The relation of the temporal muscle to the form of the coronoid process

Fgf8 dosage regulates jaw shape and symmetry through pharyngeal-cardiac tissue relationships

BACKGROUND

Asymmetries in craniofacial anomalies are commonly observed. In the facial skeleton, the left side is more commonly and/or severely affected than the right. Such asymmetries complicate treatment options. Mechanisms underlying variation in disease severity between individuals as well as within individuals (asymmetries) are still relatively unknown.

RESULTS

Developmental reductions in fibroblast growth factor 8 (Fgf8) have a dosage dependent effect on jaw size, shape, and symmetry. Further, Fgf8 mutants have directionally asymmetric jaws with the left side being more affected than the right. Defects in lower jaw development begin with disruption to Meckel's cartilage, which is discontinuous. All skeletal elements associated with the proximal condensation are dysmorphic, exemplified by a malformed and misoriented malleus. At later stages, Fgf8 mutants exhibit syngnathia, which falls into two broad categories: bony fusion of the maxillary and mandibular alveolar ridges and zygomatico-mandibular fusion. All of these morphological defects exhibit both inter- and intra-specimen variation.

CONCLUSIONS

We hypothesize that these asymmetries are linked to heart development resulting in higher levels of Fgf8 on the right side of the face, which may buffer the right side to developmental perturbations. This mouse model may facilitate future investigations of mechanisms underlying human syngnathia and facial asymmetry.

The Regrowth of Mandibular Coronoid Process After Coronoidectomy: A Retrospective Analysis of 57 Cases

PURPOSE

Coronoidectomy is carried out frequently as a part of the cranial-maxillofacial surgery procedure. There are few articles on the fate of coronoid process after coronoidectomy, except that several case reports mentioned that coronoid process had regenerated. This study aimed to radiographically access the anatomic outcomes of coronoid process and investigate which factors were associated with the outcomes after coronoidectomy.

MATERIALS AND METHODS

A retrospective cohort study included patients undergoing coronoidectomy over a 7-year period. The primary outcome variable was the new coronoid process occurrence (yes/no). Secondary outcome variable was the type of the new coronoid process by evaluating its size, shape and position. Radiograph at 1-year postoperative visit was used to determine the outcomes. The predictor variables included age, sex, surgical purpose, surgical side, surgical approach and the maximal interincisal opening. Appropriate statistics were analyzed by SPSS version 22. χ² test and binary logistic regression were used to assess the association between predictor factors and anatomic outcomes (P <.05).

RESULTS

The study sample included 57 patients. In total, 96 coronoidectomies were performed. Seventy-four coronoid processes (77.1%) showed complete (n = 44, 45.8%), nonunion (n = 19, 19.8%) or partial (n = 11, 11.5%) regrowth, whereas no evidence of regeneration in 22 sites was observed radiographically at 1-year postoperative visit. Binary logistic regression showed that a young age (odds ratio 0.704; 95% confidence interval 0.562-0.882; P = .002) was significantly associated with regeneration of coronoid process.

CONCLUSIONS

Coronoid process can mostly regenerate after coronoidectomy. A young age may contribute to regrowth of coronoid process.

Natural Compounds Attenuate Denervation-Induced Skeletal Muscle Atrophy

The weight of skeletal muscle accounts for approximately 40% of the whole weight in a healthy individual, and the normal metabolism and motor function of the muscle are indispensable for healthy life. In addition, the skeletal muscle of the maxillofacial region plays an important role not only in eating and swallowing, but also in communication, such as facial expressions and conversations. In recent years, skeletal muscle atrophy has received worldwide attention as a serious health problem. However, the mechanism of skeletal muscle atrophy that has been clarified at present is insufficient, and a therapeutic method against skeletal muscle atrophy has not been established. This review provides views on the importance of skeletal muscle in the maxillofacial region and explains the differences between skeletal muscles in the maxillofacial region and other regions. We summarize the findings to change in gene expression in muscle remodeling and emphasize the advantages and disadvantages of denervation-induced skeletal muscle atrophy model. Finally, we discuss the newly discovered beneficial effects of natural compounds on skeletal muscle atrophy.

Masticatory muscle function affects the pathological conditions of dentofacial deformities

The causes of dentofacial deformities include various known syndromes, genetics, environmental and neuromuscular factors, trauma, and tumors. Above all, the functional effects of muscles are important, and deformation of the mandible is often associated with a mechanical imbalance of the masticatory muscles. With the vertical position of the face, weakness of the sling of the masseter muscle and medial pterygoid muscle causes dilatation of the mandibular angle. In patients with a deep bite, excessive function of the masticatory muscles is reported. Myosin heavy chain　(MyHC) properties also affect jawbone morphology. In short-face patients, the proportion of type II fibers, which are fast muscles, is high. The proportions of muscle fiber types are genetically determined but can be altered by postnatal environmental factors. Orthognathic surgery may results in the transition of MyHC to type II (fast) fibers, but excessive stretching enhances the release of inflammatory mediators and causes a shift toward a greater proportion of slow muscle fibers. This feature can be related to postoperative relapse. Bones and muscles are in close crosstalk, and it may be possible to use biochemical approaches as well as biomechanical considerations for the treatment of jaw deformities.

