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An evaluation of the surface characteristics of a facial prosthetic elastomer. Part III: Wettability and hardness
An evaluation of the surface characteristics prosthetic elastomer. Part III: Wettability Eniko M. Veres, B.D.S., M.Sc., M.Dent.,* John F. Wolfaardt, B.D.S., M.Dent., Ph.D.,** Petrus J. Becker, Ph.D.*** University Research
of Witwatersrand, School of Dentistry, Council, Johannesburg, South Africa
of a facial and hardness
and
and Institute
for Biostatistics,
Medical
Silicone facial prosthetic elastomers may cause tissue damage by abrasion. Such damage is a particular concern when prostheses are mechanically retained against tissues compromised by adjunctive therapy. The hardness and wettability of Cosmesil material was compared with that of Molloplast-B material. The stone test surfaces were separated with soap, sodium alginate, silicone paste, and left untreated. A polished stainless steel surface was prepared as a control. The specimens of Cosmesil and Molloplast-B materials were processed against each of these surfaces. Ten specimens of each material were processed against the five different surfaces. Wettability was evaluated by measuring the contact angle with a profile projector. Indentation hardness was measured with a Shore-A durometer. Statistical analysis involved multiple analyses of variation and Tukey’s procedures (in all cases p < 0.06). Molloplast-B material was found to have a higher wettability than Cosmesil material (n = 3.22 degrees higher); sddium alginate separator yielded silicone specimens with the highest wettability; Molloplast-B material was found to be harder than Cosmesil material (x = 9.75 Shore-A indentation units harder). The softest silicones were processed with soap separator. Silicone grease yielded the hardest specimens. The mechanical performance of Cosmesil material would be enhanced by increasing the surface wettability. The hardness of Cosmesil material is within the ideal range for a maxillofacial prosthetic elastomer. (J
PROSTEIETDENT~~~~;~~:~~~-~~.)
I
ndentation hardnesshas been used as one of the mechanical tests applied to facial prosthetic materials.lT3 The indentation hardness of a silicone facial prosthetic material that is analogousto the softnessof living tissueis in the range of 25 to 55 Shore-A indentation unit.v.2*4 Hardness,surface texture, and wettability will determine the mechanicalpotential of a material to produce damage to tissue surfaces. A review of the literature revealedthat little researchhas been conducted on the wettability of facial prosthetic materials or resilient denture liners.5Bates and Smith6 and Louka et a1.7have discussedthe poor wettability of silicone elastomers.Murrap as well as Udagama and King9 have shownthe potentially damagingabrasive effect of silicones on tissues.It is important to consider abrasive potential when evaluating an elastomerthat will have prolonged tissuecontact. The abrasivepotential shouldbe measuredfor hardnessand wettability in relation to the surfacetexture.
*Lecturer, Department. of Prosthetic Dentistry. **Associate Professor, Division of Removable Prosthodontics, University of Alberta, Faculty of Dentistry, Edmonton, Canada. ***Specialist Statistician, Institute for Biostatistics, Medical Research Council, Johannesburg, South Africa.
1OlIl15333
466
In this study the wettability and ind,entation hardnessof Cosmesil(Cosmedica,U.W.I.S.T., Cardiff, Wales, U.K.) material wascomparedwith that of Molloplast-B (Regneri & Co., Karlsruhe, West Germany) material. The wettability and indentation hardnesswas then related to the previously establishedsurface texture of the two materials.fO The potential abrasivenessof Cosmesilmaterial was also comparedwith that of Molloplast-B, a resilient liner with a well-establishedclinical profile.
MATERIAL
AND
METHODS
The technique used to produce specimensof Cosmesil and Molloplast-B materials wasdescribedin Part II of this seriesof investigations.lo The technique used five truncated conical metal dies with a surface 2 cm in diameter that had beenpolishedto a mirror finish. The dieswere invested in dental stone under controlled conditions with standardized water/powder ratios and vacuum spatulation. The surface of the dental stone was prepared in various ways sothat the siliconeelastomerswere processedagainst the following materials: 1. Untreated stone with no separator 2. Stone treated with a soap solution (Modellglanzer, Dentaurum Pforzheim, West Germany) 3. Stone treated with sodium alginate separator (Cold Mould Seal, P.G. Smith, Johannesburg,South Africa)
APRIL
1990
VOLUME
63
NUMBER
4
FACIAL
PROSTHETIC
Table
I. &kdtiule analysis of variance Source
ELASTOMER.
PART
III
of variation*
Parameter?
