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Experimental intrusion of rat incisors with continuous loads of varying magnitude
Shulamit Steigman, D.M.D., and Yael Michaeli, D.M.D. Jerusalem, Israel The present investigation deals with the relationship between continuous intrusive loads and the rate of intrusion of rat incisors. A method for the application of constant, defined loads by means of a closed coil spring is described. In seventeen rats the left mandibular incisor was shortened to prevent occlusion. The animals were exposed to direct light, medium, and heavy intrusive loads for a period of 12 days. Light loads (1.5 to 8.0 Gm.lcm.2) did not cause active intrusion of the teeth. Medium loads (12.0 to 18.5 Gm.lcm.2) initially elicited marked intrusion, followed by a short rest period after which the intrusive movement progressed steadily at a daily rate of about 25 pm. Heavy loads (30.5 to 32.0 Gm.lcm.z) brought about active intrusion, which commenced only after 8 days of force application. The medium loads, having a magnitude in the range of rat systolic blood pressure, proved to be optimal for the intrusive movement.
Key words: Intrusion, rat incisor, coiled springs, optimal force
T
he dispute about the possibility of an active intrusion of teeth is by now a thing of the past. Lefkowitz and Waugh,” Dellinger,‘j Stenvik and Mjiir,25 and others have shown that the application of depressing forces causes tissue changes which result in an apical displacement of a tooth into the alveolus. The various orthodontic movements have received extensive attention with regard to the influence of the magnitude and duration of applied forces. The only exception is the intrusive movement, the information about its relationship to the applied forces remaining scanty, while the various load denominations used by the different investigators obscure the issue. Burstone advocates the use of approximately 25 Gm. of continuous force for intrusion of human upper incisors and recommends forces of different magnitudes for each of the anterior teeth, based upon clinical experience. Bondevik,4 applying intrusive forces of 0.29 N, 0.49 N, and 0.98 N on rat occluding molars, did not find essential differences in bone and periodontal tissue reaction of teeth subjected to different force magnitudes. However, the magnitude of the initially applied forces had been considerably reduced in the course of the experiment. Dellinger,6 in a study carried out on four monkeys, demonstrated that after force application for 60 days the greatest amount of intrusion (2.7 mm., together with other anteroposterior changes) occurred upon initial application of a load of From the Department University-Hadassah 0002-9416/81/100429+08$00.80/0
of Orthodontics and Division School of Dental Medicine. 0
1981 The C. V. Mosby Co.
of Anatomy
and Embryology,
The
Hebrew
429
Fig. 1. Schematic drawing of the experimental appliance.
50 Gm. With loads of higher magnitude, the final amount of intrusion proved to be less and root resorption was severe. These findings express the final results of extended application of different loads but do not reflect the consecutive rate of the intrusive movements. The relationship between the rate of the lateral movement of teeth and the magnitude of the applied forces was studied in guinea pig upper incisors.26 While a force in the optimal range produced steady motion of a tooth, heavy forces induced disruptive tooth movements characterized by intermittent periods of cessation and rapid movement. Although ultimately there is little difference in the total distance traveled by a tooth subjected to either light or heavy forces, the use of the latter is always accompanied by undesirable tissue changes. The aim of the present study was to observe continuously the behavior of teeth under different constant intrusive loads and to determine whether the relationship that exists between the magnitude of loads and the rate of lateral tooth movement also applies to intrusive movements. The investigation was carried out on rat lower incisors. These cylindrically shaped teeth consist of a part which lies in the oral cavity (the “clinical crown”) and of a long “root” which is situated beneath the molar roots, almost parallel to the lower alveolar ridge (Fig. 1). Rat incisors undergo continuous attrition at their incisal end, which is counterbalanced by steady eruption and growth at the basal region. The main advantage of this model lies in its quick response to intrusive forces as expressed by the easily measured length of the “clinical crown. ” Second, the proliferative region of the tooth prevents the apical root resorption which accompanies the use of intrusive forces in teeth of limited growth and which interferes with interpretation of the experimental data.” Materials and methods
Twenty-one adult female albino rats of the Wistar strain, with a mean weight of 250 grams, were used. The animals were fed a standard laboratory diet of Purina Chow* and were housed in standard metal cages. Crystalline penicillin G, 25,000 units, was injected three times weekly to prevent respiratory disease, to which these animals are susceptible. The orthodontic appliance consisted of a 0.0056 by 0.022 inch Elgiloy closed coil spring,? the force-extension curve of which, under loads applied in the current study, is a *Ralston Purina Company, St. Louis, MO. SRocky Mountain Dental Products Company,
Denver,
Colo.
