J. Dent. 1991; 19: 249-254

249

Repairability of type II glass polyalkenoate (ionomer) cements* D. G. Charlton, Department

D. F. Murchison

of Dental Materials,

and B. K. Moore Indiana University

School of Dentistry,

Indianapolis,

USA

ABSTRACT Two Type II glass polyalkenoate (ionomer) cements were evaluated for their repairability by measuring tensile

bond strength of new material added to previously placed cement. Variables included time of repair (20 min vs. 24 h) and surface condition prior to repair (smooth and unetched, smooth and etched, rough and unetched, rough and etched). Failures were evaluated using light microscopy and categorized as cohesive, adhesive, or mixed. Scanning electron microscopy was employed to view the cement surfaces following each of the four pretreatments. Surface pretreatment was found to significantly affect both materials when repaired at 20 min (P < 0.05). The highest bond strength was achieved when bonding to the smooth, unetched surface. Surface pretreatment did not significantly affect bond strength when the cements were repaired at 24 h (P > 0.05). Tensile bond strengths were greater for repairs made at 20 min than for those made at 24 h, but the differences were not always statistically significant KEY WORDS: J. Dent. 1991; 1991)

Glass polyalkenoate (ionomer) cements, Properties, Repairs 19: 249-254

(Received 21 December 1990;

reviewed 4 February 1991;

accepted 14 March

Correspondence should be addressed to: Dr D. G. Charlton, Department of Dental Materials, Indiana University School of Dentistry, Indianapolis, Indiana, USA.

INTRODUCTION Glass polyalkenoate (ionomer) cements (GIC) have been used extensively in dentistry for a variety of purposes since their commercial introduction in the mid 1970s. Although initially developed for the treatment of Class V lesions, these cements have been employed as substructures and luting agents for precision castings (Wilson et al., 1977; McComb, 1982; McComb ef al., 1984), as direct restorative materials (Mount and Makison, 1978; Charbeneau and Bozell, 1979; Lawrence, 1979; McLean, 1988), and even as pit and fissure sealants (McLean and Wilson, 1974). The beneficial physical properties of GIC have been a primary factor in their widespread acceptance by clinicians. Among these properties are acceptable film thickness, cariostatic effects (Hicksetal., 1986; Kambhu et al., 1988), high compressive strength (Tay and Lynch, 1989), low solubility (Craig, 1985), and the ability to chemically bond to both enamel and dentine (Walls, 1986). In addition, they possess a coefficient of thermal expansion similar to that of tooth structure which helps to minimize leakage between the material and the tooth. *The opinions expressed herein are those of the authors and do not necessarily reflect the opinions of the Department of Defense or the United States Air Force. @1991 Butterworth-Heinemann 0300-5712/91/040249-06

Ltd.

GIC are commonly used for the restoration of Class V lesions, either alone or in conjunction with composite resins in the ‘sandwich’ technique. During placement of direct filling forms of these cements, voids or imperfections may be noted which require repair through addition of a separate mix of the material. While bond strengths of GIC to enamel, dentine, various metals, and composites have been repeatedly studied (Hotz et al., 1977; Coury et al., 1981;Lacefield et al., 1985;Aboush and Jenkins, 1986; Hinoura ef al., 1987),little work has been done to evaluate the repairability of these cements and, in particular, the effects that repair time and surface pretreatment may have on resulting bond strengths and failure modes (Pearsonet 1989; Robbins et al., 1989; Scherer et al., 1989). The purpose of this study was to investigate the effects of repair time and surface treatment on the tensile bond strengths of two GIC restorative materials.

al.,

METHODS AND MATERIALS The two GICs tested were Ketac-Fil (ESPE-Premier, Norristown, PA, USA) and Fuji Type II (G-C International Corp., Scottsdale, AZ, USA). Forty specimens of each cement were evaluated for repairability at 20 min and at

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J. Dent. 1991; 19: No. 4

Table 1. Mean tensile bond strengths (MPa) for Fuji Type II 20 min repairs

Surface treatment Smooth, unetched Smooth, etched Rough, unetched Rough, etched

24 h repairs

Mean

s.d.

Mean

s.d.

6.37

1.70 1.19 0.85 0.78

3.13 3.52 3.36 3.09

0.85 0.88 1.10 1.19

I 5.99 4.89 3.43

n= 10. Vertical lines connect non-significantdifferences at the 0.05 level. Table II. Mean tensile bond strengths (MPa) for Ketac-Fil

Surface treatment

20 min repairs Mean s.d.

24 h repairs Mean s.d.