Degrees
of freedom
Die Treated surface
Hardness 1st Wettability 2nd Wettability
4and50 4 and 50 4 and 50 4 and 50 4 and 50 4 and 50
RS Rnax wt Hardness 1st Wettability 2nd Wettability
1 and 50 1 and 50 1 and 50 1 and 50 land50 1 and 50
R3 R mex wt Hardness 1st Wet~bility 2nd Wettability
4and50 4 and 50 4 and 50 4 and 50 4 and 50 4and.50
R, R max
wt
Material
Die X Treated surface Die X Material Treated surface X Material
Die X treated surface X material *Main effects were tested with the likelihood ratio. tParameten were tested with F-test using following Error Re Rmax wt Hardness 1st Wettability 2nd Wettability
Mean
square 0.042 25.514 34.887 0.648 28.058 32.716
of wettability
Wettability wasmeasuredby evaluating the contact anbetween the surface of a drop of liquid and the surfaceof the solid on which it wasresting. A high contact angle indicates high wettability; a low contact angle indicates low wettability. Uncontaminated high-energy surfacessuch asmetalsare wet completely by pure liquids and contact anglesof zero are obtained. Low energy solidssuch gle formed
‘i-BE
JOURNAL
OF FROSl%ETlC
0.104
mean square errors:
4. Stone treated with silicone separating grease(Masque, Harry J. Bosworth Co., Skokie, Ill.) 5. The lapped metal surfacesin the gypsum molds The experimental design allowed for 10 specimensof each material to be processedagainst the five different surfaces.All of the silicone elastomersused were of the samelot and batch numbers. The flaskswere closedunder standard pressuresof’1000 Kpa. All materials were processedaccording to the manufacturers’ instructions. After processing,the specimenswere recovered and stored in airtight containers.
Investigation
p-Value
DENTISTRY
as organic polymers show a low contact angle and poor wettabi1ity.l” A micropipette (Labystems Oy, Helsinki, Finland) was used to dispensetwo drops (5 ~1 each} of distilled water onto the surface of the samples.It must be noted that the micropipette is not absolutely accurate for such small quantities. However, it wasnot essentialfor the dropsto be exactly the samesizebecausethe magnitude of the contact angle is independent of the size of the drop for solidswith a uniform
surface.11
To achieve greater precision a secondwettability study wasconducted in which the siliconespecimenwasslicedin half with a scalpelblade. The drop of water wasplaced as near as possibleto the edgeof the cut to facilitate reading of the contact angle.After the results from the secondwettability study were reported, the samplewasplaced on the adjustablestageof a profile projector (Hilger & Watts Ltd., Hilger Division, London, England), which projected a beam of light past the sample.By a seriesof mirrors and lensesthe beamwas directed onto a circular ground glass
467
VERES,
WOLFAARDT.
AND
BECKER
(Shore-A) Molloplast-B Cosmesil
Wettability
Fig.
Hardness
Metal
1. Comparisonof hardnessand wettability.
Stone
Stone + Soap
Stone + C.M.Seal
Stone + Masque
3. Means of hardness (Shore-A indentation units) for material and treated surface. Fig. (Shore-A)
-
II. Comparisonof indentation hardnessand mean contact angles Table
-
Material
Molloplast-B
-
-
Stone
+
Metal
Stone
Soap
Stone
Stone
C.M.Seal
Masque
+
46,67
7.75
4.53
+
2. Shore-A hardnessof siliconesamplescured against five different treatment surfaces. Fig.
viewer where a magnifiedsilhouette imagewasproduced of the samplesurface and the drop of water. A x50 magnification was used for all of the samples. The horizontal axis on the screenwas aligned with the surface of the solid, and the deviation from the horizontal (0 degree) was read from the scalearound the periphery. The vertical axis wasaligned with the surface of the water drop at its contact with the solid, and the deviation from the vertical axis (0 degree) was noted. The first measurement was subtracted from the secondto give the contact angle,The angle measuredwas the external contact angle between the surface of the liquid and the surface of the solid. When the anglewaslessthan 90 degrees,the reading was a negative quantity and the drop was mushroom shaped. When the angle was more than 90 degreesthe reading was a positive quantity and the droplet was flattened. The contact angleswere measuredon both sides of each of two droplets on eachsample,thus acquiring four measurementsfor each sample.The meansof these measurementswere usedfor the analysisof the external conangle.
Investigation
of indentation
hardness
Indentation hardness was measured with a Shore-A durometer of the type specified by ASTM D2240-68 468
56,42
-
”
tact
Hardness(Shore-Aunits) Wettability (degrees)
Cosmesil
(American Society for Testing Materials, D2240-68:Standard method of test for indentation hardnessof rubber and plastics by meansof a durometer). The specimenswere placed on a granite optical flat. The pressor foot of the durometer wasapplied to the specimenasrapidly aspossible with just sufficient pressureto obtain a firm contact between the pressorfoot and the specimen.The scalewas read immediately. All measurementswere calculated in the controlled environment of a metrology laboratory with a temperature of 20’ + lo C, a relative humidity *50% and particle filtration to lessthan 2 pm. All of the diesand siliconespecimens were conditioned for a minimum of 24 hours.
RESULTS The measurementsof contact angle and indentation hardnesswere studied in a three-way MANOVA (Table I). Once the existenceof differenceswasestablished,multiple comparisonswereusedto establishwhere thesedifferences lay by using Tukey’s method for multiple comparisons.12 Repeatability studies for intraexaminer variation were conducted for both contact angleand indentation hardness measurements.In the wettability study every tenth measurement was repeated and subjected to a correlation analysisand Student’s paired t test. The correlation coefficient (r = 0.994) was highly significant (p < 0.001) and the Student paired t-test showedno significant difference (p = 0.2567)betweenthe two setsof readings.These measurementswererepeatedwithout intervening time because water droplets change shape quickly and memory could APRIL
19~20
VOLUME
63
NUMBER
4
FACIAL
PROSTHETIC
Table
III.
