Systemic patterns of trabecular bone across the human and chimpanzee skeleton

Functional morphology of trabecular system in human proximal femur: a perspective from P45 sectional plastination and 3D reconstruction finite element analysis

BACKGROUND

The trabecular architecture of proximal femur plays a crucial role in hip stability and load distribution and is often ignored in hip fracture fixation due to limited anatomical knowledge. This study analyses trabecular morphology and stress distribution, aiming to provide an anatomical foundation for optimising implant designs.

MATERIALS AND METHODS

Twenty-one formalin-fixed human pelvises (twelve male, nine female) were prepared using P45 sectional plastination. They were sliced into 3 mm sections in the coronal, sagittal, and horizontal planes and then photographed. A 3D femur model was created from computed tomographic scans and analysed for finite element analysis (FEA) using Mimics, 3-matics, and Abaqus software to simulate static and dynamic loads, visualising stress paths for compressive and tensile regions and identifying fracture-vulnerable zones.

RESULTS

Two main trabecular systems were identified: the medial and lateral systems. The medial system includes a primary vertical trabecular group extending from the femoral shaft's medial calcar to the head and two primary horizontal groups arching from the lateral shaft, greater trochanter, and femoral neck's anterolateral and posterolateral walls toward the medial side, intersecting with the primary vertical group in the head. Secondary vertical group intersects with secondary horizontal group at the neck-trochanteric junction to form the lateral system. FEA showed peak compressive stress along the vertical groups, calcar, and medial cortex, and tensile stress along the horizontal groups, greater trochanter, and lateral cortex, creating balanced support that stabilises the femoral neck and shaft.

CONCLUSION

The strength of proximal femur depends on dense cortical bone, calcar femorale, lateral and medial trabecular systems, and greater trochanter. While anterolateral and posterolateral areas of femoral neck and intertrochanteric regions are potential weak zones. Trabecular pattern follows stress paths, optimising load distribution. These insights aid in designing robotic and bionic implants that mimic natural stress patterns, reducing complications.

The Ontogeny of the Human Calcaneus: Insights From Morphological and Trabecular Changes During Postnatal Growth

OBJECTIVES

To investigate the developmental changes in the human calcaneal internal and external morphology linked to the acquisition of mature bipedal locomotion.

METHODS

Seventy seven micro-CT scans of modern juvenile calcanei (from perinates to 15 years old) are employed. The chronological period spans from the Middle/Late Neolithic (4800-4500 BCE) to the 20th century. Through a comprehensive approach that comprises geometric morphometric methods and whole-bone trabecular analysis, the calcaneal growing morphology has been explored.

RESULTS

Morphological changes reflect the development of bipedal locomotion, showing its potential when tracking the major locomotor milestones. The calcaneal shape is immature and almost featureless during the first year of life. The internal architecture is dense and isotropic with numerous thin trabeculae closely packed together. The internal architecture changes to better adapt to variations in load stimulated by a more mature gait by increasing bone mass and alignment, with fewer and thicker struts. The external morphology shows its plasticity by increasing the surface area where greater strain is expected and changing the orientation of the articular facets.

CONCLUSIONS

Analysis of morphological changes in the growing calcaneus highlights the importance of an integrative methodology when exploring developmental bone plasticity. The changes in calcaneal internal and external morphologies reflect the different loading patterns experienced during growth, gradually shifting from a more generalized morphology to a more adult-like one, reflecting major locomotor achievement. Our research shows that although initially genetically driven, calcaneal plasticity may display mechanical influences and provide precious information on tracking the main locomotor milestones.

The Virtual Database of the Documented Human Osteological Collection (DHOC) of the Certosa Cemetery of Bologna (Italy, 19th-20th Century)

This article aims to introduce a new virtual database of skeletal human remains from the Documented Human Osteological Collection (DHOC) of the Certosa Cemetery of Bologna (Emilia Romagna, northern Italy) housed at the University of Bologna. The Virtual DHOC of the Certosa Cemetery of Bologna (VirtualDHOC) is stored in the publicly accessible 3D data repository MorphoSource, and consists of 3D models or micro-Computer Tomography scans of skeletal elements of a subsample of the 425 individuals. These skeletons, mostly complete and well-preserved, pertain to individuals of different ages, ranging from fetuses/newborns to 91 years old, for which the sex is known, who died in the city of Bologna between 1898 and 1944. This collection represents an important scientific resource for the study of human skeletal remains and to test methods and techniques in the field of bioarchaeology, forensics, and paleoanthropology. The digitization of a comprehensive database of 3D scans and 3D bones is an ongoing project that will implement the Virtual DHOC of the Certosa Cemetery of Bologna over the next few years. The Virtual DHOC of the Certosa Cemetery of Bologna aims to share the data contained therein with other researchers, contributing to the dissemination of knowledge and the promotion of scientific research in anthropology, while also ensuring the virtual preservation and accessibility of this collection for future generations. This simultaneously responds to various ethical concerns and best practices about the treatment and management of human skeletal remains.

Restructuring of Femoral Cortical Bone During Growth and Locomotor Development of Wild Chimpanzees (Pan troglodytes verus)

OBJECTIVE

Chimpanzees are altricial in terms of their locomotor development and transition from being carried to engaging in suspensory and arboreal locomotor behaviors to eventually relying on terrestrial quadrupedalism as their main form of locomotion. Here, we consider the mechanical implications of femoral cortical bone restructuring during growth and locomotor development in wild chimpanzees.

MATERIALS AND METHODS

Cortical bone structure was examined in an ontogenetic sample of wild chimpanzees from a single subspecies (P. t. verus) spanning in age from 2 weeks to 12.6 years. Diaphyseal cross-sections were extracted from micro-CT scans of the femur at 35%, 50%, and 65% of total intermetaphyseal length and variation in cortical bone structure was assessed based on bending rigidity (Imax/Imin, Ix/Iy), relative medullary area, and cortical bone porosity.

RESULTS

Diaphyseal shape is relatively circular with a high amount of cortical bone porosity and a large relative medullary area during early infancy. Distinct shifts in cortical bone structure occurred for each studied parameter with the biggest changes occurring within the first 5 years. Values appear to stabilize as quadrupedal walking increases in frequency and is established as the main form of locomotion.

DISCUSSION

Collectively, the results suggest a degree of integration in which cortical bone restructures in response to rapid changes in locomotion in addition to nonmechanical influences such as hormonal, and growth factors, without compromising function and structural integrity. The extent of influence of each factor varies throughout growth and highlights the need for caution in functional interpretations of cortical bone geometry.

