A histologic study of human tooth movement

A histoEogic study of human tooth movement D. 1. Buck, Portland,

D.D.S., and

D. H. Church,

DAD.

Ore.

T

he earliest approach to the histologic investigation of tooth movement was through experiments done on various laboratory animals. Thus, much of the available data on tooth movement phenomena have been derived from studies of this kind. Most of these investigators have described a pressure-tension theory of tooth movement, with resorption of alveolar bone occurring on the leading or pressure side and bony apposition occurring on the following or tension side. Sandstedtl is given credit for the description of undermining resorption in the marrow space adjacent to the pressure zone. This occurred when pressure transmitted from the tooth to the periodontal ligament compressed blood vascular channels and destroyed the periodontal ligament’s vitality. Herzberg,2 Hemley,3 and Stuteville” reported on human orthodontic tooth movement material, but Reitan5 has been responsible for the vast majority of human studies in the literature from 1951 to the present. More recently, Edward@ and Atherton’ have reported on human gingival response, but Reitan’s description of the cell-free area has been a singular classic in the literature. Reitan’s studies have been widely quoted to explain the clinical observation of a “lag” phase of human tooth movement, and many workers have interpreted this work as the biologic basis for the need of an optimal orthodontic force to minimize this effort,. The dental literature also contains descriptions of hyalinization or cell-free areas in the periodontal ligament due to trauma caused by occlusal stress in human beings. The problem is confounded by terminology. Some interpret cellfree and hyalinixation as synonymous terms, but there is also the possibility that two or more different phenomena are coexistent (that is, a cell-free area due to pressure and a hyaline degeneration of collagen fibers occurring concomitantly). It is also possible that these events, in fact, represent a single phenonlenon and that, because of inadequate sampling or rapid cellular turnover, they appear to be either separate or related, or even missed, depending on the time sequellee studied. 507

Fig.

1. A representative

appliance

in position

Recent investigations by Rixon and co-workers* have questioned a clinical application of the optimal force range for human tooth movement that is based on histologic evidence of the timing of thrse phenomena. Animal tooth movement experimentation does not often corroborate the cell-free and hyalinization observations. It seemed pertinent to examine material on human tooth movement using the technique of R&an to observe the order of histologic events. The purpose of the present study, therefore, was to examine the reaction of a light tipping force on the periodontal ligament pressure zone at ‘i-day intervals for 28 days. Materials

and

methods

The maxillary dental arches of ten girls and two boys, 11 to 15 years of age, who required the extraction of premolars for orthodontic treat.ment were utilized in this study. A lingual appliance which created buccal tipping of the maxillary first premolars was activated with a force of ‘70 f 7 grams and stabilized by cemented molar bands (Fig. 1). The finger springs w-erc calibrated by a deatl weight loading device and fabricated from 0.018 inch standard stainless steel orthodontic wire with the design of a double helix and a 5 mm. lever arm to yield the required load at a deflection of 3 to 4 mm. The springs were recalibrated after removal from the patient to determine that the stated force was delivered. Patients were randomly divided into four groups, and the time points selected were 7, 14, 21, and 28 days following insertion of the appliance. Maxillary premolars with approximately 5 mm. of buccal alveolar bone were removed surgically in a manner similar to that described by Stuteville,4 Reitan,5 and Kohler and Ramfjord (Fig. 2). Healing was uneventful following surgical closure of the mucoperiosteal flap, and at examination 30 days postoperatively patients who had undergone this procedure were indistinguishable clinically from patients in whom routine

-Vohnte Number

Histologic

62 5

Fig.

2. Buccal

view

Fig.

of an extracted

3. Thirty-day

tooth

study of human

with

postsurgical

attached

extraction

tooth movetnent

buccal

alveolar

509

bone.

site.

extractions had been performed. It is to be emphasized that no notable postoperative swelling occurred; nor were analgesics required for any discomfort (Fig. 3). After a running water rinse, the tooth and bone were fixed in approximately ten times their own volume of neutral buffered formalin. Thirty minutes later the crown was sectioned from the root and bone by means of a high-speed dental handpiece and the tissue was returned to the fixative for 24 hours. Decalcification was accomplished in 10 days by a formic acid-sodium formate solution. Routine paraffin embedding followed. Transverse 7-micron sections were serially mounted by groups of five out of every ten cut and stained for histologic examination with hematoxylin and eosin, Mallory’s conncctivc tissue stain, Wilder’s reticulum stain, and Verhoeff’s stain,

