Cortical bone response adjacent to applied light orthodontic forces in ovariectomized rats.

Cortical bone response to experimental orthodontic forces has not been adequately described in the available literature. The aim of this study was to investigate the application of light orthodontic forces on the cortical bone adjacent to the point of their application, in normal and ovariectomized rats. At the age of eight months, twenty-four female rats were divided equally into two groups: normal (group A) and ovariectomized (group B). A 20 gr* orthodontic force was applied to the maxillary right first molars in all animals for 14 days. Histological examination of the maxillary cortical bone, at the level of the first molar of the upper jaw, was performed on both sides of all animals. It was found that light orthodontic forces applied to the upper right first molars of normal rats result in thinner cortical bone with increased osteoblastic activity, normal lamellar orientation and normal distribution of osteocytes compared to the non-affected contralateral side. In ovariectomized rats it was found that light orthodontic forces applied to the upper right first molars result in thickening of the cortex, in comparison to the respective area of the opposite side where no orthodontic forces were applied. It seems that experimental light orthodontic forces, in rats, affect cortical bone remodeling differently, according to their hormonal status, in areas adjacent to the applied forces.

Effects of experimental unilateral condylectomy followed by altered mandibular function on the maxilla and zygoma.

The effect of protruded mandibular function on the maxilla and zygoma was studied in young unilaterally condylectomized growing rats. Forty-eight-4-week-old rats were divided into two experimental and two control groups as follows: group A, 12 animals unilaterally condylectomized on the right side; the mandible was allowed to function normally; group B, 12 animals unilaterally condylectomized on the right side; the mandible was protracted forwards immediately by means of an appliance; group C, 12 animals sham-operated on the right side; no condylectomy or mandibular protraction; and group d, 12 control animals not subjected to any operation or mandibular protraction. The mandibular protraction was achieved by an appliance consisting of an acrylic collar brace fitted to the animal's neck and supporting rubber bands pulling on an intraoral part cemented on the animal's lower incisors. Twenty-five grams of pulling force and protrusion to a clinically and radiographically testes anterior crossbite was exercised for 12 hours per day. The experimental period was 30 days. Lateral and dorsoventral radiographs were taken on days 1 and 30 following condylectomies and mandibular protraction. Cephalometric analysis was performed for each animal with measurements evaluating the maxilla and zygoma. Statistical analysis and comparison of the findings in the four groups of animals can be summarized as follows: (i) condylectomy and altered mandibular function may produce remote skeletal reactions in other parts of the cranial complex; and (ii) the ipsilateral maxilla is affected by condylectomy of the mandible, but altered mandibular function by protraction compensates for the results of condylectomy.

Altered mandibular function and prevention of skeletal asymmetries after unilateral condylectomy in rats.

Unilateral condylar injury is known to be a frequent cause of mandibular asymmetry. Whether this is due to the trauma itself or to the disturbed function that follows the injury is a very important question with ramifications for clinical complications related to facial asymmetries. The aim of this study was to test the hypothesis that mandibular function in a protruded position can compensate for the absence of one condyle and prevent potential growth asymmetries. Forty-eight 4-week-old rats were divided into two experimental and two control groups consisting of 12 animals each, as follows: (A) unilateral condylectomy was performed on the right side and the mandible was left to function normally; (B) after unilateral condylectomy on the right side, the mandible was forced to function in a protruded position; (C) a sham operation was performed in the condylar area of the right side but no appliance was used; and (D) 12 animals were used as controls without any operation or appliance. Mandibular protraction was achieved by means of a specific appliance, acting via rubber bands, pulling the mandible in a straight, forward direction with a force of 25 g for 12 hours per day. The experimental period was 30 days. Dorsoventral radiographs were taken and vital dyes were administered at three time intervals, i.e. on days 1, 15 and 30, for all animals. Cephalometric analysis included 14 measurements. Findings resulting from statistical analysis and comparisons of measurements obtained in the four groups can be summarized as follows: (i) when comparing group A with groups C and D, less growth was found in the right mandibular sides in group A; (ii) when comparing group B and groups C and D, less growth was found in the right mandibular sides in group B; (iii) when comparing groups A and B, more growth was found in the right mandibular sides in group B; (iv) when comparing the right and left mandibular sides in group A, less growth was found in the right side; and (v) when comparing the right and left mandibular sides in group B, no significant growth differences were found. These findings support the hypothesis that altered mandibular function in a protruded position can compensate for the effects of unilateral condylectomy and prevent the appearance of skeletal mandibular asymmetries in growing rats.

An appliance designed for experimental mandibular hyperpropulsion in rats.

Many animal experiments dealing with function-form interaction and mandibular condylar growth have utilized hyperpropulsion of the animal's mandible. Numerous hyperpropulsive appliances have been described in the literature. However, there are significant discrepancies between reported results which are most likely due to the use of appliances that do not secure true mandibular hyperpropulsion. The specific rat craniofacial anatomy as well as the masticatory physiology of this animal requires the design of a specific appliance that would meet the species' functional characteristics. The aim of this paper is to describe a new type of appliance that can produce a controlled, stable and reproducible anterior displacement of the mandible in rats. After reviewing and evaluating the appliances used by other researchers, a totally new appliance for experimental mandibular hyperpropulsion in rats is presented. Its advantages are: (i) it produces a secure, measurable and reproducible anterior displacement of the mandible; (ii) it does not permit any retrusive or lateral mandibular movements. Avoidance of this variable can increase reliability of results and minimize involvement of other confusing parameters; and (iii) it allows the animals to be fed sufficiently for a number of hours every day. The appliance consists of an acrylic collar brace fitted to the animal's neck and carrying wire extensions that support rubber bands pulling on a band cemented to the animal's lower incisors. The pulling force depends on the thickness of the applied elastics.

Effects of altered mandibular function on mandibular growth after condylectomy.

The purpose of this study was to investigate the effect of protruded mandibular function on bilaterally condylectomized mandibles of growing rats. Sixty 4-week-old rats were divided into three experimental and two control groups each consisting of 12 animals as follows: (A) bilateral condylectomy was performed and the mandible was left to function normally; (B) the mandible was protracted forward without any condylectomy; (C) after bilateral condylectomy, the mandible was forced to function in a protruded position; (D) the animals were used as controls without any operation or appliance; and (E) a sham operation was performed in the condylar area but no appliance was used. Mandibular protraction was achieved by means of a specific appliance acting via rubber bands pulling the mandible in a straight, forward direction with a force of 25 g for 12 hours/day. The experimental period was 30 days. Lateral and dorsoventral radiographs were taken and vital dyes were administered on days 1 and 30 for all animals. Cephalometric analysis included 10 measurements. Findings resulting from statistical analysis of measurements in the five groups are summarized as follows: (i) between group A and groups D and E, less growth was found in group A; (ii) between group B and groups D and E, more growth was found in group B; (iii) between group A and group B, more growth was found in group B; (iv) between group A and group C, more growth was found in group C; and (v) between group B and group C, more growth was found in group B. These findings support the conclusion that although mandibular propulsion enhances growth even after condylectomy, the condyle seems to be an essential element for normal growth and development.