The Spine Journal 21 (2021) 1626−1634 Narrative Review Focus: Artificial Intelligence and Machine Learning Artificial intelligence for adult spinal deformity: current state and future directions Rushikesh S. Joshi, BSa,*, Darryl Lau, MDb, Christopher P. Ames, MDc a Department of Neurological Surgery, University of California San Diego, La Jolla, CA, USA b Department of Neurosurgery, New York University, New York, NY, USA c Department of Neurological Surgery, University of California San Francisco, San Francisco, CA, USA Received 17 February 2021; revised 7 April 2021; accepted 27 April 2021 Abstract As we experience a technological revolution unlike any other time in history, spinal surgery as a discipline is poised to undergo a dramatic transformation. As enormous amounts of data become digitized and more readily available, medical professionals approach a critical juncture with respect to how advanced computational techniques may be incorporated into clinical practices. Within neurosurgery, spinal disorders in particular, represent a complex and heterogeneous disease entity that can vary dramatically in its clinical presentation and how it may impact patients’ lives. The spectrum of pathologies is extremely diverse, including many different etiologies such as trauma, oncology, spinal deformity, infection, inflammatory conditions, and degenerative disease among others. The decision to perform spine surgery, especially complex spine surgery, involves several nuances due to the interplay of biomechanical forces, bony composition, neurologic deficits, and the patient’s desired goals. Adult spinal deformity as an example is one of the most complex, given its involvement of not only the spine, but rather the entirety of the skeleton in order to appreciate radiographic completeness. With the vast array of variables contributing to spinal disorders, treatment algorithms can vary significantly, and it is very difficult for surgeons to predict how patients will respond to surgery. As such, it will become imperative for spine surgeons to utilize the burgeoning availability of advanced computational tools to process unprecedented amounts of data and provide novel insights into spinal disease. These tools range from predictive models built using machine learning algorithms, to deep learning methods for imaging analysis, to natural language processing that can mine text from electronic medical records or transcribed patient visits − all to better treat the intricacies of spinal disorders. The adoption of such techniques will empower patients and propel spine surgeons into the era of personalized medicine, by allowing clinical plans to be tailored to address individual patients’ needs. This paper, which exists in the context of a larger body of literatutre, provides a comprehensive review of the current state and future of artificial intelligence and machine learning with a particular emphasis on Adult spinal deformity surgery. © 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Adult spinal deformity; Artificial intelligence; Machine learning; Predictive analytics; Predictive models; Spine FDA device/drug status: Not applicable Author disclosures: RSJ: Nothing to disclose. DL: Nothing to disclose. CPA: Royalties: Stryker (F), Biomet Zimmer Spine (C), DePuy Synthes (F), Nuvasive (B), Next Orthosurgical (F), K2M (None), Medicrea (B); Consulting: DePuy Synthes (B), Medtronic (B), Medicrea (B), K2M (B); Research Support (Investigator Salary, Staff/Materials)^: Titan Spine (E), DePuy Synthes (None), ISSG (C); Grants: SRS. *Corresponding author. Department of Neurological Surgery, University of California, San Diego, 9300 Campus Point Dr, MC-7893, La Jolla, CA 92037, USA. Tel.: (408) 507-0993. E-mail address: Rushi.Joshi.MS4@gmail.com (R.S. Joshi). https://doi.org/10.1016/j.spinee.2021.04.019 1529-9430/© 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 Introduction to artificial intelligence: Applications for spine surgery Spine surgery as a specialty is rapidly approaching a critical juncture, as medicine embraces a new era of precision medicine driven largely by unprecedented amounts of available data and advanced computational techniques. Technology and advancements within medicine are continuing to develop at a rapid rate as data becomes increasingly more digitized. Combining the massive amounts of available data with powerful new computational methods, we now have the ability to harness the power of artificial intelligence (AI). At its core, AI seeks to replicate the experience of human intelligence, or natural intelligence in computers. While the broader goal of a generalized and automated intelligence remains beyond our scope, we can now use the tools of AI to develop systems that recreate the characteristics of human intelligence − to learn from immense datasets, make decisions, provide recommendations, and adapt to new data/circumstances. Through the use of sophisticated concepts such as artificial neural networks and robust machine learning methods we can now develop systems that can dynamically learn from data and use that to inform future behavior and decision making. AI represents a vast technological goal encapsulating numerous disciplines such as natural language processing, computer vision, and robotics amongst