A new biomechanical model of the mammal jaw based on load path analysis

The primary function of the tetrapod jaw is to transmit jaw muscle forces to bite points. The routes of force transfer in the jaw have never been studied but can be quantified using load paths - the shortest, stiffest routes from regions of force application to support constraints. Here, we use load path analysis to map force transfer from muscle attachments to bite point and jaw joint, and to evaluate how different configurations of trabecular and cortical bone affect load paths. We created three models of the mandible of the Virginia opossum, Didelphis virginiana, each with a cortical bone shell, but with different material properties for the internal spaces: (1) a cortical-trabecular model, in which the interior space is modeled with bulk properties of trabecular bone; (2) a cortical-hollow model, in which trabeculae and mandibular canal are modeled as hollow; and (3) a solid-cortical model, in which the interior is modeled as cortical bone. The models were compared with published in vivo bite force and bone strain data, and the load paths calculated for each model. The trabecular model, which is preferred because it most closely approximates the actual morphology, was best validated by in vivo data. In all three models, the load path was confined to cortical bone, although its route within the cortex varied depending on the material properties of the inner model. Our analysis shows that most of the force is transferred through the cortical, rather than trabecular bone, and highlights the potential of load path analysis for understanding form-function relationships in the skeleton.

A statistical analysis of human talar shape and bone density distribution

The shape of the talus, its internal structure, and its mechanical properties are important in determining talar behavior during loading, which may be significant for the design of surgical tools and implants. Although recent studies using statistical shape modeling have described quantitative talar external shape variation, no similar quantitative study exists to describe the density distribution of internal talar structure. The goal of this study is to quantify statistical variation in talar shape and density to benefit the design of talar implants. To this end, weight-bearing computed tomography (CT) scans of the ankle were collected in neutral, bilateral standing posture, and three-dimensional models were generated for each talus. Local density derived from the Hounsfield unit of each CT voxel was extracted. A weighted spherical harmonic analysis was performed to quantify the talar external shape. One hundred and seventy-nine volumes of interest were placed in the same relative position within each talus to quantify the talar density. Additionally, a finite element analysis (FEA) was conducted on a talus with both heterogeneous and homogeneous material properties to observe the effect of these properties on the stress and strain response. Significant differences were found in the talar density in sex and age, as well as in the stress and strain response between homogeneous and heterogeneous FEA. These differences show the importance of considering heterogeneity when examining the load response of tarsal bones.

Differences in vertebral bone density between African apes

OBJECTIVES

Low-energy vertebral fractures are a common health concern, especially in elderly people. Interestingly, African apes do not seem to experience as many vertebral fractures and the low-energy ones are even rarer. One potential explanation for this difference is the lower bone density in humans. Yet, only limited research has been done on the vertebral bone density of the great apes and these have mainly included only single vertebrae. Hence the study aim is to expand our understanding of the vertebral microstructure of African apes in multiple spinal segments.

MATERIALS

Bone density in the vertebral body of C7, T12, and L3 was measured from 32 Pan troglodytes and 26 Gorilla gorilla using peripheral quantitative computed tomography (pQCT).

RESULTS

There was a clear difference between the three individual vertebrae and consequently the spinal segments in terms of trabecular density and cortical density and thickness. The variation of these bone parameters between the vertebrae differed between the apes but was also different from those reported for humans. The chimpanzees were observed to have overall higher trabecular density, but gorillas had higher cortical density and thickness. Cortical thickness had a relatively strong association with the vertebral size.

DISCUSSION

Despite the similarity in locomotion and posture, the results show slight differences in the bone parameters and their variation between spinal segments in African apes. This variation also differs from humans and appears to indicate a complex influence of locomotion, posture, and body size on the different spinal segments.

Trabecular bone volume fraction in Holocene and Late Pleistocene humans

Research suggests that recent modern humans have gracile skeletons in having low trabecular bone volume fraction (BV/TV) and that gracilization of the skeleton occurred in the last 10,000 years. This has been attributed to a reduction in physical activity in the Holocene. However, there has been no thorough sampling of BV/TV in Pleistocene humans due to limited access to high resolution images of fossil specimens. Therefore, our study investigates the gracilization of BV/TV in Late Pleistocene humans and recent (Holocene) modern humans to improve our understanding of the emergence of gracility. We used microcomputed tomography to measure BV/TV in the femora, humeri and metacarpals of a sample of Late Pleistocene humans from Dolní Věstonice (Czech Republic, ∼26 ka, n = 6) and Ohalo II (Israel, ∼19 ka, n = 1), and a sample of recent humans including farming groups (n = 39) and hunter-gatherers (n = 6). We predicted that 1) Late Pleistocene humans would exhibit greater femoral and humeral head BV/TV compared with recent humans and 2) among recent humans, metacarpal head BV/TV would be greater in hunter-gatherers compared with farmers. Late Pleistocene humans had higher BV/TV compared with recent humans in both the femur and humerus, supporting our first prediction, and consistent with previous findings that Late Pleistocene humans are robust as compared to recent humans. However, among recent humans, there was no significant difference in BV/TV in the metacarpals between the two subsistence groups. The results highlight the similarity in BV/TV in the hand of two human groups from different geographic locales and subsistence patterns and raise questions about assumptions of activity levels in archaeological populations and their relationships to trabecular BV/TV.

The deep trabecular structure of first metacarpals in extant hominids

OBJECTIVES

Recent studies have associated subarticular trabecular bone distribution in the extant hominid first metacarpal (Mc1) with observed thumb use, to infer fossil hominin thumb use. Here, we analyze the entire Mc1 to test for interspecific differences in: (1) the absolute volume of trabecular volume fraction, (2) the distribution of the deeper trabecular network, and (3) the distribution of trabeculae in the medullary cavity, especially beneath the Mc1 disto-radial flange.

MATERIALS AND METHODS

Trabecular bone was imaged using micro-computed tomography in a sample of Homo sapiens (n = 11), Pan paniscus (n = 10), Pan troglodytes (n = 11), Gorilla gorilla (n = 10) and Pongo sp., (n = 7). Using Canonical Holistic Morphometric Analysis (cHMA), we tested for interspecific differences in the trabecular bone volume fraction (BV/TV) and its relative distribution (rBV/TV) throughout the Mc1, including within the head, medullary cavity, and base.

RESULTS

P. paniscus had the highest, and H. sapiens the lowest, BV/TV relative to other species. rBV/TV distribution statistically distinguished the radial concentrations and lack of medullary trabecular bone in the H. sapiens Mc1 from all other hominids. H. sapiens and, to a lesser extent, G. gorilla also had a significantly higher trabecular volume beneath the disto-radial flange relative to other hominids.

DISCUSSION

These results are consistent with differences in observed thumb use in these species and may also reflect systemic differences in bone volume fraction. The trabecular bone extension into the medullary cavity and concentrations beneath the disto-radial flange may represent crucial biomechanical signals that will aid in the inference of fossil hominin thumb use.

Open the pores - Polydimethylsiloxane influences the porous structure of cancellous bone surrogates for biomechanical testing of osteosyntheses

Synthetic materials used for valid and reliable implant testing and design should reflect the mechanical and morphometric properties of human bone. Such bone models are already available on the market, but they do not reflect the population variability of human bone, nor are they open-celled porous as human bone is. Biomechanical studies aimed at cementing the fracture or an implant cannot be conducted with them. The aim of this study was to investigate the influence of a cell stabilizer on polyurethane-based cancellous synthetic bone in terms of morphology, compressive mechanics, and opening of the cancellous bone structure for bone cement application. Mechanical properties of cylindrical specimens of the bone surrogates were determined by static compression tests to failure. Furthermore, a morphometric analysis was performed using microcomputed tomography. To prove the open-cell nature of the bone surrogates, an attempt was made to apply bone cement. Effects on the mechanical properties of the polyurethane-based bone surrogates were observed by the addition of polydimethylsiloxane. All mechanical parameters like Young's modulus, ultimate stress and yield stress increased statistically significantly with increasing amounts of cell stabilizer (all p > 0.001), except for yield stress. The analysis of morphometric parameters showed a decrease in trabecular thickness, spacing and connectivity density, which was accompanied by an increase in trabecular number and an increase in pore size. The open-cell nature was proven by the application and distribution of bone cement in specimens with stabilizer, which was visualized by X-ray. In conclusion, the results show that by adding a cell stabilizer, polyurethane-based cancellous bone substrates can be produced that have an open-cell structure similar to human bone. This makes these bone surrogates suitable for biomechanical testing of osteosyntheses and for osteosynthesis cementation issues.

Trabecular bone structure of the proximal capitate in extant hominids and fossil hominins with implications for midcarpal joint loading and the dart-thrower's motion

OBJECTIVES

This research examines whether the distribution of trabecular bone in the proximal capitates of extant hominids, as well as several fossil hominin taxa, is associated with the oblique path of the midcarpal joint known as the dart-thrower's motion (DTM).

MATERIALS AND METHODS

We analyzed proximal capitates from extant (Pongo n = 12; Gorilla n = 11; Pan n = 10; fossil and recent Homo sapiens n = 29) and extinct (Australopithecus sediba n = 2; Homo naledi n = 1; Homo floresiensis n = 2; Neandertals n = 3) hominids using a new canonical holistic morphometric analysis, which quantifies and visualizes the distribution of trabecular bone using relative bone volume as a fraction of total volume (rBV/TV).

RESULTS

Homo sapiens and Neandertals had a continuous band of high rBV/TV that extended across the scaphoid, lunate, and hamate subarticular regions, but other fossil hominins and extant great apes did not. A. sediba expressed a distinct combination of human-like and Pan-like rBV/TV distribution. Both H. floresiensis and H. naledi had high rBV/TV on the ulnar-side of the capitate but low rBV/TV on the radial-side.

CONCLUSION

The proximal capitates of H. sapiens and Neandertals share a distinctive distribution of trabecular bone that suggests that these two species of Homo regularly load(ed) their midcarpal joints along the full extent of the oblique path of the DTM. The observed pattern in A. sediba suggests that human-like stress at the capito-scaphoid articular surface was combined with Pan-like wrist postures, whereas the patterns in H. floresiensis and H. naledi suggest their midcarpal joints were loaded differently from that of H. sapiens and Neandertals.

Serum vitamin D in sanctuary chimpanzees (Pan troglodytes) in range countries: A pilot study

BACKGROUND

Vitamin D is essential for skeletal health, calcium homeostasis and general health. The major and more stable form of vitamin D in circulation is 25-hydroxyvitamin D (25-OH-D); this is the most valuable indicator of vitamin D status. There are studies on laboratory and zoo-housed chimpanzees; however, serum vitamin D status has not been documented in chimpanzees in range countries.

OBJECTIVES

(1) Determine the range of circulating 25-OH-D concentrations in chimpanzees in range countries. (2) Assess the influence of age, sex, and sun exposure on 25-OH-D serum concentrations.

METHODS

Opportunistic blood samples were obtained from 127 clinically healthy chimpanzees. Serum 25-OH-D concentration was measured with a commercially available competitive ELISA.

RESULTS

The median overall 25-OH-D concentration for chimpanzees in range countries was 46.24 nmol/L (range: 17.10-109.23 nmol/L). Males had a significantly lower concentration (40.15 nmol/L) than females (49.61 nmol/L), and infants (37.99 nmol/L) had a significantly lower concentration than adults (46.04 nmol/L). Concentrations of 25-OH-D in chimpanzees in sunnier habitats were significantly higher compared to thick tropical forest habitat.

CONCLUSION

The present constitutes a large dataset of serum 25-OH-D concentrations in range country sanctuary chimpanzees and contributes to document normal ranges. Age, sex, and sun exposure influenced serum concentrations of 25-OH-D in sanctuary chimpanzees.

Comparison of bone density patterns of the subaxial spine between chimpanzees and gorillas - A case study

Case study on the bone density pattern of subaxial vertebral column in African apes.

INTRODUCTION

African apes have been noted to experience fewer back ailments than humans and to have higher vertebral bone density. Yet, research on the subject is quite limited and has usually included only one or few vertebrae. However, to understand vertebral column as whole and how posture and locomotion might have affected it, we need to know how bone density varies between adjacent vertebrae.

MATERIALS AND METHODS

Bone density in the vertebral body was measured for all subaxial vertebrae of five specimens including two Pan troglodytes (1 male and 1 female) and three Gorilla gorilla (2 males and 1 female) using peripheral quantitative computed tomography (pQCT).

RESULTS

The results tentatively indicated differences between species, especially in the trabecular density of the cervical segment and support the need for further studies on this subject.

The Neanderthal patellae from Krapina (Croatia): A comparative investigation of their endostructural conformation and distinctive features compared to the extant human condition

OBJECTIVES

The Neanderthal patella differs from that of extant humans by being thicker anteroposteriorly and by having more symmetric medial and lateral articular facets. However, it is still unclear to what extent these differences affect knee kinesiology. We aim at assessing the endostructural conformation of Neanderthal patellae to reveal functionally related mechanical information comparatively to the extant human condition. In principle, we expect that the Neanderthal patella (i) shows a higher amount of cortical bone and (ii) a trabecular network organization distinct from the extant human condition.

