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Retention of obturator-removable partial dentures: A comparison of buccal and lingual retention
Retention of obturator-removable partial dentures: A comparison of buccal and lingual retention David N. Firtell, D.D.S., M.A.,* and Richard J. Grisius, D.D.S., M.A.** University of California, Oakland, Calif.
School of Dentistry, San Francisco, Calif., and Naval Regional Medical Center,
T lhl e oss o f support of a removable partial denture by a patient who has had a maxillectomy causes increased pressure, torque, and lever action on the associated hard and soft tissues. The added torque and lever action are uncommon to most welldesigned removable partial dentures. Removable partial dentures designed for maxillectomy patients are further complicated by the added weight of the obturator. To help compensate for the differences of torques and levers in the maxillectomy patient, some dentists use retentive surfaces on the lingual aspect of the abutment teeth. Other dentists use the more conventional method of placing retention on the buccal aspect of the abutment teeth. A variety of methods to distribute the potentially damaging forces during function have been described.‘-!’ A maxillary defect further complicates a prosthesis design, since it is frequently located unilaterally, which increases the stresses to the supporting An in vitro study by Fiebiger and structures.‘” associates” evaluated the effect of a removable partial denture with a simulated obturator on the movement of abutment teeth. The additional weight of the obturator presents the problem of selecting the clasp design which best resists the vertical displacement of the prosthesis. An in vitro study by Firtell” has evaluated the retentive quality of various clasp designs. Controversy still exists as to whether buccal or lingual retention provides optimal retention for a unilateral removable partial denture which supports an obturator.“‘. ” The opinions or assertionscontained herein are the private ones of the author and are not to be construed as official or as reflecting the views of the Department of the Navy. *Professor and Chairman, Division of Removable Prosthodontics. **Captain (DC) USN; Chairman, Department of Dentistry.
212
PURPOSE The purpose of this in vitro study was to determine what influence the weight of an obturator would have on resistance for retention of a removable partial denture with various clasp designs. For was defined as this study, “resistance for retention” the force required to dislodge a removable partial denture from its seat in a vertical direction.
METHODS AND MATERIALS A case-hardened metal mold that simulated the abutments of an obturator retained by a unilateral partial denture was fabricated (Fig. 1). The model consisted of three spheres, each % inch in diameter, mounted on stems attached to a metal plate at the angles of an equilateral triangle. The stems were to provide clearance between the plate and the test frameworks that were to be seated on the model. The tops of all three spheres were flattened in the same plane to simulate rests and to aid in surveying. Each of the simulated rests could be identified to properly orient the test frameworks. Hard wax was applied around two of the spheres designated as abutments. The third sphere was designed as a rest to indicate that the test frameworks were seated. It was positioned in the area that would accept a simulated obturator. A surveyor with an undercut gauge was used to trim the wax and establish 0.02-inch undercuts on each of the two abutment spheres in relation to a plane common to the three rests. Additional wax was added to the base of the spheres so that the refractory casts that were made from the model would support the framework patterns. Eight duplicate refractory casts were poured in separate reversible hydrocolloid impressions of the model. This was done according to the procedure prescribed by the manufacturer of the nickel-chrome
FEBRUARY 1960
VOLUME
43
NUMBER 2
RETENTION
OF ORTURATOR-DENTURES
Fig. 1. The model with wax added prior to duplication. Simulated lingual undercuts are indicated by arrows. Fig. 2. Refractory casts prior to waxing. alloy* chosen for fabrication of the test frameworks (Fig. 2). Each cast was placed on the surveyor and oriented to a plane common to its three rests. Height of contour and limits of the clasp design were recorded, after which the refractory casts were ready for waxing. Patterns for -eight test frameworks were waxed on the refractory casts. Each sphere had a ring of 14-gauge wax placed on top of a rest preparation. The rings posit;loned on the two abutments provided attachments for the withdrawal mechanism. The third ring was attached to the rest bn the sphere that served as the location of the simulated obturator. The same obturator weigh1 could then be attached to this third ring to standardize each test framework. Two frameworks were formed with each of the following clasp designs: (1‘1 suprabulge clasps with buccal retention (retention on the side away from the obturator), (2) suprabulge clasps with lingual retention (retention on the side toward the obturator), (3) infrabulge clasps with buccal retention, and (4) infrabulge clasps with lingual retention. To keep patterns standard, prefabricated plastic patterns were used for all clasp arms., 8-gauge half-round wax was used for all rests, and 10-gauge round wax was used for major and minor connections. All casting procedures xtiere completed by the same dental laboratory technician following the manufacturer’s instructions. The castings were electrically deplated but not polished. During the removal of minimal amounts of flash and bubbles, ‘Ticonium, N. Y.
