The third joint of the canine jaw

Quantitative assessment of masticatory muscles based on skull muscle attachment areas in Carnivora

Masticatory muscles are composed of the temporalis, masseter, and pterygoid muscles in mammals. Each muscle has a different origin on the skull and insertion on the mandible; thus, all masticatory muscles contract in different directions. Collecting in vivo data and directly measuring the masticatory muscles anatomically in various Carnivora species is practically problematic. This is because some carnivorans can be ferocious, rare, or even extinct. Consequently, the most practical method to collect data on the force generated by the masticatory muscle is to estimate the force based on skulls. The physiological cross-sectional area (PCSA) of each masticatory muscle, which correlates to the maximum force that can be produced by a muscle, was quantified. Using computed tomography, we defined the three-dimensional measurement area for 32 carnivoran species based on the origin and insertion of masticatory muscles specified by observable crests, ridges, and scars. Subsequent allometric analysis relating the measurement area on skull surface to the PCSA for each masticatory muscle measured in fresh specimens revealed a strong correlation between the two variables. This finding indicates that within Carnivora, an estimation of absolute masticatory muscle PCSA can be derived from measurements area on skull surface. This method allows for the use of cranial specimens, housed in museums and research institutions, that lack preserved masticatory muscles in quantitative studies involving masticatory muscle PCSA. This approach facilitates comprehensive discussions on the masticatory muscle morphology of Carnivora, including rare and extinct species.

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

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

Symphyseal morphology and jaw muscle recruitment levels during mastication in musteloid carnivorans

In studies of mammalian mastication, a possible relationship has been proposed between bilateral recruitment of jaw adductor muscle force during unilateral chewing and the degree of fusion of the mandibular symphysis. Specifically, species that have unfused, mobile mandibular symphyses tend to utilize lower levels of jaw adductor force on the balancing (nonchewing) than the working (chewing) side of the head, when compared to related species with fused symphyses. Here, we compare jaw adductor recruitment levels in two species of musteloid carnivoran: the carnivorous ferret (unfused symphysis), and the frugivorous kinkajou (fused symphysis). During forceful chewing, we observe that ferrets recruit far more working-side muscle force than kinkajous, regardless of food toughness and that high working-to-balancing side ratios are the result of increased working-side force, often coupled with reduced balancing-side force. We propose that in carnivorans, high working-to-balancing side force ratios coupled with an unfused mandibular symphysis are necessary to rotate the hemimandible for precise unilateral occlusion of the carnassial teeth and to sustain laterally oriented force on the jaw to engage the carnassial teeth during shearing of tough foods. In contrast, the kinkajou's flattened cheekteeth permit less precise occlusion and require medially-oriented forces for grinding, thus, a fused symphysis is mechanically beneficial.

Normal variation of clinical mobility of the mandibular symphysis in cats

Introduction

The primary objective of this retrospective study was to document the normal variation of clinical mobility of the mandibular symphysis in cats and possible associations with bodyweight, age, sex, sexual status, breed and skull morphology. Secondarily, the radiographic appearance of the mandibular symphysis and possible associations with the analyzed data were evaluated.

Materials and methods

Two hundred and sixteen cats of 15 different breeds that underwent maxillofacial, oral and dental procedures from April 2015 to December 2021 were included. Clinical mobility was evaluated under general anesthesia using a 0 to 3 scale in lateromedial (LM) and dorsoventral (DV) directions. The symphysis was radiographically classified on the occlusal radiographic view of the rostral mandibles as fused or open, and with parallel or divergent margins.

Results

Bodyweight ranged from 2.2 to 12.5 kg (median 4.0 kg), age from 4 months to 17 years and 4 months (median 6 years and 4 months). At the first evaluation DV symphyseal mobility was 0 in 177 cases (82%), 1 in 32 cases (14.8%) and 2 in 7 cases (3.2%), LM mobility was 0 in 61 cases (28.3%), 1 in 110 cases (50.9%) and 2 in 45 cases (20.8%). 81.1% of the radiographs were included in the statistical analysis. Three symphyses (1.6%) were classified as fused and 190 (98.4%) as open, 129 (68.8%) having divergent margins and 61 (31.6%) parallel. One hundred and forty-eight cases (76.7%) did not show the presence of odontoclastic replacement resorption on the canine teeth (TR subgroup 1), 23 (11.9%) showed stage ≤3 lesions (TR subgroup 2) and 22 (11.4%) stage 4 lesions (TR subgroup 3). Logistic regression models exploring factors that affected DV and LM mobility were statistically significant (p < 0.0001; p < 0.0001) with an increase in LM mobility predicting an increase in DV mobility, and vice versa. An increase in DV mobility was associated with an increase in age and in having resorptive lesions. A decrease in LM symphyseal mobility was associated with being brachycephalic.

Conclusion

The great majority of cases showed some degree of LM symphyseal mobility, and 18% showed DV mobility. Symphyseal bony fusion is rare but possible.

