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The measurement of forces transmitted to abutment teeth of removable partial dentures
Jay B. Maxfield,
D.D.S., M.S.D.,*
Jack I. Nicholls,
Ph.D.,**
and Dale E. Smith, D.D.S., MS.D.***
U.S. Public Health Service Outpatient Clinic, New York, N. Y., and University of Washington, School of Dentistry, Seattle, ‘Wash.
T
he distribution of forces applied to the abutment teeth and residual ridges by distal-extension removable partial dentures remains largely undefined. A consideration of the supporting capacity of the abutment tooth and the mucosa of the residual ridge reveals the disparity in the capacity of each to withstand applied loads. As a result, designing a physiologically acceptable removable partial denture requires an understanding of the forces generated during mastication and their distribution to supporting structures. At present, the information which is necessary to determine the most acceptable removable partial denture design is unavailable. Numerous studies have been conducted to test distal-extension partial dentures, but unfortunately very few have been performed in vivo. Laboratory studies, while more easily controlled than clinical studies, may not yield accurate results due to difficulties in reproducing the physiology of the oral masticatory system. The conflicting and nonconclusive results of several laboratory studies indicate that they may be an unreliable means for testing removable partial denture designs.
LITERATURE
REVIEW
Frechette’ recognized the need for scientific testing to either prove or disprove ‘the existing theories which were based upon logical reasoning or clinical Presented and placed second in the 1977 American College of Prosthodontists Research Award Competition, New Orleans, La.; also presented at the American Prosthodontic Society, Miami Beach, Fla. *Chief, Oral Health Clinic, U.S. Public Health Service Outpatient Clinic, New York, N. Y. **Associate Professor, Department of Restorative Dentistry, University of Washington. ***Associate Professor, Department of Prosthodontics, University of Washington.
134
experience. His laboratory study showed that the loading and movement of the teeth was strongly influenced by such factors as the number and location of rests, contour and rigidity of connectors, and extension of denture bases. Metty? was one of the first investigators to utilize strain gauges in the study of removable partial dentures. His results support the concept that functionally based removable partial dentures are more stable than other types. Dutton and associates” used strain gauges to study lateral forces on five patients. The mean lateral forces to the abutment teeth during random chewing ranged from 5 to 148.7 gm. Unfortunately, the testing device used was only capable of detecting forces in the buccal-lingual direction. Lowe and associates” measured lateral forces generated by the tongue against a removable partial denture base. Mean forces of 28 to 162 gm were recorded during swallowing. Holmes5 and LeupoldG have presented evidence that the altered-cast impression technique provides the least movement of extension bases under an occlusal load when compared to bases processed on an anatomic cast. Kratochvil and Caputo’ performed photoelastic analysis of forces transmitted by distal-extension removable partial dentures to abutment teeth and to residual ridges. They found that lateral forces on the removable partial denture were distributed to all teeth contacted by the removable partial denture. They also felt that the ball and socket design of occlusal rests allowed for movement of the prosthesis, with occlusal forces applied in the region of the rest and transmitted along the long axis of the abutment tooth. Christidou and associates8 developed a device for clinically measuring the load applied to dentures
FEBRUARY 1979
VOLUME41
NUMBER
2
FORCES TRANSMITTED
TO ABUTMENT
TEETH
Fig. 1. A castsilver pontic that telescopesover the beam from the distal surfaceof the splint. and a method for measuring tooth mobility in a mesial-distal direction. They found a general tendency for abutment teeth to move mesially. They felt that the anatomic inclination of the residual ridge was largely responsible for this effect. Steffel” reported three philosophies regarding the control of stressesfrom distal-extension removable partial dentures: (1) physiologic basing, (2) stressbreakers, and (3) extensive stressdistribution. In all the articles he reviewed, substantiating evidence for any of the theories was lacking. PURPOSE The purpose of this study was to develop a means for clinically measuring the forces applied to abutment teeth by removable partial dentures. Future application of such a method could have farreaching benefits. It could provide a means for critically analyzing previous studies. If clinical testing can show laboratory results to be valid, then further research can be performed at that level with more easily controlled variables. It could provide better information for removable partial denture designs. One of our goals must be to preserve supporting structures; however, the removable partial denture design which does this best has not been conclusively demonstrated. Accurate measurement of forces applied to the structures by various designs is essential to the determination of which design is most acceptable. Finally, the results of such research should lead to a way to achieve maximum support for distal-extension removable partial dentures which is within the physiologic limits of the supporting structures. METHODS
maxillary complete dentures and mandibular bilateral distal-extension removable partial dentures. Both patients were women of ages46 and 51 years, and each was an experienced removable partial denture wearer. A 2.5 mm square beam projected 7 mm from the distal surface of the splint on the right side. A cast silver pontic was made to telescopeover the beam (Fig. l), and holes were drilled for attaching the pontic to the beam with pins. To each of four surfaces of the beam, strain gauges* were attached with cyanoacrylate adhesive,? covered with cellophane tape (Fig. 2), and sealed to render them waterpro0f.x The previously cast hollow silver pontic was then telescoped over the beam and rigidly attached with 0.5 mm stainlesssteel pins which were driven through holes drilled in the pontic and beam without touching the gauges. Four other strain gaugesfor temperature compensation were attached to two 4 X 5 X 0.75 mm silver plates, one gauge pel surface. The plates, cast with small retentive loops at one edge, were then attached to the splint with autopolymerizing resin. A series of 34-gauge insulated wires were soldered to the strain gauge leads and placed in a channel in the splint to exit the mouth and terminate in a small l&pin electrical plug (Fig. 3). With the silver splint seated on an artificial stone cast, guiding planes were milled on the distal surface of the pontic and the distal surface of the splint on the opposite side of the arch. Mesial and distal rests were prepared in the pontic on the test side, and a cingulum rest was prepared in the lingual surface of the splint on the contralateral side.
AND MATERIALS
Cast silver splints were fabricated to fit the remaining mandibular teeth of two patients wearing
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Fig. 2. Strain gaugesare attached to the beam with cyanoacrylate adhesive and covered with cellophane tape.
