IntroductionSagittal spinal morphology varies from one individual to the another and is specific to each person. Classifications proposed in the literature are based on the measurements of curvatures, angulations or other parameters, chiefly for a normal population [1-3, 7, 11, 13, 15-17]. They are purely descriptive rather than in any way analytic.Modifications of sagittal spinal curvatures have been connected with changes in pelvic orientation [7,11,12,16,17]. Nevertheless, the role of pelvic anatomy in this positioning has never been investigated. A pelvic anatomic parameter named pelvic incidence is proposed in this study as the key factor for this sagittal balance regulation. Spinal balance is conceived as the result of an optimal lordotic positioning of the vertebrae above a correctly oriented pelvis. "Optimal lordotic positioning" and "correctly oriented pelvis" values are defined in this study, performed on a normal and a scoliotic population. Materials and methods MaterialsThe data of two population groups were analysed. The first group comprised a normal population of 49 adults free of vertebral disease. There were 21 women and 28 men, with a mean age of 24 years (SD 5.8 years; range 19-50 years). Mean height was 173 cm (SD 8 cm): 177 cm (SD 7 cm) for the men and 161 cm (SD 8 cm) for the women). Mean weight was 65.8 kg (SD 10 kg): 71 kg (SD 8.6 kg) for the men and 59.2 kg (SD 7.7 kg) for the women. These 49 adults constituted the control group for assessing the relationship between spine and pelvis for a normal population. They consisted of volunteer medical and physical therapy students.The second group comprised 66 adult scoliotic women, with a mean age of 33 years (SD 19.8 years; range 10-84 years), and a mean height of 161 cm (SD 8 cm) and a mean weight of 55 kg (SD 8 kg). The frontal curves were lumbar in 24 cases and thoracolumbar in 42 cases. MethodsThe anatomical parameters (vertebral curvatures and pelvic parameters) were measured on orthogonal plain radiographs in standAbstract This paper proposes an anatomical parameter, the pelvic incidence, as the key factor for managing the spinal balance. Pelvic and spinal sagittal parameters were investigated for normal and scoliotic adult subjects. The relation between pelvic orientation, and spinal sagittal balance was examined by statistical analysis. A close relationship was observed, for both normal and scoliotic subjects, between the anatomical parameter of pelvic incidence and the sacral slope, which strongly determines lumbar lordosis. Taking into account the Cobb angle and the apical vertebral rotation confers a three-dimensional aspect to this chain of relations between pelvis and spine. A predictive equation of lordosis is postulated. The pelvic incidence appears to be the main axis of the sagittal balance of the spine. It controls spinal curves in accordance with the adaptability of the other parameters.

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 sacralslope 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°w ith 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.

The anatomic pelvic parameter "incidence" -the angle between the line perpendicular to the middle of the sacral plate and the line joining the middle of the sacral plate to the center of the bicoxofemoral axis -has been shown to be strongly correlated with the sacral slope and lumbar lordosis, and ensures the individual an economical standing position. It is important for determining the sagittal curve of the spine. The angle of incidence has also been shown to depend partly on the sagittal anatomy of sacrum, which is established in childhood while learning to stand and walk. The purpose of this study was (1) to define the relationship between the sacrum and the angle of incidence, and (2) to compare these parameters in three populations: young adults, infants before walking, and patients with spondylolisthesis. Forty-four normal young adults, 32 infants not yet walking and 39 patients with spondylolisthesis due to isthmic spondylolysis underwent a sagittal full-spine radiography. A graphic table and the software for bidimensional study of the sacrum developed by J. Hecquet were used to determine various anatomic and positional parameters. Comparison tests of means, and multiple and partial correlation tests were used. A study of

