• Alan H Daniels, David J Paller, Sarath Koruprolu, Matthew McDonnell, Mark A Palumbo, and Joseph J Crisco.
• Department of Orthopaedics, Warren Alpert Medical School of Brown University and Rhode Island Hospital, Providence, RI 02906, USA. alandanielsmd@gmail.com
• Spine. 2012 Nov 1;37(23):E1438-43.

Study DesignBiomechanical cadaver investigation.ObjectiveTo examine dynamic bending stiffness and energy absorption of the lumbar spine with and without implanted total disc replacement (TDR) under simulated physiological motion.Summary Of Background DataThe pendulum testing system is capable of applying physiological compressive loads without constraining motion of functional spinal units (FSUs). The number of cycles to equilibrium observed under pendulum testing is a measure of the energy absorbed by the FSU.MethodsFive unembalmed, frozen human lumbar FSUs were tested on the pendulum system with axial compressive loads of 181 N, 282 N, 385 N, and 488 N before and after Synthes ProDisc-L TDR implantation. Testing in flexion, extension, and lateral bending began by rotating the pendulum to 5º resulting in unconstrained oscillatory motion. The number of rotations to equilibrium was recorded and bending stiffness (N·m/º) was calculated and compared for each testing mode.ResultsIn flexion/extension, the TDR constructs reached equilibrium with significantly (P < 0.05) fewer cycles than the intact FSU with compressive loads of 282 N, 385 N, and 488 N. Mean dynamic bending stiffness in flexion, extension, and lateral bending increased significantly with increasing load for both the intact FSU and TDR constructs (P < 0.001). In flexion, with increasing compressive loading from 181 N to 488 N, the bending stiffness of the intact FSUs increased from 4.0 N·m/º to 5.5 N·m/º, compared with 2.1 N·m/º to 3.6 N·m/º after TDR implantation. At each compressive load, the intact FSU was significantly stiffer than the TDR (P < 0.05).ConclusionLumbar FSUs with implanted TDR were found to be less stiff, but absorbed more energy during cyclic loading with an unconstrained pendulum system. Although the effects on clinical performance of motion-preserving devices are not fully known, these results provide further insight into the biomechanical behavior of these devices under approximated physiological loading conditions.

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