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DENTISTRY
‘EA-13-062AP-120, Micro-Measurements. Romulus, Mich. TM-Bond 200 Adhesive, Micro-Measurements, Romulus, Mich. $M-Coat GL, Micro-Measurements, Romulus, \
135
MAXFIELD,
NICHOLLS,
Fig. 4. Interchangeable rest assemblies attached with autopolymerizing resin. An irreversible hydrocolloid impression was made of the mandibular arch with the splint in ‘place. A chrome-cobalt framework was made on the resulting master cast. Provision was made for attaching interchangeable occlusal rest assemblies with autopolymerizing resin to facilitate changing rest position on the same metal framework (Fig. 4). Primary retention of the prosthesis on the test side was attained with an I-bar placed in a O.OOl-inch undercut in the center of the middle third of the buccal surface of the test pontic. Retention on the nontest side was attained with a circumferential wrought wire clasp for one patient and an I-bar clasp for the other. Anatomic posterior teeth were set in balanced occlusion against a duplicate of the patient’s existing complete maxillary denture, and the removable partial denture was processed on the anatomic master cast. The tissue surfaces of the processed extension bases were fitted to the patient’s residual ridges using a pressure-indicating paste to evaluate adaptation. Chloroform and gold rouge were used to adjust the contact of the partial denture framework with the distal guiding planes as described by Kratochvil.lo There was contact between the lingual plate and the lingual aspect of the pontic only at the height of contour to reciprocate the direct retainer when the distal rest was used alone. The contact was adjusted to permit rotation about the fulcrum when the bases were depressed or lifted. When the mesial occlusal rest was used, the minor connector contacted the mesiolingual part of the pontic. Occlusion was adjusted after remounting the finished removable partial denture on a semiadjustable articulator, and the denture teeth opposing the pontic on the testing device were reduced to preclude any direct loading during clinical testing.
136
Calibration
AND
that
SMITH
be
testing
The 16-pin electrical plug leading from the silver splint was connected to an amplifier by 5 feet of 18-wire shielded cable. A DC power supply was attached to the system. Strain gauge output, as indicated by a voltage change from the zero load value, was recorded on a four-channel tape recorder for storage (Fig. 5). With the splint seated on a stone cast bolted to a plastic platform, a series of known loads were applied to the pontic from the occlusal, buccal, and distal directions. The point of the loading pin was placed consecutively into the mesia1 rest, distal rest, and combined mesial and distal rest, and against the buccal surface, lingual surface, and distal guiding plane region of the pontic. For each position, recordings of strain gauge output were made for loads of 0, 100, 200, 500, 1100, 1300, and 1500 gm which represented the range of most clinical load values. Loads of up to 3.5 kg were applied to the testing device to prove the linearity of the testing system.
Clinical
testing
With the patient seated in a dental chair, the testing device was placed on the mandibular teeth and followed by the removable partial denture. The necessary electrical connections were made, and the patient was instructed to rest for 10 minutes to permit temperature stabilization of all components. After 10 minutes, a recording was made with the patient resting comfortably with teeth apart to represent a zero load level. The patient was then instructed to chew dry-roasted peanuts, first on the left side, then the right side, and, finally, with peanuts placed between teeth on both sides. The
FEBRUARY
1979
VOLUME
41
NUMBER
2
FORCES
TRANSMITTED
TO ABUTMENT
TEETH
Fig. 5. The splint is connected to an amplifier recorded on a four-channel tape recorder. patient was given the opportunity to take as many peanuts as she felt capable of chewing at one time. The number varied from three to five peanuts per sequence. This chewing sequence was repeated for each of three different occlusal rest locations: mesial rest, distal rest, and combined mesial and distal rest. In addition to varying the location of the occlusal rest, the effect of extension-base adaptation to the residual ridge was examined for three conditions. First, an extension base processed on the original anatomic master cast was tested with each occlusal rest position. Next the extension bases were relieved, and an altered-cast impression” was made in polyether impression material. * The extension bases were relined on the resulting altered cast using the rests and a removable anterior repositioning guide (Fig. 4) to properly relate the metal framework to the teeth during processing on a reline jig.t The anterior repositioning guide was removed and chewing tests repeated for each rest position. Finally, the extension bases were relieved to simulate support expected after moderate ridge resorption, and a final series of chewing tests was conducted. In addition to using a test food, one patient was requested to clench and then relax several times using each of the three occlusal rest positions and the bases processed on the one-piece cast. All testing was conducted during two appointments for each patient. *Impregum, tllowmedica,
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Premier Dental Inc., Chicago,
OF PROSTHETIC
Products Ill.
Co., Philadelphia,
DENTISTRY
powered
by a DC power
supply. Data were
Occlusal --RIGHT
SIDE
-<
Euccal
Fig. 6. Forces were recorded in three axes: X, Buccallingual. Y, Occlusal-gingival. Z, Posterior-anterior. The arrows indicate the direction the forces were applied to the abutment.
Data Samples of the data recorded on magnetic tape were replayed and digitized for evaluation by a minicomputer.* A description of the data evaluation methods will be the subject of another article. The resultant strain developed in each of the four active gauges on the splint was displayed as loads in three axes by comparing clinical strain gauge output with output recorded during the calibration testing with known loads. The three axes chosen were: X = buccal-lingual; Y = occlusal-gingival; and Z = anteri-
Pa. *PDP
1 l/40,
Digital
Equipment
Corp.,
Maynard,
Mass.
137
MAXFIELD,
Table I. Load values the distal rest 2.5
Kg.
Direction
I. 5 I .5
(
,
.25
.5
75
.I
I, 25
1.5
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z
NICHOLLS,
(kg) for patient
AND
SMITH
A using
Mean
SD
Maximum
0.673 0.365 2.088
0.134
0.848 0.470 2.529
0.481
0.115 0.525
0.106
Minimum
0.180 1.011
0.156 0.347 1.801
0.334 0.443 2.295
0.020
0.106 0.414
0.447 0.504 2.056
0.207 0.119 0.479
0.849 0.677 2.986
0.153 0.208 1.249
0.110 1.320
Sec. Fig. 7. A superimposition of Z-axis tracings from each patient demonstrates the different frequency and force denotes patient magnitude during mastication. A; -------- denotes patient B. or-posterior (Fig. 6) The arrows indicate the direction the forces were applied to the abutment as recorded for both patients. The peak loads for axes X, Y, and 2’ for each chewing cycle during mastication of the test food were selected from the load value output by noting the cyclic nature of the masticatory cycle. Fig. 7 is a superimposition of two tracings, one for each patient, representing loads applied along with 2 axis using the mesial rest and bases processed on the anatomic cast. The tracings demonstrate the different frequency of mastication and force magnitude during mastication by the two patients. A total of 10 to 15 peak loads was recorded for each chewing sequence, and the mean, standard deviation, and range were computed for each sequence. Analysis of variance was then used to determine if the difference between the means for selected variables was statistically significant. RESULTS Clinical testing was performed for two patients. Due to the variability in masticatory cycles and supporting tissues between patients, a direct comparison of their data was not made. Tables I to VI contain the mean, standard deviation, and range for each pitient. Some of the forces, as indicated in the tables, were variable in direction along a given axis during that chewing sequence, i.e., the applied force was sometimes in the opposite direction of the arrows depicted in Fig. 6. A lifting force was recorded for
138
See Fig. 6 for interpretation
Table II. Load values the mesial rest Direction
of
X2 Y, Z directions.
