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  1. 1. Influence of prolonged set ting time on permanent deformation of elastomeric impression materials Markus Balkenhol, Prof Dr med dent,a Sylvia Haunschild,b Christina Erbe, Dr med dent,c and Bernd Wöstmann, Prof Dr med dentd Justus Liebig University, Giessen, Germany; University Medical Centre, Mainz, GermanyStatement of problem. Upon removal, tear forces occur in various areas of an impression. As a result, thin sulcusdetails may be permanently deformed, affecting the impression’s accuracy.Purpose. The purpose of this study was to investigate the effect of the chemistry and prolonged setting time on thepermanent deformation of light-body impression materials after stretching.Material and methods. A dumbbell-shaped mold (2 mm x 4 mm x 75 mm) was used to prepare specimens (n=20)of 6 different impression materials (Affinis Light Body, Aquasil Ultra XLV, Express 2 Light Body Flow Quick, FlexitimeCorrect Flow, P2 Polyether Light, and Impregum Garant L DuoSoft). After water storage (35°C), either for the manu-facturer’s suggested setting time or for 5 minutes, specimens were stretched by 80% using a universal testing machineat a crosshead speed of 200 mm/min. The permanent deformation (Δl (%)) was determined after 2 hours of storagein ambient conditions. A 2-way ANOVA followed by a Games-Howell test was used to analyze the influence of mate-rial and storage time (α=.05).Results. Polyether materials showed a significantly higher permanent deformation (Δl>4% to 5%) than vinyl polysi-loxane materials (P<.05). Extending the manufacturer’s suggested setting time resulted in clinically relevant improve-ments in elastic recovery for products with a polyether backbone only.Conclusions. Increasing the setting time might be necessary for impression materials with a polyether backbone toimprove elastic recovery. (J Prosthet Dent 2010;103:288-294) Clinical Implications For vinyl polysiloxane materials, an extension of the manufacturers’ recommended setting time does not provide a relevant improvement in elastic recovery. In contrast, increasing the manufacturers’ recommend- ed setting time may be advisable for polyether impression materials. Impression making is an essential rect application, the impression tray with finish lines located more than 2step during prosthodontic treatment and material combination, local con- mm subgingivally is a challenging pro-procedures, as the quality of an im- ditions in the oral cavity (periodontal cedure.pression significantly affects the accu- status, location of the finish line), For many years, impressions re-rate fit of the definitive restoration.1-3 and, finally, the properties of the im- quiring a high level of precision haveThe quality of an impression, in turn, pression material used.4-11 According been made with elastomeric impres-is influenced by many factors; namely, to the authors’ clinical experience, sion materials, such as polyether orthe impression technique and its cor- making impressions of prepared teeth polysiloxanes, due to their excellenta Assistant Professor, Department of Prosthodontics, Justus Liebig University.b Research Assistant, Department of Prosthodontics, Justus Liebig University.c Assistant Professor, Department of Orthodontics, University Medical Centre.d Professor, Department of Prosthodontics, Justus Liebig University.The Journal of Prosthetic Dentistry Balkenhol et al
  2. 2. May 2010 289chemical and physical material prop-erties.12-14 A fundamental property ofan elastomeric impression material,which significantly influences an im-pression’s accuracy, is its elastic recov-ery from deformation.2,15-17 Permanentdeformation can be related to the vis-coelastic properties inherent in elasto-meric impression materials.2,13,18-20 According to the ISO standard4823, permanent deformation of animpression material is determined aftercompression of a test specimen.21 The 1 Areas where light-body material is exposed to considerable tensile strain (typical clinical situation). A, Interproximal spaces with under-direction of load, therefore, reflects cuts. B, Undercuts beneath finish lines. C, Subgingival undercuts result-deformation that occurs horizontally ing in thin sulcus/finish line cuff of light-body impression material.in undercut areas toward the impres-sion tray upon removal of the impres-sion from the patient’s mouth.1,2,22-25 duce the risk of permanent deforma- manufacturers as vinyl polysiloxane However, tensile strain, occurring tion and inaccuracies. (VPS) materials