Medical knowledge | Dentistry » Firas-Derrick-Lloyd - A laboratory investigation of the accuracy of two impression techniques for single tooth implants

A Laboratory Investigation of the Accuracy of Two Impression Techniques for Single-Tooth Implants M. Firas Daoudi, DDS, LDS RCS, MSc (Lond)a Derrick J. Setchell, BDS, MS (Mich), LDS RCS, FDS RCSb Lloyd J. Searson, BDS, MS (Mich), FDS RCSc Purpose: This laboratory study investigated the accuracy of four implant impression procedures using two impression techniques and two different materials. Materials and Methods: A master model was used to produce 40 different stone casts incorporating laboratory implant or abutment analogues from the different combinations of two impression techniques (the repositioning impression coping technique at the implant level and the pickup impression technique at the abutment level) and materials (President polyvinyl siloxane and Impregum F polyether). Variations in the resulting working casts were measured using the Reflex Microscope to derive distances and angles from the three-dimensional coordinates of optical targets that were attached to a test

coping placed on the implant analogue and on a reference device positioned on the occlusal surfaces of the casts. Results: 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 polyvinyl siloxane and polyether impression materials for the two tested types of impression techniques. Conclusion: 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. Int J Prosthodont 2001;14:152–158 T he long-term success rate of osseointegrated implants has made implant-supported prostheses a valid option for the treatment of missing teeth.1,2 An acceptable prosthesis requires optimal accuracy in all 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 prostheses. 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 using two methods: the pickup method or the repositioning method, in which 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 benefits. Indeed, many reports advocate the use of this technique to facilitate the provision of a temporary restoration.4 Another attraction of this technique is that it allows the selection of the proper abutment in the laboratory,5 and it can enable the use of custom-made or

adjusted abutments.6–10 a Research Fellow, Conservation Department, Eastman Dental Institute for Oral Healthcare Sciences, University of London, United Kingdom. b Head of Unit and Senior Lecturer/Consultant in Restorative Dentistry, Eastman Dental Institute for Oral Healthcare Sciences, University of London, United Kingdom. cConsultant in Restorative Dentistry, Eastman Dental Institute for Oral Healthcare Sciences, University of London, United Kingdom. Reprint requests: Dr M. Firas Daoudi, 23 Blandford Court, Brondsbury Park, London NW6 7BP, United Kingdom. Fax: + 020 8459 3496. e-mail: fdaoudi@hotmailcom This study is condensed from a research report submitted by M. F Daoudi in partial fulfillment of the requirements for the Master of Science in Conservative Dentistry, University of London, 1997. The International Journal of Prosthodontics 152 Volume 14, Number 2, 2001 Daoudi et al Accuracy of Two Impression Techniques for Single Implants Impression Making and Working Cast

Fabrication 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, in which it is difficult to reposition the impression coping correctly in elastic material.12 Indeed, it has been reported that the impression copings cannot be repositioned accurately in either polyether or polyvinyl siloxane impression materials.13 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 recommended.14 As for the impression technique at the abutment level,

Schmitt et al15 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. A mold was used to produce 40 similar-sized custom trays made of autopolymerized acrylic resin (Formatray, Kerr). Two elastomeric impression materials, light- and heavy-body polyvinyl siloxane (President, Coltene) and polyether (Impregum F, ESPE), 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) with President material was applied. Then, the same impression technique was used with Impregum. After that, a new 3-mm CeraOne abutment (SDCA 334, Nobel Biocare) was connected to

the implant. A plastic impression coping (DCB 119, Nobel Biocare) was seated on the abutment, and the pickup impression technique was used with President material. Finally, the same impression technique was used with Impregum. 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 taken to make 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 digitized ten times for each implant to study the reproducibility and precision of the measurement technique and as a precaution, because the implant, once removed, could not be replaced precisely after a new implant had been placed

