Bone Loss of Edentulous Alveolar Ridges
in the rates of résorption, then perhaps one could learn to reduce or prevent this résorption in the future. It is possible to take too simplistic a view of RRR by trying to relate RRR to one or more of the mechanical procedures that dentists perform in constructing dentures (such as impression making, jaw relation records, denture tooth selection, etc.) rather than to the capacity of the bone to respond to these factors. In studies to date which have attempted to focus on one or more of these prosthetic factors, there has been wide variability of RRR between patients from minimal résorption to quite considerable résorption. To date, correlation with any one of these factors has not been so great that a causal relationship 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 elucidated or else a combination of several factors: i.e., a multifactorial disease. Furthermore, while RRR is a process of localized destruction of bone and while local factors such as dentures may well play an important role in denture wearers, we must start with the knowledge that RRR also occurs in the nondenture wearer and is therefore not dependent on prosthetic factors alone. RRR is a chronic, cumulative, localized disease of bone remodeling. Remodeling of bone is dependent on a variety of factors including the availability of viable bone cells, the biochemical factors (local and systemic) which influence bone cells, and the cell control brought about by physical loads applied to bone.12"16
by Douglas A. Atwood, m.d., d.m.d.* Following the extraction of teeth, the residual alveolar ridge undergoes bone remodeling which involves both external and internal bone changes.1 The outstanding feature of this process is a continuing reduction in the size of the residual ridge, probably due to localized periosteal osteoclastic résorption which is accompanied by endosteal bone formation. This reduction of residual ridges (RRR) occurs most rapidly in the first 6 months to 2 years following extraction, but in many individuals continues seemingly unto death, resulting in the removal of massive amounts of jaw structure. The practical significance of this continuing loss of bone lies in the fact that removable dentures which are used to substitute for the extracted teeth depend on the bony support of the residual ridge for stability, retention, comfort, function and aesthetics. If the bony base is constantly changing shape over an indefinite time period, even well-constructed dentures become unsatisfactory and require multiple retreatments to restore comfort, function and appearance. Since there are estimated to be over 20 million completely edentulous Americans, this constitutes a major socio-economic problem. Therefore, because of both the suffering caused to the individual patient and the national (worldwide) cost of these retreatments, RRR has been termed a "major oral disease entity" to be considered in the same category as caries and periodontal disease, a pathologic disease process which is worthy of extensive research for better understanding of its etiology and prevention. It is the intent of this paper to explore certain characteristics of RRR and various factors which may influence the rate of RRR. Cephalometric studies by numerous authors in different parts of the world have shown that the mean rates of RRR fall into remarkably consistent patterns1"6 which are closely allied to time since extraction and time over which rate is measured. However, in every study, a wide variation between different individuals occurred in each time frame. For example, if one graphs the raw data presented by Carlsson2 for 34 denture patients who were all studied over a 5 year period, one sees not only the change in rate relative to time postextraction, but also the range of variations from the mean. This raises the hope that if one could find the cause(s) of the differences *
Department
of Prosthetic
Dentistry,
Availability
of
Viable Bone Cells
It is known that specialized bone cells such as osteoblasts and osteoclasts have life expectancies far shorter than human life. Therefore, their continued availability depends on the genetic and environmental capacity of the progenitor cells to produce them. Since RRR is a longitudinal and cumulative process persisting for the rest of the patient's life, senescence may become an important factor in the rate of RRR at certain stages due to aging changes in the number and viability of bone cells. In addition to the loss of cells due to cell death, aging may slow down cells, diminish the amount of work they perform per day, and reduce their sensitivity and precision of response to normal regulatory stimuli. Clinical studies to date have demonstrated patients of different ages having either high or low rates of RRR. Any slight trend for slower rate in the older patients may be related more to time since extraction than to age. Nevertheless, this factor should be further explored. In addition, studies which have attempted to compare the rate of RRR in patients who had simple extractions vs. patients who had "alveolectomies" which removed significant amounts of labial cortical plate have tended to show that the rate of RRR was greater in those who had potentially osteogenic tissue removed by "alveolectomy".17"21 Carlsson et al.22 have shown that in patients with simple extractions, the labial plate of lamellar bone
Harvard School of Dental
Medicine, 188 Longwood Ave., Boston, Mass 02115. 11
The
Atwood
12
Eighth English Symposium
largely removed by osteoclastic activity by about 40 days after extraction. However, parallel to this résorption, bone formation takes place so that by 3 months both the labial plate and the socket have been replaced with new bone. In other words, the old labial plate serves is
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 of any particular type of cell could be significantly reduced as the process proceeds as seems to occur in advanced
osteoporosis. Histologie or histochemical studies of cells and cellular activity relative to RRR are limited by the availability of adequate material for study. If one is to study RRR through histological examination of biopsy material, then the specimens must be taken from the crest of the Table 1. Reduction
Reference
of
Residual Ridges: Vertical Mandibular Ridge
Number of Sub-
Mean time
ject
baseline film
postextr. y
Carlsson V Carlsson V Carlsson V Tallgren VI Carlsson V Carlsson V Carlsson V Tallgren VI
Hedegard Tallgren VI Carlsson Carlsson Carlsson Carlsson
V V V V Tallgren VI Carlsson V Tallgren VI Carlsson V Tallgren VI Atwood II Atwood II
Bergman Atwood II Atwood II Tallgren VI Tallgren VI Atwood II Tallgren VI
o o
34
34 34
o
11
o
34 34
0.17 0 0.33 0.5 0 0.2 0.33 0.5
34 11 7 11
34 34
34 34 7 34 11
34 9 8 19 54 16 15 9
6 18 20
0 0.33 1.0 1.0 0.2 2.0 0.17 20.0 10.0
Height—Anterior
Mean duration RRR
yr 0.17 0.33 0.5 0.25 0.33 1.0 0.5 0.5 3.0 1.0 1.0 1.0 5.0 2.0 3.0 1.0 7.0 3.0 13.5 2.5 2.5 1.0 2.5 2.5 6.5 1.5 2.5 15.0
Mean rate RRR
mm/yr 12.0 8.6 6.8 5.6 5.1 4.15 3.3 2.54 2.5 2.41 1.95 1.5 1.44 1.35 1.1 0.95 0.94 0.65 0.57 0.56 0.55 0.55 0.51 0.49 0.22 0.22 0.22 0.20
Figure 2. An example little reduction in size.
Figure 3. An
of well formed residual ridges with very
example of residual ridges which have undergone resulting in massive reduction in size.
gross bone résorption
Central Incisor
Genial Tubercle
Figure 1. Six orders of mandibular anterior residual ridgeform: Order I, pre-extraction; Order II, postextraction; Order III, high well-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
Edentulous Alveolar
Biology
residual ridge. For this reason, human biopsy material has been restricted in size and number. The fact that the histologie appearances in different parts of the same mouth can vary widely, probably explains the different observations and conclusions found in different biopsy studies. Furthermore, most biopsy studies of human material are of soft tissue only and therefore do not reveal information about the bone or the important interface between bone and soft tissue. Histologie studies of postmortem material are usually restricted by a lack of clinical correlation since such material is usually obtained from cadavers of unknown clinical status. Ideally, one would have sufficient biopsy material at intervals over a period of time or, at the least, post-
Ridges
13
subjects
who had been studied In each instance such of time. period clinically material is difficult to obtain. Suffice it to say, microscopic studies of residual ridges32"38 have revealed the following: (1) varying demortem material from over a
0:5
YEARS of REDUCTION of RESIDUAL RIDGES 1,5 2 2.5 3 3.5 4 4.5 1
0.13mm. /yr. Irate: min.: 2.0mm.
rate:
0.5mm./yr.
ave.z68mm.
