Continuous Attachment of the Periodontal Membrane

ALLAN G. KRAW AND DONALD H. ENLOW Department of Anatomy and The Center for Human Orowth and Development, The University of Michigan, Ann Arbor, Michigan

ABSTRACT A polychrome stain for differentiating precollagenous and collagenous fibers, and tetracycline for labeling bone changes were used in young rats to trace adjustments involved in movements of teeth within bone. Distinct differences were seen in fibrous attachments on depository and resorptive bone surfaces. These are associated with shifts of the periodontal membrane as the root and surrounding bone undergo drift. Some resorptive areas receive total destruction of fibrous attachment. Other areas involve a process of fibrous conversion as ordinary fibers of the bone matrix become uncovered during bone removal and function as periodontal fibers anchored into the receding bone surface. An intermediate zone of the periodontal membrane functions to progressively shorten distal ends of these same fibers and to relink them with newly formed precollagenous fibrils. These become continuous with mature fibers attached to the cementum. On depository surfaces, outer fibers of the membrane become embedded into new alveolar bone, and former precollagenous fibrils of the inward-shifting intermediate zone translocate to become the new outer zone. Continuous relinkage is simultaneously maintained between the inner and outer zones. Haversian remodeling occurs beneath resorptive surfaces and re-anchor- age is thereby established in specific areas where total destruction of fibrous connec- tion had previously occurred.

The purpose of this report is (1) to these movements, adjustment mecha- describe and interpret the response of the nisms necessarily exist that function to young, growing periodontal membrane to maintain firm and continuous attachment differential polychrome staining for col- of the periodontal membrane to both tooth lagen and precollagen; (2) to describe and bone. Sicher ('23, '42) reported the the nature of periodontal attachments on presence of an "intermediate plexus" in the various resorptive and depository sur- the periodontal membrane of rodents that faces of alveolar bone; and (3) to corre- is located in the approximate middle third late the distribution of underlying Haver- of the membrane. He suggested that this sian remodeling, using tetracycline label- plexus, containing an abundance of argy- ing, with periodontal anchorage on re- rophilic fibers, functions to splice col- sorptive surfaces of bone. lagenous fibers embedded in the bone on Teeth are firmly attached to the bony one side and the cementum on the other. alveolar wall by a dense connective tis- The tooth can undergo eruption and at the sue bridge, the periodontal membrane. same time maintain its attachment by a Parallel bundles of collagenous fibers ex- continuous resplicing of the new and old tend from the cementum of the tooth fibers as they become relocated during across the span between root and bone tooth movement. Orban ('27) later ex- and are embedded in the alveolar wall. tended this explanation to human denti- The continuous maintenance of this at- tion. The presence of an intermediate tachment, however, is complicated by sev- plexus has also been reported in the eral factors associated with growth, in- guinea pig (Hunt, '59) and in the spider cluding differential growth rates between monkey (Goldman, '57). Several workers, bone and teeth, progressive eruption, however, have questioned the existence of rotation and titIing movements of teeth, this plexus, including Eccles ('59), Trott drifting, and continuous rebuilding ('62), and Zwarych and Quigley ('65). changes taking place in the bone of the Sicher ('65) suggests that the presence growing maxilla and mandible. Because and relative prominence of an intermedi- spatial relationships between teeth and ate plexus is dependent upon the degree of alveolar bone constantly change during tooth movement at the time observed. Dur-

AM. J. ANAT., 120: 133-148. 133 134 ALLAN G. KRAW AND DONALD H. ENLOW ing quiescent periods, this plexus presum- specimens were fixed in formalin acetic- ably becomes obscured. alcohol ( 10 :5 :85), decalcified in Decal An important factor involved in move- (Winthrup Laboratories), double-em- ments of teeth within bone is the common bedded in celloidin, and serially sectioned occurrence of alveolar bone surfaces at 10 p. A variety of horizontal, vertical, which are resorptive in character, rather and frontal sections were prepared. Sec- than depository. This growth factor is as- tions were stained with a polychrome stain sociated with progressive movements of a (Herovici, ’63) to differentiate mature col- tooth directly into areas previously occu- lagenous fibers from formative (precol- pied by bone. Such movements take place lagenous) fibrils. This procedure was during both drift and eruption, and they used in order to identify and localize sites involve the widespread resorptive removal of collagen formation and to correlate of bone on contact surfaces between the their distribution with changes in the alveolar wall and the periodontal mem- periodontal membrane and alveolar bone brane. New bone deposition occurs on the during drift and eruption. Using this opposite (endosteal) side of the same stain, mature fibers appear red (acid alveolar wall. By this combined process, fuschin), and precollagenous fibrils ap- the entire plate of bone moves in a direc- pear blue (methylene blue). A number of tion corresponding to the movement of representative sections were also stained the tooth. It is currently believed that with standard Hematoxylin and eosin and bone resorption, in general, represents a with Mallory-Heidenhain for comparison total process of removal, including bone with the polychrome stain. cells, the entire fibrous matrix, and the Tetracycline hydrochloride (30 mg/kg ground substance. It is also generally