Three-dimensional mandibular kinematics of mastication in the marsupial Didelphis virginiana

Didelphis virginiana (the Virginia opossum) is often used as an extant model for understanding feeding behaviour in Mesozoic mammaliaforms, primarily due to their morphological similarities, including an unfused mandibular symphysis and tribosphenic molars. However, the three-dimensional jaw kinematics of opossum chewing have not yet been fully quantified. We used biplanar videofluoroscopy and the X-Ray Reconstruction of Moving Morphology workflow to quantify mandibular kinematics in four wild-caught opossums feeding on hard (almonds) and soft (cheese cubes) foods. These data were used to test hypotheses regarding the importance of roll versus yaw in chewing by early mammals, and the impact of food material properties (FMPs) on jaw kinematics. The magnitude of roll exceeds that of yaw, but both are necessary for tooth-tooth or tooth-food-tooth contact between complex occlusal surfaces. We confirmed the utility of the four vertical kinematic gape cycle phases identified in tetrapods but we further defined two more in order to capture non-vertical kinematics. Statistical tests support the separation of chew cycle phases into two functional groups: occlusal and non-occlusal phases. The separation of slow close into two (occlusal) phases gives quantitative kinematic support for the long-hypothesized multifunctionality of the tribosphenic molar. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Normal variation of clinical mobility of the mandibular symphysis in dogs

Introduction

The primary objective of this retrospective study was to document the normal variation of clinical mobility of the mandibular symphysis in dogs, and evaluate possible associations with breed, bodyweight, age, sex, and skull morphology. Secondarily, the radiographic appearance of the mandibular symphysis and possible associations with the analyzed data were also evaluated.

Methods

Medical records of dogs that underwent anesthetic procedures for maxillofacial, oral and dental evaluation from April 2015 to December 2021 were included.

Results

567 dogs of 95 different breeds were included, with a total of 695 evaluations. Body weight ranged from 0.8 kg to 79 kg (median 14.4 kg) and age from 3 months to 16 years and 4 months (median 6 years and 9 months). Clinical mobility was evaluated under general anesthesia using a 0 to 3 scale, in lateromedial (LM) and dorsoventral (DV) directions. The symphysis was radiographically classified as being fused or open. The open symphyses were further radiographically divided in having parallel or divergent margins. At the time of the first evaluation DV mobility was 0 in 551 cases (97.2%) and 1 in 16 cases (2.8%). LM mobility was 0 in 401 cases (70.7%), 1 in 148 cases (26.1%) and 2 in 18 cases (3.2%). There was not a significant change in mobility over time for cases examined more than once (P= 0.76). All cases had an intraoral radiographic examination. 83.8% of the radiographs were included in the statistical analysis. Two symphyses (0.4%) were classified as fused and 473 (99.6%) as open, 355 (74.7%) having divergent margins and 118 (24.8%) parallel margins. Logistic regression models exploring factors that affected DV and LM mobility were statistically significant (P < 0.0001; P < 0.0001), with an increase in LM mobility predicting an increase in DV mobility, and vice versa. An increase in age and in bodyweight was associated with a decrease in mobility. There was no statistical difference in clinical mobility across specific breeds or sexes. Increased probability of a divergent symphysis and increased DV mobility was found to be associated with a brachycephalic conformation. The increase in LM mobility was comparatively higher in small brachycephalic breeds compared with larger brachycephalic breed.

Discussion

The majority of the cases showed little to no mobility of the mandibular symphysis and radiographically bony fusion can be rarely seen.

Morphometry and Morphology of the Articular Surfaces of the Medial Region of the Temporomandibular Joint in the Felis Catus (Domestic cat) - A Cone Beam Computed Tomography Study

The articular surfaces of the temporomandibular joint (TMJ) in the cat consist of the articular head of the condylar process of the ramus of the mandible and the mandibular fossa of the petrous part of the temporal bone. Anatomic conformation of the TMJ articular surfaces can vary; however, this has not been studied in the cat. Thus, the aim of this study was to characterize the conformation of the medial region of the TMJ in mesocephalic adult cats as well as to determine the articular relationship by measurement of the degree of coverage of the mandibular fossa over the articular head. This was accomplished by assessing 60 TMJs from 30 mesocephalic adult cats by means of cone-beam computed tomography (CBCT). The width and depth of the mandibular fossa, and the degree of coverage of the mandibular fossa were evaluated using parasagittal reconstructions of the medial aspect of the TMJ by two observers. No statistically significant difference was observed during the intra and interobserver evaluation of the mandibular fossa width and depth and the degree of coverage of the mandibular fossa over the articular head. P-values for the studied variables were greater than 0.05. The resulting measurements demonstrated a concave mandibular fossa with a prominent retroarticular process and a poorly developed articular eminence; a well-defined rounded articular surface, and a high degree of coverage (171°) of the mandibular fossa over the articular head. In conclusion, the articular surfaces at the medial aspect of the TMJ displayed elevated geometric similarity, with an articular head of the condylar process deeply seated in the mandibular fossa.