Smooth, unetched Rough, etched Rough, unetched Smooth, etched

6.46 6.15 5.35 I 4.62

3.71 5.06 3.96 4.50

1.48 1.25 1.53 1.76

n= 10. Vertical lines connect non-significant differences at the 0.05

h. The cements were prepared according to their manufacturer’s instructions and inserted into cylindrical Teflon moulds having 6 mm lengths and 4 mm internal diameters. Glass slides were placed over the exposed cements to produce smooth surfaces and the specimens were stored in a humidor at 37 “C before being repaired at either 20 min or 24 h. The 40 specimens were divided into four groups of 10 with each group receiving a different pretreatment of the exposed cement surface prior to repair. The first group, smooth and unetched (SU), received no surface treatment. The second group, smooth and etched (SE), was acid etched for 30 s with 37 per cent phosphoric acid, rinsed for 15 s and dried with gauze. Gauze was used instead of compressed air to prevent desiccation of the cement. A third group, rough and unetched (RU), was roughened by five strokes of the exposed cement surface on wet 400 grit silicon carbide paper, rinsed for 15 s and dried with gauze. The final group, rough and etched (RE), was roughened by five strokes on wet 400 grit silicon carbide paper, rinsed for 15 s and dried with gauze. The exposed cement surface was then acid etched for 30 s with 37 per cent phosphoric acid, rinsed for 15 s and dried with gauze. Following surface preparation, all of the specimens were repaired similarly by placing a second Teflon mould in contact with the exposed cement surface and wrapping plastic tape around the outer circumference of both moulds to accurately align and immobilize them. The joined moulds were placed into a spring-loaded clamp which exerted pressure against both ends of the moulds. This prevented excess cement from being expressed into the interface between the moulds. A newly prepared mix of the same cement was then introduced into the mould and the joined moulds and clamp assembly were placed into a 37 “C humidor. At 10 min the joined moulds were removed from the clamp and returned to the humidor. After an additional 20 min of storage the plastic tape was removed from around the 24

1.05 0.94 1.44 1.08

level.

moulds and they were placed in deionized water. The 24-h repair specimens were prepared in the same manner with the exception that at 20 min after fabrication, the cement specimens were removed from the humidor and stored for 24 h in 37°C deionized water prior to being repaired. Following repair, all specimens were stored in deionized water at 37 “C for 24 h. The specimens were tested using an Instron Universal Testing Machine (Instron Corporation, Canton, MA, USA) The joined moulds, which have threaded outer surfaces, were loaded into the testing machine by screwing one of them into a threaded jig which was then fixed to the upper member of the Instron. The other mould was similarly screwed into a second threaded jig and fixed to the lower member. The jigs were capable of swivelling at their point of attachment to the Instron which reduced the possibility that forces other than tensile would be exerted on the specimens. The specimens were loaded to failure in tension at a crosshead speed of 0.5 mm/min. The specimens were examined to determine mode of failure using a binocular microscope at 25 X magnification. The mode of failure for each specimen was categorized as either cohesive, adhesive or mixed. Cohesive failure was defined as that occurring within the cement, adhesive as that occurring at the interface between the cements, and mixed as a combination of cohesive and adhesive. The data was analysed using analysis of variance. Multiple comparisons of significant differences were made using Fisher’s Least Significant Differences method. Scanning electron microscope (SEM) examination of the treated cement surfaces was performed by making replicas of representative Fuji II and Ketac-Fil specimens after surface preparation at 20 min. Four specimens of each cement were prepared as described and impressions made using a low viscosity addition polymerizing silicone

Charlton et a/.: Repairability of type II glass polyalkenoate cements

Table 111. Modes

of failure

251

for Fuji Type II 20 min repairs

Surface treatment

C

A

Smooth, Smooth,

unetched etched

6 8

;

Rough, unetched

7

Rough, etched

4

:,

24

h repairs A M

M

C

1

9 :

:

I

:

2

2

6

6

1

3

C, cohesive failure within the cement; A, adhesive failure at the interface between the cements; M, combination of cohesive and adhesive failures. Numbers indicate the number of specimens experiencing a particular type of failure.

Table IV. Modes

of failure

for Ketac-Fil

24 h repairs

20 min repairs Surface rrearmenr

C

A

Smooth, unetched Smooth, etched Rough, unetched

6 6

4 4 5

3

8

2

;

Rouah. etched

M

C

A

M

9 :

0 4 7

1 2 3

1

2

7

C, cohesive failure within the cement; A, adhesive failure at the interface between the cements; M, combination of cohesive and adhesive failures. Numbers indicate the number of specimens experiencing a particular type of failure.

impression material (Mirror 3, Sybron/Kerr, Romulus, MI, USA). The impressions were poured with a low viscosity casting resin (Stycast 1266, Adhesive Packaging Specialties, Peabody, MA, USA) and sputter coated with gold/palladium. Photomicrographs were recorded at 500 X magnification.

