What Ape Proximal Femora Tell Us About Femoroacetabular Impingement: A Comparison

Abstract

Background Human hip morphology is variable, and some variations (or hip morphotypes) such as coxa profunda and coxa recta (cam-type hip) are associated with femoroacetabular impingement and the development of osteoarthrosis. Currently, however, this variability is unexplained. A broader perspective with background information on the morphology of the proximal femur of nonhuman apes is lacking. Specifically, no studies exist of nonhuman ape femora that quantify concavity and its variability. Questions/purposes We hypothesized that, when compared with modern humans, the nonhuman apes would show (1) greater proximal femoral concavity; (2) less variability in concavity; and (3) less sexual dimorphism in proximal femoral morphology. Methods Using identical methods, we compared 10 morphological parameters in 375 human femora that are part of the Hamann-Todd collection at the Cleveland Museum of Natural History with 210 nonhuman ape femora that are part of the collection of the Royal Museum for Central Africa, Tervuren, Belgium, and the Muséum National d'Histoire Naturelle, Paris, France. Results The nonhuman apes have larger proximal femoral concavity than modern humans. This morphology is almost uniform without large variability or large differences neither between species nor between sexes. Conclusions Variability is seen in human but not in nonhuman ape proximal femoral morphology. An evolutionary explanation can be that proximal femoral concavity is more important for the nonhuman apes, for example for climbing, than for modern humans, where a lack of concavity may be related to high loading of the hip, for example in running.

Influence of evolution on cam deformity and its impact on biomechanics of the human hip joint

“…Anterosuperior concavity is decreased in humans compared to apes, represented by increased alpha angle (45.6° vs. 29.4°). Further, in humans a larger variation in alpha angles was found than in apes . Decreased posterior concavity but high anterior concavity has been described in quadrupedal runners such as horses.…”

“…This might result in improved survival of the individual into the reproductive stage. The increased risk for osteoarthritis of the hip only manifests much later in life and is not recognized as an influential factor for evolutionary selection …”

Evolution of the human hip. Part 1: the osseous framework

“…But this morphotype, although associated to development of osteoarthritis [ 25 ], is likely not important in an evolutionary sense, i.e. it has no direct effect on evolutionary fitness [ 26 ]. Conversely, low posteroinferior concavity is virtually unknown [ 27 ].…”

“…Some humans (C) have only small concavity anteriorly, others have larger anterior concavity (d), but virtually all humans have large concavity posteriorly (C, D). View is perpendicular to the superior femoral neck, from [ 26 ], with permission. …”

“…The non-human apes are yet another story in this concept. They do not run or walk long distances bipedally [ 29 ], but do need a large range of hip motion in climbing and clambering, facilitated by circumferential high concavity at the head–neck junction [ 26 ].…”