Histomorphometric study of the periodontal vasculature during and after experimental tipping of the rat incisor.

The periodontal vasculature encircling the entire length of the rat lower incisor was studied at the time of tipping movement and 3 months later. In 12 rats (212+/-4 g b.w.), loads (0.19+/-0.016 N) were applied to the lower left incisor in a linguointrusive direction. After 2 weeks of loading, six experimental animals were killed with the loading springs in place. The springs were removed in the six remaining rats, which were killed 12 weeks later. Six additional rats with intact teeth served as control. All incisors were fixed, demineralized, embedded in glycol methacrylate and cross-sectioned perpendicular to the long axis of the tooth. The distance of each section (2 microm) from the apex was calculated. A computerized image-analysis program was used to measure the width and area of the labial and lingual periodontal ligament to establish whether the measured segments corresponded to the compressed or expanded zones. In each cross-section, the various types of blood vessels were counted and the cross-sectional area of all venous vessels was measured. The results showed that after 2 weeks of loading (1) the general trend of vascular changes was similar under pressure and tension; (2) the large-diameter vessels were unaffected by loading; (3) the mean number of terminal arterioles had decreased significantly, while the number of capillaries and postcapillary venules had increased significantly in the apical tooth part; (4) the venous blood vessel area had decreased significantly in the apical tooth part; (5) the intensity of the vascular reaction was dependent on the degree of tissue distortion; and (6) after 12 weeks' recovery the vascular changes were still present, demonstrating a rebound effect. The findings suggest that microvascular alterations following tooth loading are not directly related to the spatial effect of loading itself and are of a much longer duration than expected.

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.

Effect of loading on the migration of periodontal fibroblasts in the rat incisor.

The influence of occlusal loading on periodontal fibroblasts was investigated in hypoloaded (shortened out of occlusion), functionally loaded and hyperloaded (constant linguointrusive mechanical loads of 9.4 +/- 0.06 g) lower left rat incisors. One hour following injection of 3H-thymidine, half of the animals in each group were killed, while the remaining rats were killed 2 weeks later. The decalcified incisors were embedded in glycolmethacrylate and sectioned (2 microns) serially, perpendicularly to the long tooth axis. Labeled and unlabeled fibroblasts in the tooth-related periodontal ligament were counted in 8 x 80 microns consecutive layers. Cell density (CD) and labeling index (LI) were plotted according to their location on the apico-incisal and cementum-bone axes. Loading caused a decrease in CD and a shift of cells from the cementum towards the middle of the ligament, proportionally to load intensity and duration. The average tooth-to-bone movement of the cells was 2 microns/day in the hypoloaded and 4 microns/day in the two loaded groups. The mean daily tooth eruption rate was 975 +/- 60 microns, 499 +/- 18 microns and 103 +/- 27 microns in the hypo-, functionally- and hyperloaded teeth, respectively. The respective concomitant average daily cell migration rates in the incisal direction were 786 microns, 500 microns, and 500 microns, i.e. 80%, 100% and 485% of the tooth eruption rates. The gross disparity between cell velocity and tooth movement under conditions of restrained eruption indicates active motility of the fibroblasts, rather than their passive tooth-eruption dependent translation.

Dynamics of tissue changes found after mechanical loading of the rat incisor. II. A three-dimensional longitudinal study of the histopathologic aspects.

The study comprised 60 lower left rat incisors subjected to 2 weeks of loading (19.7 +/- 1.6 gm) and 5 control teeth. After the loading period, 10 rats were killed with the springs in situ, and the remainder were killed in groups of 5 at intervals of 1 through 10 weeks after the springs were removed. The teeth were cross-sectioned serially (2 microns). The distance of each section from the apex and the time of its tissue formation in relation to load removal were calculated. Bone remodeling and dental and periodontal trauma were plotted according to their location on the tooth axis and circumference. Vigorous bone remodeling continued throughout the observation period, apposition and resorption sites intermingling around the tooth. Directionality, as expressed by the resorption/apposition ratio, for the formerly tensed and stressed areas was 0.61 +/- 0.07 and 1.79 +/- 0.52, respectively. However, in some of the groups directionality was negligible or reversed. New lesions of dentin (folding and resorption) were formed for 6 weeks after loading, whereas new damage to the enamel organ and enamel matrix appeared until week 9. The integrity of the PDL deteriorated for the first 8 weeks, as evidenced by edema, cell loss, scarring, and collagen fiber disorientation. Necrosis occurred throughout the recovery period. Fiber to bone attachment was disrupted by formation of big sinusoids adjacent to the bone. The frequency of most lesions decreased toward the end of the observation period. It is suggested that tissue recovery proceeds in subsiding cycles of tissue injury/tissue repair, a process that is much more prolonged than is generally believed.

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 microns), 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.2+_

The effects of infraocclusion: Part 3. Dental arch length and the midline.

