The role of the zygomatic arch in the statics of the skull and its adaptive shape

The zygomatic arch of mammals is usually considered a phylogenetic relic of the fenestrations of the skull roof which may be observed in morphological sequences of primitive vertebrate skulls. If this concept is correct, the element is comparable (though not homologous) to the jugal arches of diapsid reptiles. Two major questions then remain unanswered: why different elements are maintained in reptiles and mammals during evolution, and why the arches are maintained as relics of ancestral forms. It is tempting to respond to the latter question with a very simple answer, namely that the elements function in order to sustain mechanical stresses. In this paper, we raise the questions which quality of stresses occurs in a primate skull within the zygomatic arches and what relationship these stresses hold to the morphology of these bony elements. An answer has been sought by means of finite element stress analysis. We found that the zygomatic arch in primate skulls represents a structure which carries, under all biologically relevant conditions, either compressive or tensile stresses. In a very simple model of the human skull under bite forces, a strip of stresses occurs lateral to the orbits, which seems roughly comparable to the zygomatic arch. Once such a structure exists and is used as an insertion of adductor muscles, it will be exposed to bending stress in side view and in frontal view. Morphological details of the zygomatic arch (curvature, profile, suture) are well suited to sustain the evoked stresses by a minimum of material.     

Functional structure of the skull in hominoidea

Finite elements stress analysis (FESA) was used to investigate the flow of compressive forces which occur if a homogenous, three-dimensional body representing the skull is loaded by simulated bite forces against the tooth row. Model 1 represents the snout alone. Bite forces are applied simultaneously, but increase rearward. Stresses in the model concentrate along the anterior contour and the lower surface of the model, leaving unstressed a nasal opening and a wide naso-oral connection. Model 2 represents the facial region, as far as the temporomandibular joint. The orbits and the nasal cavity are assumed to be present a priori. Model 3 applies reactions to the bite forces in the temporal fossa, corresponding to the origins of the masticatory muscles. Regions of the model under compressive stress correspond closely to the arrangement of bony material in a hominoid skull. If only the stress-bearing finite elements on each section are combined, and the stress-free parts neglected, the resulting three-dimensional shape is surprisingly similar to a hominoid skull. If bite forces are applied to parts of the tooth row only, the stress patterns are lower, asymmetrical and do not spread into all regions that are stress-bearing in simultaneous biting on all teeth. In model 2, the highest stresses occur at the tooth roots and along the forehead on top of the nasal roof. There are no marked stress concentrations on top of the orbits. The resulting shape resembles that of an orang-utan. In model 3, the highest stresses also occur at the tooth roots, but the circles of force mostly close below the brain case, so that the stress concentration in the forehead region remains much less marked. In this model, however, the stress concentrations are very similar to hollow brow ridges. The entire resulting shape resembles that of gorilla or chimpanzee skulls. A typical gracile australopithecine skull (STS-5) also shows clear similarities to the patterns of stress flow in our models. Compared to our earlier study of the modern human skull, differences relate to: the relative length and width of the dental arcade, the relative size of the brain case and the position of the arcade relative to the brain case. It seems that these traits are the points of attack of selective pressures, while all other morphological details are simply consequences of stress flow.     

似卞氏兽(三列齿类爬行动物)新材料

本文记述了四川下沙溪庙组发现的三列齿类新材料,归入似卞氏兽属,建立—新种,自贡种.新材料首次揭示了该类动物的鼻腔构造.     

In-vitro results of rapid maxillary expansion on adults compared with finite element simulations

This study was mainly performed to investigate the effects of high maxillary expansion forces on the skull with fresh and thiel-fixed human skulls. The maxillary suture was not weakened except in one experiment. This study compares the strain measured on the zygomatic process of the skull with the results of a finite element model generated for this purpose. An increasing transversal force was applied on the alveolar process (teeth) until rupture. Strain on the zygomatic process, maxilla displacement and the expanding forces were registered.The results of this study show linear material behaviour of the skull before rupture. The highest stress during the experiments and FE simulation was observed on the alveolar process.Conclusions of this study are the necessity of the existence of appropriate models and that female specimens seem to rupture at a lower force than male ones. Both male and female specimens show a similar linear behaviour in the force/strain curve within each gender group. The probability of maxillary suture opening in adults during ultra-rapid maxillary expansion with tooth anchorage is very low. Complications and unwanted rupture could occur.     

