An investigation of the accuracy of 2 impression techniques

By April 27, 2011 December 2nd, 2013 No Comments




By Dr Firas Daoudi for MSc project



This laboratory study investigated the accuracy of four implant impression procedures using two impression techniques and two different materials.


A master model was used to produce forty different stone casts incorporating laboratory implant or abutment analogue from the different combinations of the following impression techniques (The repositioning impression coping technique at the implant level and the pickup impression technique at the abutment level) and materials (vinyl polysiloxane-President and polyether-Impregum F).  Variations in the resulting working casts were measured using the Reflex Microscope to derive distances and angles from the three dimensional co-ordinates of optical targets attached to a test coping placed on the implant analogue and on a reference device positioned on the occlusal surfaces of the casts.


The results showed greater variations in analogue position with the repositioning impression technique than with the pickup technique.  The rotational errors were large enough to be of clinical concern.  No significant differences were found between vinyl polysiloxane (President) and polyether (Impregum F) impression materials for the two tested types of impression techniques.


The repositioning impression technique at the implant level can produce less predictable results than the pickup technique at the abutment level. The choice of impression material made no significant difference.



The long-term success rate of osseointegrated implants has made implant supported prostheses a valid option for the treatment of missing teeth1-2.

An acceptable prostheses requires optimal accuracy needed in all the steps of fabrication.  The first step on which precision depends is accurately positioning the impression components and recording the implant position with the impression procedure.  Different impression techniques can be used for single tooth implant prosthesis.  The impression can be made either at the abutment level using a transfer coping and a pickup impression technique with the coping retained in the impression as it is removed from the mouth or at the implant level utilising two methods: the pickup method or the repositioning method where a tapered impression coping is retained on the implant and later removed from the mouth, reassembled with the implant analogue and replaced tightly into the impression 3.

Using the implant level technique can provide multiple attractions.  Indeed, many reports advocate the use of this technique to facilitate the provision of a temporary restoration 4.  Another attraction of this technique is allowing the selection of the proper abutment in the laboratory 5 and it can enable the use of custom made or adjusted abutments 6-10.

While a number of reports evaluated the implant impression techniques in general, little work has been done to investigate the accuracy of single-tooth implant impression techniques.    Acceptable fit of implant supported fixed partial dentures fabricated using the pickup impression method at the implant level at stage one surgery has been reported in dogs 11.  However, others have warned about the possible inaccuracy in the impression procedure using the implant level, repositioning technique, where it is difficult to reposition the impression coping correctly in elastic material 12.  Indeed, it has been reported that the impression copings couldn’t be repositioned accurately in either polyether or vinyl polysiloxane impression materials13.  It is assumed that similar results could take place with the pickup technique if the coping is accidentally rotated; therefore, the use of rigid impression material is recommended14.  As for the impression techniques at the abutment level, Schmitt et al 15 reported better accuracy when the transfer impression coping was picked-up in the impression material rather than splinted to the impression tray.

The purpose of this study was to investigate the accuracy of the pickup impression method at the abutment level and the repositioning method at the implant level using two different elastomeric materials.


Model preparation:

A dentate maxillary acrylic resin model missing the right central was used.  In the position of the missing tooth, a hollow tubular implant holder was fitted into which ten implants (7×3.75mm, SDA 001, Nobel Biocare, Sweden) could be placed and interchanged.  Each implant was held rigidly in position using GC acrylic resin (GC Dental Corp., Tokyo, Japan).

Three nickel chrome inserts with the shapes of a flat surface, a conical socket and V block were fitted on the occlusal surfaces of the left second molar, second premolar and the right molar teeth respectively (fig 1).  They served to apply the surveying principles whereby placing spherical ball bearings placed into these inserts would constrain the relative positions in three, two and one axis respectively.  This would provide reproducibility in the positioning of two aluminium reference plates.

Each plate had three ball bearings attached to adjustable M4 screw posts that allowed the plate to locate precisely over the inserts.

The first plate had a long titanium implant mount (DIA 282, Nobel Biocare, Sweden) attached to it as a jig for implant mounting.  The second plate carried precise optical targets to simplify the measurements (Fig 2).

Impression making and working cast fabrication:

A mould was used to produce 40 similar sized custom trays made of auto-polymerized acrylic resin (Formatray, Kerr Co., UK).  Two elastomeric impression materials, light and heavy body vinyl polysiloxane (President, Coltene AG, Switzerland) and polyether (Impregum F, ESPE, Germany) were used.

One impression per implant was recorded using the following methods:

First, the repositioning implant level method using a tapered implant impression coping combined (DCA 448, Nobel Biocare, Sweden) with president material (P.Rep) was applied.  Then, the same impression technique was used with Impregum (I.Rep).

After that, a new 3mm CeraOne abutment (SDCA 334, Nobel Biocare, Sweden) was connected to the implant.  A plastic impression coping (DCB 119, Nobel Biocare, Sweden) was seated on the abutment and the pickup impression technique was used with President material (P.Pic).  Finally, the same impression technique was used with Impregum (I.Pic).

New components were used for each procedure throughout the study to avoid any possible errors that could result from repeated use of a few components that might not be typical, or from wear.  Special care was made making sure that all components were properly oriented and completely seated.

After each set of four impressions, the position of the implant in the master model was recorded so that subsequent comparative measurements were not dependent on the precision of location of the implant in the model.  All of the measurements were digitised ten times for each implant to study the reproducibility and precision of the measurement technique and as a precaution since the implant, once removed, could not be replaced precisely after a new implant was installed into the master model.

Abutment analogues (DCA 129, Nobel Biocare, Sweden), and implant analogues (SS, DCC 084, Nobel Biocare, Sweden) were connected to the impression copings according to the manufacturer recommendations and repositioned where applicable. Each group of impressions resulting from the same implant was poured using the same mix of Silky Rock model stone (Whipmix Co., Kentucky, USA).  This resulted in 40 models, 10 with each combination of the impression material/technique.

The Measurement Phase:

The second aluminium plate was used to provide a reference plane for measurement.  Four pieces of mirror glass, each carrying an ‘X’ shaped target were fixed on the top of this plate.  Each central corner of the ‘X’ shape provided a point for measurement and the 3D centre of gravity of these was taken as the centre of the target.  This resulted in a total of 16 reference points, and four centres (A,B,C,D) which were used to compute a first reference measuring plane, the plate plane.

A custom coping, cast to be a tight sliding fit on CeraOne abutment, was used to carry a rectangular shaped glass target used for the measurement. The manufacturer’s prefabricated coping could not be used because it incorporated a predetermined cement space.  The optical target had a rectangular shape inscribed near the periphery and an ‘X’ in the centre.  Each corner of the rectangular lines provided a reference point for measurement (1, 2, 3, 4) and collectively determined a second reference plane for measurement, the coping plane.  The four corners of the central ‘X’ provided four more reference points, the 3D centre of gravity of which was regarded as analogous to the centre of the incisal edge of a prosthesis constructed on the cast and referred to as the point implant “Impl”.

Each CeraOne abutment was positioned on its corresponding implant analogue in the stone cast.  The reference coping was seated on the CeraOne abutment or on the abutment replica according to the impression method used.  The reference plate was located and the cast was then ready for measuring.

The Reflex Microscope was used for measurement according to a programmed observation plan, which also computed five analytical variables.  These variables enabled comparison of the anterio-posterior position, mesio-distal position and the axial rotation of the coping on the stone cast with its position on the implant in the master model (Fig 3).  The angle between the two reference planes and the perpendicular height offset of the point “Impl” from the plate plane indicated respectively the deviation in axial inclination (i.e. tilting) or occluso-gingival elevation from the conditions in the master model.  These dimensions were computed at the end of each observing session and compared to the relevant implant position in the master model.

A series of pilot measurements were undertaken to assess the reproducibility of the investigator and the errors involved in repositioning the measuring coping and the reference plate.  These sources of variation were considered acceptable for the present investigation. (Table 1).

The differences between each cast and the corresponding master implant were calculated in order to have pair-wise comparisons.  This enabled statistical analysis of the differences resulting from the impression methods without the confounding factor of variations in implant position arising from the use of the positioning plate to attach each implant to the master model.

Analyses of variance were undertaken using Proc GLM of the SAS system to determine whether significant differences existed between the mean discrepancies from the master model for the factors impression material and technique and their interaction.  Then Duncan’s Multiple Range test was used to perform mean separation tests for the anterio-posterior variable.