Volume 80 Number
Experimental
4
31
10
20
30,
intrusion
40
of rat
incisors
431
50 LOAD.gm
Fig. 2. Calibrated curve of the load-length relationship of the Elgiloy closed spring used in the experiment.
straight line.27 The amount of force generated by activation of the spring was precalculated by measuring the extension length under loads ranging from 0 to 50 Gm. and by plotting a force-extension curve (Fig. 2). It was found that 1 percent extension from the initial length of the spring corresponds to a load increase of 0.96 Gm. The animals were anesthetized with an intraperitoneal injection of 10 percent chloral hydrate and a twenty-two-coil spring (length 3.36 mm.) was attached between the mandibular left first molar and left incisor by means of 0.010 inch stainless steel ligatures (Fig. 1). Anteriorly, the ligature was tied around the incisor in a shallow groove cut into the buccal and distal surfaces, close to the gingival margin. The distal ligature was passed interproximally between the first and second molars and ligated around the cervical area of the first molar. The force produced by this device depends upon the length of initial spring extension. In all animals, the lower left incisor was shortened out of occlusion by means of a carborundum disk to prevent the influence of functional occlusal loads. In four control animals this procedure was repeated every other day to ensure normal unimpeded eruption of the tooth. The remaining seventeen animals were divided into three groups. In the lirst group (five animals), the shortened left incisor was subjected to light forces from springs that produced loads of 2 to 10 Gm. In the second group (nine animals) the springs generated loads of 14 to 23 Gm., and in the third group (three animals), the springs created heavy forces ranging between 38 and 40 Gm. Immediately after insertion, the length of the tied-in spring was measured intraorally with fine calipers connected to a digital voltmeter and the amount of applied force was computed using the precalculated force-extension curve. In addition, dorsoventral radiographs of the mandible were taken in the still-anesthetized animals to ascertain the spring length measurements. All measurements were repeated at the end of the experiment. No mesial movement of the five-rooted tirst molar could be detected. The length of the springs had remained unaltered, indicating a relatively constant load throughout the entire experimental period, a finding in agreement with that of Heller and Nanda.8
Table I.
Mean
body
weight
of the animals - .__.,
Day ofexperimmt
Control
251 245 243 235 236 239
2
4 6 8 10 12
Table
II. Average Control
Day of experimerit
increment
Normal
2,060 2,140
6 8 10 12 Total increment
2,240 2,320 2,310 2,220 13,290
t t t -t i +
Light
--..-__.
(fytrms)
hpermmtal
9.x7 X.2X 9.4X 11.x7 12.65 11.7X
of eruption/intrusion
group
unimais
252 245 239 236 236 234
t f t i ir ‘-
(grwn.s)
1.55 6.8X 6.X0 6.X7 6.7X 6.61
per 48 hours
loads
Medium
loads
Heabfy
loads
un-
impeded eruption (pm)
2 4
animals
+ 80 2 135 k t 2 2 ?
160 190 190 180 42
Eruption
Intrusion Intrusion
(l-m
806 i- 185 474 + 139 366 350 173 173 2,342
k 2 2 2 +
54 52 101 34 98
Eruption
-
-
-
-
-
Eruption
(W)
223t123 2260 52 57 37 371
2 -+ ” lr
Intrusion
(vunl
(vuni
303+33 297 2 58
-
187 i- 1x7 43 13 29 34
787 t
38
13 147 30 190
t -+ lr 2
73 77 55 43
The rate of eruption/intrusion was measured by the method of Michaeli and Weinreb.13 The exposed enamel surface of the incisors was marked near the gingival line with a fine carborundum disk. The migration of this notch from the gingival line indicated quantitatively the rate of eruption of the tooth, while the decrease in distance between the notch and the gingival margin designated its rate of intrusion. The measurements were performed, with the animals under ether anesthesia, at noon of every second day for 2 weeks. At the end of the experiment, the animals were decapitated. Each mandible was cut midsagittally, and lateral radiographs of the left side were taken with the spring in situ. A parallelogram of forces was drawn, and the magnitude of the load component acting in the axial direction was calculated, showing that the mean axial intrusive force equaled 0.707 + 0.058 Gm. for each gram of applied load. The root walls of the rat mandibular incisor are tubiform, with cross sections of the root being virtually the same size from gingival margin to apex. The circumference and root length of the teeth were measured, the total root surface was calculated, and the forces applied were expressed in units of grams per square centimeter of root surface. Results The animals withstood the experimental procedures well and appeared healthy. The small appliance running along the diastema between the incisor molar was tolerated satisfactorily, and none of the animals showed signs of mucosa or tongue. Nor did the spring interfere unduly with feeding activities.
normal and and the first irritation of The weight
Volume 80 Number 4
Experimental
intrusion
of rat incisors
433
I 2
4
6
6
10
DAYS
12
Fig. 3. Forty-eight-hour increments of eruption/intrusion of the lower left incisors (mean f standard error). Control group: Uppermost line with open circles. Experimental group: Triangles = light loads; squares = medium loads; closed circles = heavy loads.