and, therefore, po-in different areas of the set light-body While permanent deformation re- lymerize by a Pt-catalyzed hydrolyza-material upon removal of the impres- sulting from tensile strain has been tion reaction of vinyl end groups.13,31sion, may be even more critical with investigated in a number of prior Impregum is a conventional polyetherrespect to permanent deformation.26 studies,15,17,28,30 this clinically impor- that polymerizes by ring-openingElastic recovery resulting from ten- tant issue has only been reported in polymerization of the end groups ofsile strain reportedly decreases with a single contemporary investigation an ethylene-imine terminated poly-increasing strain and could not be that compared the influence of the ether compound.13 P2 is classifiedpredicted from results obtained from deformation’s magnitude (50% strain as a polyether by its manufacturer.compression tests.27 Tensile load clini- vs. 100% strain).27 This study did not, However, as P2 polymerizes via ancally occurs in undercut areas and in- however, cover the entire range of acid-catalyzed cross-linking reactionterproximal spaces, particularly in the frequently used impression materials of silane end groups connected tothin cuff of light-body impression ma- (vinyl polysiloxane, polyether, and hy- a polyether backbone,32,33 it shouldterial injected into the gingival sulcus of brid polyether siloxane). In addition, rather be considered a hybrid poly-the prepared tooth (Fig. 1).13,26,28 Upon the study did not test whether or not ether siloxane, similar to the productremoval of the impression, the thin sul- increasing the setting time might de- Senn (GC Corp, Tokyo, Japan).27 Allcus/finish line cuff is stretched and may crease the permanent deformation re- materials were delivered in automixingeasily tear.13,24,26,28 In clinical practice, sulting from tensile strain. Therefore, cartridges and used according to theirtorn areas are easily detectable, but the purpose of the present study was respective manufacturers’ instructionsoften require remaking the impression. to answer these important questions in ambient laboratory conditions (23Inaccuracies resulting from permanent relevant to the clinical application of ±1°C; 50 ±10% relative humidity).deformation, in contrast, can rarely be elastomeric impression materials. Before the mixing tips were at-detected, resulting in distorted impres- The following twofold null hy- tached to the automixing cartridges,sions and imperfectly fitting definitive pothesis was tested: the permanent the orifices of both chambers wererestorations.2,13 deformation of a light-body impres- inspected for plugging. The impres- The chemistry of the material used, sion material would not be affected sion materials were directly dispensedlayer thickness, duration of deforma- by (1) its chemistry or (2) the setting from the mixing tip into the dumb-tion, magnitude of strain, as well as time period at 35°C prior to stretch- bell-shaped mold (2  mm x 4  mm xthe degree of polymerization are fac- ing the specimens. 75 mm),27 which featured 2 semicir-tors which reportedly influence an cular measuring marks on each side,impression’s ability to recover from MATERIAL AND METHODS positioned 20 mm apart (Fig. 2). Thedeformation upon removal from the mold was placed between 2 glassmouth.13,15,20,22,24,28,29 Extending the in- Table I lists the materials used slides, covered with polyester foil (Ho-traoral setting time may improve the in the study to prepare dumbbell- staphan RN 50; Pütz, Taunusstein,elastic properties of the polymerized shaped specimens. The materials Af- Germany), and placed beneath a par-impression material, due to a higher finis, Aquasil, Express 2, and Flexitime allel press (Leitz GmbH, Wetzlar, Ger-degree of polymerization,13,28 and re- are characterized by their respective many) to obtain uniform specimensBalkenhol et al
