Ron N Alkaly1, Dan L Bader. 1. *Orthopedic Surgery Center for Advanced Orthopedic Studies, Beth Israel Deaconess Medical Centre, Harvard University, Boston, MA †Interdisciplinary Research Centre (IRC) in Biomedical Materials and Department of Engineering Queen Mary, University of London, London, UK.
Abstract
STUDY DESIGN: A biomechanical study using bovine thoracolumbar spines. OBJECTIVE: To study investigated whether thread design parameters aimed at altering the state of stress at the screw-bone interface increase the screw's holding power. SUMMARY AND BACKGROUND DATA: Internal spinal fixators utilizing transpedicular screw fixation are used to achieve early stabilization of the injured spine in a range of clinical conditions. Despite advances in the design of internal spinal fixation systems, implant loosening, and catastrophic failures at the screw-bone interface remains a serious complication in adult spine surgery. Although the performance of the screws in the vertebral bone critically depends on ability of screw thread design to provide and maintain adequate bone purchase, the effect of individual thread design parameters on screw performance and the failure process of the screw-bone interface, remains unclear. METHODS: On the basis of the AO Schanz thread, this parametric study used 96 lumbar bovine vertebrae instrumented with 19 screw designs to investigate the effects of pitch, ratio of major to minor diameter, screw insertion depth, and major diameter, on screw performance under pure tensile loading. The effect of vertebral morphometry on screw performance and the extent of damage within the failed screw-bone interface were evaluated. RESULTS: The increase in screw insertion depth, screw pitch, and the ratio of major to minor diameter, significantly affected screw performance under tensile loads. Complex interactions existed between the major diameter and each of the design variables. Vertebral morphometry had little effect on screw performance while the damage within the failed bone-screw interface confined to the immediate region of the screw threads. CONCLUSIONS: Design variables, able to reduce shear stresses or modify the complex stress profile at the bone-screw interface, are more effective in preventing early failure of the interface.
STUDY DESIGN: A biomechanical study using bovine thoracolumbar spines. OBJECTIVE: To study investigated whether thread design parameters aimed at altering the state of stress at the screw-bone interface increase the screw's holding power. SUMMARY AND BACKGROUND DATA: Internal spinal fixators utilizing transpedicular screw fixation are used to achieve early stabilization of the injured spine in a range of clinical conditions. Despite advances in the design of internal spinal fixation systems, implant loosening, and catastrophic failures at the screw-bone interface remains a serious complication in adult spine surgery. Although the performance of the screws in the vertebral bone critically depends on ability of screw thread design to provide and maintain adequate bone purchase, the effect of individual thread design parameters on screw performance and the failure process of the screw-bone interface, remains unclear. METHODS: On the basis of the AO Schanz thread, this parametric study used 96 lumbar bovine vertebrae instrumented with 19 screw designs to investigate the effects of pitch, ratio of major to minor diameter, screw insertion depth, and major diameter, on screw performance under pure tensile loading. The effect of vertebral morphometry on screw performance and the extent of damage within the failed screw-bone interface were evaluated. RESULTS: The increase in screw insertion depth, screw pitch, and the ratio of major to minor diameter, significantly affected screw performance under tensile loads. Complex interactions existed between the major diameter and each of the design variables. Vertebral morphometry had little effect on screw performance while the damage within the failed bone-screw interface confined to the immediate region of the screw threads. CONCLUSIONS: Design variables, able to reduce shear stresses or modify the complex stress profile at the bone-screw interface, are more effective in preventing early failure of the interface.