Superficial, suprahyoid, and infrahyoid neck musculature in naked mole-rats (Heterocephalus glaber): Relative size and potential contributions to independent movement of the lower incisors

Naked mole-rats (Heterocephalus glaber) are fossorial, eusocial rodents that exhibit the unusual capability of moving their lower incisors independently in lateral and rostroventral directions. The evolution of this trait would presumably also involve concurrent alterations in neck musculature to support and control movements of the lower incisors. In order to assess morphological adaptations that might facilitate these movements, we performed detailed dissections of the neck musculature of adult naked mole-rats. In addition to characterizing attachment sites of superficial, suprahyoid, and infrahyoid musculature, we also quantified muscle mass and mandibular features thought to be associated with gape (condyle height, condyle length, and jaw length). Based on muscle attachment sites, the platysma myoides may contribute to lateral movement of the lower incisor and hemi-mandible in naked mole-rats. The large digastric muscle is likely to be a main contributor to rostroventral movement of each lower incisor. The geniohyoid and mylohyoid muscles also likely contribute to rostroventral movements of the lower incisors, and the mylohyoid may also produce lateral spreading of the hemi-mandibles. The transverse mandibular (intermandibularis) muscle likely serves to reposition the lower incisors back to a midline orientation following a movement.

The influence of diet on masticatory motor patterns in musteloid carnivorans: An analysis of jaw adductor activity in ferrets (Mustela putorius furo) and kinkajous (Potos flavus)

Broad similarities in the timing of jaw adductor activity driving jaw movements across distantly related and morphologically disparate species have led to the hypothesis that mammalian masticatory motor patterns are conserved. However, some quantitative analyses also suggest that masticatory motor patterns have evolved in concert with dietary and/or morphological specialization. Here, we assess this relationship in two closely related carnivoran species with divergent diets and morphology: carnivorous ferrets and frugivorous kinkajous. Using electromyography to characterize jaw adductor activity during rhythmic mastication, we test the hypotheses that (1) carnivoran masticatory motor patterns differ from those of non-carnivorans based on previously published data, and (2) differences between ferret and kinkajou motor patterns are associated with dietary and morphological differences. We find that both species exhibit highly synchronous jaw adductor activity that is likely typical of most carnivorans. Kinkajous differ from ferrets, however, in having a balancing-side zygomaticomandibularis that is active later than all other adductors. The significance of these different masticatory motor patterns may relate to morphological differences in the dentition of ferrets and kinkajous. Whereas ferret cheek teeth have vertical occlusal surfaces that limit jaw closing to a primarily dorsally directed movement, kinkajous have relatively flat occlusal surfaces that allow more transverse movement, which may be essential for processing fruits. Our results suggest that some aspects of masticatory motor patterns are highly conserved yet some components are modified in concert with functional and morphological evolution of the masticatory apparatus.

Can skull form predict the shape of the temporomandibular joint? A study using geometric morphometrics on the skulls of wolves and domestic dogs

The temporomandibular joint (TMJ) conducts and restrains masticatory movements between the mammalian cranium and the mandible. Through this functional integration, TMJ morphology in wild mammals is strongly correlated with diet, resulting in a wide range of TMJ variations. However, in artificially selected and closely related domestic dogs, dietary specialisations between breeds can be ruled out as a diversifying factor although they display an enormous variation in TMJ morphology. This raises the question of the origin of this variation. Here we hypothesise that, even in the face of reduced functional demands, TMJ shape in dogs can be predicted by skull form; i.e. that the TMJ is still highly integrated in the dog skull. If true, TMJ variation in the dog would be a plain by-product of the enormous cranial variation in dogs and its genetic causes. We addressed this hypothesis using geometric morphometry on a data set of 214 dog and 60 wolf skulls. We digitized 53 three-dimensional landmarks of the skull and the TMJ on CT-based segmentations and compared (1) the variation between domestic dog and wolf TMJs (via principal component analysis) and (2) the pattern of covariation of skull size, flexion and rostrum length with TMJ shape (via regression of centroid size on shape and partial least squares analyses). We show that the TMJ in domestic dogs is significantly more diverse than in wolves: its shape covaries significantly with skull size, flexion and rostrum proportions in patterns which resemble those observed in primates. Similar patterns in canids, which are carnivorous, and primates, which are mostly frugivorous imply the existence of basic TMJ integration patterns which are independent of dietary adaptations. However, only limited amounts of TMJ variation in dogs can be explained by simple covariation with overall skull geometry. This implies that the final TMJ shape is gained partially independently of the rest of the skull.

