Free body analysis,beam mechanics,and finite element modeling of the mandible of Alligator mississippiensis

The mechanical behavior of mammalian mandibles is well‐studied, but a comprehensive biomechanical analysis (incorporating detailed muscle architecture, accurate material properties, and three‐dimensional mechanical behavior) of an extant archosaur mandible has never been carried out. This makes it unclear how closely models of extant and extinct archosaur mandibles reflect reality and prevents comparisons of structure–function relationships in mammalian and archosaur mandibles. We tested hypotheses regarding the mechanical behavior of the mandible of Alligator mississippiensis by analyzing reaction forces and bending, shear, and torsional stress regimes in six models of varying complexity. Models included free body analysis using basic lever arm mechanics, 2D and 3D beam models, and three high‐resolution finite element models of the Alligator mandible, incorporating, respectively, isotropic bone without sutures, anisotropic bone with sutures, and anisotropic bone with sutures and contact between the mandible and the pterygoid flange. Compared with the beam models, the Alligator finite element models exhibited less spatial variability in dorsoventral bending and sagittal shear stress, as well as lower peak values for these stresses, suggesting that Alligator mandibular morphology is in part designed to reduce these stresses during biting. However, the Alligator models exhibited greater variability in the distribution of mediolateral and torsional stresses than the beam models. Incorporating anisotropic bone material properties and sutures into the model reduced dorsoventral and torsional stresses within the mandible, but led to elevated mediolateral stresses. These mediolateral stresses were mitigated by the addition of a pterygoid‐mandibular contact, suggesting important contributions from, and trade‐offs between, material properties and external constraints in Alligator mandible design. Our results suggest that beam modeling does not accurately represent the mechanical behavior of the Alligator mandible, including important performance metrics such as magnitude and orientation of reaction forces, and mediolateral bending and torsional stress distributions. J.Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Determination of oral mucosal Poisson’s ratio and coefficient of friction from in-vivo contact pressure measurements

Despite their considerable importance to biomechanics, there are no existing methods available to directly measure apparent Poisson’s ratio and friction coefficient of oral mucosa. This study aimed to develop an inverse procedure to determine these two biomechanical parameters by utilizing in vivo experiment of contact pressure between partial denture and beneath mucosa through nonlinear finite element (FE) analysis and surrogate response surface (RS) modelling technique. First, the in vivo denture–mucosa contact pressure was measured by a tactile electronic sensing sheet. Second, a 3D FE model was constructed based on the patient CT images. Third, a range of apparent Poisson’s ratios and the coefficients of friction from literature was considered as the design variables in a series of FE runs for constructing a RS surrogate model. Finally, the discrepancy between computed in silico and measured in vivo results was minimized to identify the best matching Poisson’s ratio and coefficient of friction. The established non-invasive methodology was demonstrated effective to identify such biomechanical parameters of oral mucosa and can be potentially used for determining the biomaterial properties of other soft biological tissues.     

Three dimensional model of the human mandible

A new biomechanical three-dimensional (3D) model for the human mandible is proposed. A simple two-dimensional model cannot explain the biomechanics of the human mandible, where muscular forces through occlusion and condylar surfaces are in a state of dynamical 3D equilibrium. All forces are resolved into components according to a selected coordinate system. The muscular forces, which during clenching act on the jaw, along with the necessary force level for chewing, also act as some kind of stabilizers of the mandibular condyles preventing dislocation and loading of nonarticular tissues.     

A customized fixation plate with novel structure designed by topological optimization for mandibular angle fracture based on finite element analysis

Background

The purpose of this study was to design a customized fixation plate for mandibular angle fracture using topological optimization based on the biomechanical properties of the two conventional fixation systems, and compare the results of stress, strain and displacement distributions calculated by finite element analysis (FEA).

