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Properties of silicone maxillofacial elastomer processed in stone and metal
Properties of silicone maxillofacial elastomer processed in stone and metal C. N. Raptis, D.D.S., M.S.,” The
University
of Michigan.
R. Yu, Ph.D.,**
School
of Dentistry,
Ann
and J. C. Knapp, D.D.S., Arbor.
F
acial disfigurement resulting from cancer surgery or trauma can severely debilitate patients, preventing them from leading a normal life in our society. Maxillofacial prosthodontics, a subspecialty of dentistq, includes the treatment of patients with extraoral and intraoral defects with removable prostheses. As an alternative to disfigurement of the head and neck, this treatment modality has had a long history with continued improvement. Recently, many advances have been made in prosthetic materials and in material handling methods. Selection of these materials has been based on certain goal-oriented criteria. The criteria have been carefully and elaborately outlined by others.‘. z The thrust of these criteria is to provide a prosthesis which can replace the lost body part in all aspects. Realistically, no prosthesis can meet all demands. However, it should meet at least the following criteria: it should be biocompatible, esthetically soft and pliable like the tissues it acceptable. replaces? durable for a reasonable time period, and easy to fabricate. At this time, some materials currently in use are polyvinylchlorides, polyurethanes and polysiloxanes. From previous studies:‘-’ a polysiloxane. Silastic 44210,t polymerized by an addition reaction, has been shown to have excellent physical properties and color stability. The silicone has also been clinically used with success.$ The This study was supported in part by Research Grant DE 04136 and Service Award DE 07057 from the National Institute of Dental Research, National Institutes of Health. Bethesda, Md. 2001-l. *Graduate student, Department of Dental Materials. **Research Investigator, Department of Dental Materials. ***Associate Professor, Department of Hospital Dentistry Department of Complete Dentures.
and
iDow Corning Corp., Midland, Mich. $Penonal communications: Dr. N. Schaaf, Rowe11 Park Memorial Center, Buffalo. N. Y., and Captain C. E. King, U. S. Naval &dical-Dental Center, Bethesda, Md.
0027.3913/80/100447
+ 04$00.40/0’B
1980 The
C. V. Moshy
Co
M-S.***
hlich,
material can be easily polymerized and colors can be conveniently incorporated into, or later applied to, the material. In the extremes of weather chamber testing and in clinical use, the material exhibits good longevity. When all criteria are considered, Silastic 44210 is an excellent maxillofacial material. Common to all methods and materials, an impression of the defect is made and a model poured in artificial stone. On a dental stone model, a wax or clay pattern is made to conform to the defect and to simulate the shape of the missing structure. At this stage, fabrication methods differ considerably depending on the elastomer used. Some techniques require duplication of the cast and pattern in epoxy resins or in metal. A simpler stone-mold technique is used with some materials, including Silastic 44210. It has been observed clinically that using stone molds and Silastic 44210 produces a prosthesis that is weaker than that processed in metal molds, particularly on the thin edges of the periphery. When the stone molds are coated and then used to produce a prosthesis, subjective observations suggest that the physical properties are similar to those of prostheses made in a metal mold. Since the prosthodontist would like to use the simplest technique. and still provide a prosthesis with the best physical properties, the objective of this study was to test and compare the physical properties of Silastic 44210 when polymerized in metal, stone. and treated stone molds.
MATERIALS
AND METHODS
In this investigation, test specimens of Silastic 442 10 were polymerized in aluminum (A) molds and gypsum* molds with and without separators. The stone molds were prepared by first mixing improved dental stone with distilled water under vacuum Toecal,
Coe Laboratories,
THE JOURNAL
Inc.. Chicago,
Ill
OF PROSTHETIC
DENTISTRY
447
RAPTIS,
AND
KNXPI’
47!
45
“E 0 ,” ;
II!.