of the animals in both control and experimental groups remained relatively constant throughout the experimental period (Table I). The mean total surface of the bone-embedded part of the incisor was 1.24 cm.2. Accordingly, the loads applied in the experimental groups were in the range of 1.5 to 8.0 Gm./cm.2 for light forces, 12.0 to 18.5 Gm./cm.* for medium forces, and 30.5 to 32.0 Gm./cm.2 for heavy forces. The mean rates of eruption/intrusion for the control and the three experimental groups are demonstrated graphically in Fig. 3, and the amounts of active intrusion are tabulated in Table II. In the control group, the lower left incisor, unimpeded by occlusal forces, erupted continuously at a steady rate throughout the experimental period, with a normal mean increment of 2215 pm every 48 hours. Light loads (1.5 to 8.0 Gm./cm.2) restrained eruption but did not cause active intrusion of the incisors. The initial reaction of the teeth to medium forces (12.0 to 18.5 Gm./cm.2) was expressed in a considerable degree of active intrusion, followed by a short
434
.S/ci,qrntrt7
uud
Mic~llrrrli
“rest” period during which almost no movement of the incisors occurred. In the second week of the experiment, the intrusive movement started again with a constant daily rate of about 25 Frn. Active intrusion in cases of heavy forces (30.5 to 32.0 Gm./cm.‘) started only after 8 days of continuous application. Although this apical movement was of considerable magnitude, it was of short duration and decreased again toward the end of the experiment. Discussion The rate of orthodontic tooth movement is influenced by various aspects of the acting force, which includes its type, direction, duration, and magnitude. An optimal force is of such magnitude as to move the tooth in the desired direction at the most rapid possible rate, causing minimal tissue damage. Schwarz 24 defined the optimal continuous force as that approximating the capillary vessels’ blood pressure, thus preventing their occlusion in the compressed periodontal ligament. According to Schwarz, forces well below the optimal level cause no reaction in the periodontal ligament, while forces of high magnitude lead to ischemia and necrosis of the ligament. The resulting areas of ligament and bone necrosis prevent frontal bone resorption and tooth movement and delay it until the undermining resorption eliminates the necrotic tissue obstacle. 1x-90.26 Schwarz’s definition was slightly modified by Oppenheim,‘” who advocated the “use of the lightest force at long intervals’ ’ as capable of bringing about tooth movement, and by Reitan,‘” who demonstrated cell-free compressed areas within the pressure site in cases where even light forces were applied. Although the above-mentioned classic concept of optimal force based upon blood pressure level was later disputed, 1,9 2fi it still constitutes a good reference point when one wishes to analyze the force magnitude. The results of the present study show that the relationship between the rate of the apical tooth movement and the different intrusive loads is in many aspects similar to the response of tooth movement obtained by the application of various lateral forces. 15,1X-2”~24~2H The mean systolic blood pressure of vessels entering the apical region of the periodontal ligament in adult female rats is about 16 Gm. per square centimeter.l* Light forces of less than half this value did not induce intrusion. The relatively short duration of our experiment did not provide the opportunity to examine the influence of long-term light forces. The steady decline of the continuous tooth eruption (Fig. 3) leads one to speculate that, had the light forces been applied for a longer period, active intrusion of the incisors could have been finally achieved. However, normal continuous eruption of rodent incisors depends upon the integrity of periodontal ligament. l4 It is therefore possible that the vascular, cellular, and interstitial changes”-“” which occur in the ligament, even when light forces are applied, impaired the normal potential of continuous eruption. Heavy forces of about twice the value of the systolic blood pressure level induced active intrusion after only 8 days of continuous application, which is similar to the findings of Storeyz6 for lateral movements of guinea pig upper incisors. Toward the end of the experimental period, the apical movement of the incisors slowed down again and the total amount of intrusion under heavy loads was much less (190 pm) than that achieved under optimal medium loads (37 1 pm). Here, too, it may be assumed that a more prolonged application of heavy loads would have resulted in more similar values of intrusion for both groups. However, it has been shown that extended application of heavy forces is detrimental to tooth tissues.6. 25
Volume 80 Number 4
Experimental
intrusion
of rat incisors
435
The medium loads, approximating the systolic blood pressure level, proved to be optimal for intrusive movements of rat incisors. The rat alveolar bone reacts rapidly to pressure, and extensive bone resorption was shown after 12 hours of application of moderate lateral loads.16 It is therefore plausible that the extensive intrusion after the first 48 hours was caused by active bone resorption as well as by initial ligament and bone compression. 7 The subsequent “rest” period (Fig. 3) corresponds to identical delays found by Reitanlg upon application of optimal lateral forces. Furthermore, as in laterally applied loads,lg the teeth resumed their apical movement following the rest period, the uniform daily rate of intrusion indicating a constant frontal bone resorption. The rat incisor differs from the human tooth in many morphologic and physiologic aspects. However, comparison of the experimental results obtained with continuously erupting teeth23 3 with those found in human teeth17 suggests that intrusive loading produces similar responses in both systems. It is proposed, therefore, that the value of the optimal load to be used in the intrusion of any given tooth should be calculated according to the size of the root surface and the systolic blood pressure. The range of optimal forces as estimated by clinical experiences supports this proposition. Summary
Although there exists general information on intrusion of teeth, the influence of the force magnitude upon the behavior of the intruded tooth is still obscure. In the present study an attempt was made to examine the relationship between defined continuous intrusive loads of varying magnitude and the resulting rate of intrusion of rat incisors. In seventeen rats occlusion of the left mandibular incisor was eliminated and the teeth were subjected to direct light, medium, and heavy intrusive loads for a period of 12 consecutive days. The axially directed force was generated by a stretched closed coil spring running parallel to the bone-embedded part of the tooth. The increments in eruption/intrusion were measured every second day. While light loads ( 1.5 to 8 .O Gm. /cm. *) failed to cause any active intrusion, the heavy loads (30.5 to 32.0 Gm./cm.*) elicited intrusive movement that started only 8 days after force application. The medium loads (12.0 to 18.5 Gm./cm.2), having a magnitude in the range of rat systolic blood pressure (16 Gm./cm.*), caused the largest, quickest, and most uniform amount of intrusion. It is suggested that the latter loads are optimal for intrusive tooth movement. REFERENCES 1. Baumrind, 1969. 2. Bien, S. 1965. 3. Bien, S. 1966. 4. Bondevik, J. Ortbod. 5. Burstone, 6. Delhnger, monkey, 7. Grimm, 1972.
