Changes in mandibular rotation after muscular resection. Experimental study in rats

Effects on Facial Growth Following Masseter Muscle Resection in Growing Rats-A Systematic Review

An individual's facial appearance is heavily influenced by facial symmetry. In the asymmetric mandible, periosteal apposition and endochondral ossification in one of the condyles may stimulate asymmetric growth of the body. Our aim was to review the impact on the growth following masseter resection. Relevant studies up to October 2022 were retrieved from PubMed, Scopus, and Web of Science. The PICOS method was utilized to determine eligibility, and the SYRCLE risk of bias tool was utilized to provide an estimate of potential bias. A predetermined algorithm was used to search the databases. The results of our systematic review of seven studies indicate that the masseter muscle strongly impacts craniofacial growth and development. Resection of the masseter muscle significantly reduces the sagittal and vertical development of the jaw in rats. In addition, the masseter muscle excision influences the mandibular morphology, including the condylar area, angle, and development direction of the jaw.

Evaluation of trabecular bone and mandibular cortical thickness in adults with different vertical facial types

OBJECTIVES

This study investigated differences in trabecular structure and mandibular cortical thickness in adults related to vertical facial type (VFT), sex, and their interactions.

STUDY DESIGN

Lateral cephalometric radiographs (LCRs) and panoramic radiographs (PRs) of 256 patients were reviewed. The VFT classification into low-angle, normal, and high-angle groups was determined using angular and linear measurements on LCRs. Fractal dimension (FD) values and mandibular radiomorphometric indices (RMIs) were calculated on PRs.

RESULTS

Two-way analysis of variance revealed significant differences in FD overall among VFT groups in all sites (P < .001), with pairwise comparisons indicating the greatest values in the high-angle group in the condyle (P < .05) but in the low-angle group elsewhere (P < .001). RMIs were significantly different overall regarding VFT only in the posterior mandible (P = .004), with pairwise comparisons revealing low-angle and normal group values greater than high-angle group values (P < .05). Patient sex and the interaction of facial type and sex had no significant effect on any bone measurements.

CONCLUSIONS

VFT had significant effects on trabecular structure at all measured sites, but cortical thickness was affected only in 1 location.

Evaluation and Correlation Between Pharyngeal Space, Mandible, and Tongue in Two Different Facial Patterns

Introduction:

Respiratory function has a direct relationship with the pharyngeal airway dimension and mandibular morphology. The objective of the present study was to compare and correlate pharyngeal airway, mandibular morphology, and tongue position in hyperdivergent and normodivergent patterns using standard cephalometric radiographs.

Materials and Methods:

A total of 130 individuals with age 13 to 19 years, with no craniofacial skeletal abnormalities, were taken. The individuals were divided into 2 groups based on their Frankfort mandibular angle (FMA). Group I consist of 65 hyperdivergent individuals (34 males, 31 females) with FMA >28. Group II consisted of 65 normodivergent individuals (28 males, 37 females) with FMA >25 to <28. For statistical analysis, variables were analyzed by unpaired t-test, with P value <.05. Correlation between the pharyngeal space and the other variables was analyzed with the Pearson correlation test.

Results:

Upper pharyngeal airway space in hyperdivergent growth pattern exhibited a significant negative correlation with ramus width (r = −0.02 and P = .028). Gonial angle showed a statistically significant inverse correlation with lower pharyngeal airway space (r = −0.398 and P = .029). A-point-nasion-B-point (ANB), saddle angle, posterior tongue space, tongue gap, and pharyngeal airway among normodivergent subjects, showed a statistically significant negative correlation.

Conclusion:

The present study showed a significant relationship between pharyngeal airway space, craniofacial morphology, and tongue position suggesting that narrower airway space may be a causative factor for altered dentofacial skeletal patterns. Reduced ramus width, lowered tongue, and narrower posterior pharyngeal wall might be a balancing mechanism set for morphological imbalance in hyperdivergent pattern.

The role of craniofacial maldevelopment in the modern OSA epidemic: a scoping review

STUDY OBJECTIVES

There is increasing recognition that environmental factors affect human craniofacial development and our risk for disease. A scoping review of the literature was performed looking at environmental influences on craniofacial development to better understand this relationship and investigate what further study is needed to determine how this relationship may impact obstructive sleep apnea.

METHODS

A comprehensive literature search was performed using the Ovid Medline database from inception to May, 2020 with relevance to craniofacial development in 5 clinically-oriented variables: diet, secular change, breastfeeding/non-nutritive sucking habits (NNSH), nasal obstruction/mouth breathing, and masticatory muscle function. The Oxford Centre for Evidence-Based Medicine Levels of Evidence (LoE) was used to assess studies based on study design.

