title: Defining the Role of the Lower Limbs in Compensating for Sagittal Malalignment.
abstract: MINI: Despite differences in sagittal malalignment, antero-posterior pelvic translation maintained the position of T9 in line with the ankles, independently of sagittal vertical axis. Pelvic tilt was directly predicted by lower limb compensatory mechanisms. Therefore, these adaptation mechanisms being included in pelvic tilt analysis, it does not need additional consideration in the surgical planning.
  
  
  METHODS
  Retrospective review.
  
  
  OBJECTIVE
  To investigate the role of lower limbs compensation with progressive sagittal malalignment.
  
  
  BACKGROUND
  Although lower limb compensatory mechanisms are established response to progressive sagittal malalignment, their specific role and potential impact on surgical planning has not been evaluated.
  
  
  METHODS
  Single center retrospective review of full body x-rays was performed in patients of age >20 years. Parameters were measured with dedicated software. Population was stratified by 50 mm intervals of sagittal vertical axis (SVA) and one-way ANOVA was performed to compare P.shift (P.shift = anteroposterior translation of the pelvis vs. the feet) across SVA groups. Anteroposterior offset of each vertebra in relation to a vertical line extended from the distal tibial metaphysis (TM) was investigated. Linear regression was performed to predict pelvic tilt (PT) using Knee angle (KA) and P.shift, whereas controlling for pelvic incidence minus lumbar lordosis mismatch (PI-LL) and SVA.
  
  
  RESULTS
  A total of 2124 patient visits were included (PI = 55.1 ± 14.1°, PT=21.0 ± 11°, PI-LL=6.3 ± 17.3°, SVA = 29 ± 51 mm). With progressively increased SVA, P.shift decreased from 30 to -100 mm (all P < 0.005). Analysis of vertebral offset from the distal tibial metaphysis revealed that T9 was aligned with the TM line across all SVA groups. Prediction of PT based on PI-LL and SVA yielded R=0.76 (P < 0.001). Subsequent addition of KA and P.shift as independent parameters using hierarchical multiple regression led to significant improvement in R, demonstrating the independent role of lower limbs parameters in PT prediction. KA and P.shift had a positive standardized coefficient (all P < 0.05).
  
  
  CONCLUSIONS
  Lower limb compensatory mechanisms increase with progressive sagittal malalignment. Anteroposterior translation of pelvis allows the T9 vertebra to remain in line with the ankle ("conus of economy"). Lower limb compensatory mechanisms are positive predictors of PT and thus do not require additional consideration in surgical realignment planning.
  
  
  METHODS
  3.
authors.affiliation: Hospital for Special Surgery, New York, NY.
authors.forename: Renaud
authors.initials: R
authors.lastname: Lafage
authors.affiliation: Hospital for Special Surgery, New York, NY.
authors.forename: Barthelemy
authors.initials: B
authors.lastname: Liabaud
authors.affiliation: Hospital for Special Surgery, New York, NY.
authors.forename: Bassel G
authors.initials: BG
authors.lastname: Diebo
authors.affiliation: NYU Hospital for Joint Diseases, New York, NY.
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Oren
authors.affiliation: NYU Hospital for Joint Diseases, New York, NY.
authors.forename: Shaleen
authors.initials: S
authors.lastname: Vira
authors.affiliation: Hospital for Special Surgery, New York, NY.|Aix-Marseille University, French National Centre for Scientific Research, Institute Movement Science, Marseille, France.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Pesenti
authors.affiliation: NYU Hospital for Joint Diseases, New York, NY.
authors.forename: Themistocles S
authors.initials: TS
authors.lastname: Protopsaltis
authors.affiliation: NYU Hospital for Joint Diseases, New York, NY.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Errico
authors.affiliation: Hospital for Special Surgery, New York, NY.
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Schwab
authors.affiliation: Hospital for Special Surgery, New York, NY.
authors.forename: Virginie
authors.initials: V
authors.lastname: Lafage
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D000368:Aged; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008141:Lordosis / Q000150:complications / Q000000981:diagnostic imaging*; D035002:Lower Extremity / Q000000981:diagnostic imaging* / Q000502:physiology; D008297:Male; D008875:Middle Aged; D010384:Pelvic Bones / Q000000981:diagnostic imaging*; D011187:Posture / Q000502:physiology; D012189:Retrospective Studies; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging*