2

CLINICAL RESEARCH
Full=ArchImplant Framework Casting
Accuracv: Preliminam In Vitro
Observation for In Vivo Testing
Alan Brooks Carr, DMD, MS, * and Robert Bruce Stewart, DDS,MS#
Purpose: Conventional techniques for implant metal framework fabrication produce error of a
magnitude that is inconsistent with the passive-fit requirement for osseointegrated implants. To
understand the correlation between prosthesis fit and the implant-tissue response, evaluation of
the interface tissue reactions t o customary levels of fit is required. The purpose of this study is t o
determine the accuracy of torch casting full arch frameworks using a high palladium alloy and a
ringless phosphate-bonded investment technique.
Materials and Methods: Three different variables were considered relative t o casting accuracy
effect. The first variable, completeness of mold-fill, compared cast specimens where the entire
sprue system was filled as part of the casting and cast specimens without the sprue system filled.
The second variable, phosphate-bonded investment special liquid concentrations, compared
groups of castings produced from 0%. 12%. 25%. and 50% special liquid. The third variable,
investment mold shape, compared casting produced from a conventional ringless mold shape with
a modified ringless mold shape where the investment in the same horizontal plane as the pattern
was equal in thickness at the internal and external surfaces. Horizontal and vertical distances on the
wax pattern and resulting framework were measured using a machinists microscope t o determine
casting error. Combined vertical and horizontal error was used for comparison between groups
(one-way analysis of variance).
Results: No significant differences existed among the three groups compared (P >0.05). The
mean error comparison between the complete and incomplete mold-fill groups showed no
statistical difference, while the incomplete fill group was found to be more porous. The mean error
of all groups (0.130 mm] exceeded the recommended level of fit needed t o satisfy the passive fit
requirement by more than 10-fold.
Conclusions: These results verify clinical observation and suggest that the use of conventional
lost wax casting technique t o cast one-piece full arch implant frameworks is both imprecise and
inaccurate as judged against the passive fit requirement. The consequences of screw-fastening
misfitting prostheses t o osseointegrated implants is currently under investigation.
J Prosthod 2:2-8. Copyright o 1993 by the American College of Prosthodontists.

INDEX WORDS: casting accuracy, implant prostheses, implant-tissue stress

C

URRENT EKDOSSEOUS implant success has

resulted from basic principles of implant material preparation and design, atraumatic surgical
technique, provision of an unloaded healing phase,
and proper prosthetic fit and loading.' The term
passive fit characterizes the connection between the
cast framework and abutment or implant, and was
described by Brsnemark to exist at the 0.010 mm
*A.ui.stant Prafisor, Directm of Madlofacial Prosthetics, Section of
Ratoratiue und Prosthetic Dentwty, Collqe d-Dentisty, The Ohio State
Uniueni@, Columbus.
~Pricalepractice,Dttruit, MI.
Address repnnt requests to Alan Brwks Can; D M D , IMS, Collegp oJ
Dentisty, Ohw State Universip? 30.5 MJ 12th AL'e, Columbus, OH
43210-1241.
Copyright Q 1993 tp t h e i l r n ~ c a nCollege ofPmthodontists
10.59-941Xl93/0201-0001$5.00/ 0

2

level.*This level of accuracy is a reflection of the rigid
support provided b) osseointegrated implants and
underscores the requirement to achieve optimum
load di~tribution.3,~
Accurate connections sew? to
minimize prestressed loads (ie. loads inherent in the
connecting process) and favorably distribute all external loads, thus reducing unfavorable mechanical and
biological sequelae. Such connections allow an adequate stimulus for remodeling during the stage of
implant-tissue interface maturation toward steadystate.'

Introduction
Implant prosthesis fabrication technology, borrowed
from conventional prosthodontics, has not been shown
to predictably produce prosthesis fit at the 0.010 inm

