Torsion refers to the twisting of a sample due to an applied torque. Various applications requires materials to withstand torsional stress. It is thus important to test this configuration since material torsional behavior can greatly vary from what is observed under a simple axial constraint. Various mechanical properties can be calculated under this configuration, e.g. the shear modulus, the rupture shear modulus, the yield shear strength, the ultimate (rupture) shear strength and the ductility. The sample could further be evaluated for its resistance to fatigue in torsion.
ASTM F1717 - Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model
Significance and Use
5.1 Spinal implants are generally composed of several components which, when connected together, form a spinal implant assembly. Spinal implant assemblies are designed to provide some stability to the spine while arthrodesis takes place. These test methods outline standard materials and methods for the evaluation of different spinal implant assemblies so that comparison between different designs may be facilitated.
5.2 These test methods are used to quantify the static and dynamic mechanical characteristics of different designs of spinal implant assemblies. The mechanical tests are conducted in vitro using simplified load schemes and do not attempt to mimic the complex loads of the spine.
5.3 The loads applied to the spinal implant assemblies in vivo will, in general, differ from the loading configurations used in these test methods. The results obtained here cannot be used directly to predict in vivo performance. The results can be used to compare different component designs in terms of the relative mechanical parameters.
5.4 Fatigue testing in a simulated body fluid or saline may cause fretting, corrosion, or lubricate the interconnections and thereby affect the relative performance of tested devices. This test should be initially performed dry (ambient room conditions) for consistency. The effect of environment may be significant. Repeating all or part of these test methods in simulated body fluid, saline (9 g NaCl per 1000 mL water), a saline drip, water, or a lubricant should be considered. The maximum recommended frequency for this type of cyclic testing should be 5 Hz.
5.5 The location of the longitudinal elements is determined by where the anchors are clinically placed against bony structures. The perpendicular distance to the load direction (block moment arm) between the axis of a hinge pin and the anchor’s attachment-points to a UHMWPE block is independent of anchor-type. The distance between the anchor’s attachment point to the UHMWPE block and the center of the longitudinal element is a function of the interface design between the screw, hook, wire, cable, and so forth, and the rod, plate, and so forth.
5.6 During static torsion testing, the rotation direction (clockwise or counter clockwise) may have an impact on the results.
Scope
1.1 These test methods cover the materials and methods for the static and fatigue testing of spinal implant assemblies in a vertebrectomy model. The test materials for most combinations of spinal implant components can be specific, depending on the intended spinal location and intended method of application to the spine.
1.2 These test methods are intended to provide a basis for the mechanical comparison among past, present, and future spinal implant assemblies. They allow comparison of spinal implant constructs with different intended spinal locations and methods of application to the spine. These test methods are not intended to define levels of performance, since sufficient knowledge is not available to predict the consequences of the use of a particular device.
1.3 These test methods set out guidelines for load types and methods of applying loads. Methods for three static load types and one fatigue test are defined for the comparative evaluation of spinal implant assemblies.
1.4 These test methods establish guidelines for measuring displacements, determining the yield load, and evaluating the stiffness and strength of the spinal implant assembly.
1.5 Some spinal constructs may not be testable in all test configurations.
1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.