40
; z Y LL
= 0 ; 4 0
451
g
42:
z ae
2;
z >
5f: 35 * * Y I-
400
I x :
375
Y : f -
30 350
!I
CON01
Fig. 2. The tl of Silastic conditions. 3
OF
MOLDS
44210 under
various
processing
25 I CONDITION
OF
MOLDS
Fig. 1. UT5 of Silastic 44210 under various processing conditions. according to the manufacturer’s instructions and then pouring the mixture into a denture flask. The stone molds were allowed to air-dry for 72 hours before use. One group of stone molds was coated with a thin layer of petroleum jelly* (B). Another set of stone molds was coated with a silicone spray? (C), which was allowed to dry for 2 hours before the molds were used. A third set of stone molds was coated with one layer of a tin foil substitute separator$. (D), and it was also allowed to dry for 2 hours. The last set of stone molds was used without a separator to process Silastic 44210 samples (E). For each processing condition, samples were prepared by mixing the base and catalyst in a 10 to 1 ratio by weight according to the manufacturer’s instructions. The mixture was then degassed under a vacuum to eliminate porosity from the samples. All samples were allowed to polymerize at 115” C in a circulating dry-heat oven for 90 minutes. The samples were tested for ultimate tensile strength (UTS), maximum percentage of elongation (n), and tensile strength (TS) at 200% and 300% elongations by using an Instron testing machines at ‘Amajell, American Oil Co., Chicago, III. TAmco, American Handicrafts, Fort Worth, Texas. $Al-Cote, L. D. Caulk Co., Milford, Del. $Instron, floor model TT, Instron Corp.. Canton, Mass.
448
TION
a constant deformation rate of 10 cm/min. The samples used for testing tensile properties were dumbbell-shaped, 6 mm wide and 1 mm thick. Shore ,4 hardness was measured with a Shore .4 durometer* and the samples were I X 2.5 X 2.5 cm. The results obtained were first analyzed with Bartlett’s test to determine homoscedasticity and then by a one way analysis of variance. When a significant difference (p > .05) was fcmnd between mean values. a Student-Neuman-Kuel’s multiple comparison test was carried out. RESULTS The results indicate that the choice of mold materials and the method of surface conditioning do affect the UTS, and the n of processed Silastic 442 10 (Figs. 1 and 2, Table I). The UTS was significantl! affected by the test variables, with the metal mold having the highest value for UTS at 44 kg/cm’ and the uncoated stone molds lowest value at 3 I kg/cm’. These results represent a range in UTS of 29.6%. The samples processed in coated stone molds were similar in UTS. with an average value of 40 kg/cm’ or a 9.1% decrease when compared to the metal molds. The n of Silastic 44210 was also affected by processing conditions. No statistical difference was noted between samples processed in metal molds or coated stone molds. The value was 465% for samples prepared in metal molds and ranged from 445% to 465’7, *Shore A durometer, Jamaica, N. Y.
OCTOBER
1960
VOLUME
44
NUMBER
4
SILICONE
MAXILLOFACIAL
ELASTOMER
for those processed in coated stone molds. The n obtained for samples processed in uncoated stone molds at 380% was considerably lower than the results for metal molds and represented a reduction in n of 18.3%. The results obtained for TS at 200% and 300% elongations were found to be independent of processing conditions (Table II). TS values ranged from 5.16 to 5.23 kg/cm’ at 200%, and from 15.28 to 15.65 kg/cm’ at 300%, respectively. Shore A hardness was similar for all conditions and ranged from 29.8 to 30 Shore A hardness units. The overall results for UTS, n, and Shore A hardness that were obtained from aluminum molds are in good agreement with the results reported in the earlier studie?. ’ in which metal molds were used for sample preparation.
DISCUSSION
of Silastic 44210 influenced processing conditions
by the various
Processing condition
(kg/cm>)
n
A B C D E
44(1.4)* 40(1.4) 40(1.6) 40(2.8)
465(15)* 445(12) 450(10) 455(16) 380(23)
31(0.8)
*Mean of five replications with standard deviations in parentheses. No statistical difference was found between groups at the 95% confidence level.
Table II. Properties by the various
of Silastic 44210 unaffected processing conditions
Processing
When compared to the machined aluminum molds, the stone molds have surfaces with more pronounced microirregularities. The irregularities in the untreated stone mold surface may be transferred to the silicone during processing. When the processed silicone sample is elongated, these irregularities may act as the focii for concentration of stress. At high elongation, the initiation and propagation of cracks to the point of rupture is more likely to occur at one of these irregularities. However, Silastic 44210 had been found to have excellent resistance to tear.” Therefore, rupture of this material by this mode of failure is considered unlikely. The probable mechanism for lower tensile properties may be the lack of homogeneity of the polymerized silicone surface. The porosity in the untreated stone mold may selectively alter the concentration ratio of the base and catalyst by drawing the catalyst out of the unpolymerized mix. By capillary action, the less viscous catalyst may be drawn into the surface of the untreated stone mold. This could alter the base-catalyst ratio and the degree of polymerization near the surface. The changes in degree of polymerization on the surface may account for the decrease in UTS and n but may not affect Shore A hardness and TS at 200% and 300% elongations since these measurements are more dependent upon the bulk of the material. The coatings applied to the stone mold fill many of the surface microirregularities and close the capillaries between the calcium dehydrate crystals. This surface treatment improves the physical properties of Silastic 44210 processed in stone molds.