Comparative masticatory myology in anteaters and its implications for interpreting morphological convergence in myrmecophagous placentals

BACKGROUND

Ecological adaptations of mammals are reflected in the morphological diversity of their feeding apparatus, which includes differences in tooth crown morphologies, variation in snout size, or changes in muscles of the feeding apparatus. The adaptability of their feeding apparatus allowed them to optimize resource exploitation in a wide range of habitats. The combination of computer-assisted X-ray microtomography (µ-CT) with contrast-enhancing staining protocols has bolstered the reconstruction of three-dimensional (3D) models of muscles. This new approach allows for accurate descriptions of muscular anatomy, as well as the quick measurement of muscle volumes and fiber orientation. Ant- and termite-eating (myrmecophagy) represents a case of extreme feeding specialization, which is usually accompanied by tooth reduction or complete tooth loss, snout elongation, acquisition of a long vermiform tongue, and loss of the zygomatic arch. Many of these traits evolved independently in distantly-related mammalian lineages. Previous reports on South American anteaters (Vermilingua) have shown major changes in the masticatory, intermandibular, and lingual muscular apparatus. These changes have been related to a functional shift in the role of upper and lower jaws in the evolutionary context of their complete loss of teeth and masticatory ability.

METHODS

We used an iodine staining solution (I2KI) to perform contrast-enhanced µ-CT scanning on heads of the pygmy (Cyclopes didactylus), collared (Tamandua tetradactyla) and giant (Myrmecophaga tridactyla) anteaters. We reconstructed the musculature of the feeding apparatus of the three extant anteater genera using 3D reconstructions complemented with classical dissections of the specimens. We performed a description of the musculature of the feeding apparatus in the two morphologically divergent vermilinguan families (Myrmecophagidae and Cyclopedidae) and compared it to the association of morphological features found in other myrmecophagous placentals.

RESULTS

We found that pygmy anteaters (Cyclopes) present a relatively larger and architecturally complex temporal musculature than that of collared (Tamandua) and giant (Myrmecophaga) anteaters, but shows a reduced masseter musculature, including the loss of the deep masseter. The loss of this muscle concurs with the loss of the jugal bone in Cyclopedidae. We show that anteaters, pangolins, and aardvarks present distinct anatomies despite morphological and ecological convergences.

Association of Dysplastic Coronoid Process with Long-Face Morphology

Vertical malocclusion is a developmental condition, resulting from complex interactions among multiple etiological factors during the growth period. As a tricky dentofacial deformity clinically, long-face (LF) morphology is characterized by excessive vertical facial growth with severe disarrangement of jaws and teeth. Since the improvement of LF patients on facial profile and occlusion is often difficult and lacks long-term stability, it becomes important to unravel the etiology of LF pattern formation for early prevention and treatment. In the current studies, we identified a transgenic mouse model that exhibited a dysplastic coronoid process and LF morphology. Although the mutant mice exhibited jaw structures and occlusion comparable to controls at birth, they all acquired typical LF morphology with anterior open bite during postnatal growth, resembling clinical features of the selected skeletal class III patients. Since the coronoid process provides an insertion site for the temporalis attachment, we examined the initial development and differentiation of the temporalis and found identical results in both control and mutant mice before E17.5 when the temporal muscle makes attachment to the coronoid process. However, thereafter, we observed altered orientation and reduced size of the cross-sectional area of the temporalis in mutant mice, which persisted to the weaning stage. Biomechanical analysis and simulation modeling further support the idea that altered morphology of the coronoid process may impair the efficiency of the vertical temporalis contraction and appears to correlate with LF formation. Consistently, we present evidence that a dysplastic mandibular coronoid process was also seen in some human patients with skeletal III LF morphology. Taken together, the results presented in this study establish an association of the craniofacial bony structures with vertical patterning, which will have implications in earlier prediction for clinical precaution and intervention.

Osteohistological correlates of muscular attachment in terrestrial and freshwater Testudines

Sharpey's fibers are considered the anatomical structures integrated to the muscles. Since these fibers leave marks at the microscopic level, their presence and distribution are used as evidence of muscle attachment in extinct and extant forms. In recent years, studies have been focusing on muscle-bone and tendon-bone interaction mostly on mammals. The main objective of this work is to contribute to the morphological and histological knowledge of muscle attachment in other amniotes, such as reptiles, and their variation related to different locomotor habits. In this way, a study was performed on terrestrial and aquatic turtles. The musculature related to the movement of the humerus, and pectoral girdle in Chelonoidis chilensis, Phrynops hilarii and Hydromedusa tectifera was analyzed. Dissections were performed mapping the origins and insertions of each muscle and undecalcified thin sections were performed in specific muscular attachment sites. We found some differences which were not previously reported, related to the insertion of the m. pectoralis, the m. coracobrachialis magnus and the origin of the m. tractor radii. The osteohistology revealed the presence of Sharpey's fibers in the cortex of all the bone elements analyzed. Patterns were established in relation to the orientation and density of Sharpey's fibers, which were used for the categorization of each muscle attachment site. The comparative micro-anatomical study of these areas did not reveal any important differences between terrestrial and freshwater turtles in muscles involved with the rotation, abduction and adduction of the humerus. In this way, the preliminary results suggest an absence of correlation between the distribution and density of Sharpey's fibers between different habitat forms, at least in the bones and species analyzed.