Methods

A three-dimensional (3D) virtual mandible was reconstructed from CT images with a mimic angle fracture and a 1 mm gap between two bone segments, and then a FEA model, including volume mesh with inhomogeneous bone material properties, three loading conditions and constraints (muscles and condyles), was created to design a customized plate using topological optimization method, then the shape of the plate was referenced from the stress concentrated area on an initial part created from thickened bone surface for optimal calculation, and then the plate was formulated as “V” pattern according to dimensions of standard mini-plate finally. To compare the biomechanical behavior of the “V” plate and other conventional mini-plates for angle fracture fixation, two conventional fixation systems were used: type A, one standard mini-plate, and type B, two standard mini-plates, and the stress, strain and displacement distributions within the three fixation systems were compared and discussed.

Results

The stress, strain and displacement distributions to the angle fractured mandible with three different fixation modalities were collected, respectively, and the maximum stress for each model emerged at the mandibular ramus or screw holes. Under the same loading conditions, the maximum stress on the customized fixation system decreased 74.3, 75.6 and 70.6% compared to type A, and 34.9, 34.1, and 39.6% compared to type B. All maximum von Mises stresses of mandible were well below the allowable stress of human bone, as well as maximum principal strain. And the displacement diagram of bony segments indicated the effect of treatment with different fixation systems.

Conclusions

The customized fixation system with topological optimized structure has good biomechanical behavior for mandibular angle fracture because the stress, strain and displacement within the plate could be reduced significantly comparing to conventional “one mini-plate” or “two mini-plates” systems. The design methodology for customized fixation system could be used for other fractures in mandible or other bones to acquire better mechanical behavior of the system and improve stable environment for bone healing. And together with SLM, the customized plate with optimal structure could be designed and fabricated rapidly to satisfy the urgent time requirements for treatment.

     

Dietary correlates of temporomandibular joint morphology in the great apes

Behavioral observations of great apes have consistently identified differences in feeding behavior among species, and these differences have been linked to variation in masticatory form. As the point at which the mandible and cranium articulate, the temporomandibular joint (TMJ) is an important component of the masticatory apparatus. Forces are transmitted between the mandible and cranium via the TMJ, and this joint helps govern mandibular range of motion. This study examined the extent to which TMJ form covaries with feeding behavior in the great apes by testing a series of biomechanical hypotheses relating to specific components of joint shape using linear measurements extracted from three‐dimensional coordinate data. Results of these analyses found that taxa differ significantly in TMJ shape, particularly in the mandibular fossa. Chimpanzees have relatively more anteroposteriorly elongated joint surfaces, whereas gorillas tend to have relatively anteroposteriorly compressed joints. Orangutans were most commonly intermediate in form between Pan and Gorilla, perhaps reflecting a trade‐off between jaw gape and load resistance capabilities. Importantly, much of the observed variation among taxa reflects differences in morphologies that facilitate gape over force production. These data therefore continue to emphasize the unclear relationship between mandibular loading and bony morphology, but highlight the need for further data regarding food material properties, jaw gape, and ingestive/food processing behaviors. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Dietary correlates associated with the mental foramen in primates: implications for interpreting the fossil record

The mandibular nerve is a sensory and motor nerve that innervates the muscles of mastication, the lower dentition, and the lower lip and surrounding structures. Although this nerve contains both efferent and afferent fibers, the mental nerve, a terminal branch of the mandibular nerve, is a strictly sensory nerve that exits the mental foramen and innervates the lower lip, the skin overlaying the mandible, and the oral mucosa around the mandible. Osteological foramina are often used as proxies for nerve cross section area and they often correlate well with some aspect of a primate's ecology (e.g., optic foramen and visual acuity). The primary objective of this study is to explore the correlation between the mental foramen and dietary preference among primates. The mental foramen of 40 primate species (n = 180) was measured from 3‐D surface models of the mandible. Both conventional and phylogenetic tests indicate that although frugivores have larger mental foramina than folivores, the differences were not significant. These results show that while structures like the infraorbital foramen correlate well with diet and touch sensitivity, the mental foramen does not. Based on these findings, the mental foramen is not a suggested morphological character for interpreting of the fossil record. J. Morphol. 277:978–985, 2016. © 2016 Wiley Periodicals, Inc.     