as- body weight) was also administered to a sumed that the resorption of alveolar group of 14 animals in a series of three bone, into which periodontal fibers are intraperitoneal injections at 48-hour in- anchored, similarly results in a total de- tervals. This procedure was used to iden- struction of periodontal attachment in tify and localize remodeling changes in such areas. Gottlieb (’42) has estimated the bone tissue of the alveolar wall as- that as much as 20 percent of the total sociated with tooth movements and perio- alveolar bony lining can represent such dontal adjustments. Tetracycline becomes a resorptive surface with corresponding incorporated within newly formed bone, fibrous detachment. Sicher and Wein- and layers or zones of such bone can be mann (’44, ’65) suggest that intermittent, visualized by the use of ultraviolet micro- scattered spot deposition of reparative scopy (Behatia and Sognnaes, ’63; Gregg bone on such surfaces functions to tem- and Avery, ’64; Owen, ’63; Bevelander, porarily maintain partial attachment dur- Nakahara, and Rolle, ’60). ing tooth movements. In the present report, an expansion of OBSERVATIONS the concept of an intermediate plexus will Alveolar bone surfaces and fibrous be made, and additional observations and anchorage. Two general kinds of bone interpretations dealing with the nature of surfaces, depository and resorptive, line its mechanism will be presented. The crit- the alveolar cavity, and each is readily ical problem of attachment and successive identifiable in routine preparations using reattachment onto resorptive bone sur- medium power magnification. Depository faces will be examined, and new informa- surfaces are characterized by an ac- tion will be presented relative to pro- cumulation of successive layers composed cesses involved. either of lamellar or non-lamellar bone tissue. As new bone is deposited, bundles MATERIALS AND METHODS of periodontal fibers become incorporated Maxillary bones from a total of 62 fe- within the bone in a manner comparable male albino rats of the Sprague-Dawley with Sharpey’s fibers in areas of muscle strain were used for study. The animals or tendon attachment (fig. 3). The result- ranged from 60 to 75 days in age and ing bone type is distinctive in structure averaged 215 gm in weight. Maxillary and is commonly termed “bundle bone.” ATTACHMENTS OF PERIODONTAL MEMBRANE 135

Although this alveolar bone type repre- (away from the advancing root of the sents the predominant bone tissue present tooth) leaves a trail of uninterrupted and beneath depository surfaces, some scat- apparently undisturbed collagenous fibers tered areas may also be observed in which that were once a component part of the such fibrous attachment does not occur. ordinary bone matrix but which had be- In this instance, the only direct inter-con- come progressively uncovered as the re- nection between bone and the periodontal mainder of the bone around them was membrane is by vascular bundles and cell removed (fig. 4). They then become di- processes. Both types of periodontal bone, rectly incorporated into the shifting and which correspond to periosteal bone tissue, continuously reforming periodontal mem- are formed in areas where movements brane. This observation is in contradic- of teeth proceed in a general direction tion with the widespread belief that bone away from the bone surface. Progressive resorption necessarily results in a total new bone deposition on the alveolar sur- destruction of the entire bone matrix. This face serves to maintain a constant bone- can be true in areas adjacent to blood ves- to-tooth position as the tooth shifts in loca- sels, as described in the previous para- tion due to drift or eruption. graph, and in such regions bone resorp- Resorptive alveolar surfaces are charac- tion results in total fibrous detachment. In terized by a scalloped margin (fig. 4) and contrast, however, resorptive surfaces that an abundant distribution of prominent are not directly associated with perio- osteoclasts. Such surfaces are located in dontal vascular bundles are characterized areas involving progressive movement of by a retention of collagenous fibers re- the root in a direction toward the bone leased from the bone matrix as a result surface. The resorptive removal of bone of bone resorption. As they are uncovered from this outer periodontal surface, to- from the receding bone surface, these gether with new bone deposition on the bone matrix fibers become actual perio- opposite (endosteal) side, serves to move dontal fibers that insert into the remaining this entire area of the alveolar wall in a bone and retain direct continuity with direction corresponding to the movement those matrix fibers still enclosed as a part of the advancing tooth. of the bone tissue. Uninterrupted and In contrast with the depository sur- continuous attachment is thereby main- faces just described, resorptive surfaces tained in such areas. As bone fibers are can be observed in which fibrous contin- uncovered to become periodontal fibers, uity with the periodontal membrane has they subsequently undergo a process of become severed as a result of the resorp- shortening and relinkage with precol- tive destruction of alveolar bone tissue dur- lagenous fibrils within the periodontal ing tooth movements. This situation is membrane in order to accommodate mem- typically seen in those areas in advance of brane shifts and tooth movements. This vascular bundles as they enter the bone process will be outlined in later para- within formative resorption spaces (figs. graphs. 