Bone Loss of EdentulousAlveolar RidgesbyDouglas A. Atwood, m.d., d.m.d.*

Following the extraction of teeth, the residual al-veolar ridge undergoes bone remodeling which involvesboth external and internal bone changes.1 The outstand-ing feature of this process is a continuing reduction inthe size of the residual ridge, probably due to localizedperiosteal osteoclastic résorption which is accompaniedby endosteal bone formation. This reduction of residualridges (RRR) occurs most rapidly in the first 6 monthsto 2 years following extraction, but in many individualscontinues seemingly unto death, resulting in the removalof massive amounts of jaw structure. The practical sig-nificance of this continuing loss of bone lies in the factthat removable dentures which are used to substitute forthe extracted teeth depend on the bony support of theresidual ridge for stability, retention, comfort, functionand aesthetics. If the bony base is constantly changingshape over an indefinite time period, even well-con-structed dentures become unsatisfactory and requiremultiple retreatments to restore comfort, function andappearance. Since there are estimated to be over 20million completely edentulous Americans, this consti-tutes a major socio-economic problem. Therefore, be-cause of both the suffering caused to the individualpatient and the national (worldwide) cost of these retreat-ments, RRR has been termed a "major oral diseaseentity" to be considered in the same category as cariesand periodontal disease, a pathologic disease processwhich is worthy of extensive research for better under-standing of its etiology and prevention.

It is the intent of this paper to explore certain charac-teristics of RRR and various factors which may influencethe rate of RRR.

Cephalometric studies by numerous authors in differ-ent parts of the world have shown that the mean rates ofRRR fall into remarkably consistent patterns1"6 whichare closely allied to time since extraction and time overwhich rate is measured. However, in every study, a widevariation between different individuals occurred in eachtime frame. For example, if one graphs the raw datapresented by Carlsson2 for 34 denture patients who wereall studied over a 5 year period, one sees not only thechange in rate relative to time postextraction, but alsothe range of variations from the mean. This raises thehope that if one could find the cause(s) of the differences

* Department of Prosthetic Dentistry, Harvard School of DentalMedicine, 188 Longwood Ave., Boston, Mass 02115.

in the rates of résorption, then perhaps one could learnto reduce or prevent this résorption in the future.

It is possible to take too simplistic a view of RRR bytrying to relate RRR to one or more of the mechanicalprocedures that dentists perform in constructing dentures(such as impression making, jaw relation records, denturetooth selection, etc.) rather than to the capacity of thebone to respond to these factors. In studies to date whichhave attempted to focus on one or more of these pros-thetic factors, there has been wide variability of RRRbetween patients from minimal résorption to quite con-

siderable résorption. To date, correlation with any oneof these factors has not been so great that a causalrelationship has been established.1"11 At this stage of our

studies of RRR, it seems most appropriate to assume

that the cause of RRR is either a factor not yet elucidatedor else a combination of several factors: i.e., a multifac-torial disease. Furthermore, while RRR is a process oflocalized destruction of bone and while local factors suchas dentures may well play an important role in denturewearers, we must start with the knowledge that RRRalso occurs in the nondenture wearer and is therefore notdependent on prosthetic factors alone.

RRR is a chronic, cumulative, localized disease ofbone remodeling. Remodeling of bone is dependent on

a variety of factors including the availability of viablebone cells, the biochemical factors (local and systemic)which influence bone cells, and the cell control broughtabout by physical loads applied to bone.12"16

Availability of Viable Bone CellsIt is known that specialized bone cells such as osteo-

blasts and osteoclasts have life expectancies far shorterthan human life. Therefore, their continued availabilitydepends on the genetic and environmental capacity ofthe progenitor cells to produce them. Since RRR is a

longitudinal and cumulative process persisting for therest of the patient's life, senescence may become an

important factor in the rate of RRR at certain stages dueto aging changes in the number and viability of bonecells. In addition to the loss of cells due to cell death,aging may slow down cells, diminish the amount ofworkthey perform per day, and reduce their sensitivity andprecision of response to normal regulatory stimuli. Clin-ical studies to date have demonstrated patients of differ-ent ages having either high or low rates of RRR. Anyslight trend for slower rate in the older patients may berelated more to time since extraction than to age. Never-theless, this factor should be further explored.

