Multicenter validation of a formula predicting postoperative spinopelvic alignment

Clinical article

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Sagittal spinopelvic imbalance is a major contributor to pain and disability for patients with adult spinal deformity (ASD). Preoperative planning is essential for pedicle subtraction osteotomy (PSO) candidates; however, current methods are often inaccurate because no formula to date predicts both postoperative sagittal balance and pelvic alignment. The authors of this study aimed to evaluate the accuracy of 2 novel formulas in predicting postoperative spinopelvic alignment after PSO.


This study is a multicenter retrospective consecutive PSO case series. Adults with spinal deformity (> 21 years old) who were treated with a single-level lumbar PSO for sagittal imbalance were evaluated. All patients underwent preoperative and a minimum of 6-month postoperative radiography. Two novel formulas were used to predict the postoperative spinopelvic alignment. The results predicted by the formulas were then compared with the actual postoperative radiographic values, and the formulas' ability to identify successful (sagittal vertical axis [SVA] ≤ 50 mm and pelvic tilt [PT] ≤ 25°) and unsuccessful (SVA > 50 mm or PT > 25°) outcomes was evaluated.


Ninety-nine patients met inclusion criteria. The median absolute error between the predicted and actual PT was 4.1° (interquartile range 2.0°–6.4°). The median absolute error between the predicted and actual SVA was 27 mm (interquartile range 11–47 mm). Forty-one of 54 patients with a formula that predicted a successful outcome had a successful outcome as shown by radiography (positive predictive value = 0.76). Forty-four of 45 patients with a formula that predicted an unsuccessful outcome had an unsuccessful outcome as shown by radiography (negative predictive value = 0.98).


The spinopelvic alignment formulas were accurate when predicting unsuccessful outcomes but less reliable when predicting successful outcomes. The preoperative surgical plan should be altered if an unsuccessful result is predicted. However, even after obtaining a predicted successful outcome, surgeons should ensure that the predicted values are not too close to unsuccessful values and should identify other variables that may affect alignment. In the near future, it is anticipated that the use of these formulas will lead to better surgical planning and improved outcomes for patients with complex ASD.

Abbreviations used in this paper: ASD = adult spinal deformity; HRQOL = health-related quality-of-life; IQR = interquartile range; NPV = negative predictive value; PI = pelvic incidence; PPV = positive predictive value; PSO = pedicle subtraction osteotomy; PT = pelvic tilt; SVA = sagittal vertical axis.

Article Information

Address correspondence to: Virginie Lafage, Ph.D., 306 East 15th Street, Suite 1F, New York, New York 10003. email:

Please include this information when citing this paper: published online September 23, 2011; DOI: 10.3171/2011.8.SPINE11272.

© AANS, except where prohibited by US copyright law.



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    Sagittal spinal radiological parameters. Positive values for sagittal curvatures denote a lordotic alignment, while negative values denote a kyphotic alignment.

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    Pelvic radiological parameters.

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    Population pyramid illustrating the age (in years) and sex distribution of the 99 study patients.

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    Histogram of PSO levels illustrating the number of PSO resections performed at each lumbar vertebral level.

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    Case 1. Preoperative (left) and postoperative (right) radiographs obtained in a patient who underwent sagittal realignment surgery, showing a successful realignment surgery.

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    Case 2. Preoperative (left) and postoperative (right) radiographs obtained in a patient who underwent sagittal realignment surgery, showing an unsuccessful realignment surgery.


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