A Comparison of theAccuracy of Two

Removable Die SystemsWith Intact Working Casts

Philippe AramouniFormer Graduate StudentDepartment oí Prosthodontics

Philip Mitlstein, DMD, MSClinical ProfessorDepartment ol Biomateriäls

Boston UniversityGoldman School of Gtadugte Dentistry100 East Newton StreetBoston, Massachusetts 021 ¡8

This study evaluated the reproducibility of die position using two removabledie systems and two die stones. Poly(v¡nyl siloxane) impressions were madeof a stainless steel, U-shaped arch with four evenly spaced abutments. Sixgroups were evaluated: Zeiser system/Fuji Rock; Zeiser system/Die Keen;solid cast/Fuji Rock; solid cast/Die Keen; Fuji Rock/Pjndex; and DieKeen/Pindex. An optical comparator was used to measure the height of eachabutment, the distance between the anterior abutments, and the distancebetween the posterior abutments. The Zeiser system with either Fuji Rock orDie Keen yielded the greatest accuracy. Die Keen exhibited more linearexpansion than Fuji Rock, and solid casts had less distortion than the Pindexsystem, ¡nt I Prosthodont 1993;6:533-539.

R emovable die systems are frequently used tofacilitate the manipulation of dies during the

laboratory phase of fixed prosthesis fabrication.'-However, the separation of individual dies fromsolid casts requires that they be replaced in pre-cisely the same position that they had occupiedprior to removal. Both the setting expansion of thestone and the specific removabie die system willaffect die replacement accuracy, and the expansionmay create measurable shifts in die position.' '

Hard dental stones are now the most popular diematerials because of their ease of manipulation,low cost, and suitability for use with elastomericimpression material. They are, however, sub|ect tolinear setting expansion. The percentage of linearexpansion may vary from 0.05% to 0.27% depend-ing upon the die stone used."

Contemporary die systems, such as Pindex(Coltene-Whaledent, New York, NY), incorporatedie pins into a die stone cast." A stone base is thenpou red against the cast contain ing the die pins. Thestone cast Is subsequently sectioned to provideremovable dies that can, presumably, be preciselyand accurately replaced into the stone base. It

Reprint requests: Dr Millstein.

Presented at the AADR Annual Meeting Boston, MA, 1992.Presented in partial iulfillment for a Master of Science degree-

inescapable that, even with contemporarysystems such as the Pindex system, stone expan-sion will affect die position.

More recent die systems, such as the Zeiser sys-tem (Girrbach Dental, Santa Rosa, CA), incorporatethe die pins into a premachined acrylic resin basethat is placed into the die stone when it is pouredinto the impression." The dies are sectioned andreplaced into the resin base after the stone has set.According to previous research,' • the Zeiser sys-tem predetermines die pin placement, which mini-mizes the effects of stone expansion.^ '"

The purpose of this study was to measure andcompare the shitt in abutment position that arisesfrom cast expansion using two different die stonesand three different die systems.

Materials and Methods

A U shaped, cast stainless steel model consistingof four evenly spaced abutments measuring ap-proximately 6 mm in height was used for testing.The anterior and posterior abutments were 4 mmand 5 mm in diameter, respectively, and had a 5-degreetaper anda 1-mm flat circumferential shoul-der (Fig 1).

Acrylic resin custom trays (Formatray, Kerr,Romulus, Ml) were fabricated using a 3-mm spacebetween the steel cast and the acrylic resin to

Volume 6, Number 6,1993 533 The International lojcnElI ol Prostliodoritics

of Removable Die Systen

Fig 1 Distances in mm between abutments; beight of abut-ments shown in inset.

Fig 2a Standard metai model with (our abutments.

Fig 2b Impression making with oustom tray; note the threeverticai struts used as guideiines for repeatabie tray positioning.

achieve a constant impression material thickness^Three fixed vertical columns on the steel archserved as guideposts to ensure the identical place-ment of all trays during the impression procedure.The custom trays were stored at room tempe rain refor 24 hours to allow complete resin polymerization.