THE JOURNAL
Table I. Properties
OF PROSTHETIC
DENTISTRY
condition A B C D E
200%
300%
Shore A
elongation
elongation
hardness
5.2(0.3)* 5.2(0.3) 5.2(0.2) 5.2(0.2) 5.2(0.4)
15.7(0.8)* 15.4(0.8) 15.6(0.7) 15.6(0.6) 15.3(0.5)
30(0.2)* 29.9(0.2) 29.9(0.4) 30(0.3) 29.8(0.4)
*Mean of five replications wirh standard deviations in parentheses. No statistical difference was found between groups at the 95% confidence
level.
SUMMARY
AND CONCLUSIONS
1. The effect of mold conditions on the UTS, n, and TS at 200% and 300% elongation, and Shore A hardness was measured for Silastic 44210. 2. There was no difference in n, Shore A hardness, and TS at 200% and 300% elongation between Silastic 44210 samples whether produced in metal or coated stone molds. 3. The highest UTS for Silastic 442 10 was demonstrated for samples processed in aluminum molds. The UTS was slightly reduced when coated stone molds were used, and it was considerably reduced with untreated stone molds. It is felt that the slight decrease in UTS with coated stone molds would not compromise the use of stone molds clinically, particularly when ease of processing is considered. 4. Petroleum jelly, silicone spray, and tinfoil substitute were all equally effective in improving the physical properties of the elastomer when processed in a stone mold. These results demonstrate the necessity of using a mold sealant when processing Silastic 44210 in a stone mold.
449
RAI’TI$, YU, .4ND KNAPP
REFERENCES
5.
1. Chalian. \‘. .A., Drane. J. B., and Standish, S. hl.: Maxillafacial Prosthetics. Baltimore, 1971. LWliams and N’ilkins Co.. p 283. 2. Sweeney, M’. T., Fischer, T. E., Castleberry, D. J.. and Cowperthwaite. G. F.: Evaluation of improved maxillofacial prosthetic materials. J PROSTHETDENT 27:296, 1972. 3. Yu, R., Koran, .-\., and Craig. R. G.: Physical properties of elastomers for maxillofacial appliances under accelerated aging. J Dent Res (In press). 4. Craig, R. C.. Koran. .\., Yu, R., and Spencer, J.: Color stability of elastomers for maxillofacial appliances. J Dent Res 57:866, 1978.
IADR I’ROSTHODONTIC
6.
Yu, R., and Koran. .-\.: Dimensional stability of elastrmwr< for maxillofacial applications. J Dent Res 58:1908. 197‘~). Yu. R., Koran. .A., and Craig. R. G.: Physical pmperties r~f.1 pigmented silicone maxillofacial matrrial as a funcrion 4 accelerated qing. J Dent Res (In press 4
ABSTRACT
Comparison bf electromyographic of vertical rest position
and phonetic
measurements
J. D. Rugh, C. J. Drago, and N. Barghi University of Texas Health Science Center, San Antonio, Texas The definitions of and the relationships between concepts of clinical, postural, and electromyographic rest position remain vague. The purpose of this study was to compare one EMG technique to identifying rest position with a traditional clinical means. Ten dentulous subjects learned to relax the masticatory muscles at interocclusal vertical distances ranging from 0 to 16 mm in 1 mm steps. Relaxation was accomplished by feedback of EMG activity detected by an active surface electrode placed over the belly of the left and right masseter. A common electrode was
Reprinted from the Journal of Dental Research [58 (Special IssueAj, 1979 (Abst No. 899) ] with permission of the author and the editor.
450
placed over the suprahyoid muscles. This electrode placement provides a nonspecific electromyographic signal which is a function of the activity of several of the jaw positioning muscles. Interocctusal distance was measured with a K-5 kinesiograph. Clinical rest position was measured on each subject using a millimeter scale and marks on the nose and chin. Subjects were asked to say “m”, swallow and relax. A specific vertical dimension of minimal EMG activity was observed for each subject, ranging from 5 to 14 mm. In each subject, this electromyographic rest position was inferior to the clinical rest position (mean difference was 7 mm). These results suggest that the point of minimal masticatory muscle activity is very specific, but is not coincident with what is generally referred to as “clinical rest position.”