MATERIALS AND METHODS

By using micro-focus X-ray tomography, we characterized the endostructure of six adult patellae from the OIS 5e Neanderthal site of Krapina, Croatia, the largest assemblage of human fossil patellae assessed so far, and compared their pattern to the configuration displayed by a sample of 22 recent humans.

RESULTS AND DISCUSSION

The first expectation is rejected, indicating that the patellar bone might have not followed the trend of generalized gracilization of the human postcranial skeleton occurred through the Upper Pleistocene. The second prediction is at least partially supported. In Krapina the trabecular network differs from the comparative sample by showing a higher medial density and by lacking a proximal reinforcement. Such conformation indicates similar load patterns exerted in Neanderthals and extant humans by the vastus lateralis, but not by the vastus medialis, with implications on the mediolateral stabilization of the knee joint. However, the patterns of structural variation of the patellar network remain to be assessed in other Neanderthal samples.

A computational framework for canonical holistic morphometric analysis of trabecular bone

Bone is a remarkable, living tissue that functionally adapts to external loading. Therefore, bone shape and internal structure carry information relevant to many disciplines, including medicine, forensic science, and anthropology. However, morphometric comparisons of homologous regions across different individuals or groups are still challenging. In this study, two methods were combined to quantify such differences: (1) Holistic morphometric analysis (HMA) was used to quantify morphometric values in each bone, (2) which could then be mapped to a volumetric mesh of a canonical bone created by a statistical free-form deformation model (SDM). Required parameters for this canonical holistic morphometric analysis (cHMA) method were identified and the robustness of the method was evaluated. The robustness studies showed that the SDM converged after one to two iterations, had only a marginal bias towards the chosen starting image, and could handle large shape differences seen in bones of different species. Case studies were performed on metacarpal bones and proximal femora of different primate species to confirm prior study results. The differences between species could be visualised and statistically analysed in both case studies. cHMA provides a framework for performing quantitative comparisons of different morphometric quantities across individuals or groups. These comparisons facilitate investigation of the relationship between spatial morphometric variations and function or pathology, or both.

Effects of reduced mobility on trabecular bone density in captive big cats

Bone responds to elevated mechanical loading by increasing in mass and density. Therefore, wild animals should exhibit greater skeletal mass and density than captive conspecifics. This expectation is pertinent to testing bone functional adaptation theories and to comparative studies, which commonly use skeletal remains that combine zoo and wild-caught specimens. Conservationists are also interested in the effects of captivity on bone morphology as it may influence rewilding success. We compared trabecular bone volume fraction (BVF) between wild and captive mountain lions, cheetahs, leopards and jaguars. We found significantly greater BVF in wild than in captive felids. Effects of captivity were more marked in the humerus than in the femur. A ratio of humeral/femoral BVF was also lower in captive animals and showed a positive relationship to home range size in wild animals. Results are consistent with greater forelimb than hindlimb loading during terrestrial travel, and possibly reduced loading of the forelimb associated with lack of predatory behaviour in captive animals. Thus, captivity among felids has general effects on BVF in the postcranial skeleton and location-specific effects related to limb use. Caution should be exercised when identifying skeletal specimens for use in comparative studies and when rearing animals for conservation purposes.

Variation, mosaicism and degeneracy in the hominin foot

The fossil record is scarce and incomplete by nature. Animals and ecological processes devour soft tissue and important bony details over time and, when the dust settles, we are faced with a patchy record full of variation. Fossil taxa are usually defined by craniodental characteristics, so unless postcranial bones are found associated with a skull, assignment to taxon is unstable. Naming a locomotor category based on fossil bone morphology by analogy to living hominoids is not uncommon, and when no single locomotor label fits, postcrania are often described as exhibiting a 'mosaic' of traits. Here, we contend that the unavoidable variation that characterises the fossil record can be described far more rigorously based on extensive work in human neurobiology and neuroanatomy, movement sciences and motor control and biomechanics research. In neurobiology, degeneracy is a natural mechanism of adaptation allowing system elements that are structurally different to perform the same function. This concept differs from redundancy as understood in engineering, where the same function is performed by identical elements. Assuming degeneracy, structurally different elements are able to produce different outputs in a range of environmental contexts, favouring ecological robusticity by enabling adaptations. Furthermore, as degeneracy extends to genome level, genetic variation is sustained, so that genes which might benefit an organism in a different environment remain part of the genome, favouring species' evolvability.

Mineralized Cartilage and Bone-Like Tissues in Chondrichthyans Offer Potential Insights Into the Evolution and Development of Mineralized Tissues in the Vertebrate Endoskeleton

The impregnation of biominerals into the extracellular matrix of living organisms, a process termed biomineralization, gives rise to diverse mineralized (or calcified) tissues in vertebrates. Preservation of mineralized tissues in the fossil record has provided insights into the evolutionary history of vertebrates and their skeletons. However, current understanding of the vertebrate skeleton and of the processes underlying its formation is biased towards biomedical models such as the tetrapods mouse and chick. Chondrichthyans (sharks, skates, rays, and chimaeras) and osteichthyans are the only vertebrate groups with extant (living) representatives that have a mineralized skeleton, but the basal phylogenetic position of chondrichthyans could potentially offer unique insights into skeletal evolution. For example, bone is a vertebrate novelty, but the internal supporting skeleton (endoskeleton) of extant chondrichthyans is commonly described as lacking bone. The molecular and developmental basis for this assertion is yet to be tested. Subperichondral tissues in the endoskeleton of some chondrichthyans display mineralization patterns and histological and molecular features of bone, thereby challenging the notion that extant chondrichthyans lack endoskeletal bone. Additionally, the chondrichthyan endoskeleton demonstrates some unique features and others that are potentially homologous with other vertebrates, including a polygonal mineralization pattern, a trabecular mineralization pattern, and an unconstricted perichordal sheath. Because of the basal phylogenetic position of chondrichthyans among all other extant vertebrates with a mineralized skeleton, developmental and molecular studies of chondrichthyans are critical to flesh out the evolution of vertebrate skeletal tissues, but only a handful of such studies have been carried out to date. This review discusses morphological and molecular features of chondrichthyan endoskeletal tissues and cell types, ultimately emphasizing how comparative embryology and transcriptomics can reveal homology of mineralized skeletal tissues (and their cell types) between chondrichthyans and other vertebrates.

