SECTIONEDITORS

An evaluation implants Mark Jack

R.

Spector,

I. Nicholls,

[Jniversity Washington,

of impression D.D.S.,*

Terry

E. Donovan,

School Seattle,

of Dentistrv, Wash.

techniques D.D.S.,**

for osseointegrated

and

Ph.D.***

of Southern California, School of Dentistry,

Los Angeles,

Calif.,

and University

of

A passive fit between osseointegrated implants and the prosthesis they will support has been advocated. An experimental model was developed to test the accuracy of three impression techniques and the components used to make the transfer records. Statistically, no significant difference was found between the three methods tested. From this initial study, it appears that further work is needed to isolate techniques that will predictably provide accurate registration of the position of endosseous implants.(J PROSTHET DENT 1990;63:444-7.)

S

everal techniques have been advocated for transfer of implant location prior to fabrication of a prosthesis. Components have been provided to make transfers with impression plaster, hydrocolloid impression materials, and elastomeric impression materials. No scientific evidence is available to document their accuracy or superiority. In fabricating the superstructure for osseointegrated implant fixtures, the primary objective is to achieve a passively fitting prosthesis (Fig. l).l Failure to meet this objective can result in a loss of fixture integration and progressive treatment failure. Forced tightening of the superstructure can result in microfractures of bone, a zone of marginal ischemia, and healing with a nonmineralized attachment to the implant fixture.2 Adequate stress distribution may also encourage maintenance of marginal bone close to the implant fixture.3 In this study, three different impression techniques were evaluated to determine their ability to reproduce fixture positions on a working cast.

METHODS

AND

MATERIAL

A cast was designed to simulate a clinical situation in which the location of multiple implant fixtures (Branemark

Presented to the Pacific Coast Society of Prosthodontists, Corenado, Calif. *Associate Clinical Professor, Director of Implant Dentistry, I‘niversity of Southern California, School of Dentistry, **Associate Professor, Chairman, Department of Restorative Dentistry, University of Southern California, School of Dentistry. ***Professor, Department of Restorative Dentistry, Universitv of Washington, School of Dentistry. 10/l/17594

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System, Nobelpharma, Chicago, Ill.) would be recorded (Fig. 2). Impressions were made using three techniques. In technique I, guide pin-retained transfer copings (DCA 026, Nobelpharma) were united with autopolymerizing acrylic resin (Duralay, Reliance Dental Mfg., Worth, Ill.) and dental floss. An impression was then made with a polysulfide rubber impression material (Permlastic, Kerr Mfg. Co., Romulus, Mich.) in an acrylic resin open-top custom tray. In technique II, a polyvinyl siloxane (Reprosil, L.D. Caulk, Milford, Del.) impression was made in a stock tray over hydrocolloid transfer copings (DCA 025, Nobelpharma). In the third technique, a condensation silicone impression (Xantopren/Optosil, Unitek, Monrovia, Calif.) was made in a stock tray over hydrocolloid transfer copings (DCA 025). With all three impression techniques, brass implant analogues (DCA 015, Nobelpharma) were attached to the transfer copings and the impressions were poured in an improved dental stone (Die-Keen, Modern Materials, Columbus, Ohio). Five impressions and casts were made with each technique. To measure the position of the abutment replicas accurately, six hydrocolloid transfer copings were machined to form a sharply bordered circular indentation in the superior surface (Fig. 3). This indentation allowed accurate microscopic positioning of the crosshairs in recording positions in the X and Y axes. The copings were numbered to correspond to the six different implant positions on the patient model. The base of the indentation was used to measure the vertical (2 axis) position of the abutment replicas. To determine the inherent error involved with threading and removal of transfer copings, a single abutment analog

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Fig. Fig. Fig. Fig.

TECHNIQUES

1. Passively fitting prosthesis.

2. Experimental model. 3. Machined transfer copings. 4. Micrometer used to measure X-Y axis coordinates. Fig. 5. LVDT used to measure2 axis. Fig. 6. Diagrammatic representation of measurementsof relative distortion between machined transfer copings. Dl through D5, relative distortion between measuringwells.

was mounted in a resin block and measurementswere recorded as it was repeatedly removed and replaced. With a custom-madeprecision X-Y table, the X and Y coordinatesof the abutment replicason the test castswere

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measuredwith a micrometer (40x) (Fig. 4). Measurement capability was0.001mm. Three separatemeasurementsof eachX and Y coordinate were made and mean values determined.

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SPECTOR,

DONOVAN,

AND

NICHOLLS

Fig. 7. Example of impression error resulting from air entrapment and incomplete placement of impression tray. Fig. 8. Superimposed index made on experimental model demonstrates error resulting from transfer impression made on experimental model.

Table I. Mean relative distortion (mm) between implants on experimental model and master cast with impression technique I Position

X-Asi>

Y-Axis

Z-Axis

Dl D2 D3 D4

-0.16 -0.16 -0.03 0.06

-0.01 -0.06 0.08 -0.03

-0.02 0.00 0.00 0.14

D5

0.02

0.00

0.00

Table II. Mean relative distortion (mm) between implants on experimental model and master cast with impression technique II Position

Dl D2 D3 D4 D5

X-Axis

-0.05 0.25 0.05 0.24 0.12

Y-Axis

-0.07 0.02 0.08 -0.01 0.00

Z-Axis

0.00 0.00 0.03 0.34 0.00

With the use of a linear variable differential transformer (LVDT, ElOOO, Schaevitz Engineering, Pennsauken, N.J.), the 2 axis measurements were completed on a laboratorygrade ground granite slab (Doall, Des Plaines, Ill.) (Fig. 5). This instrument has a measuring capability of 0.001 mm. Three measurements for each measuring well were recorded and the mean values were determined. Analysis of distortion involved comparison of positional relationships of all six measuring points on the master test model and the gypsum casts recovered from the three test impression techniques. A reference position was arbitrarily