OCTOBER lRB0
VOLUME 44
NUMBER
4
University
of Michigan.
R. Yu, Ph.D.,**
School
of Dentistry,
Ann
and J. C. Knapp, D.D.S., Arbor.
F
acial disfigurement resulting from cancer surgery or trauma can severely debilitate patients, preventing them from leading a normal life in our society. Maxillofacial prosthodontics, a subspecialty of dentistq, includes the treatment of patients with extraoral and intraoral defects with removable prostheses. As an alternative to disfigurement of the head and neck, this treatment modality has had a long history with continued improvement. Recently, many advances have been made in prosthetic materials and in material handling methods. Selection of these materials has been based on certain goal-oriented criteria. The criteria have been carefully and elaborately outlined by others.‘. z The thrust of these criteria is to provide a prosthesis which can replace the lost body part in all aspects. Realistically, no prosthesis can meet all demands. However, it should meet at least the following criteria: it should be biocompatible, esthetically soft and pliable like the tissues it acceptable. replaces? durable for a reasonable time period, and easy to fabricate. At this time, some materials currently in use are polyvinylchlorides, polyurethanes and polysiloxanes. From previous studies:‘-’ a polysiloxane. Silastic 44210,t polymerized by an addition reaction, has been shown to have excellent physical properties and color stability. The silicone has also been clinically used with success.$ The This study was supported in part by Research Grant DE 04136 and Service Award DE 07057 from the National Institute of Dental Research, National Institutes of Health. Bethesda, Md. 2001-l. *Graduate student, Department of Dental Materials. **Research Investigator, Department of Dental Materials. ***Associate Professor, Department of Hospital Dentistry Department of Complete Dentures.
and
iDow Corning Corp., Midland, Mich. $Penonal communications: Dr. N. Schaaf, Rowe11 Park Memorial Center, Buffalo. N. Y., and Captain C. E. King, U. S. Naval &dical-Dental Center, Bethesda, Md.
0027.3913/80/100447
+ 04$00.40/0’B
1980 The
C. V. Moshy
Co
M-S.***
hlich,
material can be easily polymerized and colors can be conveniently incorporated into, or later applied to, the material. In the extremes of weather chamber testing and in clinical use, the material exhibits good longevity. When all criteria are considered, Silastic 44210 is an excellent maxillofacial material. Common to all methods and materials, an impression of the defect is made and a model poured in artificial stone. On a dental stone model, a wax or clay pattern is made to conform to the defect and to simulate the shape of the missing structure. At this stage, fabrication methods differ considerably depending on the elastomer used. Some techniques require duplication of the cast and pattern in epoxy resins or in metal. A simpler stone-mold technique is used with some materials, including Silastic 44210. It has been observed clinically that using stone molds and Silastic 44210 produces a prosthesis that is weaker than that processed in metal molds, particularly on the thin edges of the periphery. When the stone molds are coated and then used to produce a prosthesis, subjective observations suggest that the physical properties are similar to those of prostheses made in a metal mold. Since the prosthodontist would like to use the simplest technique. and still provide a prosthesis with the best physical properties, the objective of this study was to test and compare the physical properties of Silastic 44210 when polymerized in metal, stone. and treated stone molds.
MATERIALS
AND METHODS
In this investigation, test specimens of Silastic 442 10 were polymerized in aluminum (A) molds and gypsum* molds with and without separators. The stone molds were prepared by first mixing improved dental stone with distilled water under vacuum Toecal,
Coe Laboratories,
THE JOURNAL
Inc.. Chicago,
Ill
OF PROSTHETIC
DENTISTRY
447
RAPTIS,
AND
KNXPI’
47!
45
“E 0 ,” ;
II!.