In addition, studies which have attempted to comparethe rate of RRR in patients who had simple extractionsvs. patients who had "alveolectomies" which removedsignificant amounts of labial cortical plate have tendedto show that the rate of RRR was greater in those whohad potentially osteogenic tissue removed by "alveolec-tomy".17"21 Carlsson et al.22 have shown that in patientswith simple extractions, the labial plate of lamellar bone

11

12 Atwood The Eighth English Symposiumis largely removed by osteoclastic activity by about 40days after extraction. However, parallel to this résorp-tion, bone formation takes place so that by 3 monthsboth the labial plate and the socket have been replacedwith new bone. In other words, the old labial plate serves

as a nidus for new bone formation.Furthermore, since RRR is a destructive disease which

removes in toto great bulk of the bone, the number ofany particular type of cell could be significantly reducedas the process proceeds as seems to occur in advancedosteoporosis.

Histologie or histochemical studies of cells and cellularactivity relative to RRR are limited by the availability ofadequate material for study. If one is to study RRRthrough histological examination of biopsy material,then the specimens must be taken from the crest of the

Table 1. Reduction of Residual Ridges: Vertical Height—AnteriorMandibular Ridge

Number Mean time Mean dura-Reference of Sub- postextr. tion

ject baseline film RRR

Carlsson VCarlsson VCarlsson VTallgren VICarlsson VCarlsson VCarlsson VTallgren VIHedegardTallgren VICarlsson VCarlsson VCarlsson VCarlsson VTallgren VICarlsson VTallgren VICarlsson VTallgren VIAtwood IIAtwood IIBergmanAtwood IIAtwood IITallgren VITallgren VIAtwood IITallgren VI

34343411343434117

11343434347

341134

98

1954161596

1820

yoooo0.1700.330.500.20.330.500.331.01.00.22.00.17

<0.60.5-2.0

2.0-10.010.0-20.07.0

13.5>20.0

10.0

yr0.170.330.50.250.331.00.50.53.01.01.01.05.02.03.01.07.03.0

13.52.52.51.02.52.56.51.52.5

15.0

Mean rateRRR

mm/yr12.08.66.85.65.14.153.32.542.52.411.951.51.441.351.10.950.940.650.570.560.550.550.510.490.220.220.220.20

Figure 2. An example of wellformed residual ridges with verylittle reduction in size.

Figure 3. An example of residual ridges which have undergonegross bone résorption resulting in massive reduction in size.

CentralIncisor

GenialTubercle

Figure 1. Six orders of mandibular anterior residual ridge form: Order I, pre-extraction; Order II, postextraction; Order III, highwell-rounded; Order IV, knife-edge; Order V, low well-rounded; Order VI, depressed. (From Atwood, D. .: J Prosthet Dent 13: 817,1963.)

Oral Perspective on Bone Biologyresidual ridge. For this reason, human biopsy materialhas been restricted in size and number.

The fact that the histologie appearances in differentparts of the same mouth can vary widely, probablyexplains the different observations and conclusionsfound in different biopsy studies. Furthermore, most

biopsy studies of human material are of soft tissue onlyand therefore do not reveal information about the boneor the important interface between bone and soft tissue.

Histologie studies of postmortem material are usuallyrestricted by a lack of clinical correlation since suchmaterial is usually obtained from cadavers of unknownclinical status.

Ideally, one would have sufficient biopsy material atintervals over a period of time or, at the least, post-

I mo. 7mo. 50 mo.

Figure 4. Tracings of three lateral cephalographs with the max-illae and mandibles carefully superimposed. Note the changes inthe shape of the residual ridges following the extraction ofremaining teeth 50 months before. (From Atwood, D. .: JProsthet Dent 13: 811, 1963.)

Edentulous Alveolar Ridges 13

mortem material from subjects who had been studiedclinically over a period of time. In each instance suchmaterial is difficult to obtain.

Suffice it to say, microscopic studies of residualridges32"38 have revealed the following: (1) varying de-

YEARS of REDUCTION of RESIDUAL RIDGES0:5 1 1,5 2 2.5 3 3.5 4 4.5

Irate: 0.13mm. /yr.min.: 2.0mm.

rate: 0.5mm./yr.ave.z68mm.

rate: 1.8mm./yr.

max.: 14.5 mm.

•after Carls»on,1967Figure 5. Graph illustrating typical mean mandibular bone losscurve during 5-year postextraction period with maximal andminimal ranges from data published by Carlsson, G. E. andPersson, G. : Morphologic changes of the mandible after extrac-tion and wearing dentures. Odontol Revy 18: 27-54, 1967.)

