We examined the morphology of mammalian hips asking whether evolution can explain the morphology of impingement in human hips. We describe two stereotypical mammalian hips, coxa recta and coxa rotunda. Coxa recta is characterised by a straight or aspherical section on the femoral head or head-neck junction. It is a sturdy hip seen mostly in runners and jumpers. Coxa rotunda has a round femoral head with ample head-neck offset, and is seen mostly in climbers and swimmers. Hominid evolution offers an explanation for the variants in hip morphology associated with impingement. The evolutionary conflict between upright gait and the birth of a large-brained fetus is expressed in the female pelvis and hip, and can explain pincer impingement in a coxa profunda. In the male hip, evolution can explain cam impingement in coxa recta as an adaptation for running.

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.

Using 15 parameters, we provide a systematic description of mammal proximal femoral morphology. We established two types of proximal femoral morphology, termed coxa recta and coxa rotunda, characterized by low versus high concavity of the head-neck junction. Concavity is a measure of the sphericity of the femoral head as it meets the femoral neck that can be quantified by angular measurements. We asked whether the parameter of concavity corresponds with the classification of mammal proximal femoral morphology based on coalesced versus separate ossification patterns and locomotor patterns. Statistical analysis demonstrated a distinction between coxa recta and coxa rotunda with significant differences between the two groups in all but 3 of the 15 parameters examined. We found the most discriminating measurement between mammal hips to be the concavity of the posterior head-neck junction (beta angle). Coxa recta (small concavity) and coxa rotunda (large concavity) relate to the ossification pattern seen in proximal femoral development, and species-specific patterns of locomotion. We interpret the two hip types to reflect optimization for strength (recta) versus mobility (rotunda). Conceptually, both hip types can be recognized in humans, where coxa recta can be related to the development of osteoarthritis.

Morphology variants of the hip such as coxa recta (aspherical femoral head and/or reduced head-neck concavity) or coxa profunda (overcoverage, or "deep" socket) are associated with cam and pincer impingement respectively, and may ultimately lead to coxarthrosis. Several population studies have documented the prevalence of hip morphotypes, but few studies have examined this prevalence in total hip arthroplasty (THA) patients, or persons without hip symptoms or signs.
We reviewed whether coxa recta and profunda morphotypes were more prevalent in THA patients compared to normal controls. Further, we explored differences in hip abductor mechanism related to hip morphology.
We examined 113 THA patients and 83 normal controls with anteroposterior pelvic and lateral hip 
radiographs. Coxa recta and profunda were classified with alpha and lateral CE-angle, respectively. The abductor ratio (AR) was measured on AP pelvic radiographs.
Both coxa recta and profunda were more prevalent in THA patients than normal controls (coxa recta: male 42% vs 8%, female 15% vs 5%, respectively and coxa profunda: male 20% vs 1%, female 19% vs 6% respectively). AR was higher in females than males, both in THA patients (1.66 vs 1.47) and normal controls (1.68 vs 1.57). Coxa profunda was associated with a lower AR (1.54 vs 1.61).
The higher prevalence of coxa recta and profunda in arthroplasty patients supports the theory of a role of these morphotypes in the development of coxarthrosis. The higher AR in females signifies the need for increased abductor work. Coxa profunda may be an adaptation to lower the AR.

Taking costs into account, the percutaneous calcaneal tunnel technique and the open technique are the methods of choice.