40
; z Y LL
= 0 ; 4 0
451
g
42:
z ae
2;
z >
5f: 35 * * Y I-
400
I x :
375
Y : f -
30 350
!I
CON01
Fig. 2. The tl of Silastic conditions. 3
OF
MOLDS
44210 under
various
processing
25 I CONDITION
OF
MOLDS
Fig. 1. UT5 of Silastic 44210 under various processing conditions. according to the manufacturer’s instructions and then pouring the mixture into a denture flask. The stone molds were allowed to air-dry for 72 hours before use. One group of stone molds was coated with a thin layer of petroleum jelly* (B). Another set of stone molds was coated with a silicone spray? (C), which was allowed to dry for 2 hours before the molds were used. A third set of stone molds was coated with one layer of a tin foil substitute separator$. (D), and it was also allowed to dry for 2 hours. The last set of stone molds was used without a separator to process Silastic 44210 samples (E). For each processing condition, samples were prepared by mixing the base and catalyst in a 10 to 1 ratio by weight according to the manufacturer’s instructions. The mixture was then degassed under a vacuum to eliminate porosity from the samples. All samples were allowed to polymerize at 115” C in a circulating dry-heat oven for 90 minutes. The samples were tested for ultimate tensile strength (UTS), maximum percentage of elongation (n), and tensile strength (TS) at 200% and 300% elongations by using an Instron testing machines at ‘Amajell, American Oil Co., Chicago, III. TAmco, American Handicrafts, Fort Worth, Texas. $Al-Cote, L. D. Caulk Co., Milford, Del. $Instron, floor model TT, Instron Corp.. Canton, Mass.
448
TION
a constant deformation rate of 10 cm/min. The samples used for testing tensile properties were dumbbell-shaped, 6 mm wide and 1 mm thick. Shore ,4 hardness was measured with a Shore .4 durometer* and the samples were I X 2.5 X 2.5 cm. The results obtained were first analyzed with Bartlett’s test to determine homoscedasticity and then by a one way analysis of variance. When a significant difference (p > .05) was fcmnd between mean values. a Student-Neuman-Kuel’s multiple comparison test was carried out. RESULTS The results indicate that the choice of mold materials and the method of surface conditioning do affect the UTS, and the n of processed Silastic 442 10 (Figs. 1 and 2, Table I). The UTS was significantl! affected by the test variables, with the metal mold having the highest value for UTS at 44 kg/cm’ and the uncoated stone molds lowest value at 3 I kg/cm’. These results represent a range in UTS of 29.6%. The samples processed in coated stone molds were similar in UTS. with an average value of 40 kg/cm’ or a 9.1% decrease when compared to the metal molds. The n of Silastic 44210 was also affected by processing conditions. No statistical difference was noted between samples processed in metal molds or coated stone molds. The value was 465% for samples prepared in metal molds and ranged from 445% to 465’7, *Shore A durometer, Jamaica, N. Y.
OCTOBER
1960
VOLUME
44
NUMBER
4
SILICONE
MAXILLOFACIAL
ELASTOMER
for those processed in coated stone molds. The n obtained for samples processed in uncoated stone molds at 380% was considerably lower than the results for metal molds and represented a reduction in n of 18.3%. The results obtained for TS at 200% and 300% elongations were found to be independent of processing conditions (Table II). TS values ranged from 5.16 to 5.23 kg/cm’ at 200%, and from 15.28 to 15.65 kg/cm’ at 300%, respectively. Shore A hardness was similar for all conditions and ranged from 29.8 to 30 Shore A hardness units. The overall results for UTS, n, and Shore A hardness that were obtained from aluminum molds are in good agreement with the results reported in the earlier studie?. ’ in which metal molds were used for sample preparation.
DISCUSSION
of Silastic 44210 influenced processing conditions
by the various
Processing condition
(kg/cm>)
n
A B C D E
44(1.4)* 40(1.4) 40(1.6) 40(2.8)
465(15)* 445(12) 450(10) 455(16) 380(23)
31(0.8)
*Mean of five replications with standard deviations in parentheses. No statistical difference was found between groups at the 95% confidence level.
Table II. Properties by the various
of Silastic 44210 unaffected processing conditions
Processing
When compared to the machined aluminum molds, the stone molds have surfaces with more pronounced microirregularities. The irregularities in the untreated stone mold surface may be transferred to the silicone during processing. When the processed silicone sample is elongated, these irregularities may act as the focii for concentration of stress. At high elongation, the initiation and propagation of cracks to the point of rupture is more likely to occur at one of these irregularities. However, Silastic 44210 had been found to have excellent resistance to tear.” Therefore, rupture of this material by this mode of failure is considered unlikely. The probable mechanism for lower tensile properties may be the lack of homogeneity of the polymerized silicone surface. The porosity in the untreated stone mold may selectively alter the concentration ratio of the base and catalyst by drawing the catalyst out of the unpolymerized mix. By capillary action, the less viscous catalyst may be drawn into the surface of the untreated stone mold. This could alter the base-catalyst ratio and the degree of polymerization near the surface. The changes in degree of polymerization on the surface may account for the decrease in UTS and n but may not affect Shore A hardness and TS at 200% and 300% elongations since these measurements are more dependent upon the bulk of the material. The coatings applied to the stone mold fill many of the surface microirregularities and close the capillaries between the calcium dehydrate crystals. This surface treatment improves the physical properties of Silastic 44210 processed in stone molds.