Tic&urn
THE JOURNAL
Fig. 3. A test framework and withdrawal mechanism in place on the Instron machine with force being applied. The simulated obturator (weight of 64 gm) is indicated by an arrow. care was taken not to change the original dimensions or the contours of the clasps. Withdrawal mechanism 4 withdrawal mechanism was used to attach the model and the test frameworks to an Instron universal testing instrument* to measure the force required to dislodge each metal framework (Fig. 3). The mechanism consisted of the following: (1) a bolt through the model by
Division, CMP Industries Inc.. Albany, *Instron Corp., Canton, Mass
OF PROSTHETIC
DENTISTRY
213
FIRTELL AND GRISIUS
abutment. This situation would not usually occur in the mouth. Each framework was placed on the model and the force for its first instant of slippage from the model recorded in grams. First a test framework was withdrawn five times without the weight of a simulated obturator to establish a baseline of retention. Then the framework was withdrawn five times with a simulated obturator that weighed 64 gm. (This was selected as the weight which represented the mass of acrylic resin of an average-size obturator.)
RESULTS
Fig. 4. Typical graph recording patterns of force for withdrawal of a test framework: Point A, Slack being removed from chain of withdrawal mechanism before force is applied to the framework. Point B, First dislodgement of the framework. Point C, Complete removal of framework from the model, Summation of force for complete removal was not possible. which it was suspended upside down from the testing machine, (2) a spreader plate with two swiveled hooks that could be attached to the rings of the frameworks, and (3) nuts on the threaded ends of the hooks which made it possible to adapt the hooks to the frameworks and level the mechanism. A chain was attached through the center of the spreader plate by which the mechanism was secured to the lower member of the testing machine. With this device and the Instron instrument, force was exerted in a vertical direction to dislodge a test framework from the model. The frameworks were considered dislodged upon their first point of slippage from the model (Fig. 4). The reading of a force for complete dislodgement was difficult to record, since the reading mechanism of the Instron testing instrument recoiled at each point of slippage. Also, since only two abutments were used to retain a framework, it would sometimes twist when completely dislodged from one abutment and remain hanging on the other
214
The mean value of the forces required to obtain initial displacement of each framework is graphically represented in Fig. 5. The ranges of the values from which the means were derived are also given. The differences between the means in four categories are shown in Fig. 6. With a 0.02-inch undercut, the infrabulge clasp form with lingual retention (retention on the same side as the obturator) exhibited the greatest difference between initial displacement force with and without an obturator. The suprabulge clasp form with lingual retention exhibited a lesser degree of difference. Both infrabulge and suprabulge clasp forms with buccal retention (retention on the side opposite the obturator) exhibited the least difference in the force measured. The force needed to displace the test framework with lingual retention was greater than the force needed with buccal retention when the weight for the obtruator was not present. The data derived was subjected to an analysis of variance, using the type of clasp (suprabulge or infrabulge), position of retention (buccal or lingual), and presence of the obturator (with and without the obturator) as factors. A fourth factor, the variation in the test frameworks, was also considered, but it nested within the first two factors mentioned and crossed with the third. The data were considered significant at the .OOOl level.