S.: Reconsideration M.,
and Ayers,
M.:
Fluid
of propriety
H. D.: Responses
dynamic
mechanisms
of “pressure-tension” of rat maxillary which
regulate
hypothesis, incisors
tooth
to loads,
movement,
AM. .I. ORTHOD. J. Dent. Adv.
Oral
55: 12-22,
Res. 44: 517-520, Biol.
2: 173-201,
0.: Tissue changes in the rat molar periodontium following application of intrusive forces, Eur. 2: 41-49, 1980. C. R.: Deep overbite correction by intrusion, AM. J. ORTHOD. 72: l-22, 1977. E. L.: A histologic and cephalometric investigation of premolar intrusion in theMacacu speciosa AM. J. ORTHOD. 53: 325-354, 1967. F. M.: Bone bending, a feature of orthodontic tooth movement, AM. J. ORTHOD. 62: 384-393,
8. Heller. J. J., and Nanda. R.: Effect of metabolic alteration ot periodontal fibers on orthodontic tooth movement, AM. J. ORTHOD. 75: 230-258, 1979 9. Hixon, E. H., Atikian. H., Callow, G. E., McDonald, H. W.. and Tacy, R. J.: Optimal force. differential force, and anchorage. AM. J. OKTHOD. 55: 437-457, 1969. 10. Kvam, E.: Cellular dynamics on the pressure side of the rat periodontium following experimental tooth movement, Stand. J. Dent. Res. 80: 369.383. 1972. Il. Lefkowitz, W., and Waugh, L. M.: Experimental depression of teeth, AM. J. ORTHOD. 31: 21-36, 1945. 12. Main, J. H. O., and Adams, D.: Experiments on the rat incisor into the cellular proliferation and blood pressure theories of tooth eruption, Arch. Oral Biol. 11: 163-178, 1966. 13. Michaeli, Y., and Weinreb, M. M.: Role of attrition and occlusal contact in the physiology of the rat incisor. III. Prevention of attrition and occlusal contact in the non-articulating incisor, J. Dent. Res. 47: 633-640, 1968. 14. Michaeli, Y., Pitaru, S., Zajicek. G., and Weinreb, M. M.: Role of attrition and occlusal contact in the physiology of the rat incisor. IX. Impeded and unimpeded eruption in lathyritic rats. J. Dent. Res. 54: 891-896, 1975. 15. Oppenheim, A.: Human tissue response to orthodontic intervention of short and long duration, AM. J. ORTHOD.
ORAL
SURG.
28: 263-301,
1942.
16. Otero, R. L., Parodi, R. J., Ubios, A. M.. Carranza, F. study of bone resorption after tooth movement in rats, 17. Parfitt, G. J.: Measurement of the physiological mobility Res. 39: 608-618, 1960. 18. Reitan, K.: Continuous bodily tooth movement and its 115-144, 1947. 19. Reitan, K.: Some factors determining the evaluation of
A., and Cabrini, R. L.: Histologic and histometric J. Periodont. Res. 8: 327-333, 1973. of individual teeth in an axial direction, J. Dent. histological force
significance,
in orthodontics,
Acta Odontol. AM. J. ORTHOD.
Stand.
7:
43: 32-45,
1957.
20. Reitan, AM.
K.: Clinical
J. ORTHOD.
and histologic 53: 721-745,
observations
on tooth movement
during
and after orthodontic
treatment,
1967.