Forty impressions were obtained with the acrylicresin custom trays using a polylvinyl sili'xane) im-pression material (Reprosii, LD Caulk, Miiford, DE).The internal surfaces of tbe trays were painted withadhesive (LD Caulkl and allowed to dry for 10 min-utes. All impressions were made using automixprocedures. The light-body material was injectedaround the abutments using a disposable syringe,and an equal number of medium- and heavy-bodyimpression materials were loaded directly into thetrays. Impressions were made by positioning theloaded tray onto the test cast using the three verti-cal stops as a guide for tray positioning {Figs 2a and2b). Set impressions were removed from the testcast after 15 minutes and stored in air for 1 hourat 25°C.

Six groups of 10 die stone/die systems were pre-pared using two different die stones and three diesystems. Two die stones were used: Fuji Rock (GCAmerica, Scottsdale, AZ) and Die Keen (MilesColumbus, St Louis, MO). The water-powder ratioswere 100 g powder to 20 mL distilled water for FujiRock and 120 g powder to 27 mL water for Die Keen.Both die stones were vacuum mixed for 15 secondsprior to impression casting. The die systems con-sisted of solid casts and Pindex and Zeiser remov-able die systems (Fig 3), The groups were divided asfollows: group 1 = Fuji Rock/Zeiser system; group2 = Die Keen/Zeiser; group 3 = Fuji Rock/solidcasts: group 4 = Die Keen/solid casts; group 5 =Fuji Rock/Pindex system; and group 6 = Die Keen/Pindex,

The stone casts of all six groups were separatedfrom their impressions after 24 hours and stored atroom temperature. Tbe casts of groups 1 and 2were sectioned after 72 hours and evaluated. Castsfrom groups 3 and 4 were measured as being repre-sentative of solid casts, after which they werepinned using the Pindex system and sectioned.Pinning involved gluing (Krazy Glue, B |adow. NewYork, NY) brass pins into holes that bad been drilledinto the underside of the solid casts and placing theprepared cast into yellow stone (Clint Yellowstone,Clint Sales, Beverly, MA) that had been mixed with aratio of 200 g powder to 50 mL distilled water andpoured into a plastic base former (Coltene-Whaledent, New York, NY]. The casts were sec-tioned after the bases had been allowed to set for 24bours.

The International lourni l of Prosttiodontics S34 VolumeÉ, NumberÉ, 1993

son of Removable Oie Systen

Fig 3 Zeiser (¡ett> and Pindex(right) systems.

Measurements were made using an opticalcomparator (Jones and Lamson, Model FC-14,Springfield, VT) having an accuracy of ±2 n,m. Thecomparator included a screen with horizontal andvertical reference lines and a movable table thatallowed the object being studied to be positionedon the screen. The device has a light source thatprojects a magnified image of fhe object onto thescreen in the form of a shadow, so that the sharpedges of the silhouette become the referencepoints of measurement. All casts were secured to auniversally movable surveyor table INey, Hartford,CT) and adjusted to identical positions on thescreen using the horizontal reference line as aguide, A standard error of ±5 (̂ .m for repeatedpositioning and remeasurement was used for allmeasurements. A magnification of 10 x was usedfor measurement, and six different measurementswere made. Each individual measurement wasmade three times, and the means of the threereadings were used as the outcome measure. Mea-surements included;

1, The distance between the outer surfaces of thetwo anterior abutments

2, The distante between the outer surfaces of thetwo posterior abutments

3, The height of the left anterior abutment (fromshoulder to occlusai surface)

4, The height of the right anterior abutment Ifromshoulder to occlusal surface)

5, The height of the leff posterior abutment (fromshoulder to occlusal surface)

6, The height of the right posterior abutment (fromshoulder to occlusal surface)

Each measurement obtained from the casts ingroups 1 tofewascompared to its counterpart in thecontrol group of stainless steel prototypes.

The data were analyzed using a one-way analysisof variance (ANOVA) followed by the Newman-Keuls multiple comparison procedure. The ac-cepted level of significance for the ANOVA was P <,05, and the Newman-Keuls procedure routinelycompares two group means at the P < ,05 signifi-cance level.

Results

Comparison of the heights of the different abut-ments (Fig 5) showed that group 1 (Zeiser/Fuji-Rock) most accurately reproduced the prototype.

