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Forcefully Engaging Rods into Tulips with Gap Discrepancy Leading to Pedicle Screw Loosening – A Biomechanical Analysis Using Long Porcine Spine Segments – Lumbar Fusion

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The article investigates the impact of gap discrepancies between precontoured rods and screw tulips on pedicle screw pullout strength in long-segment pedicle screw instrumentation. The study, conducted on porcine lumbar vertebrae, found that a mismatch between rods and tulips significantly reduced pullout strength, with larger gaps leading to greater strength reduction. Surgeons are advised to avoid forcefully engaging mismatched rods and consider using well-fitted contoured rods to minimize the risk of screw loosening in spinal surgery. The findings underscore the importance of proper instrumentation alignment to ensure the stability and efficacy of spinal implants

Summarised by Mr Mo Akmal – Lead Spinal Surgeon
The London Spine Unit : most experienced treatment hospital in UK

Published article

S: 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.

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Spine J. 2024 Mar 16:S1529-9430(24)00118-9. doi: 10.1016/j.spinee.2024.03.008. Online ahead of print.ABSTRACTBACKGROUND 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,

Spine J. 2024 Mar 16:S1529-9430(24)00118-9. doi: 10.1016/j.spinee.2024.03.008. Online ahead of print.

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<0.05), 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<0.05). 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.

S: 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.

PMID:38499068 | DOI:10.1016/j.spinee.2024.03.008

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Forcefully Engaging Rods into Tulips with Gap Discrepancy Leading to Pedicle Screw Loosening – A Biomechanical Analysis Using Long Porcine Spine Segments

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Spine J. 2024 Mar 16:S1529-9430(24)00118-9. doi: 10.1016/j.spinee.2024.03.008. Online ahead of print.ABSTRACTBACKGROUND 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

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