Magnetic resonance imaging of the masseter muscle: a preliminary genetic study in monozygotic and dizygotic twins

Magnetic resonance images of the head were taken in five monozygotic and seven dizygotic twins in order to calculate bilaterally masseter muscle cross-sectional areas and total volume. Comparing correlation coefficients between cotwins, genetic influences could be expected for maximal cross-sectional area but not for volume measurements. Model fitting revealed that additive genetic factors explained 93.9% of the variance for the left and 82.4% for the right maximal masseter muscle cross-sections. It is anticipated that while the number of masseter muscle fibers is under strong genetic control, the length of individual fibers can be influenced by specific environmental factors. In the second part of the investigation, cephalometric measurements from lateral headplates were compared with these masseter muscle values in 10 twin pairs. Only three of the 15 angular and five (three vertical, one transversal, and one sagittal measurement) of the 20 linear measurements were significantly correlated with masseter muscle values.

[Relationship between endocrinology and craniofacial growth. I: Puberty and craniofacial growth. II: Growth of the craniofacial skeleton]

In this literature, a review is given of the endocrinology and morphology of the craniofacial complex. This article reviews in a first part the endocrinology of puberty and general growth aspects. Afterwards the adolescence growth spurt of the face and the hormonal regulation will be focused. In a second part the morphogenetic aspects together with growth area's and growth theories of the craniofacial complex will be discussed. At last a detailed description of the maxillary and mandibular growth is given.

The prevalence of cardinal TMJ dysfunction symptoms and its relationship to occlusal factors in Japanese female adolescents

Cross-sectional data were obtained from 1182 Japanese high school girls of 12 and 15 years old. The data were analysed for the prevalence of specific occlusal features and the prevalence of cardinal Temporomandibular Joint Dysfunction (TMJD) symptoms in different occlusion groups. The associations between occlusal features and TMJD symptoms were analysed by the Z-analysis. The results were as follows: the prevalence of TMJD symptoms (total) was about 23% in both age groups examined. Noise had the highest prevalence of the specific TMJD symptoms in both age groups (16% and 11%). With age and for different occlusion groups, there was a significant increase in deviation on opening (P < 0.001) and a decrease in pain and noise (P < 0.05). No significant associations were found between the occurrence of TMJD symptoms (total) and occlusal features in all the examined occlusion groups. With respect to specific TMJD symptoms, however, the investigation of the different groups revealed a significant association (P < 0.05) of intra-arch occlusal (crowding) features with the occurrence of deviation on opening and with the occurrence of the combined TMJD symptoms which include deviation on opening. This indicates that crowding may be an important sign for predisposing TMJD. For the other single disorder symptoms (except pain), we did not find a significant relationship (P < 0.05) with aspects of occlusion, which may confirm a multifactorial TMJD problem wherein occlusal features can have a contribution to TMJD.

[Preventive orthodontics]

In this article orthodontic preventive and interceptive measures are discussed. The number of environmental factors that can be influenced to prevent malocclusions is rather small.

A radiographic study of posterior apical root resorption in orthodontic patients

The purpose of this study was to determine the extent of apical root resorption, after fixed appliance therapy, in the posterior part of the dentition, and to evaluate if predisposing factors can be pointed out retrospectively. Tooth length in the posterior part of the dentition was measured on pretreatment and posttreatment orthopantomograms of 153 patients who were treated with standard edgewise appliances. Pretreatment and posttreatment tooth lengths were determined, and an attempt was made to explain the differences by sex, age, stage of root formation at onset of orthodontic treatment, extraction versus nonextraction therapy and treatment duration. Patients were divided into two groups according to their root formation. Group A consisted of patients with incompleted root formation at onset of orthodontic treatment except for the first molars. Group B consisted of patients where root formation was completed with the exception of second and third molars. Posttreatment tooth lengths of groups A and B were compared with pretreatment tooth lengths of group B with a paired t test, and correlations with the variables were calculated with multiple regression analysis only in group B. Posttreatment group A showed no significant differences with the mean root lengths of pretreatment group B. Differences between the mean pretreatment and posttreatment root lengths in group B were significant. Posterior teeth showed root shortening during active orthodontic treatment independent of sex, age, extraction versus nonextraction therapy, and duration of active treatment. Teeth with incompleted root formation at onset of orthodontic treatment showed root lengthening during active treatment, yet did not reach their "normal" tooth length.