I mo.
7mo.
50
mo. rate:
1.8mm./yr.
max.:
14.5 mm.
•after Carls»on,1967
Figure 4. Tracings of three lateral cephalographs with the maxillae and mandibles carefully superimposed. Note the changes in the shape of the residual ridges following the extraction of remaining teeth 50 months before. (From Atwood, D. .: J Prosthet Dent 13: 811, 1963.)
Graph illustrating typical mean mandibular bone loss during 5-year postextraction period with maximal and minimal ranges from data published by Carlsson, G. E. and Persson, G. : Morphologic changes of the mandible after extraction and wearing dentures. Odontol Revy 18: 27-54, 1967.) Figure 5. curve
|MEAN
E
65.2 10.1
«
.50
MEDIAN
- -1—r
.25 OO
5
"75
Figure 6. Rate
-flj
¿5
of RRR (total)
50
55
60
70
65
75
80
85
90
Age vs
age. (From Atwood, D. A. and
Coy,
W. ., J Prosthet Dent 26: 289,
1971.)
14
Atwood
grees of keratinization, acanthosis, thickness, edema, and
architectural pattern of epithelium in the same mouth and between subjects;23"29 (2) varying degrees of inflam-
of residual ridges is revealed by careful superimposition of portions of two cephalometric radiographs made 16 years apart. The actual bone loss in the anterior part of the ridge of the mandible was 13 mm in height (a 41% Figure 7. Gross bone loss
reduction) and 60 sq mm in cross-sectional area (a 24% reduction). (From Atwood, D. .: J Prosthet Dent 26: 268, 1971.)
Eighth English Symposium matory cells from clinically normal to frankly inflamed areas in both denture and nondenture wearing patients;23"29 (3) lymphocytes,23"29 plasma cells,23,27 mast The
cells29 and osteoclasts;22'23,27'29 (4) dense, fibrous connec-
tive tissue (sometimes hyalinized) frequently observed over crestal bone with fibers running parallel to epithelial surface;23"27 (5) a vascular plexus outside the periosteum in areas of bone apposition;29 (6) small blood vessels in close contact with the bone margin in areas of bone résorption, sometimes in the lacunae with positive correlation between the degree of inflammation, vascular reactions, and bone résorption;29'38 (7) marked diaphorase activity in areas of bone remodeling—either formation or résorption;29 (8) ATPase activity in areas of bone formation and acid phosphatase activity in areas of bone résorption;29 (9) the lack of evidence of bone résorption in areas which do not have inflammatory cells;29 (10) endosteal bone deposition reinforcing internal structure where external surface has been affected by résorpon tion;22' 23' 27' 29'32 (11) lack of periosteal lamellar 23'bone the external surface of the crest of the ridge;22' 27'29,32 (12) a roughened crestal bone surface which is either actively resorbing or is inactive, but without reversal lines on the external surface of the crestal bone;22,23' 27' 29'32 (13) development of secondary Haversian systems in remodelled compacted endosteal bone;32 (14) microradiographic evidence of mandibular osteoporosis 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 the trabeculae on the crest of the ridge. Also evident is an osteon within a single trabecula, and evidence of internal bone formation, and résorption of the trabeculae. (From Atwood, D. .: J Prosthet Dent 13: 816, 1963.)
Oral
Perspective on Bone Biology
Edentulous Alveolar
Ridges
15
microradiograph of the inferior border of a mandible showing evidence of moderate osteoporosis with increased variation density of osteons, increased number of incompletely closed osteons, and increased endosteal porosity. (From Atwood, D. .: J Prosthet Dent 13: 816, 1963) (Original magnification, x 15). Figure 9. A
in the
osteons, increased number of incompletely closed osteons, increased endosteal porosity and increased number of plugged osteons.31'32 It may well be that noninvasive techniques will be perfected which will provide information about the functional activities of bone cells and RRR which is additional to that available from current histologie, histochemical and microradiographic techniques. Biochemical Factors Which Influence Bone Cells
Local Biochemical Factors. Much work has been done in the study of local factors in bone résorption in trying to understand periodontal disease.39 Endotoxins from dental plaque40 (plaque can occur in edentulous mouths), osteoclast-activating factor (OAF),41 Prostaglandins,42"43 human gingival bone-resorption stimulating factor,44 etc., are all factors which could be important to the rate of RRR and deserve further study. For example, heparin, a cofactor in bone résorption, has been associated with mast cells which have been observed toward the bone margin of both residual ridges29 and gingiva.45 As with periodontal disease, local biochemical factors triggered by microorganisms could be a factor in RRR, especially when there is poor oral hygiene and when dentures are worn constantly. In addition, toxins leaching from denture 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 traumatogenic increased or decreased vascularity leading to changes in 02 tension49 or temperature.50 Systemic Biochemical factors. Just as some patients
with natural teeth and poor local factors seem to have great resistance to periodontal disease,51 there are some edentulous patients who do not have RRR and who retain good ridge form for many years even in the presence of local prosthetic factors which are felt to be unfavorable. Conversely, some patients have a high rate of RRR without any obvious unfavorable local factors. In such cases, the possibility exists that osteoporosis may be a contributing factor whether it be idiopathic or in the form of a deficiency of estrogen, calcium, or growth hormone, 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 established in the etiology of RRR.53"54 It is clear, however, that signs characteristic of osteo36'37'56 porosis have been found in human jaws.31,32' in Whether the pathophysiology of osteoporosis any specific patient is decreased bone formation or increased bone résorption or a combination of the two, the presence of an active stage of osteoporosis could very definitely have an effect on the rate of RRR.1'52,55) 57 To date, published attempts to correlate osteoporosis as determined densitometrically with the rate of RRR5 have failed, but this need not indicate a lack of relationship.
16
The
Atwood
Eighth English Symposium
250 Z2S c
0 200 '
1.75
E 1.50 E
~"
u5
£
a: a:
too
a
0£
.50 .25
.00
2
1
Figure 10. Rate of RRR (mandible) Prosthet Dent 26: 293, 1971.)
vs
3
density of anterior
mandibular
Osteoporosis is a cumulative disease whose degree of activity at any point in time may not be appropriately indicated by the radiographie densitometric techniques utilized to date. Perhaps more sophisticated diagnostic techniques, employed longitudinally, would present different
conclusions.58"62
Cell Control Brought About on Bone
by
5
4
6
7
DENSITY OF ANT. MAND.RIDGE (mm Al.)