21, Rygh, P.: Ultrastructural vascular changes in pressure zones of rat molar periodontium incident to orthodontic movement, Stand. J. Dent. Res. 80: 307-321, 1972. 22. Rygh, P.: Ultrastructural cellular reactions in pressure zones of rat molar periodontium incident to orthodontic tooth movement, Acta Odontol. Stand. 30: 575-593, 1972. 23. Rygh, P.: Ultrastructural changes of the periodontal fibers and their attachment in rat molar periodontium incident to orthodontic tooth movement, Stand. J. Dent. Res. 81: 467-480, 1973. 24. Schwartz, A. M.: Tissue changes incident to orthodontic tooth movement, INT. J. ORTHOD. 18: 331-352, 1932. 25. Stenvik, A., and MjGr, J. A.: Pulp and dentine reactions to experimental tooth intrusion: A histologic study of the initial changes, AM. J. ORTHOD. 57: 370-384, 1970. 26. Storey, E.: The nature of tooth movement, AM. J. ORTHOD. 63: 292-314, 1973. 27. Webb, R. I., Caputo, A. A., and Chaconas, S. J.: Orthodontic force production by closed coil sptigs, AM. J. ORTHOD.
74: 405-409,
1978.
Key words: Intrusion, rat incisor, coiled springs, optimal force
T
he dispute about the possibility of an active intrusion of teeth is by now a thing of the past. Lefkowitz and Waugh,” Dellinger,‘j Stenvik and Mjiir,25 and others have shown that the application of depressing forces causes tissue changes which result in an apical displacement of a tooth into the alveolus. The various orthodontic movements have received extensive attention with regard to the influence of the magnitude and duration of applied forces. The only exception is the intrusive movement, the information about its relationship to the applied forces remaining scanty, while the various load denominations used by the different investigators obscure the issue. Burstone advocates the use of approximately 25 Gm. of continuous force for intrusion of human upper incisors and recommends forces of different magnitudes for each of the anterior teeth, based upon clinical experience. Bondevik,4 applying intrusive forces of 0.29 N, 0.49 N, and 0.98 N on rat occluding molars, did not find essential differences in bone and periodontal tissue reaction of teeth subjected to different force magnitudes. However, the magnitude of the initially applied forces had been considerably reduced in the course of the experiment. Dellinger,6 in a study carried out on four monkeys, demonstrated that after force application for 60 days the greatest amount of intrusion (2.7 mm., together with other anteroposterior changes) occurred upon initial application of a load of From the Department University-Hadassah 0002-9416/81/100429+08$00.80/0
of Orthodontics and Division School of Dental Medicine. 0
1981 The C. V. Mosby Co.
of Anatomy
and Embryology,
The
Hebrew
429
Fig. 1. Schematic drawing of the experimental appliance.
50 Gm. With loads of higher magnitude, the final amount of intrusion proved to be less and root resorption was severe. These findings express the final results of extended application of different loads but do not reflect the consecutive rate of the intrusive movements. The relationship between the rate of the lateral movement of teeth and the magnitude of the applied forces was studied in guinea pig upper incisors.26 While a force in the optimal range produced steady motion of a tooth, heavy forces induced disruptive tooth movements characterized by intermittent periods of cessation and rapid movement. Although ultimately there is little difference in the total distance traveled by a tooth subjected to either light or heavy forces, the use of the latter is always accompanied by undesirable tissue changes. The aim of the present study was to observe continuously the behavior of teeth under different constant intrusive loads and to determine whether the relationship that exists between the magnitude of loads and the rate of lateral tooth movement also applies to intrusive movements. The investigation was carried out on rat lower incisors. These cylindrically shaped teeth consist of a part which lies in the oral cavity (the “clinical crown”) and of a long “root” which is situated beneath the molar roots, almost parallel to the lower alveolar ridge (Fig. 1). Rat incisors undergo continuous attrition at their incisal end, which is counterbalanced by steady eruption and growth at the basal region. The main advantage of this model lies in its quick response to intrusive forces as expressed by the easily measured length of the “clinical crown. ” Second, the proliferative region of the tooth prevents the apical root resorption which accompanies the use of intrusive forces in teeth of limited growth and which interferes with interpretation of the experimental data.” Materials and methods
Twenty-one adult female albino rats of the Wistar strain, with a mean weight of 250 grams, were used. The animals were fed a standard laboratory diet of Purina Chow* and were housed in standard metal cages. Crystalline penicillin G, 25,000 units, was injected three times weekly to prevent respiratory disease, to which these animals are susceptible. The orthodontic appliance consisted of a 0.0056 by 0.022 inch Elgiloy closed coil spring,? the force-extension curve of which, under loads applied in the current study, is a *Ralston Purina Company, St. Louis, MO. SRocky Mountain Dental Products Company,
Denver,
Colo.