(kg) for patient
A using
Mean
SD
Maximum
Minimum
0.689 1.035 2.607
0.266 0.527 0.503
1.215 1.325 3.138
0.165 0.370 1.456
0.414 1.217 1.750
0.145 0.460 0.352
0.689 2.061 2.328
0.152 0.587 1.146
0.335 1.923 1.804
0.136 0.416 0.514
0.587 2.511 2.636
0.157 1.262 1.017
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z
patient B when using the bases processed on the anatomic cast for all rest locations and for the altered-cast bases used with the combined mesial and distal rest. Tables VII and VIII are the analysis of variance for different rest positions and for different base adaptations. Statistical significance is denoted below symbols for the rests. Mean loads are included for comparison. The dentures with bases processed on the altered cast produced the least applied force on the abutment tooth during mastication. Table IX presents the total force applied to the abutment tooth for each combination of variables. The total force was computed by applying an extension of the Pythagorean theorem to the means for each of the three forces. The result is a conversion of three components into
FEBXUARY
1979
VOLUME
41
NUMBER
2
FORCES TRANSMITTED
TO ABUTMENT
TEETH
Table III. Load values (kg) for patient A using
Table V. Load values (kg) for patient R using
the mesial and distal rest
the mesial rest
Direction
Anatomic cast X Y Z Altered cast X Y z Relieved bases X Y Z
Mean
SD
Maximum
Minimum
0.725 1.365 2.729
0.274 0.568 0.445
1.300 2.454 3.467
0.421 0.656 1.805
0.321 1.433 1.966
0.127 0.266 0.678
0.590 1.877 3.545
0.101 1.105 1.363
0.236 2.131 2.008
0.109 0.273 0.415
0.373 2.443 2.681
0.045 1.431 1.404
Direction
Anatomic cast X Y Z Altered cast X Y 2 Relieved bases X Y Z *Denotes tDenotes
Table IV. Load values (kg) for patient B using the distal rest Direction
Mean
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z *Denotes tDenotes
SD
Maximum
1.385 0.938 1.187
0.870 0.176 0.498
0.586 0.3111 1.316
0.279 0.098 0.264
0.977 0.459 1.699
0.259 0.151 0.961
1.444 0.433 1.286
0.209 0.164 0.273
1.752 0.667 1.755
1.139 0.223 0.903
given
axis.
one single force. A comparison of loads recorded for various rest positions indicates that, overall, smaller forces were applied to the abutment tooth when using the distal rest. Data recorded during mastication on the nontest side and during bilateral mastication demonstrated marked variability in magnitude and direction of force applied to the abutment tooth. As expected, lateral forces on the test pontic predominated during chewing on the nontest side and included forces directed mesially when the relieved (ill-fitting) bases were used. The testing device also detected lateral forces during swallowing and speech. Reliability for the system was demonstrated with a second chewing sequence for one patient using the
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DENTISTRY
Maximum
Minimum
1.910 1.123* 1.627
0.335 0.341 0.241
2364 1.734 2 I 1c
1.435 0.679 1.297
0.482 0.446t 0.787
0.186 0.293 0.197
0681 0.936 I\?49
0.205 0.104 0.405
0.790t 1.850 0.995
0.311 0.476 0.437
1297 2571 I.558
0.366 1.172 0.399
lifting force. variable direction
along
given
axis.
Table VI. Load values (kg) for patient B using the mesial and distal rest Direction
0.151 0.206 0.256
along
SD
Minimum
1.102 0.459* 0.959
lifting force. variable direction
Mean
Mean
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z *Denotes
lifting
SD
Maximum
Minimum
1.469 0.675* 1.491
0.414 0.273 0.319
2.700 1.093 2.383
1.143 0.408 1.173
0.574 1.057* 0.877
0.254 0.438 0.295
0.966 1.784 1.504
0.178 0.517 0.451
1.083 1.560 0.737
0.368 0.771 0.377
1.589 3.024 1.290
0.545 0.576 0.221
force.
same rest position altered cast. The definite similarity tion and relative
but a base processed on a second resulting data (Table X) showed a to the original data in load direcmagnitude.
DISCUSSION While this study involved only two subjects, and conclusions must be drawn with caution, there are certain trends common to both patients. Contrary to current thinking, the distal rest generally caused the least amount of load to be placed upon the testing abutment tooth. This was perhaps due to increased freedom of movement, or the so-called ball and socket effect. As the extension base was depressed, it rotated about the distal fulcrum point, applying
139
MAXFIELD,
Table VII. Mean
force on abutment
Patient
Anatomic
tooth
(kg), comparing
three Altered
cast
rest positions
NICHOLLS,
AND
SMITH
for three base adaptations
cast
Relieved
2.131 >
bases
1.923 >
0.504
Y "1" 1.966
1.801
1.750
NS
2.056
:'.
.ol 7
2.088 >
1.804
Z 7
N:
;y
NSi
B
0.586
0.574
X
0.482
>
1.444
NS "i' 1.123 ,
0.675 ,
0.459
0.446
1.057
1.093 >
0.790
1.560 >
0.433
Dcoyg
0.311
1.850 >
Y "i' Z
.Ol
1.627 > "i
NS
yfl
.05
1.491 > r
p 0.959
.ol
"i"
1.316 >
'i
[
Legend: M, Mesial rest. D, Distal rest. MD, Both mesial and distal Notations below symbols for the rests denote statistical significance
greater force against the distal guide plane (2 axis), as is reflected in the data. One must keep in mind that the patients were capable of generating forces against the abutment tooth in excess of 2 kg during normal mastication of the test food. When the patient uses a removable partial denture, the generated forces are distributed to the supporting structures, viz, the abutment teeth and the residual alveolar ridges. One important aspect of extension-base removable partial dentures is determining how much pressure each type of tissue is capable of supporting and how to distribute that pressure both at the time of placement and later, as eventual resorption of supporting bone occurs. The data reveal the importance of maintaining good adaptation of extension base to residual ridge. The predominant lateral forces recorded during mastication on the nontest side emphasize the desirability of achieving cross-arch stabilization, primarily with rigid major connectors. The use of additional minor connectors is another excellent, yet often overlooked, way to distribute lateral forces. This study is in agreement with that done by Christidou and associates,’ showing me&al move-
140
.,:
.Ol
z
'NS
0.877 > ;
NS
p
"i
0.787
1.286
"i
NS
",p
.Ol 1
0.995 >
0.737
D&EA
rest. NS, Not significant. 1, Y, Z, Direction of force. of the differences between forces for each rest location.