RESULTS

18,196 articles were initially identified, of which 260 studies were fully reviewed and 97 articles excluded. The remaining 163 articles were categorized as follows: Secular change (n = 16), Diet (n = 33), Breastfeeding/NNSH (n = 28), Nasal obstruction/Mouth breathing (n = 57), and Masticatory muscle function (n = 35). 93% of included studies reported a significant association between craniofacial morphology and environmental factors. The majority of studies were characterized as low LoE studies with 90% of studies being LoE 4 or 5.

CONCLUSIONS

The studies in this review suggest that environmental factors are associated with changes in craniofacial development. However, most studies were heterogeneous and low-level studies, making strong conclusions about these relationships difficult. Future rigorous studies are needed to further our understanding of environmental influences on craniofacial development and OSA risk.

Study of changes in rat mandibular condyle under intermittent cyclic and continuous compressive stress

OBJECTIVES

This study aimed to investigate the effects of intermittent and continuous compressive stress on rat mandibular condyles.

DESIGN

Ninety rats were randomly divided into 5 groups: non-loading control groups and 1-day intermittent cyclic loading, 3-day intermittent cyclic loading, 7-day intermittent cyclic loading, continuous loading groups. In the loading groups, compressive mechanical stresses were loaded onto the condyles. The rats were euthanized after one, two, or four weeks. Histomorphometric and immunochemical staining of the cartilage and the micro-CT scanning of subchondral bone were investigated.

RESULTS

Under continuous loading following the first week, cartilage thickness, numbers of chondrocytes, extracellular matrix (Type-II collagen) and proliferation decreased significantly, and there was no apparent repair response after the second and fourth weeks. The cartilage of 1-day and 3-day intermittent cyclic loading groups showed similar pathological changes but better proliferative activity compared with the continuous loading group. The 7-day intermittent loading group had significant recovery after 2 weeks, including the increase of cartilage thickness and the number of chondrocytes, cell swelling and rearranging. However, the remodeling of subchondral bone showed no significant difference between the intermittent and continuous loading stress compared to the control group.

CONCLUSIONS

Under moderate mechanical stress, the condylar cartilage actively remodeled, whereas the subchondral bone, as supportive rigid structure, was less sensitive to mechanical pressure. The intermittent compressive stress protects the condylar cartilage from excessive damage, which is different from continuous compressive stress. With longer intervals, the cartilage has the potential capability for recovery as the initial state.

Mandibular Vertical Growth Deficiency After Botulinum-Induced Hypotrophy of Masticatory Closing Muscles in Juvenile Nonhuman Primates

The purpose of this study was to investigate the relationship between masticatory muscular hypotrophy and mandibular growth in juvenile nonhuman primates (cynolmolgus monkeys, Macaca fasicularis). We hypothesized that botulinum toxin (BTX)-induced neuro-muscular junctional block and its resultant hypotrophy of masticatory muscles would produce mandibular growth disturbances in size and shape. Ten male cynomolgus monkeys were divided into three groups: group I (control; n = 3), group II (unilateral BTX; n = 4), and group III (bilateral BTX; n = 3). The unilateral or bilateral muscular hypotrophy of major masticatory closing muscles was induced by synchronous BTX application to masseter, medial pterygoid, and temporal muscle. Mandibular growth was tracked by linear, angular, area and volume measurements using three-dimensional (3D) computed tomography imaging before BTX treatment and after 3 and 6 months. After unilateral hypotrophy of masticatory muscles in group II, vertical growth deficiency was prominent on the BTX side, with compensatory overgrowth on the control side. The bilateral muscular hypotrophy in group III also showed smaller ramal height and width than that of control (group I) and control side (group II). Moreover, ramal sagittal angles (posterior tilt) increased on the BTX side of both groups II and III, but coronal angles (lateral tilt) did so on the BTX side of group II, resulting in asymmetry. The results confirmed our hypothesis that functional activity of masticatory closing muscles is closely related to mandibular growth in size and shape of juvenile nonhuman primates. In addition, the focused growth disturbances on the ramal height and posterior-lateral tilt suggested the possible role of masticatory closing muscles for ramal vertical and angular growth vector of the mandible.

Compensatory dentoalveolar supraeruption and occlusal plane cant after botulinum-induced hypotrophy of masticatory closing muscles in juvenile rats

OBJECTIVE

The purpose of this study was to investigate changes in the dentoalveolus and occlusal plane associated with the hypotrophy of unilateral masticatory muscles following botulinum toxin (BTX) treatment in the juvenile period of rats.

DESIGN

We hypothesized that the loss of functional loading of masticatory muscles and occlusal force invites compensatory dentoalveolar supraeruption, accelerating occlusal cant and skeletal asymmetry. In order to confirm this hypothesis, six-week-old male rats (N = 5) were treated with BTX simultaneously at the unilateral masseter, temporalis, and medial pterygoid muscles, with a booster injection after six weeks for the experimental group. The control group (N = 6) had saline injections on both sides at the same sites and on the same schedule.

RESULTS

After 12 weeks, masseter and medial pterygoid muscles on the BTX side showed hypotrophic change. The mandibular structure was asymmetrical, with decreased size and lateral tilting. The maxillary and mandibular molars were supraerupted from the Frankfort plane or mandibular inferior border with lateral tilt. They accompanied downward occlusal plane cant resulting from the supraerupted maxillary and mandibular molars on the BTX side. The dentoalveolar structural changes included diminished alveolar bone density, narrow periodontal ligament space, and disorganized distribution of periodontal collagen fiber.

CONCLUSIONS

Unilateral hypotrophy of masticatory muscles affected the growth, symmetry, and structure of the skeletal jaws and dentoalveolus. Our hypothesis about the dentoalveolar compensation, that muscular hypotrophy was closely integrated with dentoalveolar supraeruption and an inclined occlusal plane, was confirmed.

Effects of masticatory muscle function affected by BTX on maxillofacial bone growth through the sutural modification

OBJECTIVES

To evaluate the effect of botulinum toxin type A (BTX)-induced masticatory muscle hypofunction on the maxillofacial suture bone growth of growing rats.

SETTING AND SAMPLE POPULATION

Department of Orthodontics at Taipei Medical University. Forty-eight male 4-week-old Wistar rats were divided into four groups. The N group received injections of normal saline into each of the masseter and temporalis muscles. The M group received injections of normal saline into each of the temporalis muscle and injections of BTX into each of the masseter muscle. The T group received injections of normal saline into each of the masseter muscle and injections of BTX into each of the temporalis muscle. The MT group received injections of BTX into each of the masseter and temporalis muscles.

MATERIAL & METHODS

Rats were sacrificed after 42 days of growth. Changes in body and muscle weight were measured. Anthropometric measurements of the maxillary arch, sutural bone mineral density and sutural bone deposition distances were recorded. Statistical comparisons were performed using analysis of variance.

RESULTS

No significant change in body weight was found across groups. However, significant decreases were observed in muscle weight, anthropometric measurements, sutural bone mineral density and bone apposition distance in the BTX-injected group.

CONCLUSIONS

Reduced masticatory muscle function in growing rats can affect maxillofacial suture bone growth.

Skeletal unit construction of rat mandible based on the masticatory muscle anatomy and double microcomputed tomography

This study aimed to divide the mandible into skeletal units based on three-dimensional (3D) muscular anatomy with microcomputed tomography (micro-CT) of Sprague-Dawley rat. Five normal rats were micro-CT scanned at 12 weeks of age before and after contrast enhancements for the masticatory muscles. Three-dimensional reconstruction of the mandible was performed from the initial micro-CT images, followed by segmentation of the masticatory muscles using the second enhanced micro-CT data. Bone and muscle models were superimposed based on the teeth and bony structures to evaluate muscular orientation and attachment. The mandible was divided into skeletal units using the bony structures and muscle attachments. The mandibular foramen and mental foramen were adopted as the reference points based on their anatomical and developmental significance. The skeletal units consisted of the condylar, coronoid, angular, body and symphyseal units. Further evaluation of these units in relation to development, growth, and other biology and medicine will be helpful in elucidating their biological identities.

Correction of Malocclusion by Botulinum Neurotoxin Injection into Masticatory Muscles

Botulinum toxin (BTX) is a neurotoxin, and its injection in masticatory muscles induces muscle weakness and paralysis. This paralytic effect of BTX induces growth retardation of the maxillofacial bones, changes in dental eruption and occlusion state, and facial asymmetry. Using masticatory muscle paralysis and its effect via BTX, BTX can be used for the correction of malocclusion after orthognathic surgery and mandible fracture. The paralysis of specific masticatory muscles by BTX injection reduces the tensional force to the mandible and prevents relapse and changes in dental occlusion. BTX injection in the anterior belly of digastric and mylohyoid muscle prevents the open-bite and deep bite of dental occlusion and contributes to mandible stability after orthognathic surgery. The effect of BTX injection in masticatory muscles for maxillofacial bone growth and dental occlusion is reviewed in this article. The clinical application of BTX is also discussed for the correction of dental malocclusion and suppression of post-operative relapse after mandibular surgery.

The imbalance of masticatory muscle activity affects the asymmetric growth of condylar cartilage and subchondral bone in rats

OBJECTIVE

To examine the effects of imbalance of masticatory muscle activity of the rat mandible on the condylar cartilage and subchondral bone during the growth period.

DESIGN

Forty 5-week-old male Wistar rats were randomly divided into experimental (n=20) and control (n=20) groups. In the experimental group, the left masseter muscles were resected. The rats were sacrificed at 7 or 9 weeks of age in both groups. Microcomputed tomography was used to determine the three-dimensional morphology and cancellous bone structure. For histological and histochemical examination, 5-μm-thick serial frontal sections of the condyle were stained with toluidine blue and immunostained with asporin and TGF-β1 to evaluate the promotion and inhibition of chondrogenesis.

RESULTS

In the experimental group, microcomputed tomography analysis showed asymmetric growth; the resected side condyles showed degenerative changes. Histological analysis showed that the total cartilage in the central region of the resected side was significantly thinner than in the non-resected side in the experimental group, as well as in the control group. Compared with the control group, the expression of asporin was significantly higher in the resected side, and significantly lower in the non-resected side. In contrast, the expression of TGF-β1-immunopositive cells in the non-resected side was significantly higher than in the resected side and the control group.

CONCLUSIONS

These findings imply that lateral imbalance of masseter muscle activity lead to inhibition of chondrogenesis and induce asymmetric formation of the condyle during the growth period.

Skeletal effects of the alteration of masseter muscle function

Aim

To investigate the effects of muscle denervation and the introduction of the β2-adrenoceptor agonist, formoterol, on the relationship between muscles and underlying skeletal growth.

Method

Thirty-one (4-week-old) male Sprague-Dawley rats were assigned to four groups: Surgical Sham; Denervated; Denervated +β2-agonist; and β2-agonist only. The Surgical Sham group had the left masseteric nerve exposed but not sectioned. Both of the denervated groups had the left masseteric nerve exposed and sectioned. The groups receiving the β2-agonist had formoterol directly injected into the left masseter muscle every three days for eight weeks. Sixteen angular and linear skeletal measurements were assessed in the overall craniofacial region and the mandible via standardised digital radiography in three views: lateral head, submento-vertex and right and left disarticulated hemi-mandibles.

Results

The findings indicated that, following surgical denervation of the masseter muscle, there were significant changes in the muscle and in the subsequent development of the underlying skeletal structures. The post-surgical changes were largely offset by the administration of a β2-agonist, formoterol, which attenuated muscle atrophy. However, the administration of the β2-agonist only, without surgical denervation, did not lead to changes in skeletal facial form.

Conclusions

Denervation atrophy of the masseter muscle results in statistically significant changes in the development of the underlying skeleton. The changes, however, are localised to areas of muscle attachment. The administration of the β2-agonist, formoterol, despite its effect on muscle anabolism, does not have a significant effect on underlying skeletal growth.

Changes of masseter muscle activity following injection of botulinum toxin type A in adult rats

OBJECTIVES

To investigate changes in masseter muscle function following intramuscular injection of different dose-dependent botulinum toxin type A (BTXA).

SETTING AND SAMPLE POPULATION

Department of Orthodontics at Taipei Medical University. Fifty-two, 70-day-old male Wistar rats were randomly divided into four groups. Group I received 7.5 U of BTXA (0.3 ml), Group II received 5.0 U, and Group III received 2.5 U in the right masseter muscle, respectively. Group IV is the control and received no BTXA injection.

MATERIALS AND METHODS

A wire electrode device was implanted to record muscle activity. One week after implantation, the rats were fed every 2 h and EMG signals were recorded during the first hour. All signals were recorded for 12 weeks. Thereafter, EMG data were analyzed for statistical calculation and weights of masseter muscles were measured.

RESULTS

Masseter muscle activity decreased 99% during the first week after BTXA injection and gradually recovered from the 3rd week on in Groups I-III. By the 12th week, muscle activity recovered to 41% in Groups I and II and 56.26% in Group III. No significant changes of muscle activity were observed in Group IV.

CONCLUSION

BTXA induced a reduction in masseter muscle activity and an increased toxin dose resulted in greater depression of muscle activity.

Cortical bone and ridge thickness of hyperdivergent and hypodivergent adults

INTRODUCTION

The purpose of this study was to assess differences in dentoalvolar cortical bone thickness between hyperdivergent and hypodivergent young adults.

METHODS

Pretreatment cone-beam computed tomography images of 57 patients, including 30 hypodivergent subjects (22 women, 8 men) and 27 hyperdivergent subjects (20 women, 7 men), were analyzed. The data were imported into imaging software (version 10.5; Dolphin Imaging Systems, Chatsworth, Calif); standardized orientations were used to measure buccal and lingual cortical bone thicknesses at 16 interradicular sites of the maxilla and the mandible. Total alveolar ridge thickness and medullary space thickness were measured at the same sites.

RESULTS

T tests showed significant (P <0.05) group differences, with hypodivergent subjects having significantly thicker buccal cortices. The lingual cortex of the maxilla was also significantly thicker in the hypodivergent than in the hyperdivergent subjects. Alveolar ridge thickness was significantly greater at all sites of the hypodivergent mandible and at the anterior 2 sites of the hypodivergent maxilla. Medullary thickness was significantly greater only in the hypodivergent mandibles between the first molars and the second premolars, and between the first and second premolars. Buccal cortical bone was significantly thicker than lingual cortical bone in the mandible; lingual bone was significantly thicker in the maxilla.

CONCLUSIONS

Cortical bone tends to be thicker in hypodivergent than in hyperdivergent subjects. This explains the concomitant differences in alveolar ridge thickness. Medullary space thickness is largely unaffected by facial divergence.

Bone changes in the mandible following botulinum neurotoxin injections

In this study, botulinum neurotoxin type A (BoTx/A) was injected into the temporalis and masseter muscles of growing rats to induce masticatory hypoactivity. Sixty, 30-day-old, male Long-Evans rats were randomly divided into four groups. BoTx/A was bilaterally injected in the masseter muscles in group I, in the temporalis muscles in group II, and into both the masseter and the temporalis muscles in group III. Group IV served as the control in which saline was bilaterally injected into both muscles. Forty-five days after the injections, the rats were sacrificed. Observation of cortical bone thickness from bone biopsies of the right halves of the mandibles, evaluation of the volume of masseter and temporalis muscles with a plethysmometer, and scanning of bone mineral density (BMD) of the skull and mandibular bone structure with dual-energy X-ray absorptiometry were performed. One-way analysis of variance was employed to analyse measurements of muscle volume, BMD, and cortical bone thickness among the groups. The least square difference was then used to determine significance. Reduced cortical bone thickness and BMD of the skull and mandibular bone structure were observed. The volumes of the temporalis and masseter muscles injected with BoTx/A were smaller. Masticatory hypofunction affects bone structure during development.

Using a specially designed cephalostat for longitudinal cephalometric studies in rats

To better understand and observe craniofacial growth, researchers carry out longitudinal cephalometric studies on animals. The authors constructed a cephalostat specially designed to study craniofacial growth in rats. They describe and validate a cephalometric technique using this cephalostat. They carried out cephalometric radiography and analysis of 62 Wistar rats. By using this method, the authors were able to take repeated, reproducible intracranial, intramandibular and craniomandibular measurements of the rats. The proposed method is a useful tool for studying the craniofacial growth process.

Repositioning of the masseter muscle and its effect on skeletal growth

OBJECTIVE

The aim of this study was to establish a model to aid in understanding the influences of bilateral masseter muscle relocation on the bone and muscle, and to determine the influences of bilateral masseter muscle relocation on mandibular growth pattern in rabbits.

STUDY DESIGN

Ten 3-month-old growing white New Zealand rabbits were included. Digital lateral cephalometric radiographs were obtained before operation and 6 months after surgery. The Co-Gn, gonial angle, FMA, ANS-Me, GoGn-SN, Y-axis, and Jarabak values were compared by using Student t test.

RESULTS

There was a statistically significant difference between the groups in the gonial angle (P < .05). Vertical height values (GoGn-SN, FMA, Y-axis, and ANS-Me) showed statistically significant increases in animals in the control group. In contrast, vertical height values in the experimental group did not show statistically significant increase.

CONCLUSIONS

Anterior relocation of the masseteric muscle influenced the direction of vertical growth significantly compared with the control group.

