A method of measuring pressures against tissues supporting functioning complete dentures

A

method

supporting

of measuring functioning

pressures complete

Charles C. Kelsey, D.D.S., MS.,* Fredrick Joel A. Coplowitz, B.E.E., M.S.** University of Michigan, School of Dentistry,

against

tissues

dentures D. Reid, Ann Arbor,

B.S., MS.,**

and

Mich.

F

orces generated during mastication have interested dentists since the time of Borelli’s investigations in 1681.l In 1893, Black* began his classicinvestigations using a gnathodynamometer to study maximum biting force and a phagodynamometer to study the forces required in the mastication of food. Practical usesfor improved gnathodynamometers were advocated throughout the first half of the twentieth century. By 1950, the development and availability of miniaturized variable inductive strain gagesallowed researchersto study masticatory forces and “chewing efficiency” of artificial teeth with different occlusal configurations under more normal conditions.3-6These investigators, however, studied the forces generated within the dental restoration itself. For those interested in learning the effects of extrinsic biomechanical forces (i.e., mastication with complete dentures) on the supporting tissues,it seemed that it would be more meaningful to measurethe actual forces (or pressures)exerted during function by complete dentures on the underlying tissue. It was for this purpose that the method to be described was developed. LITERATURE

REVIEW

Probably because of the difficulty of measuring pressuresat the tissue-denture base interface, while interfering minimally with the functioning of the dentures, only four major studies on this subject have been reported.‘-lo Another author approached the problem on a purely theoretical basis.ll This Michigan,

investigation was supported by the Dental Research Institute at the University of School of Dentistry, under United States Public Health Service Grant No. DE 02731, and by United States Public Health Service Research Grant No. 5501 RR 05321 from the General Research Service Branch, Division of Research Facilities and Resources. *Professor **Bioengineer.

376

of Dentistry,

Complete

Denture

Department.

Volume 35 Number

Measuring

4

pressures

in tissues under

377

dentures

EFUW-COATED 0.127

BR-CU

DISK

0 102 DIAPHRAGM

Fig. 1. A

diagram

of a pressure

transducer

indicates

placement

of the strain

gage

and

lead-in

wires.

Frechette’ and Stromberg” reported their investigations on the forces (or pressures) generated against the supporting tissues by denture teeth of varying occlusal configurations during mastication with complete dentures. Kaires,” lo using t:he method developed by Frechette,’ studied the effects of design and size of the occlusal table on the masticatory pressures generated against the residual ridges supporting a mandibular removable partial denture. Stromberg6 conducted a comparative study using nonanatomic and anatomic teeth on duplicate dentures. He used a strain gage-instrumented gold spring attached to a movable acrylic resin window in the lateral aspect of the maxillary denture base to observe pressure changes during mastication. Frechette7 compared several tooth forms using interchangeable dentitions on instrumented cast-metal mandibular

and

maxillary

denture

bases,

l’he

metal

bases

were

made

with

carefully

located islands which acted as cantilever beams. Strain gages were located at I:he junctions of the islands and the metal base for the purpose of giving an electronic signal as the islands were loaded. Lawsonl’ questioned the validity of both methods by demonstrating, with his controlled laboratory tests, that the numerical results of Stromberg’s and Frechett:e’s investigations were inaccurate. He showed that any cantilever arrangement will allow the test point to move away from the supporting tissue and will leave the surrounding base to carry the load. He also demonstrated that a more rigid test point should give more accurate results. To provide a more rigid test point, a diaphragm pressure transducer was developed for use as a pressure recording device, replacing the cantilever-beam arrangement used by Stromberg and Frechette. CONSTRUCTION

OF DIAPHRAGM

PRESSURE

TRANSDUCER

A specially designed diaphragm pressure transducer was constructed for this project. Half-hard beryllium-copper rod was machined in such a manner as to form a ruplike case with a diaphragm 0.102 mm. thick on one end (Fig. 1). A small strain gage* was attached to the center ot the interior surface of the diaphragm. After attaching the wires to the gage, the top of the case was sealed with a thin metal (Br-Cu) d’ISk an d e p ox y cement.? This formed an enc!osed air chamber which remained approximately at atmospheric pressure.

*BAE-09-015CC-120,William +GA-60

epoxy

cement

T. Beam,

kit, Automation

Inc.,

Detroit,

Industries,

Mich.

Phoenixville,

Pa.

378

J. Prosthet. Dent. April, 1976

Kelsey, Reid, and Coplowita

Fig. 2. Test specimens used to verify the calibration method. (Left) A circular specimen with interchangeable silicone rubber spacers of 2 and 5 mm. (Right) A right-angle specimen with a 2 mm. thick silicone rubber spacer.