Characters discriminating two seed husking mechanisms in finches (Fringillidae: Carduelinae) and estrildids (Passeridae: Estrildinae)

Finches have two major types of seed husking, each with two phases: crushing and seed husk disposal. Recent observations show that both Spice Finches and Greenfinches husk seeds by a crushing mechanism, but the Greenfinch, Carduelis chloris, applies mediolateral movements of the lower mandible, especially during seek husk disposal, while such movements are not-found in the Spice Finch, Lonchura punctulata. A detailed comparison of the anatomies of their jaw apparatuses elucidates that numerous discriminating characters form an integrated complex that parallels the difference in seed husking. These focus on 1) a more flattened shape of the articular facets in the quadratomandibular articulation and a lack of the postorbital ligament, resulting in decoupling of the quadratomandibular joint facets, and release of the pseudotemporal muscle from jaw elevation only and 2) adductor mandibulae and pterygoid muscles with an increased medial component in the Greenfinch. Muscles built heavier in the Greenfinch than the Spice Finch, especially mediolaterally oriented muscles, are in concurrence with muscle asymmetries found in crossbills (Loxia sp.), a finch-species employing unilateral mandibular rotation. These two groups of differences are conditions to transform a dorsoventrally oriented into an improved, mediolaterally oriented husking design. These features, though in concert with many others, are proposed as they key innovations that released the increase of crushing forces by extending forces from jaw muscles and improvement of the disposal of the crushed shell by left-right lower mandible manipulation. Such an improved husking capacity may have opened the vast food-array of hard-shelled, dicotyledon seeds, allowing extensive trophic radiation. J. Morphol 232:1–33, 1997. © 1997 Wiley-Liss, Inc.     

Comparative cranial myology and biomechanics of Plateosaurus and Camarasaurus and evolution of the sauropod feeding apparatus

Sauropodomorpha represents an important group of Mesozoic megaherbivores, and includes the largest terrestrial animals ever known. It was the first dinosaur group to become abundant and widespread, and its members formed a significant component of terrestrial ecosystems from the Late Triassic until the end of the Cretaceous. Both of these factors have been explained by their adoption of herbivory, but understanding the evolution of sauropodomorph feeding has been hampered by the scarcity of biomechanical studies. To address this, the jaw adductor musculature of the basal sauropodomorph Plateosaurus and the sauropod Camarasaurus have been reconstructed. These reconstructions provide boundary conditions for finite element models to assess differences in structural performance between the two taxa. Results demonstrate that Camarasaurus was capable of much greater bite forces than Plateosaurus, due to greater relative adductor muscle mass and shape changes to the mandible. The skull and mandible of Camarasaurus are also ‘stronger’ under static biting. The Plateosaurus mandible appears to compromise structural efficiency and force transmission in order to maintain relatively high jaw closure speed. This supports suggestions of facultative omnivory in basal sauropodomorph taxa. The expanded mandibular symphysis and ‘lateral plates’ of sauropods each lead to greater overall craniomandibular robustness, and may have been especially important in accommodating forces related to asymmetric loading. The functional roles of these characters, and observed general shape changes in increasing skull robustness, are consistent with hypotheses linking bulk‐herbivory with the origin of Sauropoda and the evolution of gigantism.     

太行山猕猴下颌的生物力学研究

采用定点方法,对太行山猕猴下颌骨有关变量作了测量、统计和分析。通过对太行山猕猴下颌生物力学、异速生长的研究表明,雄性下颌相对较长,与此有关的生物力学特点是下颌髁后缘至M1近中侧长、下颌髁后缘至M3下次小尖长和下颌支高等阻力臂减小,而下颌髁长、下颌髁宽、下颌体高Ⅱ、下颌体宽Ⅱ、下颌联合长等变量则增强,使咀嚼力增加。这些特点均与其生态对策密切相关。研究结果还与有关材料进行了比较,表明太行山猕猴与国内其他地区的猕猴存在着明显差异。     

