Biomechanical Considerations for UHMWPE in the Spine

The design of any device to be implanted in the intervertebral space must incorporate considerations of the biomechanics of the particular spinal level to be implanted. Among other factors, the primary biomechanical factors to be considered can be characterized as the kinematics (motion), kinetics (applied forces), and load sharing (distribution of stress between anatomic components). The device should allow the expected kinematics, it should be able to withstand millions of cycles of the expected loads, and it should attempt not to disrupt the distribution of the tissue level stresses and strains experienced in a healthy intervertebral joint. The kinematics, kinetics, and load sharing of the spine vary significantly as one moves from the cervical to the thoracic to the lumbar spine. 

In addition to ROM data, other characteristics of the natural joint that should be reflected in the reconstructed joint are the center of rotation (if the motion is two-dimensional), or the helical axis of motion (Kinzel, Hall, and Hillberry 1972) (if the motion is three-dimensional), and the extent and nature of the coupled motions that will accompany a specific motion. For example, because of the shape of the facet joints in the lower cervical spine (C3-C7), axial torsion 

Axes used to characterize spinal motion flexion-extension, lateral bending, and axial torsion. (Reprinted with permission White and Panjabi 1990.) 

Approximate locations of the center of rotation in the lumbar spine. E, location of the axes going from a neutral to an extended position, F, going from a neutral to a flexed position, L, center of rotation in left lateral bending and torsion, and R, right lateral bending and torsion. (Reprinted with permission White and Panjabi 1990.) 

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