Radiopacity of impression materials Sigfiis Thor E~~LISSOII,D.D S., M.S.D., ‘!’ LI~ZLIBruce Hnaskrn, ** ReykjmYk, Ic~~lnnd, and Mintwupolis, MintI. The radiographic densities of twenty-six impression materials were measured and the values expressed as an equivalent thickness of aluminum. Under simulated clinical conditions, only ten of the materials tested could be consistently distinguished from the bone structure in periapical radiographs. It was far more difficult to detect objects with beveled margins than objects of uniform thickness. The minimum radiopacity of a material which could be detected in the periodontal tissue on a periapical radiograph was estimated.

R

adiopacity is a desirable property for all dental materials used in the mouth, so that if they are inadvel-tently embedded in the oral tissue they can be located radiographically. Radiopaque restorative materials can also be distinguished from tooth structure and secondary decay; faulty proximal contours and margins can be detected. While such metals as gold and dental amalgam have adequate radiopacity for dental purposes, most resins. both denture and restorative, are radiolucent and cannot be visualized on radiographlc examination. Numerous reports of patients who ha\,e ingested or aspirated denture-base acrylics have resulted in a plea for the use of only radiopaque materials in the mouth.“’ Many worthy contributions have been made to increase the radiopacity of resins.xm”’ Recently a few reports on adverse effects of impression materials have appeared in the dental literature. Kanarek” described a severe maxillary sinusitis following penetration of an elastomeric impression material into the antrum, through an oroantral fistula. Olson.‘” Price and Whitehead.“’ Clark,” Garey and Narang.” and Eliasaon and Holtc’” all reported cases in which elastomeric impression materials were injected subperioateallv during impression making of full crow>n preparations. Severe inflammatory response occurred in all caseb. O’Leary and associates” described similar foreign bodies and extensive bone loss in two patients in whom electrosurgery had been employed prior to the making of impressions with elastomeric materials. Many of the materials that may require identification at the time of oral examination and diagnosis may be in

evidence only upon radiographic examination. It is. therefore, important to know the radiographic properties of the commonly used dental materials. Degering and Busemanlx published the radiographic densities of a number of restorative materials along with the density of dry mandibular bone and a mandibular first molar. They compared the density values obtained from the scale of a photodensitometer. Sahs’!’ reported the roentgenographic densities ot‘ various pulp-capping materials, making visual comparisons uiih an aluminum penetrometer as a standard reference. Recently De Abreu and associates”” measured the radiopacity of twenty-eight restorative materials by photodensitometry. McArthur and Taylor”’ attempted to determine the minimum radiopacity of swallowed or aspirated objects, using a cadaver. They found that an object should have a radiopacity not less than that of 0.20 mm. of copper to be detected in films of the chest or abdomen. Their results showed also that it was more difficult to locate irregularly shaped and beveled specimens than square specimens of uniform thickness. The radiographic appearance of impression materials used for making impressions of crown and inlay preparations had received little attention. The purpose of this study was to measure the radiopacity of various elastic impression materials and to determine the minimum radiopacity that a foreign body must have to be detected in a periapical radiograph under simulated clinical conditions. METHODS

AND MATERIALS

Twenty-six elastic impression materials intended for making impressions of inlay or crown preparations were used in this investigation (Table I). All the materials were mixed and handled according to the manufacturers’ recommendations. ~X~.10-112017',:050~X5~07%00

7010 0

lY7Y l'he

C

\'

Mmhy

Co

485

Table I. List of impression materials used in this investigation

!VUVl? Po/wLlJ%le e/a.~torrrer A Coe-Flex (light) B Cot-Flex (’ Flexall

(regular) (light)

BUdI .Vll Base Cat Base Cat. Base (‘31. --

050276 OS0476 042 I76 050276 I5387

04.363

I: Image (light) F Image (rcgulari G lmprex

I

(hght)

0mniHex

J Omnifex

Fast Set

K Permlastlc

(light)

Oh0876 I I35

I. Pzrmlastlc

(regular)

Oh10761 l5fl

,bl Rubheqel

(regular)

N Super Rubber

Base 76688 (‘at 7663 ( 101

1S-?7h (UK37hi349 Base--7S24X C‘at 766 (‘ Xl

B

170

Base -- BA23403 7 Cat ~ C.43 I I

Base - R760!4;1 (‘at. C75356 Revwsihl~ /~utrocollo~ri X Surgident hqdrocollold syringe material Y Cartriloids (pink) % Heav) btxlied reversible hydrocolloid impression material

101OJ1 PFO88698t PF0987OSG

Rudiopacity

Volum Numbt

Fig. 1. Impression material specimens arranged on an occlusal film, together with md radiograph of the specimens. Differences in radiopacity are obvious.