DISCUSSION Retention of a removable partial denture is not as important as stability. When an obturator is included in a removable partial denture, stability is difficult without retention. The weight of the obturator counters the retention and stability. As indicated by the results, an obturator placed on the opposite side of the rest from the retentive arm of a clasp reduces the retentive ability of that clasp. The amount of retention which is lost is almost equal to
FEBRUARY 1980
VOLUME
43
NUMBER 2
RETENTION
OF OBTURATOR-DENTURES
1
Suprabulge
Without
Suprabulge
With Simulated
Infrabulge
Without
Obturator--Lingual
I I
1
Simulated
Obturator--Lingual
Obturator--Lingual
Simulated
1
Retention I
Retention
Obturator--Lingual
Retention
Retention
Suprabulge Without Simulated Iobturator--Buccal Retention Suprabulge
With Simulated
Infrabulge
Without
Infrabulge
With Simulated
0
Simulated
530
-RANGE
1000
1500
2000
2500
3000
FORCE FOR REMOVALIN GRAMS
Fig. 5. The amount of force recorded for removal of the frameworks. the weight of the obturator, since that weight is applied in the path of withdrawal. In these situations the obturator, the rest, and the retentive arm (buccal retention) comprise a Class I lever. If allowed to rotate, the retentive arm would be levered deeper into its undercut and increase its retention in the vertical direction. Stabilizing components of the test framework appear to prevent this type of action. Placing the retentive arm on the same side as the obturator establishes a Class II lever. Although the stabilizing components of the test framework tend to keep the retentive area from being displaced out of its undercut by rotation, the vertical component of the rotational force caused by the obturator is sufficient to reduce the retentive ability of a lingually placed retentive arm. Therefore, lingual retention was reduced by the presence of the obturator. The data indicate that an infrabulge clasp was affected by the presence of the obturator more than a suprabulge clasp. A superficial evaluation of the data might support the contention that lingual retention was greater than buccal retention under the same testing circumstances. A closer observation of the experimental design and the model used to represent the abutments does not support this conclusion. The model was closely examined to help explain the wide difference in retentive ability between buccal and lingual placement of the retentive arm. We found that the angle of convergence of
THE JOURNAL
OF PROSTHETIC
DENTISTRY
the undercut on the side that represented lingual retention was greater than on the side that represented buccal retention. Although the depth of undercut was the same (0.02 inch), the retentive arm on the lingual would need to travel through a steeper incline during removal. The distance the retentive arm would travel before it was completely removed would be less on the lingual than on the buccal surface, if the angle was less and in the same relative position. If the undercuts were the same, the total force necessary to remove a clasp through a greater angle and lesser distance would equal the total force to remove a clasp through a lesser angle and a greater distance (Fig. 7). Because of the peculiarities of the Instron testing machine and the model, only the initial force for displacement and not the total force for removal of the test frameworks was recorded. The greater angle on the lingual side would require the force for initial displacement to be greater. This is true since only a short distance is needed to record the initial dislodgement from the angle and not the entire length of the undercut. The data indicate that the retentive capability of all the clasp designs tested was adversely affected (to some degree) by the addition of an obturator to the framework. Therefore, design considerations should include attempts to support the obturator with other means besides the remaining dentition to minimize the stresses to the supporting tissues. Retention as
215
FIRTELL AND GRISILJS
-------_
SuprabulSe InfrabulSe
2600 2523 2460
2200 -
-.
-.
-.
--
2190
1800 -
1400 1200 -l:::'-~~ -
\
1190
Fig. 7. The total force required to move a clasp arm from a A to B equals the total force required to move a similar clasp arm from A’ to B’. The force for initial dislodgement at A’ is greater than at A because of the increased angle. The total force for removal is a function of the angle through which the clasp must move and the distance it must move.
800 -
400 -
B Without Weight
LINGUAL With Weight
Without Weight
With Weight
Fig. 6. Comparison of the retention of test frameworks with and without the simulated obturator. Buccal retention was not affected by the obturator as much as lingual retention. No comparison can be made between buccal and lingual retention because of a discrepancy found in the model. well as support must be gained by adequate extension of the obturator to utilize the remaining anatomic structures. Desjardins”’ lists these structures as the residual soft palate, residual hard palate, anterior nasal aperture, lateral scar band, and height of the lateral wall. The results raise some questions. What is the effect of the sustained force applied by the weight of an obturator? What is the effect of the decreased force required for removal of the prosthesis because of the presence of the obturator? Which of these forces is less detrimental? Henderson’ stated that Class I levers should be avoided in designing removable partial dentures. What is the effect of an obturator? These questions might be answered by photoelastic analysis (which is currently underway at UCLA).
SUMMARY
AND CONCLUSION
An in vitro study was performed to measure the influence of a simulated obturator on the amount of 216
force required to dislodge a simulated unilateral removable partial denture with various clasp designs. The presence of an obturator reduces the retentive capability of a removable partial denture. Lingual retention appeared to provide more resistance to displacement than buccal retention. Infrabulge clasp designs appeared to be more retentive than suprabulge clasp designs.
REFERENCES
4.