E «

|MEAN 65.2 10.1

.50

.25

OO

MEDIAN - -1—r

5 "75 -flj ¿5 50 55 60 65 70 75 80 85 90

AgeFigure 6. Rate of RRR (total) vs age. (From Atwood, D. A. and Coy, W. ., J Prosthet Dent 26: 289, 1971.)

14 Atwood The Eighth English Symposiumgrees ofkeratinization, acanthosis, thickness, edema, andarchitectural pattern of epithelium in the same mouthand between subjects;23"29 (2) varying degrees of inflam-

Figure 7. Gross bone loss of residual ridges is revealed bycareful superimposition ofportions of two cephalometric radio-graphs made 16 years apart. The actual bone loss in the anteriorpart of the ridge of the mandible was 13 mm in height (a 41%reduction) and 60 sq mm in cross-sectional area (a 24% reduc-tion). (From Atwood, D. .: J Prosthet Dent 26: 268, 1971.)

matory cells from clinically normal to frankly inflamedareas in both denture and nondenture wearingpatients;23"29 (3) lymphocytes,23"29 plasma cells,23,27 mastcells29 and osteoclasts;22'23,27'29 (4) dense, fibrous connec-

tive tissue (sometimes hyalinized) frequently observedover crestal bone with fibers running parallel to epithelialsurface;23"27 (5) a vascular plexus outside the periosteumin areas of bone apposition;29 (6) small blood vessels inclose contact with the bone margin in areas of bonerésorption, sometimes in the lacunae with positive cor-

relation between the degree of inflammation, vascularreactions, and bone résorption;29'38 (7) marked diapho-rase activity in areas of bone remodeling—either for-mation or résorption;29 (8) ATPase activity in areas ofbone formation and acid phosphatase activity in areas ofbone résorption;29 (9) the lack of evidence of bone ré-

sorption in areas which do not have inflammatory cells;29(10) endosteal bone deposition reinforcing internal struc-ture where external surface has been affected by résorp-tion;22' 23' 27' 29'32 (11) lack of periosteal lamellar bone on

the external surface of the crest of the ridge;22' 23' 27'29,32

(12) a roughened crestal bone surface which is eitheractively resorbing or is inactive, but without reversallines on the external surface of the crestalbone;22,23' 27' 29'32 (13) development of secondary Haver-sian systems in remodelled compacted endosteal bone;32(14) microradiographic evidence of mandibular osteo-porosis including increased variation in the density of

Figure 8. A microradiograph of a portion of the crest of the ridge showing external résorption of both the lingual cortex and thetrabeculae on the crest of the ridge. Also evident is an osteon within a single trabecula, and evidence of internal bone formation, andrésorption of the trabeculae. (From Atwood, D. .: J Prosthet Dent 13: 816, 1963.)

Oral Perspective on Bone Biology Edentulous Alveolar Ridges 15

Figure 9. A microradiograph of the inferior border of a mandible showing evidence of moderate osteoporosis with increased variationin the density of osteons, increased number of incompletely closed osteons, and increased endosteal porosity. (From Atwood, D. .: JProsthet Dent 13: 816, 1963) (Original magnification, x 15).

osteons, increased number of incompletely closed os-

teons, increased endosteal porosity and increased num-

ber of plugged osteons.31'32It may well be that noninvasive techniques will be

perfected which will provide information about the func-tional activities of bone cells and RRR which is addi-tional to that available from current histologie, histo-chemical and microradiographic techniques.Biochemical Factors Which Influence Bone Cells

Local Biochemical Factors. Much work has been donein the study of local factors in bone résorption in tryingto understand periodontal disease.39 Endotoxins fromdental plaque40 (plaque can occur in edentulous mouths),osteoclast-activating factor (OAF),41 Prostaglandins,42"43human gingival bone-resorption stimulating factor,44etc., are all factors which could be important to the rateof RRR and deserve further study. For example, heparin,a cofactor in bone résorption, has been associated withmast cells which have been observed toward the bonemargin of both residual ridges29 and gingiva.45 As withperiodontal disease, local biochemical factors triggeredby microorganisms could be a factor in RRR, especiallywhen there is poor oral hygiene and when dentures are

worn constantly. In addition, toxins leaching from den-ture materials could be a factor.46"47 As noted previously,RRR also occurs where no dentures are worn.