  3. 3. 290 Volume 103 Issue 5 Table I. Impression materials investigated Manufacturer’s Recommended Chemical Product Manufacturer Lot Setting Time (min) Type Affinis Fast Coltène/Whaledent AG, 94622 2:00 VPS Light Body Altstätten, Switzerland Aquasil Ultra Dentsply DeTrey GmbH, 60412 3:00 VPS XLV Fast Set Konstanz, Germany Express 2 Light 3M ESPE, 242260 2:30 VPS Body Flow Quick Seefeld, Germany Flexitime Heraeus Kulzer GmbH, 230525 2:30 VPS Correct Flow Hanau, Germany P2 Polyether Heraeus Kulzer GmbH 290143 3:15 hybrid polyether Light siloxane* Impregum Garant 3M ESPE B 293554 3:30 polyether L Duo Soft C 293427 VPS: Vinyl polysiloxane (*As P2 Polyether Light polymerizes via acid-catalyzed cross-linking reaction of silane end groups connected to polyether backbone, it is described as hybrid polyether siloxane.32,33 Information provided by respective manufacturers) 2 Schematic drawing of specimen configuration (upper: top view; lower: side view). Dimensions: l = distance between measuring marks (20 mm); w = width (4 mm); h= height (2 mm); o = overall length (75 mm). of the desired thickness. The speci- yielded an adequate power (0.80) to 1454; Zwick Roell AG, Ulm, Ger- mens were placed in a water bath (Ju- detect statistically and clinically rel- many), stretched by 80% of their labo Labortechnik GmbH, Seelbach, evant significant differences. original length at a crosshead speed Germany) at 35°C, 25 seconds after After removal from the water of 200  mm/min, and released after the start of mixing, and allowed to bath, the specimens were removed 2 seconds. set for the manufacturers’ suggested from the mold and trimmed using a The specimens were placed on intraoral setting time (Table I) and for scalpel (No. 11; Feather Safety Ra- PMMA beads (Palavit G Powder; Her- 5 minutes, respectively (n=10 per im- zor Co, Osaka, Japan). Subsequently aeus Kulzer GmbH, Hanau, Germany) pression material and storage time). (1.5 minutes following removal from to allow unrestrained dimensional A sample size of 10 was chosen, since the water bath), the specimens were changes in ambient laboratory con- comprehensive preliminary tests had clamped in a tension-free manner in ditions for 2 hours (±10 minutes), demonstrated that this sample size a universal testing machine (model and aligned with a transparent plastic The Journal of Prosthetic Dentistry Balkenhol et al
  4. 4. May 2010 291pane on the XY stage of a travelling used to test the influence of the im- deformation varied greatly betweenmicroscope (M420; Leica Microsys- pression material and storage time, the different materials at the manufac-tems, Wetzlar, Germany). The dis- respectively. Due to a lack of homo- turer’s recommended intraoral settingtance between both measuring marks geneity of variances, a Games-Howell time (0.03% to 6.23%), as well as afteron each side was determined 3 times test was applied for statistical com- 5 minutes of storage (0.04% to 5.01%).at ×40 magnification, and mean val- parison of the impression materials The polyether materials showed a sig-ues were calculated. The following at each storage time. The influence nificantly greater permanent defor-formula was used to determine the of storage time was tested for each mation (P<.05) compared to the VPSpermanent deformation (Dl (%)): material using a t test for unpaired materials tested. Aquasil Ultra XLV sample groups. All statistical analyses showed a significantly greater perma- l1 - l 0 l= x100 were carried out at a significance level nent deformation than all other VPS l0 #%%! of .05 and performed using statistical materials (P<.05). A prolongation ofwhere: l1 equals the length of specimen software (SPSS 15.01; SPSS, Inc, Chi- storage time in water significantly re-after stretching  (mm), and l0  equals cago, Ill). duced the permanent deformation inthe  original length of the specimen 4 of the 6 materials tested (P<.05).(mm). RESULTS Both the impression material and Data were subjected to a Levene storage time had a significant influ-test to check for homogeneity of vari- Table II displays the mean values ence on the permanent deformationances and to a Kolmogorov-Smirnov and standard deviations of the per- (Table III). In addition, the impressiontest for normal distribution. The manent deformation (Dl  (%)) for all material and the storage time influ-mean values and standard deviations materials and includes the results of enced the permanent deformation inwere calculated. A 2-way ANOVA was the statistical analysis. The permanent interaction (P<.001). Table II. Mean values (standard deviations) of permanent deformation (Δl (%)) I (%) Manufacturer’s Recommended I (%) Product Setting Time (min) 5 min P* Affinis Light 0.03 (0.09) a – 0.04 (0.02) a .029 Body Fast Set Aquasil Ultra 1.33 (0.16) b 1.14 (0.16) b .019 XLV Fast Set Express 2 Light 0.10 (0.09) a 0.08 (0.02) c .510 Body Flow Quick Flexitime Correct 0.28 (0.10) c 0.21 (0.12) c .147 Flow Impregum Garant 5.91 (0.44) d 5.01 (0.34) d <.001 L Duo Soft P2 Polyether Light 6.23 (0.70) d 4.30 (0.63) d <.001 Same lowercase superscript letters in columns denote material groups that do not differ significantly (Games-Howell test, P >.05, n = 10) regarding permanent deformation within 1 setting time group. *P values of the pairwise comparison using t test for independent sample groupsBalkenhol et al