Physical Loads
Physical loads are placed on the alveolar bone in the individual with natural dentition through the teeth and periodontal ligaments to the lamina dura and supporting
trabecular bone.63"64 When the teeth are extracted, either there is little or no loading of the residual alveolar ridge as in the nondenture wearer, thereby perhaps resulting in a disuse atrophy of the ridge, or else the load is transmitted in the denture wearer through the artificial denture teeth, the denture base, and the mucoperiosteum to the bone.65 Studies have shown that the mean rate of RRR is three to four times greater in the mandible than in the maxilla, although for some patients the reverse is true.1"3'5 In fact, Tallgren has shown an interesting inverse relationship between the rates of RRR in the maxilla and mandible observed in 11 patients over a 7year period.3 Such differences within the same individual suggest the need to look at differences in the two jaws. In attempting to determine the basis for these differences, Woelfel, et al. showed that the average "projected" denture base area of 45 patients was 1.8 times greater for
8
ridges (site 14). (From Atwood,
D. A. and
Coy,
W. ., J
the maxilla than for the mandible.10 There follows logically from this that whenever the patient occludes the mean load per unit area applied to the mandible is 1.8 times more than that applied to the maxilla. For example, Woelfel, et al. cite a patient with projected area of the maxillary denture of 4.2 sq in and of the mandibular denture of 2.33 sq in (ratio 1.8).10 If this patient bites with a force of 50 lb, the resultant pressure would be 12 lb per sq in under the maxillary denture and 21 lb per sq in under the mandibular denture. In addition to masticatory forces, Ohashi, et al. have demonstrated that swallowing forces in 21 patients averaged 11.4 lb (2.7 lb per sq in under maxillary denture and 4.1 lb per sq in under the mandibular denture).66 Cutright, et al. have calculated that 1500 empty swallows per 24 hours could amount to 3500 to 4200 lb of loading per day.67 Further, they have demonstrated both positive and negative pressures on the residual ridges from a variety of activities, including smoking, talking and counting, as well as biting. Brewer has shown that masticatory and swallowing contacts between dentures average less than 15 minutes per waking day. What needs to be remembered is that some patients have been shown to clench and grind their teeth up to several hours per 24 hours.68 It is likely that such 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
Edentulous Alveolar Ridges
Biology
17
It is commonly assumed by dentists that a carefully fitted denture base will distribute the load through the mucoperiosteum to the bone more evenly, thereby resulting in less RRR. Workers have emphasized the importance of the thickness and the resilience of the mucoperiosteum. As a denture base is seated evenly against its tissue base, only a small amount of displacement takes place before the entire mucoperiosteum becomes a hydraulic system exerting a compressive force evenly over the denture-bearing area. It would appear that the mucous membranes exhibit viscoelastic properties.70"73 In discussing "damping" or energy absorption, in such bones as vertebral body, Frost states: "... bones which
\% Figure 13. An example of variation in the type of bone showing a broad maxillary cancellous ridge and a high but very narrow ("knife-edge") mandibular anterior ridge.
Figure 11. The even distribution of force over the denture bearing areas with well fitting dentures and equalized occlusion. (From Nagle, R. J. and Sears, V. H. : Dental Prosthetics, p. 333. C. V. Mosby Co., 1958.)
®
©
Diagrammatic representation of the process of "inwaisting" of external surface of bones subjected to compression loading which results in internal hydraulic-like effect and eventual bone remodeling. (From Frost, H. M.: The Laws of Bone Structure, 26. Springfield, Illinois, Charles C. Thomas, PubFigure 14.
lishers, 1964.)
subjected largely to compression loads, and experience no significant bending loads, are composed largely of cancellous bone which is ideally constructed for the absorption and dissipation of energy. This is because it consists 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 broad maxillary cancellous ridge may react differently from a narrow mandibular cortical ridge to the same compresare
Figure 12. An enlarged radiographie view of a vertebral body showing a trabecular pattern oriented parallel to the direction of compression deformation and with "inwaisting" on the right side of the vertebral body.
sion load. Frost refers to a process called "inwaisting" which results from the sequential events of compression, internal hydraulic-like effect, external bulging and external osteoclastic activity with resultant narrowing ("inwaisting") of the bone.75 If the internal trabeculae of cancellous bone are numerous, thick and strong, compression
18
The
Atwood
loads cause less delormation, less hydraulic-like effect, less external bulging and hence less external osteoclastic "inwaisting". Older persons who have lost trabecular thickness and number will tend to have more vertical deformation under compression and consequently more external osteoclastic résorption. Perhaps this is a factor in RRR which is characterized by external osteoclastic résorption, or a form of "inwaisting." Frost points out that trabeculae are oriented parallel to the direction of compression deformation. For years, Applegate76 recommended phasing in the loading of an edentulous ridge by first inserting a removable partial denture without the teeth and then "exercising" the ridge by intermittent gentle pressure with the goal of strengthening and reorienting the trabeculae on the crest of the ridge.77 Neufeld has demonstrated in dry human jawbones a trabecular pattern in which the trabeculae are oriented at right angles to the crest of the ridge (parallel to the direction of occlusal forces through the denture base).78 This would be optimal to resist such compression loading from a denture. The "neutrocentric" concept of occlusion (DeVan)65 attempts by a variety of means to achieve compression loading of the residual ridges, to minimize any tension or shear loads, and thereby to reduce RRR. "Anatomical" occlusal schemes seek to reduce RRR through efficiency of mastication, reduced occlusal contact area, and balanced occlusion.10 If an anatomical occlusion "misses centric" either initially or subsequently due to RRR,3 there is increased likelihood of tension and shear forces on the residual ridges. This is thought by some to increase the rate of RRR, but even a careful longitudinal study for 5 years of 45 patients which comes to the opposite conclusion has such a wide overlap of RRR in the three
Figure 15. Radiograph of thin midsaggital sections of eight mandibles illustrating a variety of types of bone and raising the question of the possible "damping" effect and "inwaisting".
Eighth English Symposium
Figure 16. An example of a craniofacial relationship which could result in different loading of the residual ridges due to the muscle attachments and direction of muscular activity.
occlusal groups as to obfuscate the conclusions.9"11 Most of this discussion of the loading of bone has focused on the residual ridge per se. In actuality, we should also be looking at the mandible as a whole bone and its functional and developmental anatomy.79"81 Tallgren has studied the possible correlation of RRR with the morphological configuration.3 It is fair to assume that the loading of the ridge is dependent on the shape of the craniofacial bones, the insertion of the muscle attachments, and the direction of muscular activ-
ity.
The pathophysiology of the overloading of bone could consist of one or more of the following mechanisms: Trauma leading to increased vascularity, compression leading to decreased blood flow or increased venous stasis, piezo-electric effect, and stimulation of bone resorbing cells through bone cell receptors activated by physical forces. Some of the most experienced and perceptive prosthodontists for decades have warned against the overly retentive denture which fits "tight" today but in a relatively short period of time loses its retention and "fit" either due to viscoelastic changes or RRR.65,69 Most warn against constant denture wearing. One additional form of possible continuous loading of the residual ridge lies in the possibility of a postoperative cicatricial mucoperiosteum seeking a reduced area.81'82 Furthermore, it is possible that the rate of RRR is predetermined by the dentist, who removes the teeth, by such surgical factors as the amount of attached gingiva that is removed, the amount of cortical bone that is removed, the amount of tissue reflection and dissection of mucoperiosteum from the bone,83 the amount of alveolectomy or alveolotomy, and the degree of tension resulting from close approximation of opposing tissues and tight suturing.