many others. However, machine learning exists as a branch of AI that utilizes computer algorithms to learn from data and prior experiences to build intelligent models. Machine learning algorithms allow the computer to extract patterns inherent within datasets without user-defined or pre-determined rules, to learn relationships from the data and then make specific predictions or determinations. As a discipline, spine surgery offers a unique niche to take advantage of the computational abilities of AI and machine learning [1]. Over the past several decades, the collective knowledge of spine disorders has increased immensely. In addition to the heterogeneity in clinical presentation, spine disorders also span various different etiologies, which can include oncology, trauma, degenerative disease, and spine deformity amongst others. With adult spinal deformity (ASD) in particular, our knowledge has increased significantly as techniques for surgical correction of ASD have become more refined and widely adopted. ASD represents an especially intriguing prospect for machine learning methods due to its nature as one of the most complex medical problems as it involves not only evaluation of the entire spinal column, but also the entirety of the skeleton for appropriate radiographic completeness [2,3]. We now know that patients can present with drastically different symptoms that cause significant disability and pain such that extent of deformity correlates with severity of symptoms [4−7]. While the literature has overall shown that surgical correction of radiographic spinopelvic measurements can significantly improve patients quality of life as measured by health-related quality of life (HRQOL) 1627 metrics, there still remains a component of unpredictability in how individual patients may respond to surgery [8−18]. Additionally, surgical correction often is invasive and requires osteotomies to achieve desired correction, which have been associated with a relatively high risk of major complications [11,19−25]. The ability to better predict postoperative outcomes for patients based on their independent profiles could greatly enhance our capability to tailor treatment plans based on individual patients’ needs and goals. Other examples showcasing the potential for advanced analytics to assist spine surgeons is with degenerative disc disease causing spinal cord and nerve compression − an extremely common clinical scenario. While the indications for surgery of intractable pain, weakness, and loss of motor coordination and strength remain familiar, the ability to predict with a high degree of certainty which patients will improve symptomatically and which ones won’t, and over how much time continues to elude us [26]. Many surgeons have experienced firsthand patients whose symptoms don’t improve despite adequate decompression, and while we can postulate that inability to regain function may be due to duration or degree of compression prior to surgery, this is impossible to monitor as patients are not serially imaged. Thus, the potential for AI tools to identify novel insights or help us determine different criteria for surgery or prognostic factors is immense. While these are only examples of some situations where the incorporation of AI can address a clear need, the opportunities are limitless in what questions may be answered beyond the scope of our current knowledge base and human limitations. Spine surgeons have historically relied on clinical judgment and experience, in addition to retrospective cohort studies based on linear and logistic regression to educate patients and inform their decision making. While these statistical methods are important for identifying associations between variables and outcomes, they represent generalizations and averages across large populations, and thus hold little value or granularity for specific cases. Additionally, regression models are far more effective at determining associations than they are at predicting future outcomes. In this review, we briefly recap some fundamental principles of machine learning algorithms and illustrate how AI can be utilized for spinal disorders, by highlighting existing studies that have begun to employ more powerful machine learning algorithms for predictive models with a particular emphasis on applications for ASD management. Machine learning principles: Strengths and limitations In order to properly implement machine learning for data analysis, it is critical that surgeons understand the methods and fundamental concepts inherent to these techniques. Proper usage will be imperative for ensuring that models are appropriately and rigorously developed for clinical deployment and to contribute to the existing literature. 