RESEARCH

AND EDUCATION

JOHN

Section editor

J. SHARRY,

The remodeling

of the edentulous

mandible

Donald H. Enlow, Ph.D.,* Henry J. Bianco, D.D.S.,* and Stephen Eklund, D.D.S.** West Virginia University, School of Dentistry, Morgantown,

W. Vu.

T

his study describes the distribution of surface fields of resorption and deposition in all parts of the edentulous mandible. These remodeling fields relate to the marked alterations in regional shape and dimensions of key parts of the whole mandible associated with complete or nearly complete loss of dentition. Some of the changes involved are consistently observed from mandible to mandible, but others are variable in individual specimens. Previous reports have described the distribution of resorptive and depository fields involved in the growth and remodeling of the mandible from childhood to adult years.l The histologic criteria used to identify the various kinds of resorptive and depository surfaces were also described. The distribution pattern of remodeling fields in the young, growing mandible is illustrated in Fig. 1. Remodeling of the child’s mandible is a basic part of the growth process itself and functions not only to enlarge the various parts of the bone but, importantly, to subsequently relocate these parts as the whole bone continues to increase in size. The ramus, for example, becomes relocated in a progressively posterior direction as the corpus simultaneously lengthens into regions previously occupied by the ramus. These remodeling changes require selective fields of surface resorption and deposition to bring about the required complex three-dimensional growth movements (Fig. 1). All other parts of the growing mandible are similarly involved in localized processes of relocation and remodeling.’ The childhood growth fields remain recognizable in adult, nongrowing mandibles with full dentitions. That is, the surface histologic characteristics related to past resorptive and depository activities can still be distinguished in the adult form even This study was supported,

in part, by United

States Public Health

Service grant DE 03761-

02. *Departments tistry. **Chief,

of Anatomy

Service Unit

Dental

and Prosthodontics, Program,

Indian

West Virginia Health

University,

Service, Many

School of Den-

Farms, Ark.

685

606

Enlow,

Bianco,

and Eklund

J. Prosthet. Dent. December, 1976

Fig. 1. The patterns of bone deposition (light stipple) and resorption (dark stipple) that carry out the growth and remodeling changes in the young mandible are shown. Compare these patterns with the distribution of remodeling fields in the edentulous mandible (Fig. 2).

though the growth changes themselves have ceased. While through the years the internal structure of the cortical bone becomes progressively replaced by osteons, the surface features as well as the underlying stratification produced during childhood remain largely identifiable. In relation to extensive losses of teeth, however, remodeling of the shape and regional size of the mandible becomes involved, and it is the pattern of these changes that will be described in this report. The distribution of remodeling fields in the edentulous mandible differs markedly from that of the young, growing, fully dentulous mandible. With loss of teeth, the functional and structural relationships of the whole adult mandible change, and specific remodeling responses are the functional result. This involves altered occlusal relationships, rotations of the entire mandible, realignment of the mandibular corpus, redesign of the morphology of the corpus, altered muscleto-bone alignments, changes in locations of muscle attachments, and decreased areas for muscle insertion. This report describes remodeling processes that produce the varied changes in regional form and size of the mandible in response to some or all of these altered functional relationships, MATERIALS AND METHODS The methods used herein are the same as those used to determine patterns of growth and remodeling in the mandible and all other bony elements of the skull in the growing child. Fifteen mandibles at various stages of tooth loss were selected for study. Of these, 10 did not involve any artificial teeth. Nine specimens were completely edentulous, and six retained two or three anterior or posterior teeth. NO significant differences in the range of variations were found among these different groups of specimens. Transverse, ground serial sections were prepared through the entire left half of each mandible from the condyle to the chin. The periosteal surface of each section was then analyzed as to the resorptive or depository nature of all areas around its perimeter. A three-dimensional mapping of the serial sections was done to show the

Remodeling

the edentulous

mandible

607

Fig. 2. The remodeling changes that take place in the mandible in relation to the loss of teeth result in major regional changes in shape and dimensions. This is a composite diagram illustrating the most commonly observed regional remodeling patterns that bring about these structural changes in the edentulous mandible. Resorption is represented by dark stippling and deposition by light stippling.

