Deux descripteurs clé des relations sacro-cotyloïdiennes : les angles d’incidence sacrée et cotyloïde: Mise en évidence par le logiciel De-Visu

RésuméLa pose des prothèses totales de hanches soulève des questions importantes concernant les relations entre l’orientation de l’articulation lombo-sacrée et celle des articulations coxo-fémorales, c’est-à-dire entre position du tronc du patient et orientation tridimensionnelle de ses cotyles. Pour élucider ces relations complexes, nous avons mené une étude morphométrique sur un échantillon de 50 bassins secs. Après la saisie des coordonnées tridimensionnelles de 47 points homologues sur chaque bassin, nous avons traité ces données grâce au logiciel de modelage statistique et visualisation graphique du bassin, le logiciel De-Visu. Nous avons mis en évidence un nouveau paramètre pelvien sagittal, que nous avons appelé « angle d’incidence cotyloïde ». Cet angle a pour sommet le centre des cotyles et pour côtés l’épaisseur pelvienne et la médiane des axes des cotyles. Il est corrélé négativement avec l’angle d’incidence sacrée, avec lequel il partage un côté commun, l’épaisseur. Il rend compte simultanément de la position sagittale du sacrum par rapport aux cotyles et du degré d’antéversion et d’inclinaison des cotyles. Nous montrons que la somme géométrique de ces deux angles, incidence sacrée et incidence cotyloïde, est égale à l’angle sacro-acétabulaire mis en évidence par Lazennec et Saillant. Ces auteurs avaient montré que l’angle sacroacétabulaire est la somme de deux paramètres positionnels, la pente sacrée et la bascule acétabulaire sagittale. Les trois angles – incidence sacrée, incidence cotyloïde et angle sacro-acétabulaire – sont, eux, des angles anatomiques qui ne dépendent pas de la position du bassin.

SummaryPurpose of the studyImplantation 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.Material et methodsThree-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.ResultsThe 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 (41.8̊) and the sacral incidence (54̊) 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 (37̊). 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.DiscussionWe 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.