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Effect of ageing on the bond strength of permanent denture soft

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									Journal of Oral Rehabilitation 2002 29; 992–996




Effect of ageing on the bond strength of a permanent
denture soft lining material
M. AL-ATHEL*, R. JAGGER† & D. JAGGER‡                                         *Department of Restorative Dentistry, King Saud University,
                      †
Riyadh, Saudi Arabia, Department of Adult Dental Health, University of Wales College of Medicine, Cardiff, U.K. and ‡Division of Restorative
Dentistry, Lower Maudlin Street, Bristol, U.K.




SUMMARY Molloplast-B is a widely used, long-term,                       had no significant effect on the measured bond
heat-cured silicone denture soft lining material.                       strength values. Longer immersion of specimens in
Adhesion failure of this material to poly (methyl                       water at 37 ± 1 ºC led to a significant reduction in
methacrylate) denture base material is a problem                        the measured tensile and shear bond strengths. The
that is encountered clinically. The purpose of this                     present study has demonstrated that the reduction
study was to investigate the effects of long-term                       in Molloplast-B bond strength that occurs as a result
immersion in water at 37 ± 1 ºC and of accelerated                      of long-term ageing in water at 37 ± 1 ºC can be
ageing in water at 50 ± 1 ºC on the tensile and shear                   achieved in a shorter period of time by ageing the
bond strength values of this denture resilient lining                   specimens in water at a higher temperature.
material bonded to a heat-cured denture base ma-                        KEYWORDS: denture soft lining material, bond
terial. Immersion in water for 1 week at 37 ± 1 ºC                      strength, ageing




                                                                        variation in methodology (Kutay, 1994; Al-Athel &
Introduction
                                                                        Jagger, 1996).
Soft lining materials are compliant, viscoelastic mate-                    The effect of ageing of soft lining materials has been
rials used to form part of the fit surface of a denture                  discussed (Amin et al., 1981; Wagner et al., 1995). The
and have been extensively reviewed (Braden, Wright                      effect of immersion in water on the bond strength of
& Parker, 1995). Adhesion failure of silicone denture                   soft lining materials has also been discussed. Ageing in
soft lining material to poly (methyl methacrylate)                      water has been reported to have no effect on the bond
denture base material is a problem that is encountered                  strength of a heat-cured silicone soft lining material
in clinical practice. Sufficient bond strength between                   (Molloplast-B*) to the denture base material (Wright,
the soft lining material and the acrylic resin denture                  1980; Amin & Ritchie, 1981). However, it has also been
base material is required to avoid interfacial separation               suggested that ageing in water can cause a marked
at the denture borders (Kutay et al., 1994). Adhesive                   reduction in Molloplast-B bond strength (Amin,
failure between the lining material and the denture                     Fletcher & Ritchie, 1981). It has been shown that the
base can create an environment for potential bacterial                  measured bond strength is highly dependent on test
growth and accelerated breakdown of the soft lining                     method (Kutay, 1994; Al-Athel & Jagger, 1996) and
material (Jacobsen et al., 1997). There have been                       could explain the differences in findings between
several studies on the bond strength of soft lining                     studies.
materials to denture base material including (Wood,                        Ageing can be accelerated by increasing the tem-
Johnson & Duncanson, 1993; Kutay, 1994; Jackley &                       perature of the solution in which the specimens are
Plummer, 1994; Emmer et al., 1995; Kawano et al.,                       immersed. Accelerated ageing is commonly used by the
1997; Baysan, Parker & Wright, 1998). However it is
difficult to compare results between studies because of                  *Regneri and Co., Karlsruhe, Germany.


