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Retention of orthodontic bands with new fluoride-re leasing cements
Retention of orthodontic bands with fluoride-releasing cements
new
D. Stephen Norris, D.D.S.,* Pamela Mclnnes-Ledoux, B.D.S., M.Sc. (Dent.),** Bernhard Schwaninger, D.D.S., Dr. Med. Dent.,*** and Roger Weinberg, Ph.D.**** New Orleans, La.
The prevalence of enamel decalcification beneath orthodontic bands has indicated the need for a fluoride-releasing, enamel-adhesive orthodontic luting cement. The purpose of this study was to compare the retentive bond strengths of orthodontic bands cemented with two new fluoride-releasing cements, a zinc polycarboxylate and a glass ionomer, with the retentive bond strength of bands cemented with the standard orthodontic cement zinc phosphate. The site of cement failure was also evaluated. One hundred eighty extracted human molar teeth were embedded in resin blocks and randomly assigned to three cement groups. Adap!ed bands were cemented by a clinically acceptable technique. The cemented teeth were then assigned to one of three time intervals--24 hours, 7 days, and 60 days--and thermocycled in synthetic saliva. The force required to initially fracture the cement bond was used as a measure of cement retention. By means of the Instron, a tensile load was app!jed to each cemented band. The maximum retentive strength (cement failure) was interpreted from the st(ess-strain curve at the point where linearity deviated. The failure site was judged subjectively: between cement and enamel, within the cement, or between cement and the band. Using stress at failure, an analysis of variance showed no significant differences among the retentive strengths of the three cements. The chi-square test revealed a significant difference (P < 0.01) between failure sites of the zinc phosPhate and glass ionomer cements. Significantly more bands cemented with the glass ionomer failed at the cement/band interface, leaving the cement adhered to the tooth. Both the glass ionomer and zinc polycarboxylate cements tested were found to be suitable as orthodontic luting agents. In addition, the favorable failure site of the glass ionomer (cement adhered to enamel) may offer clinical protection against decalcification under loose bands. (AMJ ORTHOD 89: 206-211, 1986.)
Key words: Cements, orthodontics, fluoride, retention, demineralization
S i n c e the inception of fixed orthodontic appliances, both the public and the dental profession have blamed these appliances for subsequent demineralization and caries. In 1937, Noyes ! suggested that these decalcifications resulted from failure of the cement to maintain its seal between enamel and the orthodontic band. Other authors have since agreed with this premise.2: 3 In addition to seal breakdown, inadequate structural and bonding Strength, the solubility of currently used dental cements in oral fluids, and poor hygiene also contribute to the initiation of decalcifications. 4 Because the efficacy of home prophylactic procedures is greatly reduced by the presence of fixed apFunded by Louisiana State University School of Dentistry Intramural Research Grant. *Graduate student in orthodontics, LSU School of Dentistry. **Assistant Professor of Ope~'ative Dentistry. ***Professor of Orthodontics. ****Professor of Biometry, LSU Medical Center.
206
pliances, thereis a need for protection of the adjacent enamel surfaces. It is widely accepted that fluoride has anticariogenic properties resulting from the formation of less soluble fluoroapatite in the outer enamel layer? In 1878, zinc phosphate cement was introduced as a dental cement. It has become the standard cement used to cement Orthodontic bands over the years. 6 In the 1960s, fluoride was added to zinc phosphate cement to reduce the acid solubility and to impart anticariogenic properties to tooth enamel to which stainless steel bands were cemented. 7 There are certain drawbacks inherent in the use of zinc phosphate cement. It is brittle, has a relatively high solubility in the mouth, and it does not adhere to tooth substance. 8 Zinc phosphate cement relies on mechanical interlocking for its retentive effect and on close physical adaptation for sealing restoration margins, but it does not provide any chemical bonding to tooth or metal surfaces. 9 Researchers have developed other dental cements to overcome these drawbacks.
Volume 89
Number 3
In 1968, Smith introduced zinc polycarboxylate cement, the first dental cement that did not rely solely on irregularities of the adjoining surfaces for mechanical retention. Poiycarboxylate cement is provided in a powder-liquid form. The powder is a modified zinc oxide and the liquid an aqueous solution of polyacrylic acid. Setting is accomplished by a chemical reaction in which zinc ions link adjacent polyacrylic acid molecules, producing a large cross-linked chelate structure. The polyacrylic acid molecules have the ability to chelate to calcium ions in tooth enamel as well as to stainless steel. ~° This ability to react chemically with dental enamel and stainless steel suggests that the polycarboxylate cements are highly suitable for use as a cementing medium in orthodontic treatment. 11 One disadvantage of this cement is that short setting and working times may make the cementation of more than two bands from a single mix difficult. 12In addition, the mixed cement's viscosity can make band cementation difficult. 6 Different fluoride preparations have been added to polycarboxylate cement to lend anticariogenic properties and alter mechanical properties. 13 Recent evidence indicates that fluoride is released from set, stannous fluoride-containing polycarboxylate cement, that there is a significant increase in the fluoride content of adjacent enamel, and that the solubility of the enamel is significantly reduced after 2 weeks of contact with the cement. 14 Recently, a fluoride-releasing polycarboxylate cement, Poly-F Plus,* has been introduced to the American market. The problem of high viscosity has been reduced by incorporating vacuum-dried polyacrylic acid into the powder. The liquid component is a low viscosity aqueous solution. There is a need to evaluate this product as a luting agent for orthodontic bands. In 1971, glass ionomer cements were introduced. These cements are based on the hardening reaction between an aqueous solution of homo- or copolymers of acrylic acid and a powdered calcium aluminosilicate glass. Setting results from the formation of a calcium polysalt gel matrix that is subsequently reinforced by the aluminum polysalt. Adhesion probably results from ionic or polar molecular interactions. 15 Experimentally, it has been demonstrated that glass ionomer cements chemically adhere to tooth enamel and dentin as well as tO stainless steel, suggesting their suitability as orthodontic luting cements. 16-18 As with zinc polycarboxylate cements, experiments have shown that glass ionomer cements release fluoride *Dentsply, DeTrey. For product information, contact Ash USA, Box 952, Toledo, Ohio 43697.