RESULTS

Mean tensile bond strength values for the two cements according to repair time and surface treatment are given in Tables I and II. Modes of failure for the cements are provided in Tables ZZZ and N. Bond strengths for Fuji II repaired at 20 min ranged from 6.37 MPa for the SU group to 3.43 MPa for the RE group. Analysis of variance revealed that significant differences existed (P < 0.05) for the four treatment groups. Multiple comparisons showed that the SU and SE groups had significantly higher mean tensile bond strengths (P < 0.05) than did the RU and RE groups. Additionally, the RU group had a significantly higher bond strength than did the RE group. The modes of failure for the SU, SE and RU group specimens were primarily cohesive. Only the RE group exhibited more mixed or adhesive failures than cohesive failures. For the 24 h repair specimens, values ranged from 3.52 MPa for the SE group to 3.09 MPa for the RE group. No significant differences existed between the four treatment groups repaired at 24 h. The majority of failures for the four groups were cohesive. The mean bond strengths for the treatment groups repaired at 20 min were greater than those repaired at 24 h, but the differences were not always statistically significant.

For Ketac-Fil repaired at 20 min, mean tensile bond strength values ranged from 6.46 MPa for the SU group to 4.62 MPa for the SE group. Analysis of variance revealed that significant differences existed (P < 0.05) for the four treatment groups. Multiple comparisons showed that the SU and RE groups had significantly higher mean tensile bond strengths (P < 0.05) than did the SE group. The majority of failures for the SU and SE specimens were cohesive in nature while the failures for the RU and RE group specimens were for the most part adhesive. For the 24-h repair specimens, values ranged from 5.06 MPa for the RE group to 3.71 MPa for the SU group. No significant differences existed between the four treatment groups repaired at 24 h. Failures were primarily cohesive for the SU group specimens, adhesive for the RU specimens, and mixed for the RE group specimens. As many cohesive failures as adhesive failures were exhibited by the SE specimens. The mean bond strengths for the treatment groups repaired at 20 min were greater than those repaired at 24 h, but the differences were not always statistically significant. The SEM photomicrographs of the four surfaces of Fuji II treated at 20 min are shown in Figs 1-4. The photomicrograph of the SU specimen reveals a surface characterized by voids ranging in width from 10 urn to approximately 50 urn. The larger voids bear darker centres, suggesting relatively deep dimensions. Additionally, irregular cracks join the voids and extend from other locations on their peripheries. The SE surface shows glass particles projecting from the surrounding matrix. Voids are still evident in the matrix, however, they are few in number and considerably smaller in size than those seen in the SU photomicrograph. The RU surface has a ragged appearance and few, if any, glass particles can be seen

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J.Dent. 1991; 19: No.4

Fig. 1. SEM photograph of the surface of a SU Fuji Type II specimen. The surface shows prominent voids with radiating cracks (X 364).

Fig. 2. SEM photograph of the surface of a SE Fuji Type II specimen. Etching has partially reduced the matrix and exposed glass particles (X 364).

Fig. 3. SEM photograph of the surface of a RU Fuji Type II specimen. Glass particles are largely indistinct although the surface is ragged from the roughening process (X 364).

Fig. 4. SEM photograph of the surface of a RE Fuji Type II specimen. Glass particles of varying sizes have been exposed (X 364).

protruding from the surrounding matrix. The photomicrograph of the RE specimen shows a surface similar to that of the SE specimen, although the matrix displays a slightly smoother appearance and fewer glass particles can be seen protruding from it. The SEM photomicrographs of the four surfaces of Ketac-Fil treated at 20 min are shown in Figs 5-8. The SU specimen shows a surface characterized by irregular voids with cracks of varying widths connecting them. The SE surface consists of well-defined, irregular glass particles projecting prominently from a slightly smoother matrix. The RU surface shows few glass particles and the matrix bears an irregular, ragged appearance. The RE surface is quite similar to the SE surface and is characterized by the presence of numerous glass particles. In general, the Ketac-Fil and Fuji Type II SU and RU surfaces are similar in appearance. The SE and RE

surfaces are different for the two cements with Ketac-Fil displaying larger and more numerous glass particles.

