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Dynamic Changes of the Pelvis and Spine Are Key to Predicting Postoperative Sagittal Alignment After Pedicle Subtraction Osteotomy: A Critical Analysis of Preoperative Planning Techniques

Smith, Justin S. MD, PhD*; Bess, Shay MD; Shaffrey, Christopher I. MD*; Burton, Douglas C. MD; Hart, Robert A. MD§; Hostin, Richard MD; Klineberg, Eric MD**the International Spine Study Group

doi: 10.1097/BRS.0b013e31823b0892

Study Design. Retrospective, radiographical analysis of mathe-matical formulas used to predict sagittal vertical axis (SVA) after pedicle subtraction osteotomy (PSO).

Objective. Evaluate the ability of different formulas to predict SVA after PSO.

Summary of Background Data. Failure to achieve optimal spinal alignment after spinal fusion correlates with poor outcomes. Numerous mathematical models have been proposed to aid preoperative PSO planning and predict postoperative SVA. Pelvic parameters have been shown to impact spinal alignment; however, many preoperative planning models fail to evaluate these. Compensatory changes within unfused spinal segments have also been shown to impact SVA. Predictive formulas that do not evaluate pelvic parameters and unfused spinal segments may erroneously guide PSO surgery. A formula that integrates pelvic tilt (PT) and spinal compensatory changes to predict optimal SVA has been previously proposed.

Methods. Comparative analysis of 5 mathematical models used to predict optimal postoperative SVA (<5 cm) after PSO was performed using a multicenter PSO database.

Results. Radiographs of 147 patients, mean age 52 years (SD = 15 yr), who received 147 PSOs (42 thoracic and 105 lumbar) were evaluated. Mean preoperative and postoperative SVA was 108 mm (SD = 95 mm) and 30 mm (SD = 60 mm; P < 0.001), respectively. Each mathematical formula provided unique prediction for postoperative SA (Pearson R 2 < 0.15). Formulas that neglected pelvic alignment poorly predicted final SVA and poorly correlated with optimal SVA. Formulas that evaluated pelvic morphology (pelvic incidence) had improved SVA prediction. The Lafage formulas, which incorporate PT and spinal compensatory changes, had the best SVA prediction (P < 0.05) and best correlation with optimal SVA (R 2 = 0.75).

Conclusion. Preoperative planning for PSO is essential to optimize postoperative spinal alignment. Mathematical models that do not consider pelvic parameters and changes in unfused spinal segments poorly predict optimal postoperative alignment and may predispose to poor clinical outcomes. The Lafage formulas, which incorporated PT and spinal compensatory changes, best predicted optimal SVA.

Preoperative planning for pedicle subtraction osteotomy (PSO) is essential to optimize spinal alignment. Evaluation of 5 different PSO prediction formulas demonstrated that formulas that do not evaluate pelvic alignment or account for compensatory changes in the unfused spine poorly predict optimal spinal alignment.

*Neurological and Orthopaedic Surgery, University of Virginia, Charlottesville

Orthopaedic Surgery, Rocky Mountain Hospital for Children, Denver, CO

Orthopaedic Surgery, University of Kansas Medical Center, Kansas City

§Orthopaedic Surgery, Oregon Health Sciences University, Portland

Orthopaedic Surgery, Baylor Scoliosis Center, Plano, TX; and

**Orthopaedic Surgery, University of California–Davis, Sacramento, Sacramento, CA.

Address correspondence and reprint requests to Justin S. Smith, MD, PhD, Department of Neurological Surgery, University of Virginia, P.O. Box 800212, Charlottesville, VA 22908; E-mail:

Acknowledgment date: January 13, 2011. First revision date: May 19, 2011. Second revision date: August 5, 2011. Third revision date: August 21, 2011. Acceptance date: August 30, 2011.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Corporate/Industry funds were received in support of this work. One or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript: e.g., honoraria, gifts, consultancies, royalties, stocks, stock options, decision making position.

© 2012 Lippincott Williams & Wilkins, Inc.