The impression material was not a significant factor for any of the measured variables.

The results showed significant difference for the technique type only with the anterio-posterior variable (F value 14.59, P = 0.0005).  Differences between means for the height offset of the “Impl” point approached significance  (F Value 3.9, P = 0.056).

The results from Duncan’s test at the 95% confidence level showed that the significant differences for the anterio-posterior position of the analogue were between the means for the repositioning implant level technique and those for the pickup abutment level impressions.  There were no significant differences between materials for the same impression technique.  The antero-posterior error for the repositioning technique was more than twice that for the pickup technique.

The absence of systematic mean differences from the master model dimensions for other variables were already established in the ANOVAs.  Although not significant, the ranking of mean differences for all variables consistently showed larger mean differences for the repositioning impressions and a tendency to greater differences from the master cast for the President than for Impregum.

The two angular variables, axial rotation and axial inclination were subject to substantial scatter.  Ranges observed with the repositioning impressions were greater than for the pickup impressions (Fig 4,5).


Proc Univariate of the SAS system was used to assess the normality of the data and the ratio of variances was compared with a table of the F distribution for the appropriate numerator and denominator degrees of freedom. The results are shown in (Table 2).  No significant differences between the two elastomers were found for any of the experimental variables within each technique group.  However all comparisons between the two techniques, apart from that for the offset variable, showed the repositioning implant level results to be significantly more variable than the pickup at the abutment level results at the 95% confidence level. There were no significant differences in variance between the materials or techniques for the offset variable implying that all of the impressions were subject to similar scatter in this “height” relationship.  Once again, the axial rotation of the coping and the axial inclination showed particularly large differences between the repositioning and the pickup methods (F 3.16 to 70.97; Critical level of F = 2.91).



It is suggested that CeraOne system is intended for aesthetically critical anterior situations and for single tooth replacement.  positional errors in the restorative stages are unlikely to affect passive fit.  However, any rotational or dimensional discrepancy at the impression or working cast steps is likely to be unacceptable from the point of view of appearance, approximal contact points and occlusal requirements when the superstructure is tried in.  It has been proposed that such a problem can be the result of using the repositioning impression technique12,13.  However, the possibility of rotational error can take place with the pickup technique14.

Increasingly, reports in the literature advocates the use of implant level impression technique as a method to improve on the aesthetics of the restoration, reduce the number of treatment visits, or to compensate for malpositioned implants4,6-10.  The dental laboratory is then delegated the tasks of manufacturing a custom abutment, or of selecting the appropriate stock abutment5 and using it in the build up of the final prosthesis.  Since the implant head is relatively small, has short axial surfaces and lies a long way from the occlusal plane, we suggest that this technique is particularly susceptible to rotational, axial inclination and seating errors occurring at implant level.

The possibility of accumulative discrepancies from the different steps in making the prosthesis would appear to be a practical risk particularly with the growing use of implant level and repositioning impression procedures.

The method used was to simulate a true clinical situation, with the implant positioned in the location of a missing central at a distance often seen intraorally.  All the used components were from one implant system, the Branemark system, which is a limitation of this study.

Comparison of means was used to express whether or not a systematic difference in measurements existed between the methods, but the practical implications for a clinician who is likely to record a single impression depend more on the variability.    Indeed, the more variable the results in each group, the less likely it was that a significant difference between means could be demonstrated.  Where sample numbers are small, as was unavoidable in the present study, methods to make a formal comparison of variation, as distinct from central tendency, are controversial and highly dependent on normality of the data.    The range is perhaps the most intuitive indicator of scatter and is therefore presented in addition to F ratios of variances.

The results obtained from comparing the ranges indicated a wider range of variation when the repositioning impression technique was applied.  Much less variation was noticed between the materials, although those who favour Impregum may derive some encouragement from the data, albeit not supported statistically.

For majority of the casts from the repositioning technique, a larger variation was recorded for the anterio-posterior and mesio-distal position of the coping in comparison to the pickup technique.  It is important not to forget that these results are a measurement at a level close to the incisal edges of the rest of the dentition and not at the level of the implant head.

As for the rotation aspect of the coping and the axial inclination, similar results were recorded with the repositioning technique demonstrating more variable variation in comparison to the pickup technique.