1628 R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 One of the core tenets that distinguishes the field of machine learning from other concepts is the idea of having an algorithm repeatedly “trained” on existing data and allowing the mathematical model to learn the relationships between variables in the dataset. With hypothesis-generated statistical studies, the onus lies on the user to determine which variables should be included as dependent and independent variables, limiting our ability to explore relationships within the data that may be non-intuitive. The idea behind machine learning is that by removing the necessity of hard-coding algorithmic rules or manually deciding on relevant variables, instead the algorithm will construct a model by learning the relationships inherent within the data, eliminating the user bias of simpler statistical methods. This method allows all aspects of the input data to be investigated, without ignoring information or variables that may turn out to be important. Within machine learning, there are three primary paradigms; these are supervised learning, unsupervised learning, and reinforcement learning. In supervised learning, the data has specified “labels” or outputs that have been previously designated. As such, the algorithm will learn a function that maps input variables to specific outputs based on input-output examples, so that the model can then be deployed on novel data using the inferred function. Supervised learning problems generally fall under the categories of classification or regression problems, and utilize several different algorithms including support vector machines, artificial neural networks, decision trees, and random forest models among several others. The majority of studies discussed in this review incorporate supervised learning methods to develop models for predicting various outcomes following surgical correction of ASD. In unsupervised learning, the algorithm is free to learn the intrinsic patterns within the data, as there are no labeled inputs or outputs. These methods seek to identify the natural structure of the data without any user-dictated labels. One such example of unsupervised learning is hierarchical clustering analysis, which was used by Ames et al. to identify a novel classification scheme for ASD patients, and also by Kim et al. for classification of cervical deformity patients [27,28]. Reinforcement learning is the third concept, with slightly different applications. In reinforcement learning, the machine learning model is trained in an environment to make a series of decisions based on rewards and penalties, as it seeks to maximize the total reward for the situation. Examples of reinforcement learning include developing automated systems to play games like Chess or Go, and also autonomous vehicles. The advantage of machine learning over earlier statistical methods and hypothesis-generated studies as mentioned earlier lies in its function of being able to process and analyze large amounts of heterogeneous variables to make highly accurate and reproducible predictions. While statistical models are essential for their ability to determine causative relationships and associations between variables, they lack the same predictive capabilities as machine learning models. The tradeoff, however, is that while more powerful, machine learning models can also be more difficult to interpret, and thus have a higher barrier of entry for widespread use. Additionally, machine learning generally requires large amounts of high-quality data, and models tend to perform better the more data is available for training as sparsely populated data can lead to overfitting − statistical models still retain their associative utility with relatively small amounts of data. Techniques such as ensemble learning, which is when predictions from several different models are combined either by “bagging” (train multiple complex models in parallel and average their responses for final model) or “boosting” (train simpler models in sequence to build on each other and create a final model) help mitigate some of these issues. Despite the potential pitfalls, the immense power of machine learning poses a unique opportunity for spine surgeons to capitalize on these emerging technologies to better serve the patient population. Applications for ASD surgery Early ASD predictive models ASD represents a unique niche within spine surgery that is well poised to take advantage of the immense potential offered by machine learning and AI. Given how heterogeneous the clinical presentation for ASD can be and the vast array of surgical techniques available in the surgeon’s armamentarium, the treatment algorithm can be complicated with many different possibilities. In addition, due to the high complication rates and invasiveness of surgical techniques, ASD surgery can benefit from predictive models that may offer patients additional granularity and risk stratification catered specifically towards their individual health profiles. By learning from historical data, predictive models can then be deployed prospectively to account for an individual’s specific profile and empower patients in a shared decision-making process to tailor treatment plans according to their own goals and needs. Recently, the International Spine Study Group (ISSG) and European Spine Study Group (ESSG) have been at the forefront of developing predictive models for ASD patients, using their prospectively collected, multi-institutional database. Early efforts in predictive modeling for ASD patients utilized supervised learning where the data used to build the models has a specified target outcome or variable labeled, which machine learning algorithms then attempt to predict by learning a function based on input-output examples in the data. The input variables in