Mean Dislance Between the Anterior Abutments

The one-way ANOVA in all the experiments thatmeasured the variations in the distance betweenthe two anterior abutments yielded a significanceof P < ,001, The Zeiser system was significantlymore accurate than was the solid cast, which wassignificantly more accurate than the Pindex system.

The Newman-Keuls test showed (P< ,051 that thedifferences in the distances between the two ante-rior abutments of groups 1 (Fuji Rock/Zeiser; 21.5(i.m) and 2 ¡Die Keen/Zeiser; 24,8 |j.m), though sig-nificanfly less than fhe differences observed amongall the other groups, were not significantly differ-enf from each other (Fig 4 [A], Table 1).

The distance between the two anterior abut-ments was significantly less for group 3 (Fuji Rock/Solid Cast; 39,0 p.m) than for group 6 (Die Keen/Pindex; 143,

Mean Distance Between Posterior Abutments

The Zeiser system was significantly more accu-rate than the solid cast, which was significantlymore accurate than the Pindex system. Again, the

, _ Volume 6, Number 6, 1993 535 ournEil ol Prostliodontics

Comparison ot Removable Ole Syste Aramouni/Millstp

200

150

.ÍS 100

50

Anterior Distance

r I Die Keen

T

IZeiser Solid Cast Pirflex

200

150

100

50

Posterior Distance B

^ÁI

ë

Zeiser Solid Cost Pirde«

Fig 4 Mean distance between outer suriaces ot the anterior abutments and the outer surtaces ot the posterior abutments.

GO

40

Left Anterior

Left Posterior

^ FUI I-Rock

- • Die Keen

20

10

i 1

m\m\m

Rigt)t Anterior

1

iRight Posterior

1

IZsisei Solid Casi Pindei Zeiser Solid Cost Pinflei

Fig 5 Variations in height ol thelour abutments measured tromshoulder to top surface of the abut-ment.

The international Journal o¡ Proíthodontics 5 3 6 Volume 6, Number 6,1993

Companson of Re able Die Syslen

ANOVA of the distances between the posteriorabutments for all samples was significant at P <.001. The Newman-Keuls test confirmed the signifi-cance (P < .05) of the observed disparities amongall the groups with respect to the distances be-tween the posterior abutments {Fig 4 [B], Table 1),The minimum distance recorded between poste-rior abutments with Fuji Rock/Zeiser was 9.3 |j,m,whereas the maximum distance between posteriorabutments was 176,6 \i.rr\ for Die Keen/Pindex.

Mean Height of Left Anterior Abutment

The one-way ANOVA test of the measured heightoí the left anterior abutment in each group yieldeda significance of P < .001. The Newman-Keuls testshowed that the mean height ot the left anteriorabutment in groups4 (Die Keen/solid cast; 41.2 |i.m)and 6 {Die Keen/Pindex; 41.2 \im) was significantlygreater than the mean height of the same abutmentin groups2 (Die Keen/Zeiser; 19.3 [im),3 (Fuji Rock/solid cast; 18.0 fim), and 5 (Fuji Rock/Pindex; 18.0|j.m) (Fig 5 [A], Table 2). The same findings shown inboth groups 4 and 5 are repetitions of the same dataand were not measured.

The mean height of the left anterior abutmentproduced by group 1 {Fuji Rock/Zeiser; 5.2 |xm)was

Table 1 Distance {|i.m) Between Anterior and PosteriorAbutments

Stone.'die system

Fuji Rock.'ZeiserDie Keen/ZeiserFu¡i RocK'Solid CastDie Keen/So lid CastFuji RockiPindexDie Keen/Pindex

Anterior Distance'Mean (SD)

21.5 (4.7)24.8 (7,4)39.0 (6.9)79 3(14.8)

117.6(19.8)143.0(10.6)

Posterior Distance'Mean (SD)

9,3 (4.0)29.7 (7.3)47.5 (4.9)80.4(11.9)

125.9(21.2)176.6(26.9)

•ANOVA followed by Ne*man-Keuls test (P < 05): all pairwise comparisons are signiticantly différent except Zeiser • FUJI.Rock vs Die Keen.

significantly smaller than that found for all othergroups.