Oral Perspective
on
Bone
Biology
Edentulous Alveolar Table 2. Co- Variables
Category Time Viable Bone Cells
Systemic
Biochemical Fac-
tors
19
Possibly Related to Rate of RRR
Variables which may have Direct Relation
Variables which may have Inverse Relation Time Since Extraction Progenitor Cells Osteoblasts
Progenitor Cells
Osteoclasts Senescence Local Biochemical Factors
Ridges
Endotoxins
Osteoclast-Activating Factor (OAF) Prostaglandins Human Gingival Factor Immune Complexes Heparin
Anti-Invasion Factor Indomethacin
Parathyroid Hormone (PTH) Thyroid Hormone (TH)
Calcitonin (CT) Growth Hormones
Adrenal Cortical Steroids
(AIF)
(GH)
Estrogens Androgens
Ca, Vit. D, Vit. C, F Protein Load Factors
Amount, Frequency, Duration and
Direction of Load Load Per Unit Area Occlusal Contact Area Traumatic Prosthetic Factors Craniofacial Morphology
Summary RRR is probably a multifactorial process subject to a wide number of covariables. In any given patient, these covariables will be in a unique combination which will determine the rate of RRR for that patient at that time. If the variables change in any given patient, the rate of RRR may change. The dentist who wishes to help his
edentulous patient should seek to be sensitive to all these variables and to understand and treat that unique combination of factors which exists in that patient. References
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 1.
of the mandible after extraction and wearing of dentures. OdontolRevy 18: 27, 1967. 3. Tallgren, .: The continuing reduction of the residual alveolar ridges in complete denture wearers: A mixed longitudinal study covering 25 years. J Prosthet Dent 27: 120, 1972. 4. Hedegard, B.: Some observations on tissue changes with immediate maxillary dentures. Dent Pract 13: 70, 1962. 5. Atwood, D. ., and Coy, W. .: Clinical, cephalometric and densitometric study of reduction of residual ridges. J Prosthet Dent 26: 280, 1971. 6. Bergman, B., Carlsson, G. E., and Ericson, S.: Effect of differences in habitual use of complete dentures on underlying tissues. Scand J Dent Res 79: 449, 1971. 7. Thompson, J. R.: The rest position of the mandible and its significance to dental science. J Am Dent Assoc 33: 151, 1946. 8. Atwood, D. .: A cephalometric study of the clinical rest position of the mandible. Part II: The variability in the rate of bone loss following the removal of occlusal contacts. J Prosthet Dent 7: 544, 1957. 9. Igarashi, T., Woelfel, J. B., and Winter, C. M.: Computer
analysis
Denture Bearing Area Favorable Mucoperiosteum Damping Effect of Cancellous Bone
Craniofacial
Morphology
of twelve factors related to mandibular ridge résorption. J. Prosthet Dent (IADR Abstr. 443) 30: 1971. 10. Woelfel, J. B., Winter, C. M., and Igarashi, T.: Five year cephalometric study of mandibular ridge résorption with different posterior occlusal forms. Part I: Denture construction and initial comparison. J Prosthet Dent 36: 602, 1976. 11. Winter, C. M., Woelfel, J. B., and Igarashi, T.: Five year changes in the edentulous mandible as determined on oblique cephalometric 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 Modeling Errors. Springfield, Illinois, Charles C. Thomas, Publisher, 1973. 15. Enlow, D. H.: Principles of Bone Remodeling. Springfield, Illinois, Charles C. Thomas, Publisher, 1963. 16. McLean, F. C, and Urist, M. R.: Bone: An Introduction to the Physiology of Skeletal Tissue. Chicago and London, The University of Chicago Press, 1961. 17. MacMillan, H. W.: A consideration of the structure of the alveolar process with special reference to the principles underlying its surgery and regeneration. J Dent Res 6: 251, 1924. 18. Michael, C. G, and Barsoum, W. M.: Comparing ridge résorption with various surgical techniques in immediate dentures. J Prosthet Dent 35: 142, 1976. 19. Gazabatt, C, Parra, ., and Meissner, E.: A comparison of bone résorption following intraseptal alveolotomy and labial alveolectomy. J Prosthet Dent 15: 435, 1965. 20. Lam, R. V.: Contour changes of the alveolar processes following extractions. J Prosthet Dent 10: 25, 1962. 21. Wictorin, L.: An evaluation of bone surgery in patients with immediate dentures. J Prosthet Dent 21: 6, 1969. 22. Carlsson, G. E., Thilander, H., and Hedegard, B.: Histologie changes in the upper alveolar process after extractions with or without insertion of an immediate full denture. Acta Odontol Scand 25: 1, 1967.
20
Atwood
23. 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 anatomy of 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 human edentulous alveolar mucosa and gingival mucosa. Periodontics 1: 137, 1963. 29. Wallenius, K, and Heyden, G.: Histochemical studies of flabby ridges. Odont Revy 23: 169, 1972. 30. Neufeld, J. D.: Changes in the trabecular pattern of the mandible following the loss of teeth. J Prosthet Dent 8: 685, 1958. 31. Manson, J. P., and Lucas, R. B.: A microradiographic study of age changes in the human mandible. Arch Oral Biol 7: 761, 1962. 32. Atwood, D. .: Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and serial cephalometric roentgenograms. J Prosthet Dent 13: 810, 1963. 33. Nakamoto, R. Y.: Bony defects in the crest of the residual alveolar ridge. J Prosthet Dent 19: 111, 1968. 34. Pietrokovski, J.: The bony residual ridge in man. J Prosthet Dent 34: 456, 1975. 35. VonWowern, N.: Histoquantitation of ground sections of human mandibles. Scand J Dent Res 81: 567, 1973. 36. VonWowern, N., and Stoltz, K: Sex- and age-differences in bone structure in human mandibles. J Dent Res 56: Specific Issue A, (IADR Abstr) 229, 1977. 37. Atkinson, P. J., and Woodhead, C: Changes in human mandibular structure with age. Arch Oral Biol 13: 1453, 1968. 38. Takiguchi, R.: Morphological studies on the surfaces of the alveolar ridges of edentulous mandibles with the scanning electron microscope. J Tokyo Dent Col Soc 11: 1677, 1974. 39. Hausmann, E.: Potential pathways for bone résorption in human periodontal disease. J Periodontol 45: 338, 1974. 40. Hausmann, E., Raisz, L. G, and Miller, W. .: Endotoxin: stimulation of bone résorption in tissue culture. Science 168: 862, 1970. 