Three-dimensional model of the human craniofacial skeleton: method and preliminary results using finite element analysis

The purpose of this study was to develop a three-dimensional finite element model of the craniofacial skeleton using a dry human skull. The model consisted of 2918 nodes and 1776 solid elements, and was used to investigate the biomechanical effect of a distally directed orthopaedic force on the craniofacial complex. The force was applied at the level of the maxillary first molar. The results indicated that in response to the force system applied: the nasomaxillary complex displaces in a backward and downward direction and rotates in clockwise sense; the nasomaxillary complex, including the zygomatic bone, experiences high stress levels in comparison with those at the remaining bones; the stress distribution in the maxillary basal bone area is relatively uniform; and the stress distribution across the opposing surface of the bony margins of the sutures is non-uniform.     

A new specimen of Biseridens qilianicus indicates its phylogenetic position as the most basal anomodont

A new well-preserved basal therapsid skull from the Xidagou Formation, Middle Permian of China, is identified as Biseridens qilianicus. The following synapomorphies distinguish Biseridens as an anomodont and not an eotitanosuchian as previously described: short snout; dorsally elevated zygomatic arch and septomaxilla lacking elongated posterodorsal process between nasal and maxilla. The presence of a differentiated tooth row; denticles on vomer, palatine and pterygoid; contact between tabular and opisthotic; lateral process of transverse flange of pterygoid free of posterior ramus and absence of mandibular foramen exclude it from other anomodonts. Our cladistic analysis indicates Biseridens to be the most basal anomodont, highlights separate Laurasian and Gondwanan basal anomodont clades and suggests that dicynodonts had their origins in the Gondwanan clade. The co-occurrence of the most basal anomodont (Biseridens) together with the most basal therapsid (Raranimus), basal anteosaurid dinocephalians, bolosaurids and dissorophids suggests that the earliest therapsid faunas are from China.     

Biomechanics of the masticatory apparatus of Pteropus giganteus (Megachiroptera)

Jaw mechanics in Pteropus were studied by means of a three-dimensional model. The model included several parameters of muscle architecture, combined with quantified movement and electromyographical data. Estimates of the nature of the applied forces that act upon the mandible during a chewing cycle, and subsequent estimates of reaction forces at the bite point and joints during the powerstroke, were thus obtained for different food consistencies. The resultant muscle force (relative to the palate) shifts from upward and slightly backward at large gapes to upward and markedly backward at the end of closing. The resultant simultaneously moves anteriorly. During the powerstroke it retains a constant position and orientation along the thickened anterior edge of the coronoid process. The early stages of opening are guided by the slope of the teeth and mandibular fossa; during the remaining part of opening the working line of the resultant crosses the skull behind the joint and thus acquires an opening moment. The bite force has downward and forward components, and a slight transverse component. For a given applied muscular force its magnitude is larger in more posteriorly positioned bite points. Both joints are loaded, the contralateral one more than the ipsilateral. Food consistency affects magnitude and orientation of the applied force, and hence, magnitude and orientation of the bite force and magnitude of the joint reaction forces. The magnitude of masseter activity relative to temporalis activity appears to be the key factor for the orientation of the bite force, and hence for the mechanical optimal position of the food. The adaptive value of the general topography of the masticatory muscles in Pteropus is discussed.     

3-D stress analysis in first maxillary premolar

The aim of this study was to investigate the stress distribution in a 3-D model of two-rooted tooth (first maxillary premolar) under two different occlusal force vectors by using finite element analysis. In the first model overall force of 200 N was divided into three vectors (cusp to fossa occlusion), and in the second model overall force was divided into 4 vectors (cusp to fossa and cusp to marginal ridge occlusion). The greatest compressive stress was found at the dentino- enamel junction in the cervical area of the both models (about -200 MPa). The greatest tensile stress was found at the vestibular aspect of buccal cusp in second model (about +3 MPa) and in the central fossa of the both models (about +28 MPa). Results indicate that in the both types of occlusal loadings the stress distribution was mainly compression and compressive forces were predominant over tensile stresses. In the second model with 4 vectors, stresses generated in the tooth structure were higher compared to the stresses generated in the first model with 3 vectors.     

Regional shape change in adult facial bone curvature with age

Life expectancies have increased dramatically over the last 100 years, affording greater opportunities to study the impact of age on adult craniofacial morphology. This article employs a novel application of established geometric morphometric methods to examine shape differences in adult regional facial bone curvature with age. Three-dimensional semilandmarks representing the curvature of the orbits, zygomatic arches, nasal aperture, and maxillary alveolar process were collected from a cross-sectional cranial sample of mixed sex and ancestry (male and female; African- and European-American), partitioned into three age groups (young adult = 18-39; middle-aged = 40-59 years; and elderly = 60+ years). Each facial region's semilandmarks were aligned into a common coordinate system via generalized Procrustes superimposition. Regional variation in shape was then explored via a battery of multivariate statistical techniques. Age-related shape differences were detected in the orbits, zygomatic arches, and maxillary alveolar process. Interactions between age, sex, and ancestry were also identified. Vector plots revealed patterns of superoinferior compression, lateral expansion, and posterior recession depending on the population/subpopulation, location, and age groups examined. These findings indicate that adult craniofacial curvature shape is not static throughout human life. Instead, age-related spatial modifications occur in various regions of the craniofacial skeleton. Moreover, these regional alterations vary not only through time, but across human populations and the sexes.     

