Myosin Heavy-Chain Messenger Ribonucleic Acid (mRNA) Expression and Fibre Cross-Sectional Area in Masseter, Digastric, Gastrocnemius and Soleus Muscles of Young and Adult Rats.

Different demands on the muscles of mastication may influence their functional profile (size and distribution of muscle fibre types), which may change during growth and maturation, potentially influencing craniofacial growth. The aim of this study was to evaluate mRNA expression and cross-sectional area of masticatory muscle fibres compared with limb muscles in young and adult rats. Twenty-four rats were sacrificed at two different ages, namely 12 at 4 weeks (young) and 12 at 26 weeks (adult). The masseter, digastric, gastrocnemius and soleus muscles were dissected. Gene expression of myosin heavy-chain isoforms Myh7 (MyHC-I), Myh2 (MyHC-IIa), Myh4 (MyHC-IIb) and Myh1 (MyHC-IIx) in the muscles was measured using qRT-PCR RNA analysis, and immunofluorescence staining was performed to measure the cross-sectional area of different muscle fibre types. Different muscle types and ages were compared. Significant differences were found in the functional profile between masticatory and limb muscles. For the masticatory muscles, there was an increase in Myh4 expression with age, and this change was more intense for the masseter muscles, which also presented an increase in Myh1 expression, similarly to limb muscles. The fibre cross-sectional area of the masticatory muscles was generally smaller in young rats; however, this difference was less pronounced than in limb muscles.

Longitudinal changes of the insertion of the maxillary labial frenum in children and adolescents undergoing orthodontic treatment.

INTRODUCTION: This study aimed to evaluate potential vertical changes in the position of the maxillary labial frenum (MLF) insertion in growing children and to compare these changes to the vertical growth of the dentoalveolar process and lower facial third. METHODS: This retrospective longitudinal study investigated records of 33 healthy children. Dental casts, lateral cephalograms, and photographs were evaluated at pretreatment (T0), posttreatment (T1), and 3-5 years into retention (T2). To evaluate the vertical changes of MLF insertion in relation to the vertical growth of the dentoalveolar process, the palatal plane (PP) was used as a reference. These changes were also compared between different MLF typologies (ascribed as thin or fibrous). RESULTS: The distance from MLF to PP only slightly increased from T0 to T2 by 0.6 ± 0.5 mm (P <0.001), whereas the distance between the incisal edge and PP increased significantly from T0 to T2 by 2.6 ± 0.8 mm (P <0.001). A positive correlation was found (r = 0.94; P <0.001) between the changes from the incisal edge to the PP and the MLF to the incisal edge between T0 and T2. No correlation was found between the change from the incisal edge to the PP and MLF to PP between T0 and T2. Thin MLF types showed a larger increase in distance from their insertion to the incisal edge (2.6 ± 0.8 mm) than thick MLF types (1.8 ± 0.7; P <0.03). CONCLUSIONS: The MLF remains stable compared with the PP, whereas the maxillary incisal edge moves away from the PP, indicating increased vertical growth of the alveolar process. Dentists should be aware of those changes before performing interventions such as unnecessary frenectomies.

Mandibular condyle changes in rats with unilateral masticatory function.

OBJECTIVE: Certain malocclusions or unilateral tooth loss can lead to asymmetric functional load of the two mandibular sides during mastication and induce skeletal asymmetries to the condylar process of growing individuals. However, in adults, asymmetric function may have a different impact. The aim of the present study was to investigate three-dimensionally the effects of unilateral masticatory function on the condylar process morphology in growing and adult rats and the adaptive processes to differential condylar loading. MATERIALS AND METHODS: Fifty-six growing and adult Wistar rats aged 4 and 26 weeks respectively were obtained. The maxillary right molars of the experimental animals were extracted and all animals were followed for 12 weeks. Three-dimensional images were obtained by an in-vivo microcomputed tomography (micro-CT) examination. The following measurements were studied: condylar process height, condylar base width, and condylar cross-sectional surface. RESULTS: While no differences were found with regards to condylar process height and base width, the cross-section of the condyle on the extraction side did not increase during growth in the young rats. No such differences were found in adults. Young rats had statistically significantly shorter condylar height, base width and cross-sectional surface than the adult rats and showed significant growth of these structures during the experimental period. CONCLUSION: Condylar height and base width growth are not hindered by reduced occlusal function, contrary to the average cross-sectional surface, which implies that the condyle form of growing individuals becomes thinner while maintaining its length, in the absence of occlusal stimuli. The condyle of adult rats with extractions is less affected by occlusion changes.