ELASTOMER.
Indentation
PART
III
hardness of silicone specimens cured against five different treatment surfaces* Stone + soap
Lapped metal
Stone untreated
50.18
51.35
51.38
Mean (Shore-A units) ‘Means
on same line
showed
no statistically
significant
stone + C.M. Seal
Stone + Masque
52.03
52.16
difference.
IV. Mean indentation hardness (in Shore-A units) for material and treated surface
Table
(degrees)
Material
Polished metal Stone untreated Stone + soap Stone + C.M. Seal Stone + Masque
Molloplast-B
Cosmesil
56.56 56.53 54.93 56.93 57.16
46.13 46.23 46.70 27.13 47.16
thus influence the observed correlation. For the indentation hardness data one batch of 10 samples was selected and measurements were made on them. These measurements were repeated five times and an additional batch of samples was measured between each repetition. The measurements exhibited such a high degree of repeatability that it was not necessary to use further intraoperator variability testing. For both the wettability and the indentation hardness studies intraoperator variability was not a significant factor. In the three-way MANOVA the hardness and wettability variables were studied for the three main effects: material, treated surface, and die. The levels of the main-effects material and treated surface were significantly different for both hardness and wettability (Table I).
Comparison
of two materials
The materials differed significantly for hardness and wettability (Table II, Fig. 1). The mean Shore-A hardness of Molloplast-B material was 9.75 units higher than that of Cosmesil material. The mean contact angle of MolloplastB material was 3.22 degrees higher than that of Cosmesil material.
Comparison surfaces
of the treated
investment
Hardness of the specimens was investigated to examine the effects of their being cured against the five different treatment surfaces (Table III, Fig. 2). Dental stone treated with soap yielded the softest elastomers whereas stone separated with Cold Mould Seal or Masque materials yielded harder elastomers. Because the main-effects material and treated surface interacted significantly (p < 0.001) with respect to indentation hardness, the main effects could not be interpreted alone. The mean hardness of CosTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
Stone
Stone + Soap
Stone + Masque
Metal
Stone CXvlTSeal
Fig. 4. Wettability of silicone specimens for five different treatment surfaces. mesil and Molloplast-B materials for the five different surface treatments were recorded (Table IV, Fig. 3). It is evident that interaction occurred between Cosmesil material and the sodium alginate separator, which yielded a much lower hardness than did the other treatments. The effect of the five surface treatments on the contact angle (wettability) was also investigated (Table V, Fig. 4). Silicone specimens cured against stone separated with Masque material did not differ significantly from those processed against stone separated with soap. These treatments produced wettabilities that were significantly higher than silicone surfaces processed directly against dental stone. The application of the sodium alginate separator to dental stone yielded silicone specimens with the highest wettability, which was significantly higher statistically than the wettability of the lapped martensitic steel die surface. No significant interactions occurred between the main effects with regard to wettability.
DISCUSSION Hardness Molloplast-B material was the harder of the two materials tested, a mean 9.75 Shore-A indentation units harder than Cosmesil material (Table II). Sweeney et al.’ considered a desirable range of hardness to be 48 to 52 Shore-A units. Lewis and Castleberry stated that 25 to 35 Shore-A units were ideal but 10 to 45 units were acceptable. Conroy et al.* considered 25 to 55 Shore-A units was the correct range of hardness. Cosmesil material had a mean Shore-A hardness of 46.67 units whereas that of Molloplast-B material was 56.42 units. Cosmesil material therefore had a hardness approaching the top of the desired scales. By 469
VRRES,
WOLFAARDT,
AND
BECKER
Table V. Mean external contact angles (degrees) of water droplets on silicone specimens cured against five different treatment surfaces* Stone untreated
Mean *Me&s
0.90
on same Line showed no s~tiati~l~
significant
Stone + soap
Stone + Masque
4.49
5.94
Stone iCM. Seal
7.16
11.60
difference.
varying the degree of cross-linker in Cosmesilmaterial a softer formulation can be obtained. Molloplast-B material can only be cured to a single consistency. The various methods for fiaaking the silicone samples produced a slight variation in the indentation hardnessof the samples(Fig. 2). The combinedmeansof the two elastomers rangedin hardnessbetween50.18 and 52.16ShoreA units (Table III). No difference wasfound in the hardnessof the samplescured againstpolishedmetal, untreated stone,and stonesealedwith Cold Mould Sealmaterial. The samplescured against soapedstone were softer and those cured against Masque-treated stone were harder. When the two materials were considered separately, Cosmesilwasmuch softer when cured againstCold Mould Seal-coated stone (Fig. 3). When processedin this way in Part II of the series,the Cosmesilmaterial wasalsomuch rougher with respect to two of the parameters, F&m and W,.l” This unusual behavior was statistically significant and was causedby interaction between the Cosmesilmaterial and the surface against which it wasprocessed.The Cosmesilmaterial wassoftened and roughenedby contact with stone and Cold Mould Seal material during processing. Becauseprocessingagainstplain stone did not causea similar effect, it may be presumedthat the interaction was causedby the Cold Mould Seal material.