Other possible local biochemical factors in RRR, es-

pecially under dentures, could be related to traumato-

genic increased or decreased vascularity leading to

changes in 02 tension49 or temperature.50Systemic Biochemical factors. Just as some patients

with natural teeth and poor local factors seem to havegreat resistance to periodontal disease,51 there are some

edentulous patients who do not have RRR and whoretain good ridge form for many years even in thepresence of local prosthetic factors which are felt to beunfavorable. Conversely, some patients have a high rateof RRR without any obvious unfavorable local factors.In such cases, the possibility exists that osteoporosis maybe a contributing factor whether it be idiopathic or inthe form of a deficiency of estrogen, calcium, or growthhormone, or an excess of thyroid hormone or

cortisone.12'16 52 The role of dietary intake of Ca, , Ca/ ratio, F, Vitamin D, protein, etc. is not yet establishedin the etiology of RRR.53"54

It is clear, however, that signs characteristic of osteo-

porosis have been found in human jaws.31,32' 36'37'56

Whether the pathophysiology of osteoporosis in anyspecific patient is decreased bone formation or increasedbone résorption or a combination of the two, the presenceof an active stage of osteoporosis could very definitelyhave an effect on the rate of RRR.1'52,55) 57 To date,published attempts to correlate osteoporosis as deter-mined densitometrically with the rate of RRR5 havefailed, but this need not indicate a lack of relationship.

16 Atwood The Eighth English Symposium

250

Z2S

c0 200

'

1.75

E 1.50E

~" u5 £a:a: too

a0£ .50

.25

.00

1 2 3 4 5 6 7 8DENSITY OF ANT. MAND.RIDGE (mm Al.)

Figure 10. Rate of RRR (mandible) vs density of anterior mandibular ridges (site 14). (From Atwood, D. A. and Coy, W. ., JProsthet Dent 26: 293, 1971.)

Osteoporosis is a cumulative disease whose degree ofactivity at any point in time may not be appropriatelyindicated by the radiographie densitometric techniquesutilized to date. Perhaps more sophisticated diagnostictechniques, employed longitudinally, would present dif-ferent conclusions.58"62Cell Control Brought About by Physical Loads

on Bone

Physical loads are placed on the alveolar bone in theindividual with natural dentition through the teeth andperiodontal ligaments to the lamina dura and supportingtrabecular bone.63"64 When the teeth are extracted, eitherthere is little or no loading of the residual alveolar ridgeas in the nondenture wearer, thereby perhaps resultingin a disuse atrophy of the ridge, or else the load istransmitted in the denture wearer through the artificialdenture teeth, the denture base, and the mucoperiosteumto the bone.65

Studies have shown that the mean rate of RRR isthree to four times greater in the mandible than in themaxilla, although for some patients the reverse istrue.1"3'5 In fact, Tallgren has shown an interesting in-verse relationship between the rates of RRR in themaxilla and mandible observed in 11 patients over a 7-year period.3 Such differences within the same individualsuggest the need to look at differences in the two jaws.

In attempting to determine the basis for these differ-ences, Woelfel, et al. showed that the average "projected"denture base area of 45 patients was 1.8 times greater for

the maxilla than for the mandible.10 There follows logi-cally from this that whenever the patient occludes themean load per unit area applied to the mandible is 1.8times more than that applied to the maxilla. For example,Woelfel, et al. cite a patient with projected area of themaxillary denture of 4.2 sq in and of the mandibulardenture of 2.33 sq in (ratio 1.8).10 If this patient biteswith a force of 50 lb, the resultant pressure would be 12lb per sq in under the maxillary denture and 21 lb per sqin under the mandibular denture.

In addition to masticatory forces, Ohashi, et al. havedemonstrated that swallowing forces in 21 patients av-

eraged 11.4 lb (2.7 lb per sq in under maxillary dentureand 4.1 lb per sq in under the mandibular denture).66

Cutright, et al. have calculated that 1500 empty swal-lows per 24 hours could amount to 3500 to 4200 lb ofloading per day.67 Further, they have demonstrated bothpositive and negative pressures on the residual ridgesfrom a variety of activities, including smoking, talkingand counting, as well as biting.

Brewer has shown that masticatory and swallowingcontacts between dentures average less than 15 minutesper waking day. What needs to be remembered is thatsome patients have been shown to clench and grind theirteeth up to several hours per 24 hours.68 It is likely thatsuch parafunctional forces place pathologic loads on

residual ridges in some patients.However, to date, there are no published data showing

a high correlation for either projected denture area or

parafunctional forces per 24 hours with RRR.

Oral Perspective on Bone Biology Edentulous Alveolar Ridges 17

It is commonly assumed by dentists that a carefullyfitted denture base will distribute the load through themucoperiosteum to the bone more evenly, thereby re-

sulting in less RRR. Workers have emphasized the im-portance of the thickness and the resilience of the mu-

coperiosteum. As a denture base is seated evenly againstits tissue base, only a small amount of displacement takesplace before the entire mucoperiosteum becomes a hy-draulic system exerting a compressive force evenly over

the denture-bearing area. It would appear that the mu-

cous membranes exhibit viscoelastic properties.70"73In discussing "damping" or energy absorption, in such

bones as vertebral body, Frost states: "... bones which

\%

Figure 11. The even distribution of force over the denturebearing areas with wellfitting dentures and equalized occlusion.(From Nagle, R. J. and Sears, V. H. :Dental Prosthetics, p. 333.C. V. Mosby Co., 1958.)

Figure 13. An example ofvariation in the type ofbone showinga broad maxillary cancellous ridge and a high but very narrow

("knife-edge") mandibular anterior ridge.

® ©Figure 14. Diagrammatic representation of the process of "in-waisting" of external surface of bones subjected to compressionloading which results in internal hydraulic-like effect and eventualbone remodeling. (From Frost, H. M.: The Laws of BoneStructure, 26. Springfield, Illinois, Charles C. Thomas, Pub-lishers, 1964.)

Figure 12. An enlarged radiographie view of a vertebral bodyshowing a trabecular pattern oriented parallel to the direction ofcompression deformation and with "inwaisting" on the right sideof the vertebral body.

are subjected largely to compression loads, and experi-ence no significant bending loads, are composed largelyof cancellous bone which is ideally constructed for theabsorption and dissipation of energy. This is because itconsists of innumerable tiny struts buried in a gelled,semifluid medium."74 A "damping" effect may be an

important factor in reducing the rate of RRR. A broadmaxillary cancellous ridge may react differently from a

narrow mandibular cortical ridge to the same compres-sion load.

Frost refers to a process called "inwaisting" whichresults from the sequential events of compression, inter-nal hydraulic-like effect, external bulging and externalosteoclastic activity with resultant narrowing ("inwaist-ing") of the bone.75 If the internal trabeculae of cancel-lous bone are numerous, thick and strong, compression

18 Atwood

loads cause less delormation, less hydraulic-like effect,less external bulging and hence less external osteoclastic"inwaisting". Older persons who have lost trabecularthickness and number will tend to have more verticaldeformation under compression and consequently more

external osteoclastic résorption. Perhaps this is a factorin RRR which is characterized by external osteoclasticrésorption, or a form of "inwaisting."

Frost points out that trabeculae are oriented parallelto the direction of compression deformation. For years,Applegate76 recommended phasing in the loading of an

edentulous ridge by first inserting a removable partialdenture without the teeth and then "exercising" the ridgeby intermittent gentle pressure with the goal ofstrengthening and reorienting the trabeculae on the crestof the ridge.77 Neufeld has demonstrated in dry humanjawbones a trabecular pattern in which the trabeculaeare oriented at right angles to the crest of the ridge(parallel to the direction of occlusal forces through thedenture base).78 This would be optimal to resist suchcompression loading from a denture.

The "neutrocentric" concept of occlusion (DeVan)65attempts by a variety of means to achieve compressionloading of the residual ridges, to minimize any tensionor shear loads, and thereby to reduce RRR. "Anatomi-cal" occlusal schemes seek to reduce RRR through effi-ciency of mastication, reduced occlusal contact area, andbalanced occlusion.10 If an anatomical occlusion "missescentric" either initially or subsequently due to RRR,3there is increased likelihood of tension and shear forceson the residual ridges. This is thought by some to increasethe rate of RRR, but even a careful longitudinal studyfor 5 years of 45 patients which comes to the oppositeconclusion has such a wide overlap of RRR in the three

Figure 15. Radiograph of thin midsaggital sections of eightmandibles illustrating a variety of types of bone and raising thequestion of the possible "damping" effect and "inwaisting".