these models can be diverse, including patient information such as radiographic parameters, demographic information, comorbidities, complex metrics such as frailty index and Charlson Comorbidity index, surgical characteristics, intraoperative information, and HRQOL scores. R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 Most predictive analytics in ASD pertain to the assessment of postoperative outcomes, however, some earlier studies explored the feasibility of developing predictive models for perioperative outcomes. Durand et al. used decision tree and random forest models in a cohort of over 1,000 ASD patients to predict intra- and postoperative blood transfusion requirements with AUCs of 0.79 and 0.85, respectively [29]. Influential variables for predicting transfusion requirements included operative duration, invasiveness, hematocrit, and patient weight and age. Looking at additional perioperative outcomes, Safaee et al. and Scheer et al. similarly built models to predict length of stay (LOS) and early complication risk in ASD patients [30,31]. Safaee et al. used generalized linear regression models with bootstrapping in a cohort of 653 patients and were able to estimate LOS within 2 days with predictive accuracy of 75.4%, with major predictors being staged surgery, C7 sagittal vertical axis (SVA) and number of levels fused [30]. Scheer et al. utilized an ensemble of decision trees and bootstrapping methods to predict major complications at 6 weeks postoperatively with an AUC of 0.89. Being able to predict early complication risk or LOS can help inform discharge plans and workflow, as well as identify high-risk surgical candidates at the preoperative stage [31]. The ability to predict postoperative outcomes following ASD surgery was further explored in several subsequent studies examining proximal junctional failure (PJF)/clinically relevant proximal junctional kyphosis (PJK) [32], pseudarthrosis [33], risk stratification for major complications and reoperation [34], and cervical malalignment following thoracolumbar surgery [35]. Building on the success of prior deployment of predictive models, Scheer et al. were among the first to implement a machine learningbased model to predict rates of PJF and clinically significant PJK in a cohort of 510 ASD patients [32]. Using similar methods of decision trees and bootstrapping, their final model yielded 86% accuracy with AUC of 0.89, and demonstrated the top predictors to be age, lower instrumented vertebrae (LIV) and preoperative SVA. This same analytical method was then employed for pseudarthrosis as well with equally successful results of 91% accuracy and AUC of 0.94 [33]. The model for pseudarthrosis assessed a total of 82 input variables, ultimately identifying the top contributing variables to be LIV, use of bone morphogenic protein and max coronal cobb angle, in contrast to those identified as predictors for PJK/PJF − this variance in influential input variables highlights a key advantage of machine learning to identify relationships between variables in the data that may not be readily apparent. Passias et al. in a more unique approach sought to predict cervical malalignment following thoracolumbar ASD surgery, and developed a model with AUC of 0.89 demonstrating C2-T3 cobb angle at baseline and increased number of Smith-Peterson osteotomies to be highly predictive of poor cervical compensation [35]. The most rigorous work to date assessing postoperative risk of major complications, reoperation and hospital 1629 readmission was conducted by Pellise et al. and the ISSG/ ESSG [34]. In their study, random forest models incorporating over 100 input features from a cohort of 1,612 ASD patients were built to assess risk for all three major outcomes of major complication, reoperation and hospital readmission with AUC ranging from 0.67 to 0.92. The novelty of this study lies in its development of two predictive models for each outcome: one consisting of only preoperative variables, and another including perioperative information as well. Significant predictors of hospital readmission included pelvic tilt (PT), LIV, age, and ODI walking response, while top predictors of reoperation were walking ability and site/surgeon, which accounted for much greater predictive ability for reoperation than for either readmission or major complication. The top predictors for risk of major complication in this model were LIV (with pelvic extension), age, walking ability, and sagittal imbalance radiographic parameters (most prominently SVA). Given the relatively high rates of complications in ASD surgery, having robust and accurate models to predict individual patients’ risk profiles based on their specific parameters is crucial for promoting more informed and collaborative decision-making that can take into account the patient’s own goals. To help incorporate this risk stratification model into clinical practice, the ISSG/ESSG have developed a calculator that is currently in alpha testing, to make such tools more widely accessible. In Fig. 1, the user-interface for the risk calculator is shown along with some of the input parameters that can be entered specifically for a patient to help generate their corresponding postoperative predictions. Fig. 2 demonstrates an example of some