DISCUSSION Surface treatments were found to significantly affect the bond strengths of Fuji II and Ketac-Fil when repaired at 20 min. Although the rankings of bond strength values at this early repair time were not precisely the same for the two materials, similarities existed in that the SU treatment groups had the highest mean bond strengths. The lower bond strengths recorded for the etched groups are not surprising, given the fact that considerable damage can be done to the cement matrix when it is etched for 30 s (Smith, 1988). Current recommendations are for etching times as short as 10 s (Suzuki and Jordan, 1990). The fact that the SU treatment groups had the highest bond

Charlton

et al.: Repairability

of type II glass polyalkenoate

cements

253

Fig. 5. SEM photograph of the surface of a SU Ketac-Fil specimen. The surface is characterized by voids interconnected by cracks (X 364).

Fig. 6. SEM photograph of the surface of a SE Ketac-Fil specimen. Note the prominent jagged glass particles projecting from the matrix (X 364).

Fig. 7. SEM photograph of the surface of a RU Ketac-Fil specimen. Roughening has produced an irregular surface; few glass particles are evident (X 364).

Fig. 8. SEM photograph of the surface of a RE Ketac-Fil specimen. Glass particles are clearly evident but vary by the degree to which they are still retained by the matrix (x 364).

strengths, in fact, suggests that when repairing Type II GIC soon after the restorative procedure is completed, it is best to leave the surface of the restoration untreated prior to adding additional GIC. This finding is consistent with the appearance of the SEM photomicrographs. The untreated surfaces of both the Fuji II and Ketac-Fil cements contain numerous voids and irregularities which may provide undercuts capable of mechanically retaining additional cement added during the repair process. Cracks in the matrix were probably the result of drying which occurred during preparation of the specimen immediately prior to making the repair. No varnish or unfilled resin was placed on the cement surface or around the margins of the cement and the glass slide during specimen preparation because it was felt that this layer would interfere with intimate contact of the Teflon moulds. The effect of these cracks on the bond strengths as

measured in the laboratory is difficult to determine. The cracks may have increased the bond strength by providing additional sources for mechanical retention of the added cement; however, they may also have weakened the surrounding matrix and led to premature bond failure. In either case, the number of cracks on the surface of a clinically placed restorative GIC should be minimal if proper handling procedures are followed, such as coating the newly placed cement surface with a protective varnish or unfilled resin. The finding that tensile bond strengths for the 24-h repairs were lower than those for the 20-min repairs is consistent with the results of other researchers. Robbins et al. (1989) found that the shear bond strength of repaired Ketac-Fil was 15 per cent lower when repaired at 24 h than when repaired at 30 min. Fuji II, when repaired at 24 h, had a bond strength that was 31 per cent lower than when

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J. Dent. 1991; 19: No. 4

repaired at 30 min. Scherer et al. (1989) found that the shear bond strength of Ketac-Fil was lower when repaired at 24 h than when repaired at either 5 min or 15 min. Pearson et al. (1989) measured the flexure strength of three GICs when repaired at 1 h and 7 days and found that the 7-day flexure strength values were lower than the l-h values. The repair bond strengths of Fuji II and Ketac-Fil were generally greater than their reported tensile bond strengths to untreated dentine of 2.6 and 1.6 MPa (Aboush and Jenkins, 1986). Their reported tensile bond strengths to conditioned dentine of 3.1 and 3.3 MPa were also exceeded by the values measured in this study. With the exception of several of the 24-h bond strength values for Fuji II, the repair bond strengths were greater than the reported tensile strength of unrepaired GIC (Hinoura et al., 1987). No clear trend regarding mode of failure for the two cements can be determined from examination ofthe types of failure which occurred. The majority of failures for the Fuji II specimens were cohesive, regardless of treatments or repair times, although the 20-min RE group experienced a majority of mixed failures. The failures for the Ketac-Fil were generally cohesive for both the 20-min and 24-h SU and SE groups and adhesive for the 20-min RU and RE groups. Failures were mostly adhesive for the 24-h RU specimens and mixed for the 24-h RE specimens. A significant portion of the variability in the failure modes probably stems from the difficulty encountered in categorizing them. Because of the similarity in appearance between the initial cement and the repair cement, it was not always apparent where the initial cement ended and the repair cement began. The evaluations were primarily done by examining where the failure occurred in relation to the plane of the Teflon moulds, but this assumed that the cement surfaces met at exactly that level. Although precautions were taken to adapt the moulds to each other as intimately as possible, it is conceivable that the repair level for some specimens was not precisely in the plane of the joined surfaces. The categorization of failure mode was relatively uncomplicated for the SU specimens of both cements because the initial surface to which the repair cement was added was smooth, shiny and bore no resemblance to the rough internal structure of the cement. The fact that these specimens were easy to categorize may explain the similarity in failure modes between the Fuji SU specimens at 20 min and the Ketac-Fil SU specimens at 20 min. The same similarity is evident between the Fuji II 24-h SU specimens and the Ketac-Fil 24-h SU specimens. The results of this investigation indicate that: 1. Surface pretreatment significantly affects the tensile bond strength of both Fuji II and Ketac-Fil when repaired 20 min after placement. The highest bond strength was achieved when bonding to the smooth, unetched cement surface. 2. Surface pretreatment does not significantly affect the