Jormal oJProrthodonticJ, Val 2, X o 1 (March), I993:pp 2-8

March 199.7. Volume 2. iVumber I

level of accuracy. Casting accuracy studies have
shown error in the range of 0.100 mm using conventional techniques for multiunit tooth-supported prosFurthermore, it has not been shown that
implant framework accuracy offit can be detected at
the 0.010 mm level, as exemplified by a recent report
describing a significant force introduced when connecting a prosthesis that had an acceptable fit.7It has
been shown that experienced dentists, using tactile
discrimination methods, judge as acceptable margins
exhibiting a horizontal range of opening from 0.032
mm to 0.230 mm and a vertical range of opening
from 0.043 mm to 0.196 mm.$ Skalak has described
biomechanical concerns relative to connection of
nonpassive frameworks that include stress in the
prosthesis, the fixtures, and the bone.3 The suggested biological response to excessive and undetected stress is implant interface failure. Although
failure of integratcd implants following prosthesis
connection is not a frequent complication, it is both
poorly understood and common enough to warrant
investigation of possible prosthodontic cause^.^
Despite the shortcomings associated with fabrication accuracy, implant-support ed prost he sis fabrication protocols in use are apparently effective for
various clinical situations.I0-"Although clinical protocols differ in the methods used to control the fit of
full-arch prostheses, it is common that one-piece
frameworks are used without efforts to modify their
as-cast accuracy. To understand the correlation of
prosthesis fit and interface response, evaluation of
interface reactions to customary levels of prosthesis
fit is rcquired.
The purpose of this study was to determine
discrepancies in both the horizontal and vertical

3

planes at framework locations simulating terminal
abutmcnt positions in a mandibular five-implant
arrangement for one-piece castings, where mold-fill,
special liquid concentration, and investment design
were independent variables.

Materials and Methods
The experimental model simulated a five-implant mandibular framework design (Fig 1). A gypsum master cast was
constructed to allow one horizontal and two vertical measurements at the terminal abutment locations.The horizontal measurement points were established by creating
depressions in the pattern, and vertical measurement
points were established by creating depressions on the cast
and on thc pattern lateral to the horizontal points. Such
depression allowed reproducible positioning of the reflective spheres used to determine distances for the wax
patterns and the castings, as described later. 4 single screw
for a standard gold cylinder (DCA 072; Nobelpharma USA,
Chicago, IL) was fastened at a midline abutment analog.
The single fastening point allowed the most predictable
precasting and postcasting connection and subsequent
detection of dimensional changes.
Patterns containing the gold cylinders were formed by a
split mold filled with heated wax (Blue inlay casting wax,
type C; Sybron/Kcrr, Romulus, MI). A separate split mold
was used to fabricate the wax reservoir bar and sprue
system. 'The wax pattern and sprue system was placed in
the ring so that the runner bar was at the heat center of the
investnient block and the framework arch form was 14 mm
from the investment periphery and 12 mm from the end
(Fig 2). Pattern dimensions were determincd by pilot
measurement to be stable 60 minutes following connection
of pattern parts, and precast dimensions were recorded
twice following the measurement protocol described below.

Figure 1. Experimental rnodrl with fastened framework showing (A) horizontal sphere positions and (B) vertical sphere
positions.

4

Full-Arch Implant Framework Cmtinp

Caw and Stmiart

1:

n

nm
6mrn -

II

6mm

IOyn

Figure 2. Schematic illustration of invested pattern dimensions and position. Right top view illustrates spacer design,
represented by shaded area, which provided a uniform investment coating to the pattern within the horizontal plane.
Table I outlines the pattern, investment, burnout, arid
casting protocol followed in this study. The wax patterns
were invested in fine-grain, carbon-free phosphate-bonded
investment (Cera-Fina; Whip Mix Corp, Louisville, KY)

using a plastic investing ring (Clear-Eze; M. Sullivan,
Scottsdale, AZ).In one comparison, two groups of castings,
four castings for each group, were produced to determine
the effect of completely filling the sprue system on accura-

Table 1. Pattern, Investment, Burnout, and Casting Protocol
Pattern
Material
Shape
Spruce
Investment
Material
Quantity
Met hod

Groups

Burnout
Oven
Method
Time
Casting
Material

Groups
Cooling Devesting

Blue inlay casting wax hard type/class I, Nobelpharma ‘gold’cylinder-DCA 072
Full arch, 20 mm cantilever, Lshape cross section, 6 mm in height, 9 mm wide at the cylinder,
7 mm wide at cantilever end
Reservoir ‘runner bar’ 7 mm in diameter, 2 mm connecting sprues, 5 mm in diameter attached
at 4 locations
Cera-Pina, phosphate-bonded, batch no. 10731 1100
200 gm powder: 48 mL liquid
Slow speed mix (250-550 rpm) under vacuum for 90 s, placed by brush to critical areas, positioned in ring, and filled to predetermined level in air
Bench set for 60 min and stored in sealed plastic bag (100% humidity)
CF and ICF
Special liquid concentrations: 0: 100, 12:88,25:75,50:50
Investment shape: Spacer and conventional
Spacer, CF and ICF groups all used 25:75 special liquid/water ratio
For ratio comparisons, only CF data used for 25:75 cgroup.
Accutherni 11-1000
3 rings placed in a cold oven, 15”F/min rate, 800°F for 30 min, 15”F/rnin. rate, 1500°F for 60
min
3 h 15 min
Ceramic crucible
Multiorifice torch using liquid petroleum, gas (6 psi) and oxygen (13 psi)
Broken arm centrifugal casting machine
IS 85 alloy fused 2 dwts at a time, total fusing time < 4 rnin
CF: 32 dwt. ICF: 24 dwt (no reservoir)
Bench cooled, devested, and air abraded with cylinder protection