Is there an asymmetry of the condylar and coronoid processes of the mandible in individuals with unilateral crossbite?

OBJECTIVES

To evaluate if there is a true skeletal asymmetry of the condylar and coronoid processes of the mandible in growing individuals with unilateral posterior crossbite (UPC) either functional or not.

MATERIALS AND METHODS

This cross-sectional study screened a total of 1120 cone beam computed tomography (CBCT) scans based on inclusion and exclusion criteria. The final sample comprised 20 CBCT images of individuals with UPC and 19 CBCT images of individuals without transverse malocclusion. The lengths of the condylar and coronoid processes were measured to evaluate asymmetry, as well as the magnitude of the mandibular lateral deviation in the UPC group.

RESULTS

There was a significant difference between the lengths of the affected and nonaffected sides of the coronoid processes in the UPC group (P < .01). The same was not observed in the condyle in the UPC group (P > .05). There were no significant differences between the groups (P > .05).

CONCLUSIONS

Although no differences in the condyle were observed, the coronoid process was asymmetric in individuals with UPC. However, this asymmetry was not considered to be clinically significant.

Mechanical implications of the mandibular coronoid process morphology in Neandertals

OBJECTIVES

Among the diagnostic features of the Neandertal mandible are the broad base of the coronoid process and its straight posterior margin. The adaptive value of these (and other) anatomical features has been linked to the Neandertal's need to cope with a large gape. The present study aims to test this hypothesis with regard to the morphology of the coronoid process.

MATERIALS AND METHODS

This admittedly simple, intuitive hypothesis was tested here via a comparative finite-element study of the primitive versus modified state of the coronoid process, using two-dimensional models of the mandible.

RESULTS

Our simulations demonstrate that a large gape has an unfavorable effect on the primitive state of the coronoid process: the diagonal, almost horizontal, component of the temporalis muscle resultant (relative to the long axis of the coronoid process) bends the process in the sagittal plane. Furthermore, we show that the modification of the coronoid process morphology alone reduces the process' bending in a wide gape increasing the compression to tension ratio.

DISCUSSION

These results provide indirect evidence in support of the hypothesis that the modification of the coronoid process in Neandertals is necessary for enabling their mandible to cope with a large gape.

Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid

Background

The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear.

Results

By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction.

Conclusions

The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage.

The utility of domestic dogs in assessing human morphological variation

Although distantly related to us, dogs are a highly variable cohabitating taxon that may provide clues relevant to hypotheses of human variation, evolution, and development. Here we first propose the utility of domestic dogs as a heuristic model for the study of human variation and evolution. Next we provide a case study of variation in a mandibular relationship previously suggested to distinguish Neanderthals from modern humans taxonomically. We show that for dogs varying greatly in size and form, this character (the relative heights of the condyle and coronoid process) does not produce useful group divisions. While this result cannot disprove the taxonomic value of this character for humans/hominids, it does indicate that caution is warranted in accepting it as genetically constrained. Similar explorations of other features of domestic dogs may prove fruitful in elucidating aspects of human variation and evolution not readily highlighted by primate and fossil hominid studies. Expected ranges of variation and degrees of plasticity are key questions to be investigated.

Relationship between the angle of the coronoid process of the mandible and the electromyographic activity of the temporal muscle in skeletal Class I and III individuals

The aim of the present study was to verify the relationship between the angle of the coronoid process of the mandible in the latero-lateral direction and electromyographic activity of the anterior part of the temporal muscle in skeletal Class I and III individuals. Forty-five volunteers were assessed subdivided into two groups, according to angle ANB, in Class I and III. Two radiographic examinations were performed, one lateral cephalogram to measure angle ANB and one frontal cephalogram to measure the angle of the coronoid process. The electromyographic examination of the volunteers' temporal muscles was performed. Statistical analysis of the data showed that the relationship between the angle of the coronoid process and the skeletal class was negative for both classes analysed. The relationship between electromyographic activity of the temporal muscle and angle of the coronoid process was negative for Class I individuals. In the Class III group, smaller angles of the coronoid process were related to higher values of electromyographic activity of the temporal muscle. There was no statistically significant difference between the electromyographic activity of the temporal muscle in skeletal Class I and III individuals. Based on the results obtained, it was concluded that there was no influence of the skeletal classes analysed on the angle of the coronoid; the angle of the coronoid process may be influenced by the electromyographic activity of the temporal muscle in the Class III group; the electromyographic activity of the temporal muscle is not influenced by the skeletal class in Class III individuals.

