General Session: Biomechanics
Presented by: M. Gornet - View Audio/Video Presentation (Members Only)
M. Gornet(1), J.A. Hipp(2), N.D. Wharton(2), F.W. Schranck(3), A.G. Copay(3), K. Wymore(4)
(1) The Orthopedic Center of St. Louis, St Louis, MO, United States
(2) Medical Metrics, Houston, TX, United States
(3) SPIRITT Research, St Louis, MO, United States
(4) University of Missouri, School of Medicine, Columbia, MO, United States
Introduction: “Instability” has been an exclusion criterion in every lumbar disc arthroplasty (LDA) IDE study regulated by the FDA. Although intuitive, the justification for this criterion has not been clearly described or based on scientific evidence, since the required evidence base was (and still is) lacking. The purpose of the current study was to determine, using a previously-validated stability metric, whether preoperative instability is associated with clinical outcomes following LDA.
Methods: Intervertebral motion was measured from flexion-extension radiographs for 115 single-level LDA patients, using validated, computer-assisted methods (QMA®, Medical Metrics, Inc). All patients were enrolled at a single site in an FDA-regulated IDE study. The Kellgren-Lawrence (K-L) grade of disc degeneration was recorded from PreOp X-rays. ODI data were prospectively collected following the IDE study protocol and retrospectively evaluated for correlations with assessments of mechanical stability. The mechanical stability of each treatment level was quantified using a previously-validated, normalized metric (QSIFE) based on the ratio of intervertebral translation per degree of rotation. QSIFE is simple to interpret; e.g., a QSIFE = 2 documents that the stability is two standard deviations above the level-specific average for a radiographically normal, asymptomatic population and therefore greater than the 95% confidence interval. QSIFE can only be reliably calculated when there is at least 3 degrees of intervertebral rotation.
Results: QSIFE was measured for 69 of 115 subjects with > 3 degrees of rotation at the treatment level at PreOp. Pre-operatively, the average QSIFE was higher at the treatment level in subjects who did NOT achieve at least a 15-point improvement in the ODI score at 24 months (P=0.07, Figure 1). QSIFE was approximately 0 for K-L grade 0, increased with degeneration to K-L grade 2, and then decreased (P=0.06) as the K-L grade increased to indicate severe degeneration.
Conclusions: The results support that preoperative instability may be a factor in clinical outcomes following lumbar disc arthroplasty. A larger study, where instability was not systematically excluded, would likely be needed to better understand this association, and determine if there is a threshold level of QSIFE that can be used to exclude patients who may not benefit from LDA. An improved flexion/extension protocol is needed to assure that the spine is sufficiently stressed to allow for a reliable assessment of stability/instability in most patients. The results also support the Kirkaldy-Willis hypothesis of initially increasing instability as degeneration progresses, followed by a restabilization phase with advanced degeneration.