Forcefully engaging rods into tulips with gap discrepancy leading to pedicle screw loosening—a biomechanical analysis using long porcine spine segments

Ming Kai Hsieh, De Mei Lee, Weng Pin Chen, Yun Da Li, Fu Cheng Kao, Yue Chen Lin, Tsung Ting Tsai, Po Liang Lai, Ching Lung Tai*

*Corresponding author for this work

Research output: Contribution to journalJournal Article peer-review

Abstract

BACKGROUND CONTEXT: Long-segment pedicle screw instrumentation is widely used to treat complex spinal disorders. Rods are routinely precontoured to maximize assistance on the correcting side of the deformity, but there often exists a residual gap discrepancy between the precontoured rods and screw tulips. No previous research has investigated the diminished pullout strength of the most proximal or distal pedicle screw resulting from a mismatched rod in long-segment pedicle screw instrumentation.

PURPOSE: The present study aimed to investigate the decreased pullout force of pedicle screws affected by the gap discrepancy when forcefully engaging a mismatched rod into a tulip in a normal-density porcine spine.

STUDY DESIGN: The pedicle screw fixation strength under axial pullout force was compared among three different gap discrepancies between rods and tulips using long porcine spine segments.

METHODS: Twelve porcine lumbar vertebrae (L3-L6) were implanted with pedicle screws and rods. Screws on one side had no gap between the tulip and rod (0-mm group), while the most proximal screw on the other side had an intentional gap of 3 mm (3-mm group) or 6 mm (6-mm group). Three hours after forcefully engaging the rod into the tulips at room temperature, the set screws in all specimens were loosened, and each specimen was dissected into individual vertebrae for subsequent pullout testing.

RESULTS: The control group exhibited significantly greater pullout strength (1987.68 ± 126.80 N) than the groups from different rod-tulip configurations (p<.05), with significantly greater strength in the 3-mm group (945.62 ± 97.43 N) than the 6-mm group (655.30 ± 194.49 N) (p<.05). Only 47.6% and 33.0% of the pullout strength was retained in the 3-mm and 6-mm groups, respectively, compared to the control group.

CONCLUSIONS: Gap discrepancies between rods and tulips can significantly reduce pedicle screw pullout strength, with a correlation between decreased strength and increased gaps. Surgeons should avoid forcefully engaging mismatched rods and consider well-fitted contoured rods in spinal surgery to minimize the risk of screw loosening.

CLINICAL SIGNIFICANCE: The gap discrepancy between rod and tulip significantly affected pullout strength, with greater gaps leading to reduced strength. Forcefully engaging mismatched rods into tulips in degenerative spinal surgery should be avoided to minimize the risk of early screw pullout.

Original languageEnglish
Pages (from-to)1773-1780
Number of pages8
JournalSpine Journal
Volume24
Issue number9
Early online date16 03 2024
DOIs
StatePublished - 09 2024

Bibliographical note

Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.

Keywords

  • Gap discrepancies between rods and tulips
  • Multilevel spinal fusion
  • Pedicle screw
  • Porcine model
  • Rod contouring
  • Screw pullout test
  • Biomechanical Phenomena
  • Lumbar Vertebrae/surgery
  • Animals
  • Swine
  • Spinal Fusion/instrumentation
  • Pedicle Screws

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