The effect of the Botulinum toxin-A on craniofacial development: an experimental study

In this study, we developed a novel experimental model to evaluate muscular action on bone formation and remodeling by the help of Botulinum toxin-A (BTX). Forty-nine 15-day-old male Wistar rats were put into 4 groups randomly. Group 1 was the control group. BTX 0.4 IU (0.05 mL) was injected into the right masseter muscle in group 2 and into right temporalis muscle in group 3. The same volume of sterile saline was given into the both above-mentioned muscles in group 4. At the end of the fourth month, all animals were killed. Histology and weight of the masseter and temporalis muscles were studied. Thirty different osteometric measurements were also taken from skulls. Significant atrophy in BTX injected muscles was observed in groups 2 and 3. In group 4 (saline injection), only few osteometric measurements were significantly reduced, indicating the effect of the injection itself. Both groups 2 and 3 have apparent decrease in nasal bone, premaxilla, maxilla, and zygomatic dimensions on the injected side. When masseter group was compared with control and saline groups, no significant difference was found in skull base dimensions and mandibular length. In contrast, temporal group has also shown significant decrease in skull base dimensions. Our conclusions are as follows: (1) With this model, it is possible to study muscular action on bone formation and modeling without any surgical intervention, that is, by avoiding surgical artifacts, such as scar and contracture; (2) denervation of the skeletal muscles with BTX during the growing phase does effect bone development in a negative way; (3) pediatric use of the BTX deserves reevaluation under the light of these findings.

Sutural growth restriction and modern human facial evolution: an experimental study in a pig model

Facial size reduction and facial retraction are key features that distinguish modern humans from archaic Homo. In order to more fully understand the emergence of modern human craniofacial form, it is necessary to understand the underlying evolutionary basis for these defining characteristics. Although it is well established that the cranial base exerts considerable influence on the evolutionary and ontogenetic development of facial form, less emphasis has been placed on developmental factors intrinsic to the facial skeleton proper. The present analysis was designed to assess anteroposterior facial reduction in a pig model and to examine the potential role that this dynamic has played in the evolution of modern human facial form. Ten female sibship cohorts, each consisting of three individuals, were allocated to one of three groups. In the experimental group (n = 10), microplates were affixed bilaterally across the zygomaticomaxillary and frontonasomaxillary sutures at 2 months of age. The sham group (n = 10) received only screw implantation and the controls (n = 10) underwent no surgery. Following 4 months of post-surgical growth, we assessed variation in facial form using linear measurements and principal components analysis of Procrustes scaled landmarks. There were no differences between the control and sham groups; however, the experimental group exhibited a highly significant reduction in facial projection and overall size. These changes were associated with significant differences in the infraorbital region of the experimental group including the presence of an infraorbital depression and an inferiorly and coronally oriented infraorbital plane in contrast to a flat, superiorly and sagittally infraorbital plane in the control and sham groups. These altered configurations are markedly similar to important additional facial features that differentiate modern humans from archaic Homo, and suggest that facial length restriction via rigid plate fixation is a potentially useful model to assess the developmental factors that underlie changing patterns in craniofacial form associated with the emergence of modern humans.

Relationship between the inclination of the coronoid process of the mandible and the electromyographic activity of the temporal muscle in skeletal Class I and II individuals

The aim of this study was to verify the relationship between the inclination of the coronoid process of the mandible, and electromyographic activity of the anterior part of the temporal muscle in skeletal Class I and II individuals. Forty-seven volunteers (mean age 24.5 +/- 3.9 years) were subdivided into two groups, according to angle ANB: Class I (n = 25) and II (n = 22). Two radiographic examinations were performed; one lateral cephalogram to measure angle ANB, and one frontal cephalogram to measure the inclination of the coronoid process. Electromyographic (EMG) examination of the anterior part of the subjects' temporal muscles was performed. Statistical analysis of the data showed that Class II individuals presented lower electromyographic activity of the anterior part of the temporal muscle. Based on the results obtained, it was concluded that the skeletal classes analyzed had no influence on the inclination of the coronoid process. The inclination of the coronoid process was not influenced by the electromyographic activity of the anterior part of the temporal muscle in Class I and II individuals; however, the lower electromyographic activity of the anterior part of the temporal muscle could be influenced by the skeletal class in Class II individuals.

A quantitative analysis of the Eutherian orbit: correlations with masticatory apparatus

The mammalian orbit, or eye-socket, is a highly plastic region of the skull. It comprises between seven and nine bones, all of which vary widely in their contribution to this region among the different mammalian orders and families. It is hypothesised that the structure of the mammalian orbit is principally influenced by the forces generated by the jaw-closing musculature. In order to quantify the orbit, fourteen linear, angular and area measurements were taken from 84 species of placental mammals using a Microscribe-3D digitiser. The results were then analysed using principal components analysis. The results of the multivariate analysis on untransformed data showed a clear division of the mammalian taxa into temporalis-dominant forms and masseter-dominant forms. This correlation between orbital structure and masticatory musculature was reinforced by results from the size-corrected data, which showed a separation of the taxa into the three specialised feeding types proposed by Turnbull (1970): i.e. 'carnivore-shear', 'ungulate-grinding' and 'rodent-gnawing'. Moreover, within the rodents there was a clear distinction between species in which the masseter is highly developed and those in which the temporalis has more prominence. These results were reinforced by analysis of variance which showed significant differences in the relative orbital areas of certain bones between temporalis-dominant and masseter-dominant taxa. Subsequent cluster analysis suggested that most of the variables could be grouped into three assemblages: those associated with the length of the rostrum; those associated with the width of the skull; and those associated with the relative size of the orbit and the shape of the face. However, the relative area of the palatine bone showed weak correlations with the other variables and did not fit into any group. Overall the relative area of the palatine was most closely correlated with feeding type, and this measure that appeared to be most strongly associated with the arrangement of the masticatory musculature. These results give a strong indication that, although orbital structure is in part determined by the relative size and orientation of the orbits, the forces generated by the muscles of mastication also have a large effect.

