Correlation of Translational Motion and the Ratio of “Translation Per Degree of Rotation” to Clinical and Mechanical Outcomes for Decompression with Minimally Invasive Interlaminar Stabilization vs. Posterolateral Fusion at Three Year Post-Op

Presented at SMISS Annual Forum 2014
By Hyun Bae MD
With Reginald Davis MD, Thomas Errico MD, Carl Lauryssen MD, Scott Leary MD,

Disclosures: Hyun Bae MD Reginald Davis MD None, Thomas Errico MD None, Carl Lauryssen MD None, Scott Leary MD None,

Introduction:
Posterolateral fusion is often used to treat moderate to severe lumbar spinal stenosis (LSS), particularly when instability is suspected. Surgical decompression with implantation of a minimally invasive interlaminar stabilization (ILS) device is also indicated for treatment of LSS, with up to Grade 1 spondylolisthesis. Translational motion of >3mm is known to impact LSS symptoms (Iguchi 2004, Boden 1990) though the effect of ILS on stabilizing intervertebral motion is unknown. Translation and the ratio of translation per degree of rotation (TPDR) provide objective measures of mechanical stability, which are also relatable to clinical status.

Aims/Objectives:
Evaluate data from an FDA IDE trial to determine if ILS provides mechanical stability and clinical outcomes similar to fusion for treatment of LSS, while also preserving motion. Methods: 215 ILS and 107 fusion patients were evaluated. A core lab identified a “Subset” of 21 ILS and 23 fusion patients who had higher preoperative translational motion (mean 2.0mm ILS and 1.6mm fusion) compared to the remaining 194 ILS and 84 fusion “Average” patients (translation: 0.98mm and 0.95mm). The preoperative TPDR ratio in the ILS “Subset” was 1.099 and in fusion was 0.956; in the “Average” cohort the TPDR ratio was 0.418 ILS to 0.598 fusion. Comparisons were made using 3-year Composite Clinical Success (CCS) endpoints that required absence of reoperation or steroid injection and clinically meaningful improvements on either Oswestry Disability Index (ODI), VAS or Zurich Claudication questionnaire (ZCQ).

Results:
Clinical outcomes show that in ILS 81% of the “Subset” cohort and 64% of the “Average” cohort achieved CCS constructed with an ODI component requiring =>15 point improvement (p=0.12); for fusion 70% of the “Subset” vs 50% of the “Average” (p=0.10) patients achieved this CCS. For a success rate using minimum VAS-Leg pain improvement of =>20mm, in ILS 86% “Subset” vs 57% “Average” patients achieved this CCS (p=0.01); in fusion the rates were 59% vs 61% (p=0.85); interaction p=0.01. For ILS, reoperation rates were 0% in “Subset” vs 15.5% in “Average” (p=0.05); for fusion, 17.4% vs 9.5% (p=0.28).

Conclusions:
3-year outcomes in patients with translational motion at the higher end of the normal range indicate ILS provides mechanical stability after decompression for LSS with sustainable clinical outcomes that are similar or better than fusion. This study demonstrates treating an unstable vertebral segment via decompression and implantation of a minimally invasive ILS implant portends better outcomes than fusing the segment, even in the presence of instability.