VALIDATION

OF CALIBRATION METHOD

Tests were made to determine the linearity of the pressuretransducer. Static loading. Lawson’s method of calibrating the transducer by loading it against a tissue-like material was used. Flat and angled test units with confined silicone rubber spacers* were constructed (Fig. 2). The spacers were considered to act beneath the transducer in the manner of musticatory muc0sa.t Spacer thicknessesof 2 mm. and 5 mm. were used to determine the effects of varying tissue thickness. To simulate placement in a gold denture base, transducers were mounted in the gold test plates with the diaphragms flush with the surface of the plate and in direct contact with the spacer. After the test specimenswere wetted with saliva and loaded with known weights, the pressureson the transducers were computed by dividing the applied weight by the area of the gold plate. Care was taken to be certain that the pressuredistribution across the platform

was uniform.

Using the angle specimen, force was applied at an angle of 45 degreesaway from the direction that is normal to the diaphragm. If the transducer is insensitive to the tangential component of applied pressureand respondsonly to normal pressure,the output signal should be 0.707 times its value with the flat specimen for a given applied load. Hydrostatic loading. The transducer was calibrated in an oil-filled pressure chamber where pressurewas applied at increments of 0.6 million dynes/cm.2 up to 6 million dynes/cm.2 The transducers were connected to a direct-writing oscillograph$ in a quarterbridge configuration. both

For each of the two circular test specimens, the output static pressure and hydrostatic pressure were nearly *Silastic

A RTV

mold-making

rubber,

Dow

Corning

Corp.,

signals in response to identical. The output

Midland,

Mich.

iTerm used to describe the portions of the oral mucosa that are subjected in Sicher, H.: Oral Anatomy, of pressure and friction during mastication, 1965, The C. V. Mosby Company, p. 215. land,

$,Brush Ohio.

Mark

480

eight-channel

recorder,

Gould,

Inc.,

Brush

Instruments

to strong forces ed. 4, St. Louis, Division,

Cleve-

Volume Number

0”

Measuring

35 4

I 2 PRESSURE(Million

3

4 5 Dynes/Sq.cm)

pressures

in tissues under

dentures

379

6

Fig. 3. (IA) The angle specimen-hydrostatic loading. (IB) The angle specimen-static loading. @A) The circular specimen l-hydrostatic loading. (2Bj The circuiar specimen l-static loading using a 2 mm. thick silicone rubber spacer. (3A) The circular specimen 2-hydrostatic loading. (3B) The circular specimen 2-static loading using a 5 mm. thick silicone rubber spacer. Three test specimens were calibrated (a right-angle specimen and two circular specimens) using 2 and 5 mm. thick silicone rubber spacers. For each specimen, hydrostatic applied pressure vs. output voltage and computed normal applied pressure (0.707 x applied pressure) during static loading vs. output voltage were plotted. Fig. 4. Effects of misalignment of the transducer in the test-specimen plate: a, the static loadtransducer recessed 0.25 mm. in the gold test plate; 6, the hydrostatic load; C, the static loadtransducer flush with the gold test plate; d, the static load-transducer protruding 0.25 mm. below the surface of the gold plate. A transducer was calibrated using a static load to incestigate the effects of misalignment in the gold test plate. Only those results obtained when the diaphragm of the transducer was mounted flush with the surface of the test plate are in good agreement with the results obtained while using a hydrostatic load.

signals of the right-angle specimen in response to both computed, normal, applied static pressure and hydrostatic pressure were in good agreement as well (Fig. 3). Fig. 3 shows that the maximum error between the responses to normal applied static pressure and hydrostatic pressure was 10 per cent for the three test specimens. Therefore, it was concluded that, in the presence of tangential pressures up to at least 4.2 x 10G dynes/cm,“, the transducer will respond only to normal pressures. The results obtained using the two circular test specimens indicate that the responses to static pressure and hydrostatic pressure did not differ significantly when either 2 mm. or 5 mm. silicone rubber spacers were used. As the spacers were used to simulate the masticatory mucosa beneath the transducer during clinical tewing, it was concluded that tissue thickness will not significantly affect signal outputs from the diaphragm pressure transducer. Hydrostatic loading during clinical testing. Calculations were made according to the analytical method of Fitzgerald and Hufferd13 to determine the interaction of a diaphragm and a viscoelastic material under pressure. If saliva was present as an interface between the denture base and the supporting tissues, the transducer indicated a hydrostatic load. As the saliva was forced out and the tissue became more able to support shear stress, the pressure distribution became less uniform. This effect can cause pressure gradients on the underside of

380

Kelsey,

Fig. 5. different

Reid,

and

Coplowitt

Instrumented cast-metal occlusal designs.

