Uphar Chamoli

The craniomandibular mechanics of being human

Diminished bite force has been considered a defining feature of modern Homo sapiens, an interpretation inferred from the application of two-dimensional lever mechanics and the relative gracility of the human masticatory musculature and skull. This conclusion has various implications with regard to the evolution of human feeding behaviour. However, human dental anatomy suggests a capacity to withstand high loads and two-dimensional lever models greatly simplify muscle architecture, yielding less accurate results than three-dimensional modelling using multiple lines of action. Here, to our knowledge, in the most comprehensive three-dimensional finite element analysis performed to date for any taxon, we ask whether the traditional view that the bite of H. sapiens is weak and the skull too gracile to sustain high bite forces is supported. We further introduce a new method for reconstructing incomplete fossil material. Our findings show that the human masticatory apparatus is highly efficient, capable of producing a relatively powerful bite using low muscle forces. Thus, relative to other members of the superfamily Hominoidea, humans can achieve relatively high bite forces, while overall stresses are reduced. Our findings resolve apparently discordant lines of evidence, i.e. the presence of teeth well adapted to sustain high loads within a lightweight cranium and mandible.

Toward integration of geometric morphometrics and computational biomechanics: New methods for 3D virtual reconstruction and quantitative analysis of Finite Element Models

The ability to warp three-dimensional (3D) meshes from known biological morphology to fit other known, predicted or hypothetical morphologies has a range of potential applications in functional morphology and biomechanics. One of the most challenging of these applications is Finite Element Analysis (FEA), a potentially powerful non-destructive tool in the prediction of mechanical behaviour. Geometric morphometrics is another typically computer-based approach commonly applied in morphological studies that allows for shape differences between specimens to be quantified and analysed. There has been some integration of these two fields in recent years. Although a number of shape warping approaches have been developed previously, none are easily accessible. Here we present an easily accessed method for warping meshes based on freely available software and test the effectiveness of the approach in FEA using the varanoid lizard mandible as a model. We further present new statistical approaches, strain frequency plots and landmark point strains, to analyse FEA results quantitatively and further integrate FEA with geometric morphometrics. Using strain frequency plots, strain field, bending displacements and landmark point strain data we demonstrate that the mechanical behaviour of warped specimens reproduces that of targets without significant error. The influence of including internal cavity morphology in FEA models was also examined and shown to increase bending displacements and strain magnitudes in FE models. The warping approaches presented here will be useful in a range of applications including the generation and analysis of virtual reconstructions, generic models that approximate species means, hypothetical morphologies and evolutionary intermediaries.

Finite element analysis of three patterns of internal fixation of fractures of the mandibular condyle

The most stable pattern of internal fixation for fractures of the mandibular condyle is a matter for ongoing discussion. In this study we investigated the stability of three commonly used patterns of plate fixation, and constructed finite element models of a simulated mandibular condylar fracture. The completed models were heterogeneous in the distribution of bony material properties, contained about 1.2 million elements, and incorporated simulated jaw-adducting musculature. Models were run assuming linear elasticity and isotropic material properties for bone. This model was considerably larger and more complex than previous finite element models that have been used to analyse the biomechanical behaviour of differing plating techniques. The use of two parallel 2.0 titanium miniplates gave a more stable configuration with lower mean element stresses and displacements over the use of a single miniplate. In addition, a parallel orientation of two miniplates resulted in lower stresses and displacements than did the use of two miniplates in an offset pattern. The use of two parallel titanium plates resulted in a superior biomechanical result as defined by mean element stresses and relative movement between the fractured fragments in these finite element models.

Finite element micro-modelling of a human ankle bone reveals the importance of the trabecular network to mechanical performance: New methods for the generation and comparison of 3D models

Most modelling of whole bones does not incorporate trabecular geometry and treats bone as a solid non-porous structure. Some studies have modelled trabecular networks in isolation. One study has modelled the performance of whole human bones incorporating trabeculae, although this required considerable computer resources and purpose-written code. The difference between mechanical behaviour in models that incorporate trabecular geometry and non-porous models has not been explored. The ability to easily model trabecular networks may shed light on the mechanical consequences of bone loss in osteoporosis and remodelling after implant insertion. Here we present a Finite Element Analysis (FEA) of a human ankle bone that includes trabecular network geometry. We compare results from this model with results from non-porous models and introduce protocols achievable on desktop computers using widely available softwares. Our findings show that models including trabecular geometry are considerably stiffer than non-porous whole bone models wherein the non-cortical component has the same mass as the trabecular network, suggesting inclusion of trabecular geometry is desirable. We further present new methods for the construction and analysis of 3D models permitting: (1) construction of multi-property, non-porous models wherein cortical layer thickness can be manipulated; (2) maintenance of the same triangle network for the outer cortical bone surface in both 3D reconstruction and non-porous models allowing exact replication of load and restraint cases; and (3) creation of an internal landmark point grid allowing direct comparison between 3D FE Models (FEMs).