ª 2002 Blackwell Science Ltd                                                                                                                   992
                                                          BOND STRENGTH OF SOFT LINING MATERIALS                                        993


rubber industry as a quality control procedure to study                    Shear specimens. Shear specimens consisted of Mollo-
the effect of ageing in a short period of time. The effect                 plast-B 10 · 10 · 2Æ5 mm bonded to two plates of
of accelerated ageing on the bond strength of resilient                    acrylic resin, each 50 · 10 · 3 mm. Acrylic resin plates
lining materials has not however, been investigated.                       were produced by packing Trevalon into pre-formed
   The aim of this study was to investigate the effects of                 dental moulds 50 · 60 · 3 mm using a dough mould-
ageing in water at 37 ± 1 °C for 6 months and accel-                       ing technique. The plates were then cut using a
erated ageing in water at 50 ± 1 °C for 1 month on the                     band saw machine into strips 50 · 10 · 3 m. These
tensile and shear bond strength values of Molloplast-B                     acrylic resin strips were invested into 50 ⁄ 50% stone
denture soft lining material bonded to a heat-cured                        and plaster mix, in a conventional dental flasking
acrylic resin denture base material.                                       technique, together with Perspex§ spacers 10 · 10 ·
                                                                           2Æ5 mm. The spacers were to provide space to pack
                                                                           Molloplast-B after they were removed. The bonded
Materials and methods
                                                                           acrylic resin surface was roughened with an acrylic
The resilient denture lining material used in this study                   bur and Molloplast-B primer was applied to the rough-
was Molloplast-B (Regneri and Co., Karlsruhe,                              ened surface. Liner packing and curing was according to
Germany) and the denture base material was a con-                          manufacturer’s instructions. After polymerization the
ventional heat-cured acrylic resin, Trevalon†.                             specimens were removed from the flask and liner flash
   Four test groups of specimens (n ¼ 10) were pro-                        was trimmed with scissors and a sharp blade.
duced for both tensile and shear tests. These groups
were stored in a water bath at a controlled temperature                    Test methods. All specimens were deformed at a rate of
of 37 ± 1 °C for 1 week, 1, 3 and 6 months, respect-                       20 mm min)1 using a Lloyd Instruments Testing Ma-
ively.                                                                     chine¶ which was linked to an IBM compatible com-
   Four further test groups of specimens (n ¼ 10) were                     puter. Mean and standard deviations were calculated
produced for both tensile and shear tests and these were                   for all test results. The tensile and shear bond strengths
stored in a water bath thermostatically controlled at                      were calculated as follows:
50 ± 1 °C for similar periods of time. A further group of
                                                                           Bond strength ¼ Maximum load (N)/
specimens (n ¼ 10) were produced for both tensile and
                                                                                                    Cross-sectional area ðmm2 Þ
shear tests and was used as the control.
                                                                              The type of failure was assessed visually and was
                                                                           recorded as being cohesive (through the lining mater-
Specimen preparation
                                                                           ial), adhesive (at the interface of lining material and
Tensile specimens. Tensile specimens consisted of two                      denture base) or mixed (cohesive and adhesive).
segments of acrylic, each 30 · 6 · 6 mm and a central
segment of Molloplast-B 6 · 6 · 3 mm. Acrylic resin
                                                                           Results
blocks were produced by packing Trevalon into pre-
formed moulds 66 · 6 · 6 mm using a dough moulding                         The effects of ageing specimens in water at 37 ± 1 °C
technique. A central section 3 mm thick was cut out from                   and 50 ± 1 °C on tensile bond strength are given in
each block using a band saw machine (Model BK3                             Table 1. The effects of ageing specimens in water at
MK.II‡) to provide space to pack Molloplast-B. The                         37 ± 1 °C and 50 ± 1 °C on shear bond strength are
surfaces of the acrylic blocks to be bonded to the liner                   given in Table 2.
were roughened with an acrylic bur and Molloplast-B                           The results in Tables 1 and 2 were subjected to
primer was applied to the roughened surface. Liner                         statistical analysis using a one-way analysis of variance
packing and curing was according to manufacturer’s                         (ANOVA) and where appropriate the Scheffe test.
instructions. After polymerization the specimens were                         Statistical analysis of the results revealed that there
removed from the flask and liner flash was trimmed with                      was no significant difference between the measured
scissors and a sharp blade.                                                tensile bond strength of the control specimens and

†                                                                          §
Dentsply Ltd, Weybridge, U.K.                                              ICI plc, Welwyn Garden City, U.K.
‡                                                                          ¶
Burgess Power Tools Ltd, Sapcote, U.K.                                     Lloyd Instruments Ltd, Fareham, Hants.