Retention of bands with fluoride-releasing cements
207
from the set cement. The fluoride can be eluted as a simple ion or as a complex such as a fluorophosphate and it is considered to be of clinical benefit.19 Because the cement becomes attached via ionic and polar bonds to the enamel, the intimate molecular contact facilitates fluoride ion exchanges with the hydroxyl ions in the apatite of the surrounding enamel. By contrast, a luting agent that does not adhere by molecular interactions would leave gaps between the cement and tooth. Therefore, even if such a luting agent were to release fluoride, ion exchange would be inhibited? ° An advantage of glass ionomer over zinc polycarboxylates is the fluidity or lower viscosity of the mixed cement. 21 The consistency of the mixed cement is comparable with that Of zinc phosphate cement. 2° Similar to zinc polycarboxylate, glass ionomer should have a glossy appearance. This insures that enough free carboxylic acid groups are present to provide wetting of the enamel surface sufficient for effecting an adhesive bond. 17 A high compressive strength is considered advantageous for a dental luting cement. Recent Studies have demonstrated that glass ionomer has a significantly higher compressive strength than either zinc polycarboxylate or zinc phosphate cement. 2° A disadvantage of glass ionomer is that during the initial phase of setting, moisture can adversely affect the hardness of the surface. This reduces the ionomer's ability to withstand subsequent and repeated dehydration in the mouth breather 22 and can cause a tendency to chalkiness.17 A glass ionomer recently introduced, Ketac-Cem,* has been used extensively as a luting cement in fixed prosthodontics. Its value as an orthodontic luting cement has not been reported. The aims of this investigation were 1. To compare the retentive bond strength of orthodontic bands cemented with orthodontic zinc phosphate cement, zinc polycarboxylate cement, and glass ionomer cement 2. To observe and compare the changes in bond strength with time 3. To subjectively evaluate the mode of cement failure MATERIALS AND METHODS
One hundred eighty sound, extracted human molar teeth were selected. The teeth were Cleaned of debris with nonfluoride pumice and a bristle brush, and then rinsed with deionized water. Each tooth was notched in the apical one third and then mounted up to the cervical line in a block of self-curing acrylic. Optimally sized, clinically adapted stainless steel orthodontic *Espe, Premier, Norristown, Pa.
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Norris et al.
Am. J. Orthod. March 1986
Table I. Number of bands cemented per mix of cement Cement Zinc phosphate (frozen slab technique) Polycarboxylate Glass ionomer
Fig. 1. Apparatus attached to the Instron machine to remove the cemented bands. The tips were machined to engage the undersurface of the welded lingual buttons, allowing free movement of dislodging apparatus.
bands were fitted on each mounted tooth. The bands selected had two opposing lingual buttons spot welded to facilitate the attachment of the band removal apparatus. The welding was conducted in a standard, uniform manner. Sixty banded teeth were randomly assigned to each of three groups: Group I--Cementation with orthodontic zinc phosphate cement (Ormco Gold*) Group II--Cementation with zinc polycarboxylate cement (Poly-F Plus) Group HI--Cementation with glass ionomer cement (Ketac-Cem) The cements were manipulated according to the manufacturers' recommendations. The zinc phosphate cement was mixed by means of the frozen slab technique. After each cement was correctly mixed, it was loaded into the orthodontic band and each band was seated on the selected tooth with hand pressure. 23 The *Orrnco Corporation, Glendora, Calif.
]
No. of bands 8 3-4 8
numbers of bands cemented per mix of cement are shown in Table I. The teeth were then stored in synthetic saliva24 at 37 ° C and subjected to 2500 thermocycles at a temperature differential of 400 C. 25 Two temperature baths, one at 5 ° C and the other at 45 ° C, were used. The dwell time in each bath was 30 seconds. Bond strength was tested at 24 hours, 7 days, and 60 days (20 randomly assigned teeth each time period). Using an Instron* Universal testing machine in tensile mode, the force required to initially fracture the cement bond was used as a measure of cement retention. 23 Each mounted tooth was clamped in the inferior vise grip of the instrument. The inferior vise grip consisted in part of a swivel joint that allowed lateral and rotational movements. A specially designed apparatus was attached to the superior vise grip to remove the cemented bands (Fig. i). Such an arrangement allowed the rod to freely rotate about its center point to compensate for any vertical discrepancy in the button attachment height and for self-alignment of the apparatus whenever it was placed under tension. All forces were directed parallel to the long axis of the tooth during band removal. Crosshead speed for band removal was 0.02 cm per minute. The maximum retentive strength or cement failure was interpreted from the stress-strain curve at that point at which linearity was interrupted. 10 After each cemented band failed, the failure site was judged subjectively as to its location and was graded as follows: 0---Between cement and enamel (adhesive failure) 1--Within the body of the cement (cohesive failure) 2---Between cement and stainless steel band (adhesive failure) The bands were then sectioned in halves, laid fiat on a piece of paper, and each band's profile was traced. The surface area was determined by means of an electronic planimeter (Zeiss MOP 3t). A two-way analysis of variance was used to analyze the retentive strength data. The chi-square test was used to evaluate cement failure location. The level of significance chosen was P < 0.05. *Instron Corp., Canton, Mass. "~Carl Zeiss, Inc., New York, N.Y.