The third and final article in this series examines the effect of infraocclusion on the midline of the mandibular dental arch. From clinical observation, a shift in the dental midline to the side affected by infraocclusion seemed very common, and this could be explained within the terms of the hypothesis proposed in the first article. A method was devised on a panoramic radiograph to define and to distinguish the anatomic (skeletal) midline and the dental midline. This confirmed the existence of a dental shift to the affected side. The hemiarch length of each side was measured on the film, from the distal of the second deciduous molar/second premolar to both constructed midlines, and was found to be longer on the affected side, confirming an abnormally distal position of the first molar on that side.

Dynamics of tissue changes found after mechanical loading of the rat incisor. I. A three-dimensional longitudinal study of the morphologic aspects.

A three-dimensional morphometric method was used to evaluate progressive changes in shape and size of recovering dental and periodontal tissues after orthodontic loading. In 35 female rats weighing 212 +/- 4 gm, loads of 19.7 +/- 1.6 gm generated by closed-coil springs were applied for 2 weeks to the shortened lower left incisor. The rats were killed in groups of five at 0, 1, 3, 5, 7, 9, and 10 weeks (groups 0-w to 10-w) after the springs were removed. A group of rats with normal incisors (group C-1) and one with five incisors that had been continually shortened for 10 weeks (group C-2) served as controls for groups 0-w and 10-w, respectively. Width, area, and volume of the tooth and enamel-bordering periodontal ligament (e-PDL) and cementum-bordering PDL (c-PDL) were measured. After 2 weeks of loading (group 0-w), the volume of the compressed e-PDL had decreased by 22%, and the volume of the stretched c-PDL had increased by 72%, suggesting that bone apposition lags behind the rate of tooth movement. During the recovery period, the dental and periodontal parameters tended toward a gradual return to control (C-2) values, although at the end of 10 weeks many still lagged significantly behind the controls. Recovery was slowed by repeated reversals at different sites in the various groups. The ability of the preloaded incisor to adjust to changes in occlusal function was lastingly impaired.

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 findings 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.

Effect of occlusal functional forces on incisor socket morphology and location in the rat mandible.

The effect of functional occlusal stress on dimensional alterations of the rat incisor socket and mandible were studied from roentgenograms. In 12 rats, the lower left incisor was shortened twice weekly, whereas the lower right incisor was allowed to remain in contact with both upper incisors. Thus, the right incisors were subjected to hyperfunction, and the left ones, to hypofunction. The lower incisors of 16 rats with normal occlusal contact served as control. Following an experimental period of 3 months, the animals were killed and standardized radiographs were taken of the cleaned mandibles. Socket and mandibular dimensions were measured on magnified tracings of the roentgenograms. Socket area, its posterior length, posterior mandibular length, and gonial angle changed in the same direction under both hyper- and hypofunction. The anterior socket was relocated in opposite directions: under hyperfunction, it assumed a more inferior position, whereas in hypofunction, it moved superiorly. The angulation of the socket became significantly more acute under hyperfunction, whereas in hypofunction, this parameter remained unchanged. It is concluded that altered functional demands affect the morphology of the incisor socket and its location within the mandibular borders.

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.

Three-dimensional presentation of cell migration in the periodontal ligament of the rat incisor.

The progenitor compartments and cell migration were examined in the tooth-related periodontal ligament (t-PDL) of rat incisors. A pulse injection of 3H-Tdr was administered to 15 rats (200 gm each) and the animals were killed in groups of five, at 1 hr and at 1 and 2 weeks after injection. Three-dimensional analysis of cell counts and labeling index demonstrated the existence of two progenitor compartments (PC). The apical PC (responsible for 70% of synthesizing cells) was concentrated in the apical 5 mm of the t-PDL. The paracemental PC (30% of synthesizing cells) was located along and around the cementum, occupying 24 micron of the t-PDL. The cells from the apical PC migrated incisally at a rate of 6 mm/week, which is a rate similar to that of tooth eruption. The cells from the paracemental PC moved in a transverse direction toward either bone or cementum at the much slower rate of 16 micron/week.

The effect of load duration on long-term recovery of the eruptive function in the rat incisor.

The purpose of this study was to determine the long-term recovery of the normal rate of eruption of the rat incisor following application of loads of similar magnitude but of varying duration. The daily rate of eruption of the continuously growing tooth served as indicator of the functional recovery of its periodontium. The left mandibular incisor was shortened out of occlusion in 34 experimental and 16 control rats. The experimental animals were divided into groups A, B, and C, and continuous loads (19.47 g +/- 0.42 g) were applied to the shortened incisors by means of closed-coil springs for 5, 14, and 28 days, respectively. Following removal of the springs, the daily rate of eruption was measured and calculated in all 50 animals for a period of 10 weeks. The extended impairment of eruption in the experimental animals was directly proportional to the duration of loading: at 10 weeks postloading, it was present in 18%, 67%, and 100% of the rats in groups A, B, and C, respectively. The intragroup variations in the rate of eruption of individual teeth were more pronounced following short-term loading than after prolonged load application. Short-term loading proved to be least harmful regarding recovery of eruptive function.

