The Finite Element Method (FEM) was developed within the framework of Engineering but has become a popular tool in bio-mechanical studies. It is natural that computational bio-mechanics and Finite Element Analysis (FEA) became increasingly promising in fossil studies where there are no examples of some taxa still living. To study the bio-mechanical responses of fossil hominids, modern humans and non-human primates are often used as comparative samples for which there are already known values. Despite this, precisely how accurately computational bio-mechanics compares with physical studies is still not well understood. The biological composition of bone and dentition is hard to replicate in computational terms with the cranium a mixture of trabecular and cortical bone while teeth comprise variable layers of enamel and dentine. The resolution required from Computed Tomography (CT) scans to accurately capture these finer biological compositions is not feasible for the heavy demands on software to analyze such FEA models with flow-effects for the number of specimens that can be included into any single study.
Godinho et al investigated the validity and sensitivity of Finite Element (FE) models using a direct comparison with a human cadaver. Results were particularly affected if the model was simplified by assigning all materials as cortical bone, including dentition and trabecular bone components. Results showed that the real and virtual skull showe d no differences in strain magnitude; differences in strain pattern (high or low strain distribution) were only partially different; simplifying the virtual model decreased the strain magnitude; simplifying the virtual model partially affected the strain pattern with the regions near the dentition, particularly the alveolar ridge, most affected.
For bio-mechanical studies, by not simplifying virtual models and attempting to designate dental and bone tissues properly acknowledges the underpinning biology of the cranium while potentially revealing sensitive adaptations of this biological structure. By adopting these changes, new variations between living and fossil humans, that have so-far been obscured by less time-consuming computational methods, could reveal unique adaptational trends that have real significance for human evolution.
A surface analysis on the frontal lobes in archaic humans …
[A post] [The Paper]
As the use of virtual anatomy increases, awareness of different 3D mesh (digital model) formats is useful. Simply, a 3D mesh is the geometrical representation of an anatomical structure such as a cranium, endocast or tooth. Below, are the main differences, benefits, and uses of common PLY and STL 3D formats.
PLY (Stanford Polygon Format) is a 3D file format that was commonly developed from 3D surface scanners and photogrammetry software to allow the preservation of information on surface geometry while retaining information on RGB colour. STL (Stereolithography) is a 3D format commonly generated from software using only grayscale images such as raw CT (Computed Tomography) where RGB colour is not captured. 3D Printers only require preserved information on surface geometry, not colour, leaving STL to be a preferred format for 3D printing technologies.
Even though there is no discernible difference between the quality of the 3D mesh types, PLY format offers binary encoding of all information (including RGB colour). This results in a smaller file size, allowing less space occupied on a hard-drive or cloud-storage and faster loading of the 3D mesh into software programs as employed in 3D-geometric morphometrics.
So far, the majority of new quantitative methods and approaches to investigate sexual dimorphism have focused primarily on the morphology of the five most commonly studied sexually dimorphic traits of the skull (glabella, orbital margins, mastoid process, nuchal crest, and mental eminence), while other cranial traits are still being evaluated in terms of simple subjective descriptions. One of the cranial regions showing great potential for further development of sex estimation techniques is the frontal region. Recently, Bulut and colleagues quantified the shape differences between male and female frontal bones using a novel and landmark-free 3D modeling method. Their new finding that the male frontal bone is actually more spherical than the female is in opposition to the common perception. In their study, CT scans of 80 male and 80 female Caucasian frontal bones from a Turkish population between the age of 25 and 40 years were obtained. The frontal bone was isolated by carrying out the “selection tool” in the GOM Inspect software using STL models. The frontal bone model is aligned to the CAD sphere model, using the best-fit registration method in the GOM Inspect software. Next, the difference in surface morphology between the frontal bone data and the CAD sphere was quantified, using the sphere model as the reference surface. Also, color maps were generated to show the deviations between frontal bone surface and the CAD sphere surface. Deviations of ±1mm were calculated as the overlapping areas. Color maps show that, for males, the areas exhibiting the largest discrepancy between frontal bone and CAD sphere surface are glabella, the supraorbital margins, the zygomatic processes, the superciliary arc, and the temporal face.