Volume 89
Retention o f b a n d s with fluoride-releasing c e m e n t s
Number 3
Fig. 2. Cement failure between cement and enamel stainless steel band (right).
(left),
and cement failure between cement and
Table III. C e m e n t failure location f r e q u e n c y
Table Ih M e a n retentive strength (stress) o f cemented bands
2 Cement
0 (Between cement and enamel)
1 (Within cement)
(Between cement and stainless steel band)*
Zinc phosphate Polycarboxylate Glass ionomer
46 45 34
----
11 15 25
Stress
Cement
Time
(Mn • m -e)
SE of mean
Zinc phosphate
24 hours 7 days 60 days 24 hours 7 days 60 days 24 hours 7 days 60 days
1.14 0.93 1.11 0.97 0.87 1.04 0.99 1.06 1.30
0.08 0.06 0.08 0.11 0.09 0.07 0.09 0.08 0.08
Polycarboxylate
Glass ionomer
209
There was no significant difference (P > 0.05) in the mean failure stress among cements.
RESULTS The n u m b e r o f bands c e m e n t e d with each m i x of cement is s h o w n in Table I. The w o r k i n g time and viscosity of both the zinc phosphate c e m e n t and the glass i o n o m e r c e m e n t allowed cementation o f m o r e bands per m i x than the zinc polycarboxylate cement. The latter c e m e n t b e c a m e stringy and viscous after cementation of only 3 to 4 bands. For the zinc phosphate cement, the bond strengths ranged f r o m 0 . 4 4 M N , m -2 - 1.57 M N • m -2 at 24 hours, 0.38 M N • m -2 - 1,29 M N • m -2 at 7 days, a n d 0 . 3 M N , m -2 - 1 . 9 7 M N • m - Z a t 6 O d a y s . W i t h zinc polycarboxylate c e m e n t , the bond strengths ranged from 0.31 M N • m -2 - 1.91 M N • m -2 at 24 hours, 0.32 M N - m -2 - 1.56 M N . m -2 at 7 days, and 0.5 M N . m -2 - 1.68 M N . m -2 at 60 days. The
There was no significant difference (P > 0.05) in failure location with time. *Considered most tooth-protective failure site. bond strengths for the glass i o n o m e r c e m e n t were 0.31 M N • m -2 - 1.9 M N • m -2 at 24 hours, 0.44 M N . m -z - 1.85 M N . m -2 at 7 days, and 0.83 M N • m -2 - 1.97 M N • m -2 at 60 days. T h e m e a n retentive strengths for all the cements at the three time intervals are s h o w n in Table II. T h e r e was no significant difference (P > 0.05) in the m e a n stress at failure a m o n g the cements. B o t h the glass i o n o m e r and the p o l y c a r b o x y l a t e cements tested were as e f f e c t i v e in band retention as the standard orthodontic zinc phosphate cement. The effect o f time on retentive strength o f the zinc phosphate and the polycarboxylate cements was not significant (P > 0.05). H o w e v e r , the retentive strength o f the glass i o n o m e r increased with time. This increase was not significant (P > 0.05) b e t w e e n 24 hours and 7 days but was significant (P < 0.05) b e t w e e n 24 hours and 60 days, and b e t w e e n 7 days and 60 days (Table II). T h e f r e q u e n c y of failure location is s h o w n in Table III. All failures occurred either b e t w e e n c e m e n t and
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N o r r i s e t al.
Am. J. Orthod. March 1986
enamel or between cement and the stainless steel band (Fig. 2). There were no cohesive failures. There was no significant difference (P > 0.05) in failure location with time and the data were therefore pooled. The majority of failure locations for all three cements occurred between cement and enamel. However, the glass ionomer cement had significantly more failures at the cement/stainless steel band interface than either the zinc phosphate cement (P < 0.01) or the zinc polycarboxylate cement (P < 0.05). DISCUSSION
Zinc phosphate cement is widely used for cementation of orthodontic bands. This cement is generally considered adequate for clinical band retention even though its retentive properties are purely mechanical in nature. There remains, however, the concern that enamel will decalcify beneath loose orthodontic bands. The introduction of fluoride-releasing cements with the potential for chemical adhesion and mechanical retention offers the orthodontist and the patient a theoretic advantage for band retention and enamel protection. The results of this study show that the two experimental cements tested had band retentive strengths similar to those of the standard orthodontic luting cement. The mean retentive strength values of the polycarboxylate cement were lower at all three time intervals than the zinc phosphate cement and the glass ionomer cement, although this difference was not significant (P > 0.05). When comparing the zinc phosphate and the glass ionomer, the 7-day and 60-day values for the latter were higher, although this was not significant either (P > 0.05). There appeared to be no significant increase in mean retentive strength caused by the chemical adhesive component of either experimental cement. The possible advantage of the luting cements with chemical adhesion potential was seen in cement failure location. Failure between cement and the stainless steel band was considered to be the most favorable failure location. This failure mode, leaving the fluoride-releasing cement in contact with the enamel, would offer the most clinical protection against enamel decalcification. This was most evident with the glass ionomer cement. Significant fluoride uptake by enamel from glass ionomer cements has been documented 26'27 and this would offer an additional advantage of using these cements for orthodontic band cementation. The zinc polycarboxylate cement failed to perform as expected. Even though its chemical adhesive potential is theoretically similar to that of the glass ionomer cement, its failure location frequency was no different from that observed with the zinc phosphate cement.