ment of the denture bases during loading. Apparently, this is due to the slope of the residual ridge and the downward movement of the extension base. The occlusal loading values, Y, for patient B were variable in direction with the altered-cast bases. This was interpreted as a periodic lifting force on the abutment tooth. An explanation for this phenomenon may be found by examining the relationship between the distal guiding plane and the slope of the residual ridge bearing the extension base. Patient A presented with a moderately resorbed residual ridge covered with relatively mobile soft tissue with 14 mm of ridge crest at right angles to the guiding plane, immediately adjacent to the guiding plane. The angle of the residual ridge then increased gradually up to the height of the retromolar pad. Patient B, on the other hand, presented with a very broad residual ridge, covered with dense relatively immovable soft tissue. Only a few millimeters of ridge crest were at right angles to the distal guiding plane. The angle of the slope then increased sharply, continuing to the retromolar pad. Presumably, the extension base for patient B moved down the crest during occlusal loading and, because of the angle of
FEBRUARY
1979
VOLUME
41
NUMBER
2
FORCES
TRANSMITTED
TO ABUTMENT
Table VIII. Mean
force
Patient
TEETH
on abutment Distal
tooth
(kg), comparing
rest
three Mesial
base adaptations
for three rest locations
rest
Me&l
and distal
test
A
X 0.504 Y
III
0.365 ’
I
0.347 ’
1.923 > III &kJ
1.217
1.801
2.607
1.804 >
'1
i
II
1.035
1.433 >
2.131 _:, III 1
1: ,c’
.OI
1.365 NS /
P-01N5(
2.088 >
2.056
1.760
2.729 ~,
2.008
1.966
2 1
NS
:Il
'NS
.z
111
NS
Ii
f
.ol
1:
I[
INS
B
1.469
1.093
0.574
1.057
0.675
X 0.459
0.433
0.311
Y
1.850 >
1.123
0.446
1.560
III
1
Nz
1.316
.I1
‘NS
1.286 >
I[ 0.959
1
.Ol
.;I
1.627 >
>.01
0.995 >
III
.a
I[
1
NS
0.787
1.491
i
/
,I
“.05
0.877
i 0.737
2 'i
G
1)
.Ol
/
i
.ol
11
NS
Legend: I, Anatomic cast. II, Altered cast. IZJ Relieved bases. NS, Not significant. X, Y, Z, Direction Notations below symbols for base adaptation denote statistical significance of the differences between
movement, tended to dislodge the removable partial denture occlusally and applied a lifting force to the test pontic. The degree of lifting was related to the adaptation of the extension base to the residual ridge. Because of the dense nature of the tissues covering the ridge seen in patient B, there was minimal tissue displacement, and the base adaptation for the anatomic cast was very similar to that attained with the altered cast. Regardless of the removable partial denture design used, patients need to be recalled to evaluate the support of the extension base. Data recorded during tests with relieved extension bases revealed that when support by the residual ridge was decreased there was less downward skid of the bases, and occlusal loading on the pontic increased. Greatly increased forces were expected but not observed. The patients reported some discomfort from the poor adaptation, experienced difficulty chewing the test food, and consequently used less force. Under these conditions, the distal rest caused less force to be applied to the abutment. This indicates that the rnesial rest may not be advisable under certain circumstances.
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DENTISTRY
Table IX. Total abutment Patient
.i
,I1
of force. forces for each basr adaptation
force (kg) applied
Distal
'NS'i
rest
Mesial
to test
rest
MeaWdistal
A Anatomic cast Altered cast Relieved bases
2.224 1.841 2.164
2.888 2.171 2.658
3.736 2.454 2.968
Anatomic cast Altered cast Relieved bases
1.531 1.474 1.982
2.750 1.025 2.244
2.199 1.489 2.042
B
Force
(F) calculated
by F = \/ (F,)’
t (F,)’
+ iF,!‘.
Table X. Data recorded
using two different bases and the same occlusal rest location for one patient, demonstrating reliability Initial Direction
Mean
X
0.586
Y Z
0.311 1.316
testing SD
0.279 0.098 0.264
Repeat
Mean
0.869 0.374 1.894
testing SD
0.306 0.074 0.299
141
MAXFIELD,
The results of the clenching tests revealed forces of less magnitude than found during chewing tests. Perhaps the patient did not apply as much force, or perhaps more force was directed to the residual ridge. Once again, the forces recorded in each axis using the distal rest were less than for either the mesial or combined mesial and distal rest. The variation in forces applied to the test device by a patient and between patients is readily apparent. Further study with a larger patient sample is necessary before conclusions may be drawn regarding the magnitude of force that might be expected from a specific situation. However, the data do allow comparison of the variables of impression procedures and occlusal rest placement. Lowest forces were shown with the bases processed on the altered cast. Increased force values were about the same for the anatomic cast and the relieved bases. This indicates that removable partial dentures constructed on an anatomic cast are potentially as detrimental as removable partial dentures which have lost support due to resorption.
REFERENCES 1.
2. 3.
4.
5.
AND CONCLUSIONS
Despite the limited number of patients in the study, the following conclusions may be made regarding the magnitude and direction of forces transmitted to abutment teeth of extension-base removable partial dentures. 1. A technique was presented for using strain gauges to determine the magnitude and direction of forces transmitted to abutment teeth of removable partial dentures. 2. There are variations in force magnitude with a patient and between patients from chewing cycle to chewing cycle. 3. The transmitted forces vary when different removable partial denture designs are used. 4. Improving adaptation of the extension bases to the residual ridge is an excellent means for providing
142
AND SMITH
maximum support, increasing patient comfort, and decreasing forces to abutment teeth. 5. Extension bases apply mesially directed forces to abutment teeth during mastication. 6. Further research is needed to elucidate the effects of the various philosophies of removable partial denture design.
6.
SUMMARY
NICHOLLS,
7.
8.
9. 10.
Frechette, A. R.: The influence of partial denture design on distribution of force to abutment teeth. J PROSTHET DENT 6:195, 1956. Metty, A. C.: Obtaining efficient soft tissue support for the partial denture base. J Am Dent Assoc 56:679, 1958. Dutton, D. A., Kydd, W. L., and Smith, D. E.: Lateral forces exerted on abutment teeth by partial dentures. J Am Dent Assoc 68:859, 1964. Lowe, R. D., Kydd, W. L., and Smith, D. E.: Swallowing and resting forces related to lingual flange thickness in removable partial dentures. J PROSTHET DENT 23:279, 1970. Holmes, J. B.: Influence of impression procedures and occlusal loading on partial denture movement. J PROSTHET DENT 15:474, 1965. Leupold, R. J.: A comparative study of impression procedures for distal extension removable partial dentures. J PROSTHET DENT 16:708, 1966. Kratochvil, F. J., and Caputo, A. A.: Photoelastic analysis of pressure on teeth and bone supporting removable partia1 dentures. J PROSTHET DENT 32:52, 1974. Christidou, L., Osborne, J., Chamberlain, J.: The effects of partial denture design on the mobility of abutment teeth. Br Dent J 135:9, 1973. Steffel, V. L.: Fundamental principles involved in partial denture design. J Am Dent Assoc 42:534, 1951. Kratochvil, F. J.: Influence of occlusal rest position and clasp design on movement of abutment teeth. J PROSTHET DENT 13:114, 1963.
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245 W. HOUSTON ST. NEW YORK, N. Y. 10014
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41
NUMBER
2
D.D.S., M.S.D.,*
Jack I. Nicholls,
Ph.D.,**
and Dale E. Smith, D.D.S., MS.D.***
U.S. Public Health Service Outpatient Clinic, New York, N. Y., and University of Washington, School of Dentistry, Seattle, ‘Wash.