Relationships between the size and spatial morphology of human masseter and medial pterygoid muscles, the craniofacial skeleton, and jaw biomechanics

The relationship between human craniofacial morphology and the biomechanical efficiency of bite force generation in widely varying muscular and skeletal types is unknown. To address this problem, we selected 22 subjects with different facial morphologies and used magnetic resonance imaging, cephalometric radiography, and data from dental casts to reconstruct their craniofacial tissues in three dimensions. Conventional cephalometric analyses were carried out, and the cross-sectional sizes of the masseter and medial pterygoid muscles were measured from reconstituted sections. The potential abilities of the muscles to generate bite forces at the molar teeth and mandibular condyles were calculated according to static equilibrium theory using muscle, first molar, and condylar moment arms. On average, the masseter muscle was about 66% larger in cross section than the medial pterygoid and was inclined more anteriorly relative to the functional occlusal plane. There was a significant positive correlation (P less than 0.01) between the cross-sectional areas of the masseter and medial pterygoid muscles (r = 0.75) and between the bizygomatic arch width and masseter cross-sectional area (r = 0.56) and medial pterygoid cross-sectional area (r = 0.69). The masseter muscle was always a more efficient producer of vertically oriented bite force than the medial pterygoid. Putative bite force from the medial pterygoid muscle alone correlated positively with mandibular length and inversely with upper face height. When muscle and tooth moment arms were considered together, a system efficient at producing force on the first molar was statistically associated with a face having a large intergonial width, small intercondylar width, narrow dental arch, forward maxilla, and forward mandible. There was no significant correlation between muscle cross-sectional areas and their respective putative bite forces. This suggests that there is no simple relationship between the tension-generating capacity of the muscles and their mechanical efficiency as described by their spatial arrangement. The study shows that in a modern human population so many combinations of biomechanically relevant variables are possible that subjects cannot easily be placed into ideal or nonideal categories for producing molar force. Our findings also confirm the impression that similar bite-force efficiencies can be found in subjects with disparate facial features.     

Predicted pattern of human muscle activity during clenching derived from a computer assisted model: symmetric vertical bite forces

A computer assisted three-dimensional model of the jaw, based on linear programming, is presented. The upper and lower attachments of the muscles of mastication have been measured on a single human skull and divided into thirteen independent units on each side--a total of 26 muscle elements. The direction (in three dimensions) and maximum forces that could be developed by each muscle element, the bite reaction and two joint reactions are included in the model. It is shown for symmetrical biting that a model which minimizes the sum of the muscle forces used to produce a given bite force activates muscles in a way which corresponds well with previous observations on human subjects. A model which minimizes the joint reactions behaves differently and is rejected. An analysis of the way the chosen model operates suggests that there are two types of jaw muscles, power muscles and control muscles. Power muscles (superficial masseter, medial pterygoid and some of temporalis) produce the bite force but tend to displace the condyle up or down the articular eminence. This displacement is prevented by control muscles (oblique temporalis and lateral pterygoid) which have very poor moment arms for generating usual bite forces, but are efficient for preventing condylar slide. The model incorporates the concept that muscles consist of elements which can contract independently. It predicts that those muscle elements with longer moment arms relative to the joint are the first to be activated and, as the bite force increases, a ripple of activity spreads into elements with shorter moment arms. In general, the model can be used to study the three-dimensional activity in any system of joints and muscles.     

The aquatic Lystrosaurus: An alternative lifestyle

Certain distinctive features of the skull and postcranial skeleton of Lystrosaurus are discussed. The mechanics of the skull are re‐evaluated and it is concluded that the skull was modified relative to Permian forms to produce a bigger bite force and more vertical component of the adductor muscles, while preserving a wide gape. A zone of weakness, acting as a shock absorption system, was present in the premaxillary‐nasal region of the skull. Antero‐posterior movement of the lower jaw had been reduced. It is concluded that the skull was used to crush resistant plant matter.

The external nasal opening presents no evidence for a valvular structure, but may have housed a nasal gland.