Three-dimensional presentation of the fibroblast progenitor compartment in the periodontal ligament of the rat incisor.

The purpose of this study was to determine the shape, size, and spatial configuration of the fibroblast progenitor compartment (PC) of the tooth-related periodontal ligament (PDL). Five albino rats (weight 200 gm) were killed 1 hr following intraperitoneal injection of 3H-thymidine. The incisors were decalcified, embedded in glycol-methacrylate, and sectioned serially, perpendicularly to the long axis of the tooth. Ten 2-microns-thick sections along each tooth were selected, processed autoradiographically, and the distance from the apex measured. Labeled and unlabeled fibroblasts were counted in 8 x 80-microns consecutive layers. Cell density (CD) and labeling index (LI) were plotted on a three-dimensional system of coordinates depicting the directions: apex to incisal edge, cementum to bone, and cemento-enamel junction (CEJ) to lingual tooth surface. The CD along the tooth was constant, except at 2-6 mm distant from the apex, where it was significantly higher. The highest concentration of cells around the tooth was in the 20-microns region adjacent to the cementum. The PC on the mesial and lateral tooth sides extended 1-4 mm from the apex, with the LI peak (6.5%) occurring at 2 mm. The PC on the lingual tooth side was located at 2-6 mm from apex, peaking (14%) at 4 mm. On the cementum-bone coordinate, the PC extended to about 50 microns from the cementum on all sides, the LI peak appearing at a distance of 20-40 microns from the tooth. The LI was consistently higher on the lingual side than on the mesial and lateral aspects.

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.

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.

Characteristics and stability of spaced dentition.

A serial study of changes in spacing between teeth during the period of facial maturation, finding tendencies toward closure of buccal spaces and opening of anterior spaces. Spacing in females is equally distributed between upper and lower arches; in males it is more prevalent in the upper arch.

Spaced dentition. An epidemiologic study.

This epidemiologic study deals with the incidence of spaced dentition in 1279 students between the ages of 12 and 18 years. The sample includes only those with a full complement of permanent teeth (third molars were not considered) and with no apparent etiological conditions that might lead to disruption of dental arch continuity. The sample was evenly divided by age and sex. Frequency of spaced dentition was high, occurring in 51.8% of males and in 45.5% of females. Prevalence was less in higher age groups, and the sexual dimorphism that was significant in the 14 YR-16 YR group was not found in the 16 YR-18 YR group. The mean number of spaces per subject was 6 +/- 4.3 for males and 5 +/- 3.7 for females, again displaying diminishing incidence with age and a concomitant absence of the sexual dimorphism in the highest age group. Spaces were found in both arches in 49.5% of the spaced dentitions, and in only the maxillary arch in 34.3%. Incidence of spacing in the mandibular arch alone was only 16.2%. In most subjects the spaces were distributed equally between the two quadrants of each arch. The most common sites and largest space widths in either dental arch were found between cuspids and the first bicuspids and between cuspids and lateral incisors.

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)

Odontoblast turnover in the impeded and unimpeded rat incisor derived from computerized histomorphometry.

A computerized histomorphometric method was devised to estimate the kinetics of odontoblast turnover and dentinogenesis in rat incisors. The method was applied to two groups of rats: one group with lower incisors in impeded eruption and another group with the left lower incisor in the unimpeded state. The teeth were divided into six equal segments, from which consecutive ground sections were obtained. The distance of each ground section from the posterior border of the alveolar bone was calculated. Each section was magnified, traced, and the tracings fed into a computer by a sonic digitizer. The perimeters and areas of dentine and pulp in each ground section were calculated by the computer. The mean odontoblast density along the predentine was evaluated from histological sections taken both from the same tooth segments and from teeth sectioned midsagittally. These served for the estimation of the predentine area occupied by the average odontoblast. In the impeded group, this area was 11% larger than in the unimpeded one. Outer dimensions of teeth, namely the circumference of the dentine, the labiolingual width, and the mesiolateral width remained constant and equal for both groups. Daily rates of dentine apposition were computed and were found to vary according to the age of the odontoblasts. Odontoblasts of impeded teeth started to secrete matrix at a rate of 17 microns/day, which increased slightly to 19 microns/day and later declined to 7 microns/day on the 38th day. Dentine production of unimpeded odontoblasts, on the other hand, started at a rate of 16 microns/day and gradually increased to 34 microns/day on the 17th day.(ABSTRACT TRUNCATED AT 250 WORDS)