5

March 1993, Volum 2, Number I

q.I3
The completely filled (CF) group used sufficient alloy
to fill the sprue system and the incompletely filled (ICF)
g o u p used only enough alloy to fill the framework pattern
mold space. The latter group attempted to address the
possibility that the spr-ue system may have a detrimental
effect on the resulting casting accuracy. All castings were
produced by one operator, using only new alloy. 'The CE'
group of castings required 32 dwt to fill the mold spacc and
the ICF group required 24 dwt. All castings were bench
cooled to room temperature. In another comparison, the
effect of altering special 1iquid:waterratios on accuracywas
investigated using spccial liquid concentrations of 0: 100
(two frameworks), 12238 (two Cameworks), 25:75 (four
frameworks-CF group), and 50:50 (three framcworks). In
a third comparison, three frameworks were cast to investigate a unique investment block design (Fig 2, right shaded
area), which attempted to distribute equal investment
material external and internal to the pattern and casting,
in the horizontal plane, by incorporating a spacer within
the arch form during investing. All invested patterns were
allowed to set more than 1 hour (generally overnight) and
stored in a humid environment, thcn placed in a cold oven
for burnout (Accutherm 11-1000; J.F. Jelenko, Armonk,
NY). The burnout protocol rollowed manufxturers recommendations and was identical for all casting groups (Table

1).
The alloy was melted in a ceramic crucible using a
conventional multiorifice gas/oxygen torch, and cast using
a standard broken-arm centrifugal casting machine (Kerr/
Sybron). ,411 castings were made with a high-palladium
alloy marketed for implant-supported prostheses (IS 85;
Williams Division/Ivoclar North America, Amherst, nu).
The castings were air-abraded with 50 p n alumina and
inspected for casting discrepancies using a 7-30 power
stereo microscopc. Casting cylindcrs were inspected for
casting imperfections to assure optimal reseating when
screw fastening to the abutment. Any discrepancy affecting
screw fastening or measurement depressions was considered a miscast and not used in the analysis. Measurements
were repeated as for the wax patterns.
The measurement scheme used 1.57 mm stainlcss steel
spheres placed in the depressions at the horizontal and
vertical positions as previously described and illustrated in
Fig I. Measurements were made using a machinist's
traveling microscope (Leitz Model UMW; Opto-Metric
Tools, New York, NY) with electronic digital micrometer
heads (Digirnatic Head 164 Series; Mitutoyo Mfg, Tokyo,
Japan) having a manufacturers reported accuracy of 20.003
mm and precision of ?0.001 mm. For this investigation thc
measured precision was 0.007 mm, as detcrmined by 10
measurements o f a single horizontal distance. This amount
of imprecision was caused by the variable roughness effect
of the cast framework depressions on the sphere position,
but was sufficiently precise for the measurements made.
An overhead light source was mountcd above the microscope in a fixed position to provide a reflective reference
crosshair in the spheres for spatial positioning and measurr-

ment." Data were collected as three point coordinatcs for
the horizontal and vertical measurements on each specimen. The coordinates were converted to distances by use of
the Pythagorean theorem and two measurements were
madc by one operator for each distance to be determined.
One horizontal and two vertical distances were determined
for each specimen. Four castings were produced for the
complete fill and incomplete fill groups, two castings for
the 0:100 arid 12:88investment groups, and three castings
each for the 50:50 investment and internal spacer investment group. Wax pattern and as-cast group differences
were analyzed statistically for significant differences with
the use of one-way analysis ofvariance (AYOVA).

Results
Three castings were considered miscasts and were
not measured. The bilateral vertical error was comb i n d as one vertical error. For statistical analysis,
the horizontal and vertical errors (measurement
differences between pattern and casting) were considered as the same type of error and combined,
therefore, not analyzing for individual effects. The
original measurements and measurement differences are presented in Tables 2 and 3. These include
wax pattern and as-cast measurement data for the
horizontal and vertical measurements for the six

