<h3>BACKGROUND CONTEXT</h3> Decompressive laminectomy surgery for symptomatic lumbar spinal stenosis is generally predictable and effective in providing relief of radiculopathy and neurogenic claudication. Decompression alone, without fusion, is reserved for patients with preserved lordosis and no evidence of radiographic instability. On occasion, spinal deformity or instability can occur following decompressive surgery and may lead to disabling symptoms and the need for revision surgery that is inclusive of an instrumented spine fusion. <h3>PURPOSE</h3> To evaluate the extent and nature of symptomatic deformity or instability that occurred within one year following decompressive surgery for lumbar spinal stenosis. <h3>STUDY DESIGN/SETTING</h3> A retrospective review of a consecutive series of patients treated at a single institution. <h3>PATIENT SAMPLE</h3> This study included 425 patients who underwent decompressive surgery over a 5-year period (2012-2016) to manage symptomatic lumbar spinal stenosis. Surgery was performed by one of four surgeons at a single academic medical center. All patients had preserved lordosis (no antero, lateral, or retrolisthesis) on preoperative, upright radiographs and there was no evidence of instability (>4mm) on flexion-extension films. All patients underwent open, multilevel laminectomies with lateral recess and foraminal decompression to address neural compression noted on preoperative MRI. The indication of additional surgery within one year of the index surgery were recurrent back and leg pain, and radiographic evidence of new deformity development, or instability within the lumbar spine at one of the previously decompressed levels. New deformity was defined as the presence of antero- or lateral listhesis not previously present and instability was defined as segmental motion of >4mm on flexion-extension films. <h3>OUTCOME MEASURES</h3> Pre- and postoperative VAS scores (back and leg pain), Oswestry Disability Index, and specific queries of patient satisfaction and willingness to repeat the index surgery. <h3>METHODS</h3> Multiple preoperative clinical and radiographic features were evaluated to identify a potential causal relationship with the subsequent postoperative development of clinical symptoms and radiographic deformity or instability which occurred within one year that necessitated a revision surgery. Additionally, the ultimate clinical outcome of the patients who underwent a revision surgery for deformity development were compared to the cohort of patients who had not developed a deformity after undergoing decompressive surgery during the same time period. <h3>RESULTS</h3> A total of 425 patients underwent a multilevel decompressive lumbar laminectomy for spinal stenosis over a 5-year period. Nineteen patients (4%) developed pain and radiographic evidence of a deformity or instability within one year that necessitated an additional surgery in the form of an instrumented fusion. Back pain was present in all 19 patients and radicular leg pain in 16 of 19 patients (84%). Significant factors for the development of instability or a deformity included age >50 (p=.04), body mass index >30 (p=.03), and extensive fluid with multiple facet joints on preoperative MRI (p=.04). Notable factors that approached, but did not achieve significance included 4 level laminectomy surgeries, a history of osteoporosis, and chronic preoperative use of corticosteroids. Notable factors that were not significant included presence of congenital stenosis, type of preoperative employment (physically-demanding or sedentary), use of postoperative bracing, or duration of preoperative symptoms (greater or less than 1 year). When compared with the cohort of patients who did not develop a deformity, the deformity patients had lower VAS (back) and ODI scores at 6 months following their revision surgery, although these differences became non-significant by 1 year. At 5-year follow-up, the two cohorts had similar VAS (back and leg), ODI, and in percentage of patient satisfaction and willingness to repeat the index surgery. <h3>CONCLUSIONS</h3> The development of a deformity or instability within the first postoperative year following decompressive laminectomy surgery for spinal stenosis was found to be uncommon, occurring in 4% of 425 consecutive patients treated a single institution over a 5-year period. Younger, obese patients and those with greater than 3 levels decompressed were more likely to develop instability or deformity, as were patients with diffuse fluid noted within facet joints on preoperative MRI. However, long-term follow-up revealed no ultimate difference in patient-reported outcomes of those who underwent a subsequent spine fusion, including willingness to repeat the original lumbar decompressive surgery. Lumbar decompressive laminectomy surgery is safe and predictable. When performed on patients with preserved lordosis and no pre-existing instability or deformity, the likelihood for development of early, postoperative deformity or instability is low, but if it is to occur, management with an instrumented fusion is safe and effective. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.