Biomechanical Analysis of Two-level Cervical Disc Replacement With a Stand-alone U-shaped Disc Implant

Study Design. Biomechanical study using a three-dimensional nonlinear finite element model. Objective. To analyze biomechanical changes with three prostheses based on two-level arthroplasty and to verify the biomechanical efficiency of dynamic cervical implants (DCIs) with a stand-alone U-shaped structure. Summary of Background Data. Few studies have compared biomechanical behavior of various prostheses as they relate with clinical results after two-level total disc replacement. Methods. Three arthroplasty devices Mobi-C, porous coated motion (PCM), and DCI were inserted at the C4-C6 disc space and analyzed. Displacement loading was applied to the center of the endplate at the C3 level to simulate flexion and extension motions. Results. The motion distributions in extension with DCI and in flexion with DCI and Mobi-C were relatively close to that in the intact model. Mobi-C and PCM obviously increased the combined extension range of motion at the index levels, but both resulted in about 45% decrease in extension moment. DCI showed a trend in strain energy similar to that of healthy discs. PCM exhibited a facet joint stress distribution almost similar to that of the intact model. DCI did not generate significant overloading at cartilage between the index levels, whereas the maximum facet joint stress increased with Mobi-C was about 39%. The maximum stress on a ultrahigh molecular-weight-polyethylene core was above the yield stress (42.43 MPa for Mobi-C and 30.94 MPa for PCM). Conclusion. Each prosthesis shows its biomechanical advantages and disadvantages. However, DCI has the capacity to preserve motion and store energy under external loading, similar to the behavior of normal discs. Compared with Mobi-C, both DCI and PCM showed a lower stress at cartilage between index levels, which may avoid facet joint degeneration to some extent. Such a well-controlled arthroplasty device with a stand-alone structure may be a potential candidate and needs to be investigated in future studies.