ASTM F2624—Evaluating Alternate Fixtures for Flexion/Extention, Lateral Bending, and Axial Rotation Testing of Extra Discal Motion Preserving Implants

ASTM F2624-07 prescribes applying flexion/extension (FE), lateral bending (LB), and axial rotation (AR) moments to test and evaluate extra discal motion preserving implants (EDMPI), but parameters such as fixture design and load parameters are left to the user to define. A set of fixtures to conduct FE, LB, and AR testing rotation of EDMPI devices about a fixed center of rotation were developed by Aesculap Implant Systems. Their design was shared, and a set of these fixtures were manufactured and evaluated independently at Empirical Testing Corp. Testing was conducted to quantify the loads applied to an EDMPI in the FE and LB fixtures, determine whether results are frequency dependent, and evaluate whether EDMPI of different stiffnesses can be differentiated when using the fixtures. Miniature load cells were mounted to FE and LB fixtures to quantify load applied to an EDMPI during testing at several different applied loads and at two testing frequencies (2 Hz and 5 Hz). The load cells were replaced with cylindrical test specimens made of stainless steel, titanium, polyether ether ketone (PEEK), or a polyethylene cable slipped inside a polyurethane tube and the testing repeated. Results indicated that loads measured during FE testing were similar in magnitude to those recorded during LB testing but 90 degrees out of phase. No frequency effects were observed. Differences in displacement amplitude when testing the stainless steel, titanium, and PEEK specimens in FE and LB testing at both 2 Hz and 5 Hz were not discernable. The displacement amplitude of the polyethylene (PE) cord specimen was different than the other specimens, but no frequency effect was noted. The fixtures developed by Aesculap are a reasonable attempt to conduct EDMPI testing in FE, LB, and AR motions. Potential users of these fixtures should conduct further studies to determine whether their use is appropriate to test their specific EDMPI device.