In the past one of the problems with the zinc polycarboxylate cements in general has been the precise manipulation requirements. The mixed cement must be placed on the tooth without delay to take advantage of calcium chelation with the free carboxyl groups. In the zinc polycarboxylate cement tested, the polyacrylic acid had been freeze-dried and incorporated in the powder component to mix with water. Even with this modification, the cement was viscous and working time was short. This drawback probably accounted for the poorer mean retentive strengths and unfavorable failure location frequency of the zinc polycarboxylate compared with the glass ionomer cement. (The latter also relies on chelation for chemical adhesion, but undergoes a 24-hour setting reaction.) It must be noted that failure location was evaluated with the naked eye and no cohesive failure within the cement was observed. More sophisticated techniques might have revealed cohesive failure very close to the interfaces. The retentive strength of the glass ionomer cement appeared to increase with time. CONCLUSIONS
Under the test conditions, both the zinc polycarboxylate and glass ionomer cements tested are as effective as the standard orthodontic luting cement, zinc phosphate, in retaining bands. The favorable failure site of the glass ionomer cement may offer clinical protection against decalcification under loose orthodontic bands. The adequate working time of the glass ionomer cement is equivalent to that of the zinc phosphate; however, the short working time of the zinc polycarboxylate cement may preclude its routine orthodontic use. REFERENCES 1. Noyes HJ: Dental caries and the orthodontic patient. Am Den Assc Jnl & Den Cosmos 24: 1243-1254, 1937. 2. GwinnettAJ, Matsui A: A study of enamel adhesives. The physical relationship between enamel and adhesive. Arch Oral Bi01 12: 1615-1620, 1967. 3. Balenseifen JW, Madonia JV: Study of dental plaque in orthodontic patients. J Dent Res 49: 320-324, 1970. 4. SadowskyPL, Retief DH: A comparative study of some dental cements used in orthodontics. Angle Orthod 46:171-181, 1976. 5. Levine RS: The action of fluoride in caries prevention. Br Dent J 140: 9-14, 1976. 6. Dennison JD, Powers JM: A review of dental cements used for permanent retention of restorations. Part I. Composition and manipulation. J Mich State Dent Assoc 56:116-121, 1974. 7. Wei SHY, Sierk DL: Fluoride uptake by enamel from zinc phosphate cement containing stannous fluoride. J Am Dent Ass0c 83: 621-624, 1971. 8. Smith De: A review of the zinc polycarboxylatecements. J Can Dent Assoc 1: 22-29, 1971.
Volume89 Number 3 9. Grieve AR, Jones JCG: Marginal leakage associated with four inlay cementing materials. Br Dent J 151: 331-334, 1981. 10. Rich JM, Leinfelder KF, Hershey HG: An in vitro study of cement retention as related to orthodontics. Angle Orthod 45: 219-225, 1975. 11. Mizrahi E: The recementation of orthodontic bands using different cements. Angle Orthod 49: 239-246, 1979. 12. Dennison JD, Powers JM: A review of dental cements Used for permanent retention of restorations. Part II. Properties and criteria for selection. J Mich State Dent Assoc 56: 218-225, 1974. 13. Tsukiboshi M, Tani Y: Physical properties of a polycarboxylate cement containing a tannin-fluoride preparation. J Prosthet Dent 51: 503-508, 1984. 14. Duperon DF, Jedrychowski J: Release and enamel uptake of fluoride from a fluoride-containing polycarboxylate cement. J Pedod 4: 287-294, 1980. 15. Hotz P, McLean JW, Sced I, Wilson AD: The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 142: 41-47, 1977. 16. Coury TL, Miranda FJ, Willer RD, Probst RT: Adhesiveness of glass-ionomer cement to enamel and dentin: A laboratory study. Oper Dent 7: 2-6, 1982. 17. Commission on Dental Products: Status report on the glassionomer cements. Chicago, 1981, Quintessence Publishing Co. 18. Negm MM, Beech DR, Grant AA: An evaluation of mechanical and adhesive properties of polycarboxylate and glass ionomer cements. J Oral Rehabil 9: 161-167, 1982. 19. Crisp S, Lewis BG, Wilson AD: Glass ionomer cements: Chemistry of erosion. J Dent Res 55: 1032-1041, 1976.
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20. McLean JW, Wilson AD: The clinical development of the glassionomer cement. II. Some clinical applications. Aust Dent J 22: 120-127, 1977. 21. Reisbick MH: Working qualities of glass ionomer-cements. J Prosthet Dent 46: 525-530, 1981. 22. Mount GJ, Makinson OF: Glass-ionomer restorative cements: Clinical implications of the setting reaction. Oper Dent 7:134141, 1982. 23. Bills RC, Yates JL, McKnight JP: Retention of stainless steel bands cemented with four dental cements. J Pedod 4: 273-286, 1980. 24. Shannon IL: A formula for human parotid fluid collected without exogenous stimulation. J Dent Res 46: 309, 1967. 25. Maldonado A, Swartz ML, Phillips RW: An in vitro study of certain properties of a glass ionomer cement. J Am Dent Assoc 96: 785-791, 1978. 26. Swartz ML, Phillips RW, Clark HE, Norman RD, Potter R: Fluoride distribution in teeth using a silicate model. J Dent Res 59: 1596:1603, 1980. 27. Retief DH, Bradley EL, Denton JC, Switzer P: Enamel and cementum fluoride uptake from a glass ionomer cement. Caries Res 18: 250-257, 1984. Reprint requests to: Dr. Pamela McInnes-Ledoux Department of Operative Dentistry LSU School of Dentistry 1100 Florida Ave. New Orleans, LA 70119
new
D. Stephen Norris, D.D.S.,* Pamela Mclnnes-Ledoux, B.D.S., M.Sc. (Dent.),** Bernhard Schwaninger, D.D.S., Dr. Med. Dent.,*** and Roger Weinberg, Ph.D.**** New Orleans, La.
