The Effect of Orthodontic Retention on the Mechanical Properties of the Periodontal Ligament in the Rat Maxillary First Molar R.K. HONG, A. YAMANE', Y. KUWAHARA2, and M. CHIBA Departments of Pharmacology and 2Orthodontics, School of Dental Medicine, Tsurumi University, 2-1-3 Tsurumi, Tsurumi-ku, Yokohama, Japan, 230 The load-deformation curves obtained by extraction of the rat maxillary first molar from its socket in the dissected jaw were analyzed so that the effect of orthodontic retention on the mechanical properties of the periodontal ligament could be examined. An elastic band was inserted between the rat maxillary first and second molars for four days, and then the interdental space was filled with resin for four or eight days. The average interdental spaces between the teeth ranged from 315 to 398 jm during the experimental period. The maximum shear load, elastic stiffness, and failure energy in shear decreased markedly following application of an orthodontic force, but they increased gradually and reached control levels on the 8th day after the retention. Maximum shear deformation at maximum load was not significantly different between the experimental and control teeth during the experimental period. It is suggested that, following orthodontic tooth movement, occlusal function was restored after a relatively short retention period, as was the impaired mechanical strength ofthe periodontal ligament. J Dent Res 71(7):1350-1354, July, 1992
Introduction. There have been numerous studies on the histological changes ofthe supporting tissues ofteeth following experimental tooth movement (e.g., Schwarz, 1932; Reitan, 1975; Diaz, 1978; Martinez and Johnson, 1987). For investigation oftissue reactions to mechanical stress on the molar teeth of rats, placement of a rubber band between the molar teeth has been widely used (Waldo and Rothblatt, 1954; Azuma, 1970; Roberts and Jee, 1974; Tanaka et al., 1990). It has also been reported that, following experimental tooth movement, changes in the periodontium include hemorrhage, root resorption, and increased osteoclastic activity (Waldo and Rothblatt, 1954; Zaki and Van Huysen, 1963; Rygh, 1977; Engstrbm et al., 1988; Kuitert et al., 1988/89). The mechanical properties of the periodontium supporting rat mandibular molars have been determined from the force required for extraction of the teeth from their sockets in the dissected jaw (Chiba and Ohkawa, 1980; Chiba et al., 1982; Ohkawa, 1982; Tsuruta et al., 1982). It has been suggested that the mechanical properties ofthe periodontal ligament serve as indicators ofchanges in the organization and constitution of the ligament, and that changes in the mechanical properties could be detected even when biochemical and histological differences could not be observed (Komatsu, 1988; Hong, 1990). It has been shown that the mechanical strength ofthe periodontal ligament decreased rapidly and markedly after application of an orthodontic force to the rat mandibular or maxillary molars (Ohkawa, 1982; Tsuruta et al., 1982; Hong, 1990). Restoration ofthe mechanical strength of the ligament occurred rapidly in accordance with the closure of the interdental space after removal of the elastic band (Tsuruta et al., 1982). However, changes in the mechanical strength have not been examined after orthodontically moved teeth have been fixed to prevent relapse. Received for publication October 9, 1991 Accepted for publication February 11, 1992 'To whom correspondence and reprint requests should be addressed
The purposes of the present experiment were to establish a simple experimental model for assessment of the effectiveness of orthodontic retention and to examine whether or not the reduced mechanical strength of the periodontal ligament caused by orthodontic tooth movement (Tsuruta et al., 1982) could be restored after retention in the rat maxillary first molar.
Materials and methods. Forty-one male rats of the Wistar strain, 8 weeks of age, with an average body weight of251 ± 13 (SD) g, were divided into four groups of 10-11 animals each. They were fed a powdered diet and given water adlibitum duringthe experimental period andfor aboutthree weeks beforehand. In three groups of rats, a latex elastic band (No. 404-126, Unitek, Monrovia, CA) with an average thickness of 596 ± 36 (SD) gim was inserted into the interproximal space between the left maxillary first and second molars while the animals were under ether anesthesia. In one group of rats, the elastic band was not inserted between the teeth, and these rats were killed by decapitation under ether anesthesia at the beginning of the experimental period (zero-day group). On the 4th day after insertion ofthe elastic band, one group of rats (four-day group) was killed as in the zero-day group. In the remaining two groups, the latex elastic band was removed after four days, and then light-cured resin (Transbond, Unitek/3M, CA) was placed in the interdental space between the left maxillary first and second molars. To prevent detachment of the resin, a small undercut was made between the distal side ofthe first molar and the mesial side of the second molar. The two groups of rats were killed on the 4th (eight-day group) and 8th (12-day group) days after the retentive procedure. After the death ofthe animals, their maxillae were dissected out, and the occlusal surfaces of the dental arches were photographed (Fig. 1). The distances between the mesial end of the first molar and the distal end of the third molar parallel to the midline of the dental arch were measured with a profile projector (V-16D, Nikon, Tokyo, Japan). The difference between the lengths of the left and right dental arches was determined as the interdental space (Hong, 1990). The left (experimental) and right (control) maxillary first molars were extracted by use of a tensile-strength-testing machine (Autograph, Type S-100, Shimadzu, Kyoto, Japan), as described previously (Chiba and Ohkawa, 1980; Ohkawa, 1982) at an extension rate of 5 mmper min. The load-deformation curves were obtained, and the data were stored in a computer (PC-9801 VM2, NEC, Tokyo, Japan) through an amplifier (PA-011, Star Medical, Tokyo, Japan) and an analogue-to-digital converter (DAS-1898CPC, Micro Science, Tokyo, Japan). Mechanical parameters-such as maximum shear load, maximum shear deformation (the distance pulled to reach the maximum shear load), elastic stiffness, and failure energy in shear-were estimated from the load-deformation curves (Haut, 1986; Mandel et al., 1986; Komatsu, 1988). The differences between the mean values were compared by the Student t test.