The influence of masticatory hypofunction on developing rat craniofacial structure

The purpose of this study was to use botulinum neurotoxin type A (BoNT/A) selectively to evaluate the influence of localized masticatory atrophy and paresis on craniofacial growth and development. 60 growing rats, 4 weeks old, weighing approximately 120g, were randomly divided according as follows (Long-Evans, N=15 per group): I (Mb+Tns); II (Mns+Tb); III (Mb+Tb); IV (Mns+Tns), where Mb or Tb is the BoNT/A-injected masseter or temporalis muscles (1.0U/muscle, 2.5ml) and Mns or Tns is the saline-injected muscles (2.5ml). After 7 weeks, the mature rats were killed, the muscles dissected and mean muscle mass recorded. Anthropometric cranial, maxillary and mandibular measurements were taken from the dried skulls. Changes in animal weight during the growth period were not statistically significant. The mean masticatory muscle mass was smaller for the BoNT/A-injected muscles of Mb and Tb. Anthropometric measurements of bony structures inserted by masseter and temporalis muscles revealed a significant treatment effect. The measurements showed a facial morphology typical of a dolichofacial profile: short upper face accompanied by a long lower face with an extended mandibular length and ramus height and constricted bicoronoidal and bigonial widths. The results suggest that induction of localized masticatory muscle atrophy with BoNT/A alters craniofacial growth and development.

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.

Functional Changes of Temporomandibular Joint Mechanoreceptors Induced by Reduced Masseter Muscle Activity in Growing Rats

Objective: To determine the influence of masseter muscle activity during growth on the functional characteristics of temporomandibular joint (TMJ) mechanoreceptors.

Materials and Methods: Sixty-six 3-week-old male Wistar rats were divided into an experimental group, in which the masseter muscles were bilaterally resected at 3 weeks of age, and a control group. Single-unit activities of the TMJ mechanoreceptors were evoked by indirect stimulation of passive jaw movement. Electrophysiologic recordings of TMJ units were made at 5, 7, and 9 weeks of age.

Results: During this period, the firing threshold of the TMJ units was significantly lower and the maximum instantaneous frequency of the TMJ units was significantly higher in the experimental group than in the control group.

Conclusion: Reduced masseter activity during the growth period alters the response properties of TMJ mechanoreceptors.

The aponeurotic tension model of craniofacial growth in man

Craniofacial growth is a scientific crossroad for the fundamental mechanisms of musculoskeletal physiology. Better understanding of growth and development will provide new insights into repair, regeneration and adaptation to applied loads. Traditional craniofacial growth concepts are insufficient to explain the dynamics of airway/vocal tract development, cranial rotation, basicranial flexion and the role of the cranial base in expression of facial proportions. A testable hypothesis is needed to explore the physiological pressure propelling midface growth and the role of neural factors in expression of musculoskeletal adaptation after the cessation of anterior cranial base growth. A novel model for craniofacial growth is proposed for: 1. brain growth and craniofacial adaptation up to the age of 20; 2. explaining growth force vectors; 3. defining the role of muscle plasticity as a conduit for craniofacial growth forces; and 4. describing the effect of cranial rotation in the expression of facial form.Growth of the viscerocranium is believed to be influenced by the superficial musculoaponeurotic systems (SMAS) of the head through residual tension in the occipitofrontalis muscle as a result of cephalad brain growth and cranial rotation. The coordinated effects of the regional SMAS develop a craniofacial musculoaponeurotic system (CFMAS), which is believed to affect maxillary and mandibular development.

Effects on craniofacial growth and development of unilateral botulinum neurotoxin injection into the masseter muscle

INTRODUCTION

The effects of botulinum neurotoxin type A (BoNT/A) on masseter muscles, when injected for cosmetic purposes (volumetric reduction) or treatment of excessive muscle activity (bruxism), have been investigated. However, the full anatomic effects of treatment are not known, particularly with respect to the mandible and relevant anthropometric measurements. The intent of this study was to use unilaterial BoNT/A injections to induce localized masseter atrophy and paresis and then to measure the effects of muscle influence on craniofacial growth and development.

METHODS

Growing male Wistar rats, 30 days old, were studied. The experimental group consisted of 8 rats. One side of the masseter muscle was injected with BoNT/A and the other side of the masseter muscle was injected with saline. The side with BoNT/A belonged to 1 group and the side with saline was the sham group. Three rats without injections was the control. After 45 days, the masseter muscles were dissected and weighed. Dry skulls were prepared, and anthropometric measurements determined.

RESULTS

One-way ANOVA showed that the animals maintained their weight in both groups; however, the muscles injected with BoNT/A were smaller than the sham or control muscles. Anthropometric measurements of the bony structures attached to the masseter muscle showed a significant treatment effect.

