Running, jumping, hunting, and scavenging: Functional analysis of vertebral mobility and backbone properties in carnivorans

Carnivorans are well-known for their exceptional backbone mobility, which enables them to excel in fast running and long jumping, leading to them being among the most successful predators amongst terrestrial mammals. This study presents the first large-scale analysis of mobility throughout the presacral region of the vertebral column in carnivorans. The study covers representatives of 6 families, 24 genera and 34 species. We utilized a previously developed osteometry-based method to calculate available range of motion, quantifying all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). We observed a strong phylogenetic signal in the structural basis of the vertebral column (vertebral and joint formulae, length proportions of the backbone modules) and an insignificant phylogenetic signal in most characteristics of intervertebral mobility. This indicates that within the existing structure (stabilization of which occurred rather early in different phylogenetic lineages), intervertebral mobility in carnivorans is quite flexible. Our findings reveal that hyenas and canids, which use their jaws to seize prey, are characterized by a noticeably elongated cervical region and significantly higher SB and LB mobility of the cervical joints compared to other carnivorans. In representatives of other carnivoran families, the cervical region is very short, but the flexibility of the neck (both SB and LB) is significantly higher than that of short-necked odd-toed and even-toed ungulates. The lumbar region of the backbone in carnivorans is dorsomobile in the sagittal plane, being on average ~23° more mobile than in artiodactyls and ~38° more mobile than in perissodactyls. However, despite the general dorsomobility, only some representatives of Canidae, Felidae, and Viverridae are superior in lumbar flexibility to the most dorsomobile ungulates. The most dorsomobile artiodactyls are equal or even superior to carnivorans in their ability to engage in dorsal extension during galloping. In contrast, carnivorans are far superior to ungulates in their ability to engage ventral flexion. The cumulative SB in the lumbar region in carnivorans largely depends on the mode of running and hunting. Thus, adaptation to prolonged and enduring pursuit of prey in hyenas is accompanied by markedly reduced SB flexibility in the lumbar region. A more dorsostable run is also a characteristic of the Ursidae, and the peculiar maned wolf. Representatives of Felidae and Canidae have significantly more available SB mobility in the lumbar region. However, they fully engage it only occasionally at key moments of the hunt associated with the direct capture of the prey or when running in a straight line at maximum speed.

The functional significance of aberrant cervical counts in sloths: insights from automated exhaustive analysis of cervical range of motion

Besides manatees, the suspensory extant 'tree sloths' are the only mammals that deviate from a cervical count (CC) of seven vertebrae. They do so in opposite directions in the two living genera (increased versus decreased CC). Aberrant CCs seemingly reflect neck mobility in both genera, suggesting adaptive significance for their head position during suspensory locomotion and especially increased ability for neck torsion in three-toed sloths. We test two hypotheses in a comparative evolutionary framework by assessing three-dimensional intervertebral range of motion (ROM) based on exhaustive automated detection of bone collisions and joint disarticulation while accounting for interacting rotations of roll, yaw and pitch. First, we hypothesize that the increase of CC also increases overall neck mobility compared with mammals with a regular CC, and vice versa. Second, we hypothesize that the anatomy of the intervertebral articulations determines mobility of the neck. The assessment revealed that CC plays only a secondary role in defining ROM since summed torsion (roll) capacity was primarily determined by vertebral anatomy. Our results thus suggest limited neck rotational adaptive significance of the CC aberration in sloths. Further, the study demonstrates the suitability of our automated approach for the comparative assessment of osteological ROM in vertebral series.

Truly dorsostable runners: Vertebral mobility in rhinoceroses, tapirs, and horses