into the master model. Abutment analogues (DCA 129, Nobel Biocare), and implant analogues (SS, DCC 084, Nobel Biocare) were connected to the impression copings according to the manufacturer’s recommendations and repositioned when applicable. Each group of impressions resulting from the same implant were poured using the same mix of Silky Rock model stone (Whip Mix). This resulted in 40 models, ten with each combination of the impression material and technique. Methods and Materials Model Preparation A dentate maxillary acrylic resin model missing the right central incisor was used. In the position of the missing tooth, a hollow tubular implant holder was fitted, into which ten implants (7 mm  3.75 mm, SDA 001, Nobel Biocare) could be placed and interchanged. Each implant was held rigidly in position using GC acrylic resin. Three nickel-chrome inserts with the shapes of a flat surface, a conical socket, and a V block were fit onto the occlusal surfaces of the right first molar, left

first premolar, and left second molar, respectively (Fig 1). They served to apply the surveying principles whereby placing spherical ball bearings into these inserts would constrain the relative positions in three, two, and one axes, respectively. This would provide reproducibility in the positioning of two aluminum reference plates. Each plate had three ball bearings attached to adjustable M4 screw posts that allowed the plate to be located precisely over the inserts. The first plate had a long titanium implant mount (DIA 282, Nobel Biocare) attached to it as a jig for implant mounting. The second plate carried precise optical targets to simplify the measurements (Fig 2). Volume 14, Number 2, 2001 Measurement Phase The second aluminum 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 internal angle of the X targets provided a point for measurement, and the

threedimensional center of gravity of these four points 153 The International Journal of Prosthodontics Accuracy of Two Impression Techniques for Single Implants Daoudi et al Fig 1 Master model with single-tooth implant. Note the nickelchrome inserts Fig 2 Lateral view of the master model with the aluminum reference plate and coping in situ. Coping reference plane 1 2 R Impl 4 3 AP MD B A First reference plane C D Fig 3 Occlusal view of the measuring planes. Impl = center of the incisal edge of a prosthesis that would be constructed on the cast; AP = anteroposterior position of coping; MD = mesiodistal position of coping; R = axial rotation of coping. analogous to the center of the incisal edge of a prosthesis that would be constructed on the cast; this was 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 (Reflex Measurement) was used for measurement according to a programmed observation plan, which also computed five analytic variables. These variables enabled comparison of the anteroposterior position, mesiodistal 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 was taken as the center of each target. This resulted in a total of 16 reference points and four centers (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 the CeraOne abutment, was used to carry a rectangular 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 center Each corner of the rectangle 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 three-dimensional center of gravity of which was regarded as The International Journal of Prosthodontics 154 Volume 14, Number 2, 2001 Daoudi et al Table 1 Pilot Measurement Results Anteroposterior Mesiodistal position (mm) position (mm) Mean SD Accuracy of Two Impression Techniques for Single Implants 16.33 0.003 Rotation of Axial inclination of Offset of “Impl” coping (degrees) coping (degrees) point (mm) 15.35 0.010 –9.05 0.037 1.40 0.032 –13.58 0.003 SD = standard deviation. between the means for the repositioning implantlevel technique and those for the pickup abutmentlevel impressions. There were no significant

differences between materials for the same impression technique. The anteroposterior 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 was 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 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 (Figs 4 and 5). and the perpendicular height offset of the point “Impl” from the plate plane indicated, respectively, the deviation in axial inclination (ie, tilting) and the occlusogingival 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 to allow pairwise 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 (ANOVA) 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 anteroposterior variable. Variability The 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 (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 = 291) Results The impression material was not a significant factor for any of the measured variables. The results showed a significant difference for the technique type only with the anteroposterior variable (F value 14.59, P = 00005) Differences between means for the height offset of the “Impl” point approached significance (F value 3.9, P = 0056) The results from Duncan’s test at the 95% confidence level showed that the significant differences for the anteroposterior position of the analogue were Volume 14, Number 2, 2001 155 The International Journal of Prosthodontics Daoudi et al 30 12 25 10 Inclination (degrees) Rotation (degrees) Accuracy of Two Impression Techniques for Single Implants 20 15 10 6 4 2 5 0 8 0 IRep IPic PRep IRep PPic Fig 4 Recorded ranges for

