The RMDQ consists of 24 statements related to daily physical activities such as dressing, walking, and using of stairs [36, 37]. The patient is asked to put a check mark that corresponds to his or her current situation. The check marks are added up with a total score of 24, with a higher score representing greater disability. Studies document that RMDQ and ODI have a high level of correlation to each other [38–40].

The SRS-22 is a scoliosis-specific HRQOL questionnaire. The questionnaire comprises 22 items with five domains – pain (5 items), appearance or self-image (5 items), activity or function (5 items), mental health (5 items), and satisfaction and management (2 items) [41]. Each domain score ranges from 1 to 5, with higher scores indicating better outcomes. For example: question 8 asks the respondent: “Do you experience back pain when at rest?”; question 17 asks: “In the past three months, have you taken any sick days from work/school due to back pain and, if so, how many?”

The SRS-22 is the most widely used tool to measure changes in health-related quality of life in patients with scoliosis [8, 42–47]. The SRS-22R instrument is a refinement of the SRS-22 and was created to assess quality of life following surgery in patients with adolescent idiopathic scoliosis [48]. Additionally, the SRS-22R assesses patient’s self-image; however, studies suggest that the questionnaire might not accurately assess the health status of younger patients or those with milder forms of scoliosis [49, 50].

QALYs remain the most popular measure of health benefits used in economic evaluation of health care interventions [51]. QALY measures were introduced to create a standard unit of health utility measure in order to value the length and quality of life on a single scale [52–54]. The advantage of the QALY as a measure of health outcome is that it can simultaneously capture gains from reduced morbidity (quality gains) and reduced mortality (quantity gains) and combine both into a single measure [55]. The QALY measure assumes that an additional year of life has the same value regardless of the age or other characteristics of the person who receives it, assuming that the different life years are of comparable quality [56]. A year of life extension for an infant or a 35-year-old all have the same value in QALYs and, in turn, in a cost-effectiveness analysis using QALYs, which assumes no difference in the quality of the year of life extension.

QALYs are a measure of health outcome that assigns to each period of time a weight, ranging from 0 to 1, corresponding to the health-related quality of life during that period, where a weight of 1 corresponds to optimal health and a weight of 0 corresponds to a health state judged equivalent to death; these are then aggregated across time periods [57]. QALYs are computed using Health Utilities Indexes such as the EQ-5D, or SF-6D, and estimates of the length of time a treatment benefit will last. For example, consider a patient with spinal deformity who has a health state of 0.6. Without the surgery, the patient lives for 10 years. With the surgery, the patient’s health state improves to 0.9, and his life expectancy is increased by 5 years. Thus, QALY gained with surgery = quality of life years with the surgery – quality of life years without the surgery = 0.9 * 15 − 0.6 * 10 = 7.5QALYs. See Fig. 22.2, which exemplifies QALYs gained from an intervention.

QALYs are primarily used as an outcome of interest in cost-effectiveness analysis and are typically expressed as costs divided by the QALYs gained from a treatment or intervention (cost/QALY). However, other quality-adjusted measures available in the literature are Disability-Distress Index (DDI) [58], the Quality of Well-Being (QWB) Scale [59], and disability-adjusted life years (DALYs) [60].

It is important to consider not only the clinical outcomes of care but also the costs required to achieve the outcomes associated with treatment. Over the last decade, total charges for spine deformity surgery have increased dramatically with over 20,000 discharges associated with ICD-9 diagnosis codes 737.0–737.9, which is defined as “curvature of the spine,” in 2013 [61]. See Fig. 22.3, which shows discharges and costs per year for spine deformity surgeries related to curvature of the spine.

There are multitudes of spine deformity treatments available; some treatments may be very expensive but very beneficial, while others may be inexpensive but do little to improve clinical or quality of life outcomes. Standardized methods of calculating the costs of operative and non-operative treatments for spine disorders are necessary for value-driven decision making. Therefore, determining the value of surgical treatment requires both the clinical, patient-specific, or societal outcomes and the associated costs to provide those outcomes. In addition to determining which costs to include, appropriate methods to measure and analyze costs are all equally important considerations in health economic evaluation.

