Lightning Podiums: Spinal Potpourri - 803B

Presented by: A.S. Vosoughi

Author(s):

A. Seyed Vosoughi(1), A. Kiapour(1), A. Agarwal(1), V. Goel(1), J. Zavatsky(2)

(1) University of Toledo, Bioengineering, Toledo, OH, United States
(2) Spine & Scoliosis Specialists, Tampa, FL, United States

Abstract

Introduction: A common post-operative complication of pedicle subtraction osteotomy (PSO), is rod fracture, ususally occurring at the level of the PSO often requiring revision surgery. The main objective of this project was to identify a threshold disc height that increases rod stresses placing them at higher risk for fracture. Additionally, the use of interbody spacers (IBS) were used to mitigate micromotion, to evaluate their effects on rod failure, using a finite element approach.

Methods: A validated osseoligamentous 3D T10-pelvis spinopelvic model was used to develop a PSO at L3. Four 30° L3 PSO models were analyzed: healthy discs, and 3 degenerated disc (DDD) models. Compared to the normal healthy disc heights, the 3 degenerated disc models included disc height reductions of 20% (mild), 50% (moderate), and 80% (severe), respectively. All models were instrumented from T10-pelvis. The instrumented healthy disc PSO model was also supplemented with PEEK IBS above (L2-L3) and below (L3-L4) the PSO level.

Results: With the increase in the disc degeneration, the L2-L4 global range of motion decreased in flexion, extension, lateral bending, and axial rotation. Increased DDD (decreased heights), decreased the maximum von Mises stresses on the rods at the PSO region in all loading modes. Flexion force resulted in the greatest von Mises stresses on the rods, but these stresses were mitigated as disc heights decreased (339, 321, 267, and 224 MPa, in healthy, mildly, moderately, and severely degenerated discs, respectively). The greatest change (Δ -16.8%) in von Mises stress to the rods occurred with moderately degenerated discs (50% disc height reduction). Adding inter body spacers to the L2-L3 and L3-L4 levels, reduced the flexion, extension, lateral bending, and axial rotation motions by 22%, 21%, 4%, and 11%, respectively. Supplementing the instrumented PSO model with IBS decreased the maximum von Mises stress on the rods. The use of IBS adjacent to the PSO increased the force magnitude acting on the PSO fracture site to 335.4N.

Discussion: Larger discs adjacent to the PSO lead to greater micromotion. Greater mobility adjacent to the PSO can result in a greater von Mises stress on the rods. Increased DDD (decreased disc heights), results in decreased stress values and risk of fracture to the rods. The greatest change (Δ -16.8%) in von Mises stress on the rods occurred with moderate DDD (50% disc height reduction) at L2-L3 and L3-L4, which translates to 4.92mm and 4.99 in this model, respectively. Surgeons may consider performing IBS fixation in disc heights greater than 5mm to increase stability and decrease the von Mises stresses on the rods, thereby reducing the risk of rod fracture. Additionally, greater DDD results in greater load across the osteotomy site, increasing forces required for successful fusion. Hence, adding interbody spacers above and below the L3 PSO can facilitate PSO union and decrease the risk of rod failure.

Location and magnitudes of the maximum stress