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The Deformity Angular Ratio

Does It Correlate With High-Risk Cases for Potential Spinal Cord Monitoring Alerts in Pediatric 3-Column Thoracic Spinal Deformity Corrective Surgery?

Lewis, Noah D. H.*; Keshen, Sam G. N. BSc*; Lenke, Lawrence G. MD; Zywiel, Michael G. MD; Skaggs, David L. MD§; Dear, Taylor E. BSc*; Strantzas, Samuel MSc, D ABNM*; Lewis, Stephen J. MD, MSc, FRCSC*,‡

doi: 10.1097/BRS.0000000000000984

Study Design. A retrospective analysis.

Objective. The purpose of this study was to determine whether the deformity angular ratio (DAR) can reliably assess the neurological risks of patients undergoing deformity correction.

Summary of Background Data. Identifying high-risk patients and procedures can help ensure that appropriate measures are taken to minimize neurological complications during spinal deformity corrections. Subjectively, surgeons look at radiographs and evaluate the riskiness of the procedure. However, 2 curves of similar magnitude and location can have significantly different risks of neurological deficit during surgery. Whether the curve spans many levels or just a few can significantly influence surgical strategies. Lenke et al have proposed the DAR, which is a measure of curve magnitude per level of deformity.

Methods. The data from 35 pediatric spinal deformity correction procedures with thoracic 3-column osteotomies were reviewed. Measurements from preoperative radiographs were used to calculate the DAR. Binary logistic regression was used to model the relationship between DARs (independent variables) and presence or absence of an intraoperative alert (dependent variable).

Results. In patients undergoing 3-column osteotomies, sagittal curve magnitude and total curve magnitude were associated with increased incidence of transcranial motor evoked potential changes. Total DAR greater than 45° per level and sagittal DAR greater than 22° per level were associated with a 75% incidence of a motor evoked potential alert, with the incidence increasing to 90% with sagittal DAR of 28° per level.

Conclusion. In patients undergoing 3-column osteotomies for severe spinal deformities, the DAR was predictive of patients developing intraoperative motor evoked potential alerts. Identifying accurate radiographical, patient, and procedural risk factors in the correction of severe deformities can help prepare the surgical team to improve safety and outcomes when carrying out complex spinal corrections.

Level of Evidence: 3

The deformity angular ratio (DAR) has been used to describe the acuteness of spinal deformities. In this series of patients undergoing 3-column osteotomies, sagittal DAR greater than 22° per level and total DAR greater than 45° per level were associated with high rates of intraoperative motor evoked potential changes. Preoperative identification of high-risk patients may maximize surgical outcome.

*Toronto Western Hospital, The Hospital for Sick Children, Toronto, Ontario, Canada;

Washington University in St. Louis, St. Louis, MO;

Toronto Western Hospital, Toronto, Ontario, Canada; and

§Children's Hospital Los Angeles, Los Angeles, CA.

Address correspondence and reprint requests to Stephen J. Lewis, MD, MSc, FRCSC, Division of Orthopaedic Surgery, The Hospital for Sick Children, 555 University Ave, Room S107, Service Floor, Burton Wing, Toronto, Ontario, M5G 1×8; E-mail:

Acknowledgment date: October 17, 2014. Revision date: February 13, 2015. Acceptance date: March 18, 2015.

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

No funds were received in support of this work.

Relevant financial activities outside the submitted work: board membership, consultancy, expert testimony, grants, payment for lectures, patents, royalties, payment for development of educational presentations, travel/accommodations/meeting expenses, other (fellowship grant, philanthropic research funding).

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