the axial rotation of the coping. IRep = Impregum, repositioning impression at implant level; IPic = Impregum, pickup impression at abutment level; PRep = President, repositioning impression at implant level; PPic = President, pickup impression at abutment level. Table 2 IPic PRep PPic Fig 5 Recorded ranges for the axial inclination of the coping. IRep = Impregum, repositioning impression at implant level; IPic = Impregum, pickup impression at abutment level; PRep = President, repositioning impression at implant level; PPic = President, pickup impression at abutment level. F Ratios of Variances for the Five Experimental Variables* Compared material and technique IPic vs IRep PPic vs PRep PPic vs IPic PRep vs IRep PRep vs IPic PPic vs IRep Anteroposterior Mesiodistal Rotation Inclination Offset 3.2† 3.8† 1.1 1.3 4.1† 2.9† 14.3† 7.9† 1.7 1.1 6.2† 8.6† 56.8† 39.2† 1.8 1.2 71.0† 31.4† 3.2† 4.8† 1.4 2.1 6.7† 2.3 1.1 2.5 2.6 0.9 1.0 2.9

*Critical value of F = 2.91 †Significant at P < 0.05 IPic = Impregum, pickup impression at abutment level; IRep = Impregum, repositioning impression at implant level; PPic = President, pickup impression at abutment level; PRep = President, repositioning impression at implant level. Discussion then delegated the tasks of manufacturing a custom abutment, or of selecting the appropriate stock abutment5 and using it in the buildup of the final prosthesis. Because 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 cumulative 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 intended to simulate a true clinical

situation, with the implant positioned in the location of a missing central incisor at a distance often seen intraorally. All components used were from one implant system, the Brånemark system (Nobel Biocare), which is a limitation of this study. It is suggested that the CeraOne system is intended for esthetically critical anterior situations and for singletooth 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 technique.12,13 However, rotational error can take place with the pickup technique.14 Increasingly, reports in the literature advocate the use of the implant-level impression technique as a

method to improve the esthetics of the restoration, reduce the number of treatment visits, or compensate for malpositioned implants.4,6–10 The dental laboratory is The International Journal of Prosthodontics 156 Volume 14, Number 2, 2001 Daoudi et al Accuracy of Two Impression Techniques for Single Implants Comparison of means was used to express whether 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, but those who favor Impregum may derive some encouragement from the data, although it is not supported statistically. For the majority of the casts from the repositioning technique, a larger variation was recorded for the anteroposterior and mesiodistal positions 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 rotational 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

noted carefully because 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 3 degrees for the transfer coping technique might not result in a clinical problem, given the tolerances commonly incorporated into the gold cylinders used for the buildup of the final restoration or equivalent internal relief to allow room for the cement. Indeed, a tolerance of 1.6 to 53 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 was more likely to be in supraocclusion 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 dislodge out of the impression material upon the removal of the Volume 14, Number 2, 2001 impression tray. This result coincides with other reports about loss of accuracy in the Z axis with the repositioning technique.17 The two selected materials are recommended repeatedly in the dental literature, although the use of rigid impression material has been suggested to minimize 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 favor one material over the other. The clinician appears at liberty to select which errors he or she prefers Conclusions Within the limitations 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 resulting 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. Acknowledgment Implant components used in this study were donated by Nobel Biocare UK. References 1. 2. 3. 4. 157 Adell R, Lekholm U, Rockler B, Brånemark P-I. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387–416 Priest G. Single tooth implants and their role in

preserving remaining teeth: A 10-year survival study Int J Oral Maxillofac Implants 1999;14:181–188. Parel SM. Esthetic Implant Restorations: Brånemark System Solutions for the Partially Edentulous Patient. Dallas: Taylor, 1996:179–183. Hochwald D. Surgical template impression during stage I surgery for fabrication of a provisional restoration to be placed at stage II surgery. J Prosthet Dent 1991;66:796–798 The International Journal of Prosthodontics Accuracy of Two Impression Techniques for Single Implants Daoudi et al 5. Kupeyan HK, Lang BR The role of the implant impression in abutment selection: A technical note Int J Oral Maxillofac Implants 1995;10:429–433. 6. Corrente G, Vergnano L, Pascetta R, Ramadori G A new custom made abutment for dental implants: A technique note Int J Oral Maxillofac Implants 1995;10:604–608. 7. Jemt T Modified single and short-span restorations supported by osseointegrated fixtures in the partially edentulous jaw. J Prosthet Dent