5. 6. 7.
8. 9. 10.
Demer, W.: An analysis of mesial rest-I-bar clasp designs. J PROSTHET DENT 36:243, 1976. Frechette, A.: The influence of partial design on distribution of force to abutment teeth. J PROSTHET DENT 6:195, 1956. Goodkind, R.: The effects of removable partial dentures on abutment tooth mobility: A clinical study. J PROSTHET DENT 30:139, 1973. Henderson, D., and Steffel, V. L.: Principles of removable partial denture design. In McCracken’s Removable Partial Prosthodontics, ed 5. St. Louis, 1977, The C. V. Mosby Co., pp 117-134. Javid, N., and tiadmanesh, J.: Obturator design for hemimaxillectomy patients. J PROSTHET DENT 36:77, 1976. Kaires, A. K.: Effect of partial denture design on unilateral force distribution. J PROSTHET DENT 6:526, 1956. approach to partial denture Kelly, E.: Th e physiologic design. J PROSTHET DENT 3:699, 1953. Krol, A. J.: Clasp design for extension-base removable partial dentures. J PROSTHET DENT 29:408, 1973. Perry, C.: A philosophy of partial denture design. J PROSTHET DENT 6:775, 1956. Desjardins, R.: Obturator prosthesis design for acquired maxillary defects. J PROSTHET DENT 39:424, 1978.
FEBRUARY 1980
VOLUME
43
NUMBER 2
RETENTION
OF OBTURATOR-DENTURES
partially edentulous patients. Presented at the American Academy of Maxillofacial Prosthetics, October 2-5, 1977. Orlando, Fla.
Il.
Fiebiger, G., Rahn, A.. Lundquist, D., and Morse, K.: Movement of abutments by removable partial denture frameworks with hemimaxillcctomy obturator. J PROSTHE.I DENT 34:555, 1976. 12. Firtell, iI.: EfFect of clasp design upon retention of removable partial dentures. J PROSTHET DENT 20:43. 1968. 13. Beumer. J.. Curtis. T., and Firtell, D.: Maxillofacial Rehabilitation: Pro:,thetic and Surgical Considerations. St. Louis. The C. V. Mosby Co. (To he published) 14. Aramany. M. A.: Basic principles of obturator designs for
ARTICLES TO APPEAR IN FUTURE ISSUES Applying pati.ent William
basic prosthodontic
principles in the dentulous maxillectomy
D. Gay, D.D.S., and Gordon
E. King,
Psychologic aspects of prosthodontic Donald
B. Giddon,
D.M.D..
B.S., David
Copperplating
treatment for geriatric patients
Ph.D., and Eugene Hittleman,
Posterior peripheral seal distortion Stephen Glazier,
D.D.S.
related to height of the maxillary ridge
N. Fir-tell, D.D.S.,
M.A..
copper band-compound
Rafael Grajower,
Ed.D.
Ph.D., Noah Stern, D.M.D.,
and Larry
L. Harman,
D.D.S.
impressions at constant voltage M.S.D.,
and Judith
Kaffman,
D.M.D.
An electromyographic study of masticatory and lip muscle function in patients with complete dentures Bengt Ingervall,
D.D.S.,
Odont.Dr.,
and Bjiirn
Hedeghrd,
D.D.S..
Odont.Dr.
Radiographic study of condylar position in centric relation and centric occlusion Yahia
H. Ismail,
D.M.D.,
Ph.D., and Ahmed
Rokni,
D.M.D.,
M.S.D.
Comparison of three luting agents Masaaki Iwaku, D.D.S., D.M.Sc.
D.D.S.,
D.D.Sc.,
Quantitative syndrome
electromyographic
Hiroo Kotani, D.D.S., Yasuyuki Yamada, D.D.S., D.M.Sc.
Toshio
Takatsu,
D.D.S.,
D.D.Sc.,
and Takao Fusayama.
diagnosis of myofascial pain-dysfunction
Kawazoe,
D.D.S..
Taizo
Hamada,
D.D.S.,
D.D.Sc.,
and Sanae
A technique for the delivery of complete dentures H. M. Landesman,
D.D.S.,
M.Ed.
An assessment of recent advances in external maxillofacial D. H. Lewis, Ph.D., and D. J. Castleberry,
materials
D.D.S.
