title: T1 pelvic angle (TPA) effectively evaluates sagittal deformity and assesses radiographical surgical outcomes longitudinally.
abstract: METHODS
  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.
  
  
  BACKGROUND
  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.
  
  
  CONCLUSIONS
  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.
  
  
  METHODS
  4.
authors.affiliation: *Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY †Department of Neurological Surgery, University of California, San Francisco, CA ‡Department of Orthopedic Surgery, Baylor Scoliosis Center, Plano, TX §Department of Orthopaedic Surgery, University of California, Davis, Sacramento, CA ¶Department of Orthopaedic Surgery, San Diego Center for Spinal Disorders, La Jolla, CA ‖Department of Orthopaedic Surgery, Bordeaux University Hospital, Bordeaux, France **Department of Orthopaedic Surgery, Johns Hopkins University, Baltimore, MD ††Department of Neurosurgery, University of Virginia, Charlottesville, VA ‡‡Department of Orthopaedic Surgery, Hospital for Special Surgery, New York, NY §§Department of Orthopedic Surgery, University of Kansas Medical Center, Kansas City, KS; and ¶¶Department of Orthopaedic Surgery, Oregon Health & Science University, Portland, OR.
authors.forename: Devon J
authors.initials: DJ
authors.lastname: Ryan
authors.forename: Themistocles S
authors.initials: TS
authors.lastname: Protopsaltis
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Ames
authors.forename: Richard
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authors.lastname: Hostin
authors.forename: Eric
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authors.lastname: Klineberg
authors.forename: Gregory M
authors.initials: GM
authors.lastname: Mundis
authors.forename: Ibrahim
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authors.lastname: Obeid
authors.forename: Khaled
authors.initials: K
authors.lastname: Kebaish
authors.forename: Justin S
authors.initials: JS
authors.lastname: Smith
authors.forename: Oheneba
authors.initials: O
authors.lastname: Boachie-Adjei
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Burton
authors.forename: Robert A
authors.initials: RA
authors.lastname: Hart
authors.forename: Munish
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authors.lastname: Gupta
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Schwab
authors.forename: Virginie
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authors.lastname: Lafage
mesh_terms: D016208:Databases, Factual / Q000706:statistics & numerical data; D004185:Disability Evaluation; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D010027:Osteotomy / Q000379:methods*; D017063:Outcome Assessment, Health Care / Q000706:statistics & numerical data; D010384:Pelvic Bones / Q000000981:diagnostic imaging*; D010388:Pelvis / Q000000981:diagnostic imaging; D011859:Radiography; D012189:Retrospective Studies; D012447:Sacrum / Q000000981:diagnostic imaging; D012600:Scoliosis / Q000000981:diagnostic imaging / Q000601:surgery; D013121:Spinal Curvatures / Q000601:surgery*; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging*