Location of pedicle screw hold in relation to bone quality and loads

Abstract

Introduction: Sufficient screw hold is an indispensable requirement for successful spinal fusion, but pedicle screw loosening is a highly prevalent burden. The aim of this study was to quantify the contribution of the pedicle and corpus region in relation to bone quality and loading amplitude of pedicle screws with traditional trajectories. Methods: After CT examination to classify bone quality, 14 pedicle screws were inserted into seven L5. Subsequently, Micro-CT images were acquired to analyze the screw's location and the vertebrae were split in the midsagittal plane and horizontally along the screw's axis to allow imprint tests with 6 mm long sections of the pedicle screws in a caudal direction perpendicular to the screw's surface. Force-displacement curves in combination with the micro-CT data were used to reconstruct the resistance of the pedicle and corpus region at different loading amplitudes. Results: Bone quality was classified as normal in three specimens, as moderate in two and as bad in two specimens, resulting in six, four, and four pedicle screws per group. The screw length in the pedicle region in relation to the inserted screw length was measured at an average of 63%, 62%, and 52% for the three groups, respectively. At a calculated 100 N axial load acting on the whole pedicle screw, the pedicle region contributed an average of 55%, 58%, and 58% resistance for the normal, moderate, and bad bone quality specimens, respectively. With 500 N load, these values were measured at 59%, 63%, and 73% and with 1000 N load, they were quantified at 71%, 75%, and 81%. Conclusion: At lower loading amplitudes, the contribution of the pedicle and corpus region on pedicle screw hold are largely balanced and independent of bone quality. With increasing loading amplitudes, the contribution of the pedicle region increases disproportionally, and this increase is even more pronounced in situations with reduced bone quality. These results demonstrate the importance of the pedicle region for screw hold, especially for reduced bone quality.

Keywords: bone density; instrumentation; lumbar spine; pedicle screw; primary stability; screw bone interface.

FIGURE 1

μCT reconstruction of a lumbar vertebral body to visualize the spatial variability in trabecular bone density around a 3D-printed replica of a pedicle screw.

FIGURE 2

(A) Illustration of the biomechanical testing method. (B) Specimen after potting with the holding apparatus and the 3D-printed pedicle screw still in place. (C) Cropped specimen just prior to testing. (D) Photography of the tip (left) and center (right) screw imprint probes.

FIGURE 3

Illustration of data processing: (A) The experimentally derived stress-displacement curves are assigned to the corpus and pedicle region and (B) interpolated to a single stress-displacement curve for either anatomical region. (C) The projected screw surface area of both regions is used to convert the stress-values to force-values and (D) displacement-controlled virtual screw imprint tests are performed to calculate the load distribution between the corpus and pedicle region for specific loading conditions.

FIGURE 4

Parasagittal reconstruction of the micro-CT scans of the vertebrae with normal bone quality. An image of the pedicle screw is overlaid graphically to illustrate the position of the screw and the measurement locations (blue sections). The load [MPa]-displacement [mm] curves are depicted in orange.

FIGURE 5

Continuation of Figure 3. Parasagittal reconstruction of the micro-CT scans of the vertebrae with moderate bone quality. An image of the pedicle screw is overlaid graphically to illustrate the position of the screw and the measurement locations (blue sections). The load [MPa]-displacement [mm] curves are depicted in orange.

FIGURE 6

Continuation of Figures 3, 4. Parasagittal reconstruction of the micro-CT scans of the vertebrae with bad bone quality. An image of the pedicle screw is overlaid graphically to illustrate the position of the screw and the measurement locations (blue sections). The load [MPa]-displacement [mm] curves are depicted in orange.

FIGURE 7

The mean (and the standard deviation) of the relative contribution of the pedicle and corpus region for normal, moderate, and bad bone quality for a total of 100 N, 500 N, and 1,000 N acting on the pedicle screw, which are illustrated using pie plots.