An ontogenetic perspective on symphyseal fusion, occlusion and mandibular loading in alpacas (Vicugna pacos)

A primary hypothesis for the evolution of mandibular symphyseal fusion in some mammals is that it functions to resist loads incurred during routine mastication. Anecdotal support for this hypothesis is based on the fact that when the symphysis fuses, it typically does so early during postnatal ontogeny prior to or around the time of weaning. However, little is known about the process of fusion, particularly relative to feeding behaviors and the dynamics of mastication, including occlusion and masticatory loading. In the present study, we investigate the timing and process of symphyseal fusion in alpacas (Vicugna pacos) in the context of maturation of the oral apparatus and oral behavior. We also report on in vivo strains from the symphysis and corpus in young alpacas prior to and following full fusion and M₁ occlusion. Results show that fusion begins rostrally by 1 month and is complete by 6-7 months whereas all deciduous premolars and M₁ come into occlusion by 6 months. Although symphyseal loading patterns are maintained throughout ontogeny, in young alpacas symphyseal strain magnitudes are low compared with adults but corpus strain magnitudes are comparable to those found in adults. Reduced symphyseal loading in young individuals is contrary to what might be predicted given that the symphysis is still fusing. When considered in light of the development of occlusion and rumination, strain magnitudes may be necessarily low and reflect an overall delay in the maturation of masticatory dynamics.

Morphologic and Morphometric Description of the Temporomandibular Joint in the Domestic Dog Using Computed Tomography

The temporomandibular joint (TMJ) in the domestic dog is a synovial joint with 2 articular surfaces, the mandibular fossa of the squamous portion of the temporal bone and the articular head of the condylar process of the mandible. Although different diagnostic imaging techniques have been used to study the TMJ in dogs, morphologic and morphometric studies based on computed tomography (CT) are scarce. The purpose of the present study was to describe the morphologic and morphometric features of the TMJ in domestic dogs using CT. Width and depth of the mandibular fossa and 2 different angles between the mandibular fossa and the condylar process were measured in 96 TMJs of 48 dogs of different breeds (Labrador retriever, German shepherd, cocker spaniel, boxer, English bulldog, pug, shih tzu, and Cavalier King Charles spaniel). Temporomandibular joint conformation differed between breeds. Mid- and small-sized dogs had mandibular fossae that were more shallow, less developed retroarticular processes, and irregularly shaped condylar processes. The TMJs were more congruent in large dogs, presenting with deeper mandibular fossae, prominent retroarticular processes, and more uniform condylar processes. The measurements proposed in this study demonstrated 3 different morphologic conformations for the TMJ in the dogs of this study.

Symphysiotomy, Symphysiectomy, and Intermandibular Arthrodesis in a Cat with Open-Mouth Jaw Locking — Case Report and Literature Review

A Persian cat was presented with open-mouth jaw locking. It was suspected that abnormal extrusion of the maxillary canine teeth caused contact with the mandibular canine teeth on closure of the mouth, resulting in levering forces that may have led to increased symphyseal mobility, temporomandibular joint laxity, rotational movement of the left mandibular body and locking of the left coronoid process lateral to the zygomatic arch. Symphysiotomy, symphysiectomy, circumferential and interdental wiring, and intraoral resin-based splint application were performed to achieve intermandibular arthrodesis. Following removal of all devices, no further jaw locking episodes were reported during a 33-month follow-up period.

MANDIBULAR MORPHOMETRY APPLIED TO ANESTHETIC BLOCKAGE IN THE MANED WOLF (CHRYSOCYON BRACHYURUS)

Chrysocyon brachyurus (maned wolf) is the biggest South American canid and has a high frequency of dental injuries, both in the wild and in captivity. Thus, veterinary procedures are necessary to preserve the feeding capacity of hundreds of captive specimens worldwide. The aim of this study was to investigate the mandibular morphometry of the maned wolf with emphasis on the establishment of anatomic references for anesthetic block of the inferior alveolar and mental nerves. Therefore, 16 measurements in 22 mandibles of C. brachyurus adults were taken. For extraoral block of the inferior alveolar nerve at the level of the mandibular foramen, the needle should be advanced close to the medial face of the mandibular ramus for 11.4 mm perpendicular to the palpable concavity. In another extraoral approach, the needle may be introduced for 30.4 mm from the angular process at a 20-25° angle to the ventral margin. For blocking only the mental nerve, the needle should be inserted for 10 mm from ventral border, close to the labial surface of the mandibular body, at the level of the lower first premolar. The mandibular foramen showed similar position, size, and symmetry in the maned wolf specimens examined. Comparison of the data observed here with those available for other carnivores indicates the need to determine these anatomic references specifically for each species.