41. Horton, J. E., Wezeman, F. H., and Kuettner, . E.: Regulation of osteoclast-activating factor (OAF)—stimulated bone résorption In Vitro with an inhibitor of collagenase, Proceedings, Mechanisms of Localized Bone Loss, Horton, Tarpley, and Davis (eds), Special Supplement to Calcified Tissue Abstracts, pp 127-149, 1978. 42. Gomes, B. C., Hausmann, E., Weinfeld, ., and DePuca, C: Prostaglandins: bone résorption stimulating factors released from monkey gingiva. Calcif Tissue Res 19: 285, 1976. 43. Goodson, J. M., Dewhirst, F., and Brunetti, .: Prostaglandin E2 levels in human gingival tissues. J Dent Res 52: 496, 1973. 44. Goldhaber, P., Rabadjija, L., Beyer, W. R., and Kornhauser, .: Bone résorption in tissue culture and its relevance to periodontal disease. J Am Dent Assoc 87: 1027, 1973. 45. Zachrisson, . V.: Mast cells of the human gingiva. IV. Experimental gingivitis. J Periodont Res 4: 46, 1968. 46. Axelsson, B., and Nyquist, G: The leaching and biological effect of the residual monomer of methyl methacrylate. Odontol Revy 13: 370, 1962. 47. Wictorin, L.: The leaching of residual monomer from acrylic base materials. / Dent Res (Special Issue A, No. 204)
The
Eighth English Symposium
57: 1978. 48. Irving, J. T.: Factors concerning bone loss associated with periodontal disease. J Dent Res 49: 262, 1970. 49. Goldhaber, P.: Behavior of bone in tissue culture. Calcification in Biological Systems, pp 349-372. American Association for the Advancement of Science, Washington, D. C, 1960. 50. Enlow, D. H.: Alveolar Bone. Lang and Kelsey (eds), pp 3-41, Proceedings, International Prosthodontic Workshop, 1973. 51. Glickman, I.: Clinical Periodontology, ed 4, pp 432^159. Philadelphia, W. B. Saunders, 1972. 52. Baylink, D. J., Wergedal, J. E., Yamamoto, ., and Manzke, E.: Systemic factors in alveolar bone loss. / Prosthet Dent 31: 486, 1974. 53. Wical, . E., and Swope, C. C: Studies of residual ridge résorption. Part II. The relationship of dietary calcium and phosphorous to residual ridge résorption. / Prosthet Dent 32: 13, 1974. 54. Hartsook, E. I.: Food selection, dietary adequacy and related dental problems of patients with dental prostheses. J Prosthet Dent 32: 32, 1974. 55. Atwood, D. .: Some clinical factors related to rate of résorption of residual ridges. J Prosthet Dent 12: 441, 1962. 56. Henrikson, P., Wallenius, K, and Âstrand, .: The mandible and osteoporosis. J Oral Rehab 1: 75, 1974. 57. Ward, V. J., Stephens, A. P., Harrison, ., and Lurie, D.: The relationship between the metacarpal index and the rate of mandibular ridge résorption. J Oral Rehab 4: 83, 1977. 58. Bergstrom, J., and Henrikson, C. O.: Quantitative longterm determinations of the alveolar bone mineral mass in man by 125I absorptiometry. II. Following periodontal surgery. Acta Radiol 13: 489, 1974. 59. Israel, H.: Microdensitometric analysis for study of skeletal growth and aging in the living subject. Am JPhys Anthropol 29: 287, 1968. 60. Isenberg, G., Goldman, H. M., Spira, J., Parsons, F. G., and Street, P. N.: Radiograph analysis of two dimensional microdensitometry. J Am Dent Assoc 77: 1069, 1968. 61. Garcia, D. ., and Entine, G.: Comparison of three detector systems for localization of bone abscesses. Trans Am Nucl Soc 19: 48, 1974. 62. Kaplan, M. L., Garcia, D. ., Davis, . ., Adelstein, S. J., and Goldhaber, P.: Uptake of Tc-99m-Sn-EHDP in beagles with advanced periodontal disease. Calcif Tissue Res 19: 91, 1975. 63. Kakudo, Y., and Amano, .: Strain guages used in studying strain on the human and animal jaw bones during occlusion, mastication, and swallowing. J Osaka Dent Ú 4:1, 1970. 64. Zengo, A. N., Pawluk, R. J., and Bassett, C. A. L.: Stress-induced bioelectric potentials in the dento alveolar complex. Am J Orthood 64: 17, 1973. 65. DeVan, M. M.: An analysis of stress counteraction on the part of alveolar bone with a view to its preservation. Dent Cosmos 77: 109, 1935. 66. Ohashi, M., Woelfel, J. B., and Paffenbarger, G. C: Pressures exerted on complete dentures during swallowing. / Am Dent Assoc 73: 625, 1966. 67. Cutright, D. E., Brudvik, J. S., Gay, W. D., and Selling, W. J.: Tissue pressure under complete maxillary dentures. / Prosthet Dent 35: 160, 1976. 68. Brewer, . .: Prosthodontic research in progress at the School of Aerospace Medicine. J Prosthet Dent 13: 49, 1963. 69. Hall, R. E.: Tissue atrophy resulting from compression of tissues for the retention of dentures. J Nat Dent Assoc 8: 919, 1922. 70. Kitamura, K, Sekine, H., and Miyashita, T.: Studies on the relationship between sinking of the denture bone through
Oral Perspective
on
Bone
Biology
biting pressure and muco-compressibility. J Dent 2: 25, 1965. 71. Kydd, W. L., Daly, C. H., and Nansen, M. J. E.:
Variation in the response to mechanical stress of human soft tissues as related to age. / Prosthet Dent 32: 493, 1974. 72. Jozefowicz, W.: Cushioning properties of the soft tissues forming the basal seat of dentures. J Prosthet Dent 27: 471, 1972. 73. Jozefowicz, W.: The influences of wearing dentures on residual ridges: A comparative study. / Prosthet Dent 24: 137, 1970. 74. Frost, H. M.: The Laws of Bone Structure, p. 72. Charles C. Thomas Pubi., Springfield, Illinois, 1964. 75. Frost, H. M.: Bone Modeling and Skeletal Modeling Errors, pp 19-27. Springfield, Illinois, Charles C. Thomas Pubi., 1973. 76. Applegate, O. Ci An evaluation of the support for the removable partial denture. J Prosthet Dent 10: 112, 1960. 77. Smith, F. W., and Applegate, O. C: Roentgenographic .
Edentulous Alveolar
Ridges
21
study of bone changes during exercise stimulation of edentulous areas. J Prosthet Dent 11: 1086, 1961. 78. Edwards, L. F.: The edentulous mandible. J Prosthet Dent 4: 222, 1954. 79. Enlow, D. H., Bianco, H. J., and Eklund, S.: The remodeling of the edentulous mandible. J Prosthet Dent 36: 685, 1976. 80. Lestrel, P. E., Kapur, K. K., and Chauncey, H. H.: Cortical bone loss in the mandible: dentulous vs. edentulous. / Dent Res (IADR Abstr. No. 578) 56: A187, 1977. 81. Lammie, G. .: Aging changes and the complete lower denture. J Prosthet Dent 6: 450, 1956. 82. White, S. C, Frey, . W., Blashke, D. D., Ross, M. D., Clemens, P. J., Fürst, D. E., and Paulus, H. E.: Oral radiographie changes in patients with progressive systemic sclerosis (scleroderma). J Am Dent Assoc 94: 1178, 1977. 83. Pfeifer, J. S.: The reaction of alveolar bone to flap procedures in man. Periodontics 3: 135, 1965.