CONCLUSIONS

After localized masseter muscle atrophy induced by BoNT/A injection, alterations of craniofacial bone growth and development were seen. The results agree with the functional matrix theory that soft tissues regulate bone growth.

Cephalometric changes produced by locally applied anabolic steroid in Wistar rats

BACKGROUND AND OBJECTIVE

There is a strong relationship between neuromuscular activity and facial skeletal morphogenesis. The systemic use of anabolic steroid accelerates craniofacial growth and induces high functional activity of rats' masticatory muscles. The present study examines the effect of an anabolic steroid locally applied in the masseter muscle of growing rats with the purpose of verifying possible craniofacial changes with minimum drug side effects, using cephalometric analysis.

MATERIALS AND METHODS

Forty-three 3-week-old Wistar rats were evaluated and divided into two groups: (1) NAN group (local bilateral masseter injection of nandrolone decanoate - 0.05 microg/3 microl/bi-weekly) and (2) CON group (local bilateral masseter injection of sterile oil - 3 microl). Tukey and Wilcoxon tests were used.

RESULTS

In NAN group the cephalometric measures showed horizontalized facial growth (reduce of FMA angle -p<0.01), increase of posterior facial height, decrease of anterior facial height, reduction of gonial angle (p<0.05), increase of mandibular height (p<0.01). There was no variation in the length of the mandible, in the skull and animals' weight gain, thus demonstrating that no important systemic effect occurred.

CONCLUSION

The local injection of nandrolone in the masseter muscle of rats was capable to alter the growth direction and the morphology of the craniofacial complex in Wistar rats.

Growth effects of botulinum toxin type A injected into masseter muscle on a developing rat mandible

OBJECTIVES

Botulinum toxin type A (BTX-A) reduces the muscular contractions by temporarily inhibiting the release of acetylcholine at the neuromuscular junction. The purpose of this study was to investigate the effects of the BTX-A injected into the masseter muscle of a developing rat mandible.

MATERIALS AND METHODS

Four-week-old male (no. 80) Sprague-Dawley rats were divided into four groups: control group, saline group, BTX-A group and baseline control group. Rats of baseline group were sacrificed at 0 day to provide baseline values of the mandibular measurements. The masseter muscle of rats in the saline and the BTX-A group were administered with saline and BTX-A solutions respectively. Experimental animals were sacrificed after 4 weeks.

RESULTS

The BTX-A group demonstrated smaller mandibular dimension compared with the other groups (P < 0.05). Their condylar cartilages showed increased apoptosis at the proliferation stage of the reserve zone and masseter muscle fibers demonstrated atrophic changes.

CONCLUSIONS

The result demonstrated BTX-A influence on inhibitory action of the developing mandible because of apoptosis at the proliferation stage of the reserve zone of the condylar cartilage in developing rat mandible.

Immunohistochemical identification of TGF-beta1 at the maxillaries in growing Sprague-Dawley rats and after muscle section

Growth factors are currently being extensively studied in the literature to ascertain their role during maxillofacial development. Taking into account that few investigations refer to the functions of growth in the maxillaries, our aim was to identify the TGF-beta1 immunohistochemical expression pattern in the maxillaries of growing rats. A secondary aim was to identify this pattern after orofacial function inhibition by muscle section. In the palate and the mandibular symphysis and body, we found that bone was formed through an endomembranous pathway with intense TGF-beta1 staining inside chondroid cells during the maximum development stages. At the midpalatal suture and the mandibular symphysis and condyle, endochondral ossification was detected with an intense expression of TGF-beta1 inside the chondrocytes when major growth occurred. After the muscle had been sectioned, at the mandible the maturation process was accelerated, this change being transitory until muscular function was recovered. However, at the palate, the intervention caused a greater disturbance of the growing pattern, which did not recover normality.

Effects of masticatory muscle force on temporomandibular joint disc growth in rats

Biomechanical factors in masticatory function are related to the development of the mandible and the condyle. Previous studies have reported that reduced masticatory muscle force influences mandibular bone formation and chondroblastic proliferation. We examined the importance of compressive force in temporomandibular joint (TMJ) disc growth by reducing the masticatory load. Thirty 3-week-old male Wistar rats were used in this study. In the experimental group, the masseter muscles were bilaterally resected to evaluate the influence of masticatory muscle force on morphology and composition of TMJ disc during growth. No surgery was performed in the control group. The thickness of the TMJ disc was measured to evaluate the morphological changes. The localization of type I collagen and decorin was performed by immunostaining to examine compositional change. Cell proliferation in the disc was identified by insulin-like growth factor-1 receptor (IGF-1r) immunostaining. The disc thickness at each region in the experimental group was reduced compared to the control group. While in the control group, type I collagen and decorin were identified throughout the disc, it was concentrated on the superior portion of the disc at the anterior and posterior bands in the experimental group. The expression of IGF-1r immunopositive cells in the experimental group was also significantly lower than in the control. We conclude that masticatory muscle force is closely related to TMJ disc morphology and composition during growth.