A unique feeding strategy of the extinct marine mammal Kolponomos: convergence on sabretooths and sea otters

Mammalian molluscivores feed mainly by shell-crushing or suction-feeding. The extinct marine arctoid, Kolponomos, has been interpreted as an otter-like shell-crusher based on similar dentitions. However, neither the masticatory biomechanics of the shell-crushing adaptation nor the way Kolponomos may have captured hard-shelled prey have been tested. Based on mandibular symphyseal morphology shared by Kolponomos and sabre-toothed carnivores, we hypothesize a sabretooth-like mechanism for Kolponomos prey-capture, whereby the mandible functioned as an anchor. Torque generated from jaw closure and head flexion was used to dislodge prey by prying, with prey then crushed using cheek teeth. We test this hypothesized feeding sequence using phylogenetically informed biomechanical simulations and shape analyses, and find a strongly supported, shared high mandibular stiffness in simulated prey-capture bites and mandibular shape in Kolponomos and the sabre-toothed cat Smilodon. These two distantly related taxa converged on using mandibles to anchor cranial torqueing forces when prying substrate-bound prey in the former and sabre-driving forces during prey-killing in the latter. Simulated prey-crushing bites indicate that Kolponomos and sea otters exhibit alternative structural stiffness-bite efficiency combinations in mandibular biomechanical adaptation for shell-crushing. This unique feeding system of Kolponomos exemplifies a mosaic of form-function convergence relative to other Carnivora.     

Ontogeny of material stiffness heterogeneity in the macaque mandibular corpus

Evidence is accumulating that bone material stiffness increases during ontogeny, and the role of elastic modulus in conditioning attributes of strength and toughness is therefore a focus of ongoing investigation. Developmental changes in structural properties of the primate mandible have been documented, but comparatively little is known about changes in material heterogeneity and their impact on biomechanical behavior. We examine a cross‐sectional sample of Macaca fascicularis (N = 14) to investigate a series of hypotheses that collectively evaluate whether the patterning of material stiffness (elastic modulus) heterogeneity in the mandible differs among juvenile, subadult and adult individuals. Because differences in age‐related activity patterns are known to influence bone stiffness and strength, these data are potentially useful for understanding the relationship between feeding behavior on the one hand and material and structural properties of the mandible on the other. Elastic modulus is shown to be spatially dependent regardless of age, with this dependence being explicable primarily by differences in alveolar versus basal cortical bone. Elastic modulus does not differ consistently between buccal and lingual cortical plates, despite likely differences in the biomechanical milieu of these regions. Since we found only weak support for the hypothesis that the spatial patterning of heterogeneity becomes more predictable with age, accumulated load history may not account for regional differences in bone material properties in mature individuals with respect to the mandibular corpus. Am J Phys Anthropol 153:297–304, 2014. © 2013 Wiley Periodicals, Inc.     

Analysis of human mandibular mechanics based on screw theory and in vivo data

In this paper the mechanics of human mandibular function is described in terms of the associated screws. The two distinct, yet related features of jaw mechanics, involving the motion itself as well as the forces, are both functions of the anatomical constraints, namely the contact areas that exist within the temporomandibular joint, and the forces of the muscles and tendons that allow motion to occur. The relationships that exist between these two aspects of jaw-motion are identified in this paper showing that muscle forces can be uniquely represented in terms of the action screw. This new approach to analyzing the mechanics of jaw-motion also incorporates the previously studied motion screw or helical axis. A consistent dynamic model is formulated where the action screw is used to represent the action of the closing muscle forces while the moment arms of the muscle forces are determined about the motion screw representing mandibular kinematics. The action screw formulation is verified using in vivo motion data and MR image information for a single asymptomatic subject. The results confirm the feasibility of the method and its application in dental research. A general increase in the mechanical advantage of most muscles, in the distance between action and motion screws as well as in the expended energy towards the end of the jaw-closing phase was observed. Asymmetries in the distribution of muscle force magnitudes appeared to influence the resultant force and moment of the action screw but had little effect on its spatial location. The method presented is intended to facilitate understanding of mandibular function and dysfunction.     