Results. Interdental space.-The Table shows the lengths of the dental arches after application of orthodontic force and after the retention
1350 Downloaded from jdr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on June 9, 2015 For personal use only. No other uses without permission.
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Days s iFe maimum hear load estimated Yom the loadFg. 2ChFa cn the 4tF day, the elastic band was kept between the doiaon curvs. f rmaxiary first and second iolars. On the 4th day theband was removed (aiow) and te rnterdeat spa e was filed with light-cured resin. The right rax Ilary first rolr was used as the control. Each point and vertical bha rprsentt e mean SD oftic sample. Level of sigrificarce between tie nt F p 0D00fl. eprnal nd
t I- XphotograpF Fhowiig tho occlusa surface o lef (LI and nt (r xillary iolar. After reiova of tholt c hanid liFcroed ren () Nd (M2) mcs. T efnd ofre fitmod aind the istaI end (d) ds be"een the mes1i ofehrdmola (M parle o the midline f the dern ;le (xa was (zero-day group) On the 4th day after application of orthodontic measured. The dfrence between x and y was re eid as t dinterdenta force burday group), te ma um shear load (13.4 N) decreased spae o 45c of the control value (29.9 N). Restoration of the maximum with gFh t-crd resin, and the intr'dental space detned as the shear load occured gradual y after the retention On the 4th day diierence in ne lengths between the le (expomeni a l and ignt afer the reteion (eight-day group) the maximum shear load (23.0 (on rol) denti arces The a g length ofthe dental ar in the N) returnedto 69% ofthe control value (3 33 N). On the 8th day (12 ctrol side were firmly uniform in all groups, ranging froT 6.72 to d group) he average value in the experimental teeth (29.6 N) was 6 79 mm The lergtb ofheleftdernta archir thezeodaygroupwas 91% of that in the control teeth (32.7 N) ,but the difference was not iso vitFin that tanre. le leniths of the lft denlt arces significant. Mamimam shear dfornatioa.Fig. 3 shows changes in the irnsertion ofthe elastic bard increased sgnificantly (p
Introduction. There have been numerous studies on the histological changes ofthe supporting tissues ofteeth following experimental tooth movement (e.g., Schwarz, 1932; Reitan, 1975; Diaz, 1978; Martinez and Johnson, 1987). For investigation oftissue reactions to mechanical stress on the molar teeth of rats, placement of a rubber band between the molar teeth has been widely used (Waldo and Rothblatt, 1954; Azuma, 1970; Roberts and Jee, 1974; Tanaka et al., 1990). It has also been reported that, following experimental tooth movement, changes in the periodontium include hemorrhage, root resorption, and increased osteoclastic activity (Waldo and Rothblatt, 1954; Zaki and Van Huysen, 1963; Rygh, 1977; Engstrbm et al., 1988; Kuitert et al., 1988/89). The mechanical properties of the periodontium supporting rat mandibular molars have been determined from the force required for extraction of the teeth from their sockets in the dissected jaw (Chiba and Ohkawa, 1980; Chiba et al., 1982; Ohkawa, 1982; Tsuruta et al., 1982). It has been suggested that the mechanical properties ofthe periodontal ligament serve as indicators ofchanges in the organization and constitution of the ligament, and that changes in the mechanical properties could be detected even when biochemical and histological differences could not be observed (Komatsu, 1988; Hong, 1990). It has been shown that the mechanical strength ofthe periodontal ligament decreased rapidly and markedly after application of an orthodontic force to the rat mandibular or maxillary molars (Ohkawa, 1982; Tsuruta et al., 1982; Hong, 1990). Restoration ofthe mechanical strength of the ligament occurred rapidly in accordance with the closure of the interdental space after removal of the elastic band (Tsuruta et al., 1982). However, changes in the mechanical strength have not been examined after orthodontically moved teeth have been fixed to prevent relapse. Received for publication October 9, 1991 Accepted for publication February 11, 1992 'To whom correspondence and reprint requests should be addressed
The purposes of the present experiment were to establish a simple experimental model for assessment of the effectiveness of orthodontic retention and to examine whether or not the reduced mechanical strength of the periodontal ligament caused by orthodontic tooth movement (Tsuruta et al., 1982) could be restored after retention in the rat maxillary first molar.