Wettability Although a high correlation was obtained between repeated readings,measuringthe contact angle was a difficult procedure. The problems encountered in the measuring of contact angleshas been attributed to variations in surfacetexture, presenceof poresor creviceson the surface of a solid, and contamination of the surface.” In the comparison of the two silicones,Molloplast-B had higher wettability with a contact angle 3.22 degreeshigher than that of the Cosmesilmaterial. With regard to the effect of surface treatments, stone and Cold Mould Seal material produced the highest wet~bilities on the surface of the siliconesteeted. From the study of surfacetexture,lO it was found that stone treated with Cold Mould Seal material contributed to poor primary and secondarysurfacetexture. Secondary surface texture was poorly affected when this separatorwasusedin the processingof Cosmesilmaterial. The results of the present study show that dental stone treated with soapor siliconegreaseresulted in the silicones
470
Polished surface
having wettabilities that were not significantly different (Table IV), The present investigation indicates that, in terms of hardnessand wettability, Cosmesiland Molloplast-B materials are unlikely to differ greatly in their potential to produce tissue trauma by mechanical abrasion. Cosmesil material had lower wettability whereasMolloplast-B material had higher hardness.In Part II of the study, the surface textures of Cosmesiland ~o~opl~t-B materials were similar in their potential to causesurface trauma. To determine the potential of a material to cause mechanical abrasivetrauma to surface tissues,the features of surface texture, hardness, and wettability should be considered together. On this basis,Mollop~t-B and Cosmesilmaterials seemto have similar potential for producing surface damage. From the results of the present seriesof studies,it is evident that the performance of Coemesilmaterial could be enhancedby increasingthe wettability of the surface. The study of surfacetexture indicated that the most acceptable features were achieved with a silicone greaseseparator. In this study the best resulta in terms of hardnessand wettability were achieved with a soapor silicone greaseseparator, with soap producing a marginally better result. On balance,the useof a siliconegreaseseparatorproduced the best com~ma~on of surface texture, hardness,and wettability when silicone was processedagainst a dental stone.
SUMMARY Molloplast-B is a well-tried resilient denture liner used in the construction of maxillofacial intraoral and extraoral prostheses.Cosmesilfacial prosthetic elastomer had surface characteristics as determined by surface texture, hardness,and wettability studiesthat did not differ markedly from that of Molloplast-B material. In terms of potential to prod&e mechanicalabrasionof tissues,it appearsthat Cosmesilmaterial may be usedboth extraorally and intraorally. The specific application of Cosmesilfacial prosthetic elastomerdemandsthat it will often be usedagainst compromised tissues.It is important, therefore, that biocompatability testing be conducted to enableintraoral and extraoral applications of the material with confidence. The results of bioeompa~bility testing will be presented in a future publication.
APRIL
1330
VOLUME
63
NUMBER
4
REFEREliJCES 1. Sweeney WT, Fischer TE, Castleberry DJ, Cowperthwaite BS. Evaluation of improved maxillofacial prosthetic materials. J PR~~TH~Z DENT
8.
1972;27:297-305.
2. Lewis DH, Castleberry DJ. An assessment of recent advances in external maxillofacial materials. J PROSTHET DENT 1980,43:426-32. 3. Wolfaardt JF, Chandler HD, Smith HA. Mechanical properties of a new facial prosthetic material. J PROSTHET DENT 1985;53:228-34. 4. Conroy BF, Haylock C, Hulterstrom A, Pratt G, Winter R. Report on a four year research and development programme involving the Institute of Maxillofacial Technology and the University of Wales Institute of Science and Technology aimed at the production of a new facial prosthetic system. In: Conroy BF, ed. Proceedings of the Institute of Maxillofacial Technology London: 1979;218-45. 5. Veres EM, Wolfaardt JF, Becker PJ. An evaluation of the surface characteristics of a facial prosthetic elastomer. Part I: A review of the literature on the surface characteristics of dental materials with maxillofacial prosthetic application. J PROSTHET DENT 1990;63:192-7. 6. Bates JF, Smith DC. Evaluation of indirect resilient liners for dentures. J Am Dent Assoc 1965;70:344-51. 7. Louka AN, Gesser HD, Kasslof 2. A laboratory evaluation of the effect
I
I
JOURNAL
10.
11. 12.
Reprint requests tot DR. JOHN F. WOLFAARDT FACULTY OF DENTISTRY UNIVERSITY OF ALBERTA EDMONTON, ALTA. T6G 2N8 CANADA
Bound volumes available
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Bound volumesof the JOURNAL OF PROSTHETIC DENTISTRY are available to subscribers (only) for the 1989issuesfrom the publisher at a cost of $44.00($56.00international) for Vol. 61 (January-June) and Vol. 62 (July-December). Shipping chargesare included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issuein the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Volumes 59 and 60 are also available. Payment must accompany all orders. Contact The C. V. Mosby Co., Circulation Department, 11830Westline Industrial Drive, St. Louis, MO 63146-3318,USA, phone (BOO)3254177, ext. 7351. Subscriptions must be in force to qualify. Bound volumes are not available in place of a regular JOURNAL subscription.