Allometry in the distribution of material properties and geometry of the felid skull: Why larger species may need to change and how they may achieve it

Extant members of the cat family (Felidae) have been considered behaviourally and morphologically conservative, i.e., despite great differences in size, there is relatively little variation in either the shape of the felid skull and dentition across species, or in the way in which these structures are used to kill and dismember prey. Consequently felids have been considered an appropriate focus for a number of investigations into the influence of allometry on craniomandibular mechanics and morphology. However, although previous treatments have considered the role of shape, they have not investigated the influence of differences in the distribution of relatively stiff cortical and more compliant cancellous bone on performance. Here, using models that incorporate material properties for both cortical and cancellous bone, we apply three-dimensional (3D) finite element analysis (FEA) to models representing the skulls of seven extant felid species. Our objectives being to determine allometric trends regarding both overall geometry and the relative distributions of cortical and cancellous bone tissue. We also more comprehensively assess variation in the efficiency with which muscular force is converted to bite force and the capacity to resist associated stresses. Our results show that the cheetah (Acinonyx jubatus) may be exceptional regarding both the efficiency with which muscular force is converted to bite force and the distribution of stress. We found a negative allometric trend between cortical bone volume and total skull bone volume, and positive allometry between the total skull bone volume and skull surface area. Results gained from mathematical modelling of beam analogies suggest that these trends reflect a need for larger species to respond to physical challenges associated with increased size, and, that changes in skull shape, bone composition, or a combination of both may be required to accommodate these challenges. With geometrical scaling stress increases by the same factor, and displacement by the same factor squared, but the ultimate failure stress of the material is invariant. We find that as species become larger, overall skull bone volume relative to surface area increases by adding a higher proportion of less dense and more compliant cancellous bone. This results in an increased cross-sectional area and second moment of inertia, which acts to reduce the overall stresses. An overall saving in mass is a likely additional consequence. Although we do find evidence that skull stiffness does diminish with size, we also argue that this is at least in part mitigated through the influence of these allometric trends. We further suggest that these trends and the explanations for them may be universal for vertebrates.

Comparative Biomechanical Modeling of Metatherian and Placental Saber-Tooths: A Different Kind of Bite for an Extreme Pouched Predator.

Questions surrounding the dramatic morphology of saber-tooths, and the presumably deadly purpose to which it was put, have long excited scholarly and popular attention. Among saber-toothed species, the iconic North American placental, Smilodon fatalis, and the bizarre South American sparassodont, Thylacosmilus atrox, represent extreme forms commonly forwarded as examples of convergent evolution. For S. fatalis, some consensus has been reached on the question of killing behaviour, with most researchers accepting the canine-shear bite hypothesis, wherein both head-depressing and jaw closing musculatures played a role in delivery of the fatal bite. However, whether, or to what degree, T. atrox may have applied a similar approach remains an open question. Here we apply a three-dimensional computational approach to examine convergence in mechanical performance between the two species. We find that, in many respects, the placental S. fatalis (a true felid) was more similar to the metatherian T. atrox than to a conical-toothed cat. In modeling of both saber-tooths we found that jaw-adductor-driven bite forces were low, but that simulations invoking neck musculature revealed less cranio-mandibular stress than in a conical-toothed cat. However, our study also revealed differences between the two saber-tooths likely reflected in the modus operandi of the kill. Jaw-adductor-driven bite forces were extremely weak in T. atrox, and its skull was even better-adapted to resist stress induced by head-depressors. Considered together with the fact that the center of the arc described by the canines was closer to the jaw-joint in Smilodon, our results are consistent with both jaw-closing and neck musculature playing a role in prey dispatch for the placental, as has been previously suggested. However, for T. atrox, we conclude that the jaw-adductors probably played no major part in the killing bite. We propose that the metatherian presents a more complete commitment to the already extreme saber-tooth ‘lifestyle’.

