Demographic, Surgical, and Radiographic Risk Factors for Symptomatic Adjacent Segment Disease After Lumbar Fusion A Systematic Review and Meta-Analysis Kenney K.L. Lau, BA, MSc, Dino Samartzis, BSc, MSc, DSc, Nicholas S.C. To, BSc, Garrett K. Harada, MD, Howard S. An, MD, and Arnold Y.L. Wong, PT, MPhil, PhD Investigation performed at The Hong Kong Polytechnic University, HongKong SAR, and Rush University Medical Center, Chicago, Illinois Background: Although multiple studies have investigated risk factors for symptomatic adjacent segment disease (ASD) after lumbar fusion, their findings were diverse and inconsistent. This review aimed to summarize risk factors for ASD in order to guide the management of ASD and future research. Methods: Six electronic databases were systematically searched from inception to December 2019. Two reviewers independently screened titles, abstracts, and full-text articles to identify studies investigating risk factors for ASD after lumbar fusion in humans. The methodological quality of the included studies and the strength of evidence regarding risk factors were evaluated. Results: Sixteen studies involving 3,553 patients were included. Meta-analyses revealed that high body mass index, facet joint violation, anterior shift of the preoperative and postoperative lumbosacral sagittal plumb line, decreased preoperative and postoperative lumbar lordosis, preoperative adjacent disc degeneration, decreased preoperative adjacent disc height, increased postoperative lumbopelvic mismatch, postoperative pelvic incidence, and postoperative pelvic tilt were significantly related to ASD. Conclusions: This meta-analysis addressed the limitations of prior reviews and summarized evidence with regard to risk factors for ASD following lumbar fusion. Future prospective studies should investigate whether modification of these risk factors can reduce the ASD development. Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence. L umbar fusion is one of the most common surgical pro- cedures for lumbar degenerative diseases, involving sta- bilization of spinal segments using bone graft after decompression of the spinal canal or foramina 1 . In the United States, the annual number of lumbar fusions increased by 262% from 1998 to 2015 2,3 , and the incidence of lumbar fusions for lumbar disc degeneration also increased by 136% from 7.5 per 100,000 procedures in 2000 to 18.1 per 100,000 procedures in 2009 4 . Given the growing number of lumbar fusions, the relevant medical cost surged by 177% from $3.7 billion in 2004 to $10.2 billion in 2015 3 . Importantly, up to 20% of these cases needed reoperation within 4 years 5 , placing heavy burdens on patients and the medical system. Symptomatic adjacent segment disease (ASD) is a late complication of lumbar fusion that occurs adjacent to previ- ously fused segments and is characterized by radiographic changes and associated symptoms 6 . Although the pathophysi- ology of disc degeneration is multifactorial 7 , lumbar fusion may undoubtedly accelerate the process of adjacent segment degen- eration 8 . Meta-regression analyses revealed that the pooled annual incidence rates were 6% for adjacent segment degeneration and 2% for ASD 9 . Because spinal decompression usually involves the removal of structures that may destabilize the spine 10 , lumbar fusion is used to stabilize the decompressed segments at the expense of increased stress 11 , shearing force 12 , and mobility at intervertebral discs 13 or segments adjacent to the fused construct 14 . Disclosure: This work was supported by an Early Career Scheme (251018/17M) grant by the University Grants Committee. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article ( http://links.lww.com/JBJS/G578). 1438 COPYRIGHT Ó 2021 BY THE J OURNAL OF BONE AND J OINT SURGERY,I NCORPORATED J Bone Joint Surg Am. 2021;103:1438-50 d http://dx.doi.org/10.2106/JBJS.20.00408