Trabecular structure of the elbow reveals divergence in knuckle-walking biomechanical strategies of African apes

African apes engage in a distinct form of locomotion called knuckle-walking, but there is much ambiguity as to when and how this locomotor behavior evolved. This study aims to elucidate potential differences in knuckle-walking elbow posture and loading in African apes through the study of trabecular bone. Using a whole-epiphysis approach, we quantified variation in the trabecular structure of the distal humerus of chimpanzees, western lowland gorillas, and mountain gorillas in comparison to orang-utans, siamangs, and a sample of Old and New World monkeys. Results demonstrate differences in the distribution of trabecular bone within the distal humerus that are consistent across taxa that habitually use a flexed-elbow posture in comparison to those that use an extended elbow during locomotion. Western lowland gorillas show an extended-elbow pattern consistent with the straight forelimb position during knuckle-walking, whereas chimpanzees show a flexed-elbow pattern. Unexpectedly, mountain gorillas show an intermediate pattern between their western counterparts and chimpanzees. The differences found in elbow joint posture between chimpanzees and gorillas, and between gorilla species, point to diversification in the knuckle-walking biomechanical strategies among African apes, which has implications in the debate regarding the locomotor behavior from which human bipedalism arose.

The Functional and Allometric Implications of Hipbone Trabecular Microarchitecture in a Sample of Eutherian and Metatherian Mammals

The pelvis plays an active role in weight bearing and countering the ground reaction forces incurred by the hindlimbs thus making it a critical component of the locomotor skeleton. Accordingly, this anatomical region is theoretically ideal for inferring locomotor behavior from both external skeletal morphology and trabecular microarchitecture, with the latter possibly offering nuanced insights into the mechanical loading environment given its increased plasticity and higher turnover rate. However, trabecular microarchitecture is also known to be influenced by a variety of factors including body size, sex, age, genetic regulation, diet and activity level, that collectively hinder the ability to generate consistent functional inferences. In this study, a comparative sample of mammals (42 species spanning four orders) of varying sizes, yet comparable locomotor repertoires, were evaluated to determine the effects of body size, phylogeny and locomotion on hipbone trabecular microarchitecture. This study found a weak functional signal detected in differences in bone volume fraction and the degree of anisotropy across certain pre-assigned locomotor categories, while confirming previously recognized allometric scaling trends reported for other mammalian samples based on the femur. Within primates, a more anisotropic pattern was observed for quadrupedal species attributed to their repetitive loading regimes and stereotypical limb excursions, while isotropic values were revealed for taxa utilizing more varied arboreal repertoires. Humans, despite a frequent and predictable loading environment associated with their use of bipedalism, showed relatively isotropic values. This study highlights the confounding factors that influence trabecular microarchitecture and consequently limit its utility as a method for investigating locomotor adaptation.

Locomotory behaviour of early tetrapods from Blue Beach, Nova Scotia, revealed by novel microanatomical analysis

Evidence for terrestriality in early tetrapods is fundamentally contradictory. Fossil trackways attributed to early terrestrial tetrapods long predate the first body fossils from the Late Devonian. However, the Devonian body fossils demonstrate an obligatorily aquatic lifestyle. Complicating our understanding of the transition from water to land is a pronounced gap in the fossil record between the aquatic Devonian taxa and presumably terrestrial tetrapods from the later Early Carboniferous. Recent work suggests that an obligatorily aquatic habit persists much higher in the tetrapod tree than previously recognized. Here, we present independent microanatomical data of locomotor capability from the earliest Carboniferous of Blue Beach, Nova Scotia. The site preserves limb bones from taxa representative of Late Devonian to mid-Carboniferous faunas as well as a rich trackway record. Given that bone remodels in response to functional stresses including gravity and ground reaction forces, we analysed both the midshaft compactness profiles and trabecular anisotropy, the latter using a new whole bone approach. Our findings suggest that early tetrapods retained an aquatic lifestyle despite varied limb morphologies, prior to their emergence onto land. These results suggest that trackways attributed to early tetrapods be closely scrutinized for additional information regarding their creation conditions, and demand an expansion of sampling to better identify the first terrestrial tetrapods.

Cortical and trabecular bone structure of the hominoid capitate

Morphological variation in the hominoid capitate has been linked to differences in habitual locomotor activity due to its importance in movement and load transfer at the midcarpal joint proximally and carpometacarpal joints distally. Although the shape of bones and their articulations are linked to joint mobility, the internal structure of bones has been shown experimentally to reflect, at least in part, the loading direction and magnitude experienced by the bone. To date, it is uncertain whether locomotor differences among hominoids are reflected in the bone microarchitecture of the capitate. Here, we apply a whole‐bone methodology to quantify the cortical and trabecular architecture (separately and combined) of the capitate across bipedal (modern Homo sapiens), knuckle‐walking (Pan paniscus, Pan troglodytes, Gorilla sp.), and suspensory (Pongo sp.) hominoids (n = 69). It is hypothesized that variation in bone microarchitecture will differentiate these locomotor groups, reflecting differences in habitual postures and presumed loading force and direction. Additionally, it is hypothesized that trabecular and cortical architecture in the proximal and distal regions, as a result of being part of mechanically divergent joints proximally and distally, will differ across these portions of the capitate. Results indicate that the capitate of knuckle‐walking and suspensory hominoids is differentiated from bipedal Homo primarily by significantly thicker distal cortical bone. Knuckle‐walking taxa are further differentiated from suspensory and bipedal taxa by more isotropic trabeculae in the proximal capitate. An allometric analysis indicates that size is not a significant determinate of bone variation across hominoids, although sexual dimorphism may influence some parameters within Gorilla. Results suggest that internal trabecular and cortical bone is subjected to different forces and functional adaptation responses across the capitate (and possibly other short bones). Additionally, while separating trabecular and cortical bone is normal protocol of current whole‐bone methodologies, this study shows that when applied to carpals, removing or studying the cortical bone separately potentially obfuscates functionally relevant signals in bone structure.

Novel strategies for the characterization of cancellous bone morphology: Virtual isolation and analysis

OBJECTIVES

The advent of micro-computed tomography (μCT) made cancellous bone more accessible than ever before. Nevertheless, the characterization of cancellous bone is made difficult by its inherent complexity and the difficulties in defining homology across datasets. Here we propose novel virtual methodological approaches to overcome those issues and complement existing methods.

MATERIALS AND METHODS

We present a protocol for the isolation of the whole cancellous region within a μCT scanned bone. This method overcomes the subsampling issues and allows studying cancellous bone as a single unit. We test the protocol on a set of primate bones. In addition, we describe a set of morphological indices calculated on the topological skeleton of the cancellous bone: node density, node connectivity, trabecular angle, trabecular tortuosity, and fractal dimension. The usage of the indices is shown on a small comparative sample of primate femoral heads.

RESULTS

The isolation protocol proves reliable in isolating cancellous structures from several different bones, regardless of their shape. The indices seem to detect some functional differences, although further testing on comparative samples is needed to clarify their potential for the study of cancellous architecture.