The vertebral column is a hallmark of vertebrates; it is the structural basis of their body and the locomotor apparatus in particular. Locomotion of any vertebrate animal in its typical habitat is directly associated with functional adaptations of its vertebrae. This study is the first large-scale analysis of mobility throughout the presacral region of the vertebral column covering a majority of extant odd-toed ungulates from 6 genera and 15 species. In this study, we used a previously developed osteometry-based method to calculate available range of motion. We quantified all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). The cervical region in perissodactyls was found to be the most mobile region of the presacral vertebral column in LB and SB. Rhinoceroses and tapirs are characterized by the least mobile necks in SB among odd-toed and even-toed ungulates. Equidae are characterized by very mobile necks, especially in LB. The first intrathoracic joint (T1-T2) in Equidae and Tapiridae is characterized by significantly increased mobility in the sagittal plane compared to the typical thoracic joints and is only slightly less mobile than typical cervical joints. The thoracolumbar part of the vertebral column in odd-toed ungulates is very stiff. Perissodactyls are characterized by frequent fusions of vertebrae with each other with complete loss of mobility. The posterior half of the thoracic region in perissodactyls is characterized by especially stiff intervertebral joints in the SB direction. This is probably associated with hindgut fermentation in perissodactyls: the sagittal stiffness of the posterior thoracic region of the vertebral column is able to passively support the hindgut heavily loaded with roughage. Horses are known as a prime example of a dorsostable galloper among mammals. However, based on SB in the lumbosacral part of the backbone, equids appear to be the least dorsostable among extant perissodactyls; the cumulative SB in equids and tapirs is as low as in the largest representatives of artiodactyls, while in Rhinocerotidae it is even lower representing the minimum across all odd-toed and even-toed ungulates. Morphological features of small Paleogene ancestors of rhinoceroses and equids indicate that dorsostability is a derived feature of perissodactyls and evolved convergently in the three extant families.

From dorsomobility to dorsostability: A study of lumbosacral joint range of motion in artiodactyls

This study is the first analysis of mobility in the lumbosacral joint of even-toed ungulates covering the full range of body masses and running forms. In this study, we modified a previously developed osteometry-based method to calculate the available range of motion (aROM) in the lumbosacral joint in artiodactyls. We quantified all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). This research covers extant artiodactyls from 10 families, 57 genera, and 78 species. The lumbosacral joint in artiodactyls is on average almost twice as mobile in SB as the average intralumbar joint (aROM 15.68° vs 8.22°). In all artiodactyls, the first sacral prezygapophyses are equipped with postfacet fossae determining the available range of lumbosacral hyperextension. SB aROM in the lumbosacral joint in artiodactyls varies almost sevenfold (from 4.53° to 31.19°) and is closely related to the body mass and running form. An allometric equation was developed for the first time, for the joint angular amplitude of motion, exemplified by the artiodactyl lumbosacral SB aROMs, as a power function of body mass, the power coefficient value being close to -0.15. High SB aROM at the lumbosacral joint is characteristic of artiodactyls with at least one of the following characteristics: high cumulative and average SB aROM in the lumbar region (Pearson r = 0.467-0.617), small body mass (r = -0.531), saltatorial or saltatorial-cursorial running form (mean = 16.91-18.63°). The highest SB aROM in the lumbosacral joint is typical for small antelopes and Moschidae (mean = 20.24-20.27°). Among these artiodactyls SB aROMs in the lumbosacral joint are on par with various carnivores. Large and robust artiodactyls, adapted predominantly to mediportal and stilt (running on extremely tall limbs) running forms, have 2-3 times smaller SB aROMs in the lumbosacral joint. Adaptation to endurance galloping in open landscapes (cursorial running form) is accompanied by smaller lumbar and lumbosacral SB aROMs compared to that in saltatorial-cursorial artiodactyls of the same body mass. The wide range of species studied makes it possible to significantly expand the knowledge of relations of the mobility of the lumbosacral joint in artiodactyls to body mass and running form.

How the even-toed ungulate vertebral column works: Comparison of intervertebral mobility in 33 genera

In this study, we used a previously developed osteometry-based method to calculate available range of motion in presacral intervertebral joints in artiodactyls. We have quantified all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). This research covers 10 extant families of artiodactyls from 33 genera and 39 species. The cervical region in artiodactyls is the most mobile region of the presacral vertebral column in SB and LB. Mobility is unevenly distributed throughout the joints of the neck. The posterior neck joints (C4-C7) are significantly more mobile (on average by 2.5-3.5°) to anterior joints (C2-C4) and to the neck-thorax joint (C7-T1) in SB and LB. An increase in the relative length of the cervical region in artiodactyls is accompanied by an increase in the bending amplitudes (SB: Pearson r = 0.781; LB: r = 0.884). Animals with the most mobile necks (representative of Giraffidae and Camelidae) are 2-3 times more mobile in SB and LB compared to species with the least mobile necks. The thoracic region in artiodactyls, as in other mammals, is characterized by the greatest amplitudes of AR due to the tangential orientation of the zygapophyseal articular facets. The lowest AR values in the thoracic region are typical for the heaviest artiodactyls-Hippopotamidae. The highest AR values are typical for such agile runners as cervids, musk deer, pronghorn, as well as large and small antelopes. SB mobility in the posterior part of the thoracic region can be used by artiodactyls during galloping. The highest values of SB aROM in the posterior part of the thoracic region are typical for small animals with high SB mobility in the lumbar region. The lumbar region in mammals is adapted for efficient SB. Both the cumulative and average SB values in the lumbar region showed correspondence to the running type employed by an artiodactyl. The greatest SB amplitudes in the lumbar region are typical for small animals, which use saltatorial and saltatorial-cursorial running. An increase in body size also corresponds to a decrease in lumbar SB amplitudes. The lowest SB amplitudes are typical for species using the so-called mediportal running. Adaptation to endurance galloping in open landscapes is accompanied by a decrease in lumbar SB amplitudes in artiodactyls. The consistency of the approach used and the wide coverage of the studied species make it possible to significantly expand and generalize the knowledge of the biomechanics of the vertebral column in artiodactyls.