Volume 80 Number
Experimental
4
31
10
20
30,
intrusion
40
of rat
incisors
431
50 LOAD.gm
Fig. 2. Calibrated curve of the load-length relationship of the Elgiloy closed spring used in the experiment.
straight line.27 The amount of force generated by activation of the spring was precalculated by measuring the extension length under loads ranging from 0 to 50 Gm. and by plotting a force-extension curve (Fig. 2). It was found that 1 percent extension from the initial length of the spring corresponds to a load increase of 0.96 Gm. The animals were anesthetized with an intraperitoneal injection of 10 percent chloral hydrate and a twenty-two-coil spring (length 3.36 mm.) was attached between the mandibular left first molar and left incisor by means of 0.010 inch stainless steel ligatures (Fig. 1). Anteriorly, the ligature was tied around the incisor in a shallow groove cut into the buccal and distal surfaces, close to the gingival margin. The distal ligature was passed interproximally between the first and second molars and ligated around the cervical area of the first molar. The force produced by this device depends upon the length of initial spring extension. In all animals, the lower left incisor was shortened out of occlusion by means of a carborundum disk to prevent the influence of functional occlusal loads. In four control animals this procedure was repeated every other day to ensure normal unimpeded eruption of the tooth. The remaining seventeen animals were divided into three groups. In the lirst group (five animals), the shortened left incisor was subjected to light forces from springs that produced loads of 2 to 10 Gm. In the second group (nine animals) the springs generated loads of 14 to 23 Gm., and in the third group (three animals), the springs created heavy forces ranging between 38 and 40 Gm. Immediately after insertion, the length of the tied-in spring was measured intraorally with fine calipers connected to a digital voltmeter and the amount of applied force was computed using the precalculated force-extension curve. In addition, dorsoventral radiographs of the mandible were taken in the still-anesthetized animals to ascertain the spring length measurements. All measurements were repeated at the end of the experiment. No mesial movement of the five-rooted tirst molar could be detected. The length of the springs had remained unaltered, indicating a relatively constant load throughout the entire experimental period, a finding in agreement with that of Heller and Nanda.8
Table I.
Mean
body
weight
of the animals - .__.,
Day ofexperimmt
Control
251 245 243 235 236 239
2
4 6 8 10 12
Table
II. Average Control
Day of experimerit
increment
Normal
2,060 2,140
6 8 10 12 Total increment
2,240 2,320 2,310 2,220 13,290
t t t -t i +
Light
--..-__.
(fytrms)
hpermmtal
9.x7 X.2X 9.4X 11.x7 12.65 11.7X
of eruption/intrusion
group
unimais
252 245 239 236 236 234
t f t i ir ‘-
(grwn.s)
1.55 6.8X 6.X0 6.X7 6.7X 6.61
per 48 hours
loads
Medium
loads
Heabfy
loads
un-
impeded eruption (pm)
2 4
animals
+ 80 2 135 k t 2 2 ?
160 190 190 180 42
Eruption
Intrusion Intrusion
(l-m
806 i- 185 474 + 139 366 350 173 173 2,342
k 2 2 2 +
54 52 101 34 98
Eruption
-
-
-
-
-
Eruption
(W)
223t123 2260 52 57 37 371
2 -+ ” lr
Intrusion
(vunl
(vuni
303+33 297 2 58
-
187 i- 1x7 43 13 29 34
787 t
38
13 147 30 190
t -+ lr 2
73 77 55 43
The rate of eruption/intrusion was measured by the method of Michaeli and Weinreb.13 The exposed enamel surface of the incisors was marked near the gingival line with a fine carborundum disk. The migration of this notch from the gingival line indicated quantitatively the rate of eruption of the tooth, while the decrease in distance between the notch and the gingival margin designated its rate of intrusion. The measurements were performed, with the animals under ether anesthesia, at noon of every second day for 2 weeks. At the end of the experiment, the animals were decapitated. Each mandible was cut midsagittally, and lateral radiographs of the left side were taken with the spring in situ. A parallelogram of forces was drawn, and the magnitude of the load component acting in the axial direction was calculated, showing that the mean axial intrusive force equaled 0.707 + 0.058 Gm. for each gram of applied load. The root walls of the rat mandibular incisor are tubiform, with cross sections of the root being virtually the same size from gingival margin to apex. The circumference and root length of the teeth were measured, the total root surface was calculated, and the forces applied were expressed in units of grams per square centimeter of root surface. Results The animals withstood the experimental procedures well and appeared healthy. The small appliance running along the diastema between the incisor molar was tolerated satisfactorily, and none of the animals showed signs of mucosa or tongue. Nor did the spring interfere unduly with feeding activities.
normal and and the first irritation of The weight
Volume 80 Number 4
Experimental
intrusion
of rat incisors
433
I 2
4
6
6
10
DAYS
12
Fig. 3. Forty-eight-hour increments of eruption/intrusion of the lower left incisors (mean f standard error). Control group: Uppermost line with open circles. Experimental group: Triangles = light loads; squares = medium loads; closed circles = heavy loads.