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Table III. Mean relative distortion (mm) between implants on experimental model and master cast with impression technique III Position

Dl D2 D3 D4 D5

X-Axis

Y-Axis

0.02 0.26 0.05 0.18 0.13

Z-Axis

-0.14 0.04 -0.02 -0.02 0.00

0.04 0.00 -0.01 0.21 0.00

chosen that required that point No. 1 be at the origin of the coordinate system (X = 0, Y = 0, and 2 = 0). Coordinates of all measuring wells on the master model and gypsum casts were stored in a computer (PDP 11/40) and rotated to the reference position. A specialized computer program was written by one of the authors (J.I.N.) specifically for this purpose. Distortions occurring at each of the measuring points in three axial directions (X, Y, and 2) could then be computed at one time. The relative distortions between the measuring wells were designated Dl through D5 (Fig. 6). Relative distortions were presented in the computer printout and were expressed as numeric differentials along each of the various axes. The X and Y axis positions represented relative distortions in the horizontal plane and Z axis positions represented relative distortions in the vertical axis. Means, standard deviations, and variance were calculated for each group of casts recovered from the impression techniques tested. The data were analyzed by use of a two tailed t-test.

RESULTS Measurable distortion was recorded in the resultant position of the abutment replicas with all three of the

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impression techniques evaluated. Distortion in the X and Y axes was approximately the same magnitude with each of the three techniques tested (Tables I through III). A statistically significant difference was detected (99 % level) at distance 1 and 5 for technique I versus technique II (polysulfide rubber/polyvinyl siloxane). The average variation detected in the X and Y axes ranged from 0.02 mm to 0.18 mm. The average variation in the 2 axis was 0.085 mm. In an attempt to quantify the effect of having to mechanically place and remove the transfer copings, five sets of recordings were also made of a single transfer coping and an abutment replica. The data demonstrated that this procedural sequence was a minimal source of error in the transfer process, with an average 0.0015 mm error occurring in the 2 axis.

DISCUSSION The techniques tested demonstrated distortions in the resultant positions of the transferred abutment replicas that ranged from 0.02 mm to 0.18 mm. The statistical difference detected between distance 1 and 5 for technique I and technique II is of little importance. The overall inaccuracy of both techniques is far more si~i~c~t than the statistical difference between the two techniques tested. Possible explanations for these errors can be found in the technical procedures involved and in the use of transfer copings to make the registrations. The mechanics of this system inherently impart a certain amount of error to the transfer process. Threading the transfer coping into the implant, removing it, and then attaching the abutment replica is an initial source of error. Measurements of this aspect of the transfer assembly resulted in an average vertical discrepancy of 0.0015 mm. This error, although small for a single transfer, is compounded at each subsequent transfer made thro~hout the clinical and laboratory phases. Further errors may also have occurred in technique I as a result of distortion of the autopolymerizing acrylic resin used in the transfer procedure. In this particular technique a relatively large mass of acrylic resin is used as compared with its use in the indexing of a fixed partial denture for soldering. Distortions with autopolymerizing acrylic resin increase proportionally to the mass of the resin involved. Residual stress develops in the curing resin as a result of continued polymerization, which occurs within the resin mass after the resin has attained a solid condition. Removal of the indexed transfer copings from the integrated implants allows the release of this residual stress in the autopolymerizing resin matrix. Irregularities noted in the second and third techniques could be related to the difficulty in accurate orientation of the impression coping and abutment replica assembly in

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the final impression. Sectioning of the impressions often demonstrated air entrapment and incomplete seating of the impression tray, which may have impeded accurate placement of the transfer impression coping assembly (Fig. 7). In discussing the clinical significance of the recorded distortions, it is important to unders~d that the actual relative distortions at the level of the implant fixtures are considerably less than those displayed in tables I through III. The displayed distortions are magnifications of the actual distortions because measurements were made at the ends of 9.5 mm copings that were attached to the abutment replicas. Thus, if a distortion of 0.02 mm was recorded with this system, the real distortion, at the level of the implant fixtures, would be approximately 0.002 mm. The clinical significance of distortions of this magnitude is unknown. The clinical objective is to fabricate an implant superstructure that imparts no stress to the abutment fixtures in the unloaded state. Given this objective, it seems prudent to eliminate as much distortion as possible in the impression and transfer procedure. Thus these observed distortions are undesirable, and alternative transfer procedures may be indicated (Fig. 8).

CONCLUSIONS Measurable distortions resulted from the transfer of implant positions as recorded with three impression techniques. The magnitude of the distortions were similar with all three techniques evaluated. In addition to dimensional changes in the materials used, positional errors were also attributed to the mechanical components used in the transfer process. Although the errors measured are relatively small, the study demonstrates the potential for distortions with the transfer techniques used. The objective is to achieve a passively fitting prosthesis. On the basis of results of this study, further work is indicated to isolate a technique that will reliably and predictable reproduce the intraoral relationship of implant fixtures.

1. Zarb GA, Zarb FL. Tissue integrated dental prostheses. Quintessence Int 1985;1:39-42. 2. Skalak R. Biomecbanicai considerations in osseoointegrated prostheses. J PROSTHEZ I&NT 1983;49:843-8. 3. Adell R, Lekhobn U, Rockier B, Branemark PI. A 15-year study of osseointegrated implante in the treatment of the edentulous jaw. Int J Oral Surg 1981;~~387-416. Reprint requests to: DR. MARK R. SPKZOR DIRECTOR OF IMPLANT DENTISTRY UNIYERSIlY PARK MC-0641 USC SCHOOL OF DENTISTRY Los ANGELES, CA 90089-0641

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