as the width of the vertebral body and screw insertion were found to significantly correlate with enhanced fixation. CT analysis of the instrumented vertebra demonstrated that a centrally distributed pattern of fenestrations was found to result in a cement plume with consistently predictable distribution within the vertebral body, without evidence of leak.Conclusion: Cementation of fenestrated pedicle screws increases overall pullout forces; however, there is an unclear relationship between the geometric properties of the cement plume and the overall strength of the screw-bone interface. This study demonstrates that the plume diameter, vertebral body width, and angle of screw insertion are correlated with enhanced pullout strength. Furthermore, varying the fenestration design of injectable screws resulted in a set of predictable plume patterns, which may be associated with fewer complications. Further investigation is required to clarify the optimal geometric and biomechanical properties of injectable pedicle screws and their role in establishing the cement-bone interface.

Early Experience With Novel Molded Allograft Anchors for the Management of Screw Loosening in Elderly Patients With Reduced Bone Density in Primary and Revision Lumbar Surgery

“…16 Preventive measures available to surgeons to reduce the incidence of screw loosening include optimizing bone density through pharmacological management by an endocrinologist, 6,17 encouraging smoking cessation, 5 maximizing the diameter and accuracy of pedicle screws by utilizing image guidance or robotics, 18 avoiding chromium-cobalt rods, limiting fusions to <5 levels, and using cement augmentation. 19,20 Use of wider footprint cages during anterior column surgery may also reduce subsidence and subsequent screw loosening. 1 However, all of these techniques have limitations.…”

“…Contemporary cementing techniques utilize cannulated pedicle screws with fenestrations at the distal third of the screw. 19 Polymethylmethacrylate is injected through the distal tip of the screw, typically delivering between 1.0 and 3.0 mL of cement into the vertebral body. 7 The use of cement augmentation adds time to surgery, requires frequent fluoroscopy and the wearing of lead gowns, is technically difficult, risks cement extravasation, 16 and can make future revision surgery more challenging.…”

The Potential Impact of Basivertebral Foramen Morphology and Pedicle Screw Placement on Epidural Cement Leakage With Cement-Augmented Fenestrated Pedicle Screw Fixation: A Multicenter Retrospective Study of 282 Patients and 1404 Augmented Screws

“…Although it might lead to some potential bias in comparison with direct detection of the screw tip's position, we believe that final results would not be significant. Third, there are numerous types of fenestrated pedicle screws with differing core diameters, sizes, numbers, and locations of radial holes, 18,19 which may contribute to epidural leakage. However, our study included only 1 type of fenestrated pedicle screw to prevent biased effects caused by different types of fenestrated pedicle screws.…”