Asymmetric size and shape variation in the Central European transect across the house mouse hybrid zone

We studied asymmetric variation of the mandible in the Central European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. Within introgression classes, defined by the share of diagnostic allozymes, we quantified the directional and fluctuating component of asymmetric variation, as well as skewness and kurtosis of individual asymmetry distributions. Furthermore, in the same manner we re‐analysed asymmetric variation of the ventral side of the skull. According to the quadratic polynomial model, the mandible shape‐fluctuating asymmetry, but not size‐fluctuating asymmetry, was significantly decreased in the centre of the hybrid zone (with a minimum predicted for a hybrid index of 0.41). On the contrary, the skull shape‐fluctuating asymmetry non‐monotonically increased towards the musculus side of the hybrid zone (with a peak predicted for a hybrid index of 0.86). Thus, the impact of hybridization on fluctuating asymmetry is trait‐specific in this portion of the house mouse hybrid zone. The only general feature of asymmetric variation we observed was the shift towards the platykurtosis of asymmetry distributions in the centre of the hybrid zone. Taken together, we suggest genetic variability for right–left asymmetries to be generally increased, but the developmental instability of mandible shape to be decreased, by hybridization. We hypothesize the decrease of developmental instability to be caused by overdominant effects on developmental dynamics rather than by increased heterozygosity. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 13–27.     

Development of a measurement system for the mechanical load of functional appliances

Devices called functional appliances are commonly used in orthodontics for treating maxillary protrusion. These devices mechanically force the mandible forward to apply traction force to the mandibular condyle. This promotes cartilaginous growth in the small mandible. However, no studies have clarified how much traction force is applied to the mandibular condyle. Moreover, it remains unknown as to how anatomical characteristics affect this traction force. Therefore, in this study, we developed a device for measuring the amount of force generated while individual patients wore functional appliances, and we investigated the relationship between forces with structures surrounding the mandibular condyle. We compared traction force values with cone-beam computed tomography image data in eight subjects. The functional appliance resulted in a traction force of 339–1477 gf/mm, with a mean value of 196.5 gf/mm for the elastic modulus of the mandible. A comparison with cone-beam computed tomography image data suggested that the mandibular traction force was affected by the mandibular condyle and shape of the articular eminence. This method can contribute to discovering efficient treatment techniques more suited to individual patients.     

Morphological and functional diversity of the mandible in suckermouth armored catfishes (Siluriformes: Loricariidae)

We examined the mandibles of 377 individuals representing 25 species, 12 genera, 5 tribes, and 2 subfamilies of the Loricariidae, a species‐rich radiation of detritivorous–herbivorous neotropical freshwater fishes distinguished by having a ventral oral disk and jaws specialized for surface attachment and benthic feeding. Loricariid mandibles are transversely oriented and bilaterally independent, each rotating predominantly around its long axis, although rotational axes likely vary with mandibular geometry. On each mandible, we measured three traditional and three novel morphological parameters chosen primarily for their functional relevance. Five parameters were linear distances and three of these were analogous to traditional teleost in‐ and out‐levers for mandibular adduction. The sixth parameter was insertion area of the combined adductor mandibulae muscle (AM_area), which correlated with adductor mandibulae volume across a subset of taxa and is interpreted as being proportional to maximum force deliverable to the mandible. Multivariate analysis revealed distributions of phylogenetically diagnosed taxonomic groupings in mandibular morphospace that are consistent with an evolutionary pattern of basal niche conservatism giving rise to multiple adaptive radiations within nested clades. Correspondence between mandibular geometry and function was explored using a 3D model of spatial relationships among measured parameters, potential forces, and axes of rotation. By combining the model with known loricariid jaw kinematics, we developed explicit hypotheses for how individual parameters might relate to each other during kinesis. We hypothesize that the ratio [AM_area/tooth row length²] predicts interspecific variation in the magnitude of force entering the mandible per unit of substrate contacted during feeding. Other newly proposed metrics are hypothesized to predict variation in aspects of mandibular mechanical advantage that may be specific to Loricariidae and perhaps shared with other herbivorous and detritivorous fishes. 2011. © 2011Wiley Periodicals, Inc.     

