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

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 degrees ) decreased sacral-slope and the lordosis is flattened. A high incidence (>62 degrees ) 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 degrees with the 4-item model; +/-1.73 and 3.62 degrees 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.

Fig. 1

The significant (S***, p<0.0001) chain of correlations between the positional pelvic and spinal parameters and the morphological pelvic incidence

Fig. 2

Flattened spinal curve with small incidence (38.2°), hollow spinal curve with great incidence (70.4°)

Fig. 3

The measured lordosis is strongly correlated with the predicted lordosis

Fig. 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)

Fig. 5

The positional (and non-morphological) parameters