ª 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 992–996
994   M . A L - A T H E L et al.

      Table 1. Effect of ageing specimens (n ¼ 10) in water at 37 and         further weakening effect on the bond strength although
      50 °C on tensile bond strength (N mm)2)                                 not significantly so. Failure was more toward cohesive
                                                                              in the first group and more toward adhesive in the
                      Mean                      Mean
                      N mm)2       s.d.         N mm)2        s.d.            remaining two groups.
      Specimen        37 °C        37 °C        50 °C         50 °C              Statistical analysis of shear test results showed that
                                                                              there was no significant difference between the meas-
      Control          1Æ72         0Æ07         1Æ72             0Æ07
      1 week           1Æ70         0Æ11         1Æ54             0Æ07        ured shear bond strength of the control specimens and
      1 month          1Æ55         0Æ06         1Æ47             0Æ07        specimens immersed in water for 1 week at 37 ± 1 °C.
      3 months         1Æ47         0Æ09         –                –           A highly significant decrease in Molloplast-B shear
      6 months         1Æ46         0Æ07         –                –           bond strength was demonstrated in specimens satur-
      A one way analysis of variance indicated that there was a               ated in water at 37 ± 1 °C for 1, 3 and 6 months,
      significant difference between some groups P < 0Æ0001.                   compared with the control.
      A Scheffe test was performed and the vertical tie bar in the table         Cohesive failure was observed only in some of the
      indicates values which are not significantly different between one       control specimens. Adhesive and adhesive ⁄ cohesive
      another.
                                                                              failures were the types of failures that were observed
                                                                              among the remaining specimens. In specimens satur-
                                                                              ated in water for 6 months the tendency was divided
      Table 2. Effect of ageing specimens (n ¼ 10) in water at 37 and
      50 °C on shear bond strength (N mm)2)                                   equally between cohesive and adhesive ⁄ cohesive fail-
                                                                              ures.
                   Mean                      Mean                                Statistical analysis of results showed that there was a
                   N mm)2      s.d.          N mm)2       s.d.                statistically significant decrease in shear bond strength
      Specimen     37 °C       37 °C         50 °C        50 °C
                                                                              for specimens immersed in water at 50 ± 1 °C for a
      Control       1Æ39        0Æ07          1Æ39        0Æ07                period of 1 week and 1 month compared with the
      1 week        1Æ34        0Æ07          1Æ23        0Æ07                control. The reduction in the shear bond strength of
      1 month       1Æ20        0Æ05          0Æ76        0Æ05
                                                                              specimens aged in water for 1 week and 1 month was
      6 months      0Æ80        0Æ04
      3 months      0Æ78        0Æ05
                                                                              also associated with increased tendency towards adhe-
                                                                              sive rather than cohesive failure.
      A one way analysis of variance indicated that there was a
      significant difference between some groups P < 0Æ0001.
      A Scheffe test was performed and the vertical tie bar in the table      Discussion
      indicates values which are not significantly different between one
      another.                                                                Several different types of laboratory tests are available
                                                                              to investigate the bond strength of soft linings to
                                                                              denture base materials. The most commonly used tests
      specimens saturated in water at 37 ± 1 °C for 1 week.                   till date are the peel, tensile and shear tests. Silicone
      Further ageing the specimens in water for 1 month                       lining materials have low tear strength and resist
      decreased measured tensile bond strength significantly                   stresses approaching the elastic modulus in the peel
      compared with the control. There was no significant                      test without debonding. They show more cohesive
      difference in the measured tensile bond strength                        failure (failure within the material) with a peel test
      between specimens aged for 1, 3 and 6 months.                           than with a tensile or shear test (Kutay, 1994; Al-Athel
      Cohesive failure was observed in some of the control                    & Jagger, 1996). The adhesive strength of Molloplast-B
      specimens that were stored dry and in some specimens                    is therefore possibly best investigated by tensile and
      that were saturated in water for 1 week, while adhesive                 shear testing. In the present study the effect of ageing
      and mixed types of failure were demonstrated through-                   on Molloplast-B bond strength was evaluated using
      out all the remaining specimens.                                        tensile and shear tests.
         Statistical analysis of tensile test results showed that                Bond strength test values should be interpreted with
      measured tensile bond strength of specimens immersed                    caution where specimens fail cohesively or exhibit
      in water for 1 week at 50 ± 1 °C decreased significantly                 mixed adhesive ⁄ cohesive failures. Precise bond
      compared with the control. Increasing the period of                     strength values cannot be given but rather it can be
      immersion at this temperature to 1 month had a                          said that the bond strength is greater than the failure