Table 2. Measurement Data for Wax and As-Cast
Metal Groups in Millimeters

cvm

M€X4T>

Group

Hmizonlal

Vertical

Ho~iyintal

Vertical

*CF

26.047
26.036
26.04-6
26.226
26.022
26.115
26.082
25.730

15.592
15.448
15.746
1.5.546
15.480
15.789
15.444
16.318

26.149
26.219
26.21 1
2629.5
26.159
26.241
26.419
25.708

15.707
15.332
15.777
15.668
15.687
15.636

CE'
CF
CF
ICF
ICF
ICF
ICF

Special 1iquid:waterratic
0:100
26.231
5.532

12:88

50:50

26.212
26.167
26.092
26.135
26.024
26.102
26.052

15.580

16.487
15.450
15.831
15.577
15.921
15.829

1.5.769
15.668

26.089
26.075
26.173
26.199
26.204
26.187
26.220
26.198

16.003
15.641

26.252
26.128

16.241
15.738

5.582
5.436
5.819
5.466

t

t

15.815
15.747

Investment Spacer

26.275
26.199

~~~

*CF group scrvrs as 2575 spccial1iquid:water group.
?Unable to collect vertical data.
Vertical rneasurcments (R &L) combined.

6

Full-Arch Implant Frameamrk Gating

Caw and Stewart

Table 3. Differences for Measurement Data in h*lillimeters
Absolute Mean fiaerence (SD)

Ozflereize

Hmzontal

Vertical

,102
,183
,165
.069
.I37
,127
,337
- ,022
-.I42
-.I36

.115
-.I16
.3 1
,122
,207
-.I53
,136
,169
- .08 1
249
,142
.lo3
.361

CF

ICF

0:00
1298

,007

50:50
Internal
Investment
Spacer

,108
,069
.I63
.I17
.I46
- .024
-.071

*

.046
,079
,238

Horizontal

Vertical

Pooled

,130 (.053)

,096 (.043)

.1 13 (.049)

,156 (.132)

,166 (.030)

.161 (.089)

,139 (.OM)

.165(.119)

.I52 (.070)

,058 (.071)

.123 (.028)

,090 (.058)

.1 16 (.047)

.205 (.225)

,152 (.127)

.080 (.062)

.138 (.087)

.lo9 (.074)

.098

*Unable to collect vertical data.

groups of castings. The C F group also served as the
25% special liquid group. In the statistical analysis,
the absolute magnitude of the error was used to
provide a one-sided comparison. Table 4 shows that
no significant difference existed among the three
comparisons (AKOVA,P > .05). Figure 3 shows the
grouped data, and references each to the entirr
experimental population mean (0.130 mm) and to
zero. In considering the nonpooled data relative to
the expansion and contraction, the horizontal differences showed casting expansion except €or the 0%
special liquid and spacer groups. The vertical difference values showed no consistent trends or correlation to the horizontal values.
The ICF group showed greater porosity and no
improvement in accuracy. Although the CF and ICF
groups were not significantly different from each

other (Fig 3), the descriptive data suggcst that the
C F group tended to provide better accuracy (pooled
mean: 0.1 13 m m u 0.161 mm) and precision (pooled
standard dekiation: 0.049 m m u 0.089 mm).

Discussion
Current implant prosthodontic procedures make use
of conventional techniques and materials common to
conventional fixed and removable prosthodontics.
While it is important to compare the accuracy of
implant prosthodontic techniques with the accuracy
of conventional prosthodontic techniques, the clinical
significance of such comparisons requires testing in
vivo because of the contrasting support models of
teeth and implants. Specifically, because of the qualitative and quantitative difference between the sup-

Table 4. ANOVA Table
Group

Snurce

Sum $
Squares

Mean

Square

F Ratio

Probabilip
<F

Complete fill c incomplete

Model
Error
C Total
Model
Error
C Total
Model
Error
C Total

1
14
15
1
12
13
3
17
20

0.0092
0.07 14
0.0807
0.0001
0.0042
0.0442
0.0 126
0.1058
0.1 184

0.0092
0.00.51

1.8059

0.2004

-

0.0001
0.0037

0.0117
-

0.9157
-

0.0042
0.0062

0.6710

0.5797

-

-

Complete fill z: spacer
Special liquidwater ratios

Note: CF also sewes as 25% special liquid group.

-

-

-

-

March 1993, Volume 2, Numbm I

i

I

1

0 >s

0 14
0 12
Frw.

010

Imml

0 08
0.06
004

oOW

CF

ICF

Spacer o

0%

12%

509,

*

Figure 3. Bar graph showing error (average and s t m dard deviation) by group. The overall mean error is
represented by the 0.130 mm horizontal line. The ICF
group represented the 25% special liquid ,group.
porting tissues of teeth and implants, we might not
expect the implant response to misfit to be the same
as the tooth response to misfit?
Potentially as important as the difference in
support between teeth and implants is the manner
with which the fixed partial dentures are connected
to their support. Screw fastening is a sequential,
active clamping process where each unit is independently connected that provides loads across each
screw joint (preloads),15and results in stress within
the components and at the implant-tissue interfaces
of the connected implants if any misfit relationship
exists.” Skalak” warns that connection of misfitting
prostheses produces stress that can easily escape
detection and occurs at a location where the resultant effects are not well understood. Implant prosthesis connection and the potential for implant-tissue
interface stress mandates not only detailed understanding of accuracy levels for prosthesis fabrication
but also the biological effects of connection to the
host tissue.16