1986;55:243–246. 8. Lewis SG, Beumer J, Perri GR, Hornburg WP Single tooth implant supported restoration. Int J Oral Maxillofac Implants 1988;3:25–30 9. Prestipino V, Ingber A Esthetic high-strength implant abutments Part I. J Esthet Dent 1993;5:29–36 10. Daoudi MF Case report: Temporary restoration for a single tooth implant prosthesis with adverse axial inclination of the fixture. Eur J Prosthdont Restorative Dent 1999;7:95–97 11. Henry PJ, Tan AE, Uzawa S Fit discrimination of implant-supported fixed partial dentures fabricated from implant level impressions made at stage I surgery. J Prosthet Dent 1997;77:265–270 12. Watson RM, Newman P Monitoring procedures for the singletooth implant-stabilized crown with an internally located customized abutment Quintessence Int 1996;27:347–352 13. Liou AD, Nicholls JI, Youdelis RA, Brudvik JS Accuracy of replacing three tapered transfer impression copings into two elastomeric impression materials Int J Prosthodont 1993;6:377–383

14. Wee AG Comparison of impression materials for direct multiimplant impressions J Prosthet Dent 2000;83:323–331 15. Schmitt JK, Adrian ED, Gardner FM, Caston ML A comparison of impression techniques for the CeraOne abutment. J Prosthodont 1994;3:145–148. 16. Binon PP Evaluation of three slip fit hexagonal implants Implant Dent 1996;5:235–248. 17. Barrett M, de Rijk W, Burgess J The accuracy of six impression techniques for osseointegrated implants. J Prosthdont 1993;2: 75–82. 18. Hsu CC, Millstein PL, Stein RS A comparative analysis of the accuracy of implant transfer techniques J Prosthet Dent 1993;69: 588–593. 19. Phillips KM, Nicholls JI, Ma T, Rubenstein JE The accuracy of three implant impression techniques: A 3-dimensional analysis. Int J Oral Maxillofac Implants 1994;9:533–540. 20. Assif D, Marshak B, Schmidt A Accuracy of implant impression techniques. Int J Oral Maxillofac Implants 1996;11:216–222 Literature Abstract Rehabilitation of biting abilities in

patients with different types of dental prostheses. The purposes of this study were to compare the biting abilities among subjects with different types of prostheses and to detect any difference between biting abilities without (before insertion of) and with (immediately after insertion of) renewed prostheses using a pressure-detecting sheet (Prescale). Biting abilities per person (biting force in N, biting pressure in MPa, and occlusal contact area in mm2) were assessed by the method of Miyaura et al (1999) In the first part, 590 healthy volunteers (293 men, mean age 45 years; 297 women, mean age 50 years) were divided into four groups according to the type of posterior dentition: complete denture, removable partial denture, fixed partial denture, and complete natural dentition. The results showed that the biting forces of the fixed partial, removable partial, and complete denture wearers were 80%, 35%, and 11%, respectively, of that in subjects with a natural dentition. The complete

denture wearers showed the highest biting pressure among the four groups, followed by the removable partial denture wearers. In a second part comprising a clinical intraindividual study, the biting abilities of 85 subjects without (before insertion of) and with (after insertion of) renewed prostheses were compared. No significant differences were found between biting before and immediately after insertion of the prostheses In 18 subjects examined again 2 months after insertion of the prostheses, the biting force and occlusal contact area had increased The results confirmed past clinical studies indicating an impaired masticatory function of denture wearers with some functional adaptation to new prostheses 2 months after insertion. Miyaura K, Morita M, Matsuka Y, Yamashita A, Watanabe T. J Oral Rehabil 2000;27:1073–1076 References: 11. Reprints: Prof Tasuo Watanabe, Department of Preventive Dentistry, Okayama University Dental School, 2-5-1 Shikata-cho, Okayama 700-8525, JapanAW The

International Journal of Prosthodontics 158 Volume 14, Number 2, 2001