Model sensitivity and use of the comparative finite element method in mammalian jaw mechanics: mandible performance in the gray wolf

Finite Element Analysis (FEA) is a powerful tool gaining use in studies of biological form and function. This method is particularly conducive to studies of extinct and fossilized organisms, as models can be assigned properties that approximate living tissues. In disciplines where model validation is difficult or impossible, the choice of model parameters and their effects on the results become increasingly important, especially in comparing outputs to infer function. To evaluate the extent to which performance measures are affected by initial model input, we tested the sensitivity of bite force, strain energy, and stress to changes in seven parameters that are required in testing craniodental function with FEA. Simulations were performed on FE models of a Gray Wolf (Canis lupus) mandible. Results showed that unilateral bite force outputs are least affected by the relative ratios of the balancing and working muscles, but only ratios above 0.5 provided balancing-working side joint reaction force relationships that are consistent with experimental data. The constraints modeled at the bite point had the greatest effect on bite force output, but the most appropriate constraint may depend on the study question. Strain energy is least affected by variation in bite point constraint, but larger variations in strain energy values are observed in models with different number of tetrahedral elements, masticatory muscle ratios and muscle subgroups present, and number of material properties. These findings indicate that performance measures are differentially affected by variation in initial model parameters. In the absence of validated input values, FE models can nevertheless provide robust comparisons if these parameters are standardized within a given study to minimize variation that arise during the model-building process. Sensitivity tests incorporated into the study design not only aid in the interpretation of simulation results, but can also provide additional insights on form and function.

A mechanical evaluation of three decellularization methods in the design of a xenogeneic scaffold for tissue engineering the temporomandibular joint disc

Tissue-engineered temporomandibular joint (TMJ) discs offer a viable treatment option for patients with severe joint internal derangement. To date, only a handful of TMJ tissue engineering studies have been carried out and all have incorporated the use of synthetic scaffold materials. These current scaffolds have shown limited success in recapitulating morphological and functional aspects of the native disc tissue. The present study is the first to investigate the potential of a xenogeneic scaffold for use in tissue engineering the TMJ disc. The effects of decellularization agents on the disc's mechanical properties were assessed using three common decellularization protocols: Triton X-100, sodium dodecyl sulfate (SDS) and an acetone/ethanol solution. Decellularized scaffolds were subsequently characterized through cyclic mechanical testing at physiologically relevant frequencies to determine which chemical agent most accurately preserved the native tissue properties. Results have shown that porcine discs treated with SDS most closely matched the energy dissipation capabilities and resistance to deformation of the native tissue. Treatments using Triton X-100 caused the resultant tissue to become relatively softer with inferior energy dissipation capabilities, while treatment using acetone/ethanol led to a significantly stiffer and dehydrated material. These findings support the potential of a porcine-derived scaffold decellularized by SDS as a xenograft for TMJ disc reconstruction.

Masticatory muscles and the skull: a comparative perspective

Masticatory muscles are anatomically and functionally complex in all mammals, but relative sizes, orientation of action lines, and fascial subdivisions vary greatly among different species in association with their particular patterns of occlusion and jaw movement. The most common contraction pattern for moving the jaw laterally involves a force couple of protrusor muscles on one side and retrusors on the other. Such asymmetrical muscle usage sets up torques on the skull and combines with occlusal loads to produce bony deformations not only in the tooth-bearing jaw bones, but also in more distant elements such as the braincase. Maintenance of bone in the jaw joint, and probably elsewhere in the skull, is dependent on these loads.

Imaging of the canine and feline temporomandibular joint: a review

The radiographic anatomy of the temporomandibular joint in the dog and cat is described in dorsoventral and oblique projections. The positioning for different oblique views in conventional radiography and technical details of computed tomography are reviewed. Typical radiographic features of craniomandibular osteopathy, dysplasia, luxation, subluxation, fractures, ankylosis, degenerative joint disease, infection, and neoplasia involving the temporomandibular joint are discussed.

Transverse masticatory movements, occlusal orientation, and symphyseal fusion in selenodont artiodactyls

Based on extensive experimental work on primates, two masticatory loading regimes have emerged as the likely determinants of mandibular symphyseal fusion-dorsoventral shear and lateral transverse bending (wishboning) (Ravosa and Hylander, 1994; Hylander et al., 1998, 2000). Recently, however, it has been argued that, rather than functioning to strengthen the symphysis during mastication, fusion serves to stiffen the symphyseal joint so as to facilitate increased transverse jaw movements during occlusion (Lieberman and Crompton, 2000). As part of this transverse stiffness model, it has been suggested that taxa with fused symphyses should also exhibit more horizontally oriented occlusal wear facets. Using a series of univariate and bivariate analyses, we test predictions of these three models in a sample of 44 species of selenodont artiodactyls. Consistent with the wishboning and transverse stiffness models, taxa with fused symphyses (camelids) have more horizontally oriented M(2) and M(2) occlusal wear facets, anteroposteriorly (AP) elongate symphyses, and relatively wider corpora. Contrary to the dorsoventral shear model, camelids do not have relatively deeper corpora (due to greater parasagittal bending). While taxa with ossified symphyses have relatively larger symphysis cross-sectional areas, this appears to be the byproduct of an increase in AP symphysis length due to greater lateral transverse bending of the mandible. Theoretical consideration of the biomechanics of mastication further suggests that strength, not stiffness, is the critical factor in determining symphyseal ossification. Thus, like anthropoid primates, fusion in selenodont artiodactyls appears to function in resisting increased wishboning stresses arising from an emphasis on transverse occlusal/mandibular movements and loads.