Digital tomosynthesis based digital volume correlation: A clinically viable noninvasive method for direct measurement of intravertebral displacements using images of the human spine under physiological load

PURPOSE

We have developed a clinically viable method for measurement of direct, patient-specific intravertebral displacements using a novel digital tomosynthesis based digital volume correlation technique. These displacements may be used to calculate vertebral stiffness under loads induced by a patient's body weight; this is particularly significant because, among biomechanical variables, stiffness is the strongest correlate of bone strength. In this proof of concept study, we assessed the feasibility of the method through a preliminary evaluation of the accuracy and precision of the method, identification of a range of physiological load levels for which displacements are measurable, assessment of the relationship of measured displacements with microcomputed tomography based standards, and demonstration of the in vivo application of the technique.

METHODS

Five cadaveric T11 vertebrae were allocated to three groups in order to study (a) the optimization of digital volume correlation algorithm input parameters, (b) accuracy and precision of the method and the ability to measure displacements at a range of physiological load levels, and (c) the correlation between displacements measured using tomosynthesis based digital volume correlation vs. high resolution microcomputed tomography based digital volume correlation and large scale finite element models. Tomosynthesis images of one patient (Female, 60 yr old) were used to calculate displacement maps, and in turn stiffness, using images acquired in both standing and standing-with-weight (8 kg) configurations.

RESULTS

We found that displacements were accurate (2.28 µm total error) and measurable at physiological load levels (above 267 N) with a linear response to applied load. Calculated stiffness among three tested vertebral bodies was within an acceptable range relative to reported values for vertebral stiffness (5651-13260 N/mm). Displacements were in good qualitative and quantitative agreement with both microcomputed tomography based finite element (r²  = 0.762, P < 0.001) and digital volume correlation (r²  = 0.799, P < 0.001) solutions. For one patient tested twice, once standing and once holding weights, results demonstrated excellent qualitative reproducibility of displacement distributions with superior endplate displacements increasing by 22% with added weight.

CONCLUSIONS

The results of this work collectively suggest the feasibility of the method for in vivo measurement of intravertebral displacements and stiffness in humans. These findings suggest that digital volume correlation using digital tomosynthesis imaging may be useful in understanding the mechanical response of bone to disease and may further enhance our ability to assess fracture risk and treatment efficacy for the spine.

Application of geometrical measurements in the assessment of vertebral strength

Background:

The study was aimed at the development of parameters that could be used as predictors of vertebral strength. Proposed parameters describing vertebral geometry and/or shape can be established on the basis of routine spine roentgenograms, making roentgenography a novel tool for vertebral fracture risk assessment in the future.

Material/Methods:

20 human cadaveric L3 vertebrae were included in the study. Dual-energy X-ray absorptiometry (DXA) was used for an assessment of bone mineral density (BMD). Quantitative computed tomography (QCT) was performed to measure the volumetric bone density as the most reliable parameter in vertebral fracture risk assessment. Geometrical measurements were performed on the basis of high quality and high resolution computer tomography 3-dimensional images. Biomechanical tests were performed to measure vertebral strength. Two parameters were defined on the basis of extensive research: the ratio between vertebral base area and its height (A/H), and the ratio of vertebral coronal width to coronal height (W/H). Correlations between vertebral mechanical strength – its BMD, QCT density, A/H and W/H were calculated.

Results:

The best correlation to bone durability was achieved for QCT density (r=0.882), while correlation strength for BMD (r=0.779) and A/H (r=–0.773) were comparable. W/H correlated better than BMD to mechanical strength (–0.788).

Conclusions:

Geometrical parameters of vertebrae potentially measured on spine radiograms could be used as predictors of vertebral durability. The calculated correlation coefficients suggest that one of the proposed parameters works better than the commonly used BMD.