Identifying all relevant costs associated with treatment is vital in the economic evaluation of a health care intervention. Accurate measurement of costs requires estimation of the amount of resources used in natural and comparable units of measurement. Costs related to health care interventions can be categorized into several types, including direct and indirect costs, operative and non-operative costs, and formal and informal costs. Direct costs are costs that are directly associated with the illness, procedure, or treatment or in addressing the side effects of treatment. These include costs of implants, operating room staff, tests, medications, and supplies. Indirect costs are not directly associated with the illness or treatment and may not be incurred by the individual who is receiving treatment. These often include overhead costs, such as administrative costs, as well as productivity losses associated with illness or death. It is important to note that some of these resources are challenging, if not impossible, to accurately quantify and capture. For example, how can we quantify a reestablished family routine due to reductions in pain?

The appropriate estimation of costs is varied in the literature, due to scope and specific research question being answered, not to mention the cost data that are available to the researcher. Costs have been analyzed using charges, reimbursements, payments, direct cost, total costs, allowable rates, relative value units, etc. Each of these provides some interesting information, but alone, each often fails to provide the complete cost of care.

Cost data and measurement are also constrained by the confidentiality among competing health care providers and insurers as well as by differences within the US health care system [62]. From whose perspective costs are considered is an important concept in cost evaluation. The perspective of the one performing the cost evaluation influences the methodologies incorporated and ultimately can lead to very different conclusions. For example, a health care consumer deciding whether to pay for a generic or more costly prescription may be willing to pay more or less for the medication than a hospital, insurance company, or another patient would do [63].

There are two broad categories of cost perspective, the health service perspective and the societal perspective. These can be broken down into more specific categories such as providers, payers (e.g., insurance companies and employers), patients, and policy makers. The health service perspective usually considers costs incurred by the provider or payer, while a societal perspective considers broader costs to society at large and is usually indifferent to who incurs the expense. For example, a societal perspective may consider patient expenses, including productivity loss and family disease burden. Alternatively, an individual hospital may be interested in its internal costs to treat a disease-specific population [64].

Another important concept in cost assessment is the time horizon considered. Assessment of the cost of spine deformity surgery should consider not only the cost of the surgery itself but future costs and outcomes that are realized or avoided as a result of the surgery [63].This is also related to the durability of treatment, i.e., how long an intervention will continue to provide benefits. This often manifests itself in repeat revision surgeries for spinal deformity patients. Future costs may be very substantial, and analyses may underestimate the cost if it is not incorporated in the assessment. For example, the cost of surgery includes not only the inpatient stay but preoperative visits, pain medication, postoperative follow-up, time off from work, etc. In this same vein, it is not only the costs incurred but the avoided costs of forgoing treatment (i.e., the continued disease burden on family and work life, comorbidities that were exacerbated due to spinal deformity). Non-operative costs include pain management, physical therapy, and post-acute care. Although surgery involves expensive inpatient costs, the reduction of expensive non-operative treatment may outweigh the costs of the surgery, when considered over an extended period of time. Therefore, what appears to be the more expensive initial treatment may reduce total costs over the long run.

After determining appropriate costs to include and how best to accurately capture the costs of care, analyzing cost data comes with its own pitfalls [62].The distribution of costs for surgical treatment tends to be skewed instead of normally distributed.

Due to the skewed nature of the distribution, careful consideration of the statistical approach is necessary. Frequently used methodologies include log transformation of the costs variables and generalized linear models that consider the statistical distribution. A multitude of literature has been written for those interested in learning more about these models and their assumptions [65–71].

In this section we have covered the importance of defining, accurately capturing, and modeling costs for the surgical care of spinal deformity. Ultimately the continual pursuit of the true cost of care will allow for accurate comparisons and help define value and best practices in spine deformity.

As a rule, all costs and benefits of health care programs are observed over different points in time. For example, the benefits to the individual and society of adult lumbar scoliosis surgery are incurred over the patient’s lifetime after the procedure. However, individuals value the benefits sooner rather than later in life and prefer to incur costs later in life.

Discounting accounts for the differential timing of health care costs and benefits. All future costs and benefits associated with an intervention should be discounted by computing the present value of these [72]. To calculate the present value of future costs and benefits (both monetary and nonmonetary), the following formula is applied:
where PV is the present value, $a _t is the dollar amount of cost or benefit in period t, r is the discount rate, and N is the maximum time periods. A discount rate of 5 % is prevalent in the existing literature. The US Public Health Service Panel on Cost-Effectiveness in Health and Medicine recommended that a 3 % rate be applied for health interventions [55]. Moreover an inflation-adjusted discount rate should be used if it is expected that inflation might impact health care costs and benefits.

