Finite element study of matched paired posterior disc implant and dynamic stabilizer (360° motion preservation system).

BACKGROUND

Anterior lumbar disc replacements are used to restore spinal alignment and kinematics of a degenerated segment. Compared to fusion of the segment, disc replacements may prevent adjacent segment degeneration. To resolve some of the deficiencies of anterior lumbar arthroplasty, such as the approach itself, difficulty of revision, and postoperative facet pain, 360° motion preservation systems based on posterior disc and posterior dynamic system (PDS) designs are being pursued. These systems are easier to revise and address all the pain generators in a motion segment, including the nerves, facets, and disc. However, biomechanics of the 360° posterior motion preservation system, including the contributions of the 2 subsystems (disc and PDS), are sparsely reported in the literature.nds.

METHODS

An experimentally validated 3-dimensional finite element model of the ligamentous L3-S1 segment was used to investigate the differences in biomechanical behavior of the lumbar spine. A single-level 360° posterior motion preservation system and its individual components in various orientations were simulated and compared with an intact model. Appropriate posterior surgical procedures were simulated. The PDS, a curved device with male and female components, was attached to the pedicle screws. The finite element models were subjected to 400 N of follower load plus 10Nm moment in extension and flexion.

RESULTS

The PDS restored flexion/extension motion to normal. The artificial disc led to increases in range of motion (ROM) compared with the intact model. ROM for the 360° system at the implanted and adjacent levels were similar to those of the respective intact levels. ROM was similar whether the discs were placed (a) both parallel to the midsagittal plane, (b) both angled 20° to the midsagittal plane, and (c) one at 20° and one parallel to the midsagittal plane. However, the stresses were slightly higher in the nonparallel disc configuration than in the parallel disc configuration, both in flexion and extension modes.

CONCLUSIONS

Posterior disc replacement with PDS restored the kinematics of the spine at all levels to near normal. In addition, placing the discs in a nonparallel configuration with respect to the midsagittal plane does not affect the functionality of the discs compared with parallel placement. Posterior disc replacement alone is not sufficient to restore the segment biomechanics to normal levels.

CLINICAL RELEVANCE

Finite element analysis results show that, unlike implants for fusion, PDS and posterior discs together (360° motion preservation system) are needed to preserve ROM. Such systems will prevent adjacent level degeneration and address pain from various spinal components, including facets.