The prevalence of enamel decalcification beneath orthodontic bands has indicated the need for a fluoride-releasing, enamel-adhesive orthodontic luting cement. The purpose of this study was to compare the retentive bond strengths of orthodontic bands cemented with two new fluoride-releasing cements, a zinc polycarboxylate and a glass ionomer, with the retentive bond strength of bands cemented with the standard orthodontic cement zinc phosphate. The site of cement failure was also evaluated. One hundred eighty extracted human molar teeth were embedded in resin blocks and randomly assigned to three cement groups. Adap!ed bands were cemented by a clinically acceptable technique. The cemented teeth were then assigned to one of three time intervals--24 hours, 7 days, and 60 days--and thermocycled in synthetic saliva. The force required to initially fracture the cement bond was used as a measure of cement retention. By means of the Instron, a tensile load was app!jed to each cemented band. The maximum retentive strength (cement failure) was interpreted from the st(ess-strain curve at the point where linearity deviated. The failure site was judged subjectively: between cement and enamel, within the cement, or between cement and the band. Using stress at failure, an analysis of variance showed no significant differences among the retentive strengths of the three cements. The chi-square test revealed a significant difference (P < 0.01) between failure sites of the zinc phosPhate and glass ionomer cements. Significantly more bands cemented with the glass ionomer failed at the cement/band interface, leaving the cement adhered to the tooth. Both the glass ionomer and zinc polycarboxylate cements tested were found to be suitable as orthodontic luting agents. In addition, the favorable failure site of the glass ionomer (cement adhered to enamel) may offer clinical protection against decalcification under loose bands. (AMJ ORTHOD 89: 206-211, 1986.)
Key words: Cements, orthodontics, fluoride, retention, demineralization
S i n c e the inception of fixed orthodontic appliances, both the public and the dental profession have blamed these appliances for subsequent demineralization and caries. In 1937, Noyes ! suggested that these decalcifications resulted from failure of the cement to maintain its seal between enamel and the orthodontic band. Other authors have since agreed with this premise.2: 3 In addition to seal breakdown, inadequate structural and bonding Strength, the solubility of currently used dental cements in oral fluids, and poor hygiene also contribute to the initiation of decalcifications. 4 Because the efficacy of home prophylactic procedures is greatly reduced by the presence of fixed apFunded by Louisiana State University School of Dentistry Intramural Research Grant. *Graduate student in orthodontics, LSU School of Dentistry. **Assistant Professor of Ope~'ative Dentistry. ***Professor of Orthodontics. ****Professor of Biometry, LSU Medical Center.
206
pliances, thereis a need for protection of the adjacent enamel surfaces. It is widely accepted that fluoride has anticariogenic properties resulting from the formation of less soluble fluoroapatite in the outer enamel layer? In 1878, zinc phosphate cement was introduced as a dental cement. It has become the standard cement used to cement Orthodontic bands over the years. 6 In the 1960s, fluoride was added to zinc phosphate cement to reduce the acid solubility and to impart anticariogenic properties to tooth enamel to which stainless steel bands were cemented. 7 There are certain drawbacks inherent in the use of zinc phosphate cement. It is brittle, has a relatively high solubility in the mouth, and it does not adhere to tooth substance. 8 Zinc phosphate cement relies on mechanical interlocking for its retentive effect and on close physical adaptation for sealing restoration margins, but it does not provide any chemical bonding to tooth or metal surfaces. 9 Researchers have developed other dental cements to overcome these drawbacks.
Volume 89
Number 3
In 1968, Smith introduced zinc polycarboxylate cement, the first dental cement that did not rely solely on irregularities of the adjoining surfaces for mechanical retention. Poiycarboxylate cement is provided in a powder-liquid form. The powder is a modified zinc oxide and the liquid an aqueous solution of polyacrylic acid. Setting is accomplished by a chemical reaction in which zinc ions link adjacent polyacrylic acid molecules, producing a large cross-linked chelate structure. The polyacrylic acid molecules have the ability to chelate to calcium ions in tooth enamel as well as to stainless steel. ~° This ability to react chemically with dental enamel and stainless steel suggests that the polycarboxylate cements are highly suitable for use as a cementing medium in orthodontic treatment. 11 One disadvantage of this cement is that short setting and working times may make the cementation of more than two bands from a single mix difficult. 12In addition, the mixed cement's viscosity can make band cementation difficult. 6 Different fluoride preparations have been added to polycarboxylate cement to lend anticariogenic properties and alter mechanical properties. 13 Recent evidence indicates that fluoride is released from set, stannous fluoride-containing polycarboxylate cement, that there is a significant increase in the fluoride content of adjacent enamel, and that the solubility of the enamel is significantly reduced after 2 weeks of contact with the cement. 14 Recently, a fluoride-releasing polycarboxylate cement, Poly-F Plus,* has been introduced to the American market. The problem of high viscosity has been reduced by incorporating vacuum-dried polyacrylic acid into the powder. The liquid component is a low viscosity aqueous solution. There is a need to evaluate this product as a luting agent for orthodontic bands. In 1971, glass ionomer cements were introduced. These cements are based on the hardening reaction between an aqueous solution of homo- or copolymers of acrylic acid and a powdered calcium aluminosilicate glass. Setting results from the formation of a calcium polysalt gel matrix that is subsequently reinforced by the aluminum polysalt. Adhesion probably results from ionic or polar molecular interactions. 15 Experimentally, it has been demonstrated that glass ionomer cements chemically adhere to tooth enamel and dentin as well as tO stainless steel, suggesting their suitability as orthodontic luting cements. 16-18 As with zinc polycarboxylate cements, experiments have shown that glass ionomer cements release fluoride *Dentsply, DeTrey. For product information, contact Ash USA, Box 952, Toledo, Ohio 43697.
