Spinal Implants are tested according to several ASTM standards. ASTM F1717 is intended to provide a basis for the mechanical comparison among past, present and future spinal implant assemblies. Three static and one fatigue mechanical test of a spinal construct are governed by ASTM F1717. The three static tests are compression bending, tension bending and torsion. The fatigue test is a compression bending fatigue test. The mechanical tests are conducted in vitro using simplified loading schemes and do not attempt to mimic the complex loads of the spine. An outline of the four test procedures is as follows:
Static Compression Bend Test– Load at a crosshead displacement rate not to exceed 25 mm/min and record the load vs displacement curve. Calculate displacement at 2% offset yield, elastic displacement, compressive bending yield load, compressive bending stiffness, compressive bending ultimate displacement and ultimate load.
Static Tension Bend Test– Load at a crosshead displacement rate not to exceed 25 mm/min. and record the load vs displacement curve. Calculate displacement at 2% offset yield, elastic displacement, tensile bending yield load, tensile bending stiffness, tensile bending ultimate displacement and ultimate load.
Static Torsion Test – Load at a rate not to exceed 60 deg/min and record the torque vs angle curve. Calculate angle at 2% offset yield, elastic angle, yield torque and torsional stiffness.
Compressive Bending Fatigue Test – Apply a constant load amplitude sinusoidal control profile at no faster than 5Hz. A constant load ratio, R, for all tests should be established and should be greater than or equal to 10. Evaluate two specimens at the initial fatigue loads. Determine the maximum run out load based on no samples failing prior to 5 million cycles. Continue fatigue testing pairs of specimens until the difference between the load at which a specimen fails and the run out load is no greater than 10% of the compression bending ultimate load.
ADMET biaxial universal testing systems are available for static only or for both the static and fatigue tests outlined above. These tests require specific fixturing described in the F1717 specification and can be provide by ADMET.