Functional and morphological correlates of mandibular symphyseal form in a living human sample

Variation in recent human mandibular form is often thought to reflect differences in masticatory behavior associated with variation in food preparation and subsistence strategies. Nevertheless, while mandibular variation in some human comparisons appear to reflect differences in functional loading, other comparisons indicate that this relationship is not universal. This suggests that morphological variation in the mandible is influenced by other factors that may obscure the effects of loading on mandibular form. It is likely that highly strained mandibular regions, including the corpus, are influenced by well‐established patterns of lower facial skeletal integration. As such, it is unclear to what degree mandibular form reflects localized stresses incurred during mastication vs. a larger set of correlated features that may influence bone distribution patterns. In this study, we examine the relationship between mandibular symphyseal bone distribution (i.e., second moments of area, cortical bone area) and masticatory force production (i.e., in vivo maximal bite force magnitude and estimated symphyseal bending forces) along with lower facial shape variation in a sample of n = 20 living human male subjects. Our results indicate that while some aspects of symphyseal form (e.g., wishboning resistance) are significantly correlated with estimates of symphyseal bending force magnitude, others (i.e., vertical bending resistance) are more closely tied to variation in lower facial shape. This suggests that while the symphysis reflects variation in some variables related to functional loading, the complex and multifactorial influences on symphyseal form underscores the importance of exercising caution when inferring function from the mandible especially in narrow taxonomic comparisons. Am J Phys Anthropol 153:387–396, 2014. © 2013 Wiley Periodicals, Inc.     

Glenohumeral contact forces in reversed anatomy shoulder replacement

A major requirement to design an implant is to develop our understanding of the applied internal forces during everyday activities. In the absence of any basic apparatus for measuring forces directly, it is essential to rely on modelling. The major aim of this study was therefore to understand the biomechanical function of subjects with the reversed anatomy Bayley?Walker prosthesis, using an inverse dynamic shoulder model. In this context, the muscle and joint forces of 12 Bayley–Walker subjects were compared to those of 12 normal subjects during 12 activities of daily living.Maximum glenohumeral contact forces for normal and Bayley–Walker subjects were found to be 77% (±15) and 137% (±21) body weight for lifting a 2 kg shopping bag, and the least forces 29% (±4) and 67% (±8) body weight for reaching to opposite axilla, respectively. For normal subjects, middle deltoid, supraspinatus and infraspinatus were found to be the most active muscles across the subjects and tasks. On the other hand, for implanted subjects with a lack of rotator cuff muscles, the middle deltoid and coracobrachialis muscles were found to be the most active. The biomechanical model can therefore be used in order to gain knowledge about the pathology as well as possible post surgical rehab for subjects with reversed shoulder replacement.     

Balancing function of the masticatory muscles during incisal biting in two murid rodents,Apodemus speciosus and Clethrionomys rufocanus

The functional significance of masticatory muscle direction was estimated using a mechanical model in two murid rodents: the Japanese field mouse (Apodemus speciosus) and the gray red-backed vole (Clethrionomys rufocanus). Theoretical analyses of the data suggest that a balancing mechanism among the muscle forces occurs during incisal power stroke. The activation of the large deep masseter in both murids results in marked tensile separation of two hemimandibles at the flexible mandibular symphysis. Activation of the internal pterygoid decreases this large tensile force at the symphysis more efficiently than other muscles. The lines of action of the deep masseter and internal pterygoid are aligned to produce such a balancing function in both species studied here. The resultant force generated by the deep masseter on both sides is opposite in direction to the reaction force at the lower incisor tip. Therefore, the large deep masseter forms an effective mandibular support mechanism when the reaction forces during biting push the mandible downward. Because of the area of insertion and the line of action, the posterior temporalis appears to have an important role in stabilizing the position of the mandibular condyle in the glenoid fossa during incisal biting. J. Morphol. 236:49–56, 1998. © 1998 Wiley-Liss, Inc.     