Retention of bands with fluoride-releasing cements
207
from the set cement. The fluoride can be eluted as a simple ion or as a complex such as a fluorophosphate and it is considered to be of clinical benefit.19 Because the cement becomes attached via ionic and polar bonds to the enamel, the intimate molecular contact facilitates fluoride ion exchanges with the hydroxyl ions in the apatite of the surrounding enamel. By contrast, a luting agent that does not adhere by molecular interactions would leave gaps between the cement and tooth. Therefore, even if such a luting agent were to release fluoride, ion exchange would be inhibited? ° An advantage of glass ionomer over zinc polycarboxylates is the fluidity or lower viscosity of the mixed cement. 21 The consistency of the mixed cement is comparable with that Of zinc phosphate cement. 2° Similar to zinc polycarboxylate, glass ionomer should have a glossy appearance. This insures that enough free carboxylic acid groups are present to provide wetting of the enamel surface sufficient for effecting an adhesive bond. 17 A high compressive strength is considered advantageous for a dental luting cement. Recent Studies have demonstrated that glass ionomer has a significantly higher compressive strength than either zinc polycarboxylate or zinc phosphate cement. 2° A disadvantage of glass ionomer is that during the initial phase of setting, moisture can adversely affect the hardness of the surface. This reduces the ionomer's ability to withstand subsequent and repeated dehydration in the mouth breather 22 and can cause a tendency to chalkiness.17 A glass ionomer recently introduced, Ketac-Cem,* has been used extensively as a luting cement in fixed prosthodontics. Its value as an orthodontic luting cement has not been reported. The aims of this investigation were 1. To compare the retentive bond strength of orthodontic bands cemented with orthodontic zinc phosphate cement, zinc polycarboxylate cement, and glass ionomer cement 2. To observe and compare the changes in bond strength with time 3. To subjectively evaluate the mode of cement failure MATERIALS AND METHODS
One hundred eighty sound, extracted human molar teeth were selected. The teeth were Cleaned of debris with nonfluoride pumice and a bristle brush, and then rinsed with deionized water. Each tooth was notched in the apical one third and then mounted up to the cervical line in a block of self-curing acrylic. Optimally sized, clinically adapted stainless steel orthodontic *Espe, Premier, Norristown, Pa.
208
Norris et al.
Am. J. Orthod. March 1986
Table I. Number of bands cemented per mix of cement Cement Zinc phosphate (frozen slab technique) Polycarboxylate Glass ionomer
Fig. 1. Apparatus attached to the Instron machine to remove the cemented bands. The tips were machined to engage the undersurface of the welded lingual buttons, allowing free movement of dislodging apparatus.
bands were fitted on each mounted tooth. The bands selected had two opposing lingual buttons spot welded to facilitate the attachment of the band removal apparatus. The welding was conducted in a standard, uniform manner. Sixty banded teeth were randomly assigned to each of three groups: Group I--Cementation with orthodontic zinc phosphate cement (Ormco Gold*) Group II--Cementation with zinc polycarboxylate cement (Poly-F Plus) Group HI--Cementation with glass ionomer cement (Ketac-Cem) The cements were manipulated according to the manufacturers' recommendations. The zinc phosphate cement was mixed by means of the frozen slab technique. After each cement was correctly mixed, it was loaded into the orthodontic band and each band was seated on the selected tooth with hand pressure. 23 The *Orrnco Corporation, Glendora, Calif.
]
No. of bands 8 3-4 8
numbers of bands cemented per mix of cement are shown in Table I. The teeth were then stored in synthetic saliva24 at 37 ° C and subjected to 2500 thermocycles at a temperature differential of 400 C. 25 Two temperature baths, one at 5 ° C and the other at 45 ° C, were used. The dwell time in each bath was 30 seconds. Bond strength was tested at 24 hours, 7 days, and 60 days (20 randomly assigned teeth each time period). Using an Instron* Universal testing machine in tensile mode, the force required to initially fracture the cement bond was used as a measure of cement retention. 23 Each mounted tooth was clamped in the inferior vise grip of the instrument. The inferior vise grip consisted in part of a swivel joint that allowed lateral and rotational movements. A specially designed apparatus was attached to the superior vise grip to remove the cemented bands (Fig. i). Such an arrangement allowed the rod to freely rotate about its center point to compensate for any vertical discrepancy in the button attachment height and for self-alignment of the apparatus whenever it was placed under tension. All forces were directed parallel to the long axis of the tooth during band removal. Crosshead speed for band removal was 0.02 cm per minute. The maximum retentive strength or cement failure was interpreted from the stress-strain curve at that point at which linearity was interrupted. 10 After each cemented band failed, the failure site was judged subjectively as to its location and was graded as follows: 0---Between cement and enamel (adhesive failure) 1--Within the body of the cement (cohesive failure) 2---Between cement and stainless steel band (adhesive failure) The bands were then sectioned in halves, laid fiat on a piece of paper, and each band's profile was traced. The surface area was determined by means of an electronic planimeter (Zeiss MOP 3t). A two-way analysis of variance was used to analyze the retentive strength data. The chi-square test was used to evaluate cement failure location. The level of significance chosen was P < 0.05. *Instron Corp., Canton, Mass. "~Carl Zeiss, Inc., New York, N.Y.