  5. 5. 292 Volume 103 Issue 5 Table III. Results of the 2-way ANOVA to identify influence of material and storage time on permanent deformation of materials tested Type III Mean Source Sum of Squares df Square F P Corrected model 707a 11 Intercept 503 1 Material 679 5 136 1273 <.001 Storage 8.4 1 8.4 79 <.001 Material x storage 14.4 5 2.9 27 <.001 Error 11.5 108 Total 1216 120 Corrected total 713 119 aR squared =.984 (adjusted R squared =.982) DISCUSSION influenced the results. However, as storage in water at 35°C, in addition the coefficient of thermal expansion to the manufacturers’ suggested intra- The primary purpose of this study is low for elastomeric impression ma- oral setting time, as recommended by was to evaluate whether or not an im- terials (polyether, 322 ppm/°C; VPS Nayyar et al.6 Longer storage periods pression material’s chemistry as well as light body, 208 ppm/°C), this param- prior to strain application were not the setting time period at 35°C might eter is regarded as negligible.35 used as, in the authors’ experience, affect its permanent deformation af- The magnitude of compression such time frames are regarded as clini- ter stretching. The results support or elongation reportedly influences cally unacceptable for daily practice. rejection of the first part of the null the extent of permanent deforma- The selected study design allowed hypothesis. The second part of the null tion.20,22-24,27 The amount of elongation an accurate determination of Dl for hypothesis, however, was not rejected, in the current experiment, compared to all materials apart from P2 Polyether since the permanent deformation was the study by Mansfield and Wilson,15 Light. The high standard deviation not significantly influenced by the set- provided that no rupture occurred of P2 may be related to an inhomo- ting time for all materials tested. during testing. Other investigators geneous result from mixing, which The mold used and the specimen have tested materials using greater seems to be a general problem with preparation were identical to those in strains.17,27 As manufacturers gener- this product.22 The vinyl polysiloxane the study by Lawson et al.27 However, ally recommend that impressions (VPS) materials showed low perma- in contrast to that investigation, the be removed quickly, elongation was nent deformation (Dl<0.3%), with the present study did not subtract shrink- performed rapidly.28 A longer duration exception of Aquasil Ultra XLV. In con- age effects caused by polymerization of strain reportedly results in great- trast, Lawson et al27 observed a lower contraction from the permanent de- er permanent deformation.23,28 To permanent deformation for Aquasil. formation,13,29,34 as the final perma- provide the specimens sufficient time However, it is not clear whether the nent deformation of an impression is for elastic recovery, measurements XLV version of the product was used. a combined result of viscoelastic and were performed 2 hours after strain Klooster et al28 reported similarly low shrinkage phenomena. Determining application.14,20,23,27 values measured 1  hour after strain the sum of these effects seemed to be Dental impression materials may application for VPS impression ma- more relevant to the clinical situation. not be completely polymerized after terials. Unlike in the present study, This approach explains the negative the manufacturers’ suggested intra- however, Klooster and colleagues permanent deformation obtained oral setting time and, in turn, may not stretched the specimens until rupture for Affinis Light Body Fast Set. In ad- have fully developed elastic properties and determined the permanent defor- dition, differences in the thermal ex- when removed from the mouth.5,13 mation after reassembling the ends pansion coefficient obtained for the Therefore, in this study, the tensile at various points in time after tear various materials tested may have strain was applied after 5 minutes of strength testing. The Journal of Prosthetic Dentistry Balkenhol et al