The Eighth English Symposium

Figure 16. An example of a craniofacial relationship whichcould result in different loading of the residual ridges due to themuscle attachments and direction of muscular activity.

occlusal groups as to obfuscate the conclusions.9"11Most of this discussion of the loading of bone has

focused on the residual ridge per se. In actuality, weshould also be looking at the mandible as a whole boneand its functional and developmental anatomy.79"81

Tallgren has studied the possible correlation of RRRwith the morphological configuration.3 It is fair to as-

sume that the loading of the ridge is dependent on theshape of the craniofacial bones, the insertion of themuscle attachments, and the direction of muscular activ-ity.

The pathophysiology of the overloading of bone couldconsist of one or more of the following mechanisms:Trauma leading to increased vascularity, compressionleading to decreased blood flow or increased venous

stasis, piezo-electric effect, and stimulation of bone re-

sorbing cells through bone cell receptors activated byphysical forces.

Some of the most experienced and perceptive prostho-dontists for decades have warned against the overlyretentive denture which fits "tight" today but in a rela-tively short period of time loses its retention and "fit"either due to viscoelastic changes or RRR.65,69 Mostwarn against constant denture wearing.

One additional form of possible continuous loading ofthe residual ridge lies in the possibility of a postoperativecicatricial mucoperiosteum seeking a reduced area.81'82Furthermore, it is possible that the rate of RRR ispredetermined by the dentist, who removes the teeth, bysuch surgical factors as the amount of attached gingivathat is removed, the amount of cortical bone that isremoved, the amount of tissue reflection and dissectionof mucoperiosteum from the bone,83 the amount of al-veolectomy or alveolotomy, and the degree of tensionresulting from close approximation of opposing tissuesand tight suturing.

Oral Perspective on Bone Biology Edentulous Alveolar Ridges 19

Table 2. Co- Variables Possibly Related to Rate of RRR

Category Variables which may haveDirect Relation

Variables which may haveInverse Relation

TimeViable Bone Cells

Local Biochemical Factors

Systemic Biochemical Fac-tors

Load Factors

Progenitor CellsOsteoclastsSenescence

EndotoxinsOsteoclast-Activating Factor (OAF)ProstaglandinsHuman Gingival FactorImmune ComplexesHeparinParathyroid Hormone (PTH)Thyroid Hormone (TH)Adrenal Cortical Steroids

Amount, Frequency, Duration andDirection of Load

Load Per Unit AreaOcclusal Contact AreaTraumatic Prosthetic FactorsCraniofacial Morphology

Time Since ExtractionProgenitor CellsOsteoblasts

Anti-Invasion Factor (AIF)Indomethacin

Calcitonin (CT)Growth Hormones (GH)EstrogensAndrogensCa, Vit. D, Vit. C, FProtein

Denture Bearing AreaFavorable MucoperiosteumDamping Effect of Cancellous

Bone

Craniofacial Morphology

SummaryRRR is probably a multifactorial process subject to a

wide number of covariables. In any given patient, thesecovariables will be in a unique combination which willdetermine the rate of RRR for that patient at that time.If the variables change in any given patient, the rate ofRRR may change. The dentist who wishes to help hisedentulous patient should seek to be sensitive to all thesevariables and to understand and treat that unique com-bination of factors which exists in that patient.

References1. Atwood, D. .: The reduction of residual ridges. A

major oral disease entity. J Prosthet Dent 26: 266, 1971.2. Carlsson, G. E. and Persson, G.: Morphologic changes

of the mandible after extraction and wearing of dentures.OdontolRevy 18: 27, 1967.

3. Tallgren, .: The continuing reduction of the residualalveolar ridges in complete denture wearers: A mixed longitu-dinal study covering 25 years. J Prosthet Dent 27: 120, 1972.

4. Hedegard, B.: Some observations on tissue changes withimmediate maxillary dentures. Dent Pract 13: 70, 1962.

5. Atwood, D. ., and Coy, W. .: Clinical, cephalometricand densitometric study of reduction of residual ridges. JProsthet Dent 26: 280, 1971.

6. Bergman, B., Carlsson, G. E., and Ericson, S.: Effect ofdifferences in habitual use of complete dentures on underlyingtissues. Scand J Dent Res 79: 449, 1971.

7. Thompson, J. R.: The rest position of the mandible andits significance to dental science. J Am Dent Assoc 33: 151,1946.