Volume 89
Retention o f b a n d s with fluoride-releasing c e m e n t s
Number 3
Fig. 2. Cement failure between cement and enamel stainless steel band (right).
(left),
and cement failure between cement and
Table III. C e m e n t failure location f r e q u e n c y
Table Ih M e a n retentive strength (stress) o f cemented bands
2 Cement
0 (Between cement and enamel)
1 (Within cement)
(Between cement and stainless steel band)*
Zinc phosphate Polycarboxylate Glass ionomer
46 45 34
----
11 15 25
Stress
Cement
Time
(Mn • m -e)
SE of mean
Zinc phosphate
24 hours 7 days 60 days 24 hours 7 days 60 days 24 hours 7 days 60 days
1.14 0.93 1.11 0.97 0.87 1.04 0.99 1.06 1.30
0.08 0.06 0.08 0.11 0.09 0.07 0.09 0.08 0.08
Polycarboxylate
Glass ionomer
209
There was no significant difference (P > 0.05) in the mean failure stress among cements.
RESULTS The n u m b e r o f bands c e m e n t e d with each m i x of cement is s h o w n in Table I. The w o r k i n g time and viscosity of both the zinc phosphate c e m e n t and the glass i o n o m e r c e m e n t allowed cementation o f m o r e bands per m i x than the zinc polycarboxylate cement. The latter c e m e n t b e c a m e stringy and viscous after cementation of only 3 to 4 bands. For the zinc phosphate cement, the bond strengths ranged f r o m 0 . 4 4 M N , m -2 - 1.57 M N • m -2 at 24 hours, 0.38 M N • m -2 - 1,29 M N • m -2 at 7 days, a n d 0 . 3 M N , m -2 - 1 . 9 7 M N • m - Z a t 6 O d a y s . W i t h zinc polycarboxylate c e m e n t , the bond strengths ranged from 0.31 M N • m -2 - 1.91 M N • m -2 at 24 hours, 0.32 M N - m -2 - 1.56 M N . m -2 at 7 days, and 0.5 M N . m -2 - 1.68 M N . m -2 at 60 days. The
There was no significant difference (P > 0.05) in failure location with time. *Considered most tooth-protective failure site. bond strengths for the glass i o n o m e r c e m e n t were 0.31 M N • m -2 - 1.9 M N • m -2 at 24 hours, 0.44 M N . m -z - 1.85 M N . m -2 at 7 days, and 0.83 M N • m -2 - 1.97 M N • m -2 at 60 days. T h e m e a n retentive strengths for all the cements at the three time intervals are s h o w n in Table II. T h e r e was no significant difference (P > 0.05) in the m e a n stress at failure a m o n g the cements. B o t h the glass i o n o m e r and the p o l y c a r b o x y l a t e cements tested were as e f f e c t i v e in band retention as the standard orthodontic zinc phosphate cement. The effect o f time on retentive strength o f the zinc phosphate and the polycarboxylate cements was not significant (P > 0.05). H o w e v e r , the retentive strength o f the glass i o n o m e r increased with time. This increase was not significant (P > 0.05) b e t w e e n 24 hours and 7 days but was significant (P < 0.05) b e t w e e n 24 hours and 60 days, and b e t w e e n 7 days and 60 days (Table II). T h e f r e q u e n c y of failure location is s h o w n in Table III. All failures occurred either b e t w e e n c e m e n t and
210
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Am. J. Orthod. March 1986
enamel or between cement and the stainless steel band (Fig. 2). There were no cohesive failures. There was no significant difference (P > 0.05) in failure location with time and the data were therefore pooled. The majority of failure locations for all three cements occurred between cement and enamel. However, the glass ionomer cement had significantly more failures at the cement/stainless steel band interface than either the zinc phosphate cement (P < 0.01) or the zinc polycarboxylate cement (P < 0.05). DISCUSSION
Zinc phosphate cement is widely used for cementation of orthodontic bands. This cement is generally considered adequate for clinical band retention even though its retentive properties are purely mechanical in nature. There remains, however, the concern that enamel will decalcify beneath loose orthodontic bands. The introduction of fluoride-releasing cements with the potential for chemical adhesion and mechanical retention offers the orthodontist and the patient a theoretic advantage for band retention and enamel protection. The results of this study show that the two experimental cements tested had band retentive strengths similar to those of the standard orthodontic luting cement. The mean retentive strength values of the polycarboxylate cement were lower at all three time intervals than the zinc phosphate cement and the glass ionomer cement, although this difference was not significant (P > 0.05). When comparing the zinc phosphate and the glass ionomer, the 7-day and 60-day values for the latter were higher, although this was not significant either (P > 0.05). There appeared to be no significant increase in mean retentive strength caused by the chemical adhesive component of either experimental cement. The possible advantage of the luting cements with chemical adhesion potential was seen in cement failure location. Failure between cement and the stainless steel band was considered to be the most favorable failure location. This failure mode, leaving the fluoride-releasing cement in contact with the enamel, would offer the most clinical protection against enamel decalcification. This was most evident with the glass ionomer cement. Significant fluoride uptake by enamel from glass ionomer cements has been documented 26'27 and this would offer an additional advantage of using these cements for orthodontic band cementation. The zinc polycarboxylate cement failed to perform as expected. Even though its chemical adhesive potential is theoretically similar to that of the glass ionomer cement, its failure location frequency was no different from that observed with the zinc phosphate cement.