  6. 6. May 2010 293 The significant differences ob- between oligomer chains has ad- REFERENCESserved between the VPS materials are vanced sufficiently at the manufactur- 1. Braden M, Inglis AT. Visco-elastic prop-not expected to be clinically relevant er’s recommended intraoral setting erties of dental elastomeric impressionregarding the precise fit of definitive time, exhibiting highly elastic proper- materials. Biomaterials 1986;7:45-8.restorations.26 The slightly larger per- ties at this stage. 2. Jörgensen KD. A new method of recording the elastic recovery of dental impression ma-manent deformation of Aquasil Ultra In contrast, the improvements terials. Scand J Dent Res 1976;84:175-82.XLV is presumably induced by this ma- regarding elastic recovery were no- 3. McCabe JF, Walls AW. Applied dental mate-terial’s different filler content and/or ticeably higher for the polyether rials. 9th ed. Oxford: Wiley-Blackwell; 2008. p. 136-77.lower cross-linking density compared materials. 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Wöstmann B, Blösser T, Gouentenoudis M,ally considered that the duration of deduced from other test configurations, Balkenhol M, Ferger P. Influence of marginstraining was relatively short in the this test should be included in the ISO design on the fit of high-precious alloy res-present study and can exceed that standard (ISO 4823). torations in patients. J Dent 2005;33:611-8. 10.Endo T, Finger WJ. Dimensional accuracytime period in clinical situations,16 Because this was an in vitro study, of a new polyether impression material.which would result in even greater certain limitations exist regarding Quintessence Int 2006;37:47-51.permanent deformation.15,22 Klooster the application of the results to the 11.Blatz MB, Sadan A, Burgess JO, Mercante D, Hoist S. Selected characteristics of a newet al28 recorded values of similar scale clinical situation. The specimens had polyvinyl siloxane impression material--afor a medium-bodied polyether (4.3 a cross-section of 2 x 4 mm, which is randomized clinical trial. Quintessence Int±1.1%), whereas Hondrum17 found much thicker than the thin sulcus areas 2005;36:97-104. 12.Chee WW, Donovan TE. Polyvinyl siloxanesimilarly low values for polyether and that are observed clinically. Addition- impression materials: a review of prop-VPS impression materials (<1%). ally, the amount of stretching might erties and techniques. J Prosthet Dent It is hypothesized that the dif- vary clinically depending on sulcus 1992;68:728-32. 13.Anusavice KJ. Phillips‘ science of dentalferent results for the polyether ma- depth and actual tear forces. However, materials. 11th ed. St. Louis: Elevier; 2003.terials may be due to breakage of both parameters were kept constant in p. 205-54.polymer chains and a weaker or less the current study for standardization 14.Goldberg AJ. Viscoelastic properties of sili- cone, polysulfide, and polyether impressionelastic polymer network, respective- purposes and proper testing. Notwith- materials. J Dent Res 1974;53:1033-9.ly.24 Other authors came to a similar standing these limitations, the authors 15.Mansfield MA, Wilson HJ. A new method for determining the tension set of elas-conclusion,22,27 as elastic recovery of assume that the results are clinically tomeric impression materials. Br Dent Jpolyether-based materials tended to relevant. As the current study focused 1973;135:101-5.be lower compared to VPS materials on fast-setting vinyl polysiloxane mate- 16.Blomberg PA, Mahmood S, Smales RJ, Makinson OF. Comparative elasticity testsin shear tests.22 rials, future investigations should ana- for elastomeric (non putty) impression Although prolonged polymeriza- lyze regular-set materials. materials. Aust Dent J 1992;37:346-52.tion in water for 5 minutes significantly 17.Hondrum SO. Tear and energy properties of three impression materials. Int J Prostho-reduced the permanent deformation CONCLUSIONS dont 1994;7:517-21.for the majority of the products tested, 18.McCabe JF, Wilson HJ. Polymers in den-the improvement in elastic recovery was Increasing the manufacturers’ tistry. J Oral Rehabil 1974;1:335-51. 19.Craig RG. Review of dental impressionsmall for the VPS materials. Therefore, recommended intraoral setting time materials. Adv Dent Res 1988;2:51-64.a general recommendation to increase does not provide relevant improve- 20.de Araujo PA, Jorgensen KD, Finger W.intraoral setting time is probably un- ments in elastic recovery for vinyl poly- Viscoelastic properties of setting elasto- meric impression materials. J Prosthet Dentnecessary for this group of materials.6,28 siloxane materials; however, it may for 1985;54:633-6.Presumably, the cross-linking reaction polyether materials.Balkenhol et al
