BACKGROUND CONTEXT: Manual contouring of spinal rods is often required intraoperatively
for proper alignment of the rods within the pedicle screw heads. Residual misalignments are frequently reduced by using dedicated reduction devices. The forces exerted by these devices, however, are uncontrolled and may lead to excessive reaction forces. As a consequence, screw pullout
might be provoked and surrounding tissue may experience unfavorable biomechanical loads. The
corresponding loads and induced ...
BACKGROUND CONTEXT: Manual contouring of spinal rods is often required intraoperatively
for proper alignment of the rods within the pedicle screw heads. Residual misalignments are frequently reduced by using dedicated reduction devices. The forces exerted by these devices, however, are uncontrolled and may lead to excessive reaction forces. As a consequence, screw pullout
might be provoked and surrounding tissue may experience unfavorable biomechanical loads. The
corresponding loads and induced tissue deformations are however not well identified. Additionally,
whether the forced reduction alters the biomechanical behavior of the lumbar spine during physiological movements postoperatively, remains unexplored.
PURPOSE: To predict whether the reduction of misaligned posterior instrumentation might result
in clinical complications directly after reduction and during a subsequent physiological flexion
movement.
STUDY DESIGN: Finite element analysis.
METHODS: A patient-specific, total lumbar (L1−S1) spine finite element model was available
from previous research. The model consists of poro-elastic intervertebral discs with Pfirrmann
grade-dependent material parameters, with linear elastic bone tissue with stiffness values related to
the local bone density, and with the seven major ligaments per spinal motion segment described as
nonlinear materials. Titanium instrumentation was implemented in this model to simulate a L4, L5,
and S1 posterolateral fusion. Next, coronal and sagittal misalignments of 6 mm each were introduced between the rod and the screw head at L4. These misalignments were computationally
reduced and a physiological flexion movement of 15˚ was prescribed. Non-instrumented and wellaligned instrumented models were used as control groups.
RESULTS: Pulling forces up to 1.0 kN were required to correct the induced misalignments of 6
mm. These forces affected the posture of the total lumbar spine, as motion segments were predicted
to rotate up to 3 degrees and rotations propagated proximally to and even affect the L1−2 level.
The facet contact pressures in the corrected misaligned models were asymmetrical suggesting nonphysiological joint loading in the misaligned models. In addition, the discs and vertebrae experienced abnormally high forces as a result of the correction procedure. These effects were more pronounced after a 15˚ flexion movement following forced reduction.
CONCLUSIONS: The results of this study indicate that the correction of misaligned posterior
instrumentation can result in high forces at the screws consistent with those reported to cause screw
pullout, and may cause high-tissue strains in adjacent and downstream spinal segments.
CLINICAL SIGNIFICANCE: Proper alignment of spinal posterior instrumentation may reduce
clinical complications secondary to unfavorable biomechanics
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