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RESEARCH REPORT |
Orthodontic Science, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan;
* corresponding author, yasuorts{at}tmd.ac.jp
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: occlusal stimuli • alveolar and jaw bone • growth • bone formation • double labeling
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Clinically, local factors that induce loss of occlusal function or occlusal contact—such as open-bite malocclusion, early loss of teeth, congenitally missing teeth, and/or delayed restorations—are known to result in the loss of regional alveolar bone and its support (von Wowern et al., 1979; Beckmann et al., 1998; Usami et al., 2003). In particular, occlusal hypofunction during the growth period may also influence the vertical growth of the alveolar bone (Hsieh et al., 1994).
However, the effect of occlusal hypofunction and its recovery on alveolar and jaw bone formation, especially during the growth period, is still not fully understood. Therefore, the aim of this study was to investigate the direct influence of occlusal function on the changes of spatial and temporal alveolar and jaw bone formation, by means of a hypofunction/recovered occlusal function model (Watarai et al., 2004).
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). After 2 wks of occlusal hypofunction, the devices were removed by means of orthodontic pliers. After the removal of the devices, occlusal contact of the molar area was recovered in 2 wks. Untreated normally growing rats served as controls (Fig. 1b). Rats in both the control and experimental groups were fed water and a powdered diet (CE-2, Clea Japan Inc., Shizuoka, Japan), and their body weight was monitored during the experimental period (Fig. 1c).
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Xylenol Orange/Calcein Administration and Section Preparation
During the four-week experimental period, all rats were given 2 kinds of fluorescent markers—15 mg/kg body weight calcein, and 50 mg/kg body weight xylenol orange—at seven-day intervals biweekly, by subcutaneous injection (total of 5 injections), starting from the first day of the experiment (Fig. 1b
). At the end of the study, all animals were killed by transcardiac perfusion with 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4). The right mandible was dissected and fixed in the same solution. After being embedded in polystyrene resin (Rigolac, Nisshin EM Co. Ltd., Tokyo, Japan), undemineralized ground frontal sections were processed to show the crown and both apices of buccal and lingual roots of the lower second molar. We focused on the bone around the lower second molar, because the second molar is centrally located within the mandibular arch, and the parallel alignment of the buccal and lingual roots made an easy reference when frontal sections were produced (Shimomoto et al., 2005).
Quantitative Analysis
Dynamic bone histomorphometric indices, in response to hypofunction and recovered occlusal function, were evaluated in frontal sections of the lower second molar area. We used a digitizing morphometry system to measure bone formation indices. The system consisted of a confocal laser scanning microscope (LSM510, Carl Zeiss Co. Ltd., Jena, Germany), and a morphometry program (LSM Image Browser, Carl Zeiss Co. Ltd., Germany). Bone formation indices of the periosteal surfaces of the alveolar/jaw bone included mineral apposition rate (µm/day) and bone formation rate (µm3/µm2/day), according to the standard nomenclature described by Parfitt (Parfitt et al., 1987).
Based on the reference line along the long axis of the buccal root, the area superior to the root apex was considered alveolar bone, while the area inferior to the root apex was considered the jaw bone. The lingual side of the bone was excluded, because the existence of the incisor root may influence bone formation. The periosteal surfaces of the mandible were divided into 4 regions for analysis (Fig. 1d): region 1, alveolar crest (upper 1/2 of the tooth root, near the tooth crown); region 2, alveolar bone (lower 1/2 of the tooth root, near the root apex); region 3, buccal surface of the jaw bone; and region 4, inferior border of the jaw bone. For the measurements of mineral apposition rate, the average of 3 inter-label widths at a 50-µm interval was calculated for each sample.
Statistical Analysis
The Mann-Whitney U-test was selected for evaluation of the influence of occlusal stimuli on mineral apposition rate and bone formation rate of each region and the changes in the body weight. P < 0.05 was selected as the statistically significant level.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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).
Continuous Apposition of Alveolar and Jaw Bone during the Experiment Period
Starting from the first injection (inner yellow label), continuous fluorescent lines at 5 different time points were clearly shown, indicating continuous bone apposition during the experimental period in the control group (Fig. 2a). Alveolar bone apposition occurred in the occlusal and buccal directions, while the jaw bone expanded in the lateral and inferior directions. Mineral apposition also occurred around the incisor root.
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Changes in Alveolar Bone Formation Due to Hypofunction and Recovered Occlusal Function
To evaluate further the spatial and temporal differences in bone formation in response to hypo-function and its recovery, we divided the observation area into 4 regions (Fig. 1d). Dynamic bone histomorphometric indices—such as mineral apposition rate and bone formation rate of each area during hypofunction (inner yellow -adjacent red - middle yellow)—and recovered occlusal function (middle yellow - red - outer yellow) were evaluated and compared with the values of the control group.
