Original ArticleA Novel Tool for Deformity Surgery Planning: Determining the Magnitude of Lordotic Correction Required to Achieve a Desired Sagittal Vertical Axis
Introduction
The spinal column, an essential component of the overall structural support necessary to maintain an upright posture, functions optimally when the head, shoulders, pelvis, and feet are aligned: an equilibrium termed sagittal alignment.1 Sagittal malalignment may occur because of spinal deformity: a common problem that often requires surgical intervention to improve quality of life and prevent disability.2 Postoperative measurements of the sagittal vertical axis (SVA) represent one way to assess the efficacy of deformity surgery; the typical benchmark of an effective deformity surgery is a postoperative SVA of <50 mm.3 However, SVA alone may underestimate global sagittal malalignment and should therefore be used in conjunction with measurements of pelvic tilt (PT), pelvic incidence (PI), and lumbar lordosis (LL) to determine the true extent of postoperative sagittal malalignment.4
Restoration of a normal SVA is the single most important factor in deformity surgery and positively correlates with major clinical outcomes.5 In most surgical cases, the relationship between PI and LL (PI-LL) is the primary parameter modified intraoperatively to achieve postoperative restoration of a proper SVA.6 For instance, patients with very large PIs require concomitantly large LLs to achieve optimal SVAs. Conversely, patients with low PIs have optimal SVAs with smaller LLs. Previous literature has described a goal PI-LL relationship of 10° to achieve an ideal SVA.7 Despite knowledge of this relationship, no model has been able to quantify the degree to which surgical modification of the PI-LL relationship affects subsequent SVA correction. Establishing such a model (one with the ability to predict the number of millimeters that the SVA changes for each degree of surgical correction of the PI-LL) may help surgeons plan the degree of PI-LL correction necessary to achieve a desired postoperative SVA.
Herein, we analyze a large, prospectively maintained database of adult patients with spinal deformity to create a model capable of estimating the magnitude of PI-LL correction needed to achieve a desired change in SVA.
Section snippets
Study Design and Population
A retrospective review was conducted in 2016 of a prospectively maintained multicenter adult spinal deformity database collected between 2009 and 2014. The database is composed of consecutively enrolled patients with adult spinal deformity (defined by age >18 years and at least 1 of the following: coronal Cobb angle >20°, SVA >50 mm, PT >25°, or thoracic kyphosis ≥60°) collected through the International Spine Study Group. This study was conducted in accordance with the amended Declaration of
Results
A total of 1053 adult patients were identified in the database (Figure 1). Of these patients, 463 did not undergo a surgical procedure and 87 had incomplete information on PI-LL measurement and were thus excluded. The remaining 503 patients are the focus of the present analysis.
The mean age of included patients was 57.0 years (SD, ± 15.4 years); 79.2% of these patients were male. The most common comorbidities at baseline were arthritis and major depression (Table 1). The mean SVA at baseline
Discussion
Even although depression, obesity, age, and smoking have all been identified as potential factors affecting clinically relevant outcomes after deformity surgery, the correction of the SVA remains the single most important determinant of HRQOL in these circumstances.5, 7, 8 However, outcomes after deformity surgery are highly variable, meaning that not all patients benefit from a full correction to a normal SVA of <5 cm. Therefore, surgeons must evaluate additional parameters such as PI-LL and
Conclusions
We report on a simple model that shows how changes in the PI-LL relation affect the SVA. Our model may allow surgeons to determine preoperatively how much LL is needed to achieve a desired SVA change after surgery. Further validation of our results, in the form of a prospective trial with a longer follow-up period and a patient population that includes patients with baseline spine deformity, remains warranted.
Acknowledgments
E.G. and F.A. were involved in the design and conception of the manuscript. E.G. and N.A. performed the literature search. E.G., F.A., and N.A. composed the primary manuscript. F.A. and N.A. compiled the tables and figures. B.F., P.C.G., A.S.K., D.O.O., P.P., J.S., T.P., V.L., R.L., F.S., S.B., C.A., and D.K.H. critically revised the manuscript. All authors have approved the manuscript as it is written.
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Adult spinal deformity
2019, The LancetCitation Excerpt :Several radiographical indices have been identified as pragmatic in determining patient-specific alignment targets for surgical planning.126 Work in risk stratification has augmented surgical planning to permit surgeons to identify surgically modifiable parameters to predict outcomes.127 However, other factors, such as age, should also be incorporated into planning, since Lafage and colleagues78 showed that older patients (mean age 53·7 years SD 16·4) might require less aggressive alignment goals than would younger patients.
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Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Supplementary digital content available online.