S. Steigman

A Three-dimensional Evaluation of the Effects of Functional Occlusal Forces on the Morphology of Dental and Periodontal Tissues of the Rat Incisor

We examined the effect of function on tooth and periodontal ligament (PDL) morphology in 40 lower incisors of adult female rats. Ten teeth were exposed to occlusal hyperfunction for three months, ten to hypofunction for three weeks, ten to hypofunction for three months, while ten teeth in normal occlusion served as control. Transverse ground sections were cut at various levels perpendicular to the tooth long axis, and their distances from the apex were calculated. The outlines of the tissues were traced and fed into a computer. We plotted the measurements according to their location and fitted them by second-order polynomials. We calculated tissue volume for the proximal 18 mm of bone-embedded tooth. Hyperfunction affected tooth shape, in that it became more rounded. The volumes of the dental tissues remained unchanged, while width and volume of the cementum-bordering PDL increased. Hypofunction did not alter tooth shape, but influenced its size: After three weeks, tooth circumference decreased, and after three months, it expanded. Dentin width was reduced, with concomitant increase of pulp size. The amount of enamel diminished initially, but after three months returned to normal values. The PDL bordering enamel expanded proportionally to the duration of hypofunction. The changes in socket size reflected the total dimensional variations in the tooth and its PDL. The results demonstrate that the shape and size of growing teeth and their periodontium are influenced by functional occlusal forces.

Histomorphometric Study of the Periodontal Vasculature of the Rat Incisor

This study assessed quantitatively the vascular system in the cementum-related periodontal ligament (PDL) along the rat incisor. The lower left incisors of six rats (± 200 g) were subjected to routine histological procedures and cross-sectioned serially (2 μm), and the distance between each section and the apex was computed. The PDL of five sections at different levels along the tooth was divided into mesial, lingual, and lateral parts. The number and area of small and terminal arterioles, capillaries (C), sinusoids (S), post-capillary venules (PCV), and connecting venules, as well as the area of the PDL, were established. Blood vessels (BV) occupied 47 ± 2% of the PDL area in the apical half and 4 ± 2% at the incisal end. Of the total BV area, 41%, 32%, and 27% were located on the lingual, mesial, and lateral tooth sides, respectively. The majority of BV belonged to the venous system (98.5 ± 0.6% and 82.5 ± 3.0% in the apical and incisal parts, respectively). The apical venous system comprised 95.4 ± 1.6% S and 3.2 ± 1.0% PCV, reversing to 27.2 ± 14.2% S and 55.2 ± 11.3% PCV in the incisal half. The number of arterial profiles increased gradually from 6.8 ± 1.5 at the apex to 25.3±2.4 in the incisal part and that of C from 9.0 ± 1.18 to 25.0 ± 4.3. The extensive vascularization in the apical half of the PDL is consistent with the high metabolic demands and with the need for protective cushioning of the constantly growing dental and periodontal tissues. The paucity of blood supply and the presence of numerous small BVs in the incisal end equate with the metabolic needs of the highly organized supporting tissue in this region.

Experimental intrusion of rat incisors with continuous loads of varying magnitude

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./cm.2) did not cause active intrusion of the teeth. Medium loads (12.0 to 18.5 Gm./cm.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 micrometers. Heavy loads (30.5 to 32.0 Gm./cm.2) 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.

The influence of calibrated loads upon the rate of eruption of mandibular rat incisors

Abstract A method for the application of constant intrusive loads on the rat mandibular incisor is described. In 20 rats, occlusion of the left incisor was eliminated and the tooth was subjected to direct light, medium and heavy intrusive loads. Light loads (1.8–3.2 g) reduced eruption by 34 per cent and medium loads (4.9–7.4 g) by 61 per cent on the first day of application. On the following days, the eruption decreased steadily and ceased almost entirely after 2 weeks. Heavier axial loads stopped eruption immediately. The eruption rate of the contralateral incisor was slightly retarded in all experimental animals.