Why fuse the mandibular symphysis? A comparative analysis

Fused symphyses, which evolved independently in several mammalian taxa, including anthropoids, are stiffer and stronger than unfused symphyses. This paper tests the hypothesis that orientations of tooth movements during occlusion are the primary basis for variations in symphyseal fusion. Mammals whose teeth have primarily dorsally oriented occlusal trajectories and/or rotate their mandibles during occlusion will not benefit from symphyseal fusion because it prevents independent mandibular movements and because unfused symphyses transfer dorsally oriented forces with equal efficiency; mammals with predominantly transverse power strokes are predicted to benefit from symphyseal fusion or greatly restricted mediolateral movement at the symphysis in order to increase force transfer efficiency across the symphysis in the transverse plane. These hypotheses are tested with comparative data on symphyseal and occlusal morphology in several mammals, and with kinematic and EMG analyses of mastication in opossums (Didelphis virginiana) and goats (Capra hircus) that are compared with published data on chewing in primates. Among mammals, symphyseal fusion or a morphology that greatly restricts movement correlates significantly with occlusal orientation: species with more transversely oriented occlusal planes tend to have fused symphyses. The ratio of working- to balancing-side adductor muscle force in goats and opossums is close to 1:1, as in macaques, but goats and opossums have mandibles that rotate independently during occlusion, and have predominantly vertically oriented tooth movements during the power stroke. Symphyseal fusion is therefore most likely an adaptation for increasing the efficiency of transfer of transversely oriented occlusal forces in mammals whose mandibles do not rotate independently during the power stroke.

Mandibular form and function in North American and European Adapidae and Omomyidae

Previous experimental and comparative studies among a wide variety of primate and nonprimate mammals provide a unique source of information for investigating the functional and phylogenetic significance of variation in the masticatory apparatus of Eocene primates. To provide a quantitative study of mandibular form and function in Eocene primates, the scaling of jaw dimensions and the development of symphyseal fusion was considered in a broad sample of North American and European Adapidae and Omomyidae. Statistical analyses indicate a significant size-related pattern of symphyseal fusion across Eocene primates, with larger taxa often having a greater degree of fusion than smaller species; this trend is also evident at the family level. As adapids are mostly larger than omomyids and these taxa show allometry of symphyseal fusion, this may explain why no omomyids evince complete fusion. Controlling for jaw size, species with greater symphyseal fusion tend to have more robust jaws than those with a lesser amount of fusion. Upon further examination, a primary reason why adapids have more robust mandibles than omomyids is associated with the presence of taxa with fused symphyses, and thus more robust jaws, in the adapid sample, whereas no omomyids have fused symphyses. In addition, there is little indication of a dietary effect, as measured by molar shear-crest development, on symphyseal fusion. Moreover, as there is no correlation between molar shear-crest development and skull size, this also points to the absence of a size-related pattern of dietary preference underlying the allometry of symphyseal fusion. Based on the interspecific and ontogenetic allometry of symphyseal ossification in Eocene primates, jaw-scaling patterns are used to further examine the functional determinants of fusion in this group. This study indicates that greater dorsoventral shear during mastication is a more likely factor than lateral transverse bending ("wishboning") in the evolution of symphyseal fusion among "late-fusing" mammals like adapids and omomyids. Given that wishboning is an important functional determinant of symphyseal form in recent anthropoids, apparently the evolutionary development of marked wishboning occurs only in taxa that shift the timing of fusion to a growth stage preceding the onset of weaning (before adult masticatory patterns are fully developed) and perhaps first ossified the symphysis to counter elevated dorsoventral shear stress. As early anthropoids probably consisted of members varying interspecifically and ontogenetically in the degree of ossification, it is especially informative to analyze the adaptive setting in which anthropoid symphyseal fusion evolved from a similar primitive "prosimian" perspective. Finally, since taxa with fused symphyses are widely distributed across mammals, a similar analytical framework could be directed profitably at unraveling the functional and evolutionary significance of symphyseal fusion in other mammalian clades.