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The aim of this study was to assess pelvic asymmetry (i.e. to determine whether the right iliac bone and the right part of the sacrum are mirror images of the left), both quantitatively and qualitatively, using three-dimensional measurements. Pelvic symmetry was described osteologically using a common reference coordinate system for a large sample of pelvises. Landmarks were established on 12 anatomical specimens with an electromagnetic Fastrak system. Seventy-one paired variables were tested with a paired t -test and a non-parametric test (Wilcoxon). A Pearson correlation matrix between the right and left values of the same variable was applied exclusively to values that were significantly asymmetric in order to calculate a dimensionless asymmetry index, ABGi, for each variable. Fifteen variables were significantly asymmetric and correlated with the right vs. left sides for the following anatomical regions: sacrum, iliac blades, iliac width, acetabulum and the superior lunate surface of the acetabulum. ABGi values above a threshold of ± 4.8% were considered significantly asymmetric in seven variables of the pelvic area. Total asymmetry involving the right and the left pelvis seems to follow a spiral path in the pelvis; in the upper part, the iliac blades rotate clockwise, and in the lower part, the pubic symphysis rotates anticlockwise. Thus, pelvic asymmetry may be evaluated in clinical examinations by measuring iliac crest orientation.

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IntroductionThe interdependence between the pelvis and the sagittal spinal curvatures is obvious. The leading part of the pelvic sagittal anatomy in this orientation was well established by Duval-Beaupe`re thanks to the description of the angle 'Pelvic Incidence' (PI) [14]. It is the angle between the line perpendicular to the superior plate of the first sacral vertebra (S1) at its midpoint and the line connecting this point to the middle axis of the femoral heads. It is own for every individual and independent of the position of pelvis. A strict relation was described between this anatomical parameter and the sagittal tilt of the superior plate of the sacrum, and between this sacral tilt and the amount of lumbar lordosis. So the pelvic morphology modulates the sagittal spinal alignment. During and Itoi [1,7] described also these relations using an angle complementary of the incidence. As well Jackson observed this relation between the pelvic morphology, the sacral inclination and the lumbar curvature. He expressed the pelvic sagittal shape by the 'Pelvic Lordosis' (PR-S1) [8][9][10][11][12]. The measurement of these angles is easy in most of the cases. Their reliability and their reproducibility were established. Each author agrees to use as femoral axis the middle point of the line connecting the centers of the two femoral heads.However, the exact assessment of the upper plate of the sacrum (and consequently the values of the anatomical Jean LegayeThe femoro-sacral posterior angle: an anatomical sagittal pelvic parameter usable with dome-shaped sacrum Abstract The sagittal pelvic morphology modulates the individual alignment of the spine.

The Anterior Pelvic Plane (APP), defined by the anterior superior iliac spines and the pubic tubercle, was commonly used as reference for positioning and postoperative evaluation of the orientation of the acetabular cup in total hip arthroplasty. APP was assumed to be vertical, but was not observed always so, mostly because of associated spinal diseases inducing perturbations in the harmony of the sagittal balance of the pelvi-spinal unit. Consequently a sagittal rotation of the pelvis occurs, and so a tilt of the APP which alters directly the orientation of the cup in upright position. An analysis of the APP tilt related to the sagittal balance of the spine was provided and its implication on the cup orientation. It appeared essential for an individual adjustment of the cup positioning to avoid a functional malposition which can lead to an increased risk of dislocation and impingement.Résumé Le plan antérieur pelvien (APP) définit par les crêtes iliaques et le pubis est communément appelé plan de référence pour le positionnement et l'orientation de la cupule acétabulaire dans les prothèses totales de hanche. Ce plan peut être modifié par la position verticale et du fait de pathologies associées au niveau de la colonne vertébrale entraînant une perturbation des courbures et de la balance pelvienne. En conséquence, une rotation sagittale du pelvis peut survenir avec conséquences sur le plan pelvien antérieur APP. Ceci peut avoir des conséquences directes sur l'orientation de la cupule en position debout. Une analyse de ce plan pelvien antérieur APP et des modifications entraînées par la balance pelvienne sont indispensables. Il apparaît essentiel d'ajuster le positionnement de la cupule de façon à éviter une mal position qui peut être responsable d'un risque accru de luxations ou de conflits.