The effect of retraining hypofunctional jaw muscles on the transverse skull dimensions of adult rats.

OBJECTIVE: To experimentally investigate the effects of increased masticatory muscle function on the transverse cranial dimensions on adult rats with an earlier reduced masticatory muscle function. MATERIAL AND METHOD: Sixty young male rats were used. The experimental group received soft diet for a prolonged period, so that the animals developed weak masticatory muscles. A control group received ordinary hard food during the whole experimental period (27 weeks). After 21 weeks when the animals had nearly ceased their body growth the rats in the experimental group were divided into two groups. One group continued with soft diet until the end of the experiment (hypofunctional group). The other group received ordinary hard food to get the possibility to retrain their masticatory muscles (rehabilitation group). At week 21 and at the end of the experimental period (week 27), axial cephalograms were taken. Fourteen landmarks were defined to measure seven transverse distances of the skull. RESULTS: The increase of the anterior zygomatic arch width and interzygomatic width during the experimental period were larger in the rehabilitation group compared to both the normal and the hypofunctional group. CONCLUSION: Retraining of masticatory muscles in adult rats leads to increase of some transverse cranial dimensions.

Molecular changes in detrained & retrained adult jaw muscle.

A hypofunctional masticatory system was developed in 21-day-old male rats by feeding them a soft diet for 27 weeks. Retraining of a parallel group for 6 weeks was achieved by switching back to a hard diet after 21 weeks. A control group was fed a hard diet for 27 weeks. At the end of the experimental period, the expression levels of the myosin heavy chain isoform genes MYH 1 and 2 (fast), 3 (embryonic) and 7 (slow) in the deep masseter were compared using qRT-PCR analysis. The gene expressions of MYH 3 and MYH 7 were significantly higher in the rehabilitation group compared with the normal and hypofunctional group, but no significant differences were found in regards to the gene expression of MYH 1 and 2. Retraining made it possible for the slow (MYH 7) isoform levels to adapt to the increased mechanical load. The increased level of embryonic (MYH 3) isoform could be due to the need of creation of new MYH isoforms.

Rehabilitation of masticatory function improves the alveolar bone architecture of the mandible in adult rats.

Masticatory functional changes have been shown to influence the quantity and quality of the alveolar bone during growth. This study was designed to investigate the effect of masticatory function rehabilitation on the morphology and the trabecular architecture of the mandibular alveolar bone after cessation of growth. Forty-four Sprague-Dawley male rats received soft diet in order to develop masticatory muscle hypofunction. After 21 weeks, after cessation of growth, the animals were divided into two groups: the first group continued receiving soft diet for six more weeks (hypofunction group), while the second group changed to ordinary (hard) diet with the aim to restore a normal masticatory function (rehabilitation group). A third group of 16 male rats (normal group) received ordinary (hard) diet during the whole experimental period and served as control. Micro-tomographic histomorphometry was used to evaluate the architecture of the mandibular alveolar bone (e.g. bone volume fraction, trabecular thickness, trabecular separation, etc.) at the end of the experiment (27 weeks). The height and width of the alveolar process were measured as well. The alveolar process trabecular bone volume fraction (BV/TV) was lower for the animals of the hypofunctional group as compared to those of the normal (p<0.01) and the rehabilitation (p<0.05) groups. Despite the significant improvement observed in the rehabilitation group, their BV/TV was lower in comparison to the normal group (p<0.05) at the end of this experiment. All the other micro-tomographic parameters followed the same pattern of change between groups; values of the rehabilitation group were between the values of the two other groups, differing significantly from both of them. The alveolar process was significantly shorter in the normal group in comparison to both the hypofunctional and rehabilitation groups (p<0.05). On the other hand, both the normal and rehabilitation groups were had a wider alveolar process than the hypofunctional group (p<0.05). Both alveolar height and width were significantly correlated with all micro-tomographic parameters under study. The rehabilitation of masticatory function led to a significant improvement of alveolar bone architecture in adult rats, although the negative effects of hypofunction were not totally reversed during the period under study.

Management of root resorption in a large orthodontic clinic.

The aim of this study was to investigate, describe and analyse how the problem of root resorption is handled in a large orthodontic clinic and if these approaches were in accordance with the recommendations and guidelines described in the literature, and to estimate the prevalence of root resorption found in the radiographic material. All records of patients who terminated active treatment with fixed or removable appliances during the year 2004 at the Department of Orthodontics, University Clinics of Odontology, Göteborg, Sweden were examined (902 patients). In 837 records adequate information was obtained. The factors studied included: the presence of journal recordings of predisposing factors, the presence of radiographic examinations before, during and after treatment, the frequency of radiographic examinations and the prevalence of moderate or severe root resorption reported. The most frequent registrations were for trauma, and nail biting. Before treatment 81.5% of the patients were examined with periapical radiographs. After 6 months of treatment the percentage was 54.5%, while 15.8% of the patients were examined twelve months after treatment. At the end of treatment 52.5% of the patients were examined. When moderate root resorption was diagnosed during treatment the use of lower forces, resting periods and decrease of the treatment duration were the most common preventive measures. The prevalence of light root resorption was 1.9%, 3.2%, 4.9% and 8.6% at the beginning, after 6 and 12 months and at the end of treatment, respectively. Severe root resorption was found in 1.9% at the end of treatment. In conclusion before treatment periapical radiographs were taken in most cases. The percentage of the radiographs dropped significantly at 6 and 12 months. At the end of treatment half of the patients were examined with periapical radiographs.When moderate root resorption was diagnosed the use of lower forces, resting periods and decrease of treatment time were common preventive measures. Light root resorption was found in less than 10% while severe root resorption was noted in 2% after active treatment.

Do masticatory functional changes influence the mandibular morphology in adult rats.

OBJECTIVE: This study aimed to investigate the effect of masticatory functional changes on the lateral view morphology of the mandible in adult rats. DESIGN: Sixty 21-day-old male Sprague-Dawley rats were divided into two groups. Sixteen received ordinary (hard) food during the whole experimental period (normal group). The remaining 44 received a soft diet during 21 weeks to develop a hypofunctional masticatory system. Then, the now adult animals were divided into two equal groups: one continued on a soft diet (hypofunctional group), and the other changed to an ordinary diet with the aim of functionally rehabilitating their masticatory system (rehabilitation group). After another 6 weeks all animals were sacrificed and their left mandible was dissected, photographed and customised cephalometric software was used to perform morphometric measurements. RESULTS: The area of the mandible was smaller in the hypofunctional compared to the normal group. Interestingly, the alveolar process was shorter in the normal group. Morphometric analysis revealed significant differences such as the area of the angular process and the inclination of the condylar process. The rehabilitation group was only marginally different compared to the hypofunctional group, although a general tendency to approach (catch-up) the normal group was observed, and one morphometric variable (condylar base inclination) was indeed significantly different. CONCLUSIONS: Morphometric analysis revealed only marginal changes of the adult rat mandibular morphology during a 6-week period of masticatory function rehabilitation. However, the observed catch-up tendency might suggest that a longer rehabilitation period may have significant effect on mandibular morphology.