of the output that is produced by the calculator, including major complication, reintervention and readmission rates, in addition to functional outcomes such as MCID across three different HRQOL metrics. In an effort to further promote the utility of predictive models for patient education, Ames et al. used similar techniques to also investigate the cost implications of ASD surgery given the enormous financial burden it can place on both patients and hospital systems [36]. The aim of this study was to preoperatively identify which patients may be at risk of suffering catastrophic costs (defined as > $100,000) at 90-days and 2-years following surgical correction of ASD. Using a combination of random forest models and regression trees, the group was able to achieve goodness of fit R2 scores ranging from 56% to 57% when predicting 90-day direct cost and 29% to 35% for predicting 2year direct cost. While these accuracy metrics may reflect relatively lower performance when compared to other predictive models developed for ASD, the key result was the design which allowed the authors to interpret the model results and identify variables that were highly predictive of incurring catastrophic costs. Top predictors of direct cost and catastrophic cost were determined to be number of levels fused, surgical approach, use of interbody fusion, LOS and attending surgeon. By increasing awareness of at-risk patients in the preoperative stage, patients and healthcare 1630 R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 Fig. 1. Web-based risk calculator for ASD surgery outcomes predictions (input). The calculator developed by the ISSG/ESSG allows for myriad patientrelated information including demographics, radiographic parameters and surgical plan among several others, to be input for individual patients. The calculator is then able to make predictions for several different postoperative outcomes. R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 1631 Fig. 2. Web-based risk calculator for ASD surgery outcomes predictions (output). The ISSG/ESSG risk calculator is able to predict several different outcomes for individual patients based on the input parameters entered. These outcomes include rates for major complications, reinterventions and readmission, in addition to PROs such as MCID based on three major HRQOL surveys (ODI, SRS22, and SF36). systems could both benefit tremendously from cost-sharing and bundled payments to reduce the immense economic burden of these scenarios. Predictive analytics for HRQOL scores following ASD surgery In addition to predicting postoperative outcomes and complications, machine learning also offers the potential to better prognosticate how patients will respond to surgery based on PROs and HRQOL metrics such as the Scoliosis Research Society-22 (SRS-22), Short Form-36 (SF-36) and ODI surveys. It is critical that predictive analytics move beyond solely assessing outcomes such as complication risk or MCID, because for patients, it is often far more informative to understand what their functional outcome may look like following surgical intervention. With this goal in mind, Oh et al. were among the first groups to develop a machine learning model to predict PROs following ASD surgery [37]. Using an ensemble of decision trees and bootstrapping, their predictive model was able to successfully predict MCID in 2-year ODI 1632 R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 scores with 85.5% accuracy and AUC of 0.96. Scheer et al. followed up on this work by looking at only ASD patients with preoperative ODI>30 and developed a predictive model with AUC of 0.94 and 86% accuracy. Notably, the variables identified as top predictors in these two studies were remarkably different: Oh et al. identified patient comorbidities (preoperative depression, arthritis and osteoporosis) as well as number of levels fused as the most significant predictors of MCID in 2-year ODI scores, while Scheer et al. found radiographic parameters such as SVA and pelvic incidence-lumbar lordosis (PI-LL) mismatch in addition to gender and SRS-22 scores as having greater predictive influence in their model. Similar to the efforts for developing robust predictive analytics for ASD risk stratification and outcomes, the ISSG/ESSG also led several revolutionary studies with respect to predicting PROs following ASD surgery using their large, prospectively collected database. A few key differences exist between the studies conducted by the ISSG/ESSG and the earlier described models. Most importantly, their ASD database contains highquality data that has been carefully curated and collected through collaborations across multiple countries, spine centers and surgeons. The more data that is available for a model to be trained on, the better its generalizability is for future prospective applications. In two rigorous studies, Ames et al. developed robust predictive models utilizing several different algorithmic approaches to predict probability of achieving MCID across all three major HRQOL domains (ODI, SRS-22 and SF-36) and a separate model to predict individual patient responses to each of the SRS-22 survey questions [38,39]. To predict MCID across all three different HRQOL metrics, Ames et al. in a cohort of 570 patients used a