2, 4). The resorptive surface of the bone Bone matrix fibers that become exposed is discontinuous with the periodontal as a result of the removal of bone from membrane, and fibrous attachment be- around them are not “Sharpey’s fibers” of tween the two is lacking in such areas. the kind present within bundle bone be- Another common relationship occurs neath depository surfaces. The latter are between the periodontal membrane and periodontal fibers that become embedded resorptive bone surfaces, however, that within new bone deposits as special has been previously unrecognized. A sig- anchoring fibers and represent inclusions nificant distribution of resorptive surfaces within the ordinary intercellular bone were observed in which direct fibrous con- matrix. In contrast, fibers that are un- tinuity between the bone and the adjacent covered and preserved during resorption periodontal membrane was not detached undergo a converse developmental se- or destroyed as a result of bone removal. quence. They are ordinary bone fibers In this situation, the movement of the that subsequently become converted into alveolar wall in an endosteal direction periodontal fibers. Within the matrix of 136 ALLAN G. KRAW AND DONALD H. ENLOW the bone, they are not arranged in coarse, into the cementum. These fibers are com- parallel bundles oriented in a perpendicu- posed largely of mature collagen and ap- lar plane. Rather, they enter the bone and pear red with polychrome staining. The merge directly with the general fibrous outer zone, adjacent to the alveolar wall, matrix (Compare fig. 3 with fig. 4). It is contains coarse fibers that pass directly noteworthy that a comparable process of into the bone. In general, the outer zone fibrous exposure, conversion, and subse- is more labile than the characteristically quent relinkage also occurs on resorptive stable inner zone in that extensive fiber periosteal bone surfaces in which muscles recombination is typically active in this and tendons insert (Enlow, '62b, '65b; area. Goldman ('57) has also reported the Hoyte and Enlow, '66). more stable nature of the inner as com- Four contrasting situations are, thus, pared with the outer zone. In those characteristic of various depository and regions of the alveolar wall involving de- resorptive surfaces in contact with the pository surfaces and the formation of periodontal membrane. These are (1) de- periodontal bundle bone, the fibers are pository surfaces involving embedded Sharpey's fibers (bundle bone), (2) de- colored a deep red with the differential pository surf aces in which periodontal; polychrome stain. These fibers continue fibers do not become embedded during into the bone as Sharpey's fibers and re- bone deposition, thereby leaving bone and tain their red color within the bone tis- periodontal membrane without local fi- sue. They represent mature collagen, and brous attachment, (3) resorptive surfaces they have a large diameter and are ar- involving a total destruction of fibrous ranged into coarse bundles. connection and continuity as a conse- Adjacent to resorptive surfaces, the quence of bone removal, and (4) resorp- fibers of the outer zone also stain with tive surfaces in which bone matrix fibers a red color, and they represent the mature survive bone resorption and are retained collagenous fibers of the old bone matrix as new periodontal fibers directly continu- that have been uncovered during progres- ous with remaining alveolar bone matrix sive resorption and which now serve as fibers. On depository surfaces, the first periodontal fibers. Mixed with these ma- circumstance above has been most fre- ture, red-stained fibers, however, are large quently encountered. On resorptive sur- numbers of fine, blue-stained, precol- faces, both situations described are wide- lagenous fibrils that continue directly into spread and routinely seen in varying com- the predominantly precollagenous inter- binations. They appear approximately mediate zone. The fibers of the intermedi- equal in total distribution, although quan- ate zone, thus, grade for a considerable titative determinations have not been distance into the outer zone. These over- made. As will be pointed out in later lapping precollagenous fibrils may extend paragraphs, a number of related consider- close to the resorbing bone surface, and ations are involved in each of the mechan- in this area of fiber convergence, direct isms outlined above. blending and linkage of old and new fibers Distribution of precollagen and col- can be observed. On depository surfaces, lagen. In the descriptions that follow, an overlap between the young fibrils of the fibers of the periodontal membrane the intermediate zone and the mature, will be grouped into three component coarse fibers of the outer zone is also seen, zones: the anchoring fibers of the cemen- but the degree of overlap is less marked. tum (the inner zone), the anchoring fibers The reason for this lies in the differing of the alveolar bone (outer zone), and an functions of the intermediate zone on re- intermediate zone joining the inner and sorptive as compared with depository sur- outer zones figs. 2-4). The fibers of the faces, as described in other paragraphs. intermediate zone will be termed linkage Large numbers of sizable osteoclasts are fibers. usually observed on the receding bone sur- The inner zone, adjacent to the root, face interspersed among the bundles of typically contains large collagenous fibers fibers. Using Mallory's stain, numerous, arranged into coarse bundles which insert large, eosinophilic cytoplasmic granules of ATTACHMENTS OF PERIODONTAL MEMBRANE 137 an undetermined nature are seen within dergo resorptive removal in turn, but their these osteoclasts. uncovered matrix fibers are converted in- The intermediate zone spanning be- to periodontal fibers retaining connection tween the inner and outer zones ordinar- with the fibers of the remaining bone. The ily contains a few scattered, red-stained fibers of the Haversian system matrix, mature fibers in varying stages of recom- thus, are not subject to immediate destruc- bination and shortening. The great major- tion as the bone itself undergoes removal, ity of fibers observed in the intermediate and they thereby become incorporated di- zone, however, are delicate, blue-stained rectly