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Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis

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Abstract

Pelvis and spinal curves were studied with an angular parameter typical of pelvis morphology: pelvic incidence. A significant chain of correlations between positional pelvic and spinal parameters and incidence is known. This study investigated standards of incidence and a predictive equation of lordosis from selective pelvic and spinal individual parameters. One hundred and forty nine (78 men and 71 women) healthy adults, aged 19–50 years, with no spinal disorders, were included and had a full-spine lateral X-ray in a standardised upright position. Computerised technology was used for the measurement of angular parameters. Mean-deviation section of each parameter and Pearson correlation test were calculated. A multivariate selection algorithm was running with the lordosis (predicted variable) and the other spinal and pelvic parameters (predictor variables), to determine the best sets of predictors to include in the model. A low incidence (<44°) decreased sacral-slope and the lordosis is flattened. A high incidence (>62°) increased sacral-slope and the lordosis is more pronounced. Lordosis predictive equation is based on incidence, kyphosis, sacral-slope and ±T9 tilt. The confidence limits and the residuals (the difference between measured and predicted lordosis) assessed the predicted lordosis accuracy of the model: respectively, ±1.65 and 2.41° with the 4-item model; ±1.73 and 3.62° with the 3-item model. The ability of the functional spine-pelvis unit to search for a sagittal balance depended both on the incidence and on the variation section of the other positional parameters. Incidence gave an adaptation potential at two levels of positional compensation: overlying state (kyphosis, T9 tilt), underlying state (sacral slope, pelvic tilt). The biomechanical and clinical conditions of the standing posture (as in scoliosis, low back pain, spondylisthesis, spine surgery, obesity and postural impairments) can be studied by comparing the measured lordosis with the predicted lordosis.

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Acknowledgements

We would like to thank some colleagues for their scientific contribution (C. Charbonnier, P. Fender, F. Montagnon, J. Sengler, MD) and their valuable advice (J. Dubousset, Ph D and MD). We would like to thank the Montpellier E.R.R.F. Association for their grant and contribution in the project.

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Correspondence to C. Boulay.

Appendices

Appendix

Morphologic pelvic parameters

Morphologic parameters [8, 9, 19] do not vary according to the position of the pelvis in space and they are constant for each subject (Fig.4).

Fig. 4
figure 4

Geometric relationship between pelvic incidence, pelvic tilt and sacral slope. Pelvic tilt is defined by (1) the line through midpoint of the sacral plate and midpoint of the femoral heads axis, and (2) the vertical. Sacral slope is defined as the angle between the sacral plate and a horizontal line. Pelvic incidence is defined as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the femoral heads axis. “Pelvic incidence=pelvic tilt+sacral slope” because they are formed by lines perpendicular to each other (the horizontal and the vertical, and the lines parallel and perpendicular to the sacral plate)

Pelvic incidence

Pelvic incidence is defined as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the femoral heads axis. A geometric construction by complementary angles reveals that the morphological parameter “pelvic incidence” is the algebraic sum of the “sacral slope” and the “pelvic tilting”: pelvic incidence=sacral slope+pelvic tilting (Fig.4).

Positional parameters

Positional parameters [8, 9, 19] vary according to the position of the pelvis in space (Fig.5).

Fig. 5
figure 5

The positional (and non-morphological) parameters

Spinal parameters

Kyphosis is the angle measured between the vertebrae the most forward tilted in the sagittal plane as defined by the two lines forming the kyphotic angle. Lordosis is the angle measured between the vertebrae the most backward tilted in the sagittal plane as defined by the two lines forming the lordotic angle.

The T9 tilt is the angle between the lines linking the midpoint of the femoral heads axis with the centre of the T9 vertebra body, and the vertical crossing the midpoint of the femoral heads axis. The tilt is positive when the centre of the T9 vertebral body is projected posterior to the midpoint of the femoral heads axis and vice versa.

The L1 tilt is the angle between the lines linking the midpoint of the upper endplate of the L1 vertebral body with the midpoint of the lower endplate of the L5 vertebral body, and the vertical. The tilt is positive when the centre of the upper L1 endplate is projected posterior to the midpoint of the lower L5 endplate, and vice versa.

Pelvic parameters

Sacral slope is defined as the angle between the sacral plate and a horizontal line. A vertical sacrum is described by a low value of sacral slope, a horizontal sacrum by a high value of sacral slope.

Pelvic tilt is defined by (1) the line through midpoint of the sacral plate and midpoint of the femoral heads axis, and (2) the vertical. Pelvic tilting is positive when the sacral plate is behind the hip and negative when it is in front of it.

Overhang is the geometrical range between the middle of the upper plate of the first sacral vertebra (S1) and the bi-coxo-femoral axis, measured in millimetres. It is positive when the sacral plate is behind the hip and negative when it is in front of it.

Relation between positional and morphologic parameters

A geometric construction by complementary angles reveals that the morphological parameter “Incidence” is the algebraic sum of the “sacral slope” and the “pelvic tilt”: Incidence=sacral slope+pelvic tilt (Fig.4) [8, 9, 19].

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Boulay, C., Tardieu, C., Hecquet, J. et al. Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis. Eur Spine J 15, 415–422 (2006). https://doi.org/10.1007/s00586-005-0984-5

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