Mean Height of Right Anterior Abutment

The one-way ANOVA of the measured height ofthe right anterior abutment in each group yielded asignificance of P < ,001. The Newman-Keuls testshowed that the mean height of the right anteriorabutment for groups 2 (Die Keen/Zeiser; 25.3 p.m), 3(Fuji Rock/solid cast; 27.7 (xm), and 5 {Fuji Rotk/Pindex; 27.7 |xm), and were significantly less (P <.05) than those observed for groups 4 and 6 (DieKeen/solid cast and Die Keen/Pindex; both 47.8 ¡xm).

FHowever, group 1 (Fuji Rock/Zeiser) produced amean heightof the right anterior abutment (7.5 |im)that was significantly less (P < .05) than that foundfor all the other groups (Fig 5 [B]). The same find-ings seen in both groups 4 and 5 are repetitions ofthe same data and were not measured.

Mean Height of Left Posterior Abutment

The one-way ANOVA of the measured height ofthe left posterior abutment in each group yielded asignificance of P < .001. The Newman-Keuls testshowed that the mean height of the left posteriorabutment for groups 1 (Fuji Rock/Zeiser; 6.3 |j.m), 3(Fuji Rock/solid cast; 6.1 (xm), and 5 {Fuji Rock/Pindex; 6.1 \im) were significantly less {P < .05)than that in group 2 (Die Keen/Zeiser; 12.4 |i,m),which in turn was less than that in groups 4 {DieKeen/solid cast; IS.9 ¡xm) and 6 (Die Keen/Pindex;18.9 [tm) (Fig 5 [CD.The same findings seen in bothgroups 4 and 5 are repetitions of the same data andwere not measured.

Mean Height of Right Posterior Abutment

The one-way ANOVA for the measured height ofthe right posterior abutment for each grotjp yieldeda significance of P < .OUI. The Newman-Keuls testshowed that the mean height of the right abutment

Table 2 Increase in Height {¡irr) of Abutments

Store/die systemLeft Anterior'Mean {SD)

Riglit Anterior'Mean {SD)

Lett Anterior'Mean (SD)

Bight Anterior'Mean (SD)

Fuji Roclt'ZeiserDie Keen/ZeiserFuji Rock/Solid CastDie Keen/Solid CastFuji Rock'PindexDie Keen/Pindex

5.2 (4.9)19.3(5 9)18 0(6.0)41.2(6.6)16 0(6.0)41.2(6.6)

7,6 (6.1)25.3 (6.1)27.7 (8.3)47.8(10.9)27.7 (8.3)47 3(10.9)

6.3 (5,6)12 4(3.9)6.1 (4.9)

18.9(5 5)6 1 (4.9)

18.9(5.5)

7.0 (3.0)25.7 (4.8)11 8(5.8)28 9(1.8)11.8(2 5)28.9 (1 8)

•ANOVA followed by Newman-Keuls lest Í.P < 05): all pairwise comparisons are signilicarlly ditferent

537 The International lourral of Prosthodontii

Compa able Oie Systems

for groups 2 (Die Keen/Zeiser; 25.7 n.m), 4 (DieKeen/solid cast; 28.9 ¡xm). and 6 (Die Keen/Pindex;28.9 fim) was greater than that found for groups 1(Fuji Rock/Zeiser; 7.0 (j.m), 3 {Fuji Rock/solid cast;T1.8 ^Lm), and 5 (Fuji Rock/Pindex; 11.8 |j,m) (P <.05) (Fig 5 |D|). The same findings seen in groups 4and 5 are repetitions of the same data and were notmeasured.

Discussion

The expansion of stone has been attributed tothe growth and development of the crystallinehemihydrate lattice from the supersaturated solu-tion and the accompanying outthrust of the gyp-sum crystals during setting." It has been said thatthe energy of crystallization of dental stones leavesresidual stresses in the set mass. The release ofsuch forces, however small they may be, may affectthe replacement of divided segments of casts.