J. Prosthet. Dent. April, 1976

denture

bases

with

interchangeable

posterior

segments

of

the denture base, and the pressure on the residual ridge might be considerably greater than those pressuresnear the border. However, the transducer’s accuracy was not impaired unlessthe shear modulus of the tissue (tissue stiffness) became too great. When changesin the shear modulus did occur, the diaphragm could no longer deform, as if loaded hydrostatically. Its deformation then became a function of the interaction between the diaphragm and the tissue. According to Fitzgerald and Hufferd,13 significant errors occur when Gt (shear modulus of the tissue) is greater than the normalized gage flexibility minus the Poissonratio for the tissue,or: Gt >

where :

Eg hg Ag vg vt

= = = = =

3 x lo7 p.s.i. 0.005 inch 0.0625 inch 1:3 1:2

Wx3

(I-Vt)

12(1-Vg)Ags

(diaphragm elastic modulus), (diaphragm thickness), (diaphragm radius), and (Poissonratio for the diaphragm and for most viscoelastic material),

then : 12(~f~g~Ag

(l-V*)

=

720

p.s.i.

According to Kydd and Mandley,14 palatal mucoperiosteum has a mean compressivemodulus (E,) of about 440 p.s.i. Since :

&=Et, 2(1+Vt)

Volume Number

35 4

Measuring

pressures

Fig. 6. Pressure transducers are placed in eight carefully (arrows). Each diaphragm is in direct contact with a flat and flush with the surface of the surrounding base.

in tissues under

selected surface

dentures

381

sites of the denture bases on the masticatory muco!ja

the shear modulus of the tissue (G,) is approximately 147 p.s,i., and a uniform pressure distribution across the diaphragm can be assumed. Other calculations showed that the maximum diaphragm deflection was approximately 0.01 mm. Assuming that the thickness of the masticatory mucosa is in excess of 1 mm.,‘” the shear modulus of the tissue remains relatively constant over the range of motion of the diaphragm, and the pressure distribution at the tissue-denture base interface remains uniform. Therefore, for the purpose of clinical testing, the transducers could be calibrated hydrostatically. The most significant source of error Error from misalignment of transducer. appeared to be the manner in which the transducer was mounted in the denture base (or test-specimen plate). When the space between the surface of the diaphragm and the tissue was 0.25 mm., errors were introduced. When the transducer extended 0.25 mm. from the surface of the test specimen, smaller errors were found (Fig. 411. Therefore, care must be taken to place the transducer so that its diaphragm is flush with the surface of the denture base.

DISCUSSION Earlier attempts to measure pressures against the supporting tissues under functioning dentures were made using transducers with a cantilever-beam arrangement. Lawsonl’ showed that quantitative results from these studies were inaccurate. However, the relative value of the output signal at each transducer was valid since the placement of the transducers in the denture base did not vary. Therefore, conclusions by Frechette’ and Stromberg8 regarding the effects of occlusal forms on the pressures applied to the underlying tissue were still meaningful. The method presented here yields data that are quantitatively valid. The dia.phragm pressure transducer described in this article overcomes Lawson’s criticism of the cantilever-beam arrangement by using a more rigid test point. The tramducer “sees” a hydrostatic load during function (clinical testing). Since the output

302

Kelsey,

Fig. 7. Signal magnetic tape

Reid,

outputs during

and

from clinical

Coplowitz

the transducers testing. Data

J. Prosthet. Dent. April. 1976

are recorded on a direct-writing are analyzed by computer.

oscillograph

and

signal of the transducer is linear with applied hydrostatic pressure, it can be easily calibrated in an oil bath. This method was used to compare the pressures applied to the supporting tissues when posterior teeth of various occlusal patterns are used on complete dentures. The technique developed by Trapozzano and Lazzaril” (and subsequently used by Frechette and Kaires) has been adopted to keep all denture-related factors constant except for changes in the occlusal patterns. This method consists of adapting interchangeable posterior tooth segments to the same cast-metal denture bases (Fig. 5). Carefully selected, experienced denture wearers, with healthy residual ridges of desirable form, served as subjects. Eight diaphragm pressure transducers were placed in strategic sites in the cast gold bases (Fig. 6). The embedded wires exited between the central incisors in such a manner that they were as close to the incisal edges as possible without permitting damage by incision during mastication. Pressures caused by masticating test foods using nonanatomic” and anatomic? posterior teeth were compared. During clinical testing, signal outputs from the transducers were recorded simultaneously on a writing oscillograph and magnetic tape (Fig. 7). The d a t a were analyzed by computer to determine the average pressure at the site of each transducer over the duration of each masticatory sequence. Average peak pressure and frequency of masticatory strokes were also calculated. Additional possible uses of this method include simultaneous recording of pressure and electromyograph data during mastication. Another variable that can be monitored at the same time is mandibular movement. Correlation of these variables might yield more insight into the neuromuscular mechanisms of mastication, A greater variety of tooth forms can be studied in a large number of subjects. *Trubyte