T
he distribution of forces applied to the abutment teeth and residual ridges by distal-extension removable partial dentures remains largely undefined. A consideration of the supporting capacity of the abutment tooth and the mucosa of the residual ridge reveals the disparity in the capacity of each to withstand applied loads. As a result, designing a physiologically acceptable removable partial denture requires an understanding of the forces generated during mastication and their distribution to supporting structures. At present, the information which is necessary to determine the most acceptable removable partial denture design is unavailable. Numerous studies have been conducted to test distal-extension partial dentures, but unfortunately very few have been performed in vivo. Laboratory studies, while more easily controlled than clinical studies, may not yield accurate results due to difficulties in reproducing the physiology of the oral masticatory system. The conflicting and nonconclusive results of several laboratory studies indicate that they may be an unreliable means for testing removable partial denture designs.
LITERATURE
REVIEW
Frechette’ recognized the need for scientific testing to either prove or disprove ‘the existing theories which were based upon logical reasoning or clinical Presented and placed second in the 1977 American College of Prosthodontists Research Award Competition, New Orleans, La.; also presented at the American Prosthodontic Society, Miami Beach, Fla. *Chief, Oral Health Clinic, U.S. Public Health Service Outpatient Clinic, New York, N. Y. **Associate Professor, Department of Restorative Dentistry, University of Washington. ***Associate Professor, Department of Prosthodontics, University of Washington.
134
experience. His laboratory study showed that the loading and movement of the teeth was strongly influenced by such factors as the number and location of rests, contour and rigidity of connectors, and extension of denture bases. Metty? was one of the first investigators to utilize strain gauges in the study of removable partial dentures. His results support the concept that functionally based removable partial dentures are more stable than other types. Dutton and associates” used strain gauges to study lateral forces on five patients. The mean lateral forces to the abutment teeth during random chewing ranged from 5 to 148.7 gm. Unfortunately, the testing device used was only capable of detecting forces in the buccal-lingual direction. Lowe and associates” measured lateral forces generated by the tongue against a removable partial denture base. Mean forces of 28 to 162 gm were recorded during swallowing. Holmes5 and LeupoldG have presented evidence that the altered-cast impression technique provides the least movement of extension bases under an occlusal load when compared to bases processed on an anatomic cast. Kratochvil and Caputo’ performed photoelastic analysis of forces transmitted by distal-extension removable partial dentures to abutment teeth and to residual ridges. They found that lateral forces on the removable partial denture were distributed to all teeth contacted by the removable partial denture. They also felt that the ball and socket design of occlusal rests allowed for movement of the prosthesis, with occlusal forces applied in the region of the rest and transmitted along the long axis of the abutment tooth. Christidou and associates8 developed a device for clinically measuring the load applied to dentures
FEBRUARY 1979
VOLUME41
NUMBER
2
FORCES TRANSMITTED
TO ABUTMENT
TEETH
Fig. 1. A castsilver pontic that telescopesover the beam from the distal surfaceof the splint. and a method for measuring tooth mobility in a mesial-distal direction. They found a general tendency for abutment teeth to move mesially. They felt that the anatomic inclination of the residual ridge was largely responsible for this effect. Steffel” reported three philosophies regarding the control of stressesfrom distal-extension removable partial dentures: (1) physiologic basing, (2) stressbreakers, and (3) extensive stressdistribution. In all the articles he reviewed, substantiating evidence for any of the theories was lacking. PURPOSE The purpose of this study was to develop a means for clinically measuring the forces applied to abutment teeth by removable partial dentures. Future application of such a method could have farreaching benefits. It could provide a means for critically analyzing previous studies. If clinical testing can show laboratory results to be valid, then further research can be performed at that level with more easily controlled variables. It could provide better information for removable partial denture designs. One of our goals must be to preserve supporting structures; however, the removable partial denture design which does this best has not been conclusively demonstrated. Accurate measurement of forces applied to the structures by various designs is essential to the determination of which design is most acceptable. Finally, the results of such research should lead to a way to achieve maximum support for distal-extension removable partial dentures which is within the physiologic limits of the supporting structures. METHODS
maxillary complete dentures and mandibular bilateral distal-extension removable partial dentures. Both patients were women of ages46 and 51 years, and each was an experienced removable partial denture wearer. A 2.5 mm square beam projected 7 mm from the distal surface of the splint on the right side. A cast silver pontic was made to telescopeover the beam (Fig. l), and holes were drilled for attaching the pontic to the beam with pins. To each of four surfaces of the beam, strain gauges* were attached with cyanoacrylate adhesive,? covered with cellophane tape (Fig. 2), and sealed to render them waterpro0f.x The previously cast hollow silver pontic was then telescoped over the beam and rigidly attached with 0.5 mm stainlesssteel pins which were driven through holes drilled in the pontic and beam without touching the gauges. Four other strain gaugesfor temperature compensation were attached to two 4 X 5 X 0.75 mm silver plates, one gauge pel surface. The plates, cast with small retentive loops at one edge, were then attached to the splint with autopolymerizing resin. A series of 34-gauge insulated wires were soldered to the strain gauge leads and placed in a channel in the splint to exit the mouth and terminate in a small l&pin electrical plug (Fig. 3). With the silver splint seated on an artificial stone cast, guiding planes were milled on the distal surface of the pontic and the distal surface of the splint on the opposite side of the arch. Mesial and distal rests were prepared in the pontic on the test side, and a cingulum rest was prepared in the lingual surface of the splint on the contralateral side.
AND MATERIALS
Cast silver splints were fabricated to fit the remaining mandibular teeth of two patients wearing
THE JOURNAL
Fig. 2. Strain gaugesare attached to the beam with cyanoacrylate adhesive and covered with cellophane tape.
OF PROSTHETIC
DENTISTRY
‘EA-13-062AP-120, Micro-Measurements. Romulus, Mich. TM-Bond 200 Adhesive, Micro-Measurements, Romulus, Mich. $M-Coat GL, Micro-Measurements, Romulus, \
135
MAXFIELD,
NICHOLLS,
Fig. 4. Interchangeable rest assemblies attached with autopolymerizing resin. An irreversible hydrocolloid impression was made of the mandibular arch with the splint in ‘place. A chrome-cobalt framework was made on the resulting master cast. Provision was made for attaching interchangeable occlusal rest assemblies with autopolymerizing resin to facilitate changing rest position on the same metal framework (Fig. 4). Primary retention of the prosthesis on the test side was attained with an I-bar placed in a O.OOl-inch undercut in the center of the middle third of the buccal surface of the test pontic. Retention on the nontest side was attained with a circumferential wrought wire clasp for one patient and an I-bar clasp for the other. Anatomic posterior teeth were set in balanced occlusion against a duplicate of the patient’s existing complete maxillary denture, and the removable partial denture was processed on the anatomic master cast. The tissue surfaces of the processed extension bases were fitted to the patient’s residual ridges using a pressure-indicating paste to evaluate adaptation. Chloroform and gold rouge were used to adjust the contact of the partial denture framework with the distal guiding planes as described by Kratochvil.lo There was contact between the lingual plate and the lingual aspect of the pontic only at the height of contour to reciprocate the direct retainer when the distal rest was used alone. The contact was adjusted to permit rotation about the fulcrum when the bases were depressed or lifted. When the mesial occlusal rest was used, the minor connector contacted the mesiolingual part of the pontic. Occlusion was adjusted after remounting the finished removable partial denture on a semiadjustable articulator, and the denture teeth opposing the pontic on the testing device were reduced to preclude any direct loading during clinical testing.