Manufacturers of such products have introducedmodifications and refinements in a general effort tominimize expansion, but the exacting demands ofmodern dentistry have become increasingly intol-erant of any imprecision, especially with respect tomultiple abutment prostheses. Thus, in addition toa careful choice of dependable stones, it is impera-tive to use techniques or systems that will consist-ently lead to reproducible and faithfully accurateresults.

Although Fuji Rock has less expansion than DieKeen stone, both will be consistent in their influ-ence upon die systems. The Zeiser system does notprevent the expansion of stone, but it compensatesfor it, since the acrylic resin template predeter-mines die position. Any measurable expansion inthe Zeiser system may be in the thickening of theabutments, resulting from the die stone expansion.

This study indicates that more accurate die posi-tions are attainable with the solid cast than thePindex system. There are two factors that may con-tribute to this; fa) the release of internal stresses inthe stone itself when the cast has been segmented,so that the dies, when reseated into their respectivepositions, assume a position that is different fromthat which they occupied in the solid cast, and <b),perhaps a more important factor, the unavoidableadditional expansion caused by the stone used toprovide a base for the working cast. In accordancewith ADA Specification #25,'- dental stone mayyield an expansion of 0.2%, which can significantlyinfluence the displacement of the dies from theiroriginal positions.

The combination of Fuji-Rock and Zeiser (group1) most accurately reproduced the prototype (Fig

2a). The finding that Fuji Rock die stone in associa-tion with any of the systems yielded better '^'"'•'y-^than Die Keen is readily explained by Fuji Rockslower coefficient of linear expansion. However, thesubstantial improvement in accuracy observed withgroup 1 with respect to the height of the abutmentsis probably attributable to the light pressure im-posed upon the setting die stone by the resin tem-plate. This suggestion is supported by the observa-tion that the Die Keen/Zeiser combination (group 2)also yielded greater accuracy than did the Die Keen/solid cast combination (group 4) or the Die Keen/Pindex combination (group 6) with respect to theheight of the abutments.

In the Zeiser system, even the use of a relativelyhigh-expansion die stone. Die Keen, did not pro-duce distorted casts with malpositioned abutmentsbecause of the predetermined positions of theabutments. The slight increase in volumetric ex-pansion of the abutments may provide a clinicaladvantage because of the concomitant increase incrown cementation space (the space that occursbetween the internal aspect of a crown and theprepared tooth). Adequate cementation space isrequired for the complete seating of crowns andfixed partial dentures upon luting." The expansionobtained with Fuji Rock is less than that seen withDie Keen and, accordingly, provides less cementa-tion space. A subsequent application of die spacercan satisfy cementation space requirements, butIhe use of Fuji Rock will allow a closer fit at themargin ofthe completed crown."

It is apparent that the Zeiser system more faith-fully reproduced the relationships between abut-ments than did the solid casts. The advent of im-plant dentistry has made the need for a passive fit ofmultiple abutment prostheses mandatory," and theimproved accuracy of the Zeiser system in combi-nation with Fuji Rock stone lends itself to theachievement of this objective.

Conclusion

Two die stones and three die systems were stud-ied to compare their relative accuracy. Within thelimitations of fhe study design and the materialsused, the following conclusions can be made;

1. The Zeiser system was the most accurate of thethree systems studied (Zeiser, Pindex, solid cast)with respect to the reproduction of the distancebetween abutments.

2. In combination with Fuji Rock stone, all threesystems revealed less distortion with respect to

af oí Prosthodonlii 538 re6, NiimherB. 1993

able Die Systen

abutment position than when they were com-bined with Die Keen stone.

3. Solid casts with eilher Fuji Rock or Die Keenstone yielded more accurate abutment positionreproduction than did the Pindex system.

Acknowledgments

The authors thanks to L. Gorman, DMD, PhD for his Invaluablebelp and |. Filipancic for his knowledge of the Zeiser system.

References

t. Benfieid |W, Lyons GV. Precision dies from elastic impres-sions. I Prostbet Dent 1962; 12:737-752.

2. GovolL,ZiebertG,Baltbaiary, Christensen LV Accuracy andcomparative stability of three removable die systems. | Pros-thet Dent 1988; 59:314-318.