Recovery of the dental and periodontal tissues of the rat incisor following application of continuous intrusive loads: A long-term study☆

The present investigation deals with the functional recovery of the rat incisor periodontium following application of optimal intrusive loads for a period of 2 weeks and addresses the relationship between function and integrity of dental and periodontal tissues. The daily rate of eruption served as an indicator of the functional capacity of the rat periodontal ligament (PDL), while the condition of the PDL was evaluated by means of serial histologic sections. The left mandibular incisors of thirteen experimental and seven control rats were shortened to prevent occlusion, and calibrated loads of 19 +/- 1.6 grams were applied to the teeth of the experimental animals. Following removal of the springs, the daily rate of eruption of all teeth was measured during a recovery period of 80 days. In all experimental animals, prolonged impairment of the eruptive potential had occurred: In 50% the rate of eruption was fully restored 8 weeks after removal of the loads, and in these animals the histologic evaluation showed an almost normal PDL. In the remaining 50% gross impediment in the eruption rate persisted throughout the entire experimental period. The intratissue forces operating during the recovery period had sufficient strength to cause a degree of periodontal injury equivalent to that ensuant on direct loading. Extensive newly developed necrotic lesions were evident in the initially stretched mesial PDL. Areas of dentin resorption, denuded cementum, and injured enamel appeared a considerable time after load removal. Periodontal injury included disorientation and loss of attachment of collagen fibers, presence of scarlike tissue, and infiltration of inflammatory cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effect of intrusive loads of varying magnitude upon the eruptive potential of rat incisors

The present investigation deals with the functional recovery of the periodontium following application of intrusive loads of varying magnitudes to continuously growing rat incisors. The daily rate of eruption of these teeth served as an indicator of the functional capacity of their periodontal ligament. The left mandibular incisor was shortened to prevent occlusion in thirty experimental and ten control rats. The experimental animals were divided into three groups and constant, calibrated axial loads of 8 Gm./cm.2 (light), 15 Gm./cm.2 (medium), and 26 Gm./cm.2 (heavy) were applied to their incisors by means of closed-coil springs for a period of 2 weeks. The daily eruption rate of all teeth was measured for 80 days following removal of the springs. In all experimental animals, severe and long-lasting impairment of the eruptive potential occurred. The least impairment was seen in the group with the light loads, their eruption rate being fully restored 64 days after removal of the loads, by which time 165 percent of all dental and periodontal tissues was renewed. In the medium-load group, renewal of the tissues reached 140 percent, while a gross impediment in normal eruption rate persisted throughout the entire experimental period. The teeth subjected to heavy loads received a diminished impact of the axially directed loads due to the strong horizontal component of force; consequently, they behaved like the incisors of the light-load group. The light loads proved to cause the least impairment and were the most beneficial to functional recovery.

Structural changes in the dental and periodontal tissues of the rat incisor following application of orthodontic loads

The present study deals with the morphometric changes in the loaded rat incisor and its supporting structures 3 months after load removal. The material examined was limited to that part of the tooth that is located between 8 and 20 mm from the tooth apex. Of the ten albino rats in whom laterointrusive loads (19 +/- 0.6 g) were applied to the lower left incisor for a period of 2 weeks, the rate of tooth eruption returned to normal in four animals (group A); in six rats, eruption remained grossly impaired (group B). These two groups were compared with a control group of four rats in whom the incisors were allowed to erupt normally during the 3 months of the experiment. The left mandibles were dissected and fixed; the incisors were demineralized, embedded in paraplast, and cut into transverse serial sections of 6 micron, which were used to produce a three-dimensional reconstruction of all tissue components by means of computerized histomorphometry. The parameters of pulp and dentin, and the total tooth size and shape in group A were similar to those of the control group. In group B, pulp and dentin were changed significantly, tooth size increased, and the oval shape was altered into a more rounded contour, together with a significant enlargement of the socket. Although the total PDL volumes remained similar in all three groups, the tooth/socket relation, as expressed by PDL width and PDL area at the different tooth sides and at varying distances from tooth apex, diverged from normal significantly in all teeth that had been subjected to loading.