of the animals in both control and experimental groups remained relatively constant throughout the experimental period (Table I). The mean total surface of the bone-embedded part of the incisor was 1.24 cm.2. Accordingly, the loads applied in the experimental groups were in the range of 1.5 to 8.0 Gm./cm.2 for light forces, 12.0 to 18.5 Gm./cm.* for medium forces, and 30.5 to 32.0 Gm./cm.2 for heavy forces. The mean rates of eruption/intrusion for the control and the three experimental groups are demonstrated graphically in Fig. 3, and the amounts of active intrusion are tabulated in Table II. In the control group, the lower left incisor, unimpeded by occlusal forces, erupted continuously at a steady rate throughout the experimental period, with a normal mean increment of 2215 pm every 48 hours. Light loads (1.5 to 8.0 Gm./cm.2) restrained eruption but did not cause active intrusion of the incisors. The initial reaction of the teeth to medium forces (12.0 to 18.5 Gm./cm.2) was expressed in a considerable degree of active intrusion, followed by a short
434
.S/ci,qrntrt7
uud
Mic~llrrrli
“rest” period during which almost no movement of the incisors occurred. In the second week of the experiment, the intrusive movement started again with a constant daily rate of about 25 Frn. Active intrusion in cases of heavy forces (30.5 to 32.0 Gm./cm.‘) started only after 8 days of continuous application. Although this apical movement was of considerable magnitude, it was of short duration and decreased again toward the end of the experiment. Discussion The rate of orthodontic tooth movement is influenced by various aspects of the acting force, which includes its type, direction, duration, and magnitude. An optimal force is of such magnitude as to move the tooth in the desired direction at the most rapid possible rate, causing minimal tissue damage. Schwarz 24 defined the optimal continuous force as that approximating the capillary vessels’ blood pressure, thus preventing their occlusion in the compressed periodontal ligament. According to Schwarz, forces well below the optimal level cause no reaction in the periodontal ligament, while forces of high magnitude lead to ischemia and necrosis of the ligament. The resulting areas of ligament and bone necrosis prevent frontal bone resorption and tooth movement and delay it until the undermining resorption eliminates the necrotic tissue obstacle. 1x-90.26 Schwarz’s definition was slightly modified by Oppenheim,‘” who advocated the “use of the lightest force at long intervals’ ’ as capable of bringing about tooth movement, and by Reitan,‘” who demonstrated cell-free compressed areas within the pressure site in cases where even light forces were applied. Although the above-mentioned classic concept of optimal force based upon blood pressure level was later disputed, 1,9 2fi it still constitutes a good reference point when one wishes to analyze the force magnitude. The results of the present study show that the relationship between the rate of the apical tooth movement and the different intrusive loads is in many aspects similar to the response of tooth movement obtained by the application of various lateral forces. 15,1X-2”~24~2H The mean systolic blood pressure of vessels entering the apical region of the periodontal ligament in adult female rats is about 16 Gm. per square centimeter.l* Light forces of less than half this value did not induce intrusion. The relatively short duration of our experiment did not provide the opportunity to examine the influence of long-term light forces. The steady decline of the continuous tooth eruption (Fig. 3) leads one to speculate that, had the light forces been applied for a longer period, active intrusion of the incisors could have been finally achieved. However, normal continuous eruption of rodent incisors depends upon the integrity of periodontal ligament. l4 It is therefore possible that the vascular, cellular, and interstitial changes”-“” which occur in the ligament, even when light forces are applied, impaired the normal potential of continuous eruption. Heavy forces of about twice the value of the systolic blood pressure level induced active intrusion after only 8 days of continuous application, which is similar to the findings of Storeyz6 for lateral movements of guinea pig upper incisors. Toward the end of the experimental period, the apical movement of the incisors slowed down again and the total amount of intrusion under heavy loads was much less (190 pm) than that achieved under optimal medium loads (37 1 pm). Here, too, it may be assumed that a more prolonged application of heavy loads would have resulted in more similar values of intrusion for both groups. However, it has been shown that extended application of heavy forces is detrimental to tooth tissues.6. 25
Volume 80 Number 4
Experimental
intrusion
of rat incisors
435