                                                                           ª 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 992–996
                                                          BOND STRENGTH OF SOFT LINING MATERIALS                                                   995


value. The results showed that short-term storage                          similar findings for dentures with soft linings which
(1 week) in water had no significant effect on measured                     had been present for at least 18 months. Wright (1986)
tensile and shear bond strengths, while long-term                          however, reported an increase in the hardness of 50%
storage decreased the measured bond strength signifi-                       of Molloplast-B liners that were involved in a 3-year
cantly. In addition, there was an increased tendency for                   longitudinal study.
adhesive failure (failure between lining material and                         The finding of the present study that the recorded
denture base material) to occur in both tensile and                        bond strength value was significantly reduced by long-
shear specimens as the period of immersion in water                        term exposure to water is in agreement with the
was increased. For example, after 1 week of saturation                     findings of several previous studies (Amin & Ritchie,
in water at 37 ± 1 °C, only two tensile specimens                          1981; Amin et al., 1981 and Polyzois, 1992). Wright
showed adhesive failure. This number increased to four                     (1980) and Towayer (1985), reported that water
after saturating the specimens for 1 month. After 3 and                    appeared to have little or no effect on the peel bond
6 months of ageing, adhesive failure was observed in                       strength of resilient lining materials. The effect of
five specimens. This failure pattern was also demon-                        ageing in water on Molloplast-B peel bond strength,
strated by shear test specimens. This change in the                        was not investigated in this study because previously it
nature of bonding failure was also observed by Amin                        had been shown that Molloplast-B bond failure in
et al. (1981).                                                             peeling is almost exclusively cohesive (Wright, 1986;
   Several mechanisms have been suggested in order                         Jepson et al., 1993).
to explain the effect of immersion in water on                                The present study has demonstrated that the reduc-
the measured bond strength. For example, that the                          tion in Molloplast-B bond strength that occurs as a
absorbed water may directly lead to swelling of the                        result of long-term ageing in water at 37 ± 1 °C can be
lining material and stress concentration at the bond                       achieved in a shorter period of time by ageing the
interface, which would cause reduction in the bond                         specimens in water at a higher temperature. This may
strength or the absorbed water may indirectly change                       be of value in further investigations of the bond
the viscoelastic properties of the lining material. This                   strength of silicone soft lining materials.
could possibly lead to stiffening of the material and
therefore any damaging load would be transmitted to
                                                                           Conclusions
the bond site instead of being absorbed by the liner
(Amin et al., 1981). It is possible that the absorbed                      Immersion in water for 1 week at 37 ± 1 °C had no
water could hydrolyse the cross-links that form the                        significant effect on the measured bond strength values.
adhesive bond (Towayer, 1985). The water absorption                           Longer immersion of specimens in water at 37 ± 1 °C
properties of resilient lining materials have been inves-                  led to a significant reduction in the measured tensile
tigated previously and it has been demonstrated that                       and shear bond strengths.
Molloplast-B had a low level of both water absorption                         The present study has demonstrated that the reduc-
and water solubility (Amin et al., 1981) with water                        tion in Molloplast-B bond strength that occurs as a
absorption at about two per cent at equilibrium. These                     result of long-term ageing in water at 37 ± 1 °C can be
findings would suggest that it is unlikely that the                         achieved in a shorter period of time by ageing the
reduction in Molloplast-B bond strength after ageing in                    specimens in water at a higher temperature.
water would be caused by swelling of the liner because
of water absorption. Wright (1980) reported a high
                                                                           References
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ª 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 992–996
996   M . A L - A T H E L et al.

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        123.                                                                        E-mail: d.c.jagger@bris.ac.uk




                                                                                 ª 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 992–996

								
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