Economic evaluation is used to describe a range of methods that investigate the costs and consequences of different treatments or interventions [73]. These methods are designed to identify and appropriately quantify all costs and benefits of health care interventions. There are three main types of economic evaluations: cost-utility analysis, cost-effectiveness analysis, and cost-benefit analysis.

ICERs are used to compare two or more competing health care interventions and represent the incremental cost of one unit of outcome gained by a health care intervention when compared to an alternative. An ICER is estimated using [2]:
where C ₁ and C ₀ are the mean values of the costs using Interventions 1 and Intervention 0; E ₁ and E ₀ are effectiveness values yielded by Intervention 1 and 0, respectively; and ΔC and ΔE are incremental costs and incremental effectiveness gained/lost. For CUA, ΔE is computed in terms of QALYs. ICER is increasingly used in many countries to determine which interventions to fund. An ICER of $50,000 per QALY is the conventional threshold for cost-effectiveness [77]. In the literature, health care interventions valued below this threshold are considered “cost-effective” and those above are not [78, 79]. However, the World Health Organization suggests a threshold of three times a nation’s gross domestic product per QALY, which in the USA in 2014 would be closer to $140,000 per QALY [80]. Either of these thresholds may be higher or lower than what a decision maker may deem as their true willingness to pay. Therefore, there is no clear consensus on a universal cost-effectiveness threshold [63, 81]. Instead of the threshold, a cost-effectiveness acceptability curve (CEAC) may be created to allow for different willingness to pay thresholds. For example, for the treatment under consideration in Fig. 22.4 below, if the decision maker’s willingness to pay threshold is under $100,000 per QALY, there is almost a 100 % probability the intervention is cost-effective at that threshold. If the willingness to pay threshold is $80,000, there is about a 40 % probability that the intervention is cost-effective at that threshold. See Fig. 22.4, which shows the incremental cost-effectiveness acceptability curve.

ICERs reported for spine interventions are becoming increasingly available. For example, evaluation of cost-effectiveness of surgical vs. nonsurgical treatment of lumbar disk herniation revealed that cost/QALY gained for the surgical cohort in the Medicare population was $34,355, and for general populations, it was $69,403 [82]. A cost-utility analysis comparing surgical with nonsurgical care for a lumbar disk herniation reported an ICER of $4,648 [83]. Periacetabular osteotomy performed with the goal of preventing or delaying the need for total hip arthroplasty reported an ICER of $7,856 [84].

However, this measure has its limitations mainly because value assessments are inherently subjective, and there are oversimplifications of complex processes [85]. The National Institute for Health and Clinical Excellence in England was criticized for refusing to cover four kidney cancer medications in 2008 based largely on assessments of ICERs that exceeded the $50,000 (£30,000) threshold [86]. However, despite its limitations, QALY remains the main tool for cost-effectiveness research methodology.

Decision models or trees are used formally to model a decision problem. A model reflects the question to be answered and a graphical representation of the main elements (variables and their relationships) of a clinical decision. Figure 22.5 illustrates a basic decision model related to spine surgery.

Sensitivity analysis is an essential part of economic evaluation that allows the assessment of how sensitive a study’s results are to variations in key parameters (transition probabilities, costs, utility values) that were used in the primary analysis. The goal of sensitivity analysis is to find out which variables in the model most impact the results and whether changes in parameters will result in savings or costs. Several methods to deal with uncertainty are employed: simple sensitivity analysis, threshold analysis, probabilistic sensitivity analysis, and value of information analysis. In a simple sensitivity analysis, one or more parameters are varied across a range of possible values. The purpose of threshold analysis is to identify the critical value of a parameter above or below which will change the conclusions of the study. The probabilistic sensitivity analysis (PSA) treats all input parameters as random variables with known probability distributions. PSA measures the uncertainty around a prediction of cost-effectiveness. Value of information analysis uses PSA to examine the effect of reducing the uncertainty around the model’s parameters [87].

Spine disorders are extremely expensive to treat surgically. In particular, the disorders of the lumbar spine such as lumbar stenosis, lumbar degenerative spondylolisthesis, and lumbar disk herniation are expensive to treat and cause significant disability. The evidence around cost-effectiveness of operative vs. non-operative treatment of the lumbar spine disorders is inconclusive, and the studies that suggest that surgery is advantageous over nonsurgical treatment fail to report that surgery is actually cost-effective.