A biomechanical investigation of the retentive force of pedicle screw structures for different screw tulip designs |
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| Affiliation: | 1. Department of Biomedical Engineering, National Yang-Ming University, No.155, Sec. 2, Linong St., Beitou Dist., Taipei City 11221, 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;1. Department of Mechanical Engineering, Polytechnique Montréal, P.O. Box 6079, Downtown Station, Montreal, Quebec H3C 3A7, Canada;2. Sainte-Justine University Hospital Center, 3175, Cote Sainte-Catherine Road, Montreal, Quebec H3T 1C5, Canada;3. Laboratoire de Biomécanique Appliquée, UMRT24 IFSTTAR/Aix-Marseille Université, Boulevard Pierre Dramard, 13916 Marseille Cedex 20, France;4. International Associated Lab in Biomechanics of Spine Injuries & Pathologies, France and Canada;5. Department of Surgery, Faculty of Medicine, Université de Montréal, 2900 Boulevard Edouard-Montpetit, Montreal, Quebec H3T 1J4, Canada;6. Department of Surgery, Hôpital du Sacré-Cœur de Montréal, Montreal, Quebec H4J 1C5, Canada;1. University of Louisville School of Medicine, 500 South Preston Street. Instructional Building, Room 305. Louisville, KY, USA;2. Norton Leatherman Spine Center, 210 East Gray Street, Suite 900, Louisville, KY, USA;3. Department of Orthopaedic Surgery, University of Louisville School of Medicine, 550 S. Jackson Street, 1st Floor ACB, Louisville, KY, USA |
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| Abstract: | BackgroundPedicle 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.MethodsThree 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.FindingAmong 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.InterpretationIn 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. |
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