over-all distribution of resorptive and depository fields for each mandible. The distribution patterns of these remodeling fields for all mandibles were compared, and common patterns of remodeling changes as well as major variations were determined. OBSERVATIONS Fig. 2 represents a composite diagram of the most frequently observed combinations of resorptive and depository fields for the sample. Fig. 3 is a profile overlay showing some two-dimensional effects. As a consequence of these remodeling processes, various morphologic changes (to be outlined) are produced. The regions in which the changes occur are indicated in Fig. 4 by the numbers corresponding to each of the following descriptions. (1) The over-all horizontal length of the bony mandibular arch does not decrease and may actually increase since (a) the mental protuberance is a depository type of remodeling field and (b) resorption occurs along the vertical length of the anterior border of the ramus and the temporal crest, thereby adding to the horizontal length of the corpus (see also IO). When resorption takes place on either the periosteal or the endosteal side of a bony cortex, deposition ordinarily occurs on the opposite surface of that same cortex. The whole cortical plate thus moues in either an endosteal or a periosteal direction without significant change in cortical thickness. An exception involves osteoporosis, in which noticeable thinning of the cortex can occur due to a greater extent of resorption on one side or a lesser extent of deposition on the other. While the chin region in the edentulous mandible is a “depository” surface, it is not presently known whether active deposition actually takes place following loss of teeth. This depository surface of bone may be the same surface, but in an inactive state, that was laid down during the late childhood period. However, the point to

680

Enlow,

Bianco,

and Eklund

J. Prosthet. Dent. December, 1976

Fig. 3. The structural results of the remodeling changes caused by the resorptive and depository fields illustrated in Fig. 2 are represented in this two-dimensional (before-and-after) overlay. The remodeling changes are depicted by the broken line.

be made is that the mental region does not become actively resorptive in the edentulous mandible, a process that results in a regional regression. (2) The over-all breadth of the bony arch can increase due to the presence of a depository field along its lateral (buccal) sides. The arch becomes somewhat more squared and less V-shaped. The breadth of the corpus itself from the buccal to the lingual cortex is increased due to depository fields on both sides. However, specific remodeling variations in about half of the specimens studied resulted in a decrease, not increase, in arch width (to be described later) . (3) Alveolar involution in inferior and posterior directions occurs in the incisor region of the edentulous mandible, and the alveolar bone is progressively reduced to a residual ridge. The posterior direction of this alveolar remodeling change does not significantly affect the basal bone, which actually may have an anterior direction of increased corpus growth at the chin (or at least an absence of resorption). Combined with (a) a regression of maxillary alveolar bone back to the more posteriorly located area of maxillary basal bone, (b) a forward rotation of the whole edentulous mandible at the condylar pivot, and (c) a more downward angulation of the corpus at its junction with the ramus (which has the effect of lengthening the over-all dimension of the mandible), a marked protrusion of the mandible results. While a destructive removal of the alveolar processes occurs, a constructive addition of bone takes place simultaneously within the depths of the edentulous sockets. Note the difference in the locations of the reversal lines (between the areas of resorptive alveolar and depository basal bone) on the lingual as compared with the labial sides in the anterior part of the arch. The line is much lower on the labial side. Major variations from this pattern were infrequent in the mandibles studied. The labial and lingual reversal lines represent the most inferior extent to which alveolar reduction is likely to progress unless overridden by heavy mechanical or unusua.1 physiologic factors. Resorptive reduction is less marked and characteristically more stable on the lingual in contrast to the labial side in the incisor and canine

Remodeling

Fig. 4. The numbers in these two views of the edentulous descriptions in the text.