136
Calibration
AND
that
SMITH
be
testing
The 16-pin electrical plug leading from the silver splint was connected to an amplifier by 5 feet of 18-wire shielded cable. A DC power supply was attached to the system. Strain gauge output, as indicated by a voltage change from the zero load value, was recorded on a four-channel tape recorder for storage (Fig. 5). With the splint seated on a stone cast bolted to a plastic platform, a series of known loads were applied to the pontic from the occlusal, buccal, and distal directions. The point of the loading pin was placed consecutively into the mesia1 rest, distal rest, and combined mesial and distal rest, and against the buccal surface, lingual surface, and distal guiding plane region of the pontic. For each position, recordings of strain gauge output were made for loads of 0, 100, 200, 500, 1100, 1300, and 1500 gm which represented the range of most clinical load values. Loads of up to 3.5 kg were applied to the testing device to prove the linearity of the testing system.
Clinical
testing
With the patient seated in a dental chair, the testing device was placed on the mandibular teeth and followed by the removable partial denture. The necessary electrical connections were made, and the patient was instructed to rest for 10 minutes to permit temperature stabilization of all components. After 10 minutes, a recording was made with the patient resting comfortably with teeth apart to represent a zero load level. The patient was then instructed to chew dry-roasted peanuts, first on the left side, then the right side, and, finally, with peanuts placed between teeth on both sides. The
FEBRUARY
1979
VOLUME
41
NUMBER
2
FORCES
TRANSMITTED
TO ABUTMENT
TEETH
Fig. 5. The splint is connected to an amplifier recorded on a four-channel tape recorder. patient was given the opportunity to take as many peanuts as she felt capable of chewing at one time. The number varied from three to five peanuts per sequence. This chewing sequence was repeated for each of three different occlusal rest locations: mesial rest, distal rest, and combined mesial and distal rest. In addition to varying the location of the occlusal rest, the effect of extension-base adaptation to the residual ridge was examined for three conditions. First, an extension base processed on the original anatomic master cast was tested with each occlusal rest position. Next the extension bases were relieved, and an altered-cast impression” was made in polyether impression material. * The extension bases were relined on the resulting altered cast using the rests and a removable anterior repositioning guide (Fig. 4) to properly relate the metal framework to the teeth during processing on a reline jig.t The anterior repositioning guide was removed and chewing tests repeated for each rest position. Finally, the extension bases were relieved to simulate support expected after moderate ridge resorption, and a final series of chewing tests was conducted. In addition to using a test food, one patient was requested to clench and then relax several times using each of the three occlusal rest positions and the bases processed on the one-piece cast. All testing was conducted during two appointments for each patient. *Impregum, tllowmedica,
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Premier Dental Inc., Chicago,
OF PROSTHETIC
Products Ill.
Co., Philadelphia,
DENTISTRY
powered
by a DC power
supply. Data were
Occlusal --RIGHT
SIDE
-<
Euccal
Fig. 6. Forces were recorded in three axes: X, Buccallingual. Y, Occlusal-gingival. Z, Posterior-anterior. The arrows indicate the direction the forces were applied to the abutment.
Data Samples of the data recorded on magnetic tape were replayed and digitized for evaluation by a minicomputer.* A description of the data evaluation methods will be the subject of another article. The resultant strain developed in each of the four active gauges on the splint was displayed as loads in three axes by comparing clinical strain gauge output with output recorded during the calibration testing with known loads. The three axes chosen were: X = buccal-lingual; Y = occlusal-gingival; and Z = anteri-
Pa. *PDP
1 l/40,
Digital
Equipment
Corp.,
Maynard,
Mass.
137
MAXFIELD,
Table I. Load values the distal rest 2.5
Kg.
Direction
I. 5 I .5
(
,
.25
.5
75
.I
I, 25
1.5
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z
NICHOLLS,
(kg) for patient
AND
SMITH
A using
Mean
SD
Maximum
0.673 0.365 2.088
0.134
0.848 0.470 2.529
0.481
0.115 0.525
0.106
Minimum
0.180 1.011
0.156 0.347 1.801
0.334 0.443 2.295
0.020
0.106 0.414
0.447 0.504 2.056
0.207 0.119 0.479
0.849 0.677 2.986
0.153 0.208 1.249
0.110 1.320
Sec. Fig. 7. A superimposition of Z-axis tracings from each patient demonstrates the different frequency and force denotes patient magnitude during mastication. A; -------- denotes patient B. or-posterior (Fig. 6) The arrows indicate the direction the forces were applied to the abutment as recorded for both patients. The peak loads for axes X, Y, and 2’ for each chewing cycle during mastication of the test food were selected from the load value output by noting the cyclic nature of the masticatory cycle. Fig. 7 is a superimposition of two tracings, one for each patient, representing loads applied along with 2 axis using the mesial rest and bases processed on the anatomic cast. The tracings demonstrate the different frequency of mastication and force magnitude during mastication by the two patients. A total of 10 to 15 peak loads was recorded for each chewing sequence, and the mean, standard deviation, and range were computed for each sequence. Analysis of variance was then used to determine if the difference between the means for selected variables was statistically significant. RESULTS Clinical testing was performed for two patients. Due to the variability in masticatory cycles and supporting tissues between patients, a direct comparison of their data was not made. Tables I to VI contain the mean, standard deviation, and range for each pitient. Some of the forces, as indicated in the tables, were variable in direction along a given axis during that chewing sequence, i.e., the applied force was sometimes in the opposite direction of the arrows depicted in Fig. 6. A lifting force was recorded for
138
See Fig. 6 for interpretation
Table II. Load values the mesial rest Direction
of
X2 Y, Z directions.