tensile bond strength of either Fuji II or Ketac-Fil when repaired 24 h after placement. 3. Tensile bond strengths were greater for repairs made at 20 min than for repairs made at 24 h. References Aboush Y. E. Y. and Jenkins C. B. G. (1986) An evaluation of bonding of glass-ionomer restoratives to dentin and enamel. Br. Dent. J. 161, 179-184. Charbeneau G. T. and Bozell R. R. (1979) Clinical evaluation of a glass ionomer cement for restoration of cervical erosion. J. Am. Dent. Assoc. 98,936-939. Coury T. L., Miranda F. J., Willer R. D. et aJ. (1981) Adhesiveness of glass ionomer cement to enamel and dentin: a laboratory study. Oper. Dent 7, 2-6. Craig R. G. (1985) Restorative Dental Materials, 7th edn. St Louis, C.V. Mosby, p. 187. Hicks M. J., Flaitz C. M. and Silverstone L. M. (1986) Secondary caries formation in vitro around glass-ionomer restorations. Quintessence ht. 17, 527-532. Hinoura K., Moore B. K. and Phillips R. W. (1987) Tensile bond strength between glass ionomer cements and composite resins. J. Am. Dent. Assoc. 114, 167-172. Hotz P., McLean J. W., Seed I. et al. (1977) The bonding of glass ionomer cements to metal and tooth substrates. Br. Dent. J. 142, 41-47. Kambhu P. P., Ettinger R. L. and Wefel J. S. (1988) An in vitro evaluation of artificial caries like lesions on restored overdenture abutments. J. Dent. Res. 67, 582-584. Lacefield W. R., Reindl M. C. and Retief D. H. (1985) Tensile bond strength of a glass-ionomer cement. J. Prosthet. Dent. 53, 194-198. Lawrence L. G. (1979) Cervical glass ionomer restorations: a clinical study. J. Can. Dent. Assoc. 45, 58-59, 62. McComb D. (1982) Retention of castings with glass ionomer cement. J. Prosthet. Dent. 48, 285-288. McComb D., Sirisko R. and Brown J. (1984) Comparison of physical properties of commercial glass ionomer luting cements. J. Can. Dent. Assoc. 50,699-701. McLean J. W. (1988) Glass-ionomer cements. Br. Dent. J. 164, 293-300. McLean J. W. and Wilson A. D. (1974) Fissure sealing and filling with an adhesive glass-ionomer cement. Br. Dent. .J. 136, 269-276. Mount G. J. and Makison 0. F. (1978) Clinical characteristics of a glass-ionomer cement. Br. Dent. J. 145, 67-71. Pearson G. J., Bowen G., Jacobsen P. et al. (1989) The flexural strength of repaired glass-ionomer cements. Dent. Mater. 5, 10-12. Robbins J. W., Cooley R. L., Duke E. S. et al. (1989) Repair bond strength of glass-ionomer restoratives. Oper. Dent. 14, 129-132. Scherer W., Vaidyanathan J., Kaim J. M. et al. (1989) Evaluation of the ability of glass-ionomer cement to bond to glass-ionomer cement. Oper. Dent. 14, 82-86. Smith G. E. (1988) Surface deterioration of glass-ionomer cement during acid etching: an SEM evaluation. Oper. Dent. 13, 3-7. Suzuki M. and Jordan R. E. (1990) Glass ionomer-composite sandwich technique. .I. Am. Dent. Assoc. 120, 55-57. Tay W. M. and Lynch E. (1989) Glass ionomer cements. Part II. Clinical properties. J. Jr. Dent. Assoc. 35, 59-64 Walls A. W. (1986) Glass polyalkenoate (glass-ionomer) cements: a review. J Dent. 14, 231-246. Wilson A. D., Crisp S., Lewis B. G. et al. (1977) Experimental luting agents based on the glass ionomer cements. Br. Dent. J. 142, 117-122.

Repairability of type II glass polyalkenoate (ionomer) cements.

Two Type II glass polyalkenoate (ionomer) cements were evaluated for their repairability by measuring tensile bond strength of new material added to p...
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