Rod persuading reduces primary construct stability in lumbar interbody fusion in the osteoporotic spine - A biomechanical in-vitro study.

BACKGROUND

With the introduction of the pedicle screw-rod system and intervertebral cages, posterior spinal fusion has increasingly become a standard procedure in the treatment of degenerative spinal conditions. The aim of this study is to investigate the influence of rod persuading in lumbar spinal fusion constructs on the pullout strength of pedicle screws, considering different bone qualities.

METHODS

Ten fresh-frozen lumbar spines (L1-5) from 10 cadaveric donors were initially included. All specimens were examined at the beginning of the study both macroscopically and by computed tomography scans (HR-CT, Siemens) to ensure specimen integrity, and the mineral bone density by using Hounsfield units. One specimen had to be excluded prior to biomechanical testing due to a vertebral fracture extending into the pedicle, leaving a total of 9 specimens available for the study.

FINDINGS: The displacement of the screws during the pullout test was significantly greater in the osteoporotic specimens after rod persuader use (Group 2), compared to the non-osteoporotic specimens (3.7 ± 0.7 mm vs. 5.5 ± 0.4 mm, p = 0.0486). This difference was not observed in the non-rod persuader group. Overall, rod persuading decreased the displacement distance until tensile load to screw failure by 29 %.

INTERPRETATION: The use of the rod persuader reduces the pedicle screw pullout strength, thereby compromising the construct stability. This reduction is particularly significant in osteoporotic vertebral bodies, highlighting the need for careful consideration of rod persuader use in this patient population.