Comparison of Stability and Anterior Column Load Sharing: Transfacet-Intralaminar Cortical Fixation versus Traditional Pedicle Screw Instrumentation

Introduction
A novel metal-free PEEK cortico-pedicular strap, placed after unique curvilinear drill-hole creation through the lamina-facet complex interface, has been FDA-approved. This strap provides circumferential & compressive cortical fixation of the lamina, pedicle, and facet joints. The objective of this study is to characterize the biomechanical differences between this transfacet-intralaminar cortical fixation and traditional pedicle screw system (PSS) for stability and anterior column load dynamics in conjunction with an interbody device.
Methods
A validated L1-S1 osteoligamentous spinal finite element model was simulated in eight unique scenarios consisting of L4-5 ALIF, L4-5 LLIF, L4-5 in L4-S1 ALIF, and L5-S1 in L4-S1 ALIF, each with two comparative posterior fixations. The lateral cage measured 18mm x 40mm x 8mm x 12deg, while ALIF cage hosted two integrated screws and measured 27mm x 41mm x 10mm x 12deg. Mechanical properties were assigned to physiologic tissues per published literature. Spinal weight and muscle stabilization of a 90kg male was applied through 414N follower load.
Group 1 received standard posterior Ti6AL4V PSS with 5.5mm rods, whereas PEEK strap intralaminar fixation was used for Group 2. Each lamina, pedicle and facet joint were circumferentially fixed using a ratcheting strap creating a compressive force between the facet surfaces at maximum tension of 222N. An intact model was used as the control group for range of motion (ROM). All nine models were simulated for flexion, extension, lateral bending (LB), and torsion using a maximum load of 7.5Nm. Outputs recorded were the ROM and anterior column load. Average and standard deviation was calculated for each output within the group, and one-tail paired t-test was performed between the groups for estimating the p-value.
Results
Both posterior fixation techniques (Groups 1 & 2) provided similar stabilization of the index levels with an average of 86±12% (p=0.42) reduction in ROM across all planes, compared to intact. Furthermore, the maximum ROM across all 8 instrumented models were within 0.6 degrees. The anterior column load was statistically higher in Group 2 by 172 (±59) N, 268 (±132) N, 285 (±105) N, 212 (±78) N, and 199 (±79) N in flexion, extension, lateral bending, torsion, and standing.
Conclusions
Both fixation techniques provided similar and effective reduction in index-level spinal element motion. However, anterior column stress shielding was lower in the transfacet-intralaminar strap fixation method with an average of 227 (±94) N higher anterior forces passing through the interbody space, compared to PSS. This could translate into more physiologic fusion characteristics for the interbody space via Wolff’s law.