total of 75 input features and 8 different machine learning algorithms assessed for accuracy using MAE to build their predictive models [38]. Final model selection was determined by performance as assessed by MAE (ranging from 8%−15% indicating high accuracy) and R2 goodness of fit scores. Baseline PROs were found to be the most important variable in predicting postoperative PROs, and with respect to patient-level data, age followed by comorbidities were the most important variables. Taking this study even further, Ames et al. next explored the feasibility of predicting patient responses to each of the individual SRS-22 questions to provide more granular insight into possible functional outcomes following surgery [39]. Analyzing six different machine learning algorithms and 150 input features, Ames et al. were able to develop predictive models with AUC ranging from 0.57 − 0.87. Interestingly, the models differed in their predictive ability based on which domains individual questions belonged to; SRS-22 domains of pain, disability, and social/labor function had the highest predictive accuracies while subjective responses in the domains of satisfaction, depression/anxiety and selfappearance were less accurately predicted. These efforts highlight both a paradigm shift in the ability of spine surgeons to embrace personalized/individualized medicine for ASD patients, but also underscore the difficulty of gauging functional outcomes in more subjective domains such as mental health, patient attitudes, and emotional well-being. By incorporating predictive analytics into clinical practice, spine surgeons can begin to substantiate their clinical recommendations with individualized data for patients to be more well-informed and partake in their own treatment plans. Advanced analytics for ASD surgery using unsupervised learning While predictive models that help prognosticate patient outcomes with ASD surgery are critical for empowering patients when discussing treatment plans and goals of surgery, these methods all constitute supervised learning as there is a desired output that is being predicted. In the first use of unsupervised learning for spine research, Ames et al. implemented an AI system to develop a novel classification scheme for ASD patients [27]. As described earlier, unsupervised learning represents a subset of machine learning where there are no labeled input/output pairings in the data; rather the goal is to use a mathematical model to identify inherent patterns in the natural structure of the data. Used in this context, Ames et al. wanted to employ unsupervised learning across their cohort of 570 ASD patients to identify unique patient clusters based on the available input features. The final model presented three distinct clusters of patients based on clinical information, and four distinct clusters when considering surgical characteristics. The patient-level clusters comprised of young patients with coronal deformity, older patients with higher incidence of prior spine surgery, and older patients with lower incidence of prior spine surgery. Surgery-level clusters were defined as patients with high number of levels fused and three-column osteotomies, patients with high number of levels fused and Smith-Peterson osteotomies, patients with no interbody fusions or osteotomies and patients with the highest number of interbody fusions. The hierarchical clustering algorithm determined that patients within these specific groupings shared the most similar characteristics, and represented a novel approach compared to prior classification schemes that were solely reliant on radiographic parameters. When the three patient-level clusters and four surgery-level clusters were combined into 12 sub-groups, PRO, complication and risk profiles for each sub-group could then be determined, with major complication risk ranging from 0% to 51.8%. This study presented a major stride forward in the realm of machine learning and AI, and showcased how powerful unsupervised learning techniques could help create new AI-based classification systems to facilitate treatment optimization for surgeons by highlighting treatment patterns predicted to yield optimal improvement with lowest risk. The added benefit of having more granular risk-benefit profiles during the preoperative evaluation of patients will help drive ASD surgery towards personalized medicine. Conclusion: Future directions and next steps In summary, while spine surgeons have begun to make significant strides in the realm of predictive analytics, we are just R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 barely glimpsing the power of machine learning and AI techniques, and how they can be widely incorporated to augment our ability to treat and care for spine patients. As we embrace the era of genomic medicine, combined with the digitization and widespread availability of enormous amounts of data, advanced computational techniques will become critical to help physicians analyze vast amounts of data that would be impossible without computer assistance. Oncology in particular has seen a huge advancement recently with molecular and genomic information about various tumors, and dramatically altered how patients are treated based on their individual profiles and new molecularly