the edentulous

mandible

mandible

correspond

689

to the regional

region. This region is often elevated well above the premolar and molar regions in the edentulous mandible and is associated with the higher placement of the reversal line in the anterior-lingual part of the jaw. These observations also correlate with the posterior directional tendency of alveolar reduction in the incisor region, since, due to its resorptive nature, the labial cortical plate moves in a more posterior direction than the lingual plate. The labial cortex, further, is noticeably thin in contrast to the lingual side, which retains substantial thickness. (4) An inferior course of alveolar resorption occurs on the dorsum of the corpus from the canine through the molar regions. The reversal line on the lingual side between the resorptive alveolar field and the depository field of the basal bone tends to be much lower than in the incisor region and usually occurs along the oblique . . mylohyold Ime. The residual ridge in the premolar and molar regions thus shows a pronounced dishing as it grades back from the more elevated incisor region, even in fully edentulous specimens. Cause-and-effect factors involved in the placement of these various reversal lines are not now known. (5) Along the inferior border of the corpus, bone deposition takes place throughout its length except in the antegonial region. (6) The inferior border of the corpus in the antegonial region is resorptive. This is associated with a more downward alignment of the entire corpus relative to the ramus, a change that contributes to the occlusal closure of the forward-rotated edentulous mandible. The forward and upward rotation of the whole mandible in conjunction with loss of dentition is offset, at least in part, by this remodeling adaptation of the mandibular body at its junction with the ramus. This alignment change produces a more pronounced antegonial notch, the extent of which is further increased by (a) deposition along the ventral border of the entire length of the corpus together with (b) resorption in the notch itself. Note the marked convex curvature of the ventral profile of the body as it grades into the prominent concave antegonial notch and relatively high genial region (the ventral border of the latter also tends to be

690

Enlow, Bianco, and Eklund

J, Prosthet. Dent. December, 1976

resorptive) . A typical Class III type of mandible with full dentition also often shows this antegonial notching effect. Likewise, this occurs in conjunction with a downward alignment of the corpus relative to the ramus in response to a forward and upward rotation of the whole mandible at the condylar pivot. In many patients with a Class III jaw relationship, this is associated with a relatively short, vertical midface dimension.” However, the genial angle itself is not necessarily changed by these various remodeling alterations, since differential extents of resorptive activity along the posterior border of the ramus can result in more or less constant gonial angulation.” (7) Bone deposition along the entire lateral side of the basal bone of the corpus produces a more pronounced lateral protrusion of the trihedral eminence. This adds to the squaring effect of the bony arch as a whole. (8) On the medial (lingual) side of the corpus, generalized deposition of bone occurs inferior to the mylohyoid line. This extends into the lingual fossa, and the fossa tends to become filled and leveled. (9) A reversal exists along the crest of the mylohyoid line. Superiorly, the alveolar surface is resorptive. In combination with the depository field of remodeling in the lingual fossa, just below the resorbing alveolar region, the breadth of the basal part of the body becomes widened and the overlying alveolar region becomes

markedly pattened. (10) Both the posterior (10) and the anterior borders (lb) of the ramus (and the contiguous temporal crest) are resorptive in nature. This narrows the posteroanterior breadth of the ramus. Since the extent of backward remodeling of the anterior border approximates the amount of forward remodeling on the posterior border, the relative positioning of the mandibular foramen remains fairly constant (about midway) between the two margins. The edge of the gonial region moves anteriorly and superiorly due to a resorptive field on its posterior and inferior surfaces. The occurrence of a resorptive surface on the anterior border of the ramus serves to accommodate the mass of the temporal muscle between the ramus and maxillary tuberosity as the whole ramus rotates toward the tuberosity due to dentition loss and edentulous occlusal closure. The region of the anterior ramus border that abuts the maxillary tuberosity during edentulous occlusion shows a pronounced notching. This can give the coronoid process a flared and somtimes hooklike configuration. (l.l) Following tooth loss, resorption in the inferior region of the anterior border of the ramus also functions to align the surface contour of this region with the resorptive surface on the posterior part of the dorsum of the corpus. (12) The subcondylar region on the anterior side of the neck is resorptive. This produces a localized dishing effect. The resultant cortical depression accommodates the abutment of the articular tubercle of the glenoid fossa during edentulous occlusal closure. (13) The lingual side of the coronoid process and the ramus just inferior to the si,mnoid notch are both depository regions. The contralateral buccal side is resorptive. Since the lingual surfaces of these parts face superiorly, this combination results in an elevation of the sigmoid notch, a vertical lengthening of the ramus in

