The Role of Transverse Connectors in C1-C2 fixation for Atlantoaxial Instability: Is It Necessary? A Biomechanical Study

Biomechanical properties of different anterior and posterior techniques for atlantoaxial fixation: a finite element analysis

BACKGROUND

Many techniques for atlantoaxial fixation have been developed. However, the biomechanical differences among various atlantoaxial fixation methods remain unclear. This study aimed to evaluate the biomechanical influence of anterior and posterior atlantoaxial fixation techniques on fixed and nonfixed segments.

METHODS

An occiput-C7 cervical finite element model was used to construct 6 surgical models including a Harms plate, a transoral atlantoaxial reduction plate (TARP), an anterior transarticular screw (ATS), a Magerl screw, a posterior screw-plate, and a screw-rod system. Range of motion (ROM), facet joint force (FJF), disc stress, screw stress, and bone-screw interface stress were calculated.

RESULTS

The C1/2 ROMs were relatively small in the ATS and Magerl screw models under all loading directions except for extension (0.1°-1.0°). The posterior screw-plate system and screw-rod system generated greater stresses on the screws (77.6-1018.1 MPa) and bone-screw interfaces (58.3-499.0 MPa). The Harms plate and TARP models had relatively small ROMs (3.2°-17.6°), disc stress (1.3-7.6 MPa), and FJF (3.3-106.8 N) at the nonfixed segments. Changes in disc stress and FJF of the cervical segments were not consistent with changes in ROM.

CONCLUSIONS

ATS and Magerl screws may provide good atlantoaxial stability. The posterior screw-rod system and screw-plate system may have higher risks of screw loosening and breakage. The Harms plate and TARP model may more effectively relieve nonfixed segment degeneration than other techniques. The C0/1 or C2/3 segment may not be more susceptible to degeneration than other nonfixed segments after C1/2 fixation.

Biomechanical Evaluation of the Cross-link Usage and Position in the Single and Multiple Segment Posterior Lumbar Interbody Fusion

OBJECTIVE

Previous studies have neither explored the usage of cross-links nor investigated the optimal position of the cross-links in posterior lumbar interbody fusion (PLIF). This study evaluates biomechanical properties of cross-links in terms of different fixation segments and optimal position in single- and multi-segment posterior lumbar interbody fusion.

METHODS

Two finite element (FE) models of instrumented lumbosacral spine with single-(L4/5) and multi-segment (L3-S1) PLIF surgery were simulated. On the basis of the two models, the benefits of the usage of cross-links were assessed and compared with the status of no application of cross-links. Moreover, the effects of position of cross-links on multi-segment PLIF surgery were studied in Upper, Middle, and Lower positions.

RESULTS

No significant difference was found in the range of motion (ROM), intersegmental rotational angle (IRA) of adjacent segments, and intradiscal pressure (IDP) regardless of the usage of cross-links in the single-segment PLIF surgery, while the cross-link increased the maximum von Mises stress in the fixation (MSF) under the axial rotation (53.65 MPa vs 41.42 MPa). In the multi-segment PLIF surgery, the usage of cross-links showed anti-rotational advantages indicated by ROM (Without Cross-link 2.35^o , Upper, 2.24^o ; Middle, 2.26^o ; Lower, 2.30^o ) and IRA (Without Cross-link 1.19^o , Upper, 1.08^o ; Middle, 1.09^o ; Lower, 1.13^o ). The greatest values of MSF were found in without cross-link case under the flexion, lateral bending, and axial rotation (37.48, 62.61, and 86.73 MPa). The application of cross-links at the Middle and Lower positions had lower values of MSF (48.79 and 69.62 MPa) under the lateral bending and axial rotation, respectively.

CONCLUSION

The application of cross-links was not beneficial for the single-segment PLIF, while it was found highly advantageous for the multi-segment PLIF. Moreover, the usage of cross-links at the Middle or Lower positions resulted in a better biomechanical stability.