Overdentures for treatment of severe attrition William THE JOURNAL
S. Licht,
OF PROSTHETIC
D.M.D., DENTISTRY
and Edward
E. Leveton,
D.D.S 217
School of Dentistry, San Francisco, Calif., and Naval Regional Medical Center,
T lhl e oss o f support of a removable partial denture by a patient who has had a maxillectomy causes increased pressure, torque, and lever action on the associated hard and soft tissues. The added torque and lever action are uncommon to most welldesigned removable partial dentures. Removable partial dentures designed for maxillectomy patients are further complicated by the added weight of the obturator. To help compensate for the differences of torques and levers in the maxillectomy patient, some dentists use retentive surfaces on the lingual aspect of the abutment teeth. Other dentists use the more conventional method of placing retention on the buccal aspect of the abutment teeth. A variety of methods to distribute the potentially damaging forces during function have been described.‘-!’ A maxillary defect further complicates a prosthesis design, since it is frequently located unilaterally, which increases the stresses to the supporting An in vitro study by Fiebiger and structures.‘” associates” evaluated the effect of a removable partial denture with a simulated obturator on the movement of abutment teeth. The additional weight of the obturator presents the problem of selecting the clasp design which best resists the vertical displacement of the prosthesis. An in vitro study by Firtell” has evaluated the retentive quality of various clasp designs. Controversy still exists as to whether buccal or lingual retention provides optimal retention for a unilateral removable partial denture which supports an obturator.“‘. ” The opinions or assertionscontained herein are the private ones of the author and are not to be construed as official or as reflecting the views of the Department of the Navy. *Professor and Chairman, Division of Removable Prosthodontics. **Captain (DC) USN; Chairman, Department of Dentistry.
212
PURPOSE The purpose of this in vitro study was to determine what influence the weight of an obturator would have on resistance for retention of a removable partial denture with various clasp designs. For was defined as this study, “resistance for retention” the force required to dislodge a removable partial denture from its seat in a vertical direction.
METHODS AND MATERIALS A case-hardened metal mold that simulated the abutments of an obturator retained by a unilateral partial denture was fabricated (Fig. 1). The model consisted of three spheres, each % inch in diameter, mounted on stems attached to a metal plate at the angles of an equilateral triangle. The stems were to provide clearance between the plate and the test frameworks that were to be seated on the model. The tops of all three spheres were flattened in the same plane to simulate rests and to aid in surveying. Each of the simulated rests could be identified to properly orient the test frameworks. Hard wax was applied around two of the spheres designated as abutments. The third sphere was designed as a rest to indicate that the test frameworks were seated. It was positioned in the area that would accept a simulated obturator. A surveyor with an undercut gauge was used to trim the wax and establish 0.02-inch undercuts on each of the two abutment spheres in relation to a plane common to the three rests. Additional wax was added to the base of the spheres so that the refractory casts that were made from the model would support the framework patterns. Eight duplicate refractory casts were poured in separate reversible hydrocolloid impressions of the model. This was done according to the procedure prescribed by the manufacturer of the nickel-chrome
FEBRUARY 1960
VOLUME
43
NUMBER 2
RETENTION
OF ORTURATOR-DENTURES
Fig. 1. The model with wax added prior to duplication. Simulated lingual undercuts are indicated by arrows. Fig. 2. Refractory casts prior to waxing. alloy* chosen for fabrication of the test frameworks (Fig. 2). Each cast was placed on the surveyor and oriented to a plane common to its three rests. Height of contour and limits of the clasp design were recorded, after which the refractory casts were ready for waxing. Patterns for -eight test frameworks were waxed on the refractory casts. Each sphere had a ring of 14-gauge wax placed on top of a rest preparation. The rings posit;loned on the two abutments provided attachments for the withdrawal mechanism. The third ring was attached to the rest bn the sphere that served as the location of the simulated obturator. The same obturator weigh1 could then be attached to this third ring to standardize each test framework. Two frameworks were formed with each of the following clasp designs: (1‘1 suprabulge clasps with buccal retention (retention on the side away from the obturator), (2) suprabulge clasps with lingual retention (retention on the side toward the obturator), (3) infrabulge clasps with buccal retention, and (4) infrabulge clasps with lingual retention. To keep patterns standard, prefabricated plastic patterns were used for all clasp arms., 8-gauge half-round wax was used for all rests, and 10-gauge round wax was used for major and minor connections. All casting procedures xtiere completed by the same dental laboratory technician following the manufacturer’s instructions. The castings were electrically deplated but not polished. During the removal of minimal amounts of flash and bubbles, ‘Ticonium, N. Y.