(kg) for patient
A using
Mean
SD
Maximum
Minimum
0.689 1.035 2.607
0.266 0.527 0.503
1.215 1.325 3.138
0.165 0.370 1.456
0.414 1.217 1.750
0.145 0.460 0.352
0.689 2.061 2.328
0.152 0.587 1.146
0.335 1.923 1.804
0.136 0.416 0.514
0.587 2.511 2.636
0.157 1.262 1.017
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z
patient B when using the bases processed on the anatomic cast for all rest locations and for the altered-cast bases used with the combined mesial and distal rest. Tables VII and VIII are the analysis of variance for different rest positions and for different base adaptations. Statistical significance is denoted below symbols for the rests. Mean loads are included for comparison. The dentures with bases processed on the altered cast produced the least applied force on the abutment tooth during mastication. Table IX presents the total force applied to the abutment tooth for each combination of variables. The total force was computed by applying an extension of the Pythagorean theorem to the means for each of the three forces. The result is a conversion of three components into
FEBXUARY
1979
VOLUME
41
NUMBER
2
FORCES TRANSMITTED
TO ABUTMENT
TEETH
Table III. Load values (kg) for patient A using
Table V. Load values (kg) for patient R using
the mesial and distal rest
the mesial rest
Direction
Anatomic cast X Y Z Altered cast X Y z Relieved bases X Y Z
Mean
SD
Maximum
Minimum
0.725 1.365 2.729
0.274 0.568 0.445
1.300 2.454 3.467
0.421 0.656 1.805
0.321 1.433 1.966
0.127 0.266 0.678
0.590 1.877 3.545
0.101 1.105 1.363
0.236 2.131 2.008
0.109 0.273 0.415
0.373 2.443 2.681
0.045 1.431 1.404
Direction
Anatomic cast X Y Z Altered cast X Y 2 Relieved bases X Y Z *Denotes tDenotes
Table IV. Load values (kg) for patient B using the distal rest Direction
Mean
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z *Denotes tDenotes
SD
Maximum
1.385 0.938 1.187
0.870 0.176 0.498
0.586 0.3111 1.316
0.279 0.098 0.264
0.977 0.459 1.699
0.259 0.151 0.961
1.444 0.433 1.286
0.209 0.164 0.273
1.752 0.667 1.755
1.139 0.223 0.903
given
axis.
one single force. A comparison of loads recorded for various rest positions indicates that, overall, smaller forces were applied to the abutment tooth when using the distal rest. Data recorded during mastication on the nontest side and during bilateral mastication demonstrated marked variability in magnitude and direction of force applied to the abutment tooth. As expected, lateral forces on the test pontic predominated during chewing on the nontest side and included forces directed mesially when the relieved (ill-fitting) bases were used. The testing device also detected lateral forces during swallowing and speech. Reliability for the system was demonstrated with a second chewing sequence for one patient using the
THE JOURhiAL
OF PROSTHETIC
DENTISTRY
Maximum
Minimum
1.910 1.123* 1.627
0.335 0.341 0.241
2364 1.734 2 I 1c
1.435 0.679 1.297
0.482 0.446t 0.787
0.186 0.293 0.197
0681 0.936 I\?49
0.205 0.104 0.405
0.790t 1.850 0.995
0.311 0.476 0.437
1297 2571 I.558
0.366 1.172 0.399
lifting force. variable direction
along
given
axis.
Table VI. Load values (kg) for patient B using the mesial and distal rest Direction
0.151 0.206 0.256
along
SD
Minimum
1.102 0.459* 0.959
lifting force. variable direction
Mean
Mean
Anatomic cast X Y Z Altered cast X Y Z Relieved bases X Y Z *Denotes
lifting
SD
Maximum
Minimum
1.469 0.675* 1.491
0.414 0.273 0.319
2.700 1.093 2.383
1.143 0.408 1.173
0.574 1.057* 0.877
0.254 0.438 0.295
0.966 1.784 1.504
0.178 0.517 0.451
1.083 1.560 0.737
0.368 0.771 0.377
1.589 3.024 1.290
0.545 0.576 0.221
force.
same rest position altered cast. The definite similarity tion and relative
but a base processed on a second resulting data (Table X) showed a to the original data in load direcmagnitude.
DISCUSSION While this study involved only two subjects, and conclusions must be drawn with caution, there are certain trends common to both patients. Contrary to current thinking, the distal rest generally caused the least amount of load to be placed upon the testing abutment tooth. This was perhaps due to increased freedom of movement, or the so-called ball and socket effect. As the extension base was depressed, it rotated about the distal fulcrum point, applying
139
MAXFIELD,
Table VII. Mean
force on abutment
Patient
Anatomic
tooth
(kg), comparing
three Altered
cast
rest positions
NICHOLLS,
AND
SMITH
for three base adaptations
cast
Relieved
2.131 >
bases
1.923 >
0.504
Y "1" 1.966
1.801
1.750
NS
2.056
:'.
.ol 7
2.088 >
1.804
Z 7
N:
;y
NSi
B
0.586
0.574
X
0.482
>
1.444
NS "i' 1.123 ,
0.675 ,
0.459
0.446
1.057
1.093 >
0.790
1.560 >
0.433
Dcoyg
0.311
1.850 >
Y "i' Z
.Ol
1.627 > "i
NS
yfl
.05
1.491 > r
p 0.959
.ol
"i"
1.316 >
'i
[
Legend: M, Mesial rest. D, Distal rest. MD, Both mesial and distal Notations below symbols for the rests denote statistical significance
greater force against the distal guide plane (2 axis), as is reflected in the data. One must keep in mind that the patients were capable of generating forces against the abutment tooth in excess of 2 kg during normal mastication of the test food. When the patient uses a removable partial denture, the generated forces are distributed to the supporting structures, viz, the abutment teeth and the residual alveolar ridges. One important aspect of extension-base removable partial dentures is determining how much pressure each type of tissue is capable of supporting and how to distribute that pressure both at the time of placement and later, as eventual resorption of supporting bone occurs. The data reveal the importance of maintaining good adaptation of extension base to residual ridge. The predominant lateral forces recorded during mastication on the nontest side emphasize the desirability of achieving cross-arch stabilization, primarily with rigid major connectors. The use of additional minor connectors is another excellent, yet often overlooked, way to distribute lateral forces. This study is in agreement with that done by Christidou and associates,’ showing me&al move-
140
.,:
.Ol
z
'NS
0.877 > ;
NS
p
"i
0.787
1.286
"i
NS
",p
.Ol 1
0.995 >
0.737
D&EA
rest. NS, Not significant. 1, Y, Z, Direction of force. of the differences between forces for each rest location.
