Comparison of three different screw trajectories in osteoporotic vertebrae: a biomechanical investigation

Abstract

Background Pedicle screw insertion in osteoporotic patients is challenging. Achieving more screw-cortical bone purchase and invasiveness minimization, the cortical bone trajectory and the midline cortical techniques represent alternatives to traditional pedicle screws. This study compares the fatigue behavior and fixation strength of the cement-augmented traditional trajectory (TT), the cortical bone trajectory (CBT), and the midline cortical (MC). Methods Ten human cadaveric spine specimens (L1 - L5) were examined. The average age was 86.3 ± 7.2 years. CT scans were provided for preoperative planning. CBT and MC were implanted by using the patient-specific 3D-printed placement guide (MySpine®, Medacta International), TT were implanted freehand. All ten cadaveric specimens were randomized to group A (CBT vs. MC) or group B (MC vs. TT). Each screw was loaded for 10,000 cycles. The failure criterion was doubling of the initial screw displacement resulting from the compressive force (60 N) at the first cycle, the stop criterion was a doubling of the initial screw displacement. After dynamic testing, screws were pulled out axially at 5 mm/min to determine their remaining fixation strength. Results The mean pull-out forces did not differ significantly. Concerning the fatigue performance, only one out of ten MC of group A failed prematurely due to loosening after 1500 cycles (L3). Five CBT already loosened during the first 500 cycles. The mean displacement was always lower in the MC. In group B, all TT showed no signs of failure or loosening. Three MC failed already after 26 cycles, 1510 cycles or 2144 cycles. The TT showed always a lower mean displacement. In the subsequent pull-out tests, the remaining mean fixation strength of the MC (449.6 ± 298.9 N) was slightly higher compared to the mean pull-out force of the CBT (401.2 ± 261.4 N). However, MC (714.5 ± 488.0 N) were inferior to TT (990.2 ± 451.9 N). Conclusion The current study demonstrated that cement-augmented TT have the best fatigue and pull-out characteristics in osteoporotic lumbar vertebrae, followed by the MC and CBT. MC represent a promising alternative in osteoporotic bone if cement augmentation should be avoided. Using the patient-specific placement guide contributes to the improvement of screws’ biomechanical properties.

Supraspinous ligament arc tangent guided freehand thoracic pedicle screw insertion technique: high parallelism between screws and upper endplate

“…In 2020, Swaminathan, G et al verified the Fennell technique through simulation experiments with pedicle screws and proved that inserting screws parallel to the upper endplate is safe ( 36 ). In 2021, Jarvers, JS et al experimentally proved that inserting pedicle screws parallel to the upper endplate in osteoporotic lumbar vertebrae has the best fatigue performance and the maximum resistance to pullout force ( 41 ). Therefore, different experiments also support the sagittal plane state where the screw is parallel to the upper endplate.…”

Biomechanical comparison of pedicle screw fixation strength among three different screw trajectories using single vertebrae and one-level functional spinal unit

“…Several factors affect the fixation stability of a pedicle screw, including the screw shape, diameter, length, thread type/shape, pitch width, outer/inner core difference and bone mineral density (Liu et al, 2020;Hsieh et al, 2021a). In addition to the morphometric characteristics of the pedicle screw, the trajectory of the screw also has a strong influence on fixation strength Phan et al, 2015;Delgado-Fernandez et al, 2017;Jarvers et al, 2021). Currently, three methods for the insertion of pedicle screws are widely used in the lumbar spine.…”

“…Currently, three methods for the insertion of pedicle screws are widely used in the lumbar spine. First, in the traditional trajectory (TT), the screw converges from its entry point, passes completely inside the pedicle, and reaches at least the anterior third of the vertebral body (Youssef et al, 1999;Phan et al, 2015;Wadhwa et al, 2015;Delgado-Fernandez et al, 2017;Jarvers et al, 2021). In the sagittal plane, traditional screws should be located in the superior third of the vertebral body and parallel to the superior endplate Suk et al, 2005).…”

“…However, inconsistent biomechanical results among different trajectories come from using different bone densities, different species of cadaveric specimens (Perez-Orribo et al, 2013;Lee et al, 2015;Oshino et al, 2015;Delgado-Fernandez et al, 2017), various finite element models (Matsukawa et al, 2017;Molinari et al, 2021), uncontrolled cephalad angles of CBT screws (Perez-Orribo et al, 2013;Oshino et al, 2015), and undefined MT axes . Studies to date have been predominantly based on single screw fixation stability, which may not be the actual clinical representation of implant failure Delgado-Fernandez et al, 2017;Jarvers et al, 2021). To the best of our knowledge, no study has directly compared the biomechanical performance among these three trajectories in single vertebrae and one-level functional spinal units (FSUs) in a porcine model.…”