I
THE
9.
of two surface-wetting treatments on soft denture liners. J Dent Res 1977;56:953-9. Murray CG. A resilient lining material for the retention of maxillofacial prostheses. J PROSTHET DENT 1979;42:53-7. Udagama A, King GE. Mechanically retained facial prostheses: helpful or harmful? J PROSTHET DENT 1983;49:85-6. Veres EM, Wolfaardt JF, Becker PJ. An evaluation of the surface characteristics of a facial prosthetic elastomer. Part II: The surface texture. J PROSTHET DENT 1990,63:325-31. Retief DF. The direct bonding of orthodontic attachments [Thesis]. Johannesburg: University of Witwatersrand, 1975. Guenther WC. Analysis of variance. 1st ed. New Jersey: Prentice Hall, 1985:54-7.
1 I
1
OF PROSTHETIC
DENTISTRY
471
of Witwatersrand, School of Dentistry, Council, Johannesburg, South Africa
of a facial and hardness
and
and Institute
for Biostatistics,
Medical
Silicone facial prosthetic elastomers may cause tissue damage by abrasion. Such damage is a particular concern when prostheses are mechanically retained against tissues compromised by adjunctive therapy. The hardness and wettability of Cosmesil material was compared with that of Molloplast-B material. The stone test surfaces were separated with soap, sodium alginate, silicone paste, and left untreated. A polished stainless steel surface was prepared as a control. The specimens of Cosmesil and Molloplast-B materials were processed against each of these surfaces. Ten specimens of each material were processed against the five different surfaces. Wettability was evaluated by measuring the contact angle with a profile projector. Indentation hardness was measured with a Shore-A durometer. Statistical analysis involved multiple analyses of variation and Tukey’s procedures (in all cases p < 0.06). Molloplast-B material was found to have a higher wettability than Cosmesil material (n = 3.22 degrees higher); sddium alginate separator yielded silicone specimens with the highest wettability; Molloplast-B material was found to be harder than Cosmesil material (x = 9.75 Shore-A indentation units harder). The softest silicones were processed with soap separator. Silicone grease yielded the hardest specimens. The mechanical performance of Cosmesil material would be enhanced by increasing the surface wettability. The hardness of Cosmesil material is within the ideal range for a maxillofacial prosthetic elastomer. (J
PROSTEIETDENT~~~~;~~:~~~-~~.)
I
ndentation hardnesshas been used as one of the mechanical tests applied to facial prosthetic materials.lT3 The indentation hardness of a silicone facial prosthetic material that is analogousto the softnessof living tissueis in the range of 25 to 55 Shore-A indentation unit.v.2*4 Hardness,surface texture, and wettability will determine the mechanicalpotential of a material to produce damage to tissue surfaces. A review of the literature revealedthat little researchhas been conducted on the wettability of facial prosthetic materials or resilient denture liners.5Bates and Smith6 and Louka et a1.7have discussedthe poor wettability of silicone elastomers.Murrap as well as Udagama and King9 have shownthe potentially damagingabrasive effect of silicones on tissues.It is important to consider abrasive potential when evaluating an elastomerthat will have prolonged tissuecontact. The abrasivepotential shouldbe measuredfor hardnessand wettability in relation to the surfacetexture.
*Lecturer, Department. of Prosthetic Dentistry. **Associate Professor, Division of Removable Prosthodontics, University of Alberta, Faculty of Dentistry, Edmonton, Canada. ***Specialist Statistician, Institute for Biostatistics, Medical Research Council, Johannesburg, South Africa.
1OlIl15333
466
In this study the wettability and ind,entation hardnessof Cosmesil(Cosmedica,U.W.I.S.T., Cardiff, Wales, U.K.) material wascomparedwith that of Molloplast-B (Regneri & Co., Karlsruhe, West Germany) material. The wettability and indentation hardnesswas then related to the previously establishedsurface texture of the two materials.fO The potential abrasivenessof Cosmesilmaterial was also comparedwith that of Molloplast-B, a resilient liner with a well-establishedclinical profile.
MATERIAL
AND
METHODS
The technique used to produce specimensof Cosmesil and Molloplast-B materials wasdescribedin Part II of this seriesof investigations.lo The technique used five truncated conical metal dies with a surface 2 cm in diameter that had beenpolishedto a mirror finish. The dieswere invested in dental stone under controlled conditions with standardized water/powder ratios and vacuum spatulation. The surface of the dental stone was prepared in various ways sothat the siliconeelastomerswere processedagainst the following materials: 1. Untreated stone with no separator 2. Stone treated with a soap solution (Modellglanzer, Dentaurum Pforzheim, West Germany) 3. Stone treated with sodium alginate separator (Cold Mould Seal, P.G. Smith, Johannesburg,South Africa)
APRIL
1990
VOLUME
63
NUMBER
4
FACIAL
PROSTHETIC
Table
I. &kdtiule analysis of variance Source
ELASTOMER.
PART
III
of variation*
Parameter?
