Evaluation in Strength of Spondylolisthesis Reduction Systems for Grade-I Slip

Presented at SMISS Annual Forum 2014
By Mir Hussain MS
With Brandon Bucklen PhD, Jonathan Harris MS, John Hao , Patrick Hitchon MD, Manasa Gudipally MS,

Disclosures: Mir Hussain MS E; Globus Medical. Brandon Bucklen PhD E; Globus Medical Inc., Jonathan Harris MS E; Globus Medical, Inc., John Hao E; Globus Medical, Inc., Patrick Hitchon MD A; DePuy Spine, Medtronic, Manasa Gudipally MS E; Globus Medical Inc.,

Introduction:
Low grade spondylolisthesis, or anterior slip, is typically treated conservatively, such as with braces and physical therapy. When surgical intervention is needed, reduction tools imparted with pedicle screw systems are employed. Realignment may be accomplished through anterior an approach, but has not been explicitly pursued. Indirect decompression in the treatment of degenerative conditions does impart some reduction. A novel anterior spondylolisthesis reduction system (ASR), utilizing a translational sliding mechanism, which is fully integrated with a spacer/plate, has been developed.

Aims/Objectives:
To quantify the anterior reduction of a novel integrated spacer with built-in reduction, as compared to pedicle screws with and without an anterior spacer at L5-S1.

Methods:
A grade I slip was created for each of six cadaveric specimens by simulating degeneration through a partial nucleotomy and S1 facetectomy. Specimens then underwent cyclic loading achieve a permanent anterior slip of 25%. Reduction was then performed with 50lbs, 70lbs, 90lbs, 105lbs, 130lbs, and 157.5lbs of axial load. The construct order was 1) Pedicle screw-based reduction (PSR); 2) Pedicle screw-based reduction with anterior spacer (PSRW); and 3) ASR. Testing was stopped if the mechanism allowed an incomplete reduction at the prescribed weight, denoted as final slip after reduction more than 12.5%. The final failure load was recorded as the ultimate load. Using fluoroscopic images, percentage reduction of the original anterior slip was calculated.A one-way ANOVA with repeated measures and Tukey’s post hoc analysis was performed in order to assess differences in axial loads, at which spondylisthesis reduction failed to occur between constructs (significance defined by p≤0.05) Results: The ASR reduced spondylolisthesis at significantly greater loads (average 135lbs) than PSR and PSRW (average 92.5lbs and 97.5lbs, respectively). The ASR construct also was able to achieve the maximum load without defined failure 4/6 specimens, while both PSR and PSRW reached a maximum of 105lbs. Pedicle screw systems were slightly more consistent at 50lbs, maintaining 100% reduction; however at higher loads, ASR had a higher probability of complete reduction.

Conclusions:
While the anterior spacer reduction system requires supplemental fixation, it is significantly more effective at biomechanically reducing grade I spondylolisthesis when compared to pedicle screws. Furthermore, this was achievable at L5-S1, without having to fuse two or more levels.