The influence of masseter activity on rat mandibular growth

Many studies have shown that mandibular and condylar growth is affected by compressive forces on mandibular bone and the condyle. It has been reported that chondroblastic differentiation and proliferation in chondrocytes play important roles in condylar growth. However, the influence of reduced compressive force on chondroblastic proliferation and mandibular bone formation is not fully understood. Thirty-six 3-week-old male Wistar rats were used in this study. In the experimental group, the masseter muscles were bilaterally resected to evaluate the influence of masticatory force on mandibular and condylar bone morphology. Six weeks after the operation, while the rats were in the pubertal growth stage, lateral X-rays were taken to analyze the skeletal pattern of the mandible. The form of the condyle and the thickness of the chondroblastic layers were evaluated by toluidine blue staining. Chondroblastic proliferation was identified by insulin-like growth factor-1 receptor (IGF-1r) immunostaining and bone resorption of the condyle was assessed by measuring tartrate-resistant acid phosphatase (TRAP) activity. Lateral X-rays of the mandible showed that rats in the experimental group tended to have large mandibular plane angles. The chondroblastic layer in the condyles of the experimental group rats was thinner than in the control group. The expression of IGF-1r immunopositive cells in the experimental group was significantly lower than in the control chondrocytes, and the number of TRAP-positive cells was significantly higher in the condylar bone of the experimental group. We conclude that masseter muscle activity is closely related to mandibular morphology during growth.

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.

Regeneration of the mandibular condyle after unilateral condylectomy and myotomy of the masseter in lambs

We investigated the degree of regeneration of the mandibular condyle after unilateral condylectomy and myotomy of the masseter in growing lambs. Four 10-week-old lambs had unilateral condylectomy and myotomy of the superficial layer of the masseter on the right side, and were killed 3 months later. The joints were examined radiologically and histologically. All joints showed poor regeneration of the condylar head. In the medial plane there was partial condylar and articular cartilage reformation, but in the lateral plane there was neither condylar nor cartilaginous reformation. The articular cartilage of the temporal bone was thicker in the medial plane and the disc was thicker in the central plane, than in unoperated joints. We conclude that unilateral condylectomy and myotomy of the masseter in the growing period results in poor regeneration of the condyle.

Masticatory muscle function in patients with spinal muscular atrophy

The purpose of this study was to determine whether spinal muscular atrophy affects masticatory muscle strength and mandibular range of motion. A sample of 15 subjects with spinal muscular atrophy was compared to a sample of age-matched and sex-matched controls. Maximum bite force, masticatory muscle electromyography activity, mandibular ranges of motion and masticatory muscle endurance were evaluated. Results showed that maximum bite forces were one-half as great for the sample with spinal muscular atrophy than for the controls, even though their EMG activity was not significantly different. Slopes of the relationship between electromyography activity and bite force were two to four times steeper for patients with spinal muscular atrophy than controls. Maximum opening and protrusion were reduced to approximately one-half control values. Fatigue times of patients with spinal muscular atrophy were reduced by 30% (17.9 seconds versus 11.1 seconds). We conclude that the masticatory muscles of patients with spinal muscular atrophy are weakened, that their muscles are less efficient, and that they fatigue more quickly than controls. In addition, mandibular movements of these patients take place over a more limited range than unaffected controls.

Inhibition of masseteric electromyographic activity during oral respiration

Although the effects of oral respiration on the growth and development of craniofacial structure have been studied previously, little is known about how altered respiration affects the activity of the jaw-closing muscles. Obstruction of the nasal airway in the cat significantly inhibited the masseteric stretch reflex and discharges of masseteric motor units but did not affect the electromyographic activity of the diaphragm. This inhibition was greater during inspiration than during expiration. In addition, the amplitude of the masseteric monosynaptic reflex elicited by electrical stimulation of the mesencephalic trigeminal nucleus showed no significant change in association with the altered respiratory mode. These findings suggest that masseteric electromyographic activity is inhibited during oral respiration and that the gamma-system is involved in this inhibition.