In the alveolar crest (region 1), the mineral apposition rates were similar regardless of occlusal hypofunction. However, recovered occlusal function induced significant enhancements in the mineral apposition rate compared with that in the control (Fig. 3a). The bone formation rate was significantly suppressed during the initial phase of occlusal hypofunction, but reached levels similar to those of the control group throughout recovery (Fig. 3c).
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Changes in Jaw Bone Formation Due to Occlusal Hypofunction and Recovered Occlusal Function
As the animals aged, mineral apposition rate and bone formation rate significantly decreased in both the buccal and inferior borders of the jaw bone in the control group. In response to hypofunction, mineral apposition rate and bone formation rate were significantly suppressed compared with those in the control group, in both areas (P < 0.05). Interestingly, a diverse response was detected after recovered occlusal function in these areas. While recovered occlusal function induced similar levels or delayed enhancement of mineral apposition rate and bone formation rate, compared with controls, in the buccal surface of the jaw bone (Figs. 4a, 4c), the inferior border showed rapid and dramatic enhancement throughout the recovery period, compared with the controls (P < 0.05) (Figs. 4b, 4d). The average bone formation rate of the jaw bone was nearly double that of alveolar bone. (Note the difference in the scales in Figs. 3 and 4.)
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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In growing animals, bone modeling is reported to occur principally in the cortical bone (Li and Jee, 1991). Bone apposition of the outer cortex increases the diameter and cortical cross-sectional area, and thus the stiffness and strength of the cortex can be increased (Frost, 1988). The alveolar region of the mandible is also reported to indicate active bone remodeling, similar to that in other general areas (Takagi et al., 2001). In particular, the lower molar shows remarkable eruption, with continuous root elongation along with growth, which has been reported to induce dramatic changes in these regions in five-week-old rats (Kinoshita et al., 1982). Thus, for our study, we selected five-week-old growing rats for evaluation for a four-week period, to detect the dynamics changes in growth and bone formation in response to occlusal function. Bone formation has been reported to be very active but to decline gradually with age in general (Burr and Marin, 1989; Kabasawa et al., 1996). Accordingly, we noticed maximum bone formation at the periosteal surface of the alveolar and jaw bone during the initial stage, with a gradual decline as the animals aged (normal controls).
Occlusal hypofunction significantly suppressed bone formation during this maximum growth period, as previously reported (Enokida et al., 2005; Shimomoto et al., 2005). However, our results clearly showed that the suppression of bone formation due to occlusal hypofunction can be reversed by a recovery of occlusal function. An increase in bone formation after recovered occlusal function was consistently noticed in most of the regions evaluated. Interestingly, region 4, the inferior border of the jaw bone, showed remarkable instantaneous changes after occlusal recovery, despite its considerable distance from the occlusal stimuli. Since occlusion mainly delivers vertical force to the jaw, which coincides with the direction of growth in the case of the inferior border, recovered occlusal function may have induced instantaneous response compared with that in other regions. In addition, the masseter is anatomically attached near the inferior border of the jaw in the lower molar area (Nakata, 1981). A decrease in functional forces of the masseter, with the use of an anterior biteplate appliance, has been reported to cause a decrease in mandibular bone growth (Mavropoulos et al., 2004). Thus, bone formation of the inferior border could have been stimulated by the combined effects of the increase in masseter muscle functions along with the direction of force application in response to recovered occlusal function.
In regions 1, 2, and 3 (alveolar crest, alveolar bone, buccal surface of the jaw bone), mineral apposition rate and bone formation rate showed some statistical discrepancies, despite their similar tendencies after occlusal recovery. Increased mineral apposition rate after recovered occlusal function indicates the increase in osteoblastic activity at the cellular level. The bone formation rate indicates the changes in numbers of bone-producing osteoblasts, and thus, bone formation at the tissue level (Miller et al., 1997). Our study showed that bone formation at the cellular level (mineral apposition rate) can be enhanced in response to recovered occlusal function after hypofunction, although it was not sufficient to enhance the total amount of bone formation per se (bone formation rate) during the experimental period. Previously, it was reported that active bone remodeling occurs during the growth spurt, with a decrease in the number of osteogenic cells, such as osteoblasts, at the end of the growth spurt (Takagi et al., 2001). Based on that report, we postulated that, although cellular activity was enhanced in the pre-existing osteoblasts following occlusal recovery (enhancement in MAR [mineral apposition rate]), occlusal recovery at the end stage of the growth spurt (the latter half of our experiment) may not be sufficient to enhance the total numbers of osteoblasts required for bone formation (bone formation rate, BFR). These results suggest that occlusion and timing at the proper age are important regulatory factors for bone formation and growth, especially in regions 1, 2, and 3.
In summary, our study demonstrated that alveolar and jaw bone formation is suppressed in occlusal hypofunction, while it can be enhanced following occlusal recovery during the growth period. This indicates that the recovery of the proper occlusal function during the early growing stage may induce positive changes in the pattern of bone growth.
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ACKNOWLEDGMENTS |
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Received January 6, 2006; Last revision September 15, 2006; Accepted September 29, 2006
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REFERENCES |
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