All of these rotational errors should be noticed carefully, since any rotation will affect the position of the final restoration that will be made to fit this malpositioned laboratory analogue and the error is amplified by the length of the prosthesis.

The rotation of about three degrees for the transfer coping technique might not result in a clinical problem given the tolerances commonly incorporated in the gold cylinders used for the build up of the final restoration or equivalent internal relief to allow room for the cement.  Indeed, a tolerance of 1.6-5.3 degrees was measured with different implant abutment matings16.  However, the larger observed errors certainly would be alarming.

As for the offset of the point “Impl”, the majority of the results for both techniques showed that the laboratory analogue was at an increased distance from the occlusal reference plane indicating that the final restoration is more likely to be in supra-occlusion upon fitting.  This may be an indication of the failure of complete repositioning of the implant level impression coping, and might also be explained by the tendency of the transfer coping to dislodged out of the impression material upon the removal of the impression tray.  This result coincides with other reports about loss of accuracy in the z-axis with the repositioning technique17.

The two selected materials are recommended repeatedly in the dental literature.  Although the use of rigid impression material has been suggested to minimise the possibility of rotation distortion 18-20.  The results did not show any significant differences between the two materials when the means were compared which coincides with other reports 14,17.  This is consistent with the suggestion that the errors seen are random effects.  The range recorded for the two rotational variables did not favour one material to the other.  The clinician appears at liberty to select which errors he prefers.


Within the limitation of this study the following can be concluded:

1- the repositioning impression technique at the implant level showed more variation in the position of an abutment/implant analogue assembly in the resulted casts.

2- the pickup impression technique at the abutment level can be more predictable to use than the repositioning impression technique at the implant level.

3- no significant differences were found between President and Impregum F impression materials for impressions of the types tested.

The discrepancies observed would, if produced in a clinical setting, result in a need for adjustment or in some cases with the implant level technique even remaking, of the final restoration.


Table 1: The pilot measurement results





anterio-posterior position 

value in mm

mesio-distal position 

value in mm

roatation of the coping 

value in degree

axial inclination of the coping 

value in degree

offset of Impl point 

value in mm

Mean 16.326 15.354 -9.051 1.397 -13.578
SD 0.003 0.010 0.037 0.032 0.003





Table 2: F Ratios of variances for the five experimental variables.  Critical value of F = 2.91.  Comparisons with * are significant at p <0.05.


The measured positional variables
Compared material/technique anterio-posterior mesio-disal rotation inclination offset
I.Pic vs I.Rep 3.18* 14.27* 56.8* 3.16* 1.09
P.Pic vs P.Rep 3.75* 7.91* 39.24* 4.81* 2.5
P.Pic vs I.Pic 1.09 1.66 1.8 1.39 2.62
P.Rep  vs I.Rep 1.28 1.08 1.24 2.11 0.86
P.Rep  vs I.Pic 4.09* 6.15* 70.97* 6.69* 0.95
P.Pic vs I.Rep 2.91* 8.56* 31.4* 2.27 2.87


I.Pic: Impregum/pickup impression technique at the abutment level.


P.Pic: President/ pickup impression technique at the abutment level.


I.Rep: Impregum/ repositioning impression technique at the implant level.


P.Rep: President/ repositioning impression technique at the implant level.



Figure 3: Occlusal view of the measuring planes.


AP: anterio-posterior position of the coping

MD: mesio-distal position of the coping

R: axial rotational of the coping

Figure 4: The recorded ranges of the axial rotation of the coping.











I.Pic: Impregum/pickup impression technique at the abutment level.


P.Pic: President/ pickup impression technique at the abutment level.


I.Rep: Impregum/ repositioning impression technique at the implant level.


P.Rep: President/ repositioning impression technique at the implant level.



Figure 5: The recorded ranges for the axial inclination of the coping.









I.Pic: Impregum/pickup impression technique at the abutment level.


P.Pic: President/ pickup impression technique at the abutment level.


I.Rep: Impregum/ repositioning impression technique at the implant level.


P.Rep: President/ repositioning impression technique at the implant level.




Figure 1: the used master model with single tooth implant.  Note the nickel chrome inserts.

Figure 2: Lateral view of the master model with the aluminium reference plate and coping in situe.


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