Many studies have reported of satisfactory outcomes in patients with degenerative spinal diseases who have undergone lumbar interbody fusion.10,11 Many biomechanical studies have proven that TLIF or PLIF with one cage can acquire effective spinal stability similar to that achieved using two cages, however, one cage TLIF has the limitations of leading to a slight increase in screw stress and cage-endplate interface comparing to two cages TLIF.12,13 Previous studies have compared PLIF and TLIF, however the cages in the two methods were different, and different cages can lead to inaccurate results.3,14 In the current study, we first used the finite-element method to analyze differences between the PLIF and TLIF techniques with the same two cages.
In our study, instrumented TLIF and PLIF were found to be able to achieve effective and similar spinal segment stability, compared with the intact spine model. We must pay attention to these results because insufficient stabilization and increased motion can elevate implant stresses that may lead to mechanical failure and a lack of fusion under repetitive physiological loading.
Cage dislodgement has been reported to be a complication of transforaminal lumbar interbody fusion, especially when spinal construct stability is not sufficient.15 Aoki et al16 conducted a clinical study and found that TLIF performed using a bullet-shaped cage could increase the cage dislodgement, since the geometrical structure of a bullet-shaped cage is less able to prevent migration than a box-shaped cage. In our study, we found the maximal cage stress produced by PLIF was high, except under r-rotation, increasing the possibility of cage dislodgement and cage collapse.
In our study, we found that the maximal stress in the bone graft in TLIF was high in extension, l-rotation and l-bending. TLIF may offer less stress-shielding between intervertebral bodies, resulting in the fusion bone receiving greater stress. The stress in the bone may be helpful for the fusion process in accordance with Wolff’s law. Zhang et al.17 conducted a finite element study on instrumented PLIF using one spacer and two spacers. They found that one spacer PLIF resulted in better bone formation than two spacer PLIF under physiological loading.
Screw breakage has been reported in 12.4% of the patients treated with lumbar interbody fusion.18 In our study, TLIF produced greater pedicle screw stress than PLIF except under extension and l-bending. This may be related to the TLIF procedure with complete resection of one side of the articular processes. The TLIF model placed more stress on the pedicle screw, which may explain why the breakage of pedicle screw often occurred in this model. Under flexion motion, we found the largest pedicle screw stress to be induced by TLIF. The results of our study indicated that patients should exercise caution in lumbar flexion movements when treated with TLIF surgery in order to avoid pedicle screw breakage.
Cage subsidence is a complication arising from spinal interbody fusion surgery, which can result in a decrease of normal intervertebral height, and can cause nerve oppression.19 Stress on the L4 lower endplate was higher in PLIF. The largest stress on the endplate was found to be exerted in the PLIF model under flexion loading. This indicated that cage subsidence could easily occur as a result of PLIF under flexion motion.
There are some limitations in our study. First, the material properties used for the finite element analysis of the lumbar spine did not exactly model lumbar tissue. For example spinal ligaments show nonlinear behavior, viscoelasticity of intervertebral disc, and spinal orthotropic characteristics. Second, our study did not simulate spinal muscle contraction, and perhaps it can induce certain errors that affect the final outcome. Third, our study did not consider differences in the mineral density of the lumbar spine between patients, such as patients with osteoporotic spinal bone and other positions and sizes of spacers. Therefore, further precise modelling studies need to be performed in future to obtain accurate conclusions.