[Two key describers of the sacro-acetabular relationships: the angles of sacral and acetabular incidence].

Abstract Implantation of total hip arthroplasties raises several important questions concerning the relationship between the orientation of the lumbosacral joint and the acetabular-femoral joint; in other words, between the position of the patients trunk and the orientation of the acetabula. To elucidate better these complex relationships, we conducted a morphometry study on a sample of 51 dry pelves: pelv 26 female and 25 male specimens. Three-dimensional coordinates of 47 homologous points were recorded for each pelvis. Data were then processed with De-Visu, a graphic visualization software. Seven parameters were compared: sacral slope, sacral incidence, and five parameters quantifying the three-dimensional orientation of the acetabula. The graphic modelization enabled an integral 3-D visualization of each pelvis. The sagittal view enabled simultaneous visualization of the sacrum, the sacroiliac joints, the acetabula, and their alignments, as well as the variability of their spatial relation. The position reference chosen to simulate the upright position aligned the anterior iliac spines and the superior pubic point. This position was found pertinent because the mean value of the sacral slope (41.8) and the sacral incidence (54) were not different from published series. The sacral slope was the most strongly correlated with the acetabular parameters. It exhibited a positive correlation with sagittal acetabular slope (r=0.59) and acetabular inclination (r=0.59). It exhibited a negative correlation with acetabular anteversion (R=0.45). The correlation with the sagittal acetabular slope was very strong for anteversion (r=0.92), and rose with acetabular inclination (r=-0.66). The angle formed by the two acetabular axes was highly variable (37). The correlation between this angle and inclination was very high in males (r=-0.88) and non-significant in females. There was however a very strong correlation with anteversion in females (r=-0.74) which was non-significant in males. This contrasting finding was related to the wide spread of the inclination values in males and anteversion values in females. We demonstrated a new sagittal parameter: the acetabular incidence. The summit of this angle is the center of the acetabulum. The sides are the pelvic thickness and the acetabular axes. This parameter was negatively correlated with the sacral incidence. It account simultaneously for the sagittal position of the sacrum in relation to the acetabula and for the degree of acetabular anteversion and inclination. We have demonstrated that the geometric sum of these two angles, sacral incidence and acetabular incidence, is equivalent to the sacro-acetabular angle demonstrated by Lazennec and Saillant. These authors showed that the sacro-acetabular angle is the sum of two positional parameters, the sacral slope and the sagittal acetabular tilt (or slope). The three angles -- sacral incidence, acetabular incidence, sacro-acetabular angle -- are anatomic angles which do not vary with the pelvic position.