Remodeling

the edentulous

mandible

691

a superior direction, and an elongation of the coronoid process. The latter usually rises well above the level of the condyle in many edentulous mandibles (a much less frequently encountered variation involves an actual shortening of the coronoid process). (14) The presence of bone resorption on the lingual side of the gonial region along with deposition on the buccal side produces a tendency toward a flaring and buccal protrusion of this region. However, a similar tendency can also be found in many adult, tooth-bearing mandibles. VARIATIONS Of the 15 mandibles analyzed in this study, four showed variable regions of deposition in the posterior part of the ramus on the lingual side. This remodeling variation results in a lingual movement of this region that brings it more into line with the axis of the corpus (in the dentulous mandible, this region of the ramus lies well to the lateral of the arch). In six specimens, a large field of resorption occurred on the lateral side of the corpus anterior to the trihedral eminence (which remains depository) . The remodeling effect is to narrow rather than widen the arch of basal bone and to make the arch more V-shaped. Both the buccal and lingual sides of the basal bone cortex of the corpus anterior to the trihedral eminence (i.e., the former premolar and canine areas) were resportive in four specimens. This reduces the transverse thickness of each corpus and narrows the over-all arch width, The more stable inferior border of the corpus, dimension becomes however, remains depository in nature, so that the vertical reduced only by the resorption of the alveolar area. In three specimens, the buccal side of the basal bone cortex in the former premolar and canine regions is resorptive, but the lingual side is depository. This combination reduces arch width in this part of the mandible, although the transverse breadth of each corpus is not decreased to the extent seen in the previous remodeling variation. A resorptive surface in the entire region of the lingual fossa was found in seven specimens. Thereby, a reduction in transverse corpus thickness from the lingual cortex results in this region. The depth of the fossa is increased in these particular mandibles in conjunction with a marked resorptive flattening of the overlying alveolar process previously housing the molars. In six of the specimens, the inferior part of the ramus on the buccal side was entirely resorptive. Because the contralateral lingual side also was resorptive in these same mandibles, the effect was to reduce the cortex-to-cortex thickness of each ramus by reducing the volume of the medullary area between the cortical plates (the endosteal sides of each cortex are depository). Seven specimens showed a resorptive lingual surface in the region anterior to the mandibular foramen. This results in a thinning of the region and a depression of this area, reducing and flattening the edge of the temporal crest. It also results in a decreased area for temporal muscle attachment. Resorption on the lingual side of the basal bone cortex in the former premolar