Long-term effect of loading on the fibroblast population of the periodontal ligament in the rat lower incisor.

The number and density of the cells in the tooth-related periodontal ligament (t-PDL) were related to the recovery of eruptive potential 2.5 months after load removal. Four left incisors in which eruption returned to normal (group A) and six in which eruption remained grossly impaired after a two-week application of latero-intrusive loads of 18.5 +/- 0.69 (group B) were compared to four control rats with unimpeded eruption. The incisors were demineralized, embedded in paraplast and cut into 6 micron transverse serial sections from which a three-dimensional reconstruction of the PDL was made by computerized histomorphometry, and the fibroblast population counted. The t-PDL volumes in controls and in groups A and B reached 1.15, 1.05 and 1.53 mm3, respectively; the estimated number of cells in the same PDL volumes were 2244 X 10(3), 1.659 X 10(3) and 2094 X 10(3) cells, respectively. The mean cell count/segment of PDL and the calculated number of cells/unit of periodontal tissue were significantly lower (p less than 0.001) in the formerly stressed, lateral periodontium of both experimental groups. In group B, these values were also decreased in the mesial periodontium.

Effect of repeated orthodontic treatment on the dental and periodontal tissues of the rat incisor.

This study evaluated the response of treated teeth to renewed orthodontic force. Thirty female rats (201 +/- 2.7 g) were divided into groups A and B. Linguointrusive loads (20.58 +/- 1.88 g) generated by springs were applied to the lower left incisor for 2 weeks and then removed to allow recovery during 27 weeks (group A). Identical loading was then repeated in group A and applied as primary treatment in group B. Five animals from each group were killed with the springs in situ (A-1 and B-1), while the remaining 20 animals were killed after a 3-month recovery (A-2, B-2). The decalcified incisors were cross-sectioned serially (2 microm), and the distance of each section from the apex was computed. Dental and periodontal injuries were evaluated by light microscopy and plotted according to their location on the tooth axis. The intrusion of the teeth in group A-1 was significantly greater, whereas recovery of the normal eruption rate in group A-2 was significantly slower compared with groups B-1 and B-2. The histopathologic lesions in groups A-1 and B-1 did not differ. However, group A-2 showed a higher frequency of injured enamel organ, tissue infiltration by inflammatory cells, necrotic areas, and dentin resorption than group B-2. Initial orthodontic loading had a detrimental effect on the ability of the periodontal and dental tissues to cope with, and to recover from, repeated stress, probably because of a decrease in the number of periodontal fibroblasts and damage to the dentin-protecting cementoblastic layer.

Long-term Effects of Orthodontic Forces on the Morphology of the Rat-incisor Socket and its Location in the Mandible

The effect of orthodontic force application on the rat-incisor socket and mandible was studied on roentgenograms. A mean linguo-intrusive force of 19 ± 0.6 g was applied continuously to the shortened left lower incisor for a period of two (group A) and four weeks (group B). A third group of rats, subjected to shortening of the left mandibular incisor only, served as a hypofunctional control (group C). A fourth group of normal rats constituted the intact control (group D). After a recovery period of three months, the animals were killed, and standardized roentgenograms of the cleaned mandibles were taken. Socket and mandibular dimensions were measured on magnified tracings of the roentgenograms. Comparison of groups A and B with the control groups, on the one hand, and of group C with group D, on the other, facilitated isolation of the hypofunctional factor. In groups A and B, the orthodontic forces caused changes in the parameters affected (i.e., socket area, alveolar bone thickness, mandibular dimensions) and not affected by hypofunction (i.e., socket angulation and location, anterior-socket length). The former finding implies modification of the adaptive capability of dental structures to functional demands. It is concluded that mechanical loading of the incisor for two to four weeks causes long-lasting changes in the socket and its surrounding bone.