Osteoclast activation and recruitment after application of orthodontic force.

In this study, whole body radioactive cobalt 60 (Co60) irradiation was used in an orthodontic tooth movement model to investigate osteoclast activation and recruitment in adult rats. Seventy-five rats were divided into three groups and were irradiated with Co60 ranging from 10 to 26 Gray. An orthodontic appliance was fitted to each rat 12 days after initiation of irradiation. Identical appliances were fitted on an additional 25 unirradiated rats. Groups of rats were sacrificed 1 week before and on the day of appliance placement as well as 1, 2, 3, 4, or 5 weeks after appliance placement. Histologic sections from decalcified maxillary processes were prepared and osteoclasts were counted. Results showed that regardless of irradiation dosage, osteoclast number did not decrease during the first 3 weeks of orthodontic treatment; animals receiving lower (ie, 10 Gray) total irradiation actually showed a transient increase in the osteoclast number. In addition, regardless of dose in the irradiated rats, the total time osteoclasts were present in the periodontal membrane after orthodontic activation was reduced from 4 to 3 weeks. These findings lead us to hypothesize that osteoclasts involved in appliance-induced remodeling are initially from precursors in the periodontal membrane. During prolonged periods of orthodontic force application, replacement osteoclasts originate from bone marrow precursors. Although an orthodontic bone resorption cycle lasts 4 weeks, the life span of individual osteoclasts is limited to 9 to 10 days. The clinical significance of this finding remains to be elucidated.

The effect of cobalt-60 irradiation on bone marrow cellularity and alveolar osteoclasts.

The purpose of this study was to investigate the mechanisms of bone resorption in the rat dentoalveolar complex occurring as a result of orthodontic appliance therapy. Utilizing whole body radioactive cobalt 60 (Co60) irradiation in combination with orthodontic tooth movement, the activation, recruitment, and life span of osteoclasts was studied. Thirty-four adult Fischer 344 rats were irradiated with 10 and 20 Gray of Co60. Twelve days after irradiation, each rat was fitted with an orthodontic appliance; the rats were sacrificed 7, 14, or 21 days after appliance placement. To serve as controls, another group of 12 rats was subjected to orthodontic treatment only. Histologic sections were prepared from decalcified maxillary alveolar process, and osteoclasts were counted. In the control group, osteoclasts were presence in the periodontal membrane for four weeks after appliance placement, and the largest number of osteoclasts was observed in the second week. The smaller total irradiation dose (10 Gray, administered in 5 daily fractions) induced a transient reduction in the bone marrow cell count of more than 90%, followed by a complete rebound. The peak osteoclst number also was increased 110%. In contrast, the higher total dose (20 Gray, administered in 10 daily fractions) resulted in a reduction of 60% in bone marrow cellularity and a decrease in the peak osteoclast number by 40%. Fluctuations in bone marrow cellularity generally corresponded to similar variations in the osteoclast number, but there was an apparent lack of correspondence between bone marrow cell and white blood cell values. The total period of osteoclastic presence in the periodontal membrane following orthodontic activation, normally four weeks in duration, was reduced by one week in the irradiated animals. These findings lead us to speculate that the mechanisms of osteoclast activation and recruitment following orthodontic appliance therapy may involve three consecutive waves of osteoclast maturation. The total duration of osteoclastic bone resorption lasts 4 weeks, but the osteoclast lie span is calculated to be 9 to 10 days (9 days x 3 waves = 27 days). If the results of this study are proved to be reproducible, a scientific basis may have been provided to support the common practice in orthodontics wherein the duration between orthodontic appointments is 4-5 weeks.

A study of the suitability of two methods used for describing the occlusal plane.

In this study an effort was made to determine which of the two commonly used methods of occlusal plane determination more precisely depicts changes induced by treatment or growth. Longitudinal lateral cephalometric radiographs of 70 individuals, 40 orthodontically treated and 30 with clinically acceptable occlusion, were used. Following an analysis of outcomes obtained through the conventional application of the Downs occlusal plane method and the functional occlusal plane method, there are indications that: 1) the Downs method may provide a better description of what happens to the occlusion following treatment; 2) the notion that the plane reconstructed using the functional method actually represents the location at which the occlusal function takes place is unsubstantiated; and 3) since it is possible that the results of this study could be influenced by the types of malocclusion, skeletal patterns, and methods of orthodontic treatment, further analyses are called for in order to make more conclusive determinations about these methods.

A biomechanical analysis of craniofacial form and bite force.

In vivo measurements of bite force and bone strain obtained in growing African green monkeys were used to study skull biology and geometry. Strain values and distributional patterns seen in association with forceful jaw elevation were inconsistent with most conventional explanations that link upper facial morphology with masticatory function and use beam models of craniofacial architecture. The results mandate careful use of concepts about skeletal geometry based on static analysis that have not been experimentally verified with in vivo procedures. In particular, a reevaluation of conventional ideas about the generation and dissipation of forces during contraction of the jaw elevator muscles seems called for.