The flared scapula produced a slightly greater mechanical advantage in the limb protractors and retractors.

The manus was short and broad, suitable for digging, but the claws were flat and rounded.

The wide knee‐joint indicated powerful foot‐moving muscles.

The palaeoenvironment of the Lystrosaurus‐Thrinaxodon Assemblage Zone is examined. It was probably drier than usually described. The fauna contained many terrestrial elements.

Lystrosaurus was probably a fully terrestrial animal which may have excavated burrows for itself, but was not a committed burrower like Cistecephalus.     

Intraoral approach for reduction malarplasty: a simple method

Young Korean women with prominent zygoma may experience stress in daily life because the Oriental physiognomy often associates prominent zygoma with bad luck. Moreover, prominent zygoma in a wide Oriental face has the effect of making a person appear older and stubborn. Zygomatic reduction is often necessary to relieve stress from self-consciousness about facial appearance and to obtain younger and softer features. As such, most zygomatic procedures are cosmetic; therefore, an entirely intraoral approach with no skin incision is desirable. The current operative method of zygomatic reduction consists of two steps. The zygomatic body and arch are exposed through a mucoperiosteal incision from the maxillary canine to the first molar area. The first step is to grind and file the zygomatic body. The second step is made on the zygomatic arch. Using an oscillating saw, a partial-thickness osteotomy is made just posterior to the orbital rim, and a full-thickness osteotomy is made just anterior to the articular tubercle of the zygomatic arch. Light pressure on the posterior part of the arch produces a greenstick fracture of the anterior osteotomy site and a complete fracture of the posterior osteotomy site, resulting in inward repositioning of the zygomatic arch. This method of zygomatic reduction is simple, easy, effective, and leaves no conspicuous scars on the face.     

以乳突切迹和翼钩为基点的侧颅底分区方法

目的:探讨以乳突切迹和翼钩为基点的侧颅底分区新方法.方法:在乳突切迹后缘、翼钩、枕骨大孔前缘中点和颧根四个结构间相互连线,区分侧颅底并测量连线的长度.结果:乳突切迹与侧颅底重要结构的关系密切,切迹后缘与翼钩连线和正中线将侧颅底分成内、外侧两个大的三角区,每个区再分成前后两个三角区共四个三角区,即腭和颞下三角、咽三角、关节和听三角、血管神经三角,其中血管神经三角的三边长度左右侧分别为(74.52±5.47)mm和(74.66±5.41)mm、(59.77±3.84)mm和(59.67±3.56)mm、(42.23±3.11)mm和(42.48±2.60)mm.结论:本研究提供了新的侧颅底分区方法,且血管神经三角的区域划分更为科学,为临床侧颅底手术入路和定位提供了解剖学参考.     

The mammalian postorbital bar as a torsion-resisting helical strut

The mammalian skull is asymmetrically loaded during mastication because most of these animals chew on only one side at a time. This loading regime tends to twist the braincase relative to the rostral, tooth bearing part of the skull at the zone of potential weakness between the orbits. This torsional effect is exaggerated, and a postorbital bar is present, in those animals with very large masseter and pterygoid muscles. The lines of action of these muscles are oriented at large angles to the long axis of the skull in lateral view, providing large components of force that twist the skull segments relative to one another. When the temporalis is the dominant muscle, the torsional effect is usually less important, and the bar is absent, because this muscle acts at a smaller angle to the skull axis. The postorbital bar exhibits the predicted three dimensional spatial orientation required to resist torsional forces: it is a segment of an imaginary 45° helix that is wound around the skull, if the skull is idealized as a cylinder. This orientation is significant because, in general, maximum compressive and tensile shear stresses lie along 45° helices on a cylinder loaded in torsion; to resist torsion, material should be placed far from the axis of torsion and along a helix oriented at 45° to the deforming forces. Each half of a supraorbital ridge is also a segment of a 45° helix that is perpendicular to the helix passing through the postorbital bar. This model suggests that the postorbital bar is loaded in compression on the chewing side and in tension on the non-chewing side; the supraorbital ridge is loaded in tension on the chewing side and in compression on the non-chewing side.     