Tic&urn
THE JOURNAL
Fig. 3. A test framework and withdrawal mechanism in place on the Instron machine with force being applied. The simulated obturator (weight of 64 gm) is indicated by an arrow. care was taken not to change the original dimensions or the contours of the clasps. Withdrawal mechanism 4 withdrawal mechanism was used to attach the model and the test frameworks to an Instron universal testing instrument* to measure the force required to dislodge each metal framework (Fig. 3). The mechanism consisted of the following: (1) a bolt through the model by
Division, CMP Industries Inc.. Albany, *Instron Corp., Canton, Mass
OF PROSTHETIC
DENTISTRY
213
FIRTELL AND GRISIUS
abutment. This situation would not usually occur in the mouth. Each framework was placed on the model and the force for its first instant of slippage from the model recorded in grams. First a test framework was withdrawn five times without the weight of a simulated obturator to establish a baseline of retention. Then the framework was withdrawn five times with a simulated obturator that weighed 64 gm. (This was selected as the weight which represented the mass of acrylic resin of an average-size obturator.)
RESULTS
Fig. 4. Typical graph recording patterns of force for withdrawal of a test framework: Point A, Slack being removed from chain of withdrawal mechanism before force is applied to the framework. Point B, First dislodgement of the framework. Point C, Complete removal of framework from the model, Summation of force for complete removal was not possible. which it was suspended upside down from the testing machine, (2) a spreader plate with two swiveled hooks that could be attached to the rings of the frameworks, and (3) nuts on the threaded ends of the hooks which made it possible to adapt the hooks to the frameworks and level the mechanism. A chain was attached through the center of the spreader plate by which the mechanism was secured to the lower member of the testing machine. With this device and the Instron instrument, force was exerted in a vertical direction to dislodge a test framework from the model. The frameworks were considered dislodged upon their first point of slippage from the model (Fig. 4). The reading of a force for complete dislodgement was difficult to record, since the reading mechanism of the Instron testing instrument recoiled at each point of slippage. Also, since only two abutments were used to retain a framework, it would sometimes twist when completely dislodged from one abutment and remain hanging on the other
214
The mean value of the forces required to obtain initial displacement of each framework is graphically represented in Fig. 5. The ranges of the values from which the means were derived are also given. The differences between the means in four categories are shown in Fig. 6. With a 0.02-inch undercut, the infrabulge clasp form with lingual retention (retention on the same side as the obturator) exhibited the greatest difference between initial displacement force with and without an obturator. The suprabulge clasp form with lingual retention exhibited a lesser degree of difference. Both infrabulge and suprabulge clasp forms with buccal retention (retention on the side opposite the obturator) exhibited the least difference in the force measured. The force needed to displace the test framework with lingual retention was greater than the force needed with buccal retention when the weight for the obtruator was not present. The data derived was subjected to an analysis of variance, using the type of clasp (suprabulge or infrabulge), position of retention (buccal or lingual), and presence of the obturator (with and without the obturator) as factors. A fourth factor, the variation in the test frameworks, was also considered, but it nested within the first two factors mentioned and crossed with the third. The data were considered significant at the .OOOl level.
