High assimilation of the sacrum in a sample of American skeletons: Prevalence, pelvic size, and obstetrical and evolutionary implications

Abstract

High assimilation sacrum is fusion of the caudal-most lumbar vertebra to the first sacral vertebra. Previous studies have shown that high assimilation is associated with clinical problems, including obstetrical difficulty. This study used adult American males (n = 1,048) and females (n = 1,038) of the Hamann-Todd and Terry skeletal collections to determine the prevalence of high assimilation and its effect on pelvic size, and to consider the obstetrical and evolutionary implications of high assimilation. The prevalence of high assimilation in this sample is 6.3%, with males and females not differing significantly from one another in their prevalence. This prevalence is near the median for that reported in 41 other samples. In both males and females, individuals with high assimilation have significantly longer anteroposterior and posterior sagittal diameters of the inlet, and shorter sacrum compared to those with a nonassimilated sacrum. Females with high assimilation have a significantly narrower sacral angulation (i.e., reduced inclination of ventral axis of sacrum), and shorter posterior sagittal diameter of the outlet compared to those with a nonassimilated sacrum. A short posterior sagittal diameter of the outlet is associated with childbirth difficulty. As high assimilation is partial homeotic transformation of a lumbar vertebra, this study supports previous research that homeotic transformation of vertebrae is selectively disadvantageous.

Sacralization is not associated with elongated cervical costal process and cervical rib

“…The bodies may or may not be connected to one another by a bony bridge. Fourth, the sacralized lumbar vertebra does not contribute to the auricular surface in the majority of cases (Tague, 2009). In contrast, the first sacral vertebra always contributes to the auricular surface in a nonsacralized sacrum (Scheuer and Black, 2000;Standring, 2005b).…”

“…Rather in this study, the sacrum was evaluated for fusion of the last lumbar and first sacral vertebrae before studying the remainder of the vertebral column. Morphologies used to identify sacralization by fusion included the following (Tague, 2009). First, the body of the sacralized lumbar vertebra is entirely superior to the arcuate line, whereas in the absence of sacralization the body of the first sacral vertebra is only partly superior to this line (see earlier).…”

“…1; Moore, 1924;Cushway and Maier, 1929;Young, 1951;Schinz et al, 1952;Allbrook, 1955;Elster, 1989). Studies differ on whether there is a sexual difference in prevalence of sacralization (Williams, 1938;Magora and Schwartz, 1978;Leboeuf et al, 1989;Erken et al, 2002;Tague, 2009).…”

“…The prevalence of sacralization in adults ranges from 2% to 28.4% with a median of 7.4%, based on 29 samples reported in the literature in which the samples consisted of individuals who did not have back or leg pain, or were obstetrical patients, or were skeletonized (Paterson, 1893;Moore, 1924;Cushway and Maier, 1929;Giles, 1931;Thoms and Greulich, 1940;Young and Ince, 1940;Allen and Lindem, 1950;Southworth and Bersack, 1950;Colcher and Hursh, 1952;Splithoff, 1953;Andrew, 1954;Hult, 1954;Runge, 1954;Allbrook, 1955;Berger and Stoll, 1965;Leverett, 1966;LaRocca and Macnab, 1969;Tilley, 1970;Tini et al, 1977;Magora and Schwartz, 1978;Bustami, 1989;Otani et al, 2001;Santiago et al, 2001;Kamanli and Genc, 2002;Masnicová and Beňuš, 2003;Steinberg et al, 2003;Tague, 2009). Erken et al (2002) reported a prevalence of 35.9%.…”

Analyse anthropométrique du bassin obstétrical datant du Néolithique : conséquences obstétricales. Étude préliminaire

“…2 -5) [6,7]. Lorsque les bassins é taient incomplets, nous n'avons pas fait de symé trisation artificielle afin d'é viter un biais d'interpré tation.…”