CONCLUSIONS

The approaches presented overcome some of the difficulties of trabecular bone studies. The methods presented here represent an alternative or supporting method to the existing tools available to address the biomechanics of cancellous bone.

Musculoskeletal modelling of the Nile crocodile (Crocodylus niloticus) hindlimb: Effects of limb posture on leverage during terrestrial locomotion

We developed a three-dimensional, computational biomechanical model of a juvenile Nile crocodile (Crocodylus niloticus) pelvis and hindlimb, composed of 47 pelvic limb muscles, to investigate muscle function. We tested whether crocodiles, which are known to use a variety of limb postures during movement, use limb orientations (joint angles) that optimise the moment arms (leverages) or moment-generating capacities of their muscles during different limb postures ranging from a high walk to a sprawling motion. We also describe the three-dimensional (3D) kinematics of the crocodylian hindlimb during terrestrial locomotion across an instrumented walkway and a treadmill captured via X-ray Reconstruction of Moving Morphology (biplanar fluoroscopy; 'XROMM'). We reconstructed the 3D positions and orientations of each of the hindlimb bones and used dissection data for muscle lines of action to reconstruct a focal, subject-specific 3D musculoskeletal model. Motion data for different styles of walking (a high, crouched, bended and two types of sprawling motion) were fed into the 3D model to identify whether any joints adopted near-optimal poses for leverage across each of the behaviours. We found that (1) the hip adductors and knee extensors had their largest leverages during sprawling postures and (2) more erect postures typically involved greater peak moment arms about the hip (flexion-extension), knee (flexion) and metatarsophalangeal (flexion) joints. The results did not fully support the hypothesis that optimal poses are present during different locomotory behaviours because the peak capacities were not always reached around mid-stance phase. Furthermore, we obtained few clear trends for isometric moment-generating capacities. Therefore, perhaps peak muscular leverage in Nile crocodiles is instead reached either in early/late stance or possibly during swing phase or other locomotory behaviours that were not studied here, such as non-terrestrial movement. Alternatively, our findings could reflect a trade-off between having to execute different postures, meaning that hindlimb muscle leverage is not optimised for any singular posture or behaviour. Our model, however, provides a comprehensive set of 3D estimates of muscle actions in extant crocodiles which can form a basis for investigating muscle function in extinct archosaurs.

Assessing patterns of variation in BV/TV in the calcaneus and C2 vertebra of Gorilla gorilla, Pan troglodytes, and populations of Homo sapiens from the Pleistocene and Holocene that differ in physical activity levels

OBJECTIVES

Because trabecular bone volume fraction (BV/TV) is influenced by variations in physical activity recent declines in BV/TV in humans are often attributed to modern sedentary lifestyles. This study tests the hypothesis that presumed variations in mechanical loading between groups can predict the observed BV/TV patterns in humans, chimpanzees and gorillas in two bones: the calcaneus which experiences high and well characterized impact forces, and the C2 vertebrae which experiences reduced locomotor forces.

MATERIALS AND METHODS

BV/TV and other structural variables were quantified from high-resolution microCT scans in gorillas, chimpanzees, and four Homo sapiens populations: Pleistocene, semi-sedentary Natufians; Holocene hunter-gatherers from Point Hope, Alaska; Holocene nomadic pastoralists from medieval Europe; and modern, sedentary Americans.

RESULTS

In the calcaneal tuberosity, Natufian BV/TV was 36, 46, and 46% greater than Alaskans (p = .02), Europeans (p = .005) and modern Americans (p = .002), respectively, but not significantly different from apes. BV/TV was not significantly different between modern Americans and Alaskans or Europeans. In the C2, Natufian BV/TV was 53 and 25% greater than in the Alaskan (p = .0001) and European (p = .048) populations.

DISCUSSION

These results suggest that phenomena other than or in addition to variations in physical activity are needed to explain BV/TV patterns observed in H. sapiens, and point to a systemic decline in H. sapiens BV/TV after the Pleistocene.

Combinations of trabecular and cortical bone properties distinguish various loading modalities between athletes and controls

OBJECTIVES

Variation in trabecular and cortical bone properties is often used to infer habitual behavior in the past. However, the structures of both types of bone are rarely considered together and may even contradict each other in functional interpretations. We examine trabecular and cortical bone properties in various athletes and sedentary controls to clarify the associations between combinations of cortical and trabecular bone properties and various loading modalities.

MATERIALS AND METHODS

We compare trabecular and cortical bone properties using peripheral quantitative computed tomography scans of the tibia between groups of 83 male athletes (running, hockey, swimming, cricket) and sedentary controls using Bayesian multilevel models. We quantify midshaft cortical bone rigidity and area (J, CA), midshaft shape index (Imax/Imin), and mean trabecular bone mineral density (BMD) in the distal tibia.

RESULTS

All groups show unique combinations of biomechanical properties. Cortical bone rigidity is high in sports that involve impact loading (cricket, running, hockey) and low in nonimpact loaded swimmers and controls. Runners have more anteroposteriorly elliptical midshafts compared to other groups. Interestingly, all athletes have greater trabecular BMD compared to controls, but do not differ credibly among each other.

DISCUSSION

Results suggest that cortical midshaft hypertrophy is associated with impact loading while trabecular BMD is positively associated with both impact and nonimpact loading. Midshaft shape is associated with directionality of loading. Individuals from the different categories overlap substantially, but group means differ credibly, suggesting that nuanced group-level inferences of habitual behavior are possible when combinations of trabecular and cortical bone are analyzed.

Using point clouds to investigate the relationship between trabecular bone phenotype and behavior: An example utilizing the human calcaneus

OBJECTIVES

The objective of this study is to demonstrate a new method for analyzing trabecular bone volume fraction and degree of anisotropy in three dimensions.

METHODS

We use a combination of automatic mesh registration, point-cloud correspondence registration, and P-value corrected univariate statistical tests to compare bone volume fraction and degree of anisotropy on a point by point basis across the entire calcaneus of two human groups with different subsistence strategies.

RESULTS

We found that the patterns of high and low bone volume fraction and degree of anisotropy distribution between the Black Earth (hunter-gatherers) and Norris Farms (mixed-strategy agriculturalists) are very similar, but differ in magnitude. The hunter-gatherers exhibit higher levels of bone volume fraction and less anisotropic trabecular bone organization. Additionally, patterns of bone volume fraction and degree of anisotropy in the calcaneus correspond well with biomechanical expectations of relative forces experienced during walking and running.