Mandibular coronoid hyperplasia in pediatric patients

Bilateral coronoid hyperplasia is a relatively rare condition in the pediatric population and yet may be an unrecognized cause of limited mouth opening in children. There are multiple theories as to the causes of the hyperplasia, which include temporalis hyperactivity, hormonal stimulus, and genetic inheritance. The resulting excess growth of the coronoids results in impingement on the zygomatic processes leading to mandibular hypomobility. The diagnosis is confirmed with plain films and computed tomography scans. Treatment involves bilateral coronoidectomies to relieve impingement on the zygoma. Postoperative physical therapy is crucial for success; the therapy focuses on maintaining the mouth opening achieved at the time of surgery. Outcome reports have been variable despite good physical therapy, suggesting that the exact pathology of the condition is not well understood.

Biomineralization and adaptive plasticity of the temporomandibular joint in myostatin knockout mice

Mice lacking myostatin (GDF-8), a negative regulator of skeletal muscle growth, show a significant increase in muscle mass versus normal mice. We compared wild-type and myostatin deficient mice to assess the postnatal effect of elevated masticatory loads due to increased jaw-adductor muscle activity and greater bite forces on mandibular condyle morphology. Microcomputed tomography (microCT) was used to provide details of internal condylar morphology and quantify bone density in three condylar regions. Biomineralization levels, as well as external mandibular dimensions, were used to characterize within-slice, within-joint, within-group and between-group variation. Dimensions of the mandible and mandibular condyle were similar between the myostatin knockout and normal mice. Knockout mice exhibited significantly more biomineralization on the outer surface of the condylar subchondral bone and along the condylar neck, most notably on the buccal side of the condylar neck. The buccal side of the inner aspect of the condyle was significantly less biomineralized in knockout mice, both for the pooled data and for the posterior and anterior condylar slices. Whilst normal mice had symmetric subchondral bone surfaces, those of knockout mice were asymmetric, with a lower, less convex surface on the buccal side versus the lingual side. This appears related to the ontogenetic effects of increased masticatory stress in the mandibles of knockout mice as compared to normal mice. Significant differences in biomineralization between normal and myostatin knockout mice, coupled with the lack of significant differences in certain external dimensions, underscores a need for information on the external and internal morphology of mineralized tissues vis-à-vis altered or excessive mechanical loads.

Relationship between the direction of mandibular growth and masseter muscle conduction velocity

INTRODUCTION

The purpose of this study was to investigate the relationship between muscle conduction velocity (MCV) of the masseter muscle and the direction of mandibular growth.

METHODS

Longitudinal cephalometric X-rays taken at the prepubertal and postpubertal periods of 16 Japanese girls were analyzed. MCV was calculated from the delay in myoelectric signals obtained by using multiple surface electrode arrays placed along the fibers of the left masseter muscle in the postpubertal period. The direction of mandibular growth was evaluated by superimposition of the lateral cephalometric X-rays at the prepubertal and postpubertal periods. The relationship between MCV and the direction of mandibular growth was analyzed statistically.

RESULTS

MCV was significantly correlated with the vertical facial height at the postpubertal period and the direction of mandibular condyle growth.

CONCLUSIONS

If the relationship between prepubertal and postpubertal of MCV is clarified, it might be possible to predict the direction of mandibular growth and the vertical facial proportions at the postpubertal period from MCV of the masseter muscle at the prepubertal period.

Role of suprahyoid musculature on mandibular morphology and growth orientation in rats

This study was undertaken in rats to study the influence of the suprahyoid muscles on mandibular growth, morphology, and orientation. The aim was to investigate the effect of bilateral suprahyoid muscle myectomy on the skeletal growth and orientation of the mandible and on its orientation in rats. Forty-eight 4-week-old rats were divided into 2 experimental and 2 control groups as follows: A, 12 animals in which bilateral excision of the anterior digastric muscle was performed; B, 12 animals in which bilateral excision of the anterior digastric, transverse mandibular, and mylohyoid muscles was performed; C, 12 animals that were sham-operated bilaterally without any muscular excision; D, 12 control animals not subjected to any operation. The experimental period was 30 days. Lateral and dorsoventral radiographs were taken on days 1 and 30, after muscular excision. Cephalometric analysis was performed for each animal according to a method and procedure established in our laboratory and described earlier. The findings support the occurrence of decreased mandibular growth in the group in which bilateral digastric, transverse mandibular, and mylohyoid excision was performed as compared with controls. In addition, the mandible in the bilateral digastric excision group exhibited a more upward orientation. It was thus proven that the absence of the suprahyoid musculature does affect both skeletal growth and orientation of the mandible.