In the past one of the problems with the zinc polycarboxylate cements in general has been the precise manipulation requirements. The mixed cement must be placed on the tooth without delay to take advantage of calcium chelation with the free carboxyl groups. In the zinc polycarboxylate cement tested, the polyacrylic acid had been freeze-dried and incorporated in the powder component to mix with water. Even with this modification, the cement was viscous and working time was short. This drawback probably accounted for the poorer mean retentive strengths and unfavorable failure location frequency of the zinc polycarboxylate compared with the glass ionomer cement. (The latter also relies on chelation for chemical adhesion, but undergoes a 24-hour setting reaction.) It must be noted that failure location was evaluated with the naked eye and no cohesive failure within the cement was observed. More sophisticated techniques might have revealed cohesive failure very close to the interfaces. The retentive strength of the glass ionomer cement appeared to increase with time. CONCLUSIONS
Under the test conditions, both the zinc polycarboxylate and glass ionomer cements tested are as effective as the standard orthodontic luting cement, zinc phosphate, in retaining bands. The favorable failure site of the glass ionomer cement may offer clinical protection against decalcification under loose orthodontic bands. The adequate working time of the glass ionomer cement is equivalent to that of the zinc phosphate; however, the short working time of the zinc polycarboxylate cement may preclude its routine orthodontic use. REFERENCES 1. Noyes HJ: Dental caries and the orthodontic patient. Am Den Assc Jnl & Den Cosmos 24: 1243-1254, 1937. 2. GwinnettAJ, Matsui A: A study of enamel adhesives. The physical relationship between enamel and adhesive. Arch Oral Bi01 12: 1615-1620, 1967. 3. Balenseifen JW, Madonia JV: Study of dental plaque in orthodontic patients. J Dent Res 49: 320-324, 1970. 4. SadowskyPL, Retief DH: A comparative study of some dental cements used in orthodontics. Angle Orthod 46:171-181, 1976. 5. Levine RS: The action of fluoride in caries prevention. Br Dent J 140: 9-14, 1976. 6. Dennison JD, Powers JM: A review of dental cements used for permanent retention of restorations. Part I. Composition and manipulation. J Mich State Dent Assoc 56:116-121, 1974. 7. Wei SHY, Sierk DL: Fluoride uptake by enamel from zinc phosphate cement containing stannous fluoride. J Am Dent Ass0c 83: 621-624, 1971. 8. Smith De: A review of the zinc polycarboxylatecements. J Can Dent Assoc 1: 22-29, 1971.
Volume89 Number 3 9. Grieve AR, Jones JCG: Marginal leakage associated with four inlay cementing materials. Br Dent J 151: 331-334, 1981. 10. Rich JM, Leinfelder KF, Hershey HG: An in vitro study of cement retention as related to orthodontics. Angle Orthod 45: 219-225, 1975. 11. Mizrahi E: The recementation of orthodontic bands using different cements. Angle Orthod 49: 239-246, 1979. 12. Dennison JD, Powers JM: A review of dental cements Used for permanent retention of restorations. Part II. Properties and criteria for selection. J Mich State Dent Assoc 56: 218-225, 1974. 13. Tsukiboshi M, Tani Y: Physical properties of a polycarboxylate cement containing a tannin-fluoride preparation. J Prosthet Dent 51: 503-508, 1984. 14. Duperon DF, Jedrychowski J: Release and enamel uptake of fluoride from a fluoride-containing polycarboxylate cement. J Pedod 4: 287-294, 1980. 15. Hotz P, McLean JW, Sced I, Wilson AD: The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 142: 41-47, 1977. 16. Coury TL, Miranda FJ, Willer RD, Probst RT: Adhesiveness of glass-ionomer cement to enamel and dentin: A laboratory study. Oper Dent 7: 2-6, 1982. 17. Commission on Dental Products: Status report on the glassionomer cements. Chicago, 1981, Quintessence Publishing Co. 18. Negm MM, Beech DR, Grant AA: An evaluation of mechanical and adhesive properties of polycarboxylate and glass ionomer cements. J Oral Rehabil 9: 161-167, 1982. 19. Crisp S, Lewis BG, Wilson AD: Glass ionomer cements: Chemistry of erosion. J Dent Res 55: 1032-1041, 1976.
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20. McLean JW, Wilson AD: The clinical development of the glassionomer cement. II. Some clinical applications. Aust Dent J 22: 120-127, 1977. 21. Reisbick MH: Working qualities of glass ionomer-cements. J Prosthet Dent 46: 525-530, 1981. 22. Mount GJ, Makinson OF: Glass-ionomer restorative cements: Clinical implications of the setting reaction. Oper Dent 7:134141, 1982. 23. Bills RC, Yates JL, McKnight JP: Retention of stainless steel bands cemented with four dental cements. J Pedod 4: 273-286, 1980. 24. Shannon IL: A formula for human parotid fluid collected without exogenous stimulation. J Dent Res 46: 309, 1967. 25. Maldonado A, Swartz ML, Phillips RW: An in vitro study of certain properties of a glass ionomer cement. J Am Dent Assoc 96: 785-791, 1978. 26. Swartz ML, Phillips RW, Clark HE, Norman RD, Potter R: Fluoride distribution in teeth using a silicate model. J Dent Res 59: 1596:1603, 1980. 27. Retief DH, Bradley EL, Denton JC, Switzer P: Enamel and cementum fluoride uptake from a glass ionomer cement. Caries Res 18: 250-257, 1984. Reprint requests to: Dr. Pamela McInnes-Ledoux Department of Operative Dentistry LSU School of Dentistry 1100 Florida Ave. New Orleans, LA 70119