Finite Element Analysis On Different Posterior Stabilization Construct For Mini-TLIF Of Lumbar Spine

Objective:A biomechanical study using a finite element model of L3-4comparing stress distribution and stability of TLIF constructs augmented by:(1) unilateral pedicle screw fixation,(2) unilateral pedicle screw fixation supplemented with contralateral facet screw construct, or (3) bilateral pedicle screw fixation.Methods:A finite element model of L3-4segment was generated, and then UPS+TLIF, UPS+CFS+TLIF and BPS+TLIF finite element models were further established. Forces were applied to simulate status in standing, flexion, extension, axial rotation, and lateral bending. The stress distribution and range of motion of different posterior stabilization models for TLIF were recorded and analyzed.Results:1. The maximum principal stresses of posterior structure of vertebrae were located on the pars interarticularis, which reached the peak value in extension status. The higher stresses were found at the ventral aspect than at the dorsal aspect in axial compression loading mode.2. The stress distribution of posterior structure of vertebrae is most symmetrical in BPS+TLIF model. High von Mises stresses were concentrated at left side (contralateral facet screw side) for posterior element of UPS+CFS+TLIF model, and high von Mises stresses were concentrated at right side (unilateral pedicle screw side)for posterior element of UPS+TLIF model.3. The stress distribution is very asymmetrical in UPS+TLIF model, which was concentrated at right side (unilateral pedicle screw side). It may result in off-axis movement.4. When compared the peak von Mises stresses of posterior stabilization construct, the sequence was UPS+TLIF>BPS+TLIF>UPS+CFS+TLIF. The peak von Mises stresses of posterior stabilization construct of UPS+TLIF model was far beyond the yield strength value of titanium in extension and left bending mode.5. The stresses distribution of interbody cage in BPS+TLIF model were asymmetrically, which is higher at right side (facetomy side) than left side.6. When compared the range of motion of different posterior stabilization construct plus TLIF, the unilateral pedicle screw construct alone was significantly less stiff than either bilateral pedicle screws or unilateral pedicle screws with a supplemental facet screw, especially in extension and left bending. The fixation following TLIF of unilateral pedicle screws with a supplemental contralateral facet screw showed similar stiffness with the standard bilateral pedicle screw construct.Conclusions1. Lumbar spondylolysis is a stress fracture of the pars interarticularis result from mechanical stresses in the neural arch due to repetitive overload during daily activities. According to the finite-element analysis, incomplete stress fractures of the pars should be noted which is most occurred in ventral aspect.2. The UPS+TLIF model had asymmetrical stress distribution with off-axis movement and the peak von Mises stresses of posterior stabilization construct was far beyond the yield strength value of titanium in extension and left bending model. It may result higher hard construct failure. The facet screw in UPS+CFS+TLIF model significantly decreased the peak von Mises stresses of right pedicle screw and rod. It can protect right hard construct and the peak von Mises stresses of itself was not exceed the yield strength value of titanium.3. The stresses would shift to posterior side after posterior stabilization with TLIF. Different posterior stabilization construct will result different stresses distribution and concentration. The stresses of posterior structure of vertebrae in UPS+TLIF model was concentrated at right side and UPS+CFS+TLIF model was concentrated at left side (contralateral facet screw side), it may result different adjacent segment degenerative changes in future.4. The stresses distribution of interbody cage in BPS+TLIF model were asymmetrically, which is higher at right side (facetomy side) than left side. It indicated that the preserved facet joint contributed to load sharing even with posterior bilateral pedicle screw fixation. Thus, we should try to avoid facetomy if possible.5. When compared of range of motion of different posterior stabilization construct plus TLIF, the unilateral pedicle screw construct alone was significantly less stiff than either bilateral pedicle screws or unilateral pedicle screws with a supplemental facet screw, especially in extension and left bending. The UPS+CFS+TLIF model showed similar stiffness with the standard and BPS+TLIF model.