Analysis of the Stress and Displacement Distribution of Inferior Tibiofibular Syndesmosis Injuries Repaired with Screw Fixation: A Finite Element Study

Background

Studies of syndesmosis injuries have concentrated on cadaver models. However, they are unable to obtain exact data regarding the stress and displacement distribution of various tissues, and it is difficult to compare models. We investigated the biomechanical effects of inferior tibiofibular syndesmosis injuries (ITSIs) and screw fixation on the ankle using the finite element (FE) method.

Methodology/Principal Findings

A three-dimensional model of a healthy ankle complex was developed using computed tomography (CT) images. We established models of an ITSI and of screw fixation at the plane 2.5 cm above and parallel to the tibiotalar joint surface of the injured syndesmosis. Simulated loads were applied under three conditions: neutral position with single-foot standing and internal and external rotation of the ankle. ITSI reduced contact forces between the talus and fibula, helped periarticular ankle ligaments withstand more load-resisting movement, and increased the magnitude of displacement at the lower extreme of the tibia and fibula. ITSI fixation with a syndesmotic screw reduced contact forces in all joints, decreased the magnitude of displacement at the lower extreme of the tibia and fibula, and increased crural interosseous membrane stress.

Conclusions/significance

Severe syndesmosis injuries cause stress and displacement distribution of the ankle to change multidirectional ankle instability and should be treated by internal fixation. Though the transverse syndesmotic screw effectively stabilizes syndesmotic diastasis, it also changes stress distribution around the ankle and decreases the joint''s range of motion (ROM). Therefore, fixation should not be performed for a long period of time because it is not physiologically suitable for the ankle joint.     

The expanded mandibular condyle of the Megaladapidae

The Megaladapidae have a posterior expansion of the articular surface of the mandibular condyle. Several other strepsirhine species exhibit a similar condylar surface. In this study, I propose two behavioral scenarios in which the posterior articular expansion might function: 1) contact with the postglenoid process and resistance to joint stress during browsing, and 2) movement against the postglenoid process during the fast closing and power strokes of mastication, as a consequence of large transverse jaw movements and associated with a strong mandibular symphysis. These models are evaluated through dissection of the TMJ in Lepilemur and from comparative anatomical observations on strepsirhines and ungulates. In Lepilemur the mandibular symphysis is unfused, but compared to the unfused symphyses of other strepsirhines is strengthened by interlocking bony projections (Beecher [1977] Am. J. Phys. Anthropol. 47:325–336). An accessory articular meniscus is found between the posterior articular expansion and the postglenoid process in Lepilemur, suggesting that significant movement occurs in this part of the TMJ. The symphysis is fused in adult specimens of Megaladapis. A posterior articular expansion is common among ungulates, and its presence is associated not with browsing but with symphyseal fusion. This supports the second model and suggests that the posterior articular expansion functions as a movement surface during mastication. Schwartz and Tattersall ([1987] J. Hum. Evol. 16:23–40) cite the posterior articular expansion as a synapomorphy uniting an Adapis-Leptadapis clade with a Megaladapidae-Daubentonia-Indridae clade. The comparative evidence suggests that the posterior articular expansion has evolved convergently in adapines, notharctines, megaladapids, hapalemurids, and indrids as part of a functional complex related to herbivory. However, close morphological similarity of the posterior articular expansion among genera within these strepsirhine subfamilies and families indicates that it is probably a reliable synapomorphy at lower taxonomic levels. Am J Phys Anthropol 103:263–276, 1997. © 1997 Wiley-Liss, Inc.