Fig 1.
Lumbar vertebrae anatomy and placement of pedicle screws (grey), bi-cortical pedicle screws (blue), and cortical pedicle screws (purple).
The contact area with the cortical layer using such screws is indicated in red. A) Lumbar vertebrae (L1-L5), intervertebral discs, spinal cord nerves and pedicle screw placement (sagittal plane). B) Lumbar vertebra (transverse plane). C) pedicle screw placement (transverse plane). Illustration adapted from Servier Medical Art by Servier, licensed under a Creative Commons Attribution 3.0 Unported License.
Fig 2.
L-shaped spinal bone anchor design and placement.
A) Possible expansion mechanism that allows placement of the L-shaped anchor through a single entry point as well as the removal of the anchor. B) Photograph of the L-shaped spinal bone anchor without screw thread, and a reference pedicle screw. The L-shape anchor comprises a central pin with a series of grooves that serve as indication of the insertion depth. C) Four orientations (A-D) of the L-shaped anchor that were evaluated in the experiment. The proximal cortex of the vertebral body is indicated in red. Illustration adapted from Servier Medical Art by Servier licensed under a Creative Commons Attribution 3.0 unported license.
Fig 3.
Vertebra preparation steps before placement of the L-shaped anchor.
Fig 4.
Experimental facility consisting of the actuator that pulls the slider and the anchor out of the vertebra that is contained in the container.
The force is measured using the force sensor and the displacement of the L-shaped anchor with respect to the vertebra is measured using the linear potentiometer.
Fig 5.
Pull-out force of the L-shaped anchor in different anchor orientations (A-D) for different vertebra (L1-L5).
The maximum pull-out force is indicated with an ‘X’. The close-up shows the determination of the initial pull-out force indicated with an ‘O’.
Table 1.
Pull-out test results for L-shaped anchor in four orientations and the pedicle screw.
Fig 6.
Pull-out force linked to the cross-section of the vertebra.
Left: Double force peak profile linked to Orientation A in the L2 vertebra, Middle: Single force peak profile linked to Orientation C for the L4 vertebra, Right: No force peak profile linked to Orientation B in the L4 vertebra.
Fig 7.
Pull-out force of the pedicle screw and L-shaped anchor in Orientation (A-D) each for four repetitions in a L2 vertebra phantom.
The maximum force is indicated with an ‘X’ and the initial pull-out force is indicated with an ‘O’ in the close up.
Fig 8.
Boxplot presenting the initial (blue) and maximum pull-out force (orange) for the three identified degrees of cortical bone damage: 1) clean pull-out, 2) posterior breach and 3) pedicle breach for the experiments performed with the L-shaped anchor.
Fig 9.
Schematic representation of the varying shape of the L1-L5 pedicle cross-section, with the L-shaped anchor in four orientations (Orientation A: blue, Orientation B: red, Orientation C: yellow, Orientation D: purple).
Based om pedicle measures reported by Zindrick et al. [23].
Fig 10.
Future vision of an L-shaped anchor to increase the pull-out resistance of spinal bone anchors with the utilisation of multiple L-shaped anchors to create a larger contact area with the proximal cortex of the vertebral body which could increase the pull-out resistance further.
Illustration adapted from Servier Medical Art by Servier, licensed under a Creative Commons Attribution 3.0 Unported License.