Copyright © 2016 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited. Nonfusion Does Not Prevent Adjacent Segment Disease: Dynesys Long-term Outcomes With Minimum Five-year Follow-up Godefroy H. St-Pierre, MD, FRCSC,  Andrew Jack, MD,  M. Mashfiqul A. Siddiqui, MBBS, FRCS (Edin),  Ronald L. Henderson, MD, FRCSC, y and Andrew Nataraj, MD, FRCSC  Study design. Case series. Objective. The aim of this study was to determine the relationship between fusion and adjacent segment disease via Dynesys long-term outcomes. Summary of Background Data. Dynesys is a dynamic stabilization system meant to improve symptoms by stabilizing the spine without fusion and avoiding the development of adjacent segment disease. However, few studies have evaluated long-term outcomes. Methods. All patients were operated on with Dynesys from 2006 to 2009 by a single surgeon at a single institution. We prospectively collected 18 variables among the following categories: patient characteristics, comorbidities, surgical indica- tions, and OR variables. We analyzed two primary endpoints: solid fusion on X-ray and clinical adjacent segment disease (ASD) both at 5 years. Secondary endpoints were time to fusion, time to ASD, reoperation, Oswestry disability index (ODI), and visual analogue scale (VAS) leg pain. We conducted a multi- variate analysis via the random forest method. Mann-Whitney U test and Fisher exact test were then used to qualify relationship between variables. Results. We had 52 patients to review in the database. Eight had preexisting ASD. Mean follow-up was 92 months (median 87 months). Fifteen had ASD (29%) during follow-up at a mean 45 months (Median 35 months). Nine had a solid fusion (17%), 2 of which also had ASD. Mean time to fusion was 65 months (median 71 months). Differences in improvement of ODI (P ¼ 0.005) and VAS leg pain (P ¼ 0.002) were significant favoring patients without ASD. The multivariate analysis revealed four variables associated with ASD: prior ASD (OR 11.3, P ¼ 0.005), neurological deficit (OR 8.5, P ¼ 0.018), revision OR (OR 8.5, P ¼ 0.018), and multilevel degeneration (OR 0.184, P ¼ 0.026). No variable was associated with fusion. Conclusion. Dynesys was associated with a high rate of ASD over long-term follow-up despite maintaining a low fusion rate. Prior ASD was the strongest predictor of progressive ASD. Key words: clinical adjacent segment disease, dynamic stabilization, dynesys, long term outcomes, motion preservation, non fusion, progression of adjacent segment disease, random forest algorithm, re-operation, retrospective cohort study Level of Evidence: 3 Spine 2016;41:265–273 I n recent years, increasing concerns have arisen with the durability of fusion procedures. Initially described in the 1950s, 1,2 adjacent segment disease (ASD) was then felt to be a relatively uncommon phenomenon. However, in recent years, both the increasing number of fusion procedures being completed as well as increased awareness of ASD have led to its occurrence as a relatively common complication of spinal arthrodesis. 3 Reported incidence for ASD varies substantially depending on the definition used, ranging from 5% to 100%. 4 More specifically, clinical ASD describes symptomatic disease usually due to spinal stenosis at the adjacent segments potentially leading to re-operation. 5–7 The incidence of clinical ASD is lower, reported to range from 5.2% to 16.5% at 5 years and 10.6% to 36.1% at 10 years. 4,9–13 This entity is believed to be secondary to excessive motion at the adjacent segments, 14,15 although its pathophysiol- ogy remains controversial. 16 Another leading theory links it with a patient‘s propensity to develop degenerative spine disease. 14 From the  Division of Neurosurgery, University of Alberta; and y Meadowlark Health Centre, University of Alberta, Edmonton, Alberta, Canada. Acknowledgment date: April 30, 2015. First revision date: July 9, 2015. Second revision date: August 4, 2015. Acceptance date: August 15, 2015. The legal regulatory status of the device(s)/drug(s) that is/are the subject of this manuscript is not applicable in my country. No funds were received in support of this work. No relevant financial activities outside the submitted work. Address correspondence and reprint requests to Godefroy H. St-Pierre, MD, FRCSC, 417 – 636 McAllister loop, Edmonton, Alberta, Canada T6W 1N4; E-mail: guilderk@gmail.com DOI: 10.1097/BRS.0000000000001158 Spine www.spinejournal.com 265 SPINE Volume 41, Number 3, pp 265–273 ß 2016 Wolters Kluwer Health, Inc. All rights reserved. SURGERY