All slides were coded and read by two observers. Irleasnrcments reported wor( made on a Zeiss GL4 bright-field microscope with a calihratctl ocular micromctcr. and photomicrographs were made on a Zeiss photoscopc. After sliclc rcatling, clata were compiled according to time intervals and patient,s. Thcl clcgre~ ()t cellular loss within the periodontal ligament space and linear cstcnt from thcX alveolar crest apically were recorded. The width of the periodontal ligamt‘nt space was indicated as the narrowest region in those specimens which demonstrated compression or as the widest region in those which cxhihitctl cvidencc of a frontal resorption phenomenon. Histologic

observations

Seven days. Compression of the periodontal ligament space was cxhibitetl in all specimens. The mean of the maximum compression of the periodontal ligament was 64 microns, with a range of 28 to 98 (Fig. 4). In the sections stained with hematoxylin and eosin, all of the samples showed at least some portion of the periodontal ligament in which there was a condensation of collagenous fibers with an absence of observable cells. The mean gingival-apical extent of the cell-free areas in all specimens was 1.15 mm., with a range of 0.18 mm. to 1.61 mm. In general, this phenomenon of cell loss was most prominent near the alveolar crest and in regions of compression and lessened as the sections proceeded apically. Undermining resportion from the marrow spaces toward the periodontal ligament space was a universal finding (Fig. 5). In more apical sections, frontal resorption of the alveolar process was present. Similarly, the appearances of occluded blood vessel channels diminishccl as the serial sections were examined from the cervicalmost pressure area to the apex. All but one of the samples studied demonstrated the presence of fibrous immature bone, indicating new bone proliferation. The Mallory-stained sections also demonstrated the cell-free areas but, in areas of extreme compression, showed a color change from t,he characteristic aniline bluo reaction of the c01lagen to yellow, indicating an altered collagen metabolism or structure change that may have indicated hyalinization of the fibers (Fig. 6). This was further corroborated by the other stains studied. Fourteen days. Of the four patients examined, three exhibited similar characteristics. There was only a localized compression of the periodontal ligament, and approximately one half of the specimens showed a very limited cell-free area with absence of fibroblasts, nerve and blood vessel elements, and bone cell types. Proportions of bone type again varied with individual specimens, but all exhibited the presence of immature fibrous bone. In general, all specimens from these patients showed a widened periodontal ligament space with active frontal alveolar bone resorption and patent blood vessel channels. The resorbing alveolar bone front may have extended nearly into previous bone marrow spaces (Fig. 7). One patient of this group exhibited histologic changes similar to those described in the 7-day group, that is, extensive cell-free areas, undermining bone resorption, and obliterated or occluded blood vessel channels. Twenty-one days. After 21 days, individual variance was more pronounced. A widened periodontal ligament space (a mean of 338 microns, with a range of

Histologic

Fig. and

4. Photomicrograph of undermining resorption.

Fig. 5. Undermining lin and eosin stain.)

osteoclastic

.ctdy

periodontal ligament Scale equals 100 resorption

of human

teeth movement

511

compression at 7 days with loss of cells microns. (Hematoxylin and eosin stain.)

at 7 days.

Scale

equals

20

microns.

(Hematoxy-

280 to 420 microns) was found. There was little or no evidence of osteoclastic activity, either frontal or undermining. All specimens exhibited osteoblastic activity and areas of fibrous immature bone. Reorganization of the periodontal ligament was characterized by capillary budding, fibroblasts with basophilic cytoplasm and collagen bundles with a faint basophilia, and fiber bundles less compact or separated. Two of the three specimens examined exhibited an unusual vacuolated or perforated periodontal ligament with signet ring cells characteristically similar to fatty degeneration (Figs. 8 and 9). The earliest evidence of lateral root resorption was seen in these specimens, in areas ecrvical to the area of fatlike deposition (Fig. 10). Twenty-eight days. The principal finding at 28 days was one of fiber and cell

Fig.

6.