The area displaying most overlap with the sphere is the upper frontal region, including the frontal squama and the frontal eminences. For females, the frontal squama showed the main congruence with the sphere surface, while the largest deviations were observed for glabella, the supraorbital margins, the zygomatic processes, the superciliary arch, the frontal eminences, and the temporal face. The amount of frontal bone overlapping with the sphere was found to range from 30.1% to 56.1% for males, and from 19.6% to 48.3% for females. The difference in average values between males (43.2 ± 6.5%) and females (33.9 ± 6.6%) was found to be statistically significant, i.e. p < 0.0001, using the double-sided version of student’s t-test. This finding is in opposition to the common perception that the male frontal bone is more inclined than the female, which is described as more vertical and rounded, convex, smooth, and broad. Using the overlapping surface parameter to develop linear discriminant functions, sex was accurately predicted for 61 of 80 females (76.3%) and 63 of 80 males (78.8%) after leave-one-out cross-validation, yielding an average of 77.5% correct classifications.
Sex assessment is crucial in any survey on human remains. Musilová et al, have recently published a new method for sexual identification using virtual scans of both male and female individuals. They found that the size of the cranial surface was significantly different between both sexes, being the male skulls larger than the females in some areas, such as the nasal root, external occipital protuberance and mastoids. The most pronounced areas with sexual cranial differences are those linked to muscle attachment, such as supraorbital, frontal and nuchal regions. Sexual dimorphism was significantly lower in senile skulls. This article provides a new and successful method using 3D techniques and geometric morphometrics, interesting for different applications in anthropology.
Gizéh Rangel de Lázaro
The Rhoton Collection is composed by an outstanding anatomical presentations of the brain created by the renowned surgeon and educator Dr. Albert Rhoton Jr throughout his life. These presentations were made using bright blue and red dyes in the blood vessels, so that surgeons could easily visualize and explore the brain and vascular structures for planning surgical interventions.
[Here a post in Spanish]
Gizéh Rangel de Lázaro
Dimitri Neaux and colleagues have published a series of comprehensive analyses on the influence of the cranial base in facial morphology of humans and apes. In one of the papers, they assessed the integration between the face and the two basicranial modules: the sagittal and the lateral cranial base. They tested the covariation between the three sets of 3D landmarks (face vs. midline base and face vs. lateral base) on modern humans and chimpanzees, separately. Only the correlation between the face and the lateral cranial base was significant, confirming the important role of the lateral cranial base in facial morphology. Though the levels of covariation were comparable, the patterns differed between the two species, as taller faces were associated with wider and shorter cranial fossae in chimps and with longer and narrower cranial fossae in humans. In another article, they assessed the relationship between cranial base flexion, facial orientation, and facial shape in modern humans, chimpanzees, and gorillas. Using 3D landmark analysis, they evaluated the within-species patterns of covariation, confirming the intraspecific relationship between facial structures and base flexion. Base flexion is associated with downward rotation of the facial block in both humans and chimps (confirming previous works) but not in gorillas. On the other hand, an upward rotation of the facial block is associated with anterior face vertical elongation on the three species. In humans, facial elongation is also associated with base flexion, which might have been selected during evolution to match the elongation of the nasomaxillary complex, as proposed before. The authors further tested whether increased base flexion is associated with the shortening of the facial length or with the decrease of facial projection. The relationship between base flexion and facial length was only observed in chimps, while facial projection was not related with cranial base flexion in chimpanzees and gorillas. In humans, contrary to what was expected, basicranial flexion was associated with increasing facial projection, which the authors attribute to sexual dimorphism, as males have increased base flexion and facial projection. Again, the main patterns of correlation differed between the species. Cranial base angle is negatively correlated with facial projection in modern humans, with facial length in chimps, and with the angle between the posterior-maxillary plane and the anterior facial plane in gorillas. As the authors conclude, these differences in the patterns of integration might reflect changes in the structural relationships between the face and the cranial base during hominoid evolution.
Mask of Xiutecuhlti, god of fire; 1325-1521 CE, Aztec-Mixtec, Mexico
[Museum of Artifacts]