Cranial anatomy of Shunosaurus, a basal sauropod dinosaur from the Middle Jurassic of China

Shunosaurus, from the Middle Jurassic of China, is probably the best‐known basal sauropod and is represented by several complete skeletons. It is unique among sauropods in having a small, bony club at the end of its tail. New skull material provides critical information about its anatomy, brain morphology, tooth replacement pattern, feeding habits and phylogenetic relationships. The skull is akinetic and monimostylic. The brain is relatively small, narrow and primitively designed. The tooth replacement pattern exhibits back to front replacement waves in alternating tooth position. The teeth are spatulate, stout and show well‐developed wear facets indicative of coarser plant food. Upper and lower tooth rows interdigitate and shear past each other. Tooth morphology, skull architecture, and neck posture indicate that Shunosaurus was adapted to ground feeding or low browsing. Shunosaurus exhibits the following cranial autapomorphies: emargination of the ventral margin of the jugal/quadratojugal bar behind the tooth row; postorbital contains a lateral pit; vomers do not participate in the formation of the choanae; pterygoid is extremely slender and small with a dorsal fossa; quadrate ramus of the pterygoid is forked; quadratojugal participates in the jaw articulation; tooth morphology is a combination of cylindrical and spatulate form; basipterygoid process is not wrapped by the caudal process of the pterygoid; trochlear nerve has two exits; occlusal level of the maxillary tooth row is convex downward, whereas that of the dentary is concave upward, acting like a pair of garden shears; dentary tooth count is 25 or more; and the replacing teeth invade the labial side of the functional teeth. Cranial characters among the basal sauropods are reviewed. As Shunosaurus is the earliest sauropod for which cranial remains are known, it occupies an important position phylogenetically, showing the modification of skull morphology from the prosauropod condition. Although the skull synapomorphies of Sauropoda are unknown at present, 27 cranial synapomorphies are known for the clade Eusauropoda. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136 , 145?169.     

A fossil baboon skull from the lower Omo basin,southwest Ethiopia

A well-preserved fossil skull assignable to the genusPapio was discovered in the late Pliocene deposits at the “White Sands locality” of the lower Omo basin, southwest Ethiopia in November 1978. This skull obviously belongs to a male baboon which is morphologically somewhat different from the extant species ofPapio and closely resemblesPapio baringensis R. E. F. Leakey, 1969. Its muzzle is not only long but comparatively wide and robust with a very flat dorsum, an almost quadrate vertical cross section, short and obtuse maxillary ridges and virtually absent maxillary fossae, and its zygomatic portion flares laterally to widen the large temporal process of the zygomatic bone. The post-orbital constriction is very marked, the temporal fossa is squarish in basal view, and the sagittal crest is placed rather posteriorly and meets a large nuchal crest, all of which suggest heavy temporal musculature. The incisors are lacking but were probably not as small as inP. baringensis. As a result of morphological comparisons with extant and fossil species ofPapio, this newly discovered skull was considered to represent a new species,Papio quadratirostris.     

FOSSORIAL ADAPTATIONS OF A TAENIOLABIDOID MULTITUBERCULATE MAMMAL FROM THE EOCENE OF CHINA

<正> Fragments of the postcranial skeleton of s taeniolabidoid multituberculate,? Lambdopsalis bulla, from the Eocene of Chins are described. These consist of cervical vertebrae C2-C3 (fused), a fragment of the neural arch of C4, and complete C5-C7, thoracic vertebrae T1-T3, and two humeri, which are the first Complete multituberculate humeri ever described. The fusion of C2 and C3, and the stout structure of the humerus with a very large deltopectoral crest, a wide distal end, and prominent radial and ulnar condyles, indicate a fossorial mode of life for? L. bulla. These postcranial elements and a wide flattened, skull with a vertical occipital plate, suggest adaptstions seen in some modern golden moles and cricetid rodents such as Myospalax that dig with claws and displace the soil with the head.     

Stress distribution and displacement analysis during an intermaxillary disjunction--a three-dimensional FEM study of a human skull

The goal of this study was to contribute to an understanding of how much expansion force is needed during a maxillary expansion (ME) and where bony reaction takes place. A finite element (FE) model of a dry human male skull was generated from CT scans. The FE model, which consists of cortical and cancellous bone and teeth, was loaded with the same force magnitudes, directions and working points as in rapid maxillary expansion (RME). A three-dimensional finite element stress analysis (FESA) of the forces and displacement was performed. The highest stress was observed in the maxilla in the region where the forces were applied, and spreads more or less throughout almost the whole frontal skull structures. The displacement distribution which causes stress in the skull is highly dependant on the thickness of the bone and its structure. All areas with high compressive and tensile stress are exactly the regions which determine the maximal amount of force to be used during the maxillary expansion and should be examined in case of any complication during a patient's treatment. Regions with significant compressive and tensile stress are the regions observed to have an increase in cellular activity. Further simulations with a given displacement (0.5mm) showed that displacement simulations need extra caution otherwise they will lead to very high forces which are not realistic in an orthodontic treatment.