Skip Navigation LinksHome > July 01, 2014 - Volume 39 - Issue 15 > T1 Pelvic Angle (TPA) Effectively Evaluates Sagittal Deformi...
doi: 10.1097/BRS.0000000000000382

T1 Pelvic Angle (TPA) Effectively Evaluates Sagittal Deformity and Assesses Radiographical Surgical Outcomes Longitudinally

Ryan, Devon J. BA*; Protopsaltis, Themistocles S. MD*; Ames, Christopher P. MD; Hostin, Richard MD; Klineberg, Eric MD§; Mundis, Gregory M. MD; Obeid, Ibrahim MD; Kebaish, Khaled MD**; Smith, Justin S. MD, PhD††; Boachie-Adjei, Oheneba MD‡‡; Burton, Douglas C. MD§§; Hart, Robert A. MD¶¶; Gupta, Munish MD§; Schwab, Frank J. MD*; Lafage, Virginie PhD*; International Spine Study Group

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Study Design. Retrospective review of a multicenter database of consecutive patients undergoing 3-column osteotomy for treatment of adult spinal deformity (ASD).

Objective. To rigorously develop a T1 pelvic angle (TPA) categorization paradigm and use it to assess the surgical management of patients with ASD.

Summary of Background Data. TPA, the angle between the hips-T1 line and hips-S1 endplate line, is a novel spinopelvic parameter that assesses the combined effect of a loss of lordosis on trunk inclination and pelvic retroversion.

Methods. A prospective, multicenter database of consecutive patients with ASD was queried to identify the severe deformity threshold and meaningful change values for TPA by correlation with Oswestry Disability Index. A separate multicenter, consecutive, retrospective database of patients with ASD treated with single lumbar 3-column osteotomy was then analyzed at baseline, 3-month, and 1-year follow-up. Subjects were classified into well-aligned or poorly aligned groups at 3 months on the basis of TPA. Patients “deteriorated” if they lost more than 1 meaningful change in TPA between 3 months and 1 year and had TPA more than deformity threshold at 1 year.

Results. The severe deformity threshold for TPA was 20° (Oswestry Disability Index > 40) and the meaningful change was 4.1° (Oswestry Disability Index change = 15). Review of the 3-column osteotomy database identified 179 patients with preoperative severe deformity; 63 were well-aligned (TPA < 15.9°) and 73 were poorly aligned (TPA > 20°) at 3-month follow-up. This newly developed TPA categorization mechanism grouped patients in a manner comparable with the Scoliosis Research Society-Schwab Classification. Subjects who were well-aligned at 3 months had less severe baseline deformity, but received more correction, than poorly aligned subjects. Four well-aligned patients and 13 poorly aligned patients deteriorated between 3 months and 1 year after surgery.

Conclusion. TPA accounts for sagittal vertical axis and pelvic tilt and shows great promise as a classification tool. Longitudinal analysis demonstrated undercorrection among patients with more severe preoperative deformity. We propose a surgical target of 10° for TPA.

Level of Evidence: 4

© 2014 by Lippincott Williams & Wilkins

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