Pullout strength of different pedicle screws after primary and revision insertion: an in vitro study on polyurethane foam

Lien-Chen Wu; Yueh-Ying Hsieh; Fon-Yih Tsuang; Yi-Jie Kuo; Chia-Hsien Chen; Chang-Jung Chiang

doi:10.1186/s12891-023-07015-3

Pullout strength of different pedicle screws after primary and revision insertion: an in vitro study on polyurethane foam

BMC Musculoskelet Disord. 2023 Nov 7;24(1):863. doi: 10.1186/s12891-023-07015-3.

Authors

Lien-Chen Wu^{1

2

3}, Yueh-Ying Hsieh^{1

2}, Fon-Yih Tsuang^{4

5}, Yi-Jie Kuo^{2

6}, Chia-Hsien Chen^{1

2

7}, Chang-Jung Chiang^{8

9}

Affiliations

¹ Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, New Taipei City, 23561, Taiwan.
² Department of Orthopaedics, School of Medicine, College of Medicine, Taipei Medical University, Taipei, 11031, Taiwan.
³ Graduate Institute of Biomedical Materials and Tissue Engineering, College of Biomedical Engineering, Taipei Medical University, Taipei, 11031, Taiwan.
⁴ Division of Neurosurgery, Department of Surgery, National Taiwan University Hospital, Taipei City, 10022, Taiwan.
⁵ Spine Tumor Center, National Taiwan University Hospital, Taipei City, 10022, Taiwan.
⁶ Department of Orthopedic Surgery, Wan Fang Hospital, Taipei Medical University, Taipei City, 11696, Taiwan.
⁷ School of Biomedical Engineering, College of Biomedical Engineering, Taipei Medical University, Taipei, 11031, Taiwan.
⁸ Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, New Taipei City, 23561, Taiwan. cjchiang@s.tmu.edu.tw.
⁹ Department of Orthopaedics, School of Medicine, College of Medicine, Taipei Medical University, Taipei, 11031, Taiwan. cjchiang@s.tmu.edu.tw.

Abstract

Background: Surgeons are routinely required to remove loose or failed pedicle screws and insert a new screw in their place. However, inserting a new screw into an existing hole may compromise the holding capacity of the pedicle screw. The purpose of this study is to evaluate the pullout strength of pedicle screws with different thread designs after the primary insertion and revision surgery in a synthetic bone model.

Methods: Four pedicle screws with different thread designs (single-lead-thread (SLT) screw, dual-lead-thread (DLT) screw, mixed-single-lead-thread (MSLT) screw, and proximal-unthreaded-dual-thread (PUDL) screw) were inserted into pre-drilled, untapped holes (ø 4.2 mm, length 35 mm) in Sawbone blocks of density 20 pcf. In the first sequence, a 6.0 mm screw was inserted into the predrilled foam block and the primary pullout strength of the screw was measured according to ASTM F543. In the second sequence, a 6.0 mm screw was inserted and removed, and then either a 6.5 mm screw of the same design or a different screw design was inserted into the same hole and the pullout strength recorded.

Results: In the first sequence, the mean pullout strength of the MSLT screw was significantly (p < 0.05) greater than all other screw designs. In the second sequence, when the MSLT screw was the primary screw, using a larger MSLT screw (6.5 mm) as the revision screw did not lead to a higher pullout strength than if a 6.0 mm diameter PUDL screw was used for the revision. Using a larger DLT screw (6.5 mm) as the revision screw resulted in a significantly (p < 0.05) greater pullout strength than a 6.0 mm STL, DLT, MSLT, or PUDL screw.

Conclusions: Our results indicate that employing classic oversizing of the same screw design is a safe choice for maintaining screw purchase in the bone after revision. In cases where oversizing with the same screw design is not practical, opting for a PUDL screw with the same original diameter can provide enough purchase in the bone to maintain stability.

Keywords: Biomechanics; Pedicle screw; Pullout strength; Screw thread.

MeSH terms

Biomechanical Phenomena
Bone and Bones
Humans
Materials Testing
Pedicle Screws*
Polyurethanes

Substances

polyurethane foam
Polyurethanes