3. Millstein P. Determining the accuracy of gypsum cast madefrom Type IV dental stone. | Orai Rehabil 1992;19:239-243.

4. Myers M, Hembree |H. Relative accuracy of four removabledie systems. | Prosthet Dent 1982;43:163-165.

5. Dilts WE, Podshadley AG, Sawyer HF, Neiman R. Accuracy offour removable die techniques. | Am Dent Assoc1971:83:1081.

6. Gettieman L, Ryge G. Accuracy of stone, metal and piastic diematerials. | Caiif Dent Assoc t970;46:28.

7. Hofstee E, Sbiu A, Renner R. The use of the Pindex system inrestorative dentistry. QDT Yearbook 1988;f2:107-n3.

a. Miilstein P, Filapancic |. The Zeiser system: A method foraccurate die placement Qurntessence Dent Technol 1990;14:18a-192.

9. Lehmann KM, WengelerV. Untersuchungen zur Genauigkeitverschiedener zahntechnischer modell Systeme. Dental-labor 1985;5:613-617.

10. Zeiser MP Ein pinloses Prazisions-Arbeitsmodall fur diekronen und Bruckentechnik ZWR 1986;95:10-13.

11. Gypsum compounds. In: Graig RG (edl. Restorative DentalMaterials, ed 6. St Louis. Mosby, 1980.

t2. Council on Dental Materials, American Dental Association.ANSI/ADA Specification No. 25 for Dentai Gypsum Products.Chicago: Am Dent Assoc, 1987.

t3. WangCMillsteinP, Nathanson D. Effecis of cement, cementspace and closing force on crown seating (abstract]. | DentRes 1990;69(special issue).

14. Passon C, Lambert RF, Lambert RL, Newman SM. Effect ofmultipie iayers of die spacer on crown retention [abstract!. IDent Res 1990:69fspeciai issue}.

15. VigoioP, Millstein P Evaluation of master cast techniques formultiple abutment implant prostbeses. Int | Oral MaxiiiofacImplants t993;8:439-446.

Literature Abstraa-

Differences Two Years After Tooth Exttaclion inMandibular Bone Reduction in Patients Treated WithImmediate Overdentures or Immediate CompleteDentures

Desire for improved denture function and reduced residual bone résorption bas made overdenturetherapy one ot the main preventive prosthodontic strategies. Tbe present investigation compared thedegree of mandibular résorption in patients who were treated with immediate overdentures (lOD) ontwo lower canine abutments to tbat ot patients who received immediate complete dentures (IGD).Tbree experimental groups were observed, one group received lODs supported by two mandibularcanine abutments: one group received lODs supported and retained with magnetic attachments onthe canine abutments: and one group received ICDs. An ICD was indicated or a complete denture wasalready present in patients' maxillae Reduction of the edentulous mandibie was calculated bycomparing oblique lateral cephalometric radiographs that had been made 1 and 2 years postplacementwith baseline measurements that were made directly following extraction of the remaining dentitionand piacement of tbe dentures. The average verticai bone ioss in the mandibular canine region in thefirst year was 0.9 mm in tbe lOD groups and 1.8 mm in tbe IGD group. Bone reduction in tbe posteriormandible measured 0.7 mm and 1.9 mm for each group. No significant differences were foundbetween bone reduction values in the first and second years foilowing denture piacement. Theauthors conclude that retaining the roots of canines as overdenlure abutments reduces alveolar bone lossduring the immediate postplacement pieriod, during which bone reduction is (ypically the grealest.

Van WaasMAI, lonkman REG, Kalk W, Van't Hof MA, Plooii f. Van OB |H. / Den! Hes ig93:7í:1ÜÜ1-ltJ04. Reference*: 15.Reprints: M.A.|. Van Waas, Department of Oral Function snd Prosthetic Dentistry, University of Nijmegen, PO Bo>:<i^O^,bS0a^\B Niimegen, The Netherlands.— David ß. Cagna, DMD, Department o< FrosthodoiMs, Ttie (Jnirersit)'oí Texas Health Science Center at Sin Antonio. San Anlania. Texas

Volume6, Number 6, 1993 539 Vhe International lournal of ProsthodontK