Fig. 2. The radiographic xpcuimen~ in the DXTTR

setup of the DXTTR head

head to simulate

clinical

qf impression

muterial.s

487

a five-step aluminum StcpwedPe

condition5

and the position

of the

thickness. was placed on the films and exposed with the specimen\ (Fig. 1). Two supplementary aluminum blocks, -1 mm. thick. increased the thickness of the

penetrometer to the ranges of 4.5 to 8 mm. and 8.5 to 12 mm. The radiographic density of all the materials tested fell within the opacity limits of one of the three penetrometer ranges used. The radiation source used was a General Electric dental x-ray unit,* long cone, with a 2.7 mm. total aluminum filtration equivalence. The films were exposed for 24 seconds at 15 mA and 75 kVp. and at a target-to-film distance of 1 meter. Six exposures were made of each specimen, two with each thickness range of the penetrometer. Double-film packets were used, but only the film nearest to the object was utilized. The films were pro-

“Eastman

*Model 46- lS888061.

Five specimens of each material were made in a Teflon mold. This resulted in test pieces measuring 20 mm. long, 5 mm. wide, and 1.6 2 0.02 mm. thick. The hydrocolloid specimens were made just prior to testing, but the elastomeric materials were stored at room temperature from 3 to 6 hours. The specimens were arranged on Kodak Ultra-Speed occlusal dental films’~ with one specimen of each material on every film. A five-step aluminum penetrometer, the so-called

Kodak

stepwedge,

Company.

0.5,

Rochester.

1. 2, 3, and 4 mm.

N. Y.

in

General Electric Company.

Milwaukee.

Ws.

Mafertal

cexsed immediately in an Auveloper automatic x-t-a) film processor.” for 4.S minutes at 29” C. A MacBcth Quantalog dcnsitometerf was used to measure the inherent film base and emulsion fog densit!,. the radiographic density of the films under each impression material. and each step of the penetromctcr. The densitometer was allowed to warm up for I hour before LIX. For each exposure, a curve was plotted with the densitometer readings corresponding to the steps 01’ the penetrometer on the ordinate against the thickness of aluminum on the abscissa. When these curvt’s U’CK~ used. the density measured under the specimens could bc expressed as an equivalent thickness of alumimum. The density readings for the materials on each film wccrc recorded only when the densit) full within the range of the penetrometer. Ten densit! readings \+cri‘ obtained f‘or c\cry material. A dental x-ray teaching and tralmng head rcplicil. Model DXTTR 11:~LIas used to simulutc clinical condi tions (Fis. 2). The DXTTR head consists of a natural human skull covered with rubber and plastic foam settions for correct eutcrnal anatomic form and spcciull~ formulated tissue-equivalent isoc\.anatc rubber to rcproduce ;I dental radiogmphy setting. The DXTTR head is supported on 21structure which can be attached to the back 01‘a dental c~hair. Ont: specimen of each material was positioned on the left mandibular huc~~~lside in the molar region of the DXTTR head and secured in place with ;I w~all piece ot radiolucent cloublc-coated tape. Small rcf’erencc lint\

Fig. 4. Periapical radiographs from the simulated clinical study. a, Impression material qmAmen of uniform thicknesr as ;I foreign body. with radiopacity equivalent to 5.99 mm. aluminun. h, Same xpecimcn as in a afk~ the margins had been bweled. c, Impression material spccirnen of uniform thicktws as a foreign body. with radiopacity qui\ alent to I .36 mm. aluminum. d, Same specimen ;IX in c after the margins had been beveled. A t’ol-cign hod! in this radiograph L+:IS not detected by any of the cxamincrs. and ranked according to radiopacity in Table II. The radiopacity is expressed as an equivalent thickness of aluminum for the specimen thickness of I.6 mm. Samples of the radiographs from the simulated clinical study are presented in Fig. 4. The evaluation of the radiograph{ is summarized in Table II. The values presented are mean percent observability of the specimen on each film for both the beveled and the unbevcled test groups. None of the examiners w’as able to detect a foreign body image on the film in either test group when the radiopacity of the material was equivalent to 0.71 mm. of aluminum or less. All of the examiners diagnosed the presence of a foreign body, for the nonbeveled group, when the radiopacity was equal to or (rreater than that of 1.31 mm. of aluminum. A raGiopacity equivalent to at least 4.22 mm. of aluminum was required for a 100 percent diagnosis of foreign body presence Mhen the specimens were beveled, Thcrc wax some disagreement among the examiners in detecting the specimens with a radiopacity range ot 1.21 to I .X5 mm. of aluminum, especially in the beveled group.