Degrees
of freedom
Die Treated surface
Hardness 1st Wettability 2nd Wettability
4and50 4 and 50 4 and 50 4 and 50 4 and 50 4 and 50
RS Rnax wt Hardness 1st Wettability 2nd Wettability
1 and 50 1 and 50 1 and 50 1 and 50 land50 1 and 50
R3 R mex wt Hardness 1st Wet~bility 2nd Wettability
4and50 4 and 50 4 and 50 4 and 50 4 and 50 4and.50
R, R max
wt
Material
Die X Treated surface Die X Material Treated surface X Material
Die X treated surface X material *Main effects were tested with the likelihood ratio. tParameten were tested with F-test using following Error Re Rmax wt Hardness 1st Wettability 2nd Wettability
Mean
square 0.042 25.514 34.887 0.648 28.058 32.716
of wettability
Wettability wasmeasuredby evaluating the contact anbetween the surface of a drop of liquid and the surfaceof the solid on which it wasresting. A high contact angle indicates high wettability; a low contact angle indicates low wettability. Uncontaminated high-energy surfacessuch asmetalsare wet completely by pure liquids and contact anglesof zero are obtained. Low energy solidssuch gle formed
‘i-BE
JOURNAL
OF FROSl%ETlC
0.104
mean square errors:
4. Stone treated with silicone separating grease(Masque, Harry J. Bosworth Co., Skokie, Ill.) 5. The lapped metal surfacesin the gypsum molds The experimental design allowed for 10 specimensof each material to be processedagainst the five different surfaces.All of the silicone elastomersused were of the samelot and batch numbers. The flaskswere closedunder standard pressuresof’1000 Kpa. All materials were processedaccording to the manufacturers’ instructions. After processing,the specimenswere recovered and stored in airtight containers.
Investigation
p-Value
DENTISTRY
as organic polymers show a low contact angle and poor wettabi1ity.l” A micropipette (Labystems Oy, Helsinki, Finland) was used to dispensetwo drops (5 ~1 each} of distilled water onto the surface of the samples.It must be noted that the micropipette is not absolutely accurate for such small quantities. However, it wasnot essentialfor the dropsto be exactly the samesizebecausethe magnitude of the contact angle is independent of the size of the drop for solidswith a uniform
surface.11
To achieve greater precision a secondwettability study wasconducted in which the siliconespecimenwasslicedin half with a scalpelblade. The drop of water wasplaced as near as possibleto the edgeof the cut to facilitate reading of the contact angle.After the results from the secondwettability study were reported, the samplewasplaced on the adjustablestageof a profile projector (Hilger & Watts Ltd., Hilger Division, London, England), which projected a beam of light past the sample.By a seriesof mirrors and lensesthe beamwas directed onto a circular ground glass
467
VERES,
WOLFAARDT.
AND
BECKER
(Shore-A) Molloplast-B Cosmesil
Wettability
Fig.
Hardness
Metal
1. Comparisonof hardnessand wettability.
Stone
Stone + Soap
Stone + C.M.Seal
Stone + Masque
3. Means of hardness (Shore-A indentation units) for material and treated surface. Fig. (Shore-A)
-
II. Comparisonof indentation hardnessand mean contact angles Table
-
Material
Molloplast-B
-
-
Stone
+
Metal
Stone
Soap
Stone
Stone
C.M.Seal
Masque
+
46,67
7.75
4.53
+
2. Shore-A hardnessof siliconesamplescured against five different treatment surfaces. Fig.
viewer where a magnifiedsilhouette imagewasproduced of the samplesurface and the drop of water. A x50 magnification was used for all of the samples. The horizontal axis on the screenwas aligned with the surface of the solid, and the deviation from the horizontal (0 degree) was read from the scalearound the periphery. The vertical axis wasaligned with the surface of the water drop at its contact with the solid, and the deviation from the vertical axis (0 degree) was noted. The first measurement was subtracted from the secondto give the contact angle,The angle measuredwas the external contact angle between the surface of the liquid and the surface of the solid. When the anglewaslessthan 90 degrees,the reading was a negative quantity and the drop was mushroom shaped. When the angle was more than 90 degreesthe reading was a positive quantity and the droplet was flattened. The contact angleswere measuredon both sides of each of two droplets on eachsample,thus acquiring four measurementsfor each sample.The meansof these measurementswere usedfor the analysisof the external conangle.
Investigation
of indentation
hardness
Indentation hardness was measured with a Shore-A durometer of the type specified by ASTM D2240-68 468
56,42
-
”
tact
Hardness(Shore-Aunits) Wettability (degrees)
Cosmesil
(American Society for Testing Materials, D2240-68:Standard method of test for indentation hardnessof rubber and plastics by meansof a durometer). The specimenswere placed on a granite optical flat. The pressor foot of the durometer wasapplied to the specimenasrapidly aspossible with just sufficient pressureto obtain a firm contact between the pressorfoot and the specimen.The scalewas read immediately. All measurementswere calculated in the controlled environment of a metrology laboratory with a temperature of 20’ + lo C, a relative humidity *50% and particle filtration to lessthan 2 pm. All of the diesand siliconespecimens were conditioned for a minimum of 24 hours.
RESULTS The measurementsof contact angle and indentation hardnesswere studied in a three-way MANOVA (Table I). Once the existenceof differenceswasestablished,multiple comparisonswereusedto establishwhere thesedifferences lay by using Tukey’s method for multiple comparisons.12 Repeatability studies for intraexaminer variation were conducted for both contact angleand indentation hardness measurements.In the wettability study every tenth measurement was repeated and subjected to a correlation analysisand Student’s paired t test. The correlation coefficient (r = 0.994) was highly significant (p < 0.001) and the Student paired t-test showedno significant difference (p = 0.2567)betweenthe two setsof readings.These measurementswererepeatedwithout intervening time because water droplets change shape quickly and memory could APRIL
19~20
VOLUME
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NUMBER
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FACIAL
PROSTHETIC
Table
III.