Seven-day

acteristic 50

Fig. fatty

microns. 7.

tooth

ligament

(Mallory

connective

Fourteen-day

marrow

movement

periodontal

site.

tooth Scale

equals

showing collagen tissue

movement 100

extreme staining

compression depicting

site

and

“hyalinization.”

loss Scale

of

charequals

stain.) with microns.

receding (Wilder

alveolar

bone

front

approaching

stain.)

reorganization. Active bone resorption was not apparent, and few osteoclasts root resorption was relatively extensive. There was a marked were seen. Lateral increase in extravascular blood elements. Macrophages were seen in several locations that appeared to bc adjacent to sites of presumptive fatty deposition. To summarize the histologic findings, periodontal ligament compression was greatest at 7 days, localized only by 14 days, and nonexistent thereafter. Osteoclastic activity was closely correlated to the degree of compression, with undermining at 7 da.ys and frontal resorption thereafter except in one specimen. Lateral root resorption was found at 21 and 28 days and was cervical to areas

Histologic

Fig. Scale

Fig.

8. Twenty-one-day equals 9.

movement microns.

Higher-power

morphologic eosin

100

evidence

showing

(Hematoxylin view of

of signet

21-day ring

stltdy

of

periodontal and tooth

cells.

eosin

human

ligament

tooth moummt

vacuolization

and

513

spaces.

stain.) movement

Scale

equals

reaction 20

microns.

showing

spaces

(Hematoxylin

and and

stain.)

of possible fatty deposition. The 2%day specimens showed phagocytic activity. All specimens showed vigorous osteoblastic activity evidenced by fibrous immature bone which decreased at the 28-day level. The T-day specimens showed an absence of cells in the compression zone and also, with special stains, an altered collagen metabolism or structure reaction in areas of extreme compression. After 7 days, there were progressive reorganization changes of the periodontal ligament. Discussion

The histologic effect of a light tipping force after 7 days was dramatic. Extreme compression of the periodontal ligament was apparent, with a loss of

Fig.

10.

more matoxylin

Twenty-one-day

apical

to and

tooth

possible eosin

previous

movement cell-free

showing pressure

root zone.

resorption Scale

occurring equals

150

at microns.

a

level (He-

stain.]

cellular elements and an increased density of collagen fibers. It appears likely that physical compression occludes the vascular channels, creating ischemia, possible pressure atrophy, degeneration, cell death, and subsequent cell disappearance. The probability that this chain of events leads to cell death rather than simple cell migration was substantiated by t,he appearance of cells adjacent to areas of cell loss which showed pyknotic nuclei. Furthermore, ghostlike structures were seen which appeared to be former lumina of blood vessel channels filled with red blood cell fragments and degenerated endothelium. We chose not to term this cell-free area hynlhtixatio~t. Byalinixation is a term used in pathology to mean “clear or glasslike.” When used in histology, it signifies a loss of the fibrillar nature of collagen fibers as demonstrated by hematosylin and eosin stain. I~‘urthermore, it is an indication that tissue must bc replaced for r&oration of the normal function of the organ system. The cell-free loss that we observed in this stage of tooth movement does not appear to result in a hyalinixation change of the fibers, and this biologic mechanism should be a subject for further investigation. While this cell loss may represent lytic degeneration or necrosis due to pressure, ischemia, autolysis, or other mechanisms, it seems best at this time to tlcscribc the altIered structure simply as a “cell-free area.” Evidence of collagen fiber loss in areas of extreme compression was seen, but it was rare. The sections stained with hematoxylin and eosin showed a loss of the fibrillar nature of the collagen fibers. This was corroborated by an inability of this compressed zone to pick up the aniline blue component of the Mallory connective tissue stain as well as by loss of fibrillar structure with the Wilder’s and Verhoefl’s stains. Therefore, this area was termed “a hyalinization change.” Osteoclastic activity was undermining in nat,ure. Since no frontal resorption was occurring, any tooth movement must have been due to periodontal ligament compression or alveolar bone (or tooth) bending.

