title: Design of spinous process hooks for flexible fixation of the lumbar spine.
abstract: A prototype flexible fixation system for the lumbar spine was subjected to tensile testing to failure and cyclic tensile testing in order to determine any regions of weakness. The system consisted of a spinous process hook and two laminar hooks made of stainless steel (316L). Each laminar hook was attached to the spinous process hook by a loop of polyester braid secured by a crimped metal sleeve. In five tensile tests, the system failed by irreversible deformation of the spinous process hook at 2.5 +/- 0.3 kN (mean +/- standard deviation). In three cyclic tests, in which the applied tension varied sinusoidally between 0.04 and 0.4 kN at a frequency of 5 Hz, failure occurred after less than 400,000 loading cycles. This occurred as a result of fatigue crack initiation and propagation in the spinous process hook. A finite element model showed a stress concentration in the region where the crack occurred, which raised the applied stress above the tensile fatigue strength of this stainless steel. The spinous process hook was redesigned for manufacture in a titanium alloy (Ti-6AI-4V ELI) to minimize artefacts in magnetic resonance imaging. Further finite element models showed no unacceptable stress concentrations.
authors: Department of Bio-Medical Physics and Bio-Engineering, University of Aberdeen, Scotland, UK.
authors: J C
authors: JC
authors: Leahy
authors: K J
authors: KJ
authors: Mathias
authors: A
authors: A
authors: Heaton
authors: D E
authors: DE
authors: Shepherd
authors: D W
authors: DW
authors: Hukins
authors: W F
authors: WF
authors: Deans
authors: M W
authors: MW
authors: Brian
authors: D
authors: D
authors: Wardlaw
mesh_terms: D000497:Alloys; D016477:Artifacts; D003198:Computer Simulation; D017076:Computer-Aided Design; D004867:Equipment Design; D004868:Equipment Failure; D020342:Finite Element Analysis; D006801:Humans; D016268:Internal Fixators / Q000592:standards*; D017116:Low Back Pain / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000601:surgery*; D008279:Magnetic Resonance Imaging; D008422:Materials Testing; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D013718:Tensile Strength; D014025:Titanium
chemical_list: D000497:Alloys; C031462:titanium alloy (TiAl6V4); D014025:Titanium