8. Atwood, D. .: A cephalometric study of the clinicalrest position of the mandible. Part II: The variability in therate of bone loss following the removal of occlusal contacts. JProsthet Dent 7: 544, 1957.

9. Igarashi, T., Woelfel, J. B., and Winter, C. M.: Computer

analysis of twelve factors related to mandibular ridge résorp-tion. J. Prosthet Dent (IADR Abstr. 443) 30: 1971.

10. Woelfel, J. B., Winter, C. M., and Igarashi, T.: Five yearcephalometric study of mandibular ridge résorption with dif-ferent posterior occlusal forms. Part I: Denture constructionand initial comparison. J Prosthet Dent 36: 602, 1976.

11. Winter, C. M., Woelfel, J. B., and Igarashi, T.: Five yearchanges in the edentulous mandible as determined on obliquecephalometric radiographs. J Dent Res 53: 1455, 1974.

12. Frost, H. M.: Bone Remodeling Dynamics. Springfield,Illinois, Charles C. Thomas, Publisher, 1963.

13. Frost, H. M.: The Laws of Bone Structure. Springfield,Illinois, Charles C. Thomas, Publisher, 1964.

14. Frost, H. M.: Bone Modeling and Skeletal ModelingErrors. Springfield, Illinois, Charles C. Thomas, Publisher,1973.

15. Enlow, D. H.: Principles of Bone Remodeling. Spring-field, Illinois, Charles C. Thomas, Publisher, 1963.

16. McLean, F. C, and Urist, M. R.: Bone: An Introductionto the Physiology of Skeletal Tissue. Chicago and London, TheUniversity of Chicago Press, 1961.

17. MacMillan, H. W.: A consideration of the structure ofthe alveolar process with special reference to the principlesunderlying its surgery and regeneration. J Dent Res 6: 251,1924.

18. Michael, C. G, and Barsoum, W. M.: Comparing ridgerésorption with various surgical techniques in immediate den-tures. J Prosthet Dent 35: 142, 1976.

19. Gazabatt, C, Parra, ., and Meissner, E.: A comparisonofbone résorption following intraseptal alveolotomy and labialalveolectomy. J Prosthet Dent 15: 435, 1965.

20. Lam, R. V.: Contour changes of the alveolar processesfollowing extractions. J Prosthet Dent 10: 25, 1962.

21. Wictorin, L.: An evaluation of bone surgery in patientswith immediate dentures. J Prosthet Dent 21: 6, 1969.

22. Carlsson, G. E., Thilander, H., and Hedegard, B.: His-tologie changes in the upper alveolar process after extractionswith or without insertion of an immediate full denture. ActaOdontol Scand 25: 1, 1967.

20 Atwood The Eighth English Symposium23. Pendleton, E. C: Changes in the denture supporting

tissues. J Am Dent Assoc 42: 1, 1951.24. Ostlund, S. G.: The effect of complete dentures on the

gum tissues. Acta Odontol Scand 16: 1, 1958.25. Kapur, . K., and Shklar, G.: The effect of complete

dentures on alveolar mucosa. / Prosthet Dent 13: 1030, 1963.26. Turck, D.: A histologie comparison of the edentulous

denture and non-denture bearing tissues. J Prosthet Dent 15:419, 1965.

27. Pudwill, M. L., and Wentz, F. M.: Microscopic anatomyof edentulous residual alveolar ridges. J Prosthet Dent 34: 448,1975.

28. Kapur, K. K, Chauncey, H. H., Shapiro, S., and Shklar,G.: A comparative study of enzyme histochemistry of humanedentulous alveolar mucosa and gingival mucosa. Periodontics1: 137, 1963.

29. Wallenius, K, and Heyden, G.: Histochemical studiesof flabby ridges. Odont Revy 23: 169, 1972.

30. Neufeld, J. D.: Changes in the trabecular pattern of themandible following the loss of teeth. J Prosthet Dent 8: 685,1958.

31. Manson, J. P., and Lucas, R. B.: A microradiographicstudy of age changes in the human mandible. Arch Oral Biol7: 761, 1962.

32. Atwood, D. .: Postextraction changes in the adultmandible as illustrated by microradiographs of midsagittalsections and serial cephalometric roentgenograms. J ProsthetDent 13: 810, 1963.

33. Nakamoto, R. Y.: Bony defects in the crest of theresidual alveolar ridge. J Prosthet Dent 19: 111, 1968.

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