The Relationship between T1 Sagittal Angle and Sagittal Balance: A Retrospective Study of 119 Healthy Volunteers.

T1 sagittal angle has been reported to be used as a parameter for assessing sagittal balance and cervical lordosis. However, no study has been performed to explore the relationship between T1 sagittal angle and sagittal balance, and whether T1 sagittal angle could be used for osteotomy guidelines remains unknown. The aim of our study is to explore the relationship between T1 sagittal angle and sagittal balance, determine the predictors for T1 sagittal angle, and determine whether T1 sagittal angle could be used for osteotomy guidelines to restore sagittal balance. Medical records of healthy volunteers in our outpatient clinic from January 2014 to August 2015 were reviewed, and their standing full-spine lateral radiographs were evaluated. Demographic and radiological parameters were collected and analyzed, including age, gender, T1 sagittal angle, maxTK, maxLL, SS, PT, and PI. Correlation coefficients between T1 sagittal angle and other spinopelvic parameters were determined. In addition, multiple regression analysis was performed to establish predictive radiographic parameters for T1 sagittal angle as the primary contributors. A total of 119 healthy volunteers were recruited in our study with a mean age of 34.7 years. It was found that T1 sagittal angle was correlated with maxTK with very good significance (r = 0.697, P<0.001), maxLL with weak significance (r = 0.206, P = 0.024), SS with weak significance (r = 0.237, P = 0.009), PI with very weak significance (r = 0.189, P = 0.039), SVA with moderate significance (r = 0.445, P<0.001), TPA with weak significance (r = 0.207, P = 0.023), and T1SPI with weak significance (r = 0.309, P = 0.001). The result of multiple regression analysis showed that T1 sagittal angle could be predicted by using the following regression equation: T1 sagittal angle = 0.6 * maxTK-0.2 * maxLL + 8. In the healthy population, T1 sagittal angle could be considered as a useful parameter for sagittal balance; however, it could not be thoroughly replaced for SVA. maxTK was the primary contributor to T1 sagittal angle. According to this equation, we could restore sagittal balance by surgically changing thoracic kyphosis and lumbar lordosis, which could serve as a guideline for osteotomy.