in the rates of résorption, then perhaps one could learn to reduce or prevent this résorption in the future. It is possible to take too simplistic a view of RRR by trying to relate RRR to one or more of the mechanical procedures that dentists perform in constructing dentures (such as impression making, jaw relation records, denture tooth selection, etc.) rather than to the capacity of the bone to respond to these factors. In studies to date which have attempted to focus on one or more of these prosthetic factors, there has been wide variability of RRR between patients from minimal résorption to quite considerable résorption. To date, correlation with any one of these factors has not been so great that a causal relationship 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 elucidated or else a combination of several factors: i.e., a multifactorial disease. Furthermore, while RRR is a process of localized destruction of bone and while local factors such as dentures may well play an important role in denture wearers, we must start with the knowledge that RRR also occurs in the nondenture wearer and is therefore not dependent on prosthetic factors alone. RRR is a chronic, cumulative, localized disease of bone remodeling. Remodeling of bone is dependent on a variety of factors including the availability of viable bone cells, the biochemical factors (local and systemic) which influence bone cells, and the cell control brought about by physical loads applied to bone.12"16
by Douglas A. Atwood, m.d., d.m.d.* Following the extraction of teeth, the residual alveolar ridge undergoes bone remodeling which involves both external and internal bone changes.1 The outstanding feature of this process is a continuing reduction in the size of the residual ridge, probably due to localized periosteal osteoclastic résorption which is accompanied by endosteal bone formation. This reduction of residual ridges (RRR) occurs most rapidly in the first 6 months to 2 years following extraction, but in many individuals continues seemingly unto death, resulting in the removal of massive amounts of jaw structure. The practical significance of this continuing loss of bone lies in the fact that removable dentures which are used to substitute for the extracted teeth depend on the bony support of the residual ridge for stability, retention, comfort, function and aesthetics. If the bony base is constantly changing shape over an indefinite time period, even well-constructed dentures become unsatisfactory and require multiple retreatments to restore comfort, function and appearance. Since there are estimated to be over 20 million completely edentulous Americans, this constitutes a major socio-economic problem. Therefore, because of both the suffering caused to the individual patient and the national (worldwide) cost of these retreatments, RRR has been termed a "major oral disease entity" to be considered in the same category as caries and periodontal disease, a pathologic disease process which is worthy of extensive research for better understanding of its etiology and prevention. It is the intent of this paper to explore certain characteristics of RRR and various factors which may influence the rate of RRR. Cephalometric studies by numerous authors in different parts of the world have shown that the mean rates of RRR fall into remarkably consistent patterns1"6 which are closely allied to time since extraction and time over which rate is measured. However, in every study, a wide variation between different individuals occurred in each time frame. For example, if one graphs the raw data presented by Carlsson2 for 34 denture patients who were all studied over a 5 year period, one sees not only the change in rate relative to time postextraction, but also the range of variations from the mean. This raises the hope that if one could find the cause(s) of the differences *
Department
of Prosthetic
Dentistry,
Availability
of
Viable Bone Cells
It is known that specialized bone cells such as osteoblasts and osteoclasts have life expectancies far shorter than human life. Therefore, their continued availability depends on the genetic and environmental capacity of the progenitor cells to produce them. Since RRR is a longitudinal and cumulative process persisting for the rest of the patient's life, senescence may become an important factor in the rate of RRR at certain stages due to aging changes in the number and viability of bone cells. In addition to the loss of cells due to cell death, aging may slow down cells, diminish the amount of work they perform per day, and reduce their sensitivity and precision of response to normal regulatory stimuli. Clinical studies to date have demonstrated patients of different ages having either high or low rates of RRR. Any slight trend for slower rate in the older patients may be related more to time since extraction than to age. Nevertheless, this factor should be further explored. In addition, studies which have attempted to compare the rate of RRR in patients who had simple extractions vs. patients who had "alveolectomies" which removed significant amounts of labial cortical plate have tended to show that the rate of RRR was greater in those who had potentially osteogenic tissue removed by "alveolectomy".17"21 Carlsson et al.22 have shown that in patients with simple extractions, the labial plate of lamellar bone
Harvard School of Dental
Medicine, 188 Longwood Ave., Boston, Mass 02115. 11
The
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Eighth English Symposium
largely removed by osteoclastic activity by about 40 days after extraction. However, parallel to this résorption, bone formation takes place so that by 3 months both the labial plate and the socket have been replaced with new bone. In other words, the old labial plate serves is
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 of any particular type of cell could be significantly reduced as the process proceeds as seems to occur in advanced
osteoporosis. Histologie or histochemical studies of cells and cellular activity relative to RRR are limited by the availability of adequate material for study. If one is to study RRR through histological examination of biopsy material, then the specimens must be taken from the crest of the Table 1. Reduction
Reference
of
Residual Ridges: Vertical Mandibular Ridge
Number of Sub-
Mean time
ject
baseline film
postextr. y
Carlsson V Carlsson V Carlsson V Tallgren VI Carlsson V Carlsson V Carlsson V Tallgren VI
Hedegard Tallgren VI Carlsson Carlsson Carlsson Carlsson
V V V V Tallgren VI Carlsson V Tallgren VI Carlsson V Tallgren VI Atwood II Atwood II
Bergman Atwood II Atwood II Tallgren VI Tallgren VI Atwood II Tallgren VI
o o
34
34 34
o
11
o
34 34
0.17 0 0.33 0.5 0 0.2 0.33 0.5
34 11 7 11
34 34
34 34 7 34 11
34 9 8 19 54 16 15 9
6 18 20
0 0.33 1.0 1.0 0.2 2.0 0.17 20.0 10.0
Height—Anterior
Mean duration RRR
yr 0.17 0.33 0.5 0.25 0.33 1.0 0.5 0.5 3.0 1.0 1.0 1.0 5.0 2.0 3.0 1.0 7.0 3.0 13.5 2.5 2.5 1.0 2.5 2.5 6.5 1.5 2.5 15.0
Mean rate RRR
mm/yr 12.0 8.6 6.8 5.6 5.1 4.15 3.3 2.54 2.5 2.41 1.95 1.5 1.44 1.35 1.1 0.95 0.94 0.65 0.57 0.56 0.55 0.55 0.51 0.49 0.22 0.22 0.22 0.20
Figure 2. An example little reduction in size.
Figure 3. An
of well formed residual ridges with very
example of residual ridges which have undergone resulting in massive reduction in size.
gross bone résorption
Central Incisor
Genial Tubercle
Figure 1. Six orders of mandibular anterior residual ridgeform: Order I, pre-extraction; Order II, postextraction; Order III, high well-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
Edentulous Alveolar
Biology
residual ridge. For this reason, human biopsy material has been restricted in size and number. The fact that the histologie appearances in different parts of the same mouth can vary widely, probably explains the different observations and conclusions found in different biopsy studies. Furthermore, most biopsy studies of human material are of soft tissue only and therefore do not reveal information about the bone or the important interface between bone and soft tissue. Histologie studies of postmortem material are usually restricted by a lack of clinical correlation since such material is usually obtained from cadavers of unknown clinical status. Ideally, one would have sufficient biopsy material at intervals over a period of time or, at the least, post-
Ridges
13
subjects
who had been studied In each instance such of time. period clinically material is difficult to obtain. Suffice it to say, microscopic studies of residual ridges32"38 have revealed the following: (1) varying demortem material from over a
0:5
YEARS of REDUCTION of RESIDUAL RIDGES 1,5 2 2.5 3 3.5 4 4.5 1
0.13mm. /yr. Irate: min.: 2.0mm.
rate:
0.5mm./yr.
ave.z68mm.
I mo.
7mo.
50
mo. rate:
1.8mm./yr.
max.:
14.5 mm.
•after Carls»on,1967
Figure 4. Tracings of three lateral cephalographs with the maxillae and mandibles carefully superimposed. Note the changes in the shape of the residual ridges following the extraction of remaining teeth 50 months before. (From Atwood, D. .: J Prosthet Dent 13: 811, 1963.)
Graph illustrating typical mean mandibular bone loss during 5-year postextraction period with maximal and minimal ranges from data published by Carlsson, G. E. and Persson, G. : Morphologic changes of the mandible after extraction and wearing dentures. Odontol Revy 18: 27-54, 1967.) Figure 5. curve
|MEAN
E
65.2 10.1
«
.50
MEDIAN
- -1—r
.25 OO
5
"75
Figure 6. Rate
-flj
¿5
of RRR (total)
50
55
60
70
65
75
80
85
90
Age vs
age. (From Atwood, D. A. and
Coy,
W. ., J Prosthet Dent 26: 289,
1971.)
