The distributions of the lumbar lordosis, the sacral slope, and the pelvic incidence with regard to gender were not normal. The mean values for women were higher than those obtained for men. No significant difference was detected, on the basis of the numbers available, for the T9 sagittal offset or for the pelvic tilt with regard to gender (Table III). No significant correlation was found, on the basis of the numbers, between the body mass index and any of the angular parameters. No significant correlation was found, on the basis of the numbers, between the subject age and the T9 sagittal offset, the sacral slope, and the pelvic tilt. We found small correlations between age and the following parameters: thoracic kyphosis (r = 0.2), pelvic incidence (r = 0.14), and lumbar lordosis (r = 0.14). When gender was considered in the multivariate analysis, age no longer influenced the thoracic kyphosis, the pelvic incidence, or the lumbar lordosis.
We observed a very strong correlation between the maximum thoracic kyphosis and the T9 sagittal offset, between the maximum lumbar lordosis and the sacral slope (Fig. 3), between the maximum lumbar lordosis and the pelvic incidence, between the sacral slope and the pelvic incidence, and between the pelvic tilt and the pelvic incidence. The correlation matrix among all of the angular parameters is shown in Table IV. Several simple or multiple regression models were then established (Table V). Principal components analysis indicated that 90% of the variation of all of the parameters considered could be explained by three uncorrelated new variables. The first variable was the pelvic incidence, the maximum lumbar lordosis, and the sacral slope; the second one was the thoracic kyphosis; and the third one was the pelvic tilt. A multivariate regression on principal components showed a very good correlation (r = 0.8, R2 = 0.7) between the T9 sagittal offset and the set of the three new variables.
We think that it is important to make radiographs in a standardized fashion. According to Vedantam et al.28, positioning the arms at 90° rather than 30° resulted in a negative shift of the sagittal vertical axis. According to Marks et al.29, shoulder flexion of 45° is the best position to use when a lateral radiograph is made, in order to repeatedly measure the sagittal vertical axis. We believe that it is possible to use these positions in daily practice for the evaluation and treatment of spinal disorders affecting sagittal balance.
Because of the large number of patients in this study, we were able to provide physiological values for the segmental angulations from T9 to S1. We consider these values important in the management of spinal trauma. In a study on the long-term follow-up after treatment of lumbar or thoracolumbar fractures30, the authors stressed the importance of obtaining anatomic realignment in the sagittal plane from a functional point of view. According to that study, the parameter called “traumatic regional angulation” is dependent on the regional kyphosis and on the physiological angulation of the considered segment. The values obtained in the current study should be useful in establishing goals and planning an optimal therapeutic course.
The mean values for lumbar lordosis and sacral slope were different between women and men. Gelb et al.24, Legaye et al.10, and Korovessis et al.7,8 came to the same conclusion. On the contrary, lumbar lordosis as well as thoracic kyphosis were independent of gender in studies on fifty adult volunteers by Jackson et al.3-6. The variations in lumbar lordosis and sacral slope observed in those studies may be explained by a pelvic incidence that was slightly higher in women than in men.
Providing correct sagittal balance by surgical correction of a spinal deformity is of paramount importance. In the short term, it ensures a good position of the fused segment with regard to the gravity plumb line, allowing the best conditions for fusion. In the long term, good sagittal balance facilitates preservation of the adjacent levels16. During et al.1 showed that there is a relationship between the pelvic inclination angle and the sacral slope, between the sacral slope and the lumbar lordosis, and between the pelvisacral angle and the pelvic inclination angle. Legaye et al.10 defined the spatial position of the pelvis by means of the pelvic incidence and the pelvic tilt. In a cohort of thirty-eight volunteers, they demonstrated an interdependence between a complete set of parameters describing the spinal sagittal balance. This interdependence was directly linked to the pelvic incidence, which influences, by means of the sacral slope, the “ideal” lumbar lordosis allowing the maintenance of an efficient standing position. Statistical analysis (principal components analysis) of the data in the present study also showed that a well-balanced position is obtained by means of three uncorrelated factors: the pelvic incidence, the sacral slope, and the lumbar lordosis. This finding confirms the conclusions of During et al.1. Within this group of parameters, the pelvic incidence has a predominant role, since its value is not supposed to vary in an individual. Furthermore, the sacral slope also influences the lumbar lordosis.
The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
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