CONCLUSIONS

We conclude that comparing site-specific, localized differences in trabecular bone variables such as bone volume fraction and degree of anisotropy in three-dimensions is a powerful analytical tool. This method makes it possible to determine where similarities and differences between groups are located within the whole skeletal element of interest. The visualization of multiple variables also provides a way for researchers to see how the trabecular bone variables interact within the morphology, and allows for a more nuanced understanding of how they relate to one another and the broader mechanical environment.

The position of Australopithecus sediba within fossil hominin hand use diversity

The human lineage is marked by a transition in hand use, from locomotion towards increasingly dexterous manipulation, concomitant with bipedalism. The forceful precision grips used by modern humans probably evolved in the context of tool manufacture and use, but when and how many times hominin hands became principally manipulative remains unresolved. We analyse metacarpal trabecular and cortical bone, which provide insight into behaviour during an individual's life, to demonstrate previously unrecognized diversity in hominin hand use. The metacarpals of the palm in Australopithecus sediba have trabecular morphology most like orangutans and consistent with locomotor power-grasping with the fingers, while that of the thumb is consistent with human-like manipulation. This internal morphology is the first record of behaviour consistent with a hominin that used its hand for both arboreal locomotion and human-like manipulation. This hand use is distinct from other fossil hominins in this study, including A. afarensis and A. africanus.

Trabecular architecture of the capitate and third metacarpal through ontogeny in chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla)

Chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) both knuckle-walk in adulthood but are known to develop their locomotor strategies differently. Using dentally defined age-groups of both Pan and Gorilla and behavioral data from the literature, this study presents an internal trabecular bone approach to better understand the morphological ontogeny of knuckle-walking in these taxa. Capitate and third metacarpal bones were scanned by μCT at 23-43 μm resolution with scaled volumes of interest placed centrally within the head of the capitate and base of the third metacarpal. Trabecular measures related to activity level (size-adjusted bone volume/total volume, trabecular number, and bone surface area/bone volume) met expectations of decreasing through ontogeny in both taxa. Degree of anisotropy did not show statistical support for predicted species differences, but this may be due to the sample size as observed changes through ontogeny reflect expected trends in the capitate. Analyses of principal trabecular orientation corroborated known behavioral differences related to variation of hand use in these taxa, but only Pan showed directional patterning associated with suggested wrist posture. Assessment of allometry showed that the trabecular bone of larger animals is characterized by fewer and thinner trabeculae relative to bone size. In combination, these findings confirm the efficacy of trabecular bone in reflecting locomotor ontogeny differences between closely related taxa. These techniques show promise for use within the hominin fossil record, particularly for taxa hypothesized to be arboreal in some capacity.

Trabecular variation in the first metacarpal and manipulation in hominids

OBJECTIVES

The dexterity of fossil hominins is often inferred by assessing the comparative manual anatomy and behaviors of extant hominids, with a focus on the thumb. The aim of this study is to test whether trabecular structure is consistent with what is currently known about habitually loaded thumb postures across extant hominids.

MATERIALS AND METHODS

We analyze first metacarpal (Mc1) subarticular trabecular architecture in humans (Homo sapiens, n = 10), bonobos (Pan paniscus, n = 10), chimpanzees (Pan troglodytes, n = 11), as well as for the first time, gorillas (Gorilla gorilla gorilla, n = 10) and orangutans (Pongo sp., n = 1, Pongo abelii, n = 3 and Pongo pygmaeus, n = 5). Using a combination of subarticular and whole-epiphysis approaches, we test for significant differences in relative trabecular bone volume (RBV/TV) and degree of anisotropy (DA) between species.

RESULTS

Humans have significantly greater RBV/TV on the radiopalmar aspects of both the proximal and distal Mc1 subarticular surfaces and greater DA throughout the Mc1 head than other hominids. Nonhuman great apes have greatest RBV/TV on the ulnar aspect of the Mc1 head and the palmar aspect of the Mc1 base. Gorillas possessed significantly lower DA in the Mc1 head than any other taxon in our sample.

DISCUSSION

These results are consistent with abduction of the thumb during forceful "pad-to-pad" precision grips in humans and, in nonhuman great apes, a habitually adducted thumb that is typically used in precision and power grips. This comparative context will help infer habitual manipulative and locomotor grips in fossil hominins.

Trabecular bone architecture in the stylopod epiphyses of mustelids (Mammalia, Carnivora)

Mustelidae, a carnivoran clade that includes for instance weasels, badgers, otters and martens, has undergone several evolutionary transitions of lifestyle, resulting in specializations for fossorial, natatorial and scansorial locomotion, in addition to more generalized species. The family is therefore regarded as offering an adequate framework for morpho-functional analyses. However, the architecture of the epiphyseal trabecular bone, which is argued to be particularly responsive to the biomechanical environment, has never been studied. Here, we quantify trabecular bone parameters of the proximal and distal epiphyses of the humerus and femur in 29 species of mustelids and assess the differences of these parameters among groups defined a priori based on the aforementioned locomotor types. The parameters are assessed in a phylogenetic framework, taking into account the potential effect on an individual's body mass. The range of variation described by the acquired parameters is relatively restricted when compared to that of other clades. Generalists, however, are featuring a wider range of variation than the other types. While clear discrimination of locomotor types is difficult, some differences were highlighted by our analysis, such as a greater bone fraction associated with the natatorial taxa, which we discuss in a functional context.

Metacarpal trabecular bone varies with distinct hand-positions used in hominid locomotion

Trabecular bone remodels during life in response to loading and thus should, at least in part, reflect potential variation in the magnitude, frequency and direction of joint loading across different hominid species. Here we analyse the trabecular structure across all non-pollical metacarpal distal heads (Mc2-5) in extant great apes, expanding on previous volume of interest and whole-epiphysis analyses that have largely focused on only the first or third metacarpal. Specifically, we employ both a univariate statistical mapping and a multivariate approach to test for both inter-ray and interspecific differences in relative trabecular bone volume fraction (RBV/TV) and degree of anisotropy (DA) in Mc2-5 subchondral trabecular bone. Results demonstrate that whereas DA values only separate Pongo from African apes (Pan troglodytes, Pan paniscus, Gorilla gorilla), RBV/TV distribution varies with the predicted loading of the metacarpophalangeal (McP) joints during locomotor behaviours in each species. Gorilla exhibits a relatively dorsal distribution of RBV/TV consistent with habitual hyper-extension of the McP joints during knuckle-walking, whereas Pongo has a palmar distribution consistent with flexed McP joints used to grasp arboreal substrates. Both Pan species possess a disto-dorsal distribution of RBV/TV, compatible with multiple hand postures associated with a more varied locomotor regime. Further inter-ray comparisons reveal RBV/TV patterns consistent with varied knuckle-walking postures in Pan species in contrast to higher RBV/TV values toward the midline of the hand in Mc2 and Mc5 of Gorilla, consistent with habitual palm-back knuckle-walking. These patterns of trabecular bone distribution and structure reflect different behavioural signals that could be useful for determining the behaviours of fossil hominins.