targeted therapies. AI will especially help with ASD as our knowledge of biomarkers increases, and as we acquire additional information including with the use of wearable technology pertaining to various aspects of a person’s biometrics and biophysical health profile. A strong foundation of computer science tools will be imperative to analyze this disparate data. The age of physicians making decision based on clinical judgment in isolation will slowly fade away as technology continues to advance, providing physicians with novel insights and data to supplement and augment decades of clinical experience. The information we are poised to gain through the implementation of machine learning and AI into clinical practice will empower patients in a shared decision-making and push spine surgeons confidently into the era of individualized medicine. Acknowledgment [9] [10] [11] [12] [13] [14] [15] [16] No funding was provided for this article. Declarations of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. [17] [18] References [1] Joshi RS, Lau D, Ames CP. Artificial intelligence and the future of spine surgery. Neurospine 2019;16(4):637–9. [2] Joshi RS, Haddad AF, Lau D, Ames CP. Artificial intelligence for adult spinal deformity. Neurospine 2019;16(4):686–94. [3] Lenke LG. Commentary: Artificial intelligence for adult spinal deformity. Neurospine 2019;16(4):695–6. [4] Jackson RP, Simmons EH, Stripinis D. Incidence and severity of back pain in adult idiopathic scoliosis. Spine (Phila Pa 1976) 1983;8 (7):749–56. [5] Robin GC, Span Y, Steinberg R, Makin M, Menczel J. Scoliosis in the elderly a follow-up study. Spine (Phila Pa 1976) 1982;7(4):355– 9. [6] Lowe T, Berven SH, Schwab FJ, Bridwell KH. The SRS classification for adult spinal deformity: Building on the King/Moe and Lenke classification systems. Spine (Phila Pa 1976) 2006;31(19 SUPPL.). [7] Terran J, Schwab F, Shaffrey CI, Smith JS, Devos P, Ames CP, et al. The SRS-schwab adult spinal deformity classification: Assessment and clinical correlations based on a prospective operative and nonoperative cohort. Neurosurgery 2013;73(4):559–68. [8] Bridwell KH, Glassman S, Horton W, Shaffrey C, Schwab F, Zebala LP, et al. Does treatment (nonoperative and operative) improve the two-year quality of life in patients with adult symptomatic lumbar [19] [20] [21] [22] [23] 1633 scoliosis: A prospective multicenter evidence-based medicine study. Spine (Phila Pa 1976) 2009;34(20):2171–8. Bridwell KH, Baldus C, Berven S, Edwards C, Glassman S, Hamill C, et al. Changes in radiographic and clinical outcomes with primary treatment adult spinal deformity surgeries from two years to three- to five-years follow-up. Spine (Phila Pa 1976) 2010;35(20):1849–54. Smith JS, Klineberg E, Schwab F, Shaffrey CI, Moal B, Ames CP, et al. Change in classification grade by the srs-schwab adult spinal deformity classification predicts impact on health-related quality of life measures; prospective analysis of operative and nonoperative treatment. Spine (Phila Pa 1976) 2013;38(19):1663–71. Smith JS, Shaffrey CI, Berven S, Glassman S, Hamill C, Horton W, et al. Operative versus nonoperative treatment of leg pain in adults with scoliosis: A retrospective review of a prospective multicenter database with two-year follow-up. Spine (Phila Pa 1976) 2009;34(16):1693–8. Smith JS, Shaffrey CI, Berven S, Glassman S, Hamill C, Horton W, et al. Improvement of back pain with operative and nonoperative treatment in adults with scoliosis. Neurosurgery 2009;65(1):86–93. Smith JS, Shaffrey CI, Glassman SD, Carreon LY, Schwab FJ, Lafage V, et al. Clinical and radiographic parameters that distinguish between the best and worst outcomes of scoliosis surgery for adults. Eur Spine J 2013;22(2):402–10. Smith JS, Shaffrey CI, Lafage V, Schwab F, Scheer JK, Protopsaltis T, et al. Comparison of best versus worst clinical outcomes for adult spinal deformity surgery: A retrospective review of a prospectively collected, multicenter database with 2-year follow-up. J Neurosurg Spine 2015;23(3):349–59. Smith JS, Singh M, Klineberg E, Shaffrey CI, Lafage V, Schwab FJ, et al. Surgical treatment of pathological loss of lumbar lordosis (flatback) in patients with normal sagittal vertical axis achieves similar clinical improvement as surgical treatment of elevated sagittal vertical axis: Clinical article. J Neurosurg Spine 2014;21(2):160–70. Scheer JK, Hostin R, Robinson C, Schwab F, Lafage V, Burton DC, et al. Operative management of adult spinal deformity results in significant increases in QALYs gained compared to nonoperative management. Spine (Phila Pa 1976) 2018;43(5):339–47. Liu S, Schwab F, Smith JS, Klineberg E, Ames CP, Mundis G, et al. Likelihood of reaching minimal clinically important difference in adult spinal deformity: A comparison of operative and nonoperative treatment. Ochsner J 2014;14(1):67–77. Scheer JK, Smith JS, Clark AJ, Lafage V, Kim HJ, Rolston JD, et al. Comprehensive study of back and leg pain improvements after adult spinal deformity surgery: Analysis of 421 patients with 2-year follow-up and of the impact of the surgery on treatment satisfaction. J Neurosurg Spine 2015;22(5):540–53. Smith JS, Lafage V, Shaffrey CI, Schwab F, Lafage R, Hostin R, et al. Outcomes of operative and nonoperative treatment for adult