Lumbopelvic parameters can be used to predict thoracic kyphosis in adolescents

Purpose Distal thoracic kyphosis (DTK) equivalent to proximal lumbar lordosis (PLL) is the sum of pelvic tilt (PT) and the difference (Delta) between lumbar lordosis (LL) and pelvic incidence (PI): PLL = DTK = PT + Delta. With the assumption that proximal thoracic kyphosis (PTK) is similar to DTK, we propose the equation TK = 2(PT + LL - PI) to express the relationship between thoracic kyphosis (TK) and pelvic parameters. The objective of this work is to verify this relationship in a normal population. Methods Full spine radiographs of 100 adolescents and young adults (13 to 20 years old), free from vertebral pathology, were analyzed. Measurements included pelvic parameters, LL, PLL, DLL, TK, PTK, DTK and C7 global tilt. The measured global TK was compared with the theoretical TK calculated according to the formula TK = 2(PT + LL - PI). Results The difference between measured TK and calculated TK was + 2.3 degrees and correlated with the C7 global tilt (r = 0.86). There was a significant linear regression between TK and PT + increment (p < 0.0001). Given radiographs' inter-rater reliability of 5 degrees for angled measurements, the p value (0.047) between measured TK and calculated TK is statistically significant to support the hypothesis. Conclusion This work validates the formula TK = 2(PT + LL - PI) which allows the calculation of global TK as a function of PT, LL and PI. This calculated TK can be used as a target for sagittal correction of adolescents with spine deformities. Graphic abstract These slides can be retrieved under Electronic Supplementary Material. [GRAPHICS] .