Histologic

study

of

humax

tooth

movement

515

The 1Cday findings were a logical sequel. Breakthroughs of undermining resorption resulted in a return of cellular elements. A great deal of ost’eoblastic activity resulted in the formation of fibrous immature boric. The events observed up to this time point were similar to but occurred more rapidly than what has been reported previously. We believe that two different phenomena were present : (1) that of cell loss due to pressure and (2) in some instances, an altered physiochemical change of the collagen fibers which is termetl “hyalinization.” The cell loss was rapidly reversed by increased vitality, and thtX small areas of collagen loss do not appear extensive enough to be of clinical significance in this study. Unfortunatcl\-, our time points dicl not inform us whether these two phenomena were concurrent or different in their time sequence. At 21 days, periodontal ligament reorganization was the dominant finding. The appearance of fatlike tissue in the periodontal ligament space was an unexpected finding. Periodontal ligament fat has been described by Kronfeld’” around teeth in limited function. To our knowledge, this is the first report of fatlike deposition in tooth movement experiments. It should be emphasized that the stains used demonstrated only morphologic evidence of fat, and positive confirmation cannot be made at this time. Possibly this potentially reversible degeneration process was brought on by local ischemia, as it is most likely due to a change in the resident cells of the periodontal ligament following anaerobic glycolysis from reduced blood flow, rather than an infiltration of cells from other areas. Another possibility might be the movement of the tooth into an area of bone marrow spaces occupied by fatty marrow. The reaction may be sirnilar t,o the regenerative fat formation in an area of reduced circulation reported by Clark and Clark’l in experiments on the rabbit ear. Finally, the possibility of artifactual spaces must not be discounted. Extensive lateral root resorption was also found adjacent to these areas, but at this time one cannot speculate as to the cause-effect relationship. The 28-day specimens show periodontal ligament reorganization, including capillary budding, fiber reorganization, and orientation. In addition, the phagocytic activity that was present substantiates the reversibility of the earlier possible fat deposition changes. The clinical significance of these findings remains to be determined. The rapidity of cellular changes suggests that no significant lag phase of tooth movement would be associated with this force range. Frontal resorption and tooth movement through bone should be a clinical possibility after 7 days. Periodontal ligament cell loss and, in rare instances, observable collagen changes occur as separate phenomena but are readily reversible. In the situation of reduced oxygen tension due to partial ischemia, a phenomenon of fatty deposition may result. The movement of the tooth into a fatty marrow site may be a more likely possibility. In either case, this change is also readily reversible. The lateral root resorption could possibly be the result of excessive pressure. Also, the fat could be involved in some manner in the resorption phenomenon. The effect of repeated activations of an appliance before sufficient time for complete tissue repair is not known, but it is apparent from the foregoing that

an experimental design could be applied to answer the question. h’inally, clifferent force ranges and types of movement, may yield different findings which wcrc riot? investigated in this st,utly. Conclusions

1. Light tipping Sorces created areas of cell loss in the pressure zone of the periodontal ligament. 2. In areas of extreme compression, the rare phenomenon of an altered collagen metabolism or structure react,ion may be present, which is termed “hyalinixation. ” 3. Morphologic evidence of signet ring cells characteristic of fatty tissue nmy appear in the periodontal ligament following light tipping force application. 4. These changes are all readily reversible, and the rapidity of ccl1 and fiber reorganization appears to render these phenomena relatively nonsignificant in a clinical tooth movement time sequence. Surgical removal of Ralph G. Merrill appreciated.

of tooth and of the University

overlying buccal bone was performed under the direction of Oregon Dental School, whose assistance was gratefully

REFERENCES

1. Sandstedt, C.: Nsgra bidgag til tandregleringens teori, Stockholm, 1901; Einige bcitrage zur theorie der zahnregulierung, Nord. Tandl. Tid. 5: 236, 1904; 6: 1, 1905. 2. Herzberg, B.: Bone changes incident to orthodontic tooth movement in man, J. Am. Dent. sssoc. 19: 1777, 1932. 3. Hemley, S.: Bite plates, their application and action, A&I. J. ORTHOD. 24: 721, 1938. 4. Stuteville, 0.: A summary review of tissue changes incident to tooth movement, Angle Orthod. 8: 3, 1938. 5. Reitan, K.: The initial tissue reaction incident to orthodontic tooth movement, Scta Odont. Scan. Supp. 6: l-240, 1951. 6. Edwards, J. G.: A study of the periodontium during orthodontic rotation of teeth, Anr. J. 0~~~0~54: 441,191X 7. Atherton, J. D.: The gingival response to orthodontic tooth movement, A,\r. J. ORTHOD. 58: 179, 1970. 8. Hixon, E., a,nd others: Optimal force, differential force, and anchorage, AM. J. ORTHOD. 55: 437, 1969. 9. Kohlcr, C., and Ramfjord, S.: Healing of gingival mucoperiosteal flaps, Oral Rurg. 13: 89, 1960. 10. Kronfeld, R.: Histopathology of the teeth and their surrounding structures, ed. 4, Philadelphia, 1955, Lea & Febiger, p. 334. II. Clark, E. R., and Clark, E. L.: Microscopic studies of the new formnt,ion of fat in living adult rabbits, Am. J. Anat. 67: 255, 1940. 611 S. W. Crcmpus

Dr.