ELASTOMER.
Indentation
PART
III
hardness of silicone specimens cured against five different treatment surfaces* Stone + soap
Lapped metal
Stone untreated
50.18
51.35
51.38
Mean (Shore-A units) ‘Means
on same line
showed
no statistically
significant
stone + C.M. Seal
Stone + Masque
52.03
52.16
difference.
IV. Mean indentation hardness (in Shore-A units) for material and treated surface
Table
(degrees)
Material
Polished metal Stone untreated Stone + soap Stone + C.M. Seal Stone + Masque
Molloplast-B
Cosmesil
56.56 56.53 54.93 56.93 57.16
46.13 46.23 46.70 27.13 47.16
thus influence the observed correlation. For the indentation hardness data one batch of 10 samples was selected and measurements were made on them. These measurements were repeated five times and an additional batch of samples was measured between each repetition. The measurements exhibited such a high degree of repeatability that it was not necessary to use further intraoperator variability testing. For both the wettability and the indentation hardness studies intraoperator variability was not a significant factor. In the three-way MANOVA the hardness and wettability variables were studied for the three main effects: material, treated surface, and die. The levels of the main-effects material and treated surface were significantly different for both hardness and wettability (Table I).
Comparison
of two materials
The materials differed significantly for hardness and wettability (Table II, Fig. 1). The mean Shore-A hardness of Molloplast-B material was 9.75 units higher than that of Cosmesil material. The mean contact angle of MolloplastB material was 3.22 degrees higher than that of Cosmesil material.
Comparison surfaces
of the treated
investment
Hardness of the specimens was investigated to examine the effects of their being cured against the five different treatment surfaces (Table III, Fig. 2). Dental stone treated with soap yielded the softest elastomers whereas stone separated with Cold Mould Seal or Masque materials yielded harder elastomers. Because the main-effects material and treated surface interacted significantly (p < 0.001) with respect to indentation hardness, the main effects could not be interpreted alone. The mean hardness of CosTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
Stone
Stone + Soap
Stone + Masque
Metal
Stone CXvlTSeal
Fig. 4. Wettability of silicone specimens for five different treatment surfaces. mesil and Molloplast-B materials for the five different surface treatments were recorded (Table IV, Fig. 3). It is evident that interaction occurred between Cosmesil material and the sodium alginate separator, which yielded a much lower hardness than did the other treatments. The effect of the five surface treatments on the contact angle (wettability) was also investigated (Table V, Fig. 4). Silicone specimens cured against stone separated with Masque material did not differ significantly from those processed against stone separated with soap. These treatments produced wettabilities that were significantly higher than silicone surfaces processed directly against dental stone. The application of the sodium alginate separator to dental stone yielded silicone specimens with the highest wettability, which was significantly higher statistically than the wettability of the lapped martensitic steel die surface. No significant interactions occurred between the main effects with regard to wettability.
DISCUSSION Hardness Molloplast-B material was the harder of the two materials tested, a mean 9.75 Shore-A indentation units harder than Cosmesil material (Table II). Sweeney et al.’ considered a desirable range of hardness to be 48 to 52 Shore-A units. Lewis and Castleberry stated that 25 to 35 Shore-A units were ideal but 10 to 45 units were acceptable. Conroy et al.* considered 25 to 55 Shore-A units was the correct range of hardness. Cosmesil material had a mean Shore-A hardness of 46.67 units whereas that of Molloplast-B material was 56.42 units. Cosmesil material therefore had a hardness approaching the top of the desired scales. By 469
VRRES,
WOLFAARDT,
AND
BECKER
Table V. Mean external contact angles (degrees) of water droplets on silicone specimens cured against five different treatment surfaces* Stone untreated
Mean *Me&s
0.90
on same Line showed no s~tiati~l~
significant
Stone + soap
Stone + Masque
4.49
5.94
Stone iCM. Seal
7.16
11.60
difference.
varying the degree of cross-linker in Cosmesilmaterial a softer formulation can be obtained. Molloplast-B material can only be cured to a single consistency. The various methods for fiaaking the silicone samples produced a slight variation in the indentation hardnessof the samples(Fig. 2). The combinedmeansof the two elastomers rangedin hardnessbetween50.18 and 52.16ShoreA units (Table III). No difference wasfound in the hardnessof the samplescured againstpolishedmetal, untreated stone,and stonesealedwith Cold Mould Sealmaterial. The samplescured against soapedstone were softer and those cured against Masque-treated stone were harder. When the two materials were considered separately, Cosmesilwasmuch softer when cured againstCold Mould Seal-coated stone (Fig. 3). When processedin this way in Part II of the series,the Cosmesilmaterial wasalsomuch rougher with respect to two of the parameters, F&m and W,.l” This unusual behavior was statistically significant and was causedby interaction between the Cosmesilmaterial and the surface against which it wasprocessed.The Cosmesilmaterial wassoftened and roughenedby contact with stone and Cold Mould Seal material during processing. Becauseprocessingagainstplain stone did not causea similar effect, it may be presumedthat the interaction was causedby the Cold Mould Seal material.
