The effect of mechanical vibration on orthodontically induced root resorption.

OBJECTIVE: To investigate the effect of low-frequency mechanical vibration (LFMV) on orthodontically induced root resorption. MATERIALS AND METHODS: Forty male CD1, 12-week-old mice were used for the study. The mice were randomly divided into five groups: group 1 (baseline)-no spring and no mechanical vibration, group 2-orthodontic spring but no vibration, group 3-orthodontic spring and 5 Hz of vibration applied to the maxillary first molar, group 4-orthodontic spring and 10 Hz of vibration applied to maxillary first molar, and group 5-orthodontic spring and 20 Hz of vibration applied to maxillary first molar. In the different experimental groups, the first molar was moved mesially for 2 weeks using a nickel-titanium coil spring delivering 10 g of force. LFMVs were applied at 5 Hz, 10 Hz, and 20 Hz. Microfocus X-ray computed tomography imaging was used to analyze root resorption. Additionally, to understand the mechanism, we applied LFMV to MC3T3 cells, and gene expression analyses were done for receptor activator of nuclear factor kappa-B ligand (RANKL) and osteoprotegerin (OPG). RESULTS: Orthodontic tooth movement leads to decreased root volume (increased root resorption craters). Our in vivo experiments showed a trend toward increase in root volume with different frequencies of mechanical vibration. In vitro gene expression analyses showed that with 20 Hz of mechanical vibration, there was a significant decrease in RANKL and a significant increase in OPG expression. CONCLUSION: There was a trend toward decreased root resorption with different LFMVs (5 Hz, 10 Hz, and 20 Hz); however, it was not more statistically significant than the orthodontic-spring-only group.

Effect of low-frequency mechanical vibration on orthodontic tooth movement.

INTRODUCTION: Our objective was to investigate the effect of low-frequency mechanical vibration (LFMV) on the rate of tooth movement, bone volume fraction, tissue density, and the integrity of the periodontal ligament. Our null hypothesis was that there would be no difference in the amount of tooth movement between different values of LFMV. METHODS: Sixty-four male CD1 mice, 12 weeks old, were used for orthodontic tooth movement. The mice were randomly divided into 2 groups: control groups (baseline; no spring + 5 Hz; no spring + 10 Hz; and no spring + 20 Hz) and experimental groups (spring + no vibration; spring + 5 Hz; spring + 10 Hz; and spring + 20 Hz). In the experimental groups, the first molars were moved mesially for 2 weeks using nickel-titanium coil springs delivering 10 g of force. In the control and experimental groups, LFMV was applied at 5, 10, or 20 Hz. Microfocus x-ray computed tomography analysis was used for tooth movement measurements, bone volume fraction, and tissue density. Additionally, immunostaining for sclerostin, tartrate-resistant acid phosphatase (TRAP) staining, and picrosirius red staining were used on the histologic sections. Simple descriptive statistics were used to summarize the data. Kruskal-Wallis tests were used to compare the outcomes across treatment groups. RESULTS: LFMV did not increase the rate of orthodontic tooth movement. Microfocus x-ray computed tomography analysis showed increases in bone volume fractions and tissue densities with applications of LFMV. Sclerostin expression was decreased with 10 and 20 Hz vibrations in both the control and experimental groups. Additionally, the picrosirius staining showed that LFMV helped in maintaining the thickness and integrity of collagen fibers in the periodontal ligament. CONCLUSIONS: There was no significant increase in tooth movement by applying LFMV when compared with the control groups (spring + no vibration).

Sex differences in tolerance to visually-induced motion sickness.

BACKGROUND: Women report a history of motion sickness (MS) about twice as frequently as men, but the results of experimental studies are equivocal. In the present investigation, we sought to replicate previous findings that women report a greater history of MS than men when interrogated with MS history questionnaires. We examined the hypothesis that those reporting that they are prone to MS are less likely to volunteer for MS provocative experiments than those who are MS resistant. Finally, using a subset of these participants, we exposed men and women, during two separate sessions, to visually elicited apparent motion, with and without voluntary head motion (pseudo-Coriolis stimulation), to examine any differences in MS elicited between these two groups on exposure to such motion stimulation. METHOD: Experiment 1 used a MS History Questionnaire, which included an opportunity for male and female participants to volunteer for "psychology experiments" in the coming semester. This instrument was used to determine effects of sex and volunteer status on motion sickness susceptibility (MSS). Experiment 2 involved exposing a subset of these participants to rotation of a vertically striped rotating drum under static and head movement conditions. Measures of vection and MS were recorded. RESULTS: We found higher MSS scores in women vs. men, particularly when looking at participants who elected to volunteer. Women in the second experiment reported significantly more MS, but they exhibited less tolerance with head movement. No significant differences in vection were observed. CONCLUSIONS: The results indicate that laboratory manipulations that are more provocative of MS reveal reliable sex differences.