Mandibular antegonial notch depth in postpubertal individuals: A longitudinal cohort study.

OBJECTIVES: To perform an epidemiological analysis of the antegonial notch depth in postpubertal individuals and to analyze the development of deep antegonial notches longitudinally in growing individuals. MATERIAL AND METHODS: Lateral cephalograms of 302 untreated 17/18-year-old subjects (171 males; 131 females), from the craniofacial growth legacy collection, were analysed to measure antegonial notch depth along the mandibular plane. Sex and sagittal malocclusion were investigated as possible factors influencing notch depth. In subjects with deep antegonial notches (>1.5 standard deviation) at the age of 17/18 years, earlier lateral cephalograms at 7/8 and 13/14 years were obtained, and the magnitude of notch depth analyzed longitudinally. Linear regression analyses were used to assess correlations between antegonial notch depth and other recorded variables. RESULTS: Antegonial notch depth ranged from 0 to 5.3 mm (mean 2.0 ± 1.0 mm). Antegonial notches were significantly deeper in males (2.3 ± 1.1 mm) than females (1.5 ± 0.7 mm) (p < .001). Notch depth was on average 0.3 mm deeper in Class I than in Class II or III individuals (p = .019). Twenty-one subjects (all male) were judged to have deep antegonial notches at the age of 17/18. In these subjects, notch depth deepened from 13/14 to 17/18 years (p < .001), whereas no change was observed between 7/8 and 13/14 years. CONCLUSIONS: Antegonial notch depth shows important variation in postpubertal individuals, with males having deeper notches than females on average. In those with deep antegonial notches (all males in the present sample), notch depth increases not during prepubertal growth but during the pubertal growth spurt.

Monitoring Overeruption Pattern of Young vs Adult Unopposed Molars in the Rat.

PURPOSE: To investigate the overeruption of unopposed molars and the adaptation of alveolar bone height in young and adult rats. MATERIALS AND METHODS: A total of 56 4-week-old (young) and 26-week-old (adult) male Wistar rats were followed up longitudinally. In each age group (n = 28), 16 rats were included in the experimental subgroup, in which all the maxillary right molars were extracted, and 12 rats were included in the control subgroup. All rats were scanned at regular intervals with in vivo microcomputed tomography for a 12-week period, and the eruption rate of the mandibular first molars and the surrounding bone were measured, with the reference point at the mandibular canal. The molar categories were unopposed right molars and overloaded left molars in the experimental group and control molars in the control group. RESULTS: The young unopposed molars had the highest mean eruption rate (172 ± 67 µm/day). The overeruption was most marked during the first 3 weeks postextraction, thereafter gradually returning to a level comparable to controls (12.9 ± 6.8 µm/day). The overloaded adult molars did not erupt during the experimental period. Alveolar bone height also increased in young unopposed molars, but at a slower pace than overeruption, causing an increase in clinical crown height. CONCLUSION: Unopposed young molars overerupt at a higher pace than adult molars during the first weeks postextraction. The alveolar bone grows in response to the tooth eruption but to a lesser extent, which leads to a constant increase in clinical crown height at the same rate in young and adult rats. The increase of clinical crown height was 4.5-fold higher in young unopposed molars compared to control molars.

A longitudinal study on timing and velocity of rat molar eruption: Timing of rat molar eruption.