In conventional prosthodontic marginal-discrepancy investigations, the marginal error that is measured results from a combination of horizontal and
vertical inaccuracy. For the comparisons made in this
study, the single fastening point at the anterior
abutment positioned each specimen on the cast for
measurement. Therefore, the error attributable to
casting is cxpresscd as pooled horizontal and vertical
data (Table 3 ) . The two were considered as similar
types of error and represent error in two planes that
would be expressed as marginal error in conventional
marginal “sleeve fit” accuracy studies. Therefore, for
comparison purposes, pooling the errors allows better correlation with precedent casting accuracy liter-

7

ature.3,6 From the perspective of the cellular response to stress, all such stress is seen as strain by the
cell.” It is impossible to isolate the various strains
and correlate them with an observed tissue response
in vivo. Therefore, the cumulative error was used,
realizing that it may be an oversimplification.
The results of this casting investigation show that
for all variations of the casting procedure evaluated,
the data are inaccurate and imprecise as judged
against the 0.010 mm requirement. The first part of
the study examined the effect of the sprue system on
~
~
distortion of the framework
during solidification.’3
Patterns of the size required to cast full-arch restorations require large reservoirs to ensure an adequate
sourcc of molten alloy for the solidifying framework.
Placement of the reservoirs in the vertical heat
center of the investment mold allows for the reservoir to solidify last and also ensures optimum density
in the casting. Under these conditions, the solidifying
framework develops rigidity before the reservoir. If
the reservoir were to solidify first, sufficient stress
might be placed on the solidifying framework, causing distortion.
Under the conditions of this study, incomplete
filling of the reservoir increased the porosity in the
rrameworks and did not improve casting accuracy. It
is believed that position of the sprue systcm within
the investment block is most critical for accuracy
related to mold filling. Different data would be
expected if the sprue system were placed outside the
heat center, thereby reversing the solidifying sequence from the current model and promoting
porosity. Porosity would also be promoted by not
providing a molten reservoir source during framework solidifying.
Expansion of phosphate-bonded investments can
be controlled through alteration of the special liquidwater ratio.’*In this study, special liquid proportions
were varied to provide four groups: O%, 12%, 25%,
and 50%. The complete fill group represented the
25% special liquid group. A ringless investment mold
technique was used to allow optimum expansion.
The two best special liquid groups, along with the
spacer group, showed data below the entire experimental mean (0.130 mm), yet still with an average
error 10 times the required level of accuracy. The 0%
group showed contraction in three of the four measurements (Table 3).
The 12% group showed expansion of all dimensions measured, with a mean expansion of 0.0130 mm.
The internal investment spacer group (25% special
liquid) was designed to offset the external expansion

~

8

Fu'ull-Arch Implant F r a m e m k Casting

effect seen in the CF group by allowing inward
expansion at the level of the mold space. This oKkt
was accomplished in 2 of 3 horizontal errors that
showed contraction (Table 3 ) . Further study is required to determine the usefulness of such an alternative investment block shape. Ail data regarding
investment tcchniqucs are specific to the ringless
method as described. Use of metal rings may restrict
expansion in the horizontal plane and would bc
expected to provide differing results.
Given the above findings, strategies for improving
the span fit ofprosthcses need to be used and include
techniques similar to conventional prosthodontics as
well as those unique to implant superstructure^.'^-'^
In spite of the limited sample size of this study, thc
overall finding of casting error is in agreement with
previous conventional fixed prosthodontic casting
accuracy research? Frameworks produced at accuracy levels similar to this study cannot be considercd
passive. As described earlier, the nature of the
implant-tissue interface, and the scret\-rastening
method of rigidly connecting a prosthesis to the
supporting implants, requires an understanding of
the biological consequences of connecting a misfitting prosthesic to oswointegrated implants. Consequently, the data from this study are being applied in
ongoing animal studies investigating the implanttissue intcrfacc rcsponse to prosthesis misfit.

Acknowledgment
T h e authors acknowledge t h e gracious support of t h e
Williams Division of Tvoclar N o r t h Amcrica. Without such
material support (40 oz do!), this project would not have
brrn possible.

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