as the new outer zone of the perio- precollagenous fibrils. They form a rela- dontal membrane as the latter follows the tively dense mesh and are interspersed drift of the bone surface. The red-stained, with abundant fibroblasts and scattered mature fibers released from the resorbing vessels. The functional significance of osteones blend directly with the elongating the intermediate zone with its character- precollagenous fibrils of the intermediate istic overlap into both the inner and outer zone. This sequence is progressive, and zones will be evaluated in the later dis- the continued formation of resorption cussion. canals and new secondary osteones con- In some scattered areas, the three-part tinues in advance of the resorbing sur- zoning just described is not evident. Here, face of the alveolar wall. Such internal the majority of fibers throughout the en- Haversian reconstruction deep to a resorp- tire thickness of the periodontal mem- tive surface is characteristic, in contrast brane are all coarse-bundled, mature col- to the unremodeled, primary nature of lagenous fibers and respond with a typical the bone beneath depository surfaces. This red color to the polychrome stain. Such general process is comparable with the areas may be interpreted as stable at that situation seen in long bones in which particular time, and bone deposition or Haversian remodeling occurs within bone resorption was not then active. Since in relation to muscle and tendon attach- shifts of the periodontal membrane and ment on resorptive surfaces (Enlow, linkage adjustments in response to bone '62b). movements were not involved, the entire In tetracycline labeled bone specimens, fibrous component of the intermediate the remodeling sequence outlined above is zone had progressed to maturity. effectively demonstrated (fig. 1). Floures- Remodeling changes in alveolar bone. cence is observed (1) on the surface of In contrast to the unmodified and pri- the depository side of a tooth socket, and mary nature of periodontal bone beneath (2) within the enlarged canals and re- depository alveolar surfaces, endosteal sorption spaces deep to the bone surface bone tissues underlying resorptive sur- on the resorptive side of the socket. Using faces are often characterized by marked H3 Proline, Stallard ('63) also demon- internal remodeling changes (figs. 2, 4). strated bone deposition within the spaces Just beneath the surface, large resorption of bone associated with resorptive alveolar canals following the course of pre-existing surfaces. primary vascular canals extend in a pro- Haversian reconstruction in tooth-bear- gressive direction deeper into the bone, ing bone is most widespread beneath those thereby keeping pace with the advancing resorptive surfaces in which the under- resorptive front of the entire receding lying bone is relatively thick. In some bone surface. These erosion canals subse- areas, the intervening wall of alveolar quently receive new deposits of bone with- bone between a resorptive surface on one in their enlarged lumina and are thereby side and a depository surface on the other converted into secondary osteones (Haver- can be quite thin (fig. 4). In such loca- sian systems). The fibrous matrix of these tions, the remodeling sequence described newly formed osteones is directly continu- above does not occur since the plate of ous with the fibers of the adjacent perio- bone is not sufficiently thick to contain dontal membrane. As the resorptive front large resorption canals or osteones. In (resorbing alveolar surface) continues to these locations, the functional role of advance into the bone, the osteones un- Haversian remodeling is not involved be- 138 ALLAN G. KRAW AND DONALD H. ENLOW cause the relatively rapid formation and the periodontal membrane are shown, in- destruction of the entire thickness of the cluding the inner and outer layers contain- alveolar wall obviates the functional role ing large numbers of coarse, mature fiber of internal remodeling, as described be- bundles. The intermediate zone, with its low. mesh of delicate precollagenous fibrils, overlaps and blends well into both the in- DISCUSSION ner and outer zones. This overlap is par- Movements of teeth and alveolar bm. ticularly prominent between the intermedi- The basic movements of teeth involve ate and the outer zones. In diagrams 1, eruption and a progressive drifting in 2, and 3 of figure 5, the tooth is drifting in either a mesial or a distal direction de- a direction toward the right (away from pending upon species considered. The the bone surface shown). Periodontal mechanism of drift functions to maintain fibers that were once a component part of firm contact between adjacent teeth as the outer zone have now become embedded contact surfaces undergo wear, thereby within the periodontal bundle bone as bracing the entire dental arch. Teeth also Sharpey’s fibers. As mentioned earlier, experience tilting and rotating movements scattered areas of new bone formation can during drift and eruption. Another kind also occur in which fibers of the perio- of tooth movement occurs which is not dontal membrane do not become bound as generally realized. The growth of any inclusions within formative bone tissue, bone, including the mandible and maxilla, and direct fibrous connection between the involves a process of continuous relocation alveolar wall and the membrane is lack- of its component areas as the entire bone ing in such locations. The arrangement increases in overall size. As the various shown in the growth stages 1-3, however, regions of a bone undergo such consecu- is more commonly found. Progressive in- tive repositioning, the teeth located with- clusion of periodontal fibers has occurred in these areas experience corresponding as new bone is laid down on the alveolar and simultaneous shifts in position in surface by the periodontal membrane. As order to retain their relative position in seen in this