THE JOURNAL
Table I. Properties
OF PROSTHETIC
DENTISTRY
condition A B C D E
200%
300%
Shore A
elongation
elongation
hardness
5.2(0.3)* 5.2(0.3) 5.2(0.2) 5.2(0.2) 5.2(0.4)
15.7(0.8)* 15.4(0.8) 15.6(0.7) 15.6(0.6) 15.3(0.5)
30(0.2)* 29.9(0.2) 29.9(0.4) 30(0.3) 29.8(0.4)
*Mean of five replications wirh standard deviations in parentheses. No statistical difference was found between groups at the 95% confidence
level.
SUMMARY
AND CONCLUSIONS
1. The effect of mold conditions on the UTS, n, and TS at 200% and 300% elongation, and Shore A hardness was measured for Silastic 44210. 2. There was no difference in n, Shore A hardness, and TS at 200% and 300% elongation between Silastic 44210 samples whether produced in metal or coated stone molds. 3. The highest UTS for Silastic 442 10 was demonstrated for samples processed in aluminum molds. The UTS was slightly reduced when coated stone molds were used, and it was considerably reduced with untreated stone molds. It is felt that the slight decrease in UTS with coated stone molds would not compromise the use of stone molds clinically, particularly when ease of processing is considered. 4. Petroleum jelly, silicone spray, and tinfoil substitute were all equally effective in improving the physical properties of the elastomer when processed in a stone mold. These results demonstrate the necessity of using a mold sealant when processing Silastic 44210 in a stone mold.
449
RAI’TI$, YU, .4ND KNAPP
REFERENCES
5.
1. Chalian. \‘. .A., Drane. J. B., and Standish, S. hl.: Maxillafacial Prosthetics. Baltimore, 1971. LWliams and N’ilkins Co.. p 283. 2. Sweeney, M’. T., Fischer, T. E., Castleberry, D. J.. and Cowperthwaite. G. F.: Evaluation of improved maxillofacial prosthetic materials. J PROSTHETDENT 27:296, 1972. 3. Yu, R., Koran, .-\., and Craig. R. G.: Physical properties of elastomers for maxillofacial appliances under accelerated aging. J Dent Res (In press). 4. Craig, R. C.. Koran. .\., Yu, R., and Spencer, J.: Color stability of elastomers for maxillofacial appliances. J Dent Res 57:866, 1978.
IADR I’ROSTHODONTIC
6.
Yu, R., and Koran. .-\.: Dimensional stability of elastrmwr< for maxillofacial applications. J Dent Res 58:1908. 197‘~). Yu. R., Koran. .A., and Craig. R. G.: Physical pmperties r~f.1 pigmented silicone maxillofacial matrrial as a funcrion 4 accelerated qing. J Dent Res (In press 4
ABSTRACT
Comparison bf electromyographic of vertical rest position
and phonetic
measurements
J. D. Rugh, C. J. Drago, and N. Barghi University of Texas Health Science Center, San Antonio, Texas The definitions of and the relationships between concepts of clinical, postural, and electromyographic rest position remain vague. The purpose of this study was to compare one EMG technique to identifying rest position with a traditional clinical means. Ten dentulous subjects learned to relax the masticatory muscles at interocclusal vertical distances ranging from 0 to 16 mm in 1 mm steps. Relaxation was accomplished by feedback of EMG activity detected by an active surface electrode placed over the belly of the left and right masseter. A common electrode was
Reprinted from the Journal of Dental Research [58 (Special IssueAj, 1979 (Abst No. 899) ] with permission of the author and the editor.
450
placed over the suprahyoid muscles. This electrode placement provides a nonspecific electromyographic signal which is a function of the activity of several of the jaw positioning muscles. Interocctusal distance was measured with a K-5 kinesiograph. Clinical rest position was measured on each subject using a millimeter scale and marks on the nose and chin. Subjects were asked to say “m”, swallow and relax. A specific vertical dimension of minimal EMG activity was observed for each subject, ranging from 5 to 14 mm. In each subject, this electromyographic rest position was inferior to the clinical rest position (mean difference was 7 mm). These results suggest that the point of minimal masticatory muscle activity is very specific, but is not coincident with what is generally referred to as “clinical rest position.”
OCTOBER lRB0
VOLUME 44
NUMBER
4