Fatigue Testing of a Titanium Tapered Rod versus Two Rods Connected by a Parallel Connector across the Cervicothoracic Junction: A Biomechanical Study

This study compares the fatigue failure of a tapered titanium-rod construct against two connected titanium rods (domino construct) across the cervicothoracic junction. All testing was carried out in a simulated flexion-extension plane. The 3.5-mm/6.0-mm tapered titanium rod and the 3.5-mm titanium rod, connected to a 6.0-mm titanium rod with a connector, were compared for their fatigue failures. Six specimens of each construct were tested in a cantilever displacement control method using 6 different amplitudes. Each specimen was cycled to failure or to 2.5 million cycles (run out) at 10 Hz. Failure was defined as rod fracture. The domino construct reached the test limit of 2.5 million cycles at± 0.45 mm (72 N) but failed in all the other tested amplitudes. The tapered rod construct reached the test run out limit of 2.5 million cycles at higher amplitudes than the domino construct at ± 0.9 mm and failed in all the other tested amplitudes. The study showed that the tapered-rod construct across the cervicothoracic junction is more fatigue resistant than the domino construct.