figure, the outer zone is com- the bone as a whole. This process of re- posed of coarse, mature collagenous fibers. location in conjunction with bone changes This same area was formerly located as due to growth contributes to the func- the intermediate zone of a previous growth tional basis for the process of mesial drift stage. They were once precollagenous, (Enlow, ’65a). finely-bundled fibrils formerly positioned The drifting process of the periodontal in the intermediate zone but have become membrane. The structure of the perio- mature, coarsely-bundled fibers trans- dontal membrane provides an intrinsic located into the new outer zone. The con- mechanism by which the membrane it- tinuously shifting intermediate zone has self can continuously move in a manner moved in a direction to the right. It is corresponding to the complex variety of composed of newly formed fibrils that re- tooth and bone movements outlined above. present an elongation of the fibrils of the Just as both teeth and bone drift together old intermediate zone which, in turn, have in a horizontal plane, the intervening become the mature, coarse fibers in the periodontal membrane also undergoes new outer zone. As the root of the tooth “drift.” At the same time, it permits dif- drifts to the right, the mature fibers of ferential vertical movements between a the inner zone, attached to the cementum, tooth and the surrounding bony alveolar move to the right with the moving tooth. wall. Throughout these progressive These fibers are stable and undergo rela- changes, the periodontal membrane main- tively little turnover. The young fibrils of tains firm attachment between teeth and the continuously reforming intermediate supporting bone. zone become elongated behind the moving The horizontal drift of a tooth and the fibers of the inner zone, and they keep con- corresponding movements of its bony stant bond with them. The overlap be- socket and periodontal membrane is sche- tween these two zones represents the con- matized in figure 5. The three zones of stantly moving site of linkage between ATTACHMENTS OF PERIODONTAL MEMBRANE 139 their component fibers. The entire perio- whether a process of progressive molecular dontal membrane on this side of the tooth recombination and conversion takes place (the “tension” side) thereby keeps propor- is not now known. However, direct con- tionate position between bone and cemen- tinuity can be traced between the matrix tum by its own mechanism of drifting. In of the resorbing alveolar bone, the fibers of this process, thus, the fibrils of the inter- the outer zone, and the new fibrils of the mediate zone elongate on one side to keep intermediate zone. Fibrous connection, pace with the fibers of the moving inner thus, is not interrupted as a consequence zone, and on the other side they become of surface bone removal. converted into the fibers of the outer zone The inner zone of the periodontal mem- which, in turn, become progressively em- brane, anchored into the cementum, is bedded into newly forming alveolar bone. composed of mature, coarse-bundled fi- On the opposite side of the root (the bers. As the root drifts, these same fibers “pressure” side), an equivalent but fun- are carried in a corresponding direction. damentally different process is involved. Continuous relinkage occurs in the over- As the root drifts to the right in diagrams lap area between the stable inner and the 4-6 of figure 5, the alveolar wall simul- labile intermediate zones as the constantly taneously shifts in a like direction. This is reforming precollagenous fibrils of the lat- brought about by the resorptive removal ter layer become bonded as extensions of of bone from this bone surface together the inner mature fibers. with corresponding bone deposition on the The complex sequence of adjustments opposite endosteal side. The proportion- involved in the drift of the membrane in a ate thickness of the alveolar wall is there- direction toward the alveolar bone surface by maintained. As previously described, involves, in summary, a direct conversion inward-advancing, resorptive bone sur- of former bone matrix fibers released dur- faces involve two general circumstances. ing resorption into the new fibers of the First, some areas of surface resorption re- outer periodontal zone. Since these fibers sult in a total destruction of the bone, in- retain direct continuity with the fibrous cluding its fibrous connection with the matrix of the remaining bone deep to the periodontal membrane. These areas typi- resorptive surface, attachment is main- cally occur around vascular bundles as tained between bone and membrane. they course into the alveolar bone from Second, in the overlap zone between the the periodontal membrane. Second, resorp- outer and intermediate zones, a progres- tive removal of surface bone in other areas sive shortening (or de-differentiation) of leaves a distinct trail of mature col- these same fibers occurs and relinkage lagenous fibers which were once a part of takes place with a simultaneous elongation the bone matrix, prior to resorption, but of the intermediate precollagenous fibrils. which have become translocated as the These newly formed young fibrils, in turn, fibrous outer zone in contact with the become successively reformed and se- resorptive bone surface. As the root moves quentially relinked with the stable fibers progressively in a direction toward the of the inner zone attached to the cemen- receding bone surface, a constant shorten- tum of the tooth. This combined sequence ing of these same fibers occurs as they of changes permits a “drift” of the entire grade into the labile intermediate zone. At periodontal membrane in such a manner the same time, continued elongation of that it maintains proportionate thickness precollagenous fibrils takes place within during such movement. It provides, fur- the outer part of the intermediate zone, ther, continued attachment of teeth