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Anterior Lumbar Interbody Fusion (ALIF) has been widely used to treat internal disc degeneration. However, different cage positions and their orientations may affect the initial stability leading to different fusion results. The purpose of the present study is to investigate the optimum cage position and orientation for aiding an ALIF having a transfacet pedicle screw fixation (TFPS). A three-dimensional finite element model (ALIF with TFPS) has been developed to simulate the stability of the L4/L5 fusion segment under five different loading conditions. The Taguchi method was used to evaluate the optimized placement of the cages. Three control factors and two noise factors were included in the parameter design. The control factors included the anterior-posterior position, the medio-lateral position, and the convergent-divergent angle between the two cages. The compressive preload and the strengths of the cancellous bone were set as noise factors. From the results of the FEA and the Taguchi method, we suggest that the optimal cage positioning has a wide anterior placement, and a diverging angle between the two cages. The results show that the optimum cage position simultaneously contributes to a stronger support of the anterior column and lowers the risk of TFPS loosening.

Based on elastic beam-column theory, antegrade and retrograde femur-nail-screw constructs were modeled to perform stress analysis on the locking screws. The effects of the nail-cortical and nail-subchondral support, the geometrical properties of the femur-nail-screw construct, and the nailing types on the locking screw stress were evaluated. The cortex that contacts the nail was simulated as a linearly elastic quasifoundation to support the medially deflected nail. The contact reaction of the femoral subchondral bone was assumed to serve as a moment-reliever to the distal screws. The current study predicted the following findings. 1) The axial compression along the nail dominated the distal screw stress (70%~80%). However, a little increase in the bending moment resulted in a drastic increase in the distal screw stress. 2) The nail-cortical contact served as the significant moment-reliever to the locking screws. Thus, the stresses at locking screw for both antegrade and retrograde nailing decreased as length of nail-cortical contact increased. 3) Due to protection of the subchondral bone and the comparatively shorter nail used, the distal screw stress of retrograde nailing was much less than the counterpart for antegrade nailing. 4) The length of the locking screw played a significant role in the stress upon it within the region without bony support to the nail. Within the supracondylar region, the increased nail and screw lengths drastically increased the distal screws stress.

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