Pedicle screw-based posterior dynamic stabilizers for degenerative spine:In vitrobiomechanical testing and clinical outcomes: Pedicle Screw-based Posterior Dynamic Stabilizers for Degenerative Spine

Dynamic stabilization in a degenerate symptomatic spine may be advantageous compared with conventional fusion procedures, as it helps preserve motion and minimizes redistribution of loads at instrumented and adjacent segments. This article presents a systematic review of biomechanical and clinical evidence available on some of the pedicle screw based posterior dynamic stabilization (PDS) devices. Using Medline, Embase, and Scopus online databases, we identified four pedicle-screw-PDS devices for which both, biomechanical testing and clinical follow-up data are available: Graf artificial ligaments, Isobar TTL, Polyetheretherketone rods, and Dynesys. The current state-of-the-art of pedicle-screw-PDS devices is far from achieving its desired biomechanical efficacy, which has resulted in a weak support for the posited clinical benefits. Although pedicle-screw-PDS devices are useful in salvaging a moderately degenerate functionally suboptimal disc, for severe disc degeneration cases fusion is still the preferred choice. We conclude that a pedicle-screw-PDS device should aim at restoring load sharing amongst spinal elements while preserving the qualitative and quantitative nature of spinal motion, especially minimize posterior shift of the helical axis of motion. More precise and objective assessment techniques need to be standardized for in vivo evaluation of intervertebral motion and load sharing amongst spinal elements across different pedicle-screw-PDS devices.

Interpedicular kinematics in an in vitro biomechanical assessment of a bilateral lumbar spondylolytic defect.

BACKGROUND A spondylolytic defect in lumbar vertebra is a common condition during early childhood and adolescence, and is considered a precursor to spondylolisthesis. This study examined whether a bilateral spondylolytic defect in lumbar spine intrinsically results in increased intervertebral translations during different bending motions. METHODS Seven fresh frozen cadaveric kangaroo lumbar (L1-L6) spine specimens were tested in a kinematic spine simulator; first in their intact state, followed by creating a bilateral spondylolytic defect at L4 and retesting. In addition to recording global and segmental range of motions, the pedicles at L3, L4, and L5 vertebrae were digitized bilaterally and virtually tracked throughout testing. Interpedicular kinematic metrics were employed to capture any changes in translatory motions during flexion-extension, bilateral bending, and axial torsion testing modes. FINDINGS Following the defect, range of motion at the defect level (L4-L5) increased significantly in all the three motion planes. At L4-L5, normalized interpedicular displacement increased significantly in flexion-extension (median change +156%) and bilateral bending (median change +58%) motions, but changes in bending-plane and out-of-plane intervertebral translations were not significant in any of the testing modes. INTERPRETATION In the absence of any significant changes in bending-plane and out-of-plane intervertebral translations at L4-L5, changes in interpedicular displacement would directly correspond with the stretching of posterior annulus of the L4-L5 intervertebral disc. A bilateral spondylolytic defect at L4 may result in significant overstretching of the posterior annulus of the L4-L5 disc during flexion-extension and bilateral bending motions.

Finite element analysis of patient-specific condyle fracture plates: a preliminary study.

Various patterns of internal fixation of mandibular condyle fractures have been proposed in the literature. This study investigates the stability of two patient-specific implants (PSIs) for the open reduction and internal fixation of a subcondylar fracture of the mandible. A subcondylar fracture of a mandible was simulated by a series of finite element models. These models contained approximately 1.2 million elements, were heterogeneous in bone material properties, and also modeled the muscles of mastication. Models were run assuming linear elasticity and isotropic material properties for bone. The stability and von Mises stresses of the simulated condylar fracture reduced with each of the PSIs were compared. The most stable of the plate configurations examined was PSI 1, which had comparable mechanical performance to a single 2.0 mm straight four-hole plate.

The Role of Sacral Slope in the Progression of a Bilateral Spondylolytic Defect at L5 to Spondylolisthesis: A Biomechanical Investigation Using Finite Element Analysis

Study Design: A biomechanical study using finite element analysis. Objectives: The main objective of this study was to investigate the role of sacral slope in the progression of a L5 bilateral spondylolytic defect to spondylolisthesis. Methods: A 3-dimensional model of lumbosacral spine was built using computed tomography (CT) data procured from an anonymized healthy male subject. The segmented CT data was manipulated to generate 3 more models representing L5 bilateral spondylolytic defect with normal sacral slope (SS), sacral slope increased by 10° (SS+10), and sacral slope decreased by 10° (SS-10). The 3D models were imported into finite element modelling software Strand7 for preprocessing, running nonlinear static solves, and postprocessing of the results. Results: Directional biomechanical instabilities were induced in the lumbosacral spine as a result of changes in the L5-S1 disc shape secondary to the changes in sacral slope. Compared with the normal L5 lytic model, wedging of the L5-S1 disc (SS+10) resulted in a significantly greater range of motion in flexion (18% ↑) but extension motion characteristics were similar. Conversely, flattening of the L5-S1 disc (SS-10) resulted in a significantly greater range of motion in extension (16% ↑) but flexion motion characteristics were similar to that of the normal L5 lytic model. Conclusions: Variations in sacral slope while preserving the L5-S1 mid-disc height and orientation of the L5 vertebra resulted in variations in the L5-S1 disc shape. The results suggest that for such extremities in the L5-S1 disc shape different pathomechanisms exist for the progression of the L5 lytic defect to spondylolisthesis.