The role of vection, eye movements and postural instability in the etiology of motion sickness.

Motion sickness is a term that is commonly used to describe the ill effects of many provocative motion (e.g. seagoing or air travel) and apparent motion (e.g. IMAX movies and virtual reality) environments on human well-being and performance. It can be extremely debilitating and yet we do not have a precise understanding of its cause. This study evaluates the importance of three factors that are purported to be involved in the etiology of motion sickness (MS). Most provocative motion environments cause three distinct, but possibly related, responses: reflexive eye movements (EM), sensory conflict (SC), and postural instability (PS). Three current theories, concerning the etiology of motion sickness, emphasize one of these responses, but deny the importance of the others. Such theoretical approaches preclude the possibility of a synergistic interaction of these factors. This experiment employed a three-factor experimental design wherein each factor was manipulated alone or in combination with the others. The independent variables involved two levels of: PS (induced by having the subject stand on a stationary platform or on a posturally challenging platform mounted atop a partially inflated rubber inner tube); SC (with or without illusory self movement elicited visually by whole field stimulation); and EM (unrestricted or controlled by a stable fixation point). Analysis of measures of PS, SC and EM confirmed the effectiveness of these manipulations. Analysis of MS measures (questionnaires, magnitude ratings, tolerance times) revealed a main effect of SC (p < 0.01), increased MS found with illusory self motion conditions. In addition, measures of MS symptomatology revealed a significant three-way interaction between SC, PS and EM (p < 0.05), greater amounts of MS found with conditions of illusory self motion, postural challenge, and unrestricted EM. This suggests support for a multi-factorial approach to the study of MS etiology. These findings suggest a major role of SC in the elicitation of MS, but also suggest important contributions from the EM and PS mechanisms.

The influence of visual reference on stance and walking on a moving surface.

BACKGROUND: It is apparent that visual information is used in maintaining stable posture and ambulating throughout the stationary world. Considerable previous research has indicated that significant perturbations of posture can be induced with a shift in the entire visual scene. In motion environments, it is assumed that posture and ambulatory ability are controlled more by vestibulo-spinal reflexes, but the role of visual reference has not been extensively studied. METHOD: In the present study, the frequency of motion-induced interruptions (MIIs) under conditions which did and did not provide a stable visual reference were compared. Subjects were tested on a motion platform driven by a simulated ship-motion profile. Two independent observers recorded MIIs for standing and walking under two conditions. In one, the subjects were allowed to see the walls and ceiling of the stationary test cubicle that housed the motion platform. In the other, curtains attached to the motion platform prevented this view. Force plate recordings were also obtained during the standing tasks. RESULT: Observers reported significantly more MIIs, and force plate recordings indicated more postural instability under the condition involving curtains. CONCLUSIONS: These results suggest that manipulations, such as artificial horizons, that provide stable visual representations of the static environment within which the motion occurs may provide important health and safety measures for individuals working in modern vehicular conveyances.

Optokinetic nystagmus, vection, and motion sickness.

BACKGROUND: Two current theories concerning the etiology of motion sickness (MS)-the eye movement hypothesis and sensory conflict theory-were evaluated under conditions that manipulated the degree of optokinetic nystagmus (OKN) and/or vection. METHOD: Eye movement and perceptual responses were elicited with whole field stimulation in a vertically striped rotating drum and modulated with fixation and/or a restriction of the field of view (FOV). Measures of OKN, vection, and MS were recorded under the various conditions. RESULTS: Both visual field restriction and/or fixation diminished circular vection, OKN, and MS. Conditions involving both fixation and restricted FOV resulted in greater reductions in MS than did either restriction alone. CONCLUSIONS: These findings lend support to a multi-factor explanation of MS, involving both sensory conflict and eye movement.