Mandibular growth and function in Archaeolemur

Ontogenetic changes in the morphology of the mandibular symphysis are described in Archaeolemur so as to infer the functional significance of symphyseal fusion in this subfossil Malagasy lemur. The first regions of the symphysis to show a more complex morphology were the lower and anterior borders of the joint and, to a lesser extent, the lingual borders of the superior and inferior transverse tori. During growth, these regions became increasingly rugose and encroached upon a centrally located, smooth, "oval" region, which may have been a principal pathway for neurovascular structures communicating with the unfused joint. In subadults, the symphysis was completely fused except for the lingual surface of the inferior transverse torus, where a patent suture and potential space were present between dentaries. Thus, in Archaeolemur there was an age- and size-related pattern of increased symphyseal ossification or fusion that was complete by adulthood. The morphology of the interlocking bony processes and the sequence of ossification in the symphysis suggest that increased dorsoventral shear stress during mastication was the most likely determinant of symphyseal fusion in Archaeolemur. The allometric pattern of greater symphyseal fusion may be linked to the presence of relatively greater dorsoventral shear in adults due to an increased recruitment of balancing-side jaw-muscle force. There is little indication that the symphysis of juvenile Archaeolemur was buttressed to resist forces associated with "wishboning" during mastication or vertical bending during incision. Our observations, as well as those of others, suggest that symphyseal fusion in primates occurs initially as a response to increased dorsoventral shear during mastication. Therefore, wishboning stress might only become a major determinant of symphyseal form and function in those taxa that develop a fused symphysis to counter increased dorsoventral shear.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

Ultrastructural quantification of collagen fibrils in the central region of the articular disc of the temporomandibular joint of the cat and the guinea pig

This quantitative ultrastructural analysis was made on articular discs from four guinea pigs and four cats. Mean diameters of collagen fibrils were small (approximately 45 nm) and showed unimodal distributions. These features are consistent for connective tissues subjected to compressional forces. The relatively high percentage volume of the extracellular matrix occupied by collagen in the articular disc of the guinea pig (approximately 60 per cent) and the presence of crimping is, however, more typical of a connective tissue subjected to tension. Two differences were discernible between the collagen in the articular discs of the guinea pig and cat. First, the percentage volume occupied by collagen for the guinea pig disc was significantly higher than for the cat. Second, the frequency distribution of collagen fibril diameters for the cat, although unimodal, was skewed. These differences possibly reflect the different types of movement at the temporomandibular joints in the two species.

A functional consequence of an ossified mandibular symphysis

According to most recent workers, the presence of fused symphyses in some mammals is explained by the common view that muscle force is transmitted better across a fused, as opposed to an unfused, mandibular symphysis. Recent theoretical work has cast doubt on the importance of fusion for simple force transmission by suggesting that force can also be transmitted efficiently across an unfused symphysis, an expectation that has since been confirmed by a number of observational studies. Perhaps the real significance of symphyseal fusion is that, in animals with upper and lower incisor tooth rows that apply large forces to relatively small resistant food items, muscle force from both sides of the head is reliably available only when the symphysis is fused. Independent movement between the two sides of the lower incisor row, permitted by a patent symphysis, allows the possibility that one side of the lower row will come into contact with the upper incisor row, dissipating all of the muscle force from that side. The dissipation of approximately half of the available jaw muscle force, allowed by a patent symphysis, cannot be ignored when attempting to explain the presence of fused symphyses if one accepts the idea that strong incisor biting is an important element in the masticatory apparatus of those primates and other mammals with fused mandibular symphyses.

The effect of partial rostral hemimandibulectomy on mandibular mobility and temporomandibular joint morphology in the dog

Partial rostral hemimandibulectomy was performed in 10 adult dogs. The temporomandibular joints (TMJs) were examined radiographically and tomographically before surgery, and mandibular stability was evaluated before and immediately after surgery. Radiographic, tomographic, and hemimandibular mobility assessments were made again at months 3 and 6. The TMJs were examined grossly and histologically in five dogs euthanatized at month 3 and in five dogs euthanatized at month 6. Statistically significant hemimandibular instability (p less than 0.05) persisted in all subjects throughout the study. The radiographic appearance of the joints remained unaltered; however, space asymmetry was identified in postoperative tomograms of three dogs at month 3 and four dogs at month 6. The TMJs were grossly normal at necropsy. Histologically, there were degenerative changes in articular cartilage and subchondral bone in all of the joints. The authors conclude that partial rostral hemimandibulectomy causes TMJ degeneration, as a consequence of hemimandibular instability or abnormal loading, or both.

Gross anatomy of the craniomandibular joint and masticatory muscles of the dog

The craniomandibular joint (CMJ) and masticatory muscles in 15 dogs (Canis familiaris) were dissected. The major structures were fossa, disc and condyle surrounded by a thin fibrous capsule. The horizontally orientated temporal fossa had a pronounced tubercle anteriorly. Posteriorly, a retroarticular process curved backwards. This fossa and process closely surrounded the condyle. The disc was thin. The cylindrically-shaped condyle was elongated in the medio-lateral direction, situated at right angles to the sagittal plane. The lateral pterygoid muscle was poorly developed; the temporalis was by far the largest masticatory muscle. These findings support the assumption that the movements of the canine mandible are mainly hinge-like. Furthermore, the canine CMJ differs in many respects from the corresponding anatomical structures in the domestic pig, as well as in man.