DISCUSSION Retention of a removable partial denture is not as important as stability. When an obturator is included in a removable partial denture, stability is difficult without retention. The weight of the obturator counters the retention and stability. As indicated by the results, an obturator placed on the opposite side of the rest from the retentive arm of a clasp reduces the retentive ability of that clasp. The amount of retention which is lost is almost equal to
FEBRUARY 1980
VOLUME
43
NUMBER 2
RETENTION
OF OBTURATOR-DENTURES
1
Suprabulge
Without
Suprabulge
With Simulated
Infrabulge
Without
Obturator--Lingual
I I
1
Simulated
Obturator--Lingual
Obturator--Lingual
Simulated
1
Retention I
Retention
Obturator--Lingual
Retention
Retention
Suprabulge Without Simulated Iobturator--Buccal Retention Suprabulge
With Simulated
Infrabulge
Without
Infrabulge
With Simulated
0
Simulated
530
-RANGE
1000
1500
2000
2500
3000
FORCE FOR REMOVALIN GRAMS
Fig. 5. The amount of force recorded for removal of the frameworks. the weight of the obturator, since that weight is applied in the path of withdrawal. In these situations the obturator, the rest, and the retentive arm (buccal retention) comprise a Class I lever. If allowed to rotate, the retentive arm would be levered deeper into its undercut and increase its retention in the vertical direction. Stabilizing components of the test framework appear to prevent this type of action. Placing the retentive arm on the same side as the obturator establishes a Class II lever. Although the stabilizing components of the test framework tend to keep the retentive area from being displaced out of its undercut by rotation, the vertical component of the rotational force caused by the obturator is sufficient to reduce the retentive ability of a lingually placed retentive arm. Therefore, lingual retention was reduced by the presence of the obturator. The data indicate that an infrabulge clasp was affected by the presence of the obturator more than a suprabulge clasp. A superficial evaluation of the data might support the contention that lingual retention was greater than buccal retention under the same testing circumstances. A closer observation of the experimental design and the model used to represent the abutments does not support this conclusion. The model was closely examined to help explain the wide difference in retentive ability between buccal and lingual placement of the retentive arm. We found that the angle of convergence of
THE JOURNAL
OF PROSTHETIC
DENTISTRY
the undercut on the side that represented lingual retention was greater than on the side that represented buccal retention. Although the depth of undercut was the same (0.02 inch), the retentive arm on the lingual would need to travel through a steeper incline during removal. The distance the retentive arm would travel before it was completely removed would be less on the lingual than on the buccal surface, if the angle was less and in the same relative position. If the undercuts were the same, the total force necessary to remove a clasp through a greater angle and lesser distance would equal the total force to remove a clasp through a lesser angle and a greater distance (Fig. 7). Because of the peculiarities of the Instron testing machine and the model, only the initial force for displacement and not the total force for removal of the test frameworks was recorded. The greater angle on the lingual side would require the force for initial displacement to be greater. This is true since only a short distance is needed to record the initial dislodgement from the angle and not the entire length of the undercut. The data indicate that the retentive capability of all the clasp designs tested was adversely affected (to some degree) by the addition of an obturator to the framework. Therefore, design considerations should include attempts to support the obturator with other means besides the remaining dentition to minimize the stresses to the supporting tissues. Retention as
215
FIRTELL AND GRISILJS
-------_
SuprabulSe InfrabulSe
2600 2523 2460
2200 -
-.
-.
-.
--
2190
1800 -
1400 1200 -l:::'-~~ -
\
1190
Fig. 7. The total force required to move a clasp arm from a A to B equals the total force required to move a similar clasp arm from A’ to B’. The force for initial dislodgement at A’ is greater than at A because of the increased angle. The total force for removal is a function of the angle through which the clasp must move and the distance it must move.
800 -
400 -
B Without Weight
LINGUAL With Weight
Without Weight
With Weight
Fig. 6. Comparison of the retention of test frameworks with and without the simulated obturator. Buccal retention was not affected by the obturator as much as lingual retention. No comparison can be made between buccal and lingual retention because of a discrepancy found in the model. well as support must be gained by adequate extension of the obturator to utilize the remaining anatomic structures. Desjardins”’ lists these structures as the residual soft palate, residual hard palate, anterior nasal aperture, lateral scar band, and height of the lateral wall. The results raise some questions. What is the effect of the sustained force applied by the weight of an obturator? What is the effect of the decreased force required for removal of the prosthesis because of the presence of the obturator? Which of these forces is less detrimental? Henderson’ stated that Class I levers should be avoided in designing removable partial dentures. What is the effect of an obturator? These questions might be answered by photoelastic analysis (which is currently underway at UCLA).
SUMMARY
AND CONCLUSION
An in vitro study was performed to measure the influence of a simulated obturator on the amount of 216
force required to dislodge a simulated unilateral removable partial denture with various clasp designs. The presence of an obturator reduces the retentive capability of a removable partial denture. Lingual retention appeared to provide more resistance to displacement than buccal retention. Infrabulge clasp designs appeared to be more retentive than suprabulge clasp designs.
REFERENCES
4.
5. 6. 7.
8. 9. 10.