[Orthopedic aspects of orthodontic treatment of patients with labio-palatine clefts]

Cleft lip and palate patients often show a distorted facial growth which can be orthodontically treated by means of "growth guidance". The treatment can be applied during three different phases of the dental development. First phase is immediately after birth where the growth of the alveolar segments are aligned together in order to facilitate the closure of lip and palate. Subsequently, a protraction of the upper jaw is carried out during the first transitional stage; As a result of this, the permanent incisors can erupt in a good sagittal relation. Finally, a transversal expansion of the upper jaw can be applied which is mostly done before alveolar bone grafting. The timing of the grafting procedure depends on the eruption of the upper permanent canine at the cleft side.

The interaction between soft tissues and the sagittal development of the dentition and the face

The purpose of this study was to investigate whether there are relationships between the cross-sectional surface area (CSSA) of the lips and the sagittal structure of the skeletal parts of the face and the dentition in the midsagittal plane and whether the sagittal structure of the face and the dentition is related to the class of malocclusion. The CSSA of the upper and lower lip was measured with a planimeter on the lateral cephalograms of 156 patients before their orthodontic treatment with a closed lip relation in rest. They were divided into 18 groups according to their age, sex, and Angle classification. The Quick Ceph program (Orthodontic Processing, Chula Vista, Calif.) was used to digitize and analyze 23 dental and skeletal parameters. The parameters were tested by an analysis variance to show differences between the three classes of malocclusion. The regression analysis was used to look for relations between the CSSA and the skeletal or dental parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of twins in dentofacial genetic research

A literature review is given on the different results obtained with twin and family studies in relationship to the development and structure of the dentofacial complex. Recent advances in twin and family studies are mentioned. Attention is focused on functional components considered to be of primary importance in craniofacial growth.

[Orthodontic treatment for orthognathic surgery: indications, possibilities and limitations]

The relation between the maxilla and the mandible can be so discordant that it cannot be corrected with orthodontic treatment only in adults, neither with orthodontic and jaw orthopedic in children. In these cases, only a combination of orthodontic treatment and orthognathic surgery can give the most satisfying results. A careful analysis of all patient records and an accurate planning of all the consecutive procedures should precede this treatment. Nowadays, computer-aided systems can provide substantial help in analysis and planning. In this article, possibilities and limitations of the orthodontic part of these treatments are elaborated.

The validity of the masseteric post-stimulus electromyographic complex: a twin study

The twin model was used to assess the validity of an electromyographically recorded, masseter muscle reflex by measuring the sensitivity and specificity. Results were satisfying, implying that in future studies this reflex could be used to calculate heritability estimates between monozygotic and dizygotic twins.

Interactive periodontal and acoustic influences on the masseteric post-stimulus electromyographic complex in man

Post-stimulus electromyogram (EMG) complexes (PSECs) were studied in the full-wave rectified and averaged EMGs of the masseter muscles in 15 subjects, who clenched at a controlled level. The PSECs, a series of downward- and upward-going waves reflecting inhibitory and excitatory influences upon the masseteric motoneurones, were elicited by mechanical stimulation of a tooth. The stimuli selectively activated mechanoreceptors in the periodontium and, by bone-conduction, acoustic receptors. Application of acoustic masking during the periods of stimulation revealed a series of inhibitory and excitatory acoustic influences in the PSEC, which were absent after local electrical stimulation of receptors in the periodontium or their afferents. By applying local anaesthesia to the periodontium of a mechanically stimulated tooth, the durations of the acoustic influences were on the average reduced by 76%. In subjects whose PSECs consistently included a second inhibitory period, the duration of the acoustic influences with respect to that of the PSEC (30%) was larger than otherwise (13%), suggesting a central gating of periodontal pathways which can block both periodontal and acoustic influences. The acoustic influences, of which the appearance in the PSEC largely depends upon activated periodontal pathways, represent a new finding of audio-motor reflexes.

