Design of spinous process hooks for flexible fixation of the lumbar spine

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 an d two laminar hooks made of stainless steel (316L). Each laminar hook was a ttached 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 irreve rsible deformation of the spinous process hook at 2.5 +/- 0.3 kN (mean +/- standard deviation). In three cyclic tests, in which the applied tension va ried sinusoidally between 0.04 and 0.3 kN at a frequency of 5 Hz, failure o ccurred after less than 400 000 loading cycles. This occurred as a result o f fatigue crack initiation and propagation in the spinous process hook. A f inite element model showed a stress concentration in the region where the c rack occurred, which raised the applied stress above the tensile fatigue st rength of this stainless steel. The spinous process hook was redesigned for manufacture in a titanium alloy (Ti-6Al-4V ELI) to minimize artefacts in m agnetic resonance imaging. Further finite element models showed no unaccept able stress concentrations.