Rat molar eruption and occlusion data were compiled from several studies but several inconsistencies were found, rendering the planning of eruptional studies difficult and imprecise. Our aim was to measure eruption and occlusion days, as well as eruption velocity, in the upper and lower three molars from infancy to end of adolescence in the rat. A total of 19 male and female Wistar rats were scanned daily by micro-computed tomography (CT) from day 15 to 70. We measured the eruption of all maxillary and mandibular molars with reference points at the hard palate and mandibular canal at three stages: pre-emergent, pre-occlusal, and functional. Statistical analysis was performed with a mixed-model analysis of variance (ANOVA) and a Sidak post hoc test. The first molar erupts on average on day 17, the second molar on day 20, and the third molar on day 33. The eruption velocity of the first molar was the highest at 90.9 microns/day (standard error (se) = 12.80), followed by the second molar at 65.9 microns/day (se = 5.80), and the lowest was the third at 47.0 microns/day (se = 3.28), ( p < 0.001). On average, the pre-occlusal phase had the highest velocity at 97.2 microns/day (se = 1.72), the pre-emergent was lower at 84.9 (se = 2.29), and the functional was the lowest at 21.7 (se = 0.45), ( p < 0.001). The eruption rate decreased from the first to third molar and was also different between phases: the pre-occlusal phase had the highest rate, closely followed by the pre-emergent phase while the functional eruption rate was significantly lower than the other phases.

Effect of different masticatory functional demands on the 3D mandibular condyle morphology of growing rats using posterior bite-blocks.

Background and objectives: Apart from the primary effect of bite-blocks on tooth position, they may also influence the mandibular condylar growth. Our aim was to investigate their influence on the condylar morphology, with variable masticatory forces. Material and methods: Fifty-two 4-week old male rats were divided into hard and soft diet groups in order to create individuals with different masticatory muscle capacity. Two weeks later, they were equally divided into bite-block and control groups. After a total of 6 weeks experimental time, the animals were sacrificed. The mandibles were scanned with high-resolution micro-CT and 3D analysis was performed on the condylar neck and head of the condyle. The volume and the length of the condylar process were measured. Statistical analysis was done with a one-way analysis of variance. Results: The use of bite-blocks decreased the length of the condylar process (P = 0.001) as well as the volume of the condylar neck (P = 0.001) and head (P = 0.006). The soft diet decreased the volume of the condylar neck (P < 0.001) and head (P < 0.001) two to three times more than the bite-blocks but did not affect the condylar process length. The interaction between the two variables was not statistically significant. Conclusions: Both the bite-block appliance and weak masticatory muscle function reduced the volume at all regions of the condylar process, although the functional factor had a substantially greater effect. However, only the bite-block appliance affected the condylar process length. In the presence of both factors, an additive effect was found but no interaction detected.

Influence of masticatory hypofunction on the alveolar bone and the molar periodontal ligament space in the rat maxilla.

Previous studies have established that complete absence of masticatory function results in a narrower alveolar process and periodontal ligament (PDL). The aim of our study was to investigate, for the first time, both the alveolar process and the PDL in masticatory hypofunction. Twenty-six rats, 3 wk of age, were randomly assigned to either a hard- or a soft-diet group (n = 13 each group). The rats were killed after 6 wk and their skulls were scanned using micro-computed tomography (micro-CT). We measured the cross-sectional width of the space occupied by the PDL, as well as the cross-sectional alveolar socket surface (AS) and the cross-sectional root surface (RS). We also measured the width of the alveolar process. The alveolar process was narrower, the PDL width was thinner, and the AS was smaller in rats fed a soft diet compared with rats fed a hard diet. The PDL width was correlated to the alveolar process width and the AS. The narrower alveolar process found in rats fed a soft diet is the result of alterations to both the alveolar bone and the PDL. The correlation between them provides evidence that a reduction of occlusal loading induces a simultaneous response in both tissues.