Concepts on functional appliances' mode of action

Several working hypotheses have been conceived to explain the mode of action of various functional appliances. A review of the different concepts is presented and these concepts are tested by means of data from basic scientific research. Furthermore, a general concept is postulated that could unite the working principles of several functional appliances. Finally, the clinical relevance of scientific research in this area is evaluated.

Changes in neuromuscular reflexes in the masseter muscles during functional jaw orthopedic treatment in children

Masseteric reflexes following standardized taps on an upper incisor were recorded electromyographically (1) in children with Class II, Division 1 malocclusions at different times before and during the treatment with the Bionator functional appliance and (2) during an observation period in control subjects who were not being treated. In each experimental session, 72 taps were delivered during clenching at 10% of the maximal electromyographic output. Statistical criteria provided by a previously developed computer program allowed the objective determination of inhibitory and excitatory periods in the electromyographic output following the stimulus, which together constitute the poststimulus electromyographic complex (PSEC). The influence of the change in occlusal relations on the PSEC was determined in ten subjects by comparing pretreatment results with those obtained after 1, 3, and 6 months of treatment. Concurrent with an improvement of the sagittal jaw relation in the patients with a pronounced disharmony, a short latency excitatory peak appeared in their PSECs. It was demonstrated by averaging nonrectified electromyographic activity that this peak was brought about through synchronization of motor units. In the control group, however, the morphology of the PSEC was reproducible over time. Successful functional therapy is thus accompanied by specific transient changes in the reflex response of the masseter muscles to standardized tooth taps. The neural pathways contributing to these observations originate in periodontal and muscular receptors, and might provide a clue for the mechanism underlying successful functional treatment.

Post-stimulus EMG complexes in the masseter muscle following gingival electrical stimulation in man

Post-stimulus EMG complexes (PSECs) were elicited by means of electrical stimulation of the gingiva in six subjects during clenching in three biting positions, namely the maximal occlusion, the incisor edge-to-edge and the hyperpropulsive biting position. No interrelationship was found between the mechanically and electrically elicited PSECs. With electrical stimulation the PSEC morphology was not related to the biting position in contrast with the PSECs elicited with mechanical stimulation. Following gingival electrical stimulation, a P wave was not present in the PSEC.

The influence of jaw position and antagonistic tooth relations on the appearance of a short-latency excitatory reflex in the human masseter muscles following mechanical tooth stimulation

Post-stimulus electromyographic complexes (PSEC) were elicited by standardized tapping of a central upper incisor in 12 subjects during isometric voluntary biting in two positions, the incisor edge-to-edge (IEE) or the maximal occlusion (MO). In IEE a short latency excitatory peak (P wave) was present in the PSEC; this was absent in MO. By superimposing the rough EMG sweeps without full-wave rectification, it appeared that this P wave was brought about by synchronous masseter motor-unit firing.

Interobserver and intraobserver agreement of clinical orthodontic judgments based on intraoral and extraoral photographs

The purpose of this study was to calculate the agreement between and within observers for orthodontic judgments based on intraoral and extraoral photographs at two separate occasions in ten twin pairs. Eighteen variables were scored by two orthodontic students according to well-defined rating scales. Interobserver and intraobserver proportion of agreement as well as the agreement which could be expected only by chance and the remaining agreement beyond chance were calculated. The calculated agreement beyond chance was not significant (alpha = 0.05%) for middle and upper facial height, anterior apical area in lower jaw and posterior apical area in both jaws within the first observer and for upper facial height within the second observer. Interobserver reliability was not acceptable at the 5% level for judging asymmetry, facial animation, posterior apical area in the upper and lower jaws, sagittal lip position and the upper facial height. Lower facial height, sagittal lip position and middle apical area in the lower jaw agreed significantly at this level for only one interobserver comparison.