Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P48. What is the normal distribution of kyphosis across the thoracic spine in the adult population?

BACKGROUND CONTEXT The usual definition of thoracic kyphosis (TK) is the angle between T4 and T12. However, little is known about the overall shape of the TK in adults. The aim of this study was to describe TK in a normative population and to evaluate the influence of TK magnitude on its shape. PURPOSE To evaluate whether the shape of kyphosis of the thoracic spine is independent of kyphotic magnitude. STUDY DESIGN/SETTING Retrospective analysis. PATIENT SAMPLE A total of 119 asymptomatic volunteers. OUTCOME MEASURES T1–12 TK, T4–12 TK, maxTK, vertebral orientation, cumulative TK and centered kyphosis in T7. METHODS Asymptomatic volunteers were recruited and demographic data along with full body standing radiographs were captured. Radiographic data such as lateral T1–12 and T4–12 angles were collected. Maximum TK (MaxTK) and vertebral orientation were also collected. Cumulative TK, defined as the total kyphosis measured at every level starting from L1 and ending at T1, and centered kyphosis at T7, defined as the kyphosis starting from T7 and measured at another vertebrae from either direction (ie T5–T7 or T7–L1) were also calculated. The cohort was stratified by T1–12 value ( 60), where comparisons and regressions were performed, afterwards. RESULTS A total of 119 subjects were included (51 yo, 81 women). Mean T1–12 kyphosis was 49.5°, mean T4-12 kyphosis was 41.5°, and mean maximum kyphosis was 52.6°. Analysis of vertebral orientation demonstrated that T1 was the most anteriorly tilted vertebra (25˚), L1 the most posteriorly tilted (−21˚), T7 was horizontal on an average, independent of T1–12 value or age. Cumulative kyphosis resulted in a sigmoid progression (slow increase, rapid increase, slow increase) with a maximum increase around T6–T7, revealing the apex of kyphosis, again independently of TK group. Centered analysis produced a double bell curve with two maximum values located at T1–T7 and T7–L1. Stratification showed that across TK groups, the T1–T7 kyphosis value increased (−19.2vs. −26.8vs. −33.5, p CONCLUSIONS Classic definition of thoracic kyphosis using T4−T12 tends to underestimate the total amount of kyphosis. Changes in kyphosis distribution in the normative population suggest that, like lumbar lordosis, TK is not a simple circle arc but rather closer to a sinusoidal shape. With low TK, 2/3 of the kyphosis is located in the upper part and when TK increases, the distribution of kyphosis will be symmetric around T7. It is possible to predict the amount of kyphosis in the upper part using total kyphosis value. It could be used to estimate preoperative compensation and predict reciprocal change.