Trabecular architecture of the great ape and human femoral head

Studies of femoral trabecular structure have shown that the orientation and volume of bone are associated with variation in loading and could be informative about individual joint positioning during locomotion. In this study, we analyse for the first time trabecular bone patterns throughout the femoral head using a whole-epiphysis approach to investigate how potential trabecular variation in humans and great apes relates to differences in locomotor modes. Trabecular architecture was analysed using microCT scans of Pan troglodytes (n = 20), Gorilla gorilla (n = 14), Pongo sp. (n = 5) and Homo sapiens (n = 12) in medtool 4.1. Our results revealed differences in bone volume fraction (BV/TV) distribution patterns, as well as overall trabecular parameters of the femoral head between great apes and humans. Pan and Gorilla showed two regions of high BV/TV in the femoral head, consistent with hip posture and loading during two discrete locomotor modes: knuckle-walking and climbing. Most Pongo specimens also displayed two regions of high BV/TV, but these regions were less discrete and there was more variability across the sample. In contrast, Homo showed only one main region of high BV/TV in the femoral head and had the lowest BV/TV, as well as the most anisotropic trabeculae. The Homo trabecular structure is consistent with stereotypical loading with a more extended hip compared with great apes, which is characteristic of modern human bipedalism. Our results suggest that holistic evaluations of femoral head trabecular architecture can reveal previously undetected patterns linked to locomotor behaviour in extant apes and can provide further insight into hip joint loading in fossil hominins and other primates.

Trabecular architecture and joint loading of the proximal humerus in extant hominoids, Ateles, and Australopithecus africanus

OBJECTIVES

Several studies have investigated potential functional signals in the trabecular structure of the primate proximal humerus but with varied success. Here, we apply for the first time a "whole-epiphyses" approach to analysing trabecular bone in the humeral head with the aim of providing a more holistic interpretation of trabecular variation in relation to habitual locomotor or manipulative behaviors in several extant primates and Australopithecus africanus.

MATERIALS AND METHODS

We use a "whole-epiphysis" methodology in comparison to the traditional volume of interest (VOI) approach to investigate variation in trabecular structure and joint loading in the proximal humerus of extant hominoids, Ateles and A. africanus (StW 328).

RESULTS

There are important differences in the quantification of trabecular parameters using a "whole-epiphysis" versus a VOI-based approach. Variation in trabecular structure across knuckle-walking African apes, suspensory taxa, and modern humans was generally consistent with predictions of load magnitude and inferred joint posture during habitual behaviors. Higher relative trabecular bone volume and more isotropic trabeculae in StW 328 suggest A. africanus may have still used its forelimbs for arboreal locomotion.

DISCUSSION

A whole-epiphysis approach to analysing trabecular structure of the proximal humerus can help distinguish functional signals of joint loading across extant primates and can provide novel insight into habitual behaviors of fossil hominins.

Trabecular bone patterning across the human hand

Hand bone morphology is regularly used to link particular hominin species with behaviors relevant to cognitive/technological progress. Debates about the functional significance of differing hominin hand bone morphologies tend to rely on establishing phylogenetic relationships and/or inferring behavior from epigenetic variation arising from mechanical loading and adaptive bone modeling. Most research focuses on variation in cortical bone structure, but additional information about hand function may be provided through the analysis of internal trabecular structure. While primate hand bone trabecular structure is known to vary in ways that are consistent with expected joint loading differences during manipulation and locomotion, no study exists that has documented this variation across the numerous bones of the hand. We quantify the trabecular structure in 22 bones of the human hand (early/extant modern Homo sapiens) and compare structural variation between two groups associated with post-agricultural/industrial (post-Neolithic) and foraging/hunter-gatherer (forager) subsistence strategies. We (1) establish trabecular bone volume fraction (BV/TV), modulus (E), degree of anisotropy (DA), mean trabecular thickness (Tb.Th) and spacing (Tb.Sp); (2) visualize the average distribution of site-specific BV/TV for each bone; and (3) examine if the variation in trabecular structure is consistent with expected joint loading differences among the regions of the hand and between the groups. Results indicate similar distributions of trabecular bone in both groups, with those of the forager sample presenting higher BV/TV, E, and lower DA, suggesting greater and more variable loading during manipulation. We find indications of higher loading along the ulnar side of the forager sample hand, with high site-specific BV/TV distributions among the carpals that are suggestive of high loading while the wrist moves through the 'dart-thrower's' motion. These results support the use of trabecular structure to infer behavior and have direct implications for refining our understanding of human hand evolution and fossil hominin hand use.

Trabecular bone patterning in the hominoid distal femur

BACKGROUND

In addition to external bone shape and cortical bone thickness and distribution, the distribution and orientation of internal trabecular bone across individuals and species has yielded important functional information on how bone adapts in response to load. In particular, trabecular bone analysis has played a key role in studies of human and nonhuman primate locomotion and has shown that species with different locomotor repertoires display distinct trabecular architecture in various regions of the skeleton. In this study, we analyse trabecular structure throughout the distal femur of extant hominoids and test for differences due to locomotor loading regime.

METHODS

Micro-computed tomography scans of Homo sapiens (n = 11), Pan troglodytes (n = 18), Gorilla gorilla (n = 14) and Pongo sp. (n = 7) were used to investigate trabecular structure throughout the distal epiphysis of the femur. We predicted that bone volume fraction (BV/TV) in the medial and lateral condyles in Homo would be distally concentrated and more anisotropic due to a habitual extended knee posture at the point of peak ground reaction force during bipedal locomotion, whereas great apes would show more posteriorly concentrated BV/TV and greater isotropy due to a flexed knee posture and more variable hindlimb use during locomotion.

RESULTS

Results indicate some significant differences between taxa, with the most prominent being higher BV/TV in the posterosuperior region of the condyles in Pan and higher BV/TV and anisotropy in the posteroinferior region in Homo. Furthermore, trabecular number, spacing and thickness differ significantly, mainly separating Gorilla from the other apes.

DISCUSSION

The trabecular architecture of the distal femur holds a functional signal linked to habitual behaviour; however, there was more similarity across taxa and greater intraspecific variability than expected. Specifically, there was a large degree of overlap in trabecular structure across the sample, and Homo was not as distinct as predicted. Nonetheless, this study offers a comparative sample of trabecular structure in the hominoid distal femur and can contribute to future studies of locomotion in extinct taxa.