  7. 7. 294 Volume 103 Issue 5 21.International Organization for Standard- 28.Klooster J, Logan GI, Tjan AH. Effects of 35.McCabe JF, Storer R. Elastomeric impres- ization. ISO Specification No. 4823:2000. strain rate on the behavior of elastomeric sion materials. The measurement of some Dentistry - elastomeric impression materi- impression. J Prosthet Dent 1991;66:292-8. properties relevant to clinical practice. Br als. 3rd ed. Geneva: ISO; 2000. Available at: 29.Kim KM, Lee JS, Kim KN, Shin SW. Dimen- Dent J 1980;149:73-9. http://www.iso.org/iso/store.htm sional changes of dental impression materi- 22.Endo T, Finger WJ. Evaluation of the elastic als by thermal changes. J Biomed Mater Res Corresponding author: recovery of polyether impression materials. 2001;58:217-20. Dr Markus Balkenhol Am J Dent 2005;18:355-60. 30.Kaloyannides TM. Elasticity of elastometer Justus Liebig University 23.Inoue K, Wilson HJ. Viscoelastic properties impression materials. II. Permanent defor- Department of Prosthodontics of elastomeric impression materials. III. mation. J Dent Res 1973;52:719-24. Schlangenzahl 14, 35392 The elastic recovery after removal of strains 31.Rosenstiel SF, Land MF, Fujimoto J. Con- Giessen applied at the setting time. J Oral Rehabil temporary fixed prosthodontics. 4 th ed. St. GERMANY 1978;5:323-7. Louis: Elsevier; 2006. p. 431-65. Fax: +49-641-9946 139 24.Jamani KD, Harrington E, Wilson HJ. The 32.Balkenhol M, Wöstmann B, Kanehira E-mail: markus.balkenhol@dentist.med.uni- determination of elastic recovery of impres- M, Finger WJ. Shark fin test and impres- giessen.de sion materials at the setting time. J Oral sion quality: a correlation analysis. J Dent Rehabil 1989;16:89-100. 2007;35:409-15. Acknowledgments 25.Sotiriou M, Hobkirk JA. An in vivo investi- 33.Kanehira M, Finger WJ, Endo T. Volatil- The authors thank Michael Köhl, Dipl Ing, and gation of seating and removal forces associ- ization of components from and water Dr Jürgen Riehl for their assistance with the ated with recording impressions in dentate absorption of polyether impressions. J Dent statistical analysis. In addition, the authors patients. J Prosthet Dent 1995;74:455-62. 2006;34:134-8. thank the respective manufacturers for the 26.Laufer BZ, Baharav H, Ganor Y, Cardash 34.Lampé I, Marton S, Hegedüs C. Effect donation of materials. HS. The effect of marginal thickness on the of mixing technique on shrinkage rate of distortion of different impression materials. one polyether and two polyvinyl siloxane Copyright © 2010 by the Editorial Council for J Prosthet Dent 1996;76:466-71. impression materials. Int J Prosthodont The Journal of Prosthetic Dentistry. 27.Lawson NC, Burgess JO, Litaker MS. Tensile 2004;17:590. elastic recovery of elastomeric impression materials. J Prosthet Dent 2008;100:29-33. Noteworthy Abstracts of the Current Literature In vitro cytotoxicity evaluation of elemental ions released from different prosthodontic materials Elshahawy WM, Watanabe I, Kramer P. Dent Mater 2009;25:1551-5. Epub 2009 Aug 13. Objectives. This study investigated the cytotoxicity of elemental ions contained in four fixed prosthodontic materials (gold, nickel-chromium, stainless-steel alloys and CAD-CAM ceramics). Materials and methods. According to the determination of elements released from prosthodontic materials by using inductively coupled plasma mass spectroscopy, similar amounts of elements Pd, Ag, Zn, Cu, Ni, Cr, Mo, Be, Fe, Al, and K were prepared as salt solutions. Wells with a tenfold higher concentration of the tested elements were used as positive controls, while a well without any tested element was used as a negative control. These salt solutions were tested for cytotoxicity by culturing mouse L-929 fibroblasts in the salt solutions for a 7-day period of incubation. Then, the percentage of viable cells for each element was measured using trypan blue exclusion assay. The data (n=5) were statistically analyzed by ANOVA/Tukey test (p<0.05). Results. The results showed a statistically significant difference for the cytotoxic effect of the tested elements salt solutions. For the released element concentrations the lowest percentage of viable cells (mean ± SD) was evident with Zn, Cu or Ni indicating that they are the highly toxic elements. Be and Ag were found to be intermediate in cytotoxic effect. Fe, Cr, Mo, Al, Pd or K were found to be the least cytotoxic elements. Significance. Zn and Cu released from gold alloys, and Ni released from nickel-chromium alloys, which are commonly used as fixed prosthodontic restorations, show evidence of a high cytotoxic effect on fibroblast cell cultures. Reprinted with permission of the Academy of Dental Materials. The Journal of Prosthetic Dentistry Balkenhol et al