692

Enlow, Bianco, and Eklund

J. Prosthet. Dent. December, 1976

and canine regions together with deposition on the contralateral buccal side was found in three mandibles. This combination increased arch width in these specimens but did not materially affect the transverse cortex-to-cortex breadth of each side of the arch itself. SUMMARY In a sample of mandibles having complete or nearly complete loss of dentition, the left half of each mandible was serially sectioned. The entire perimeter of each section was analyzed for the distribution of resorptive and depository periosteal surfaces, and from this information, the fields of remodeling were mapped for the mandible as a whole. The most common patterns of combined resorption-deposition and the range of variations were then determined. The over-all distribution of remodeling fields in the edentulous mandible differs markedly from that in the young, growing mandible. In most of the edentulous specimens, the surface of the basal bone on both the medial and lateral sides of the corpus is of a depository nature. The overlying alveolar regions on both the lingual and buccal sides, however, are characteristically resorptive. Significantly, the placement of the reversal line between the alveolar resorptive and the basal depository areas is much lower (i.e., at the level of the mental foramen) on the buccal side. Except for its inferior part, the lateral side of the ramus tends to be largely resorptive in character, and the posterior half of the lingual side also tends to be resorptive. Unlike the child’s mandible, the posterior border of the ramus is resorptive, becomes and the posteroanterior dimension of the ramus (not the whole mandible) reduced and narrowed in conjunction with resorption along the anterior border. However, the amount removed from the anterior ramus is actually added to the dimension of the corpus, which becomes longer. Further, removal from the posterior ramus border does not affect the over-all length of the mandible unless condylar reduction is also involved. Also, over-all arch length is not decreased, because the surface of the mental protuberance is retained as a depository type of field (or at least does not become actively resorptive). The corpus-ramus angle (not gonial angle) is increased in the antegonial region. Because of the opening of this angle, over-all mandibular length as well as arch length is increased. In about half of the specimens, arch width was not decreased, because the lateral side of the corpus is usually of a depository nature. Notching of the anterior side of the condylar neck and the inferior part of the anterior ramus border is associated with resorptive fields in these regions, changes that are presumed to be a consequence of pressure contacts made with the articular tubercle and the maxillary tuberosity, respectively, in conjunction with a forward rotation of the whole mandible. The inferior direction of corpus realignment relative to the basal part of the ramus also increases the notching effect in the antegonial region, an effect augmented by the presence of the resorptive field in the notch itself. Certain specific variations commonly occur in several major regions of the mandible on both the lateral and medial sides. These remodeling variations affect the placement of the ramus relative to the corpus, the extent of flaring of the gonial region, the width of the arch, and the thickness of the basal part of the body. The illustrations were prepared by Roger Posten.

Volume 36 Number 6

Remodeling

the edentulous

mandible

693

References 1. Enlow, D. H.: The Human Face, New York, 1968, Harper & Row, Publishers. 2. Enlow, D. H., Kuroda, T., and Lewis, A. B.: The Morphological and Morphogenetic Basis for Craniofacial Form and Pattern, Angle Orthod. 41: 161-188, 1971. 3. Enlow, D. H.: A Handbook of Facial Growth, Philadelphia, 1975, W. B. Saunders Company. DR. EKLUND SERVICE UNIT DENTAL PROGRAM INDIAN HEALTH SERVICE MANY FARMS, ARIZ. 86503

DRS. ENLOW AND BIANCO WEST VIRGINIA UNIVERSITY SCHOOL OF DENTISTRY MORGANTOWN, W. VA. 26506

ARTICLES

TO APPEAR

Centric relation, arrangement Ellsworth

Kelly,

Outline

centric

of standards

A theoretical semiprecision

prosthetic

D.D.S.,

patients

eyes in orbital

of psychogenic D.D.S.,

of polymeric

The measurement Jack I. Nicholls,

An overdenture

aspects

of the Thompson

dowel

D.D.S.

and treatment

of bruxism

M.Ed.

materials

for maxillofacial

R. Glaser,

Ph.D.,

of distortion-Theoretical

prosthetics

Michael

J. Tabacco,

D.D.S.,

and

considerations

Ph.D.

survey:

V. Reitz, D.D.S.,

artificial

R. Roraff,

prostheses

M.S.

B.D.S., L.D.S.R.C.S.,

B. Mikami,

Arranging

and

for overdentures

for handicapped

Dorsey J. Moore, D.D.S., Zorach Michael G. Linebaugh, D.D.S.

Arthur

forms

F.A.C.P.

analysis of the mechanics intracoronal retainer

Neil S. McLeod,

Phillip M.Ed.

patients

MS.,

prosthesis

Aids for positioning

Evaluation

tooth

D.M.D.

D. Ray McArthur,

A review

and posterior

for evaluating

B.D.S., D.M.D.,

A new mouthstick

Donald

occlusion,

ISSUES

D.D.S.

Keki R. Kotwal,

Ellis Lutwak,

IN FUTURE

Preliminary

M.Ed.,

Marshall

teeth according

D.D.S.,

MS.

report G. Weiner,

to anatomic

D.D.S.,

and Bernard

landmarks

Levin, D.D.S.,

The remodeling of the edentulous mandible.

In a sample of mandibles having complete or nearly complete loss of dentition, the left half of each mandible was serially sectioned. The entire perim...
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