Friday, September 28, 2018 4:05 PM–5:05 PM abstracts: basic science of spinal fusion: 238. The effects of varenicline (Chantix) on lumbar spinal fusion in a rat model

BACKGROUND CONTEXT Smoking is detrimental to obtaining a solid spinal fusion mass with previous studies demonstrating that tobacco use is associated with pseudoarthrosis in patients undergoing spinal fusions. Additionally, smoking is an important predictor of increased dissatisfaction, decreased walking ability, increased pain medication use, and poorer quality of life scores in patients after spine surgery when compared to nonsmokers. Accordingly, physicians make significant efforts toward helping patients discontinue their use of nicotine products. Varenicline is a nonscheduled prescription medication that works as a partial agonist of nicotinic acetylcholine receptors, the same receptors activated during smoking and tobacco use, and is used as a pharmacologic adjunct to aid patients in smoking cessation. However, no clinical or basic science studies to date have characterized if varenicline has the same negative effects as nicotine on spinal fusion, consolidation, and bone healing. PURPOSE As a partial agonist of nicotine receptors, our study aimed to elucidate whether varenicline affects the rate or quality of spinal fusion. STUDY DESIGN/SETTING Manual, radiographic, and biomechanical testing. PATIENT SAMPLE A total of 14 Lewis rats undergoing L4-5 lumbar spinal fusion. OUTCOME MEASURES Gross motion, radiographic density, peak load, and stiffness of fusion mass METHODS According to our IACUC approved protocol, fourteen male Lewis rats underwent tail amputation to create an autogenous bone graft from the tailbone followed by a lumbar L4-5 posterior spinal fusion procedure. Postsurgery, rats were randomly separated into two experimental groups receiving either daily subcutaneous injections of human dose (1mg/kg daily in saline) varenicline or saline (placebo) for 12 weeks postsurgery. Spine samples were then explanted and fusion was determined via manual palpation of segments by two independent blinded observers. Radiographic analysis of fusion was performed via high-resolution radiographs evaluating bridging fusion mass. Rat lumbar spinal fusion units were then tested in 4-point bending to evaluate stiffness and peak load before failure. Comparisons were made between groups (significance at p RESULTS At three months postsurgery, twelve out of fourteen rats demonstrated lumbar spine fusion (86% fused) with no difference in fusion rate between the varenicline and control groups (6/7, 86%). Fusion rate as detected by manual palpation indicated no motion in the desired fusion segment. Findings were further supported with faxitron radiography demonstrating six out of seven rats (86%) having complete fusion consisting of contiguous transverse processes bilaterally at a single level in each of the varenicline and placebo control groups. Of the six successfully fused specimens, five from each group underwent successful biomechanical testing. The average stiffness at the fusion site of the varenicline group and control group was 49.2±17.5N/mm and 69.1±32.6N/mm, respectively (p=.271). The average peak load at the fusion site of the varenicline group and control group was 51.9±25.2N and 60.5±20.4N, respectively (p=.574). CONCLUSIONS Varenicline has no significant effect on the rate and quality of spinal fusions and remains an appropriate pharmaceutical adjunct for tobacco cessation in individuals undergoing spinal fusion.