to the and these young fibrils link with the con- alveolar wall. stantly shortening, mature fibers of the The movements and changes just de- outer zone as the latter become successive- scribed represent, in part, the special ly uncovered from the bone matrix as a mechanism adapted to the horizontal drift- result of surface resorption. Whether such ing of teeth within bone. Two separate and shortening involves an actual dissolution distinct processes are involved, one as- and subsequent reattachment of the ma- sociated with the tension side and the ture fibers with precollagenous fibrils, or other with the pressure side of a moving 140 ALLAN G. KRAW AND DONALD H. ENLOW root. Differential movements between teeth ized areas where former attachment was and the adjacent alveolar wall, as in erup- destroyed. Haversian remodeling may also tion and rotation, are also brought about be involved in regions where firm fibrous by this sequence of progressive fibrous re- anchorage was previously lacking. This linkage between the intermediate and the situation, noted earlier, has been found in inner and outer zones of the periodontal some scattered areas where formative membrane. periodontal bone does not embody perio- Another basic process contributes to the dontal fibers as it is laid down. AIthough constant maintenance of tooth attachment not widespread in the rat, secondary bone to bone during their respective movements. reconstruction with Haversian system for- This mechanism involves the continuous mation apparently can provide fibrous con- formation and reformation of secondary tinuity between the alveolar wall and the (Haversian) bone in advance of resorptive membrane where such regions become in- surfaces (figs. 2, 4). As a bone surface in contact with the periodontal membrane is volved in subsequent bone and tooth move- destroyed, erosive penetration of resorp- ments. tion canals into the bone take place, and It is noteworthy that Haversian remodel- new bone is subsequently laid down within ing also occurs in long bones on those par- these enlarged spaces. The resulting sec- ticular surfaces where muscles and ten- ondary bone has a fibrous matrix that is dons are attached (Enlow, '62b). Signifi- directly continuous with the fibers of the cantly, secondary Haversian systems are outer zone of the shifting periodontal characteristically lacking in the bone of membrane and provides localized attach- rats (and many other species) except in ment between the membrane and the specific areas, such as the periodontal alveolar wall during this process of resorp- membrane, which involve resorptive sur- tive movement. As older Haversian sys- faces in association with fibrous reattach- tems are destroyed by the advancing re- ment, In the skeleton of very young in- sorptive front, fibers are uncovered from dividuals, bone tissues are typically fine- their matrix in the manner described in cancellous in structure. The canals within previous paragraphs. They become incor- this type of bone are large, and a process porated in turn into the outer periodontal of compaction by bone deposition within zone where they undergo linkage with the these available spaces results in the forma- fibrils of the correspondingly moving in- tion of primary osteones (Enlow, '63). termediate zone. As this occurs, new gen- This developmental sequence functions in erations of resorption spaces develop in fibrous reattachment in a manner compar- advance of the blood vessels entering the able with secondary Haversian remodeling bone from the periodontal membrane, and in that both involve the deposition of new new Haversian systems are then formed bone within large canals or spaces. In within these spaces. This process of very young individuals, the distribution of Haversian remodeling functions as a reat- primary osteones parallels that of the sec- tachment mechanism in two specific in- ondary osteones in older individuals. stances. First, it occurs beneath those par- ticular surface areas deep to a resorptive CONCLUSIONS front in which bone removal has resulted Sections of decalcified maxillary bones in a total destruction of the bone matrix, from a group of young, rapidly growing including its fibers. This characteristically white rats were stained with a differential takes place in locations next to and on collagenous and precollagenous poly- either side of periodontal blood vessels. chrome stain. A second group of rats was Continuations of these vessels pass into treated with tetracycline, a vital bone- the resorption spaces and eventually be- marking dye, and their maxillae were then come the central blood vessels within Hav- sectioned and studied by fluorescence ersian canals following the formation of microscopy. The purpose of these pro- secondary osteones. The overall process cedures was to trace the sequence of ad- provides a means by which reattachment justments that take place in the perio- of fibers becomes established in those local- dontal membrane and alveolar bone dur- ATTACHMENTS OF PERIODONTAL MEMBRANE 141 ing facial growth and the complex move- ences corresponding movement. Drifting ments of teeth. of the periodontal membrane involves two The different kinds of bone surfaces lin- basic and different processes. One repre- ing the alveolar socket are described. Of sents a mechanism associated with deposi- special interest is the finding that resorp- tory surfaces and the other with resorptive tive surfaces adjacent to the root are as- surfaces. In all areas, the actively moving sociated with two separate circumstances. periodontal membrane is composed of First, approximately half of the total three zones. The inner zone is attached resorptive surface area involves total to the cementum and contains mature col- fibrous destruction and loss of attachment. Iagenous fibers which are relatively stable. This occurs primarily in regions adjacent The outer zone also contains mature, to vascular bundles