14
Atwood
grees of keratinization, acanthosis, thickness, edema, and
architectural pattern of epithelium in the same mouth and between subjects;23"29 (2) varying degrees of inflam-
of residual ridges is revealed by careful superimposition of portions of two cephalometric radiographs made 16 years apart. The actual bone loss in the anterior part of the ridge of the mandible was 13 mm in height (a 41% Figure 7. Gross bone loss
reduction) and 60 sq mm in cross-sectional area (a 24% reduction). (From Atwood, D. .: J Prosthet Dent 26: 268, 1971.)
Eighth English Symposium matory cells from clinically normal to frankly inflamed areas in both denture and nondenture wearing patients;23"29 (3) lymphocytes,23"29 plasma cells,23,27 mast The
cells29 and osteoclasts;22'23,27'29 (4) dense, fibrous connec-
tive tissue (sometimes hyalinized) frequently observed over crestal bone with fibers running parallel to epithelial surface;23"27 (5) a vascular plexus outside the periosteum in areas of bone apposition;29 (6) small blood vessels in close contact with the bone margin in areas of bone résorption, sometimes in the lacunae with positive correlation between the degree of inflammation, vascular reactions, and bone résorption;29'38 (7) marked diaphorase activity in areas of bone remodeling—either formation or résorption;29 (8) ATPase activity in areas of bone formation and acid phosphatase activity in areas of bone résorption;29 (9) the lack of evidence of bone résorption in areas which do not have inflammatory cells;29 (10) endosteal bone deposition reinforcing internal structure where external surface has been affected by résorpon tion;22' 23' 27' 29'32 (11) lack of periosteal lamellar 23'bone the external surface of the crest of the ridge;22' 27'29,32 (12) a roughened crestal bone surface which is either actively resorbing or is inactive, but without reversal lines on the external surface of the crestal bone;22,23' 27' 29'32 (13) development of secondary Haversian systems in remodelled compacted endosteal bone;32 (14) microradiographic evidence of mandibular osteoporosis 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 the trabeculae on the crest of the ridge. Also evident is an osteon within a single trabecula, and evidence of internal bone formation, and résorption of the trabeculae. (From Atwood, D. .: J Prosthet Dent 13: 816, 1963.)
Oral
Perspective on Bone Biology
Edentulous Alveolar
Ridges
15
microradiograph of the inferior border of a mandible showing evidence of moderate osteoporosis with increased variation density of osteons, increased number of incompletely closed osteons, and increased endosteal porosity. (From Atwood, D. .: J Prosthet Dent 13: 816, 1963) (Original magnification, x 15). Figure 9. A
in the
osteons, increased number of incompletely closed osteons, increased endosteal porosity and increased number of plugged osteons.31'32 It may well be that noninvasive techniques will be perfected which will provide information about the functional activities of bone cells and RRR which is additional to that available from current histologie, histochemical and microradiographic techniques. Biochemical Factors Which Influence Bone Cells
Local Biochemical Factors. Much work has been done in the study of local factors in bone résorption in trying to understand periodontal disease.39 Endotoxins from dental plaque40 (plaque can occur in edentulous mouths), osteoclast-activating factor (OAF),41 Prostaglandins,42"43 human gingival bone-resorption stimulating factor,44 etc., are all factors which could be important to the rate of RRR and deserve further study. For example, heparin, a cofactor in bone résorption, has been associated with mast cells which have been observed toward the bone margin of both residual ridges29 and gingiva.45 As with periodontal disease, local biochemical factors triggered by microorganisms could be a factor in RRR, especially when there is poor oral hygiene and when dentures are worn constantly. In addition, toxins leaching from denture 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 traumatogenic increased or decreased vascularity leading to changes in 02 tension49 or temperature.50 Systemic Biochemical factors. Just as some patients
with natural teeth and poor local factors seem to have great resistance to periodontal disease,51 there are some edentulous patients who do not have RRR and who retain good ridge form for many years even in the presence of local prosthetic factors which are felt to be unfavorable. Conversely, some patients have a high rate of RRR without any obvious unfavorable local factors. In such cases, the possibility exists that osteoporosis may be a contributing factor whether it be idiopathic or in the form of a deficiency of estrogen, calcium, or growth hormone, 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 established in the etiology of RRR.53"54 It is clear, however, that signs characteristic of osteo36'37'56 porosis have been found in human jaws.31,32' in Whether the pathophysiology of osteoporosis any specific patient is decreased bone formation or increased bone résorption or a combination of the two, the presence of an active stage of osteoporosis could very definitely have an effect on the rate of RRR.1'52,55) 57 To date, published attempts to correlate osteoporosis as determined densitometrically with the rate of RRR5 have failed, but this need not indicate a lack of relationship.
16
The
Atwood
Eighth English Symposium
250 Z2S c
0 200 '
1.75
E 1.50 E
~"
u5
£
a: a:
too
a
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.50 .25
.00
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Figure 10. Rate of RRR (mandible) Prosthet Dent 26: 293, 1971.)
vs
3
density of anterior
mandibular
Osteoporosis is a cumulative disease whose degree of activity at any point in time may not be appropriately indicated by the radiographie densitometric techniques utilized to date. Perhaps more sophisticated diagnostic techniques, employed longitudinally, would present different
conclusions.58"62
Cell Control Brought About on Bone
by
5
4
6
7
DENSITY OF ANT. MAND.RIDGE (mm Al.)
Physical Loads
Physical loads are placed on the alveolar bone in the individual with natural dentition through the teeth and periodontal ligaments to the lamina dura and supporting
trabecular bone.63"64 When the teeth are extracted, either there is little or no loading of the residual alveolar ridge as in the nondenture wearer, thereby perhaps resulting in a disuse atrophy of the ridge, or else the load is transmitted in the denture wearer through the artificial denture teeth, the denture base, and the mucoperiosteum to the bone.65 Studies have shown that the mean rate of RRR is three to four times greater in the mandible than in the maxilla, although for some patients the reverse is true.1"3'5 In fact, Tallgren has shown an interesting inverse relationship between the rates of RRR in the maxilla and mandible observed in 11 patients over a 7year period.3 Such differences within the same individual suggest the need to look at differences in the two jaws. In attempting to determine the basis for these differences, Woelfel, et al. showed that the average "projected" denture base area of 45 patients was 1.8 times greater for
8
ridges (site 14). (From Atwood,
D. A. and
Coy,
W. ., J
the maxilla than for the mandible.10 There follows logically from this that whenever the patient occludes the mean load per unit area applied to the mandible is 1.8 times more than that applied to the maxilla. For example, Woelfel, et al. cite a patient with projected area of the maxillary denture of 4.2 sq in and of the mandibular denture of 2.33 sq in (ratio 1.8).10 If this patient bites with a force of 50 lb, the resultant pressure would be 12 lb per sq in under the maxillary denture and 21 lb per sq in under the mandibular denture. In addition to masticatory forces, Ohashi, et al. have demonstrated that swallowing forces in 21 patients averaged 11.4 lb (2.7 lb per sq in under maxillary denture and 4.1 lb per sq in under the mandibular denture).66 Cutright, et al. have calculated that 1500 empty swallows per 24 hours could amount to 3500 to 4200 lb of loading per day.67 Further, they have demonstrated both positive and negative pressures on the residual ridges from a variety of activities, including smoking, talking and counting, as well as biting. Brewer has shown that masticatory and swallowing contacts between dentures average less than 15 minutes per waking day. What needs to be remembered is that some patients have been shown to clench and grind their teeth up to several hours per 24 hours.68 It is likely that such 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
Edentulous Alveolar Ridges
Biology
17
It is commonly assumed by dentists that a carefully fitted denture base will distribute the load through the mucoperiosteum to the bone more evenly, thereby resulting in less RRR. Workers have emphasized the importance of the thickness and the resilience of the mucoperiosteum. As a denture base is seated evenly against its tissue base, only a small amount of displacement takes place before the entire mucoperiosteum becomes a hydraulic system exerting a compressive force evenly over the denture-bearing area. It would appear that the mucous membranes exhibit viscoelastic properties.70"73 In discussing "damping" or energy absorption, in such bones as vertebral body, Frost states: "... bones which
\% Figure 13. An example of variation in the type of bone showing a broad maxillary cancellous ridge and a high but very narrow ("knife-edge") mandibular anterior ridge.
