Increasing Bending Strength and Pullout Strength in Conical Pedicle Screws: Biomechanical Tests and Finite Element Analyses

Study Design Comparative in vitro biomechanical study and finite element analysis. Objectives To investigate the bending strength and pullout strength of conical pedicle screws, as compared with conventional cylindrical screws. Summary of Background Data Transpedicle screw fixation, the gold standard of spinal fixation, is threatened by screw failure. Conical screws can resist screw breakage and loosening. However, biomechanical studies of bending strength have been lacking, and the results of pullout studies have varied widely. Methods Ten types of pedicle screws with different patterns of core tapering and core diameter were specially manufactured with good control of all other design factors. The stiffness, yielding strength, and fatigue life of the pedicle screws were assessed by cantilever bending tests using high-molecular-weight polyethylene. The pullout strength was assessed by pullout tests using polyurethane foam. Concurrently, 3-dimensional finite element models simulating these mechanical tests were created, and the results were correlated to those of the mechanical tests. Results In bending tests, conical screws had substantially higher stiffness, yielding strength, and fatigue life than cylindrical screws (P<0.01), especially when there was no step at the thread-shank junction. In pullout tests, pullout strength was higher in screws with a conical core and smaller core diameter and also in situations with higher foam density (P<0.01). In finite element analysis, the maximal deflection and maximal tensile stress were closely related to yielding strength (r=−0.91) and fatigue life (r=−0.95), respectively, in the bending analyses. The total reaction force was closely related to the pullout strength in pullout analyses (r=0.84 and 0.91 for different foam densities). Conclusions Conical screws effectively increased the bending strength and pullout strength simultaneously. The finite element analyses reliably predicted the results of the mechanical tests.

[1]  K. Okuyama,et al.  Influence of bone mineral density on pedicle screw fixation: a study of pedicle screw fixation augmenting posterior lumbar interbody fusion in elderly patients. , 2001, The spine journal : official journal of the North American Spine Society.

[2]  R. Castelein,et al.  Complications of pedicle screws in lumbar and lumbosacral fusions in 105 consecutive primary operations , 2002, European Spine Journal.

[3]  R. Gaines,et al.  Successful short-segment instrumentation and fusion for thoracolumbar spine fractures: a consecutive 41/2-year series. , 2000, Spine.

[4]  R. Fraser,et al.  Short segment fixation of thoracolumbar burst fractures without fusion , 1999, European Spine Journal.

[5]  J. Moran,et al.  Transpedicular screw fixation , 1989, Journal of orthopaedic research : official publication of the Orthopaedic Research Society.

[6]  Tain‐Hsiung Chen,et al.  Biomechanical Effects of the Body Augmenter for Reconstruction of the Vertebral Body , 2004, Spine.

[7]  Shing-Sheng Wu,et al.  Mechanical Performance of the New Posterior Spinal Implant: Effect of Materials, Connecting Plate, and Pedicle Screw Design , 2003, Spine.

[8]  M. Brown,et al.  Triangulated pedicle screw construct technique and pull-out strength of conical and cylindrical screws. , 2001, Journal of spinal disorders.

[9]  R. Kaya,et al.  Modified transpedicular approach for the surgical treatment of severe thoracolumbar or lumbar burst fractures. , 2004, The spine journal : official journal of the North American Spine Society.

[10]  C. Reitman,et al.  Physical Characteristics of Polyaxial-Headed Pedicle Screws and Biomechanical Comparison of Load With Their Failure , 2003, Spine.

[11]  G. Sapkas,et al.  Complications and Problems Related to Pedicle Screw Fixation of the Spine , 2003, Clinical orthopaedics and related research.

[12]  Ching-Chi Hsu,et al.  Multiobjective optimization of tibial locking screw design using a genetic algorithm: Evaluation of mechanical performance , 2006, Journal of orthopaedic research : official publication of the Orthopaedic Research Society.

[13]  R L Huston,et al.  Strength and fatigue resistance of thoracolumbar spine implants: an experimental study of selected clinical devices. , 1999, Journal of spinal disorders.

[14]  E. Schneider,et al.  Comparison of the in vitro holding strengths of conical and cylindrical pedicle screws in a fully inserted setting and backed out 180 degrees. , 2000, Journal of spinal disorders.

[15]  R. Winter,et al.  Complications associated with pedicle screws. , 1999, The Journal of bone and joint surgery. American volume.

[16]  S. Esterby.  American Society for Testing and Materials , 2006 .

[17]  G. Andersson,et al.  Augmentation of Pedicle Screw Fixation Strength Using an Injectable Calcium Phosphate Cement as a Function of Injection Timing and Method , 2004, Spine.

[18]  C. Ames,et al.  Kyphoplasty-augmented short-segment pedicle screw fixation of traumatic lumbar burst fractures: initial clinical experience and literature review. , 2005, Neurosurgical focus.

[19]  A. Vaccaro,et al.  Potential Large Vessel Injury During Thoracolumbar Pedicle Screw Removal: A Case Report , 1997, Spine.

[20]  A. Alanay,et al.  Short-Segment Pedicle Instrumentation of Thoracolumbar Burst Fractures: Does Transpedicular Intracorporeal Grafting Prevent Early Failure? , 2001, Spine.

[21]  R. Gaines.  The Use of Pedicle-Screw Internal Fixation for the Operative Treatment of Spinal Disorders* , 2000, The Journal of bone and joint surgery. American volume.

[22]  Darrel S Brodke,et al.  Diagnosis and Management of Thoracolumbar Spine Fractures , 2003 .

[23]  D. Pienkowski,et al.  Multicycle Mechanical Performance of Titanium and Stainless Steel Transpedicular Spine Implants , 1998, Spine.

[24]  T C Hearn,et al.  Insertional Torque and Pull‐out Strengths of Conical and Cylindrical Pedicle Screws in Cadaveric Bone , 1996, Spine.

[25]  D. Cho,et al.  Treatment of Thoracolumbar Burst Fractures with Polymethyl Methacrylate Vertebroplasty and Short-segment Pedicle Screw Fixation , 2003, Neurosurgery.

[26]  D. Burton,et al.  Treatment of Thoracolumbar Burst Fractures with Variable Screw Placement or Isola Instrumentation and Arthrodesis: Case Series and Literature Review , 2004, Journal of spinal disorders & techniques.

[27]  J. Christoforakis,et al.  Treatment of unstable thoracolumbar and lumbar spine injuries using Cotrel-Dubousset instrumentation. , 1999, Spine.

[28]  J. Corin,et al.  Biomechanical Analysis of Pedicle Screw Instrumentation Systems in a Corpectomy Model , 1989, Spine.

[29]  R. Huston,et al.  Biomechanical assessment of titanium and stainless steel posterior spinal constructs: effects of absolute/relative loading and frequency on fatigue life and determination of failure modes. , 1997, Journal of spinal disorders.

[30]  B. Cunningham,et al.  Static and Cyclical Biomechanical Analysis of Pedicle Screw Spinal Constructs , 1993, Spine.

[31]  O. Böstman,et al.  Complications of transpedicular lumbosacral fixation for non-traumatic disorders. , 1997, The Journal of bone and joint surgery. British volume.

[32]  C. Chao,et al.  Increase of pullout strength of spinal pedicle screws with conical core: Biomechanical tests and finite element analyses , 2005, Journal of orthopaedic research : official publication of the Orthopaedic Research Society.

[33]  W. Walsh,et al.  Multiaxial Pedicle Screw Designs: Static and Dynamic Mechanical Testing , 2004, Spine.

[34]  A. Valdevit,et al.  Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out. , 2001, The spine journal : official journal of the North American Spine Society.