spinal deformity: A prospective, multicenter, propensity-matched cohort assessment with minimum 2-year follow-up. Neurosurgery 2016;78 (6):851–61. Lau D, Deviren V, Joshi RS, Ames CP. Comparison of perioperative complications following posterior column osteotomy versus posterior-based 3-column osteotomy for correction of rigid cervicothoracic deformity: a single-surgeon series of 95 consecutive cases. J Neurosurg Spine 2020;33(3):297–306. Joshi RS, Lau D, Haddad AF, Deviren V, Ames CP. Risk factors for determining length of intensive care unit and hospital stays following correction of cervical deformity: Evaluation of early severe adverse events. J Neurosurg Spine 2021;34(2):178–89. Smith JS, Kasliwal MK, Crawford A, Shaffrey CI. Outcomes, expectations, and complications overview for the surgical treatment of adult and pediatric spinal deformity. Spine Deform 2012;1(1):4–14. Bianco K, Norton R, Schwab F, Smith JS, Klineberg E, Obeid I, et al. Complications and intercenter variability of three-column osteotomies for spinal deformity surgery: A retrospective review of 423 patients. Neurosurg Focus 2014;36(5). 1634 R.S. Joshi et al. / The Spine Journal 21 (2021) 1626−1634 [24] Lau D, Deviren V, Ames CP. The impact of surgeon experience on perioperative complications and operative measures following thoracolumbar 3-column osteotomy for adult spinal deformity: Overcoming the learning curve. J Neurosurg Spine 2020;32(2):207–20. [25] Smith JS, Shaffrey CI, Klineberg E, Lafage V, Schwab F, Lafage R, et al. Complication rates associated with 3-column osteotomy in 82 adult spinal deformity patients: Retrospective review of a prospectively collected multicenter consecutive series with 2-year follow-up. J Neurosurg Spine 2017;27(4):444–57. [26] Perez-Breva L, Shin JH. Artificial intelligence in neurosurgery: A comment on the possibilities. Neurospine 2019;16(4):640–2. [27] Ames CP, Smith JS, Pellisé F, Kelly M, Alanay A, Acaroǧlu E, et al. Artificial intelligence based hierarchical clustering of patient types and intervention categories in adult spinal deformity surgery: towards a new classification scheme that predicts quality and value. Spine (Phila Pa 1976) 2019;44(13):915–26. [28] Kim HJ, Virk S, Elysee J, Passias P, Ames C, Shaffrey CI, et al. The morphology of cervical deformities: A two-step cluster analysis to identify cervical deformity patterns. J Neurosurg Spine 2020;32 (3):353–9. [29] Durand WM, Depasse JM, Daniels AH. Predictive modeling for blood transfusion after adult spinal deformity surgery. Spine (Phila Pa 1976) 2018;43(15):1058–66. [30] Safaee MM, Scheer JK, Ailon T, Smith JS, Hart RA, Burton DC, et al. Predictive modeling of length of hospital stay following adult spinal deformity correction: analysis of 653 patients with an accuracy of 75% within 2 days. World Neurosurg 2018;115: e422–7. [31] Scheer JK, Smith JS, Schwab F, Lafage V, Shaffrey CI, Bess S, et al. Development of a preoperative predictive model for major complications following adult spinal deformity surgery. J Neurosurg Spine 2017;26(6):736–43. [32] Scheer JK, Osorio JA, Smith JS, Schwab F, Lafage V, Hart RA, et al. Development of validated computer-based preoperative predictive [33] [34] [35] [36] [37] [38] [39] model for proximal junction failure (PJF) or clinically significant PJK with 86% accuracy based on 510 ASD patients with 2-year follow-up. Spine (Phila Pa 1976) 2016;41(22):E1328–35. Scheer JK, Oh T, Smith JS, Shaffrey CI, Daniels AH, Sciubba DM, et al. Development of a validated computer-based preoperative predictive model for pseudarthrosis with 91% accuracy in 336 adult spinal deformity patients. Neurosurg Focus 2018;45(5). Pellisé F, Serra-Burriel M, Smith JS, Haddad S, Kelly MP, VilaCasademunt A, et al. Development and validation of risk stratification models for adult spinal deformity surgery. J Neurosurg Spine 2019;31 (4):587–99. Passias PG, Oh C, Jalai CM, Worley N, Lafage R, Scheer JK, et al. Predictive model for cervical alignment and malalignment following surgical correction of adult spinal deformity. Spine (Phila Pa 1976) 2016;41(18):E1096–103. Ames CP, Smith JS, Gum JL, Kelly M, Vila-Casademunt A, Burton DC, et al. Utilization of predictive modeling to determine episode of care costs and to accurately identify catastrophic cost nonwarranty outlier patients in adult spinal deformity surgery: a step toward bundled payments and risk sharing. Spine (Phila Pa 1976) 2020;45(5):E252–65. Oh T, Scheer JK, Smith JS, Hostin R, Robinson C, Gum JL, et al. Potential of predictive computer models for preoperative patient selection to enhance overall quality-adjusted life years gained at 2year follow-up: A simulation in 234 patients with adult spinal deformity. Neurosurg Focus 2017;43(6). Ames CP, Smith JS, Pellisé F, Kelly MP, Gum JL, Alanay A, et al. Development of deployable predictive models for minimal clinically important difference achievement across the commonly used health-related quality of life instruments in adult spinal deformity surgery. Spine (Phila Pa 1976) 2019;44(16):1144–53. Ames CP, Smith JS, Pellisé F, Kelly M, Gum JL, Alanay A, et al. Development of predictive models for all individual questions of SRS-22R after adult spinal deformity surgery: a step toward individualized medicine. Eur Spine J 2019;28(9):1998–2011.