within the periodontal coarse-bundled fibers that are anchored in membrane. On remaining surfaces of the alveolar bone as attachment fibers. The resorptive bone, some fibers within the intermediate zone, comparable with the in- original bone matrix are not destroyed but termediate plexus of Sicher, contains an become uncovered during bone removal abundant distribution of young, precol- and are subsequently incorporated into the lagenous fibrils. Adjacent to depository laterally-moving periodontal membrane. surfaces of alveolar bone, these young These fibers retain direct connection with fibrils are converted into the coarse fiber the fibrous matrix of the alveolar bone and bundles of the outer zone as the entire provide fibrous attachment and anchorage membrane shifts and as former outer between the resorptive surface of the drift- fibers become embedded into new surface ing bone and the contiguous periodontal bone. The young, labile precollagenous membrane. fibrils of the intermediate zone also pro- Deep to resorptive surfaces of alveolar vide continuous linkage with the mature, bone, enlarged resorption canals contain- stable fibers of the inner zone attached to ing blood vessels extend into the bone in the cementum. Adjacent to resorptive advance of the resorptive front. Progres- bone surfaces, fibers released from the sive deposition of bone within these ero- resorbing bone matrix become progres- sion spaces takes place, and the fibrous sively shortened within the intermediate matrix of the resulting secondary Haver- zone as the periodontal membrane drifts sian systems is in direct continuity with in a direction toward the receding bone the fibers of the periodontal membrane. surface. Continuous relinkage occurs be- Repetition of this remodeling process oc- tween the newly-formed fibrils of the in- curs as resorption of the alveolar surface termediate zone and these fibers of the re- continues. This mechanism provides at- sorbing bone matrix which serve as the tachment and subsequent reattachment be- transient outer fibrous zone of the perio- tween bone and tooth in those particular dontal membrane. Linkage adjustments areas which previously had involved total simultaneously occur in the overlap area destruction of fibrous connection between between the intermediate and inner zones. the periodontal membrane and alveolar Actual mechanisms of linkage and subse- bone. As each Haversian system under- quent re-linkage at the molecular level goes removal, in turn, the fibers of the are not now known. Haversian matrix become released and The combination of processes outlined serve as transient periodontal fibers that above provides continuous attachment of retain continuity with the remaining bone. the tooth to its alveolar wall during the Surface remnants of such secondarily- complex variety of movements that take formed bone, as it undergoes progressive place as teeth undergo relocation due to destruction during resorption, represent in maxillary and mandibular growth, erup- part the localized areas previously attri- tion, drifting, rotation, and tilting. buted to “spot” deposition and repair dur- ing resorption. ACKNOWLEDGMENTS Just as the root of a tooth and its sur- This study was supported in part by rounding bone undergo a process of drift, U.S.P.H.S.grant DE-01903 and by summer the periodontal membrane itself experi- student fellowships awarded by the Uni- 142 ALLAN G. KRAW AND DONALD H. ENLOW versity of Michigan School of Dentistry. Herovici, C. 1963 A polychrome stain for dif- Diagrams used in this report were pre- ferentiating precollagen from collagen. Stain pared by Mr. William L. Brudon. Tech., 38: 204-205. Hoyte, D. A. N., and D. H. Enlow 1966 Wolf€'s LITERATURE CITED law and the problem of muscle attachment on resorptive surfaces of bone. Am. J. Phys. Behatia, H. L. and R. F. Sognnaes 1963 Tetra- Anthro., cycline discoloration and labelling of teeth and 24: 205-214. bone. A review. J. South. California D. A., Humason, G. L. 1962 Animal Tissue Tech- 31: 215. niques. W. H. Freeman and Co., San Francisco. Bevelander, G., H. Nakahara and G. Rolle 1960 151-152. The effect of tetracycline on the development Hunt, A. M. 1959 Description of the molar of the skeletal systems of the chick embryo. teeth and investing tissues of normal guinea Develop. Biol., 2: 298. pigs. J. Dent. Res., 38: 216-231. Bevelander, G., G. Rolle and S. Cohlan 1961 Orban, B. 1927 Beziehungen zwischen zahn The effect of administration of tetracycline on und knochen. Bewegung der zahn keime. 2. F. the development of teeth. J. Dent. Res., 40: Anat. u. Entw., 83: 804. 1020. Owen, L. N. 1963 The effects of administering Eccles, J. D. 1959 Studies on the development tetracyclines to young dogs with particular of the periodontal membrane: The principal reference to localization of drugs in the teeth. fibers of the molar teeth. Dental Practit. Dent. Arch. Oral Biol., 8-715. Rec., 10: 31-35. Sicher, H. 1923 Bau und funktion des fixation- Enlow, D. H. 1962a A study of the post-natal sapparatus der meerschweinchenmolaren. 2. f. growth and remodeling of bone. Am.-J. Anat., Stomat., 21: 580. 110: 79-102. 1942 Tooth eruption: The axial move- 1962b Functions of the haversian sys- ment of continuously growing teeth. J. Dent. tem. Am. J. Anat., 110: 269-306. Res., 21: 201-210. 1963 Principles of Bone Remodeling. Sicher, H., and J. P. Weinmann 1944 Bone Charles C Thomas, Publisher, Springfield, Ill. growth and physiologic tooth movement. Am. 1964 A study of the postnatal growth J. Ortho. and Oral Surg., 30: 109. of the human mandible. Am. J. Orthodontics, 1954 Principal fibers of the periodontal 50: 25-50. membrane. The Bur., 55: 2-4. 1965a Growth and remodeling of the 1965 Oral Anatomy. C. V. Mosby, St. human maxilla. Am. J. Orthodontics, 51: 446- Louis. 276-277. 464. Stallard, R. E. The utilization H3-pro- 1965b The problem of muscle tension 1963 of and the stimulation of bone growth. Anat. Rec., line by the connective tissue elements of perio- dontium. Periodontics, 1: 131: 3, 451. 