Wettability Although a high correlation was obtained between repeated readings,measuringthe contact angle was a difficult procedure. The problems encountered in the measuring of contact angleshas been attributed to variations in surfacetexture, presenceof poresor creviceson the surface of a solid, and contamination of the surface.” In the comparison of the two silicones,Molloplast-B had higher wettability with a contact angle 3.22 degreeshigher than that of the Cosmesilmaterial. With regard to the effect of surface treatments, stone and Cold Mould Seal material produced the highest wet~bilities on the surface of the siliconesteeted. From the study of surfacetexture,lO it was found that stone treated with Cold Mould Seal material contributed to poor primary and secondarysurfacetexture. Secondary surface texture was poorly affected when this separatorwasusedin the processingof Cosmesilmaterial. The results of the present study show that dental stone treated with soapor siliconegreaseresulted in the silicones
470
Polished surface
having wettabilities that were not significantly different (Table IV), The present investigation indicates that, in terms of hardnessand wettability, Cosmesiland Molloplast-B materials are unlikely to differ greatly in their potential to produce tissue trauma by mechanical abrasion. Cosmesil material had lower wettability whereasMolloplast-B material had higher hardness.In Part II of the study, the surface textures of Cosmesiland ~o~opl~t-B materials were similar in their potential to causesurface trauma. To determine the potential of a material to cause mechanical abrasivetrauma to surface tissues,the features of surface texture, hardness, and wettability should be considered together. On this basis,Mollop~t-B and Cosmesilmaterials seemto have similar potential for producing surface damage. From the results of the present seriesof studies,it is evident that the performance of Coemesilmaterial could be enhancedby increasingthe wettability of the surface. The study of surfacetexture indicated that the most acceptable features were achieved with a silicone greaseseparator. In this study the best resulta in terms of hardnessand wettability were achieved with a soapor silicone greaseseparator, with soap producing a marginally better result. On balance,the useof a siliconegreaseseparatorproduced the best com~ma~on of surface texture, hardness,and wettability when silicone was processedagainst a dental stone.
SUMMARY Molloplast-B is a well-tried resilient denture liner used in the construction of maxillofacial intraoral and extraoral prostheses.Cosmesilfacial prosthetic elastomer had surface characteristics as determined by surface texture, hardness,and wettability studiesthat did not differ markedly from that of Molloplast-B material. In terms of potential to prod&e mechanicalabrasionof tissues,it appearsthat Cosmesilmaterial may be usedboth extraorally and intraorally. The specific application of Cosmesilfacial prosthetic elastomerdemandsthat it will often be usedagainst compromised tissues.It is important, therefore, that biocompatability testing be conducted to enableintraoral and extraoral applications of the material with confidence. The results of bioeompa~bility testing will be presented in a future publication.
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63
NUMBER
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REFEREliJCES 1. Sweeney WT, Fischer TE, Castleberry DJ, Cowperthwaite BS. Evaluation of improved maxillofacial prosthetic materials. J PR~~TH~Z DENT
8.
1972;27:297-305.
2. Lewis DH, Castleberry DJ. An assessment of recent advances in external maxillofacial materials. J PROSTHET DENT 1980,43:426-32. 3. Wolfaardt JF, Chandler HD, Smith HA. Mechanical properties of a new facial prosthetic material. J PROSTHET DENT 1985;53:228-34. 4. Conroy BF, Haylock C, Hulterstrom A, Pratt G, Winter R. Report on a four year research and development programme involving the Institute of Maxillofacial Technology and the University of Wales Institute of Science and Technology aimed at the production of a new facial prosthetic system. In: Conroy BF, ed. Proceedings of the Institute of Maxillofacial Technology London: 1979;218-45. 5. Veres EM, Wolfaardt JF, Becker PJ. An evaluation of the surface characteristics of a facial prosthetic elastomer. Part I: A review of the literature on the surface characteristics of dental materials with maxillofacial prosthetic application. J PROSTHET DENT 1990;63:192-7. 6. Bates JF, Smith DC. Evaluation of indirect resilient liners for dentures. J Am Dent Assoc 1965;70:344-51. 7. Louka AN, Gesser HD, Kasslof 2. A laboratory evaluation of the effect
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11. 12.
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I
THE
9.
of two surface-wetting treatments on soft denture liners. J Dent Res 1977;56:953-9. Murray CG. A resilient lining material for the retention of maxillofacial prostheses. J PROSTHET DENT 1979;42:53-7. Udagama A, King GE. Mechanically retained facial prostheses: helpful or harmful? J PROSTHET DENT 1983;49:85-6. Veres EM, Wolfaardt JF, Becker PJ. An evaluation of the surface characteristics of a facial prosthetic elastomer. Part II: The surface texture. J PROSTHET DENT 1990,63:325-31. Retief DF. The direct bonding of orthodontic attachments [Thesis]. Johannesburg: University of Witwatersrand, 1975. Guenther WC. Analysis of variance. 1st ed. New Jersey: Prentice Hall, 1985:54-7.
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