Materials and methods. Forty-one male rats of the Wistar strain, 8 weeks of age, with an average body weight of251 ± 13 (SD) g, were divided into four groups of 10-11 animals each. They were fed a powdered diet and given water adlibitum duringthe experimental period andfor aboutthree weeks beforehand. In three groups of rats, a latex elastic band (No. 404-126, Unitek, Monrovia, CA) with an average thickness of 596 ± 36 (SD) gim was inserted into the interproximal space between the left maxillary first and second molars while the animals were under ether anesthesia. In one group of rats, the elastic band was not inserted between the teeth, and these rats were killed by decapitation under ether anesthesia at the beginning of the experimental period (zero-day group). On the 4th day after insertion ofthe elastic band, one group of rats (four-day group) was killed as in the zero-day group. In the remaining two groups, the latex elastic band was removed after four days, and then light-cured resin (Transbond, Unitek/3M, CA) was placed in the interdental space between the left maxillary first and second molars. To prevent detachment of the resin, a small undercut was made between the distal side ofthe first molar and the mesial side of the second molar. The two groups of rats were killed on the 4th (eight-day group) and 8th (12-day group) days after the retentive procedure. After the death ofthe animals, their maxillae were dissected out, and the occlusal surfaces of the dental arches were photographed (Fig. 1). The distances between the mesial end of the first molar and the distal end of the third molar parallel to the midline of the dental arch were measured with a profile projector (V-16D, Nikon, Tokyo, Japan). The difference between the lengths of the left and right dental arches was determined as the interdental space (Hong, 1990). The left (experimental) and right (control) maxillary first molars were extracted by use of a tensile-strength-testing machine (Autograph, Type S-100, Shimadzu, Kyoto, Japan), as described previously (Chiba and Ohkawa, 1980; Ohkawa, 1982) at an extension rate of 5 mmper min. The load-deformation curves were obtained, and the data were stored in a computer (PC-9801 VM2, NEC, Tokyo, Japan) through an amplifier (PA-011, Star Medical, Tokyo, Japan) and an analogue-to-digital converter (DAS-1898CPC, Micro Science, Tokyo, Japan). Mechanical parameters-such as maximum shear load, maximum shear deformation (the distance pulled to reach the maximum shear load), elastic stiffness, and failure energy in shear-were estimated from the load-deformation curves (Haut, 1986; Mandel et al., 1986; Komatsu, 1988). The differences between the mean values were compared by the Student t test.
Results. Interdental space.-The Table shows the lengths of the dental arches after application of orthodontic force and after the retention
1350 Downloaded from jdr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on June 9, 2015 For personal use only. No other uses without permission.
V OF OR ~~EFFLCL Vol. 71 77Th. No.
-
YJODONLIC REJANIION ONILERIODONJIUM
35 1351
40
-o-
Cont
-*-
Exp
0-
0
30
10 ta o 0
20
E
x
a
10 0
Li 0
4
8
12
Days s iFe maimum hear load estimated Yom the loadFg. 2ChFa cn the 4tF day, the elastic band was kept between the doiaon curvs. f rmaxiary first and second iolars. On the 4th day theband was removed (aiow) and te rnterdeat spa e was filed with light-cured resin. The right rax Ilary first rolr was used as the control. Each point and vertical bha rprsentt e mean SD oftic sample. Level of sigrificarce between tie nt F p 0D00fl. eprnal nd
t I- XphotograpF Fhowiig tho occlusa surface o lef (LI and nt (r xillary iolar. After reiova of tholt c hanid liFcroed ren () Nd (M2) mcs. T efnd ofre fitmod aind the istaI end (d) ds be"een the mes1i ofehrdmola (M parle o the midline f the dern ;le (xa was (zero-day group) On the 4th day after application of orthodontic measured. The dfrence between x and y was re eid as t dinterdenta force burday group), te ma um shear load (13.4 N) decreased spae o 45c of the control value (29.9 N). Restoration of the maximum with gFh t-crd resin, and the intr'dental space detned as the shear load occured gradual y after the retention On the 4th day diierence in ne lengths between the le (expomeni a l and ignt afer the reteion (eight-day group) the maximum shear load (23.0 (on rol) denti arces The a g length ofthe dental ar in the N) returnedto 69% ofthe control value (3 33 N). On the 8th day (12 ctrol side were firmly uniform in all groups, ranging froT 6.72 to d group) he average value in the experimental teeth (29.6 N) was 6 79 mm The lergtb ofheleftdernta archir thezeodaygroupwas 91% of that in the control teeth (32.7 N) ,but the difference was not iso vitFin that tanre. le leniths of the lft denlt arces significant. Mamimam shear dfornatioa.Fig. 3 shows changes in the irnsertion ofthe elastic bard increased sgnificantly (p