Rational

tTrubyte

Pilkington-Turner

posteriors,

Dentsply

International,

30 degree

posteriors,

York, Dentsply

Pa. International,

York,

Pa.

VObmlC Number

35 4

Measuring

pressure5

in tissues under

dentures

383

Pressuresagainst the supporting tissuesunder distal-extension removable partial denture basescan be compared when occlusal form, framework, and clasp designsare altered. It would also be possibleto determine any correlation between average pressure and chewing frequency for a large number of foods. This could lead to a new classification of foods for purposesof testing in future investigations. SUMMARY

A method for measuring pressuresexerted by denture basesagainst the supporting tissuesduring mastication which yields valid quantitative results was developed. Use of a specially designed diaphragm pressure transducer overcomes the disadvantages of a cantilever-beam arrangement which gave inaccurate results in earlier studies. Ongoing research using this method was described. Several suggestionsfor additional applications of this method for prosthodontic research were presented. References 1. Rowlett, A. E.: The Gnathodynamometer and Its Use in Dentristry, Proc. R. Sot. Med. 26: 463-471, 1933. 2. Black, G. V.: A Work on Operative Dentistry, vol. I, Chicago, 1908, Medico-Dental Publishing Company, pp. 16 l- 17 1. 3. Howell, A. H., and Manly, R. S.: 4n Electronic Strain Gage for Measuring Oral Forces, J. Dent. Res. 27: 705-712, 1948. 4. Howell, A. H., and Brudevold, F.: Vertical Forces Used During Chewing of Food, J. Dent. Res. 29: 133-136, 1950. 5. Brudevold, F.: A Basic Study of the Chewing Forces of a Denture Wearer, J. Am. Dent. Assoc. 43: 45-51, 1951. 6. Yurkstas, A., and Curby, W. A.: Force Analysis of Prosthetic Appliances During Function, J. PROSTHET. DENT. 3: 82-87, 1953. 7. Frechette, A. R.: Masticatory Forces Associated With the Use of Various Types of Artificial Teeth, J. PROSTHET. DENT. 5: 252-267, 1955. 8. Stromberg, W. R.: A Method of Measuring Forces of Denture Bases Against Supporting Tissues, J. PROSTHET. DENT. 5: 268-288, 1955. 9. Kaires, A. K.: Partial Denture Design and Its Relation to Force Distribution and Masticatory Performance, J. PROSTHET. DENT. 6: 672-683, 1956. 10. Kaires, A. K.: A Study of Partial Denture Design and Masticatory Pressures in a Mandibular Bilateral Distal Extension Case, J. PROSTHET. DENT. 8: 340-350, 1958. 11. Ledley, R. S.: Theoretical Analysis of Displacement and Force Distribution for the Tissue-Bearing Surface of Dentures, J. Dent. Res. 47: 318-322, 1968. 12. Lawson, W. A. L.: The Validity of a Method Used for Measuring Masticatory Forces, J. PROSTHET. DENT. 10: 99-111, 1960. 13. Fitzgerald, J. E., and Hufferd, W. L.: Interaction of a Diaphragm Pressure Gage With a Viscoelastic Halfspace, Experimental Mechanics, July, 1970, pp. 257-265. 14. Kydd, W. L., and Mandley, J.: The Stiffness of Palatal Mucoperiosteum, J. PROSTHET.

DENT. 18: 116-121, 1967. 15. 16.

Kydd, W. L., Daly, C. H., and Wheeler, J. B., III: The Thickness Measurement of Masticatory Mucosa In Vivo, Int. Dent. J. 21: 430-441, 1971. Trapozzano, V. R., and Lazzari, J. B.: An Experimental Study of the Testing of Occlusal Patterns on the Same Denture Bases, J. PROSTHET. DENT. 2: 440-457, 1952. SCHOOL

OF DENTISTRY

UNIVERSITY ANN ARBOR,

OF MICHIGAN MICH. 48104