Demer, W.: An analysis of mesial rest-I-bar clasp designs. J PROSTHET DENT 36:243, 1976. Frechette, A.: The influence of partial design on distribution of force to abutment teeth. J PROSTHET DENT 6:195, 1956. Goodkind, R.: The effects of removable partial dentures on abutment tooth mobility: A clinical study. J PROSTHET DENT 30:139, 1973. Henderson, D., and Steffel, V. L.: Principles of removable partial denture design. In McCracken’s Removable Partial Prosthodontics, ed 5. St. Louis, 1977, The C. V. Mosby Co., pp 117-134. Javid, N., and tiadmanesh, J.: Obturator design for hemimaxillectomy patients. J PROSTHET DENT 36:77, 1976. Kaires, A. K.: Effect of partial denture design on unilateral force distribution. J PROSTHET DENT 6:526, 1956. approach to partial denture Kelly, E.: Th e physiologic design. J PROSTHET DENT 3:699, 1953. Krol, A. J.: Clasp design for extension-base removable partial dentures. J PROSTHET DENT 29:408, 1973. Perry, C.: A philosophy of partial denture design. J PROSTHET DENT 6:775, 1956. Desjardins, R.: Obturator prosthesis design for acquired maxillary defects. J PROSTHET DENT 39:424, 1978.
FEBRUARY 1980
VOLUME
43
NUMBER 2
RETENTION
OF OBTURATOR-DENTURES
partially edentulous patients. Presented at the American Academy of Maxillofacial Prosthetics, October 2-5, 1977. Orlando, Fla.
Il.
Fiebiger, G., Rahn, A.. Lundquist, D., and Morse, K.: Movement of abutments by removable partial denture frameworks with hemimaxillcctomy obturator. J PROSTHE.I DENT 34:555, 1976. 12. Firtell, iI.: EfFect of clasp design upon retention of removable partial dentures. J PROSTHET DENT 20:43. 1968. 13. Beumer. J.. Curtis. T., and Firtell, D.: Maxillofacial Rehabilitation: Pro:,thetic and Surgical Considerations. St. Louis. The C. V. Mosby Co. (To he published) 14. Aramany. M. A.: Basic principles of obturator designs for
ARTICLES TO APPEAR IN FUTURE ISSUES Applying pati.ent William
basic prosthodontic
principles in the dentulous maxillectomy
D. Gay, D.D.S., and Gordon
E. King,
Psychologic aspects of prosthodontic Donald
B. Giddon,
D.M.D..
B.S., David
Copperplating
treatment for geriatric patients
Ph.D., and Eugene Hittleman,
Posterior peripheral seal distortion Stephen Glazier,
D.D.S.
related to height of the maxillary ridge
N. Fir-tell, D.D.S.,
M.A..
copper band-compound
Rafael Grajower,
Ed.D.
Ph.D., Noah Stern, D.M.D.,
and Larry
L. Harman,
D.D.S.
impressions at constant voltage M.S.D.,
and Judith
Kaffman,
D.M.D.
An electromyographic study of masticatory and lip muscle function in patients with complete dentures Bengt Ingervall,
D.D.S.,
Odont.Dr.,
and Bjiirn
Hedeghrd,
D.D.S..
Odont.Dr.
Radiographic study of condylar position in centric relation and centric occlusion Yahia
H. Ismail,
D.M.D.,
Ph.D., and Ahmed
Rokni,
D.M.D.,
M.S.D.
Comparison of three luting agents Masaaki Iwaku, D.D.S., D.M.Sc.
D.D.S.,
D.D.Sc.,
Quantitative syndrome
electromyographic
Hiroo Kotani, D.D.S., Yasuyuki Yamada, D.D.S., D.M.Sc.
Toshio
Takatsu,
D.D.S.,
D.D.Sc.,
and Takao Fusayama.
diagnosis of myofascial pain-dysfunction
Kawazoe,
D.D.S..
Taizo
Hamada,
D.D.S.,
D.D.Sc.,
and Sanae
A technique for the delivery of complete dentures H. M. Landesman,
D.D.S.,
M.Ed.
An assessment of recent advances in external maxillofacial D. H. Lewis, Ph.D., and D. J. Castleberry,
materials
D.D.S.
Overdentures for treatment of severe attrition William THE JOURNAL
S. Licht,
OF PROSTHETIC
D.M.D., DENTISTRY
and Edward
E. Leveton,
D.D.S 217