Morphologic determinants in the etiology of class III malocclusions: a review

Morphospatial disharmony of the craniomaxillary and mandibular complexes may yield apparent mandibular prognathism, but Class III malocclusions can exist with any number of aberrations of the craniofacial complex. Deficient orthocephalization of the cranial base allied with a smaller anterior cranial base component has been implicated in the etiology of Class III malocclusions. Whereas the more acute cranial base angle may affect the articulation of the condyles resulting in their forward displacement, the reduction in anterior cranial size may affect the position of the maxilla. As well, intrinsic skeletal elements of the maxillary complex may be responsible for maxillary hypoplasia that may exacerbate the anterior crossbite seen in the Class III condition. Conversely, with an orthognathic maxilla, condylar hyperplasia and anterior positioning of the condyles at the temporo-mandibular joint may produce an anterior crossbite. Aside from the skeletal components, soft tissue matrices, particularly labial pressure from the circumoral musculature, may influence the final outcome of craniofacial growth of a child skeletally predisposed to Class III conditions. Indeed, as some Asian ethnic groups demonstrate an increased prevalence of Class III malocclusions, it is likely that the skeletal components and soft tissues matrices are genetically determined. Presumably, the co-morphologies of the craniomaxillary and mandibular complexes are likely dependent upon candidate genes that undergo gene-environmental interactions to yield Class III malocclusions. The identification of such genes is a desirable step in unraveling the complexity of Class III malocclusions. With this knowledge, the clinician may elect an early course of dentofacial orthopedic and orthodontic treatments aimed at preventing the development of Class III malocclusions.

Procrustes, Euclidean and cephalometric analyses of the morphology of the mandible in human Class III malocclusions

The role of mandibular phenotype in the development of Class III malocclusion remains unclear. The purpose of this study was to determine whether the form of the mandible differed between prepubertal individuals with Class I and Class III malocclusions. Lateral cephalographs of 73 children of European-American descent aged between 5-11 years with Class III malocclusion were compared to those of 60 counterparts with a normal, Class I molar occlusion. The cephalographs were traced and checked, and eight homologous mandibular landmarks were digitized. Average mandibular geometries, scaled to an equivalent size, were generated using Procrustes superimposition. Euclidean distance matrix analysis (EDMA) was undertaken to corroborate the Procrustes analysis, and bivariate analysis utilizing eight linear and five angular measurements was also performed. Residuals and F-values from Procrustes analysis indicated that mandibular configurations differed statistically for Class I and Class III types. EDMA confirmed that the Class I and Class III geometries were significantly different, revealing that the greatest differences in morphology arose in the anterior-most mandibular regions. As well, most variables showed statistically significant differences when the Class I and Class III mandibular types were compared. When the sample was subdivided into seven age- and sex-matched groups, nearly all age-based comparisons were significantly different. It is concluded that the morphology of the mandible differs in individuals with Class III malocclusions when compared to the normal Class I configuration, and that these alterations may indicate dichotomous postnatal mandibular ontogeny.

The effects of muscular dystrophy on craniofacial growth in mice: a study of heterochrony and ontogenetic allometry

Mechanical loading of muscles on bones at their sites of attachment can regulate skeletal morphology. The present study examined the effects of muscle degeneration on craniofacial growth, using two strains of muscular dystrophic mice, Mus musculus, differing in pathological severity. We collected radiographic and weight data longitudinally and digitized radiographs to obtain distances between anatomical landmarks in different functional regions of the skull. We then quantified heterochronic and allometric differences among genotypes and between sexes. Because growth is nonlinear with respect to time, we first used the Gompertz model to obtain heterochronic growth parameters, which were then tested with ANOVA. Ontogenetic allometric analyses examined the scaling relationships between various measurements with linear regressions. For most measurements the severely dystrophic mice are significantly smaller in final size than both the control and the mildly dystrophic mice, which are statistically indistinguishable. Measures of total growth and the neurocranium exhibit more differences among groups in heterochronic parameters of early ontogeny because growth in these regions is controlled primarily by brain expansion that ceases early in development. In contrast, the face and mandible exhibit more differences in later growth parameters possibly because of the increased influence of muscles on these regions as growth progresses. The severely dystrophic mice have flatter, more elongate skulls and mandibles than those of the other two genotypes, concurrent with an absence of muscular forces to stimulate growth in a superior-inferior direction.

The medial axis branch point in the human mandible

The medial axis method was applied to radio-cephalometric images of the mandible in 20 adults and 18 children and to panoramic X-ray images of these children, and also directly to 50 halves of dry mandibles. It was found that the location of the posterior branch point coincided almost invariably with the mandibular foramen/lingula. The foramen may be regarded as the posterior limit of the mandibular body, from which the condylar and coronoid processes branch off. The medial axis appears to reflect the developmental and functional anatomy of the human mandible.

