A biomechanical investigation of the retentive force of pedicle screw structures for different screw tulip designs

Po-Yi Liu; Po-Liang Lai; Chun-Li Lin

doi:10.1016/j.clinbiomech.2019.06.003

A biomechanical investigation of the retentive force of pedicle screw structures for different screw tulip designs

Clin Biomech (Bristol, Avon). 2019 Dec:70:23-30. doi: 10.1016/j.clinbiomech.2019.06.003. Epub 2019 Jul 10.

Authors

Po-Yi Liu¹, Po-Liang Lai², Chun-Li Lin³

Affiliations

¹ Department of Biomedical Engineering, National Yang-Ming University, No.155, Sec. 2, Linong St., Beitou Dist., Taipei City 11221, Taiwan; Department of Orthopedic Surgery, Bone and Joint Research Center, Chang Gung Memorial Hospital at Linkou, College of Medicine, Chang Gung University, No. 5, Fuxing St., Guishan Dist., Taoyuan City 33305, Taiwan.
² Department of Orthopedic Surgery, Bone and Joint Research Center, Chang Gung Memorial Hospital at Linkou, College of Medicine, Chang Gung University, No. 5, Fuxing St., Guishan Dist., Taoyuan City 33305, Taiwan. Electronic address: polianglai@gmail.com.
³ Department of Biomedical Engineering, National Yang-Ming University, No.155, Sec. 2, Linong St., Beitou Dist., Taipei City 11221, Taiwan. Electronic address: cllin2@ym.edu.tw.

PMID: 31382200
DOI: 10.1016/j.clinbiomech.2019.06.003

Abstract

Background: Pedicle screw based spinal fixation systems have been widely used for treating a variety of spinal diseases. The retentive force is an important factor that determines structural stability. The screw tulip design and the magnitude of nut tightening torque influence the retentive force. This study investigated the influences of varied tilt angles between the shaft-rod interface and varied nut tightening torques on the retentive force of the monoaxial, polyaxial, and uniplanar screws.

Methods: Three types of tulip constructs were biomechanically tested. Two parameters that affect the retentive force include the tilt angle and the nut tightening torque. The retentive force was investigated by an axial gripping capacity test and axial torque gripping capacity test.

Finding: Among all combinations of screw designs and tilt angles, the 12 Nm nut tightening torque offered a greater retentive force than the 8 Nm, except for monoaxial screws with a 0 degree tilt angle. For monoaxial screws, the retentive force was negatively correlated with increasing tilt angles. For polyaxial and uniplanar screws, the retentive forces remained constant with increasing tilt angles.

Interpretation: In monoaxial screws, when the axis of the shaft isn't perpendicular to the axis of the rod, a gap is formed between the tulip-rod interface. This results in a decreased retentive force. In polyaxial and uniplanar screws, the contact surfaces were the same in different tilt angles, therefore, the retentive force remained constant, which was attributed to the adjustable tulips always being perpendicular to the axis of the rods.

Keywords: Nut tightening torque; Pedicle screw; Screw tulip design; Tilt angle.

MeSH terms

Alloys
Biomechanical Phenomena
Equipment Design
Materials Testing
Pedicle Screws*
Stress, Mechanical*
Tensile Strength
Titanium / chemistry*
Torque

Substances

Alloys
titanium alloy (TiAl6V4)
Titanium