ment of the denture bases during loading. Apparently, this is due to the slope of the residual ridge and the downward movement of the extension base. The occlusal loading values, Y, for patient B were variable in direction with the altered-cast bases. This was interpreted as a periodic lifting force on the abutment tooth. An explanation for this phenomenon may be found by examining the relationship between the distal guiding plane and the slope of the residual ridge bearing the extension base. Patient A presented with a moderately resorbed residual ridge covered with relatively mobile soft tissue with 14 mm of ridge crest at right angles to the guiding plane, immediately adjacent to the guiding plane. The angle of the residual ridge then increased gradually up to the height of the retromolar pad. Patient B, on the other hand, presented with a very broad residual ridge, covered with dense relatively immovable soft tissue. Only a few millimeters of ridge crest were at right angles to the distal guiding plane. The angle of the slope then increased sharply, continuing to the retromolar pad. Presumably, the extension base for patient B moved down the crest during occlusal loading and, because of the angle of
FEBRUARY
1979
VOLUME
41
NUMBER
2
FORCES
TRANSMITTED
TO ABUTMENT
Table VIII. Mean
force
Patient
TEETH
on abutment Distal
tooth
(kg), comparing
rest
three Mesial
base adaptations
for three rest locations
rest
Me&l
and distal
test
A
X 0.504 Y
III
0.365 ’
I
0.347 ’
1.923 > III &kJ
1.217
1.801
2.607
1.804 >
'1
i
II
1.035
1.433 >
2.131 _:, III 1
1: ,c’
.OI
1.365 NS /
P-01N5(
2.088 >
2.056
1.760
2.729 ~,
2.008
1.966
2 1
NS
:Il
'NS
.z
111
NS
Ii
f
.ol
1:
I[
INS
B
1.469
1.093
0.574
1.057
0.675
X 0.459
0.433
0.311
Y
1.850 >
1.123
0.446
1.560
III
1
Nz
1.316
.I1
‘NS
1.286 >
I[ 0.959
1
.Ol
.;I
1.627 >
>.01
0.995 >
III
.a
I[
1
NS
0.787
1.491
i
/
,I
“.05
0.877
i 0.737
2 'i
G
1)
.Ol
/
i
.ol
11
NS
Legend: I, Anatomic cast. II, Altered cast. IZJ Relieved bases. NS, Not significant. X, Y, Z, Direction Notations below symbols for base adaptation denote statistical significance of the differences between
movement, tended to dislodge the removable partial denture occlusally and applied a lifting force to the test pontic. The degree of lifting was related to the adaptation of the extension base to the residual ridge. Because of the dense nature of the tissues covering the ridge seen in patient B, there was minimal tissue displacement, and the base adaptation for the anatomic cast was very similar to that attained with the altered cast. Regardless of the removable partial denture design used, patients need to be recalled to evaluate the support of the extension base. Data recorded during tests with relieved extension bases revealed that when support by the residual ridge was decreased there was less downward skid of the bases, and occlusal loading on the pontic increased. Greatly increased forces were expected but not observed. The patients reported some discomfort from the poor adaptation, experienced difficulty chewing the test food, and consequently used less force. Under these conditions, the distal rest caused less force to be applied to the abutment. This indicates that the rnesial rest may not be advisable under certain circumstances.
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OF PROSTHETIC
DENTISTRY
Table IX. Total abutment Patient
.i
,I1
of force. forces for each basr adaptation
force (kg) applied
Distal
'NS'i
rest
Mesial
to test
rest
MeaWdistal
A Anatomic cast Altered cast Relieved bases
2.224 1.841 2.164
2.888 2.171 2.658
3.736 2.454 2.968
Anatomic cast Altered cast Relieved bases
1.531 1.474 1.982
2.750 1.025 2.244
2.199 1.489 2.042
B
Force
(F) calculated
by F = \/ (F,)’
t (F,)’
+ iF,!‘.
Table X. Data recorded
using two different bases and the same occlusal rest location for one patient, demonstrating reliability Initial Direction
Mean
X
0.586
Y Z
0.311 1.316
testing SD
0.279 0.098 0.264
Repeat
Mean
0.869 0.374 1.894
testing SD
0.306 0.074 0.299
141
MAXFIELD,
The results of the clenching tests revealed forces of less magnitude than found during chewing tests. Perhaps the patient did not apply as much force, or perhaps more force was directed to the residual ridge. Once again, the forces recorded in each axis using the distal rest were less than for either the mesial or combined mesial and distal rest. The variation in forces applied to the test device by a patient and between patients is readily apparent. Further study with a larger patient sample is necessary before conclusions may be drawn regarding the magnitude of force that might be expected from a specific situation. However, the data do allow comparison of the variables of impression procedures and occlusal rest placement. Lowest forces were shown with the bases processed on the altered cast. Increased force values were about the same for the anatomic cast and the relieved bases. This indicates that removable partial dentures constructed on an anatomic cast are potentially as detrimental as removable partial dentures which have lost support due to resorption.
REFERENCES 1.
2. 3.
4.
5.
AND CONCLUSIONS
Despite the limited number of patients in the study, the following conclusions may be made regarding the magnitude and direction of forces transmitted to abutment teeth of extension-base removable partial dentures. 1. A technique was presented for using strain gauges to determine the magnitude and direction of forces transmitted to abutment teeth of removable partial dentures. 2. There are variations in force magnitude with a patient and between patients from chewing cycle to chewing cycle. 3. The transmitted forces vary when different removable partial denture designs are used. 4. Improving adaptation of the extension bases to the residual ridge is an excellent means for providing
142
AND SMITH
maximum support, increasing patient comfort, and decreasing forces to abutment teeth. 5. Extension bases apply mesially directed forces to abutment teeth during mastication. 6. Further research is needed to elucidate the effects of the various philosophies of removable partial denture design.
6.
SUMMARY
NICHOLLS,
7.
8.
9. 10.
Frechette, A. R.: The influence of partial denture design on distribution of force to abutment teeth. J PROSTHET DENT 6:195, 1956. Metty, A. C.: Obtaining efficient soft tissue support for the partial denture base. J Am Dent Assoc 56:679, 1958. Dutton, D. A., Kydd, W. L., and Smith, D. E.: Lateral forces exerted on abutment teeth by partial dentures. J Am Dent Assoc 68:859, 1964. Lowe, R. D., Kydd, W. L., and Smith, D. E.: Swallowing and resting forces related to lingual flange thickness in removable partial dentures. J PROSTHET DENT 23:279, 1970. Holmes, J. B.: Influence of impression procedures and occlusal loading on partial denture movement. J PROSTHET DENT 15:474, 1965. Leupold, R. J.: A comparative study of impression procedures for distal extension removable partial dentures. J PROSTHET DENT 16:708, 1966. Kratochvil, F. J., and Caputo, A. A.: Photoelastic analysis of pressure on teeth and bone supporting removable partia1 dentures. J PROSTHET DENT 32:52, 1974. Christidou, L., Osborne, J., Chamberlain, J.: The effects of partial denture design on the mobility of abutment teeth. Br Dent J 135:9, 1973. Steffel, V. L.: Fundamental principles involved in partial denture design. J Am Dent Assoc 42:534, 1951. Kratochvil, F. J.: Influence of occlusal rest position and clasp design on movement of abutment teeth. J PROSTHET DENT 13:114, 1963.
Reprint requests to: DR. JAY MAXFIELD U.S. PUBLIC HEALTH OUTPATIENT
SERVICE
CLINIC
245 W. HOUSTON ST. NEW YORK, N. Y. 10014
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