DISCUSSION The radiopacity values obtained in this study are relative and apply only to a 1.6 mm. specimen thickness. The expression of radiopacity is a difficult and complex matter, because radiopacity depends. among other things. on the quality of the x-ray beam. In diagnostic radiography. the beam is of mixed wavelengths. and fluctuations in line voltage further offset the consistency of the beam. The method of exposing a pcnetrometer together with the specimens and of plotting a graph for each film minimizes these variation\ in measuring the radiopacity. A pilot study of the method used for measuring the radiopacity revealed that when the target-to-film distance was kept at 1 meter, the films used were exposed evenly throughout. The precision of the densitometer was also tested. The warm-up time for the type of densitometer used is at least 20 minutes. Repeated measurements of the density of the same spot without moving the film showed some fluctuations in the readings for additional 1.5to 20 minutes. The warm-up time for this study was set at 1 hour. The small standard devia-

490

Eliusson urd Haaskrn

tion for each material indicates that the sources of error had a minimal effect on the final results of the radiopacity measurements. The results from the second part of the study make it possible to divide the materials into three categories according to the observability of the materials (Table II). The first category, consisting of the materials which none of the ten examiners detected. had radiopacity values equivalent to or less than 0.71 mm. of aluminum. The materials in this group were all the reversible hydrocolloids tested, both of the polyether materials, and three of the silicone-base materials. The materials in the second category were those which could not be distinguished, radiographically, with certainty from the mandibular bone structure. These materials had radiopacity values equivalent to 1.21 to 1.85 mm. of aluminum. The results in this group clearly demonstrate that it is far more difficult to locate or distinguish from the bone structure specimens with beveled edges than specimens of uniform thickness. These tindings are in accord with those of McArthur and Taylor.” Most of the silicone materials and four of the polysulfide brands belong to this group. The last group consists of the materials that could be detected easily on the radiographs, with radiopacity values equivalent to or greater than 4.22 mm. of aluminum. All but four of the polysulfide materials belong to this group. The high lead peroxide content of the accelerator paste is rcsponsible for the radiopacity of many brands of the polysultide materials.” Since there is a large gap ( 1.X5 to 4.22 mm. ot aluminum) in the radiopacity values obtained in this study, from the highest value in the hard-to-locate category to the lowest value for the easily located categor! it is difficult to determine the exact minimum radiodensity for a material to be detected (easily) in a periapical radiograph. However. it can be estimated from the results presented in Table II that radiopacitj equivalent to at least 2 mm. of aluminum is needed. Of the twenty-six materials tested. only ten were detected on the radiographs by all the examiners when the specimens had been beveled. Even the most t’:idiopaque of the impression materials might not be dctected if the retained section was sufficiently thin. A section of material B 0.1 mm. thick would be roughlq equivalent. in radiopacity. to 0.5 mm. of aluminum. well within the undetectable range. With the trend toward replacement of the lead peroxide in the polysultide materials with radiolucent organic peroxides and the introduction of the very accurate but radiolucent polyether materials, this number is likely to decrease.“” The increasing number of reported cases in which inlpression materials have been embedded in the gingival

tissues, creating postoperative problems. and in which diagnosis has been made only by a roentgenologic method makes adequate radiopacity of these materials an important issue. CONCLUSIONS

1. Available impression materials Intended for making impressions for crown and inlay preparations display a large range of radiopacities. 2. The minimum radiopacity required of an impression material for detection as a foreign body in the gingival tissue was estimated to be not less than that of 2 mm. of aluminum 3. II is much more diflicult to detect an object with beveled or tapering margins than an equally radiopaque object of uniform thickness. 4. Reversible hydrocolloids. polyether tnaterials. and some silicone-base materials have a r:!diopacity which is too low to allow detection, on periapical radiographs. against the background of the mandibular bone structure, 5. Most of the polysultide material< Are radiopaque enough for radicgidpnlc detection. However. there appear; to be a trend toward replacement of the radiopa’lue lead peroxide in this group of mixterials with a more radiolucent organic peroxide. The

authors

~~tentl their apprecliitloll IO l)r. Richard .1

Schnell.

Professor

School

of Denttstrq.

prqxtration

01’ tilt\

of

Dental

Materials.

Indianapolis.

In&and.

Indiana

Llniversit)

t‘or l~clp

in the

manuscript

REFERENCES Bunker. I’