Variable positions of the sacral auricular surface: classification and importance.

OBJECT: Although the area at the auricular surface defines the magnitude of weight transmission to the hip bones, this study proposes that the position of the auricular surfaces may also significantly influence load bearing patterns at the sacrum. This study attempts to investigate and classify variable positions of the auricular surfaces that may cause vertical shifts in weight-bearing patterns between the L-5 and S-1 segments, altering weight distribution at the lumbosacral and sacroiliac regions.
METHODS: Three hundred human sacra were studied to determine the position and extent of their auricular surfaces in relation to the sacral segments. Specimens were grouped as "normal," "high-up," and "low-down" auricular surface-bearing sacra. All bones were also scrutinized for the presence of accessory articulating facets on the ala of the sacrum and sacralization of the L-5 segment or lumbarization of the S-1 segment. Seven dimensions and 5 articular areas were measured in all sacra. Nine indices were calculated to show proportional representation of dimensions and areas in the bones. Obtained data were analyzed for differences in groups of sacra bearing different auricular surface positions.
RESULTS: Thirty-nine of the sacra (13%) showed auricular surfaces that occupied a high-up position (from upper S-1 to low S-2 segments). Forty-four of the sacra (15%) exhibited a low-down auricular surface (from the low S-1 to low S-3 sacral segments). The remaining bones demonstrated a normal position of the surface (from the S-1 to the middle of the S-3 segments). Twenty of the high-up sacra demonstrated unilateral or bilateral accessory articulating facets on the alae that articulated with extended transverse processes of the L-5 vertebrae. The low-down sacra transmitted load predominantly via lower (S2-3) segments and exhibited stouter, broader, and efficient weight-bearing lower sacral elements, and a prominent gap between the S-1 segment and the rest of the sacrum. The high-up sacra: 1) were shorter and broader in comparison with the normal sacra; 2) at times presented accessory articular facets on their alae; 3) had a smaller body span and a wider ala; 4) were found to have the plane of the facet joints nearer to the posterior aspect of the S-1 body; and 5) had the smallest of the facet areas. The low-down sacra were longer than they were broad, had a substantially broad body span at S-1, possessed the smallest interauricular distance, and showed considerable depth of the plane of the facet joints.
CONCLUSIONS: The position of the auricular surface varies in human sacra. These variations are associated with differential load bearing at the sacral joints. Only the high-up sacra demonstrated the presence of accessory articulating facets between L-5 and S-1. The position of the auricular surface can explain or possibly predict low-back pain situations.