Relationship between bone and muscles of mastication in hemifacial microsomia

The relationship between the bone and muscles of mastication in hemifacial microsomia was studied using three-dimensional volumetric computed tomography scans and image processing techniques. High resolution head computed tomography scans were obtained from 31 patients with unilateral hemifacial microsomia and eight normal patients. Using three-dimensional volume renderings of bone, mandibular deformities in patients with hemifacial microsomia were classified using the Pruzansky system. For each patient, specific craniofacial bones (temporal bone, maxilla mandible) and the muscles of mastication (masseter, temporalis and lateral and medial pterygoid) were segmented bilaterally from the image volume for independent display and volume measurement. Volumes were expressed as the ratio of the affected: unaffected sides. For the masseter and temporalis, the relationship between muscular hypoplasia and osseous hypoplasia in its origin and insertion was studied by plotting affected:unaffected bone volume as a function of affected:unaffected muscle volume for each muscle, bone of origin, bone of insertion triplet. The volumes of the pterygoid muscles were compared with hemimandibular volumes. The precision of object segmentations was examined by repetitive definition tasks, whereas the accuracy of volume measurement was tested by scanning custom-made phantom objects and comparing digital to physical object volume measurements. Volume measurements performed using these techniques were both accurate and precise. In hemifacial microsomia, the extent of hypoplasia of specific muscles of mastication predicted the extent of dysplasia in their osseous origin and insertion. However, the reverse was not true. The extent of hypoplasia of the facial bones did not necessarily predict the extent of hypoplasia in the attached muscles of mastication. Pruzansky grade of the mandible described the degree of mandibular hypoplasia on the affected side, but was inconsistent in its prediction of volume decrease of the other facial bones.

Hyperplasia of the mandibular coronoid process: an analysis of 31 cases and a review of the literature

PURPOSE

This study was undertaken to investigate the demographic and clinical features of mandibular coronoid hyperplasia and to assess the response to current treatments.

PATIENTS AND METHOD

Case notes were studied and records made of age, sex, duration of symptoms, operative procedures, and response to treatment in all patients presenting at the Queen Victoria Hospital, East Grinstead, over a 20-year period from 1970 to 1990 with a confirmed diagnosis of mandibular coronoid hyperplasia.

RESULTS

Thirty-one cases were recorded, 23 bilateral and 8 unilateral. The average age on presentation was 27.8 years for bilateral and 23.6 years for unilateral cases, with symptoms predating presentation by an average of 9 years and 6.75 years, respectively. Surgery was disappointing in terms of improving mouth opening.

CONCLUSION

The results of this large series are supported by meta-analysis of the previous literature. They suggest that surgical management may be improved by a greater understanding of the pathogenesis of this condition.

Jaw protruder muscles and condylar cartilage growth in the rat

Many studies have explored the role of the protrusive musculature in promoting growth at the condylar cartilage and the overall lengthening of the lower jaw, with emphasis on the lateral pterygoid muscle (LPM). The largely anteroposterior orientation of the superficial part of the masseter muscle (SM) in the rat suggests that it may also function as a protruder of the lower jaw. Accordingly, it is possible that the action of the SM may play a part in the regulation of growth of the condylar cartilage and the lower jaw. To examine this hypothesis, bilateral resection of the superficial portion of the masseter muscle was performed in male Sprague-Dawley rats at 26 days of age. At 5 days after surgery, [3H]-thymidine incorporation in the condylar cartilage was increased (F = 6.93, p less than or equal to 0.01) in the SM myectomy group relative to the surgical control and unoperated control groups. However, by 20 days after surgery no differences were present. At this sacrifice interval, lower jaw dimensions relating to areas of muscle attachment, as well as ramus height, were significantly reduced in the SM myectomy group, but overall jaw length (mental foramen to condyle) was unaffected. In contrast, myotomy of the LPM resulted in a significant decrease in mitotic activity of the cartilage 4 days after surgery. This decrease was present, but not more pronounced, in animals subjected to both SM myectomy and LPM myotomy. Hence, myotomy or myectomy of these two muscles, each with a protrusive orientation, produces opposite effects on proliferative activity at the condylar cartilage.

Bilateral coronoid hyperplasia. Report of two cases and review of the literature

Two cases of bilateral hyperplasia of the coronoid processes of the mandible are presented. The patients were men and first noticed their symptoms at 14 and 10 years of age, respectively. They were treated successfully by intraoral bilateral coronoidectomies. A review of the literature revealed only forty-nine cases of the condition reported; 43 cases in western countries and 6 in Japan. As described previously, the majority of the cases were men and their symptoms occurred under the age of 19 years.

Coronoid process elongation in rhesus monkeys (Macaca mulatta) after experimentally induced mandibular hypomobility. A cephalometric and histologic study

The present study provided an experimental model that allowed a cephalometric and histologic analysis of craniofacial growth in monkeys with induced translatory impairment of the mandibular condyle. Cauterization was performed anterior to the joint in nine experimental rhesus monkeys, while eight animals served as control subjects. The experimental procedure produced mandibular hypomobility in six animals, in three by means of temporomandibular joint ankylosis and in three by means of dense scar tissue formation anterior to the joint. In the remaining three experimental animals no restriction of mandibular mobility was created. Mandibular hypomobility was found to induce elongation of the coronoid process and was also associated with bone deposition in the gonial region. In contrast, normal remodeling of the gonial region was found in the experimental animals with normal range of movements and in the control animals.

