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 10 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 as 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 1,500 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, 1,510 cycles, and 2,144 cycles, respectively. 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 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 guide contributes to improve screws’ biomechanical properties.

Bone density optimized pedicle screw insertion

“…Fixation of lumbar motion segments with pedicle screws is the surgical gold standard for a variety of pathologies such as scoliosis, degenerative deformities, fractures, infections, and tumors (Wang et al, 2014;Tschugg et al, 2017;Chan et al, 2020;Perna et al, 2022). The stability of pedicle screws is known to depend on a variety of factors, including screw shape, diameter, length, thread shape, pitch width, difference between inner and outer cortex, bone mineral density, and screw trajectory (Phan et al, 2015;Delgado-Fernandez et al, 2017;Liu et al, 2020;Hsieh et al, 2021;Jarvers et al, 2021). In the search for an optimal solution, screw trajectories have been extensively studied clinically and biomechanically in recent years (Chang et al, 2021;Tai et al, 2022); comparing the most common methods such as the traditional trajectory, a modified trajectory directed caudally toward the antero-inferior margin of the vertebral body, and the cortical bone trajectory, which attempts to maximize contact between the screw thread and the cortical bone (Tai et al, 2022).…”