Notch sensitivity of titanium alloy, commercially pure titanium, and stainless steel spinal implants

Study Design. This is a biomechanical study comparing the fatigue life of stainless steel (SS), titanium alloy (TI), and commercially pure titanium (CP) rods that were notched using techniques essential to spine surgery. Objectives. To show the difference in fatigue life of the three rods and the effect on fatigue life of marks caused by techniques unavoidable in spine surgery. Summary of Background Data. Past studies have shown a decrease in the fatigue life of titanium when there is a defect or notch in the surface. This has been demonstrated with spinal and fracture implants, prosthetic joints, and in the materials science literature. Previous authors have recommended avoidance of techniques that mark the surface of titanium implants. Methods. Four Texas Scottish Rite Hospital rods of each metal were prepared in four different ways: 1) no notch, 2) a French bender notch, 3) a top-loading connector-left on, 4) a top-loading connector-removed. The rods were fatigued in four point bending up to 1 million cycles. Before fatigue testing, two samples of each preparation were examined with electron microscopy. Results. No SS rods failed. No TI rods without a notch or connector removed failed. One TI rod with the connector left in place failed (106,635 cycles). Three of the TI rods in the French-bender group failed (average, 87,663 cycles). One CP rod without a notch failed (358,957 cycles). No CP rods with the connector left in place failed. All CP rods with the connector removed failed (average, 230,094 cycles). All CP rods in the French bender group failed (average 120,817 cycles). The following differences were significant (P < 0.05): SS-TI and SS-CP in the French bender group; SS-CP and TI-CP in the connector removed group. Electron micrographs performed before fatigue testing showed cracks in the TI and CP rods that were marked with the French bender. Conclusion. Caution is recommended when bending TI or CP rods, or placing a connector and removing it when using CP rods in situations in which fatigue failure is a concern.