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

The importance of the sacral pelvic incidence (SPI) in relation to individual variations of sagittal spinal curvature has become well-recognised. We attempted to determine the relationship between SPI and acetabular orientation. The three-dimensional coordinates of 47 homologous points were observed on 51 adult anatomical pelvises (26 female and 25 male). The reference vertical plane was Lewinnek's anterior pelvic plane. 10 angular parameters and 11 linear parameters were defined and calculated. These were expressed both in absolute value (in millimetres) and in "acetabular unit" (relative to the mean value of the right and left acetabular rays). Mean values of the parameters were calculated for all pelvises and according to gender. There were two dominant parameters: the "sacral slope" and the "V pubic angle". "Acetabular tilting" was primarily dependent on the "sacral slope" and its intermediary on the "SPI", while "acetabular anteversion" dependent on the "V pubic angle" via the "angle of prow". It is recommended that positioning of the acetabular cup in total hip arthroplasty relates to anatomical parameters, and to the global sagittal balance of the pelvi-spinal unit.

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