The medium loads, approximating the systolic blood pressure level, proved to be optimal for intrusive movements of rat incisors. The rat alveolar bone reacts rapidly to pressure, and extensive bone resorption was shown after 12 hours of application of moderate lateral loads.16 It is therefore plausible that the extensive intrusion after the first 48 hours was caused by active bone resorption as well as by initial ligament and bone compression. 7 The subsequent “rest” period (Fig. 3) corresponds to identical delays found by Reitanlg upon application of optimal lateral forces. Furthermore, as in laterally applied loads,lg the teeth resumed their apical movement following the rest period, the uniform daily rate of intrusion indicating a constant frontal bone resorption. The rat incisor differs from the human tooth in many morphologic and physiologic aspects. However, comparison of the experimental results obtained with continuously erupting teeth23 3 with those found in human teeth17 suggests that intrusive loading produces similar responses in both systems. It is proposed, therefore, that the value of the optimal load to be used in the intrusion of any given tooth should be calculated according to the size of the root surface and the systolic blood pressure. The range of optimal forces as estimated by clinical experiences supports this proposition. Summary
Although there exists general information on intrusion of teeth, the influence of the force magnitude upon the behavior of the intruded tooth is still obscure. In the present study an attempt was made to examine the relationship between defined continuous intrusive loads of varying magnitude and the resulting rate of intrusion of rat incisors. In seventeen rats occlusion of the left mandibular incisor was eliminated and the teeth were subjected to direct light, medium, and heavy intrusive loads for a period of 12 consecutive days. The axially directed force was generated by a stretched closed coil spring running parallel to the bone-embedded part of the tooth. The increments in eruption/intrusion were measured every second day. While light loads ( 1.5 to 8 .O Gm. /cm. *) failed to cause any active intrusion, the heavy loads (30.5 to 32.0 Gm./cm.*) elicited intrusive movement that started only 8 days after force application. The medium loads (12.0 to 18.5 Gm./cm.2), having a magnitude in the range of rat systolic blood pressure (16 Gm./cm.*), caused the largest, quickest, and most uniform amount of intrusion. It is suggested that the latter loads are optimal for intrusive tooth movement. REFERENCES 1. Baumrind, 1969. 2. Bien, S. 1965. 3. Bien, S. 1966. 4. Bondevik, J. Ortbod. 5. Burstone, 6. Delhnger, monkey, 7. Grimm, 1972.
S.: Reconsideration M.,
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8. Heller. J. J., and Nanda. R.: Effect of metabolic alteration ot periodontal fibers on orthodontic tooth movement, AM. J. ORTHOD. 75: 230-258, 1979 9. Hixon, E. H., Atikian. H., Callow, G. E., McDonald, H. W.. and Tacy, R. J.: Optimal force. differential force, and anchorage. AM. J. OKTHOD. 55: 437-457, 1969. 10. Kvam, E.: Cellular dynamics on the pressure side of the rat periodontium following experimental tooth movement, Stand. J. Dent. Res. 80: 369.383. 1972. Il. Lefkowitz, W., and Waugh, L. M.: Experimental depression of teeth, AM. J. ORTHOD. 31: 21-36, 1945. 12. Main, J. H. O., and Adams, D.: Experiments on the rat incisor into the cellular proliferation and blood pressure theories of tooth eruption, Arch. Oral Biol. 11: 163-178, 1966. 13. Michaeli, Y., and Weinreb, M. M.: Role of attrition and occlusal contact in the physiology of the rat incisor. III. Prevention of attrition and occlusal contact in the non-articulating incisor, J. Dent. Res. 47: 633-640, 1968. 14. Michaeli, Y., Pitaru, S., Zajicek. G., and Weinreb, M. M.: Role of attrition and occlusal contact in the physiology of the rat incisor. IX. Impeded and unimpeded eruption in lathyritic rats. J. Dent. Res. 54: 891-896, 1975. 15. Oppenheim, A.: Human tissue response to orthodontic intervention of short and long duration, AM. J. ORTHOD.
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16. Otero, R. L., Parodi, R. J., Ubios, A. M.. Carranza, F. study of bone resorption after tooth movement in rats, 17. Parfitt, G. J.: Measurement of the physiological mobility Res. 39: 608-618, 1960. 18. Reitan, K.: Continuous bodily tooth movement and its 115-144, 1947. 19. Reitan, K.: Some factors determining the evaluation of
A., and Cabrini, R. L.: Histologic and histometric J. Periodont. Res. 8: 327-333, 1973. of individual teeth in an axial direction, J. Dent. histological force
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21, Rygh, P.: Ultrastructural vascular changes in pressure zones of rat molar periodontium incident to orthodontic movement, Stand. J. Dent. Res. 80: 307-321, 1972. 22. Rygh, P.: Ultrastructural cellular reactions in pressure zones of rat molar periodontium incident to orthodontic tooth movement, Acta Odontol. Stand. 30: 575-593, 1972. 23. Rygh, P.: Ultrastructural changes of the periodontal fibers and their attachment in rat molar periodontium incident to orthodontic tooth movement, Stand. J. Dent. Res. 81: 467-480, 1973. 24. Schwartz, A. M.: Tissue changes incident to orthodontic tooth movement, INT. J. ORTHOD. 18: 331-352, 1932. 25. Stenvik, A., and MjGr, J. A.: Pulp and dentine reactions to experimental tooth intrusion: A histologic study of the initial changes, AM. J. ORTHOD. 57: 370-384, 1970. 26. Storey, E.: The nature of tooth movement, AM. J. ORTHOD. 63: 292-314, 1973. 27. Webb, R. I., Caputo, A. A., and Chaconas, S. J.: Orthodontic force production by closed coil sptigs, AM. J. ORTHOD.
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