Craniofacial alterations following electrolytic lesions of the trigeminal motor nucleus in actively growing rats

The objective of this study was to define further the role of the trigeminal motor nucleus (TMNu) in the postnatal ontogeny of the mammalian craniofacial skeleton. To that end, 42 male Sprague-Dawley rats underwent stereotaxic surgery at 40 days of age; 21 received small electrolytic lesions to their left-side TMNu (lesioned group) while 21 had TMNu stimulation with no actual electrolytic lesion produced (sham group). Seven rats from each group were killed at 28, 56, and 84 days postoperative to analyze trigeminal motoneuron (TMNe) count, masticatory muscle weight, and osteological growth vector data. At all three time periods, lesioned animals showed significant differences 1) between the surgery and nonsurgery sides, and 2) from sham animals. However, sham animals also demonstrated significant between-side differences for medial pterygoid muscle weight (56 days), mandibular height (28 and 56 days), and mandibular length data (84 days); these data suggested that even relatively slight damage to TMNe can create morphological changes within the craniofacial complex. Snout deviation in a lesioned rat towards the opposite side from all other lesioned animals was correlated with unique damage to its pontine reticular formation; this suggested that the observed morphological alterations of the craniofacial complex may have been due not only to TMNu damage, but also to changed expressions of the masticatory central pattern generator (CPG). Morphological alterations of the craniofacial skeleton resulting from lesions to the TMNu were likely due to changed neuromuscular activity patterns of the masticatory muscles and their biomechanical effects upon bone.

A cephalometric analysis of patients with coronoid process enlargement and locking

Two patient groups with radiographically verified coronoid process locking were cephalometrically examined. In eight patients the locking was of congenital origin, and in eight patients the locking had developed as a result of internal derangement of the temporomandibular joint. When compared with that in a control group, the height of the coronoid process was statistically significantly greater in both patient groups, but there was no difference on condylar height between any of the groups. Neither were there any differences between groups regarding the upper face height, which, if present, could have been expected to contribute to the development of coronoid process locking. In the group with congenital enlargement, the mandibular configuration was strikingly square-shaped, which was expressed by a statistically significantly smaller gonion angle than was present in the other two groups. The finding was interpreted as a growth effect due to the mandibular locking.

Human dental reduction: natural selection or the probable mutation effect

Dental reduction has been sufficiently widespread among human populations to render the phenomenon of reduced tooth size worthy of scientific explanation. One of the most controversial models invoked to explain structural reduction in organisms is referred to as the "probable mutation effect" (PME). According to this model, structures no longer functional owing to ecological or cultural changes will experience a relaxation of selection pressure, permitting an accumulation of mutations in the population that inevitably will result in the reduction in size or the loss of the concerned structure. Although the PME continues to be offered as a viable explanation of human dental reduction, it is based upon several premises that modern dental clinical experience fails to support. Known enzyme defects resulting from mutations, factors predisposing to dental infections, and the deleterious effects of teeth that are too large or too small reveal that the PME does not logically account for the reduction of tooth size. Given such information, this paper proposes models of dental reduction based upon natural selection, which, unlike the PME, are testable in both modern and archaeological populations. The integration of clinical and skeletal data permits a more thorough understanding of dental reduction in the hominid fossil record.

Craniofacial sequelae of lesions to facial and trigeminal motor nuclei in growing rats

Unilateral electrolytic lesions were made in the left-side facial motor nucleus (FMNu) of six Sprague-Dawley rats at 35 days of age in order to correlate craniofacial sequelae with changed motoneuron function. Experimental and control rats were killed at 22, 32, 42, and 52 days postoperatively to provide muscle weight, brain histology, and dry skull preparations for analyses. Dissection, muscle weight, motoneuron count, and osteometric data revealed that lesion-side facial and masticatory muscles were affected by the lesions. Paired t-tests indicated that significant differences existed between weights of experimental lesion- and nonlesion-side anterior digastric, temporalis, masseteric complex, and medial pterygoid muscles, numbers of facial and trigeminal motoneurons, and several skeletal dimensions of the skull. Basi-cranial dimensions of experimental animals were least affected by the lesion, whereas zygomatic arch, dorsal facial region, and mandibular condyle dimensions were most affected. Statistical analyses also detected significant differences between experimental and control groups for several skeletal dimensions of the skull. Data indicated that damage to the trigeminal motor nucleus (TMNu) was secondary to the primary lesion in the FMNu. Motoneurons within the facial and trigeminal neuromuscular complexes (FNC and TNC) play an important role in craniofacial growth and development.

Mandibular coronoid process locking: a prospective study of frequency and association with internal derangement of the temporomandibular joint

Coronoid process locking has been regarded a rare condition. The literature reveals 59 reports each of one or two cases and an additional two papers reporting on four and six patients, respectively, the latter including hereditary cases. Coronoid process elongation tends to be overlooked when limitation of mouth-opening ability is investigated, since most interest is focused on the joint. To study the frequency of coronoid process hyperplasia and its possible association with TMJ disk displacement, 163 patients with limited mouth-opening ability were examined. All patients were referred for radiographic examination of the TMJs. In eight patients (5%), the limitation of mouth-opening ability was caused by elongation of the coronoid process. Four cases were of congenital origin, and four were secondary to long-standing disk displacement without reduction. The results of this study indicate that elongation of the coronoid process is more common than previously thought. In cases of limited mouth opening, attention should be paid to the possibility of coronoid process locking.