Introduction: When a residual mismatch occurs between the rod and pedicle screw head, the use of a rod persuasion device has been found to have deleterious consequences on pedicle screw pull-out strength (POS) in the thoracic spine. However, posterior-only Ponte osteotomies are performed during many deformity surgeries to improve flexibility/correction of the spine, and may offset forces of the rod reduction maneuver. The objective of this study is to evaluate the biomechanical effect of the rod reduction technique on pedicle screw pull-out strength in the thoracic spine following Ponte osteotomies.
Methods: Fifteen (n=15) three-segment, fresh-frozen human cadaveric thoracic specimens were prepared. Specimens were instrumented with titanium pedicle screws, after Ponte’s were performed, and the left side served as the control with perfect screw-rod alignment. The right side rod was intentionally contoured with a 5 mm residual gap between the ventral aspect of the rod and the inner bushing of the pedicle screw, followed by use of a rod reduction device. The pedicle screws were pulled out “in-line” with the screw axis, with peak POS measured in Newtons (N).
Results: After rod reduction, pedicle screws had significantly decreased POS compared to controls (419±426 N versus 708±462 N), with 40% lower mean POS. Eleven (73%) of the pedicle screws had visible pull-out/failure during the reduction attempt, and occurred irrespective of BMD. Although exhibiting a significant difference in insertional torque, there was no significant difference in POS between re-inserted screws compared to control screws (768±375 N versus 819±502 N).
Conclusions: Despite Ponte osteotomies to improve the flexibility of the spine, the rod reduction device significantly decreases overall pedicle screw pullout strength; typically resulting in outright failure of the screw-bone interface. Therefore, the rod reduction technique should be performed with caution as it frequently results in pedicle screw fixation failure and suboptimal fixation strength.
Patient Care: Surgeons will avoid the rod reduction maneuver, or perform it with caution to avoid suboptimal pedicle screw fixation strength.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the Ponte osteotomy, 2) Discuss the factors affecting pedicle screw fixation strength, 3) Describe the biomechanical consequences of rod reduction in the thoracic spine, 4) Identify alternatives to the rod reduction technique