185-188. Finerman, G., and R. A. Milch 1963 In vitro Stein, G., and J. P. Weinmann 1925 Die phy- binding- of tetracycline to calcium. Nature, siologische wanderung der Zahne. Z. f. Sto- 198: 486. mat., 23: 733. Goldman, H. M. 1957 Histologic structure of Tomes, C. S. 1923 A Manual of Dental Anat- the attachment apparatus. Alpha Omegan, 25: omy. Ed. by H. W. Tims and C. B. Henry. 102. MacMillan Co., New York. 128-232. Goto, S. 1952 Studien dber die fastern des Trott, J. R. 1962 The development of the perio- plexus intermedius der zahnwurgelhaut. Anat. dontal attachment in the rat. Acta anat. Inst. Med. Fak., University of Okazama, 4: (Basel), 51: 313-328. 177. Weinmann, J. P. 1954 The adaptation of the Gottlieb, B. 1942 Some histologic facts useful periodontal membrane to physiologic and patho- in orthodontic practice. Am. J. Orthodontics, logic changes. J. Oral Sur., Oral Med. and Oral 28: 167. Path., 8: 977. Gregg, J. M., and J. K. Avery 1964 Studies of Zwarych, P. D., and M. B. Quigley 1965 The alveolar bone growth and tooth eruption using intermediate plexus of the periodontal liga- tetracycline-induced fluorescence. J. Oral. Ther. ment: History and further investigations. J. and Pharm., 1: 268-281. Dent. Res., 44: 383-391. PLATES PLATE 1

EXPLANATION OF FIGURES

1 Photomicrograph of a tetracycline-injected specimen using ultraviolet light. New bone deposition (light-appearing band) occurs on the alveolar surface (A) of the tension side of the socket. In contrast, bone formation on the pressure side takes place deep to the surface within resorption spaces (B). x 50. 2 The periodontal membrane is composed of an inner zone (A) adjacent to the cementum, an intermediate zone (B), and an outer zone (C) contiuous with the fibrous matrix of the alveolar wall. The bone surface seen in this section is resorptive, and resorption spaces and secondary Haversian systems (osteones) form and continuously reform (D) deep to the surface in advance of the drifting resorptive front. X 100. 3 On the depository alveolar surface (tension side of socket), the inner zone (A), intermediate zone (B), and the outer zone (C) of the periodontal membrane are apparent, although the sequence of changes involved in periodontal drift differs from that on the pressure (resorptive) side. Compare with figure 5. Note the embedded periodontal fibers within the bone. X 200. 4 A thin alveolar wall separates two adjacent sockets. The bone surface on the left (C) is resorptive, and that on the right (E) is depository. The inner (A), inter- mediate (B), and outer (C) zones of the periodontal membrane are seen. As the resorptive surface drifts toward the right, bone matrix fibers are uncovered and become the fibers of the outer zone (C). A trail of such fibers can be seen exiting from primary alveolar bone tissue as well as from secondary Haversian bone. Note that total fiber destruction and loss of attachment occurs, however, in ad- vance of blood vessels (D). Such an area will later become partially filled with bone to become a secondary Haversian system. Note also that Haversian remodel- ing occurs only in the thicker areas of the alveolar wall and is absent from very thin, transient regions (compare area D with E). X 100.

144 ATTACHMENTS OF PERIODONTAL MEMBRANE PLATE I Allan G. Kraw and Donald H. Enlow

145 PLATE 2

EXPLANATION OF FIGURE

5 This schematic series of diagrams is an interpretation of adjustment mechanisms involved in the movements of the periodontal membrane during bone and tooth shifts. They illustrate successive and sequential changes that occur in the alveolar bone (A), the outer periodontal zone (B), intermediate periodontal zone (C), inner periodontal zone (D), and the cementum of the root (E) as both the bone and tooth drift to the right (arrows). The increment“U” designates an arbitrary distance moved during each growth stage. The illustrations on the left (1, 2, 3) represent the series of drift changes on the depository (tension) side of the alveolar socket, and the illustrations on the right (4, 5, 6) show changes on the resorptive (pressure) side. The Foints “x” and “y” are included as random posi- tion markers so that movements relative to a fixed point can be visualized. On the tension side (1, 2, 3), increments of new bundle bone are deposited on the alveolar surface, thereby progressively enclosing the fibers of the outer zone B. At the same time, the peripheral ends of the fibrils in the intermediate zone C are converted into the new fibers of the outer zone as the latter shifts to the right. The stable fibers of the inner zone D are pulled to the right and maintain constant linkage with the labile fibrils of the intermediate zone C. These latter fibrils constantly elongate in a direction toward the right as they simultaneously become converted to outer zone fibers on the left. On the pressure side (4, 5, 6), the stable fibers of the inner zone are moved to the right and similarly maintain constant linkage with the shortening inner ends of the fibrils in the intermediate zone. Bone-matrix fibers are constantly un- covered from the resorbing surface of A to become the fibers of the outer perio- dontal zone B. The inner ends of these same fibers are then converted successively into the elongating outer ends of the linkage fibrils in the intermediate zone. Direct fibrous continuity can be traced through each zone from A to E. Extensive bone reconstruction is seen deep to the resorptive alveolar surface, in contrast to the depository side, and resorption canals and secondary osteones are formed. This remodeling mechanism provides re-attachment in those localized areas that previously involved total resorptive destruction of fibrous anchorage. Such alter- nating areas are shown along the contact between A and B. ATTACHMENTS OF PERIGDONTAL MEMBRASE PLATE 2 Allan G. Kraw and Donald H. Enlow

1 4 U

2 5

3 6