Figure 11. The even distribution of force over the denture bearing areas with well fitting dentures and equalized occlusion. (From Nagle, R. J. and Sears, V. H. : Dental Prosthetics, p. 333. C. V. Mosby Co., 1958.)
®
©
Diagrammatic representation of the process of "inwaisting" of external surface of bones subjected to compression loading which results in internal hydraulic-like effect and eventual bone remodeling. (From Frost, H. M.: The Laws of Bone Structure, 26. Springfield, Illinois, Charles C. Thomas, PubFigure 14.
lishers, 1964.)
subjected largely to compression loads, and experience no significant bending loads, are composed largely of cancellous bone which is ideally constructed for the absorption and dissipation of energy. This is because it consists 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 broad maxillary cancellous ridge may react differently from a narrow mandibular cortical ridge to the same compresare
Figure 12. An enlarged radiographie view of a vertebral body showing a trabecular pattern oriented parallel to the direction of compression deformation and with "inwaisting" on the right side of the vertebral body.
sion load. Frost refers to a process called "inwaisting" which results from the sequential events of compression, internal hydraulic-like effect, external bulging and external osteoclastic activity with resultant narrowing ("inwaisting") of the bone.75 If the internal trabeculae of cancellous bone are numerous, thick and strong, compression
18
The
Atwood
loads cause less delormation, less hydraulic-like effect, less external bulging and hence less external osteoclastic "inwaisting". Older persons who have lost trabecular thickness and number will tend to have more vertical deformation under compression and consequently more external osteoclastic résorption. Perhaps this is a factor in RRR which is characterized by external osteoclastic résorption, or a form of "inwaisting." Frost points out that trabeculae are oriented parallel to the direction of compression deformation. For years, Applegate76 recommended phasing in the loading of an edentulous ridge by first inserting a removable partial denture without the teeth and then "exercising" the ridge by intermittent gentle pressure with the goal of strengthening and reorienting the trabeculae on the crest of the ridge.77 Neufeld has demonstrated in dry human jawbones a trabecular pattern in which the trabeculae are oriented at right angles to the crest of the ridge (parallel to the direction of occlusal forces through the denture base).78 This would be optimal to resist such compression loading from a denture. The "neutrocentric" concept of occlusion (DeVan)65 attempts by a variety of means to achieve compression loading of the residual ridges, to minimize any tension or shear loads, and thereby to reduce RRR. "Anatomical" occlusal schemes seek to reduce RRR through efficiency of mastication, reduced occlusal contact area, and balanced occlusion.10 If an anatomical occlusion "misses centric" either initially or subsequently due to RRR,3 there is increased likelihood of tension and shear forces on the residual ridges. This is thought by some to increase the rate of RRR, but even a careful longitudinal study for 5 years of 45 patients which comes to the opposite conclusion has such a wide overlap of RRR in the three
Figure 15. Radiograph of thin midsaggital sections of eight mandibles illustrating a variety of types of bone and raising the question of the possible "damping" effect and "inwaisting".
Eighth English Symposium
Figure 16. An example of a craniofacial relationship which could result in different loading of the residual ridges due to the muscle attachments and direction of muscular activity.
occlusal groups as to obfuscate the conclusions.9"11 Most of this discussion of the loading of bone has focused on the residual ridge per se. In actuality, we should also be looking at the mandible as a whole bone and its functional and developmental anatomy.79"81 Tallgren has studied the possible correlation of RRR with the morphological configuration.3 It is fair to assume that the loading of the ridge is dependent on the shape of the craniofacial bones, the insertion of the muscle attachments, and the direction of muscular activ-
ity.
The pathophysiology of the overloading of bone could consist of one or more of the following mechanisms: Trauma leading to increased vascularity, compression leading to decreased blood flow or increased venous stasis, piezo-electric effect, and stimulation of bone resorbing cells through bone cell receptors activated by physical forces. Some of the most experienced and perceptive prosthodontists for decades have warned against the overly retentive denture which fits "tight" today but in a relatively short period of time loses its retention and "fit" either due to viscoelastic changes or RRR.65,69 Most warn against constant denture wearing. One additional form of possible continuous loading of the residual ridge lies in the possibility of a postoperative cicatricial mucoperiosteum seeking a reduced area.81'82 Furthermore, it is possible that the rate of RRR is predetermined by the dentist, who removes the teeth, by such surgical factors as the amount of attached gingiva that is removed, the amount of cortical bone that is removed, the amount of tissue reflection and dissection of mucoperiosteum from the bone,83 the amount of alveolectomy or alveolotomy, and the degree of tension resulting from close approximation of opposing tissues and tight suturing.
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Edentulous Alveolar Table 2. Co- Variables
Category Time Viable Bone Cells
Systemic
Biochemical Fac-
tors
19
Possibly Related to Rate of RRR
Variables which may have Direct Relation
Variables which may have Inverse Relation Time Since Extraction Progenitor Cells Osteoblasts
Progenitor Cells
Osteoclasts Senescence Local Biochemical Factors
Ridges
Endotoxins
Osteoclast-Activating Factor (OAF) Prostaglandins Human Gingival Factor Immune Complexes Heparin
Anti-Invasion Factor Indomethacin
Parathyroid Hormone (PTH) Thyroid Hormone (TH)
Calcitonin (CT) Growth Hormones
Adrenal Cortical Steroids
(AIF)
(GH)
Estrogens Androgens
Ca, Vit. D, Vit. C, F Protein Load Factors
Amount, Frequency, Duration and
Direction of Load Load Per Unit Area Occlusal Contact Area Traumatic Prosthetic Factors Craniofacial Morphology
Summary RRR is probably a multifactorial process subject to a wide number of covariables. In any given patient, these covariables will be in a unique combination which will determine the rate of RRR for that patient at that time. If the variables change in any given patient, the rate of RRR may change. The dentist who wishes to help his
edentulous patient should seek to be sensitive to all these variables and to understand and treat that unique combination of factors which exists in that patient. References
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Morphology
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20
Atwood
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The
Eighth English Symposium
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Oral Perspective
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Bone
Biology
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Edentulous Alveolar
Ridges
21
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