Morphology of the tongue in the tuatara, Sphenodon punctatus (Reptilia: Lepidosauria), with comments on function and phylogeny

The morphology and histology of the tongue in Sphenodon punctatus are described and used to infer function and to determine character state polarities in lepidosaurs. The tongue lacks an anterior notch and is covered with filamentous papillae, including specialized gustatory papillae containing taste buds. Lingual glands are restricted to mucocytes covering the papillae. Three intrinsic tongue muscles are identified and shown to be discrete fiber systems and not merely elaborations of the M. hyoglossus. These muscles interact with a connective tissue skeleton, particularly three septal planes, to cause changes in tongue shape. Tongue protrusion is probably caused by hyoid protraction and contraction of posterior genioglossus fibers; retraction by hyoid retraction, hyoglossus contraction, and contraction of anterior genioglossus fibers. It is argued that taste is important in prey discrimination and possibly in courtship. Vomeronasal function is probably mediated by inhalation and not tongue movement. Insertion of genioglossus fibers into the buccal floor is a derived feature of lepidosaur tongues. Derived features of squamate tongues include an anterior bifurcation, a divided genioglossus comprising medial and lateral portions, ventral transverse and circular muscle fiber systems around the hyoglossus, and the presence of a median septum. The tongue of the squamate family Iguanidae shares many plesiomorphic features with Sphenodon.

Mechanical analysis of the malformed, yet functional, mandibular joints of a wild timber wolf, Canis lupus

In spite of grossly malformed jaw joints, presumably from birth, the animal survived in the wild for at least 3 years. Instead of a deep glenoid fossa folded over a cylindrical condyle to form a hinge, both joints had freely open articulating surfaces and a mandibular condyle without neck. The neckless condyle produced a shorter moment arm of resistance in all biting positions. The moment arm of the masseter, the jaw-adductor tending to disarticulate the jaw, was longer as a result of an elongated angular process. The combined effect at the carnassials was a 36 per cent improvement in the efficiency of the lever for which the joint is the fulcrum and thus an equivalent reduction in the disarticulating force. The joints were held together by an extra stout ligament about 5 mm in diameter and 8 mm long, linking the lateral pole of the condyle to the zygomatic arch.

A cineradiographic and electromyographic study of mastication in Tenrec ecaudatus

Regular chewing was studied in the specialized Malagasy insectivore Tenrec ecaudatus with the aid of precisely correlated electromyography of the main adductors, digastrics, and two hyoid muscles and cineradiography for which metallic markers were placed in the mandibles, tongue, and hyoid bone. During the power stroke the body of the mandible moves dorsally and medially. The medially directed component of movement at this time is greatly increased by simultaneous rotation of the mandible about its longitudinal axis. The highly mobile symphysis, spherical dentary condyle, loss of superficial masseter muscle and zygoma, and the simplified zalamnodont molars all appear to be related to the large amount of mandibular rotation that occurs during occlusion. The balancing side lateral pterygoid muscle (inferior head) apparently shifts the working side mandible laterally during the last part of opening and the first part of closing. The working side temporalis and the superficial masseter muscle are both responsible for the shift back to the midline. The temporalis is usually active to the same extent on the working and balancing sides during the power stroke. The level of activity (amplitude) of the temporalis and duration of the power stroke increase with harder foods. Whenever soft foods are chewed, the superficial masseter is only active on the working side; whenever foods of increasing hardness are chewed, its level of activity on the balancing side increases to approach that of the working side. Mandibular rotation is greatly reduced when hard foods are chewed.

Aspects of masticatory form and function in common tree shrews, Tupaia glis

Tree shrews have relatively primitive tribosphenic molars that are apparently similar to those of basal eutherians; thus, these animals have been used as a model to describe mastication in early mammals. In this study the gross morphology of the bony skull, joints, dentition, and muscles of mastication are related to potential jaw movements and cuspal relationships. Potential for complex mandibular movements is indicated by a mobile mandibular symphysis, shallow mandibular fossa that is large compared to its resident condyle, and relatively loose temporomandibular joint ligaments. Abrasive tooth wear is noticeable, and is most marked at the first molars and buccal aspects of the upper cheek teeth distal to P2. Muscle morphology is basically similar to that previously described for Tupaia minor and Ptilocercus lowii. However, in T. glis, an intraorbital part of deep temporalis has the potential for inducing lingual translation of its dentary, and the large medial pterygoid has extended its origin anteriorly to the floor of the orbit, which would enhance protrusion. The importance of the tongue and hyoid muscles during mastication is suggested by broadly expanded anterior bellies of digastrics, which may assist mylohyoids in tensing the floor of the mouth during forceful tongue actions, and by preliminary electromyography, which suggests that masticatory muscles alone cannot fully account for jaw movements in this species.